nanomaterials

Review Full-Color Realization of Micro-LED Displays

Yifan Wu, Jianshe Ma, Ping Su * , Lijun Zhang and Bizhong Xia

Tsinghua Shenzhen International Graduate School, Tsinghua University, Shenzhen 518055, China; [email protected] (Y.W.); [email protected] (J.M.); [email protected] (L.Z.); [email protected] (B.X.) * Correspondence: [email protected]



Received: 25 October 2020; Accepted: 7 December 2020; Published: 10 December 2020


Abstract: Emerging technologies, such as smart wearable devices, augmented reality (AR)/virtual reality (VR) displays, and naked-eye 3D projection, have gradually entered our lives, accompanied by an urgent market demand for high-end display technologies. Ultra-high-resolution displays, flexible displays, and transparent displays are all important types of future display technology, and traditional display technology cannot meet the relevant requirements. Micro-light-emitting diodes (micro-LEDs), which have the advantages of a high contrast, a short response time, a wide color gamut, low power consumption, and a long life, are expected to replace traditional liquid-crystal displays (LCD) and organic light-emitting diodes (OLED) screens and become the leaders in the next generation of display technology. However, there are two major obstacles to moving micro-LEDs from the laboratory to the commercial market. One is improving the yield rate and reducing the cost of the mass transfer of micro-LEDs, and the other is realizing a full-color display using micro-LED chips. This review will outline the three main methods for applying current micro-LED full-color displays, red, green, and blue (RGB) three-color micro-LED transfer technology, color conversion technology, and single-chip multi-color growth technology, to summarize present-day micro-LED full-color display technologies and help guide the follow-up research.

Keywords: micro-LED; full-color display; mass transfer; color conversion; monolithic growth

1. Introduction Augmented reality (AR) and virtual reality (VR) technologies are developing rapidly with the maturity of artificial intelligence and image recognition technology. Among them, near-eye displays (NEDs) and head-mounted displays (HMDs) are key devices for AR/VR products. At present, mainstream near-eye displays are mostly liquid-crystal displays (LCDs) and organic light-emitting diodes (OLEDs) [1]. However, LCDs have a slow response speed, poor conversion efficiency, and low color saturation and are gradually being replaced by OLED displays that have the advantages of self-luminescence, wide viewing angles, high contrast, low power consumption, and fast response speeds. In addition, the excellent flexibility and transparency of OLEDs also provides a broad space for emerging electronic products, such as curved displays, smart watches, and folding displays [2]. However, due to the characteristics of OLED’s organic light-emitting materials, defects, such as their rapid aging, short life span, and low color purity, have been gradually exposed [3]. Therefore, both LCD and OLED technologies have technical limitations and cannot perfectly adapt to the display performance of AR/VR. Mini-LEDs and micro-LEDs, which inherited the characteristics of inorganic LEDs and have micron-level dimensions, have become popular as the next generation of display technology. Micro-LED technology was first proposed by a research team at Texas Tech University in 2000 [3]. The pixel pitch limit to distinguish Mini-LEDs and micro-LEDs is 100 µm, and those smaller than 100 µm are defined as micro-LEDs [4]. With the advancement of the manufacturing process,

Nanomaterials 2020, 10, 2482; doi:10.3390/nano10122482 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 2482 2 of 33 the pixel size and pixel pitch of micro-LEDs have gradually decreased from 100 µm. In terms of functionality, Mini-LEDs are mainly used as backlight displays [5,6], while micro-LEDs [7], which have the advantages of a high contrast, a high response speed, a wide color gamut, low power consumption, and a long life, are considered to be an ideal display technology for AR/VR. Table1 provides a comparison of micro-LED with other display technologies.

Table 1. Performance comparison of micro-light-emitting diodes (micro-LEDs) with liquid-crystal displays (LCD) and organic light-emitting diodes (OLED).

Display Technology LCD OLED Micro-LED Luminous mode Backlight module Self-luminous Self-luminous Luminous efficiency Low Medium High Brightness (cd/sqm) 3000 1000 10,000 Contrast 5000:1 10,000:1 1,000,000:1 Life (hours) 60 K 20–30 K 80–100 K Color rendering 75% NTSC 124% NTSC 140% NTSC Response time Millisecond level Microsecond level Nanosecond level Energy consumption High About 60–85% of LCD About 30–40% of LCD Pixels per inch (PPI) 800 500 >2000 NTSC—National Television Standards Committee.

The world’s first micro-LED display was launched by Sony at the 2012 Consumer Electronics Show (CES) exhibition and was called crystal light-emitting diodes (CLED). This display has a size of 55 inches, with unprecedented picture color, response speed, and high contrast. In the following years, Sony officially launched this technology to the market, successfully opening the door to the commercialization of micro-LED display technology. In 2018, Samsung showed off the world’s first spliced 146–inch micro-LED TV, “the Wall”; X-Celeprint also demonstrated their 5.1–inch micro-LED full-color display, and LG introduced a 175–inch micro-LED display in the same year. In 2020, many micro-LED companies, such as Jade Bird Display, Lumiode, Plessey, PlayNitride, etc., have begun to gradually launch their own products, and Vuzix is expected to launch its first micro-LED glasses next year. PlayNiride also plans to build its second micro-LED production line, which will increase its productivity by 2–3 times. It can thus be seen that micro-LED has gradually stepped onto the stage of the display field. In addition, micro-LEDs also have great potential in neural photosensitization [8,9], microscopic imaging [10], flexible display backlighting [5,11], biological detection [12], cochlear implants [13], and visible light communication (VLC) [14–17]. However, despite such excellent performance, micro-LED display face many difficulties in their commercial mass production. The two main technical problems of this technology are the mass transfer of LED chips [18] and the production of a full-color display [19]. For inorganic LEDs, the size of the grown wafer is limited, usually to 4 inches or 8 inches [20]. Therefore, to realize a large-area micro-LED display, it is necessary to transfer the LED array grown on the wafer to a circuit substrate that can drive it to illuminate. This process requires the support of mass transfer technology. The mass transfer technologies applied to micro-LEDs mainly include pick and place technology, self-assembly technology, and selective release and transfer technology, each of which has its own advantages and limitations; moreover, the accuracy and yield have a great influence on the performance of the display. A 4K full-color TV includes (3840 2160 3) × × pixels. If the mass transfer technology used can guarantee a 99.99% yield, 2488 pixels may still be damaged. This will undoubtedly increase the difficulty and cost of later maintenance. Therefore, the yield and cost of the massive transfer limit the development of micro-LED technology to a certain extent. Another difficulty of micro-LED displays is the realization of full color. Many teams have reported on the development of micro-LEDs [3,21–28], but these studies only briefly describe the realization of a full-color micro-LED solution. This review will introduce in detail the three main methods for achieving a full-color micro-LED: (1) integrating red, green, and blue (RGB) micro-LED chips through transfer Nanomaterials 2020, 10, 2482 3 of 33 and bonding, (2) adding a color conversion layer on a mono-color or bi-color array, and (3) directly growingNanomaterials multi-color 2020, 10, x FOR micro-LEDs PEER REVIEW on the same substrate. 3 of 33

2.2. Chip Transfer andand BondingBonding TheThe mostmost directdirect solutionsolution toto achieveachieve aa full-color micro-LED isis toto epitaxially grow RGB micro-LED chipschips on GaN and and GaAs GaAs wafers. wafers. Then, Then, through through large-area large-area and and high-precision high-precision transfer transfer technology, technology, the thethree-color three-color micro-LEDs micro-LEDs are bonded are bonded onto ontothe same the samedrive drivesubstrate substrate to form to an form RGB an micro-LED RGB micro-LED display displayarray. Independent array. Independent addressing addressing and Pulse and Pulse width width modulation modulation (PWM) (PWM) technology technology can can control control thethe brightnessbrightness andand grayscalegrayscale ofof eacheach RGBRGB pixel,pixel, whichwhich cancan realizerealize aa full-colorfull-color ofof thethe micro-LEDmicro-LED display.display.

2.1. Horisontally and Vertically Stacked Micro-LED Structures 2.1. Horisontally and Vertically Stacked Micro-LED Structures There are two main arrangement structures for obtaining a full-color micro-LED array in this There are two main arrangement structures for obtaining a full-color micro-LED array in this way: a horizontal arrangement structure and a vertical stack structure. In 2016, Deng et al. used way: a horizontal arrangement structure and a vertical stack structure. In 2016, Deng et al. used chips chips on board (COB) technology to integrate RGB three-color micro-LEDs on a transparent quartz on board (COB) technology to integrate RGB three-color micro-LEDs on a transparent quartz substrate (LEDoTS) with addressable pixels to form a color display [29]. The display substrate area substrate (LEDoTS) with addressable pixels to form a color display [29]. The display substrate area was 5 5 mm2, and the resolution was 5 5 3. The side length of each RGB pixel was 1 mm, and the was 5 ×× 5 mm2, and the resolution was 5 ×× 5 × 3. The side length of each RGB pixel was 1 mm, and the emission wavelengths of the RGB three-color LEDs were 450 nm, 565 nm, and 630 nm, respectively. emission wavelengths of the RGB three-color LEDs were 450 nm, 565 nm, and 630 nm, respectively. The planar structure is shown in Figure1a. Field-programmable gate array (FPGA), as the main control The planar structure is shown in Figure 1a. Field-programmable gate array (FPGA), as the main unit of a LEDoTS color display, controls the cathode constant current driver and the anode scan driver control unit of a LEDoTS color display, controls the cathode constant current driver and the anode through dynamic scanning and PWM modulation to realize the independent addressing and dimming scan driver through dynamic scanning and PWM modulation to realize the independent addressing of each RGB sub-pixel. Its working principle is shown in Figure1b. In 2017, the authors used the same and dimming of each RGB sub-pixel. Its working principle is shown in Figure 1b. In 2017, the authors technology to develop a smaller 8 8 4 R-G-B-IR micro-LED array [30]. The dimensions of the blue, used the same technology to develop× × a smaller 8 × 8 × 4 R-G-B-IR micro-LED array [30]. The green, red, and infrared LEDs were 230 µm 385 µm, 210 µm 210 µm, 225 µm 225 µm, and 285 µm dimensions of the blue, green, red, and infrared× LEDs were 230× μm × 385 μm, ×210 μm × 210 μm, 225 285 µm, respectively. The peak wavelengths of the emission spectrum were 460 nm, 572 nm, 630 nm, ×μm × 225 μm, and 285 μm × 285 μm, respectively. The peak wavelengths of the emission spectrum andwere 850 460 nm, nm, and 572 the nm, maximum 630 nm, and half-wavelength 850 nm, and the was maximum less than 30half-wavelength nm. was less than 30 nm.

FigureFigure 1.1. (a(a)) Horizontally Horizontally arranged arranged red, red, green, green, and, and, blue bl (RGB)ue (RGB) three-color three-color light-emitting light-emitting diodes diodes (LED) pixel(LED) plane pixel structure; plane structure; (b) horizontally (b) horizontally arranged RGBarranged micro-LED RGB micr arrayo-LED working array principle. working Reproduced principle. fromReproduced [29], with from permission [29], with from permi Institutession from of Electrical Institute and of Electrical Electronics and Engineers, Electronics 2016. Engineers, 2016.

VerticallystackedVertically stacked LED LED structures structures have have been been studied studied for for a long a long period period of time of time [31,32 [31,32].]. For micro-LEDFor micro- displaysLED displays with highwith resolutionhigh resolution on a limitedon a limited substrate, substrate, the area the ofarea the of substrate the substrate occupied occupied by each by RGBeach pixelRGB ofpixel the of vertically the vertically stacked stacked RGB micro-LED RGB micro-LED array is reducedarray is byreduced 2/3 compared by 2/3 compared to the horizontal to the arrangementhorizontal arrangement structure. Instructure. 2017, Kang In 2017, et al. Kang proposed et al. aproposed color-tunable a color-tunable display composed display composed of blue and of greenblue and passive green matrix passive micro-LED matrix arraysmicro-LED stacked arrays vertically stacked [33 ].vertically This display [33]. arrayThis candisplay independently array can addressindependently each pixel, address and, each by modulating pixel, and, theby PWMmodulati dutyng cycles the PWM of the duty blue cycles or green of the micro-LED blue or drivegreen voltages,micro-LED the drive light-emitting voltages, wavelengthsthe light-emitting of each wavele pixel canngths be of changed each pixel from can 450 be nm changed to 540 nm, from as 450 shown nm into Figure540 nm,2a. as shown in Figure 2a. TheThe preparationpreparation ofof thisthis arrayarray isis shownshown inin FigureFigure2 2b.b. BlueBlue andand greengreen micro-LEDs micro-LEDs areare grown grown on on double-side-polisheddouble-side-polished (DSP) (DSP) sapphire. sapphire. After After that, that, the th entiree entire wafer wafer is decomposed is decomposed into individual into individual pixels pixels with sizes of 75 μm × 75 μm by inductively coupled plasma (ICP) etching, and each pixel chip is bonded onto the receiving substrate through mass transfer technology. Each sub-pixel is connected with a metal cross line, and the column line and the row line are electrically isolated. The green passive matrix in the lower layer needs to be slightly larger than the blue passive matrix in the upper layer (their dimensions are 0.8 × 0.8 cm2 and 1.2 × 1.2 cm2, respectively). Finally, after accurately

Nanomaterials 2020, 10, 2482 4 of 33 with sizes of 75 µm 75 µm by inductively coupled plasma (ICP) etching, and each pixel chip is × bonded onto the receiving substrate through mass transfer technology. Each sub-pixel is connected with a metal cross line, and the column line and the row line are electrically isolated. The green passiveNanomaterials matrix 2020 in, 10 the, x FOR lower PEER layer REVIEW needs to be slightly larger than the blue passive matrix in the upper4 of 33 layer (their dimensions are 0.8 0.8 cm2 and 1.2 1.2 cm2, respectively). Finally, after accurately × × aligningaligning eacheach pixelpixel ofof thethe twotwo arrays,arrays, thethe photoresistphotoresist isis usedused forfor bondingbonding underunder aa temperaturetemperature ofof 2 250250 ◦°CC andand aa pressurepressure ofof 11 kgkg/cm/cm 2.. TheThe devicedevice hashas aa transmittancetransmittance ofof 63%63% andand fourfour modes:modes: (1)(1) greengreen independentindependent lightlight emission,emission, (2)(2) blueblue independentindependent lightlight emission,emission, (3)(3) blueblue andand greengreen lightlight emissionemission inin didifferentfferent positions,positions, andand (4)(4) blueblue andand greengreen lightlight emissionemission inin thethe samesame pixelpixel positionposition superimposed.superimposed. TheThe resultsresults of of the the optical optical performance performance tests tests of theseof these four four modes modes are shownare shown in Figure in Figure2c. When 2c. When only blueonly lightblue islight emitted, is emitted, the peak the peak of its of electroluminescence its electroluminescence (EL) spectrum(EL) spectrum is at ais wavelength at a wavelength of 450 of nm, 450 and,nm, whenand, when only greenonly green light islight emitted, is emitted, the peak the ofpeak its ELof its spectrum EL spectrum is at a is wavelength at a wavelength of 540 of nm. 540 Each nm. pixelEach ofpixel the deviceof the device can be addressedcan be addressed independently, independently, and the lightand ofthe di lightfferent of wavelengthsdifferent wavelengths can be emitted can atbe diemittedfferent at pixels different at the pixels same time.at the Itsame is also time. possible It is also to adjust possible the PWMto adjust duty the cycle PWM at theduty same cycle pixel at the to changesame pixel the emissionto change wavelength the emission from wavelength 450 nm to from 540 nm.450 However,nm to 540 thenm. performance However, the of performance such vertically of stackedsuch vertically LEDs is severelystacked limitedLEDs is by severely the light absorptionlimited by andthe poorlight thermal absorption conductivity and poor of adjacentthermal narrow-bandconductivity gapof adjacent quantum narrow- wellsband (QWs). gap quantum wells (QWs).

FigureFigure 2.2. ((aa)) ElectroluminescenceElectroluminescence (EL)(EL) spectraspectra measuredmeasured byby changingchanging thethe PulsePulse widthwidth modulationmodulation (PWM)(PWM) dutyduty ratiosratios ofof thethe greengreen sub-pixelssub-pixels andand blueblue sub-pixels,sub-pixels, andand microscopemicroscope imagesimages atat thethe samesame b pixelpixel positionspositions of the micro-LED arra arrayy under under different different PWM PWM duty duty ratios; ratios; (b () manufacturing) manufacturing steps steps of of vertically stacked passive matrix micro-LED arrays; (c) four emission modes of vertically stacked vertically stacked passive matrix micro-LED arrays; (c) four emission modes of vertically stacked passive matrix micro-LED arrays. Reproduced from [33], The Optical Society, 2017. passive matrix micro-LED arrays. Reproduced from [33], The Optical Society, 2017. In 2020, Ostendo introduced a multi-color addressable micro-LED display—the Quantum Photonic In 2020, Ostendo introduced a multi-color addressable micro-LED display—the Quantum Imager (QPI), with a vertical structure to produce a full-color micro-LED display [34]. The pixel pitch Photonic Imager (QPI), with a vertical structure to produce a full-color micro-LED display [34]. The of the RGB three-color micro-LED chips are less than 10 µm, where the blue and green layers are based pixel pitch of the RGB three-color micro-LED chips are less than 10 μm, where the blue and green on GaN, as well as the red layers are based on AlInGaP. The RGB three-color single chips are stacked layers are based on GaN, as well as the red layers are based on AlInGaP. The RGB three-color single on the same substrate (from top to bottom: red, green, and blue) through wafer-wafer fusion bonding chips are stacked on the same substrate (from top to bottom: red, green, and blue) through wafer- techniques, and the RGB light emitting shares the same optical aperture. The micro-LED RGB array is wafer fusion bonding techniques, and the RGB light emitting shares the same optical aperture. The controlled by the underlying silicon Complementary Metal Oxide Semiconductor (CMOS) substrate, micro-LED RGB array is controlled by the underlying silicon Complementary Metal Oxide which can provide digital control logic and power for each pixel RGB micro-LED. Through PWM Semiconductor (CMOS) substrate, which can provide digital control logic and power for each pixel modulation, color correction, luminance correction, and uniformity correction, electrical and optical RGB micro-LED. Through PWM modulation, color correction, luminance correction, and uniformity information are transmitted in multiple bonding interfaces of the stack structure so that the multicolor correction, electrical and optical information are transmitted in multiple bonding interfaces of the emissions and displays of a single vertical structure chip can be realized. The vertical structure of stack structure so that the multicolor emissions and displays of a single vertical structure chip can be the micro-LED RGB array first needs RGB three-color micro-LED chips to be separately epitaxially realized. The vertical structure of the micro-LED RGB array first needs RGB three-color micro-LED grown to prepare wafers of the same size with a pitch of 5–10 µm. The three processed wafers are then chips to be separately epitaxially grown to prepare wafers of the same size with a pitch of 5–10 μm. sequentially bonded to the same drive substrate, and the alignment accuracy between the bonded The three processed wafers are then sequentially bonded to the same drive substrate, and the alignment accuracy between the bonded wafers is better than 1 μm each time. Afterward, the substrate and part of the P- or N-doped layer are removed by laser lift-off technology or grinding and wet etching technology (depending on the growth substrate). Finally, the RGB three-color wafers are bonded by wafer–wafer fusion technology. A QPI display has the advantages of full color, high resolution, high brightness, high contrast, and low power consumption, and its vertical structure enables it to achieve higher pixel density on small size displays, which is very suitable for wearable AR/VR devices.

Nanomaterials 2020, 10, 2482 5 of 33 wafers is better than 1 µm each time. Afterward, the substrate and part of the P- or N-doped layer are removed by laser lift-off technology or grinding and wet etching technology (depending on the growth substrate). Finally, the RGB three-color wafers are bonded by wafer–wafer fusion technology. A QPI display has the advantages of full color, high resolution, high brightness, high contrast, and low power consumption, and its vertical structure enables it to achieve higher pixel density on small size displays, which is very suitable for wearable AR/VR devices. At present, RGB micro-LED chips can be accurately and efficiently transferred from one substrate to another through multiple transfer methods, such as elastomer stamp technology, laser induction technology, fluid transportation technology, electrostatic transfer technology, and roll-to-roll (roll-to-panel) technology. In the following, we briefly describe these methods for micro-LED mass transfer and analyze their theory, features, and application scope.

2.2. Elastomer Stamp Transfer Elastomer stamp transfer technology can transfer a large number of microstructures from one substrate to another at high speed [35–38]. Rogers’ team initially used elastomer stamps for large-scale transfer printing [11]. This technology closely contacts the micro devices on the donor substrate with an elastomer stamp array matching the pitch of the micro devices on the donor substrate. Through the van der Waals force between the elastomer stamp and the latter, the micro device is picked up and transferred to the receptor substrate. The picking and placing of the elastomer stamp on the microstructure are determined by the peeling speed after the two contact each other. By pulling the stamp away from the donor substrate at a sufficiently high peeling speed (usually faster than 10 cm/s), the elastomer stamp can generate strong adhesive force to adhere the microstructure to the surface of the stamp and remove the target device from the donor substrate. When the peeling speed of the stamp is less than 1 mm/s, the microstructure will be separated from the stamp and remain on the receptor substrate instead of being adsorbed by the stamp. In 2005, Lee et al. proposed two elastomer stamp transfer methods that can realize the selective transfer of thin film transistors (TFT) to rigid or flexible substrates [39]. The transfer processes of these two methods are shown in Figure3. The first method uses polydimethylsiloxane (PDMS) as an elastomer stamp, which comes into contact with the micro-silicon structure on the silicon-on-insulator (SOI). Due to the strong automatic adhesion of the PDMS, the Si microstructure in the corresponding area of the protruding PDMS part will be selectively detached from the SOI surface. After that, the PDMS part comes into contact with a polyethylene terephthalate (PET) receiving plate coated with polyurethane (PU), and then ultraviolet/ultraviolet ozone (UV/UVO) exposure is performed on the PET side to fully cure the PU and strengthen its bond with the Si microstructure. Lastly, the PDMS is peeled off, and the microstructures are bonded. The second method is to use decal-transfer lithography (DTL) technology based on method one, using a flat, unmolded PDMS plate for selective transfer. The adhesion strength of the plate is enhanced by photochemical treatment. Afterward, selective masking and an UVO treatment are performed on the surface of the PDMS plate. By exposing the plate, the PDMS coating with changed photochemical properties comes into contact with the SOI wafer and is heated to 70 ◦C for 30 min. Finally, the PDMS is stripped from the SOI, thereby selectively transferring the Si microstructure from the SOI wafer to the plate with the PDMS. The elastomer stamp has excellent flexibility and stickiness, allowing it to make large-area physical contact on the surface of an incompletely flat substrate and transfer fragile, thin, and micro-scale devices without damaging the devices. Therefore, elastomer stamp transfer technology can transfer micro-LED chips from one substrate to another in a cost-effective manner. Moreover, the substrate of the stamp is usually a rigid material (such as glass), which can make it difficult to bend in the lateral dimension and reduce lateral deformation, thereby obtaining good transfer yield and accuracy. In 2017, Radauscher et al. used elastomer stamp technology to manufacture active and passive color micro-LED arrays and proved that this method has a high transfer yield greater than 99.99% [11]. In 2019, Kim et al. found that elastomer stamp transfer technology controls the adhesion force of the stamp by changing Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 33

At present, RGB micro-LED chips can be accurately and efficiently transferred from one substrate to another through multiple transfer methods, such as elastomer stamp technology, laser induction technology, fluid transportation technology, electrostatic transfer technology, and roll-to- roll (roll-to-panel) technology. In the following, we briefly describe these methods for micro-LED mass transfer and analyze their theory, features, and application scope.

2.2. Elastomer Stamp Transfer Elastomer stamp transfer technology can transfer a large number of microstructures from one substrate to another at high speed [35–38]. Rogers’ team initially used elastomer stamps for large- scale transfer printing [11]. This technology closely contacts the micro devices on the donor substrate with an elastomer stamp array matching the pitch of the micro devices on the donor substrate. Through the van der Waals force between the elastomer stamp and the latter, the micro device is picked up and transferred to the receptor substrate. The picking and placing of the elastomer stamp on the microstructure are determined by the peeling speed after the two contact each other. By pulling the stamp away from the donor substrate at a sufficiently high peeling speed (usually faster than 10 cm/s), the elastomer stamp can generate strong adhesive force to adhere the microstructure to the surface of the stamp and remove the target device from the donor substrate. When the peeling speed of the stamp is less than 1 mm/s, the microstructure will be separated from the stamp and remain on the receptor substrate instead of being adsorbed by the stamp. In 2005, Lee et al. proposed two elastomer stamp transfer methods that can realize the selective transfer of thin film transistors (TFT) to rigid or flexible substrates [39]. The transfer processes of these two methods are shown in Figure 3. The first method uses polydimethylsiloxane (PDMS) as an elastomer stamp, which comes into contact with the micro-silicon structure on the silicon-on- insulator (SOI). Due to the strong automatic adhesion of the PDMS, the Si microstructure in the corresponding area of the protruding PDMS part will be selectively detached from the SOI surface. After that, the PDMS part comes into contact with a polyethylene terephthalate (PET) receiving plate Nanomaterials 2020, 10, 2482 6 of 33 coated with polyurethane (PU), and then ultraviolet/ultraviolet ozone (UV/UVO) exposure is performed on the PET side to fully cure the PU and strengthen its bond with the Si microstructure. Lastly,the contact the PDMS force is or peeled peeling off, speed and ofthe the microstr polymeructures stamp are but bonded. has a The problem second of method repeatability is to use [40]. decal-transferTherefore, the lithography authors chose (DTL) to use technology a magnetorheological based on method elastomer one, asusing the stampa flat, andunmolded controlled PDMS the plateadsorption for selective force by transfer. changing The the adhesion magnetic streng field nearth of the the stamp plate duringis enhanced the transfer. by photochemical This method treatment.successfully Afterward, optimized selective the elastomer masking stamp and an transfer UVO treatment technology. are Inperformed 2020, Lu on et the al. optimizedsurface of the the PDMSmicro-LED plate. elastomer By exposing stamp the transferplate, the technology PDMS coating using with the chang supported vectorphotochemical machine properties (SVM) model comes [41]. intoThe contact results showedwith the thatSOI thewafer SVM and model is heated and ato large 70 °C number for 30 min. of transfer Finally, signal the PDMS feature is databasesstripped from offer the85% SOI, classification thereby selectively prediction transferring accuracy for the the Si elastomer microstructure stamp transfer from the process, SOI wafer which to can thesignificantly plate with theimprove PDMS. the transfer efficiency of elastomer stamp transfer technology.

FigureFigure 3. 3. ((aa)) The transfertransfer printing printing step step using using polydimethylsiloxane polydimethylsiloxane (PDMS) (PDMS) and and (b) the (b) elastomer the elastomer stamp stamptransfer transfer step using step decal-transfer using decal-transfer lithography lithograp (DTL)hy technology. (DTL) technology. Reproduced Repr fromoduced [39], from with permission[39], with permissionfrom John Wileyfrom John and Sons,Wiley 2005. and Sons, 2005. 2.3. Laser-Induced Transfer Laser-induced forward transfer (LIFT) is a high-resolution single-step direct printing technology. LIFT can achieve surface micro-patterning or the partial deposition of solid or liquid materials and is widely used in selective printing of electronic devices, such as organic TFT, OLED, Micro-Electro-Mechanical System (MEMS), and sensors [42,43]. The steps for transferring solid-state pixels are shown in Figure4a. The transfer of solid-state pixels is done to irradiate a thin layer of absorbing material deposited on a transparent substrate with a pulsed laser. After irradiation, light–matter interactions occur at the interface between the substrate and the absorbing material, causing the local pressure of the irradiated area to increase rapidly. In this way, a small pixel on the absorbing material film is separated from the donor substrate and deposited on the opposite receiving substrate. Because the size and shape of the emitted material are controlled by the size and shape of the incident laser spot, LIFT can not only deposit typically sized devices but can also transfer microstructures. In 2013, on the basis of LIFT technology, Uniqarta introduced Massively Parallel Laser-Enabled Transfer (MPLET) technology to realize the large-scale transfer of a micro-LED [44]. MPLET technology is based on the single-beam laser transmission process, and its working principle is shown in Figure4b. The micro-LEDs are deposited on a glass substrate plated with a dynamic release layer (DRL). When an ultraviolet (UV) beam irradiates a certain area of the substrate, bubbles will be generated between the substrate and the DRL layer. Under the force of bubble expansion and gravity, the micro devices will be transferred to the receiving substrate with a pitch of 10–300 µm. After that, the MPLET technology uses a diffractive optical element to diffract a single laser beam into multiple sub-beams, the number of which depends on the laser power, as shown in Figure4c. Each sub-beam corresponds to the transfer of a micro-LED. In this way, the micro-LEDs can be transferred in batches under one irradiation, which greatly shortens the process of single beam scanning transmission. At present, the transfer Nanomaterials 2020, 10, 2482 7 of 33 speed of MPLET technology for large panels can reach 100 M units/hour, and the transfer speed of Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 33 smallNanomaterials panels 2020 can, 10 exceed, x FOR PEER 500 M REVIEW units/ hour, with a transfer accuracy of 34 µm. 7 of 33 ±

Figure 4. (a) Principle of laser-induced forward transfer technology. Reproduced from [43], with Figure 4. (a()a )Principle Principle of oflaser-induced laser-induced forward forward transfer transfer technology. technology. Reproduced Reproduced from from[43], with [43], permission from Elsevier, 2016; (b) principle of massively parallel laser transmission technology withpermission permission from from Elsevier, Elsevier, 2016; 2016; (b) (bprinciple) principle of ofmassively massively parallel parallel laser laser transmission transmission technology launched by Uniqarta and (c) Massively Parallel Laser-Enabled Transfer (MPLET) technology that launchedlaunched by UniqartaUniqarta andand ((c)) MassivelyMassively ParallelParallel Laser-EnabledLaser-Enabled Transfer (MPLET)(MPLET) technologytechnology thatthat uses diffractive optical elements to transmit multiple pixels in parallel. Reproduced from [44], with uses diffractive diffractive optical optical elements elements to to transmit transmit multiple multiple pixels pixels in parallel. in parallel. Reproduced Reproduced from from[44], with [44], permission from John Wiley and Sons, 2018. withpermission permission from fromJohn JohnWiley Wiley and Sons, and Sons, 2018. 2018. 2.4. Fluidically Self-Assembled Transfer 2.4. Fluidically Self-Assembled Transfer Fluidically Self-Assembled (FSA) technology can freely assemble microstructures on a substrate Fluidically Self-Assembled (FSA) technology can freely assemble microstructures on a substrate in a large area with a simple operation and offers a small interconnection parasitic effect compared to in a large area with a simple operation and offers a small interconnection parasitic effect compared other transfer methods. In 1994, Yeh et al. realized the transfer of trapezoidal GaAs LED devices from to other transfer methods. In 1994, Yeh et al. realized the transfer of trapezoidal GaAs LED devices a growth wafer to an Si substrate via FSA [45,46], the process of which is shown in Figure5a. To use from a growth wafer to an Si substrate via FSA [45,46], the process of which is shown in Figure 5a. FSA technology to transfer microstructures, the first step is to use photolithography to etch an inverted To use FSA technology to transfer microstructures, the first step is to use photolithography to etch an trapezoidal hole with a certain inclination on the silicon substrate and then deposit an Al-Cr-Sn contact inverted trapezoidal hole with a certain inclination on the silicon substrate and then deposit an Al- layer on the hole bottom as a receiving substrate for the chip transfer. Then, trapezoidal LED chips Cr-Sn contact layer on the hole bottom as a receiving substrate for the chip transfer. Then, trapezoidal are grown on the GaAs wafer via molecular beam epitaxy (MBE), and a Cr-Ni barrier layer and Au LED chips are grown on the GaAs wafer via molecular beam epitaxy (MBE), and a Cr-Ni barrier layer contact layer are deposited on the n-doped GaAs layer for subsequent bonding with the Si substrate. and Au contact layer are deposited on the n-doped GaAs layer for subsequent bonding with the Si Afterwards, the dispersed micro-LED devices flow through the Si substrate via the ethanol carrier substrate. Afterwards, the dispersed micro-LED devices flow through the Si substrate via the ethanol liquid. Because the substrate has inverted trapezoidal holes, under the action of gravity and fluid carrier liquid. Because the substrate has inverted trapezoidal holes, under the action of gravity and vibration, the micro-LED devices in the carrier liquid can be accurately captured. After the Au layers of fluid vibration, the micro-LED devices in the carrier liquid can be accurately captured. After the Au the LED come into contact with the Al-Cr-Sn layers on the bottom of the Si substrate’s holes, by heating layers of the LED come into contact with the Al-Cr-Sn layers on the bottom of the Si substrate’s holes, the substrate, the temperature of Sn becomes higher than its melting point, allowing the Sn to diffuse by heating the substrate, the temperature of Sn becomes higher than its melting point, allowing the to the Au and form an alloy with the Au. Finally, through cooling, the bottom of the micro-LEDs and Sn to diffuse to the Au and form an alloy with the Au. Finally, through cooling, the bottom of the the Si substrate base are solidified to complete the bonding. micro-LEDs and the Si substrate base are solidified to complete the bonding.

Figure 5. a Figure 5. (a() )The The loading loading process process of offluid fluid self-assembl self-assemblyy technology. technology. Reproduced Reproduced from from[45], with [45], withFigure permission 5. (a) The from loading Institute process of Electrical of fluid andself-assembl Electronicsy technology. Engineers, 1994;Reproduced (b) Principle from diagram [45], with of permission from Institute of Electrical and Electronics Engineers, 1994; (b) Principle diagram of electrostaticpermission from transfer Institute technology of Electric [47]. al and Electronics Engineers, 1994; (b) Principle diagram of electrostatic transfer technology [47]. electrostatic transfer technology [47].

In 2007, Ehsan Saeedi et al. used FSA technology to transfer micro-LEDs grown on AlGaAs substrates to flexible substrates [48]. In their latest experiment, the authors were able to achieve a yield of 65% by placing the LED at the correct binding site through FSA technology. The main

Nanomaterials 2020, 10, 2482 8 of 33

In 2007, Ehsan Saeedi et al. used FSA technology to transfer micro-LEDs grown on AlGaAs substrates to flexible substrates [48]. In their latest experiment, the authors were able to achieve a yield of 65% by placing the LED at the correct binding site through FSA technology. The main problems in reducing the yield are solder changes of the metal contact on the LED and the rapid degradation of solder on the contact pad in an acidic environment. Although FSA technology can place scattered microstructures on the receiving substrate compactly and on a large scale, this technology also has many defects in the integration process. FSA technology has imposed strict requirements for the carrier solution and bonding material when transferring micro devices, which will limit its application scope to a certain extent. Moreover, due to the manufacturing process of the microstructure and the material properties of the substrate, the transferred microstructure may not match the holes of the receiving substrate. In addition, determining how to bond more effectively after the microstructure enters the hole represents another area for FSA’s subsequent optimization.

2.5. Electrostatic Transfer Electrostatic transfer technology was proposed by the LuxVue company in 2012, who successfully fabricated a micro device array using the electrostatic transfer method. The working principle of this method is shown in Figure5b [ 47]. By applying a voltage to a silicon electrostatic head, the electrostatic transfer head array picks up the micro device array from the carrier substrate via a charged adsorption force. After that, the receiving substrate is brought into contact with the micro device array, and the control voltage of the electrostatic transfer head is removed, thereby releasing the micro device array onto the receiving substrate. The advantage of the electrostatic transfer technology is that it can selectively transfer individual components or parts of components, and the pitch of the transfer electrostatic head array and the pitch of the micro-LED on the receiving substrate need not be the same, so the transfer is very flexible. However, the voltage applied to the transfer head during electrostatic induction can produce a charging phenomenon, which may damage the micro-LEDs.

2.6. Roll-to-Roll or Roll-to-Panel Imprinting Transfer Roll-to-roll (R2R) or roll-to-panel (R2P) imprinting technology can realize low-cost, high-throughput, and high-efficiency micro-device imprinting or patterning on flexible substrates or rigid substrates [49–51]. The whole system includes two embossing rollers and a conveyor belt supported by the two rollers, which can ensure that the system has a larger curing surface and a constant curing pressure, thereby speeding up the transfer of micro devices. In 2013, Korea institute of machinery & materials developed an R2R transmission system that can control the contact load in real time and transfer In-Ga-Zn-O (IGZO) TFTs from rigid substrates to flexible substrates [50]. The R2R transfer speed can be maintained at 1 mm/s and can effectively prevent the device from breaking due to fluctuations in the load contact during the transfer process of the film structure. In 2017, KIMM demonstrated an R2R transfer printing process suitable for micro-LEDs and successfully fabricated an active matrix micro-LED flexible display driven by TFT [51]. The whole embossing process is divided into three stages, as shown in Figure6. First, the Si-TFT array is transferred from the silicon-on-insulator (SOI) growth wafer to the transition substrate through the first R2R. Then, in the second R2R, the micro-LED is transferred from the GaAs substrate to the transition substrate. Finally, after the two are bonded, both are transferred from the transition substrate to the flexible substrate through the third R2R to integrate the active matrix light emitting diode (AMLED) display. The entire transfer process is performed by an automatic R2R transfer device with an overlay alignment function, and the transfer rate of the Si-TFT and micro-LED is close to 99.9%. By using two microscopes for precise position matching, the entire system can maintain a precise alignment of up to 3 µm. However, the maximum mechanical stretch ability of the flexible display prepared by this system can only reach 40%, and, when the TFT is measured on a soft stretchable substrate, the hard probe can Nanomaterials 2020, 10, 2482 9 of 33 easily tear the electrode, which will cause its electrical performance to decrease. These defects can be further improved by using optimized interconnected structures or reduced pixel sizes. Nanomaterials 2020, 10, x FOR PEER REVIEW 9 of 33

FigureFigure 6. 6.Schematic Schematic diagram diagram of of the the steps steps for for assembling assembling a a flexible flexible AMLED AMLED display display using using three-roll three-roll transfertransfer technology. technology. Reproduced Reproduced from from [51 [51],], with with permission permission from from John John Wiley Wiley and and Sons, Sons, 2017. 2017.

2.7. SummaryThe entire transfer process is performed by an automatic R2R transfer device with an overlay alignmentIn summary, function, Mass and transfer the transfer technology rate of hasthe Si-T greatFT limitations and micro-LED in both is equipmentclose to 99.9%. and By technology. using two Asmicroscopes the size of for micro-LEDs precise position decreases, matching, the accuracy the entire of thesystem transfer can maintain equipment, a precise the matching alignment degree of up ofto the 3 μ transferm. However, head, the and maximum the high cost mechanical of equipment stretch investment ability of the make flexible the mass display transfer prepared technology by this disystemfficult can to improve. only reach The 40%, production and, when capacity, the TFT yield,is measured bad dies on detectiona soft stretchable and maintenance substrate, the will hard all aprobeffect the can transfer easily cost.tear the To prepareelectrode, a 4Kwhich full-color will ca micro-LEDuse its electrical display, performance the resolution to decrease. is 3840 2160,These × anddefects each can pixel be mustfurther be improved composed by of using three RGBoptimized colors. interconnected Therefore, 22,550,400 structures micro-LED or reduced chips pixel need sizes. to be correctly transferred and bonded to the driving substrate, which is very demanding for current mass2.7. Summary transfer technology. Fluid self-assembly technology is suitable for low cost and large spacing micro-LED.In summary, However, Mass until transfer its low technology transfer e ffihasciency great andlimitations unstable in bonding both equipment methods and are technology. improved, FSAAs the can size only of stay micro-LEDs in lab. In decreases, contrast, elastomerthe accuracy stamp of the transfer transfer technology, equipment, with the itsmatching unique degree soft and of stickythe transfer characteristics, head, and can the effi cientlyhigh cost transfer of equipmen micro-devicest investment to rigid ormake flexible the substrates.mass transfer Therefore, technology it is thedifficult most commonlyto improve. used The massproduction transfer capacity, technology yield, at bad present. dies detection Both laser-induced and maintenance transfer will technology all affect andthe electrostatictransfer cost. transfer To prepare technology a 4K full-color have good micro-LED repeatability, display, which the resolution greatly reduces is 3840 the × 2160, transfer and cost.each Poorpixel repeatability must be composed is a weak of point thre ofe RGB stamp colors. transfer Therefore, technology 22,550 that,400 cannot micro-LED be ignored. chips And need the highto be precisioncorrectly oftransferred LIFT makes and it bonded even more to the prominent driving substrate, in the face which of ultra-small is very demanding spacing micro-LED for current arrays. mass Thetransfer advantage technology. of R2R Fluid technology self-assembly lies in its technology large throughput. is suitable For for the low future cost and production large spacing of large-size micro- micro-LEDLED. However, displays, until its its advantages low transfer are efficiency obvious. Various and unstable transfer bonding technologies methods are tryingare improved, to efficiently FSA andcan accuratelyonly stay in transfer lab. In micro-LEDs contrast, elastomer from the stamp growth transfer wafer totechnology, the drive substrate.with its unique Table2 soft summarizes and sticky andcharacteristics, compares the can mainstream efficiently transfer technologies micro-devices currently toused rigid foror flexible the mass substrates. transfer ofTherefore, micro-devices. it is the Bymost comparison, commonly we used are concernedmass transfer that technology elastomer stampat present. transfer Both technology laser-induced is still transfer the main technology transfer methodand electrostatic for the future transfer micro technology and small devices.have good Replacing repeatability, the conventional which greatly stamp reduces transfer the head transfer with cost. an electromagneticPoor repeatability head is ora weak other point flexible of headstamp to transfer improve technology its repeatability that cannot has been be provenignored. to And be a feasiblethe high solution,precision which of LIFT improves makes it the even reliability more prominent of the stamp in the transfer face of technology ultra-small and spacing reduces micro-LED its transfer arrays. cost. However,The advantage manufacturers of R2R technology still need tolies choose in its thelarge appropriate throughput. transfer For the technology future production according of to large-size the cost performancemicro-LED displays, and application its advantages scenarios. are obvious. Various transfer technologies are trying to efficiently and accurately transfer micro-LEDs from the growth wafer to the drive substrate. Table 2 summarizes and compares the mainstream technologies currently used for the mass transfer of micro-devices. By comparison, we are concerned that elastomer stamp transfer technology is still the main transfer method for the future micro and small devices. Replacing the conventional stamp transfer head with an electromagnetic head or other flexible head to improve its repeatability has been proven to be a feasible solution, which improves the reliability of the stamp transfer technology and reduces its transfer cost. However, manufacturers still need to choose the appropriate transfer technology according to the cost performance and application scenarios.

Nanomaterials 2020, 10, x FOR PEER REVIEW 10 of 33

Table 2. Comparison of various mass transfer technologies.

Mass Transfer NanomaterialsTransfer 2020, 10, 2482 Advantage Disadvantage10 of 33 Performance Method Table 2. ComparisonSoftness of and various stickiness mass transfer of elastomer technologies. Elastomer Transfer yield stamp ensure that it can transfer the Poor repeatability. Massstamp Transfer Methodin 99.99% Transfer Performancemicrostructures in an economical Advantage and Disadvantage Softnessefficient and stickinessway. of elastomer stamp ensure that it can transfer the Elastomer stamp Transfer yield in 99.99% Poor repeatability. High resolutionmicrostructures and impurities in an economical will and The transfer stability is Laser-induced ≤500 million not be introduced oneffi thecient substrate way. affected by the laser transfer units/hour The transfer stability is High resolutionsurface. and impurities will not be source. Laser-induced transfer 500 million units/hour affected by the ≤ introduced on the substrate surface. Transfer yield Easy to operate, small parasitic effects Low transferlaser source. efficiency FSA transfer in 65% Easyand to operate,low cost. small parasitic effects and Lowand transfer accuracy. efficiency FSA transfer Transfer yield in 65% low cost. Electrostaticand accuracy. phenomena Electrostatic ~1 Electrostaticcan damage phenomena can damage Electrostatic transfer ~1 million/hourFlexible and Flexible good and repeatability. good repeatability. transfer million/hour microdevices;microdevices; small transfertransfer area. area. Low cost, high throughput and It can damage larger R2R (R2P)Transfer Transfer yield yield inLow ~99.9% cost, high throughput and high It can damage larger R2R (R2P) high efficiency. microdevices. in ~99.9% efficiency. microdevices.

In addition,addition, sincesince the the growth growth materials materials of of blue blue and and green green LEDs LEDs are basedare based on GaN, on GaN, but the but red the LEDs red areLEDs based are based on GaAs on orGaAs GaP. or Therefore, GaP. Therefore, different different growth growth materials materials lead to lead different to different performance performance of RGB micro-LEDsof RGB micro-LEDs in terms ofin temperature, terms of temperature, lifetime and drivinglifetime voltage. and driving Using unprocessedvoltage. Using general-purpose unprocessed driversgeneral-purpose will cause drivers the output will current cause the of theoutput driver current circuit of to the have driver a certain circuit deviation to have froma certain the theoretical deviation current,from the which theoretical will yield current, color which differences will yield in the color micro-LED differences display. in the To micro-LED ensure the display. uniformity To ensure of the full-colorthe uniformity micro-LED of the display,full-color specific micro-LED driving display, conditions specific need driving to be establishedconditions forneed that to display,be established which makesfor that the display, integration which process makes too the complicated. integration Therefore, process beforetoo complicated. the breakthrough Therefore, of mass before transfer the technology,breakthrough the of use mass of thetransfer same technology, material or substratesthe use of to the grow same RGB material three-color or substrates micro-LEDs to grow would RGB be thethree-color best solution micro-LEDs to achieve would full-color be the displays.best solution to achieve full-color displays. 3. Color Conversion Technology 3. Color Conversion Technology Color conversion technology is a method that can realize a full-color micro-LED display without Color conversion technology is a method that can realize a full-color micro-LED display without a mass transfer and is utilized in two main ways. One is to use a UV or blue micro-LED array as the a mass transfer and is utilized in two main ways. One is to use a UV or blue micro-LED array as the excitation light source and place colored phosphors, quantum dots (QDs), or other light-emitting nano excitation light source and place colored phosphors, quantum dots (QDs), or other light-emitting organic materials onto an RGB color conversion layer covering the micro-LED light source to achieve nano organic materials onto an RGB color conversion layer covering the micro-LED light source to multi-wavelength light emissions, as shown in Figure7. If the UV micro-LED is the light source, it is achieve multi-wavelength light emissions, as shown in Figure 7. If the UV micro-LED is the light necessary to use three conversion materials to produce the RGB three-color conversion layers; if the source, it is necessary to use three conversion materials to produce the RGB three-color conversion excitation light source is a blue LED, only the corresponding red and green conversion layers are layers; if the excitation light source is a blue LED, only the corresponding red and green conversion needed to realize the full-color display of the micro-LED. layers are needed to realize the full-color display of the micro-LED.

Figure 7. SchematicSchematic diagram of color conversion technology.

An alternativealternative methodmethod of of implementing implementing a full-colora full-color display display is tois firstto first convert convert the bluethe blue micro-LED micro- intoLED whiteinto white light light and and then then cover cover the the RGB RGB filter filter on on the the white white light light source source to to obtain obtain thethe RGBRGB tricolor sub-light source. This method does not need to di distinguishstinguish between the RGB pixels of the micro-LED array but only needs to prepare the corresponding RGB pixel filters,filters, making the preparation simple and low-cost.low-cost. InIn 2019, 2019, Xu Xu et al.et proposedal. proposed a full-color a full-color LED micro-displayLED micro-display system system based onbased time on division time multiplexingdivision multiplexing with dynamic with dynamic color filters color [52 filters]. The [52]. whole The system whole issystem composed is composed of a micro-LED of a micro-LED display module,display module, a color conversiona color conversion layer, a microcontrollerlayer, a microcontroller unit (MCU), unit and(MCU), a dynamic and a dynamic color filter color module, filter as shown in Figure8a. The area of the micro-LED display module is 7 mm 11 mm, the resolution is × 920 480, and the pixel pitch is 12.8 µm; when constructed, this module can emit 450 nm blue light. × The color conversion layer with a thickness of less than 3 µm is composed of QDs, which are coated on Nanomaterials 2020, 10, x FOR PEER REVIEW 11 of 33

Nanomaterialsmodule, as2020 shown, 10, 2482 in Figure 8a. The area of the micro-LED display module is 7 mm × 11 mm,11 ofthe 33 resolution is 920 × 480, and the pixel pitch is 12.8 μm; when constructed, this module can emit 450 nm blue light. The color conversion layer with a thickness of less than 3 μm is composed of QDs, thewhich micro-LED are coated array on the to convertmicro-LED blue array light to into convert white blue light. light The into core white component light. The of this core system component is the dynamicof this system color is filter, the dynamic which consists color filter, of four-color which consists blocks of of red, four-color green, blue,blocks and of colorless.red, green, By blue, rotating and thecolorless. color filter,By rotating the MCU the outputs color filter, a gray-scale the MCU image outputs of the a correspondinggray-scale image channel of the according corresponding to the dutychannel ratio according of the di fftoerent the duty colors, ratio thus of displaying the different the colors, color image thus displaying using a time-division the color image multiplexing using a method.time-division The workingmultiplexing process method. is shown The inworking Figure 8processb. However, is shown the in filter Figure absorbs 8b. However, 2 /3 of the the emitted filter light,absorbs which 2/3 of greatly the emitted reduces light, the which luminous greatly efficiency reduces of the the luminous micro-LED efficiency array, so of the the first micro-LED method witharray, separate so the lightfirst conversionmethod with materials separate is morelight suitableconversion for thematerials development is more of colorsuitable conversion for the micro-LEDdevelopment displays. of color conversion micro-LED displays.

Figure 8.8. (a(a)) Dynamic Dynamic color color filter filter full-color full-color LED LED micro-display micro-display system system structure structure based onbased time on division time multiplexing and (b) the working process of the system. Reproduced from [52], with permission from division multiplexing and (b) the working process of the system. Reproduced from [52], with John Wiley and Sons, 2019. permission from John Wiley and Sons, 2019. The material of the color conversion layer is generally phosphors or QDs. The phosphor is The material of the color conversion layer is generally phosphors or QDs. The phosphor is composed of some wide band gap materials (such as oxides, nitrides, and sulfides) and a small composed of some wide band gap materials (such as oxides, nitrides, and sulfides) and a small number of doping ions (transition metals or rare earth compounds). Although phosphors have number of doping ions (transition metals or rare earth compounds). Although phosphors have mature preparation technology, high thermal and chemical stability, and a good quantum yield [53], mature preparation technology, high thermal and chemical stability, and a good quantum yield [53], their particle size is in the micron level, making them difficult to uniformly coat on micro-LED pixels at their particle size is in the micron level, making them difficult to uniformly coat on micro-LED pixels the same size level, which will affect the color conversion efficiency and uniformity of the micro-LED at the same size level, which will affect the color conversion efficiency and uniformity of the micro- display. In contrast, the rapidly developed QDs of recent years have a nanometer size, a narrow LED display. In contrast, the rapidly developed QDs of recent years have a nanometer size, a narrow emission spectrum, a wide absorption spectrum, higher luminous efficiency, etc. Therefore, QDs are emission spectrum, a wide absorption spectrum, higher luminous efficiency, etc. Therefore, QDs are more suitable for full-color micro-LED displays. This section will introduce the methods used for more suitable for full-color micro-LED displays. This section will introduce the methods used for achieving full-color micro-LED displays through the phosphor color conversion layer, the QD color achieving full-color micro-LED displays through the phosphor color conversion layer, the QD color conversion layer, and other color conversion technologies over the past ten years. conversion layer, and other color conversion technologies over the past ten years. 3.1. Phosphor Color Conversion LEDs 3.1. Phosphor Color Conversion LEDs Phosphors have the advantages of a high and stable quantum yield, high thermal stability, and highPhosphors chemical have stability the advantages (humidity of resistance), a high and sostable they quantum are often yield, used forhigh UV thermal LEDs orstability, blue LEDs and tohigh synthesize chemical white stability light (humidity through downresistance), conversion so they [54 are]. Commonly often used used for UV phosphors LEDs or include blue LEDs yellow to synthesize white light through down conversion3+ [54]. Commonly used phosphors2+ include yellow2+ emission: yttrium aluminum garnet (YAG): Ce , red emission: Ca1-xSrxS: Eu , Sr2Si5N8: Eu , emission: yttrium3+ aluminum garnet (YAG): Ce23++, red emission: Ca21-x+ SrxS: Eu2+, Sr2Si5N8: Eu2+, CaSiN2+2: CaSiN2: Ce , green emission: SrGa2S4: Eu , SrSi2O2N2: Eu , blue emission: LiCaPO4:Eu , Ce3+, green emission:2+ SrGa2S4: Eu2+, SrSi2O2N2: Eu2+, blue emission: LiCaPO4:Eu2+, Sr5(PO4)3Cl: Eu2+, etc. Sr5(PO4)3Cl: Eu , etc. [53]. Converting blue micro-LEDs into white light via yellow phosphor and then[53]. Converting dividing each blue RGB micro-LEDs sub-pixel withinto white a color light filter via is oneyellow method phosphor used toand realize then dividing a full-color each display. RGB However,sub-pixel with as mentioned a color filter above, is one the method color filter used will to absorb realize most a full-color of the emitted display. light, However, and the as scattering mentioned on theabove, color the filter color will filter produce will absorb crosstalk most between of the eachemitted sub-pixel. light, and Therefore, the scattering preparing on the the color RGB phosphorfilter will asproduce a color crosstalk conversion between layer and each coating sub-pixel. the phosphor Therefore, on eachpreparing light pixel the isRGB a more phosphor efficient as method. a color conversionIn 2011, layer Zhao and et al.coating used the phosphors phosphor to manufactureon each light thepixel first is a full-color more efficient active method. LED array displays on silicon (LEDoS) [55,56]. These displays used a 380 nm UV LED array to excite red, green, and blue

Nanomaterials 2020, 10, x FOR PEER REVIEW 12 of 33 Nanomaterials 2020, 10, 2482 12 of 33 In 2011, Zhao et al. used phosphors to manufacture the first full-color active LED array displays on silicon (LEDoS) [55,56]. These displays used a 380 nm UV LED array to excite red, green, and blue phosphors to achieve a full-color display. This type of LED array has a resolution of 8 8 and a phosphors to achieve a full-color display. This type of LED array has a resolution of 8 × 8 and× a pixel pixelpitch pitchof 550 of μ 550m. µEachm. Eachcolor color pixel pixel is composed is composed of two of twogreen, green, one onered, red, and and one one blue blue sub-pixels, sub-pixels, as asshown shown in inFigure Figure 9a.9a. The The color color conversion conversion layer layer is is made made by by deep reactive-ion etchingetching (DRIE)(DRIE) onon aa singlesingle crystalcrystal siliconsilicon wafer.wafer. Then, phosphors with a separate emission wavelength are dropped into thethe correspondingcorresponding etchingetching holesholes onon thethe colorcolor conversionconversion layer.layer. After the phosphor is cured, the color conversion layer can be excited by the UV LED arrayarray to achieve a full-color display, asas shownshown in Figure9 9b.b.

Figure 9.9. (a) SingleSingle colorcolor pixelpixel composition;composition; (b) Full-colorFull-color display using UV LED to exciteexcite thethe RGBRGB phosphor.phosphor. ReproducedReproduced fromfrom [[55],55], withwith permissionpermission fromfrom JohnJohn WileyWiley andand Sons, Sons, 2012. 2012.

However, for micro-LEDsmicro-LEDs with sizes below 100 µμm,m, oversizedoversized phosphorsphosphors cannot be used to prepareprepare aa uniformuniform colorcolor conversionconversion layerlayer thatthat conformsconforms toto thethe micro-LEDmicro-LED displaydisplay pixels.pixels. The size ofof thethe phosphorphosphor willwill varyvary according toto the preparationpreparation process. Table 3 3 shows shows thethe variousvarious processesprocesses forfor preparingpreparing thethe phosphorphosphor andand thethe sizesize ofof thethe phosphorphosphor thatthat cancan bebe obtainedobtained underunder thisthis process.process. It cancan be concluded that by using advanced synthesis te technology,chnology, nano-sized phosphors can be prepared. However, althoughalthough smaller-sized smaller-sized phosphors phosphors are are beneficial beneficial to improveto improve the colorthe color uniformity uniformity of the of color the conversioncolor conversion layer, theirlayer, luminous their luminous efficiency efficiency is proportional is proportional to their size,to their which size, also which means also that means the lightthat ethefficiency light efficiency of the color of the conversion color conversion layer prepared layer prepared from small-sized from small-sized phosphors phosphors will be reduced. will be reduced. In 2013, ChenIn 2013, et al.Chen found et al. that found mixing that large-sized mixing large-si and small-sizedzed and small-sized phosphors phosphors together cantogether greatly can improve greatly theimprove color the uniformity color uniformity of mixed of phosphors mixed phosphors at the expense at the expense of a small of a small amount amount of light of light efficiency efficiency [57]. The[57]. authors The authors mixed 4mixedµm small-sized 4 μm small-sized phosphor phosphor particles andparticles 22 µm and large-sized 22 μm phosphorlarge-sized particles phosphor in a ratioparticles of 3:2 in toa ratio obtain of the3:2 bestto obtain performance the best withperformance 91.6% angular with 91.6% color angular uniformity color and uniformity 95.7% normalized and 95.7% lightnormalized efficiency. light Therefore, efficiency. this Therefore, method can this effectively method improve can effectively the uniformity improve and the luminous uniformity efficiency and ofluminous phosphor efficiency LEDs. of phosphor LEDs.

Table 3. Overview of the phosphor particle sizes obtainedobtained by didifferentfferent preparationpreparation processesprocesses [[58].58].

PreparationPreparation Solid State Sol gel/ Spray Sol gel/ PechiniCo-Precipitation Co-Precipitation Hydrothermal Hydrothermal Combustion Combustion Spray Pyrolysis ProcessProcess Reaction Pechini Pyrolysis sizesize >5>5 μµm 10 nm–210 nm–2µm μm 10 nm–110 nm–1µm μm 10 nm–110 nm–1µm μm 500500 nm–2 nm–2µm μm 100100 nm–2 nm–2µm μm

Nevertheless, as the sizes of micro-LED pixels shrink, the relatively large sizes of the phosphors willwill limitlimit the the color color uniformity. uniformity. Although Although this this color color uniformity uniformity can be can improved be improved by preparing by preparing a smaller a sizesmaller phosphor size phosphor or mixing or phosphors, mixing phosphors, quantum quantum efficiency efficiency will still decrease will still withdecrease size. with This reductionsize. This willreduction eventually will eventually affect the displayaffect the of thedisplay micro-LED. of the micro-LED. Therefore, itTherefore, is necessary it is to necessary find a conversion to find a materialconversion with material a smaller with size a andsmaller a higher size and conversion a higher effi conversionciency to replace efficiency the phosphor,to replace forthe which phosphor, QDs arefor thewhich best QDs candidates. are the best candidates.

3.2. Quantum Dot Color Conversion LEDs QDs are aa newnew typetype ofof semiconductorsemiconductor nanomaterialnanomaterial that havehave thethe advantagesadvantages of aa nanometernanometer size,size, aa highhigh quantumquantum yield, yield, a a broad broad absorption absorption spectrum, spectrum, and and a narrow a narrow emission emission spectrum. spectrum. When When the

Nanomaterials 2020, 10, 2482 13 of 33 Nanomaterials 2020, 10, x FOR PEER REVIEW 13 of 33 theQDs QDs are are excited excited by electricityby electricity or light,or light, they they can ca emitn emit light light with with diff erentdifferent wavelengths wavelengths related related to their to theirsize, size, shape, shape, and and composition. composition. In In this this way, way, the the emission emission wavelength wavelength cancan bebe adjusted by by changing changing thesethese parameters, parameters, which which can can be be used used to to realize realize a a fu full-colorll-color display display with with mi micro-LEDscro-LEDs [59–65]. [59–65]. QDs QDs are are usuallyusually II-VI II-VI group group compounds, compounds, III- III-VV group group compounds, compounds, or I-III-VI or I-III-VI compounds, compounds, such suchas CdSe, asCdSe, InP, andInP, CuInS and CuInS2 [66]. 2The[66 ].deposition The deposition methods methods of QDs mainly of QDs include mainly spin include coating spin [67], coating aerosol [67 ],printing aerosol technologyprinting technology [68], photolithography [68], photolithography technology technology [69], inkjet [ 69printing], inkjet [70,71], printing and [70 the,71 ],pulse and spraying the pulse methodspraying [72]. method In addition, [72]. In since addition, QDs have since a QDsshorter have light-emitting a shorter light-emitting life, they have life, a faster they havemodulation a faster responsemodulation speed, response and their speed, narrow and their emission narrow linewi emissiondth gives linewidth them gives have them a good have color a good purity, color which purity, provideswhich provides the possibility the possibility of wavelength of wavelength division division multiplexing multiplexing for visible for light visible communication. light communication. Thus, QD-convertedThus, QD-converted full-color full-color micro-LEDs micro-LEDs are not areonly not used only for used displays for displays but also but widely also applied widely appliedin visible in lightvisible communication light communication [73–75]. [73–75]. InIn 2015, 2015, Han Han et et al. al. prepared prepared a a full-color full-color display display in in which which the the RGB RGB QDs QDs conversion conversion layer layer was was excitedexcited by by a aUV UV micro-LED micro-LED array array [71]. [71]. This This micro-LED micro-LED display display array array had had a resolution a resolution of of128 128 × 128,128, a × pixela pixel size size of of35 35μm,µm, and and a pixel a pixel pitch pitch of of40 40 μm.µm. RGB RGB three-color three-color QDs QDs are are composed composed of of CdSe/ZnS CdSe/ZnS and and CdS,CdS, which which can emitemit lightlight with with wavelengths wavelengths of of 450 450 nm, nm, 520 520 nm, nm, and 630and nm,630 respectively,nm, respectively, when excitedwhen excitedby 365 by nm 365 ultraviolet nm ultraviolet light. The light. QDs The are QDs sprayed are sprayed on the micro-LEDon the micro-LED array by array aerosol by inkjetaerosol printing inkjet printingtechnology, technology, the process the of process which isof shown which in is Figure show 10n a.in First,Figure the 10a. QD solutionFirst, the at QD a concentration solution at ofa concentrationabout 5 mg/mL of isabout atomized 5 mg/mL and is entrained atomized in and the entrained air flow jetted in the by air the flow print jetted head; by then, the print the process head; then,parameters the process of aerosol parameters inkjet of printing aerosol areinkjet adjusted printing to are obtain adjusted a spray to obtain linewidth a spray of 35 linewidthµm. For of QDs 35 μofm. di Forfferent QDs sizes, of different to ensure sizes, the uniformityto ensure ofthe spraying, uniformity the of spraying spraying, needs the tospraying be performed needs to under be performeddifferent flow under rates different of gas, flow which rates means of gas, that which the larger means QD that is, the fasterlarger theQD gas is, the flow faster rate mustthe gas be. flowAfter rate the RGBmustQDs be. areAfter sprayed the RGB onto QDs the micro-LEDare sprayed array onto in the sequence, micro-LED a distributed array in Bragg sequence, reflector a distributed(DBR) is deposited Bragg reflector on the top(DBR) to reflectis deposited the ultraviolet on the top light to thatreflect leaked the ultraviolet back to the light color that conversion leaked backlayer to for the reuse. color conversion layer for reuse. TheThe optical optical performance of thisthis QD-basedQD-based micro-LED micro-LED display display is is shown shown in in Figure Figure 10b–d. 10b–d. Figure Figure 10b 10bshows shows the reflectancethe reflectance spectrum spectrum of the of DBR the structureDBR struct placedure placed on top on of thetop micro-LED of the micro-LED array. This array. structure This structurehas a reflectivity has a reflectivity of 90% at of 395 90% nm, at 395 as well nm, asas awell reflectivity as a reflectivity of only 10%of only at 45010% nm at 450 (B), nm 520 (B), nm 520 (G), nmand (G), 630 and nm 630 (R). nm Therefore, (R). Therefore, the leaked the leaked UV light UV can light be can effectively be effectively recovered, recovered, and other and lightother can light be canemitted. be emitted. Figure Figure10c demonstrates 10c demonstrates that the that QD th micro-LEDe QD micro-LED display withdisplay a DBR with structure a DBR structure can effectively can effectivelyinhibit the inhibit leakage the of leakage ultraviolet of ultraviolet light and increaselight and the increase luminous the luminous intensity byintensity 194% (B),by 194% 173% (B), (G), 173%and 183%(G), and (R). 183% Figure (R). 10d Figure provides 10d a provides comparison a comparison between the between QD micro-LED the QD displaymicro-LED and display the National and theTelevision National Standards Television Committee Standards (NTSC) Committee International (NTSC) CommissionInternational on Commiss Illuminationion on (CIE) Illumination 1976 color (CIE)space 1976 chromaticity color space diagram. chromaticity The QD diagram. micro-LED The display QD micro-LED has a wider display color gamut,has a wider which color is 1.52 gamut, times whichthat of is NTSC. 1.52 times that of NTSC.

FigureFigure 10. 10. (a(a) )Quantum Quantum dots dots (QDs) (QDs) printing printing process; process; (b (b) )distributed distributed Bragg Bragg refl reflectorector (DBR) (DBR) reflection reflection spectrum;spectrum; (c (c)) emission emission spectrum spectrum with with and and without without a a DBR DBR structure; structure; ( (dd)) the the NTSC NTSC CIE CIE 1976 1976 color color space space chromachroma diagram. diagram. Reproduced Reproduced from from [71], [71], The The Optical Optical Society, Society, 2012. 2012. As the size of the micro-LED decreases, the optical crosstalk between each sub-pixel will seriously As the size of the micro-LED decreases, the optical crosstalk between each sub-pixel will affect the contrast, color purity, and saturation of the display. To reduce the optical crosstalk between seriously affect the contrast, color purity, and saturation of the display. To reduce the optical crosstalk the sub-pixels in the QD micro-LED display, Kuo et al. used photolithography and photoresist (PR) between the sub-pixels in the QD micro-LED display, Kuo et al. used photolithography and photoresist (PR) to prepare an anti-crosstalk window mold in 2017. The structure of this window

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Nanomaterials 2020, 10, x FOR PEER REVIEW 14 of 33 to prepare an anti-crosstalk window mold in 2017. The structure of this window mold is shown moldin Figure is shown 11a [in76 ]Figure and is 11a composed [76] and ofis acomposed window forof a QD window injection for QD and injection a barrier and wall a forbarrier reducing wall forcrosstalk. reducing The crosstalk. window The size window is the same size as is the the sub-pixel same as the size sub-pixel of the micro-LED size of the array. micro-LED The barrier array. wall The is barriersilver-plated wall is to silver-plated prevent emission to prevent light leakage. emission Figure light 11 b,cleakage. are the Figure fluorescence 11b,c are microscope the fluorescence images of microscopethe QD micro-LED images of array the QD with micro-LED the anti-crosstalk array with mold the anti-crosstalk and without themold crosstalk and without mold, the respectively. crosstalk mold,Here, thererespectively. is no clear Here, isolation there between is no clear each isolation sub-pixel between in the QD each micro-LED sub-pixel array in withoutthe QD themicro-LED crosstalk arrayprevention without mold. the crosstalk The blue prevention QDs overlap mold. with The the redblue and QDs green overlap QDs, with and the the red crosstalk and green degree QDs, is about and the32.8%. crosstalk In contrast, degree the is about QD micro-LED 32.8%. In arraycontrast, with the anti-crosstalk QD micro-LED molds array has with a clear anti-crosstalk boundary, andmolds the hascrosstalk a clear degree boundary, is almost and zero,the crosstalk which eff degreeectively is reducesalmost thezero, optical which crosstalk effectively between reduces each the sub-pixel optical crosstalkin the QD between micro-LED each display. sub-pixel in the QD micro-LED display.

FigureFigure 11. 11. ((aa)) QD QD micro-LED micro-LED array with thethe anti-crosstalkanti-crosstalk mold;mold; ( b(b)) fluorescence fluorescence microscope microscope image image of ofthe the QD QD micro-LED micro-LED array array without without the the crosstalk crosstalk prevention prevention mold; mold; (c ()c fluorescence) fluorescence microscope microscope image image of ofthe the QD QD micro-LED micro-LED array array with with the the anti-crosstalk anti-crosstalk mold. mold. Reproduced Reproduced from from [76], [76], Photonics Photonics Research, Research, 2017. 2017. Another problem for QD micro-LED displays is the stability and quantum yield of QDs. The dry QD conversionAnother problem layer su forffers QD from micro-LED a low quantum displays yield, is the plottingstability di andfficulties, quantum and yield an unstable of QDs. structure,The dry QDwhich conversion will increase layer with suffers the from thickness a low of quantum the dry QDyield, film plotting [77,78]. difficulties, Therefore, and it is an necessary unstable to structure, mix QDs whichwith other will substancesincrease with and the then thickness distribute of thethe mixturedry QD into film LED [77,78]. molds Therefore, or place theit is mixture necessary onto toLEDs mix QDsto manufacture with other substances QD-LEDs [and66]. then In 2018, distribute Zhou et the al. mixture used a microwaveinto LED molds assisted or place heating the method mixture to onto mix LEDsQDs andto manufacture sodium silicate QD-LEDs aqueous [66]. solution In 2018, within Zhou 30 et s. al. The used quantum a microwave yield of theassisted prepared heating QD method solution to(69%) mix QDs was moreand sodium than twice silicate that aq ofueous the original solution QD within solution 30 s. The (33%) quantum [75]. In yield 2020, of Ho the et prepared al. prepared QD solutionan RGB color(69%) conversionwas more filmthan usingtwice salt-sealedthat of the QDoriginal [79]. TheQD saltsolution layer (33%) covering [75]. the In QD2020, successfully Ho et al. preparedprotected an the RGB QD fromcolor oxygen,conversion moisture, film using heat, salt-sealed and other external QD [79]. environments, The salt layer thereby covering prolonging the QD successfullythe reliability protected and service the QD life from of the oxygen, QD-LED. moisture The,photoresist heat, and other hybrid external QD providedenvironments, a method thereby for prolongingpatterning multi-colorthe reliability QD and arrays. service This life method of the can QD-LED. not only The control photoresist the thickness hybrid and QD size provided of the QD a methodconversion for layerpatterning but also multi-color retain the QD characteristics arrays. This of method photolithography can not only technology, control the thereby thickness providing and sizea cost-e of theffective QD conversion solution for layer the developmentbut also retain of the high-resolution, characteristics large-area of photolithography micro-LED displaystechnology, [80]. therebyIn 2020, providing Chen et al. a preparedcost-effective an RGB solution full-color for the micro-LED development array of using high-resolution, a photoresist large-area color conversion micro- LEDlayer displays mixed with [80]. QD In [81 2020,]. The Chen experiments et al. prepared show that QDsan RGB fused ful withl-color photoresist micro-LED can reach array a quantumusing a photoresistyield of 60–70%, color andconversion the optical layer density mixed of with red and QD green[81]. The mixed experiments QDs can reach show 1.5 that and QDs 0.8, respectively.fused with photoresistIn addition can reach to using a quantum high-quality yield QDs of 60–70%, or micro-LED and the excitation optical density light sources, of red optimization and green mixed of the QDsstructures can reach can also1.5 and improve 0.8, respectively. the light effi ciency of QD micro-LEDs. Chen et al. prepared a monolithic RGBIn QD addition micro-LEDs to using with high-quality black matrix QDs photoresist or micro-LED spacers, excitation hybrid light Bragg sources, reflector optimization (HBR) structures, of the structuresand distributed can also Bragg improve reflector the light (DBR) efficiency structures. of QDThese micro-LEDs. structures Chen are et shownal. prepared in Figure a monolithic 12a [82]. RGBThe QD average micro-LEDs transmittance with black of the matrix black photoresist matrix photoresist spacers, hybrid in the Bragg visible reflector light band (HBR) is onlystructures, 0.56%. andSpin-coating distributed this Bragg photoresist reflector on (DBR) the sidewall structures. of each Th RGBese structures sub-pixel notare onlyshown prevents in Figure crosstalk 12a [82]. between The averagepixels but transmittance also blocks theof the leakage black ofmatrix blue light,photoresis therebyt in increasing the visible the light contrast band is ratio only of 0.56%. the RGB Spin- QD coatingmicro-LED this photoresist from 11 to 22.on Tothe further sidewall improve of each the RGB color sub-pixel purity andnot only light prevents output intensity crosstalk of between red and pixelsgreen but light, also a distributed blocks the Braggleakage reflector of blue (DBR)light, thereby with high increasing reflectivity the under contrast blue ratio light of was the fabricatedRGB QD micro-LEDon top of the from QD 11 micro-LED to 22. To further along withimprove a hybrid the color Bragg purity reflector and (HBR)light output composed intensity of a of DBR red and and a greensilver-plated light, a distributed layer at the Bragg bottom. reflector This structure (DBR) with can reflecthigh reflectivity the blue light under leaking blue backlight towas the fabricated QD color on top of the QD micro-LED along with a hybrid Bragg reflector (HBR) composed of a DBR and a silver-plated layer at the bottom. This structure can reflect the blue light leaking back to the QD color conversion layer, allowing the red and green QDs to be excited by more blue light. According to the

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Nanomaterials 2020, 10, x FOR PEER REVIEW 15 of 33 conversion layer, allowing the red and green QDs to be excited by more blue light. According to the authors’ tests, comparedcompared toto QD QD micro-LED micro-LED without without HBR, HBR, the the red red and and green green output output intensity intensity of the of QDthe QDmicro-LED micro-LED with with HBR wasHBR increasedwas increased by about by 23%.about Moreover, 23%. Moreover, the red andthe greenred and output green intensities output intensitiesof the QD of micro-LED the QD micro-LED with both with HBR both and HBR DBR and increased DBR increased by about by more about 27% more compared 27% compared to the QD to themicro-LED QD micro-LED with HBR with only. HBR only. Gou et al. proposed aa funnelfunnel tubetube arrayarray for for a a micro-LED micro-LED display display for for color color conversion, conversion, as as shown shown in inFigure Figure 12 b12b [83 [83,84].,84]. The The simulation simulation showed showed that that the the inner inner wall wall of of the the funnel funnel tube tube structure structure is madeis made of ofabsorptive absorptive or reflectiveor reflective material, material, and and each each sub-light sub-light source source is completely is completely isolated isolated by the by funnel the funnel tube tubestructure, structure, which which can e ffcanectively effectively reduce reduce color crosstalk color crosstalk between between pixels. Inpixels. addition, In addition, as the cone as the angle cone of anglethe funnel of the tube funnel becomes tube smaller,becomes the smaller, luminous the eluminousfficiency of efficiency the micro-LED of the ismicro-LED gradually increasedis gradually by increasedup to three by times up to that three without times thethatfunnel without tube the structure. funnel tube Therefore, structure. this Therefore, structure this can structure improve can the improveenvironmental the environmental contrast of the contrast QD micro-LED. of the QD micro-LED. In 2020, Kang et al. applied applied a a nanoporous (NP) (NP) GaN structure to the QD color conversion of a micro-LED display [[85,86].85,86]. Compared Compared to ordinary QD films, films, the GaN GaN NP NP structure structure has high high structural structural stability and improves the light absorption rate of QD through its unique type of multiple light scattering. The The preparation preparation process process of of the the GaN GaN NP NP st structureructure conversion conversion layer layer is is shown shown in in Figure Figure 12c.12c. The NP GaN conversion layer with a pitch of 40 µμm and an area of 3636 × 36 µμm2 was prepared by × conventional photolithography and and chlorine-based chlorine-based inductively inductively coupled coupled plasma plasma etching etching technology, technology, followed by loading the red and greengreen QDsQDs intointo thethe correspondingcorresponding NPNP GaNGaN holes.holes. To To prevent crosstalk between the the RGB sub-pixels, sub-pixels, a a Ni/Al Ni/Al laye layerr was was deposited deposited on on the the sidewalls sidewalls of of the the NP NP GaN. GaN. The experiments showshow thatthat the the green green and and red red QDs QDs filling filling the the GaN GaN NP NP structure structure can can achieve achieve about about 96% 96%and 100%and 100% light light conversion conversion efficiency efficiency under under the excitation the excitation of 370 of nm 370 blue nm light.blue light.

Figure 12. ((a)) Monolithic RGB RGB QD QD micro-LED micro-LED with with a a black matrix photoresist sp spacer,acer, hybrid Bragg reflectorreflector (HBR) and DBR structure. Re Reproducedproduced from [82], [82], with permissi permissionon from Institute of Electrical and ElectronicsElectronics Engineers, Engineers, 2018; 2018; (b) RGB(b) RGB QD micro-LED QD micro-LED with a funnelwith tubea funnel array structure.tube array Reproduced structure. Reproducedfrom [84], with from permission [84], with frompermission John Wileyfrom John and Sons,Wiley 2019;and Sons, (c) GaN 2019; nanoporous (c) GaN nanoporous (NP) structure (NP) structureconversion conversion layer preparation layer preparation process. Reprinted process. Repr withinted permission with permission from [85]. Copyrightfrom [85]. Copyright (2020) American (2020) AmericanChemical Society.Chemical Society. 3.3. Other Color Conversion Technologies 3.3. Other Color Conversion Technologies In addition to using phosphors or QD conversion layers to achieve full-color micro-LED displays In addition to using phosphors or QD conversion layers to achieve full-color micro-LED displays with micro-LEDs, other conversion materials and external conversion structures can also be used. with micro-LEDs, other conversion materials and external conversion structures can also be used. Liu et al. proposed a micro-LED full-color projection system on silicon (LEDoS) in 2013, which is Liu et al. proposed a micro-LED full-color projection system on silicon (LEDoS) in 2013, which is called a 3-LEDoS projector [87,88]. In this system, the RGB three-color micro-LED array on the silicon called a 3-LEDoS projector [87,88]. In this system, the RGB three-color micro-LED array on the silicon is mounted onto a trichroic prism, as shown in Figure 13a–c. Each monochromatic micro-LED array is mounted onto a trichroic prism, as shown in Figure 13a–c. Each monochromatic micro-LED array on the silicon is bonded to an addressable active silicon substrate via flip chip technology with a on the silicon is bonded to an addressable active silicon substrate via flip chip technology with a resolution of 30 30 and a pixel pitch of 140 µm. The monochromatic emission wavelengths of the resolution of 30 × 30 and a pixel pitch of 140 μm. The monochromatic emission wavelengths of the RGB three-color micro-LED array are 630 nm (R), 535 nm (G), and 445 nm (B), respectively. RGB three-color micro-LED array are 630 nm (R), 535 nm (G), and 445 nm (B), respectively.

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Figure 13. (a) Triangular prism projection source with an RGB three-color micro-LED array on silicon Figure 13. (a) Triangular Triangular prism projection source with an RGB three-color micro-LED array on silicon (3-LEDoS); (b) 3-LEDoS projector prototype and (c) schematic diagram of the optical architecture; (d) (3-LEDoS);(3-LEDoS); ( (b)) 3-LEDoS 3-LEDoS projector projector prototype prototype and and (c ()c schematic) schematic diagram diagram of the of the optical optical architecture; architecture; (d) The image projected on the wall by the 3-LEDoS. Reproduced from [87], with permission from (Thed) The image image projected projected on onthe the wall wall by by the the 3-LEDoS. 3-LEDoS. Reproduced Reproduced from from [87], [87], with with permission fromfrom Institute of Electrical and Electronics Engineers, 2013. Institute ofof ElectricalElectrical andand Electronics Engineers,Engineers, 2013.2013. TheThe image image signals signals are are transmitted transmitted to the to the three three arra arrayy drives drives through through the external the external control control unit, and unit, The image signals are transmitted to the three array drives through the external control unit, and aand full-color a full-color image image can then can then be projected be projected using using a trichroic a trichroic prism, prism, as asshown shown in inFigure Figure 13d. 13d. The The team team a full-color image can then be projected using a trichroic prism, as shown in Figure 13d. The team subsequentlysubsequently manufactured manufactured a ahigher higher quality quality LEDoS LEDoS chip chip with with a aresolution resolution of of 100 100 × 100100 and and a apixel pixel subsequently manufactured a higher quality LEDoS chip with a resolution of 100 ×× 100 and a pixel pitchpitch of of 50 50 μµm.m. In In 2018, 2018, Zhang Zhang et et al. al. prepared prepared ultra-high ultra-high-brightness-brightness micro-LED micro-LED display display panels panels with with pitch of 50 μm. In 2018, Zhang et al. prepared ultra-high-brightness micro-LED display panels with displaydisplay resolutions resolutions of of 640 640 × 480480 and and pixel pixel pitches pitches of of 20 20 μµmm through through the the monolithic monolithic hybrid hybrid integration integration display resolutions of 640 ×× 480 and pixel pitches of 20 μm through the monolithic hybrid integration technologytechnology of of Beida Beida Jade Jade Bird Bird Co., Co., Ltd. Ltd. [89]. [89]. The The co colorlor combination combination prism prism integrates integrates the the light light emitted emitted technology of Beida Jade Bird Co., Ltd. [89]. The color combination prism integrates the light emitted byby the the red, red, green, green, and and blue blue monochromatic monochromatic micro-LED micro-LED display display panels panels to to form form a a full-color full-color image. image. by the red, green, and blue monochromatic micro-LED display panels to form a full-color image. InIn 2014, 2014, Joao Joao et et al. al. used used liquid liquid capillary capillary forc forcee to to transfer transfer a a micron-sized micron-sized ZnCdSe/ZnCdMgSe ZnCdSe/ZnCdMgSe In 2014, Joao et al. used liquid capillary force to transfer a micron-sized ZnCdSe/ZnCdMgSe multiplemultiple quantum quantum well well (MQW) (MQW) color color conversion conversion film film to to a a micro-LED micro-LED with with an an emission emission wavelength wavelength multiple quantum well (MQW) color conversion film to a micro-LED with an emission wavelength ofof 450 450 nm nm and and prepared prepared a a hybrid hybrid micro-LED micro-LED that that can can emit emit 540 540 nm nm light light waves waves [90]. [90]. The The II-VI II-VI MQW MQW of 450 nm and prepared a hybrid micro-LED that can emit 540 nm light waves [90]. The II-VI MQW colorcolor conversion conversion film film is is grown grown on on an an InP InP substrat substrate,e, and and the the InGaAs InGaAs growth growth buffer buffer layer layer is is produced produced color conversion film is grown on an InP substrate, and the InGaAs growth buffer layer is produced viavia MBE. MBE. This This layer layer is is composed composed of of nine nine ZnCdSe ZnCdSe QWs QWs with with CdMgZnSe CdMgZnSe barriers, barriers, and and its its thickness thickness is is via MBE. This layer is composed of nine ZnCdSe QWs with CdMgZnSe barriers, and its thickness is lessless than than 2.5 2.5 μµm.m. Its Its structure structure is is shown shown in in Figure Figure 14a 14a and and is is bonded bonded to to the the sapphire sapphire substrate substrate of of the the less than 2.5 μm. Its structure is shown in Figure 14a and is bonded to the sapphire substrate of the micro-LEDmicro-LED via via liquid liquid capillary capillary force. force. The The luminous luminous ef efectffect under under the the excitation excitation of of a a 450 450 nm nm light light source source micro-LED via liquid capillary force. The luminous effect under the excitation of a 450 nm light source isis shown shown in in Figure Figure 14b. 14b. By By changing changing the the materials materials of this of this MQW MQW conversion conversion filmfilm structure, structure, such such as III- as is shown in Figure 14b. By changing the materials of this MQW conversion film structure, such as III- VIII-V AlGaInP AlGaInP (yellow (yellow to red) to red) and and InGaN InGaN (green), (green), this this structure structure can can be be extended extended to to other other wavelengths, wavelengths, V AlGaInP (yellow to red) and InGaN (green), this structure can be extended to other wavelengths, allowingallowing a a full-color full-color display display to to be be realized realized using using micro-LEDs. micro-LEDs. allowing a full-color display to be realized using micro-LEDs.

Figure 14. (a) II-VI multiple quantum well (MQW) structure design, (b) top view micrograph of the Figure 14. (a) II-VI multiple quantum well (MQW) structure design, (b) top view micrograph of the mixingFigure 14. device (a) II-VI and themultiple effect ofquantum II-VI film well when (MQW) excited structure by a blue design, LED. Reproduced(b) top view frommicrograph [90], Institute of the mixing device and the effect of II-VI film when excited by a blue LED. Reproduced from [90], Institute ofmixing Physics, device 2015. and the effect of II-VI film when excited by a blue LED. Reproduced from [90], Institute of Physics, 2015. of Physics, 2015. The deposition of QDs will face problems, such as low printing efficiency, the inability to pattern, The deposition of QDs will face problems, such as low printing efficiency, the inability to pattern, the coTheffee deposition ring effect, of and QDs a roughwill face and problems, uneven thickness such as low of theprinting QD color efficiency, conversion the inability layer. Techniques, to pattern, the coffee ring effect, and a rough and uneven thickness of the QD color conversion layer. Techniques, suchthe coffee as spin ring coating, effect, fogand coating, a rough and and spray uneven printing, thickn cannotess of the effi QDciently color and conversion controllably layer. deposit Techniques, QDs or such as spin coating, fog coating, and spray printing, cannot efficiently and controllably deposit QDs phosphorssuch as spin on coating, each sub-pixel. fog coating, Therefore, and spray a stable, printing, low cannot cost, high efficiently resolution, and uniform,controllably and deposit patternable QDs or phosphors on each sub-pixel. Therefore, a stable, low cost, high resolution, uniform, and depositionor phosphors technology on each is sub-pixel. required. InTherefore, 2020, Kim a etstable, al. proposed low cost, to deposithigh resolution, a color conversion uniform,layer and patternable deposition technology is required. In 2020, Kim et al. proposed to deposit a color patternable deposition technology is required. In 2020, Kim et al. proposed to deposit a color conversion layer made of a mixture of light-cured acrylic and nano-organic color conversion conversion layer made of a mixture of light-cured acrylic and nano-organic color conversion

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Nanomaterials 2020, 10, x FOR PEER REVIEW 17 of 33 made of a mixture of light-cured acrylic and nano-organic color conversion materials onto a blue materialsmicro-LED onto through a blue traditionalmicro-LED photolithographythrough traditional technology photolithography to achieve technology a full-color to achieve display a full- [91]. colorThrough display traditional [91]. Through photolithography traditional technology, photolithography the conversion technology, material the wasconversion accurately material deposited was accuratelyonto each sub-pixel,deposited and onto the each position, sub-pixel, size, and thickness, the position, and patterning size, thickness, of the and color patterning conversion oflayer the color was conversionable to be controlled. layer was able The to structure be controlled. of the color The st conversionructure of devicethe color array conversion based on device blue micro-LEDsarray based onis shown blue micro-LEDs in Figure 15 is. Theshown chip in sizeFigure is 10 15. The10 mm chip2, thesize resolution is 10 × 10 ismm 1002, the100, resolution and the is size 100 of × each100, × × andsub-pixel the size is 60of each100 sub-pixelµm2 with is a 60 pixel × 100 pitch μm of2 with 300 µam. pixel Here, pitch green of 300 and μ redm. Here, perylene green imides and red are × perylenemixed with imides organic are insulator mixed with materials, organic acrylic insulator resin ma solutions,terials, acrylic and photoactive resin solutions, compounds and photoactive containing compoundsa positive tone containing of diazonaphthoquinone a positive tone of diazonapht to form ahoquinone color conversion to form layera color that conversion is deposited layer ontothat isa bluedeposited micro-LED onto a arrayblue micro-LED via traditional array photolithography via traditional photolitho technology.graphy In addition,technology. to In avoid addition, color tocrosstalk avoid color between crosstalk each sub-pixel, between each a black sub-pixel, matrix is a alsoblack deposited matrix is between also deposited the blue between micro-LED the chipsblue micro-LEDusing photolithography. chips using photolithography.

Figure 15.15. SchematicSchematic diagram diagram of aof full-color a full-color micro-LED micro-LED based onbased light-curing on light-curing color-changing color-changing materials. materials.Reproduced Reproduced from [91], MDPI,from [91], 2020. MDPI, 2020. The team studied the thickness, transmittance, conversion efficiency, and mixing rate of the color The team studied the thickness, transmittance, conversion efficiency, and mixing rate of the color conversion layer and concluded that the transmittance of the color conversion layer was almost 100% conversion layer and concluded that the transmittance of the color conversion layer was almost 100% in the visible light range. Under the excitation of 365 nm blue light, the green and red conversion in the visible light range. Under the excitation of 365 nm blue light, the green and red conversion materials absorbed the blue light and emitted the light at 550 nm and 620 nm, respectively. Finally, materials absorbed the blue light and emitted the light at 550 nm and 620 nm, respectively. Finally, by comparing the different mixing ratios of the acrylic and nano-organic color conversion materials, by comparing the different mixing ratios of the acrylic and nano-organic color conversion materials, the best conversion efficiency of the green and red conversion materials was determined to be obtained the best conversion efficiency of the green and red conversion materials was determined to be with 0.6% organic color conversion materials (the conversion efficiency of the green and red mixtures obtained with 0.6% organic color conversion materials (the conversion efficiency of the green and red was 47% and 35%, respectively). mixtures was 47% and 35%, respectively). 3.4. Summary 3.4. Summary In summary, color conversion technology is an effective solution to achieve full-color micro-LEDs. In summary, color conversion technology is an effective solution to achieve full-color micro- It does not require complex mass transfer technology to bond the RGB three-color LED chips to the LEDs. It does not require complex mass transfer technology to bond the RGB three-color LED chips same substrate to achieve full-color micro-LEDs. Thus, the chip defect rates caused by mass transfer to the same substrate to achieve full-color micro-LEDs. Thus, the chip defect rates caused by mass technology and the uneven color and brightness caused by RGB LED chips of different materials can transfer technology and the uneven color and brightness caused by RGB LED chips of different be avoided. materials can be avoided. Phosphor is a mature color conversion substance used for solid-state lighting. However, as the Phosphor is a mature color conversion substance used for solid-state lighting. However, as the sizes of micro-LED chips gradually shrink, defects caused by the small size of phosphors lead to sizes of micro-LED chips gradually shrink, defects caused by the small size of phosphors lead to degradation of light efficiency, wide emission spectra, and uneven color distribution, making them degradation of light efficiency, wide emission spectra, and uneven color distribution, making them difficult to be considered a high-quality choice for micro-LED color displays in the future. Due to difficult to be considered a high-quality choice for micro-LED color displays in the future. Due to their nanometer size, high quantum yield, wide absorption spectrum, and narrow emission spectrum, their nanometer size, high quantum yield, wide absorption spectrum, and narrow emission spectrum, quantum dots give the QD color conversion layer an adjustable wavelength, high color accuracy, quantum dots give the QD color conversion layer an adjustable wavelength, high color accuracy, high color saturation, and high color uniformity. Such excellent performance makes QDs the best high color saturation, and high color uniformity. Such excellent performance makes QDs the best choice to replace phosphors in the next generation of micro-display color conversion. choice to replace phosphors in the next generation of micro-display color conversion. Color purity shows the ability of the display to restore the nature origin colors, and it is an important indicator in evaluating the displays. An LCD illuminated by the backlight, so, its color

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Color purity shows the ability of the display to restore the nature origin colors, and it is an important indicator in evaluating the displays. An LCD illuminated by the backlight, so, its color purity is affected by the spectral composition of the backlight and the RGB color filter. An OLED realizes spontaneous light through the design of organic molecular structure, so it has a wider color gamut and higher color purity than LCD. However, due to the low lifespan of OLEDs, their various properties will become unstable over time, including the color purity. A full-color micro-LED display based on QD relies on the narrow and continuous emission spectrum of QDs, and the stability, long life and strong energy saving of the LED itself. So, it has unparalleled color expression ability. According to the Recommendation BT.2020 published by The International Telecommunication Union Radiocommunication Sector (ITU-R), QD micro-LED displays are in the leading position with their 83% color purity, while OLED displays can only reach 75% at most. However, QDs are not perfect, as the conversion efficiency and stability of QDs remain severely limited. On the one hand, these limitations come from the lack of mature spraying technology for QDs. On the other hand, the synthesis quality of QD materials cannot be guaranteed. The current QD spraying process has low efficiency and uneven spray thickness, and the QD layers cannot be patterned. And due to its nature, QD is often metastable when mixed with other solutions, which will result in low conversion efficiency. The conversion efficiency of QDs will affect the power consumption and life of QD displays. Although some external structures can be used to increase the external quantum efficiency (EQE) of QD micro-LEDs and hybrid QDs to increase their yields, QDs with high conversion efficiency also need to be further studied. Any QD material will be extremely sensitive to external conditions, such as light, heat, oxygen, and moisture. This instability will cause the degradation of the QD materials, which will quench the luminescence of the micro-LEDs. The stability of QDs will directly affect the reliability and life of color micro-LED displays, so determining how to improve the stability of QD materials is the research focus of color-transfer micro-LED displays. Fortunately, existing studies have shown that QD stability can be improved by methods, such as ion doping, engineering the design of the QD ligand surface, and encapsulation of the QDs with polymers or oxide materials. Finally, QD contain heavy metal elements that are harmful to the human body and have a certain level of toxicity. Thus, the next step of research needs to prepare high-quality QDs without heavy metals or find other high-quality color conversion materials.

4. Monolithic Multi-Color Growth Technology As described above, a full-color micro-LED display can be achieved through mass transfer and color conversion technology. However, as the sizes of the micro-LED chips shrink, and the demand for display resolution increases, the costs and difficulties of transfer and bonding technology will undoubtedly increase. Although color transfer technology can avoid these problems and realize full-color micro-LEDs, the color conversion layer faces the problems of low external quantum efficiency, a short lifetime, poor reliability, and complicated preparation. Therefore, realizing the growth of multi-color LEDs on a single wafer is the ultimate solution for realizing full-color micro-LEDs. Theoretically, InGaN/GaN MQWs LEDs can change the wavelengths of monochromatic LEDs by modulating the indium (In) content in the MQW. The higher the In content is, the longer the emission wavelength will be. However, as the In content increases, the internal quantum efficiency (IQE) of the LED will drop rapidly, especially when the wavelength is larger than 520 nm, which also means that high-efficiency green and red LEDs cannot be generated using traditional InGaN/GaN MQWs. This decrease in IQE is mainly caused by surface defects in the MQW, the internal polarization’s electric field and poor miscibility between InN and GaN [92]. Excessive In content will significantly aggravate the lattice and thermal mismatch between the GaN barrier and the InGaN well, resulting in increased strain in this light-emitting heterostructure and producing defects at the surface interface, such as dislocations, related V-defects, different point defects, etc. These defects are likely to become the center of non-radiative recombination, which will increase the probability of non-radiative recombination, thereby reducing the IQE of the LED. In addition, due to the low thermodynamic stability of InGaN Nanomaterials 2020, 10, 2482 19 of 33 with a high In content, phase separation in this layer will reduce the chemical uniformity of the MQW, whichNanomaterials will lead2020, to10, the x FOR degradation PEER REVIEW of the MQW active region. Another factor affecting the low quantum 19 of 33 efficiency of long-wavelength LEDs is the piezoelectric polarization electric field caused by lattice inmismatches nitride materials and the [93]. spontaneous MQW containing polarization too electricmuch In field will in have nitride a large materials internal [93 polarization]. MQW containing electric field,too much which In will will separate have a large the electron-hole internal polarization wave fu electricnction in field, the MQW, which willthereby separate reducing the electron-hole the effective waveluminescence function radiation in the MQW, recombination thereby reducing rate. This the is also effective known luminescence as the quantum radiation confined recombination Stark Effect (QCSE)rate. This [94]. is In also addition, known the as low the quantumquantum confinedefficiency Starkof long-wavelength Effect (QCSE) LEDs [94]. Inmay addition, also be affected the low quantumby the Auger efficiency recombination of long-wavelength [95], carrier LEDs(electron/ mayhole) also asymmetry be affected by[96], the and Auger carrier recombination leakage from [ 95the], activecarrier region (electron [97]./hole) Considering asymmetry the [96 ],challenges and carrier in leakage the implementation from the active of region long-wave [97]. Considering InGaN-based the LEDs,challenges red LEDs in the usually implementation use GaP/GaAs of long-wave as their InGaN-based growth substrate. LEDs, However, red LEDs usuallystudies usehave GaP shown/GaAs that as IQEtheir and growth EQE substrate. will also However,decrease significantly studies have as shown the chip that size IQE decreases and EQE will[98]. also decrease significantly as theIn chip the sizepast decreasesten years,[ 98although]. multi-color micro-LEDs grown on the same substrate using the standardIn the approaches past ten years, have althoughnot been prepared, multi-color the micro-LEDs emergence of grown some ontechnologies the same substrate has made using this type the ofstandard growth approaches possible. For have example, not been a growth prepared, buffer the emergencelayer could ofbe some used technologiesto alleviate the has lattice made and this thermaltype of growth mismatch possible. between For GaN example, and InGaN, a growth allowi bufferng layerthree could kinds be of usedRGB tomicro-LEDs alleviate the to latticebe grown and onthermal the substrate. mismatch Moreover, between GaNnanowi andres InGaN, or nanoring allowing structures three kinds could of be RGB prepared micro-LEDs on GaN to to be achieve grown micro-LEDson the substrate. with Moreover,a tunable wavelength. nanowires or In nanoring addition structures, the wavelength could be of prepared a single-chip on GaN LED to could achieve be tunedmicro-LEDs through with a multi-active a tunable wavelength. QW structure. In This addition, section the will wavelength introduce ofthe a LED single-chip technologies LED could that are be expectedtuned through to realize a multi-active single-chip QW multi-color structure. light This emissions section will and introduce provide research the LED directions technologies for thatthe futureare expected realization to realize of the single-chip single-chip multi-color growth of lightRGB emissionsmulti-color and micro-LED provide researchchips. directions for the future realization of the single-chip growth of RGB multi-color micro-LED chips. 4.1. Growth Buffer Technology 4.1. Growth Buffer Technology To grow RGB three-color LEDs on the same substrate, it is necessary to use epitaxial QW structuresTo grow with RGB different three-color In content. LEDs on For the green same substrate,and red LEDs, it is necessary the large to useIn content epitaxial in QW the structuresQW will withinevitably different cause In an content. excessive For lattice green mismatch and red LEDs,between the the large GaN In barrier content layers in the and QW the will InGaN inevitably wells, whichcause anwill excessive affect the lattice performance mismatch of between the LEDs. the Studies GaN barrier have shown layers andthat thethe InGaNmaximum wells, In whichcontent will in traditionalaffect the performance InGaN/GaN ofQW the structure LEDs. Studies is limited have to 25% shown [99]. that Therefore, the maximum it is necessary In content to find in traditional a suitable growthInGaN/ GaNbuffer QW layer structure as a seed is limited layer to 25%grow [ 99heterostructures]. Therefore, it iswith necessary high crystallinity, to find a suitable as shown growth in Figurebuffer layer16. as a seed layer to grow heterostructures with high crystallinity, as shown in Figure 16.

FigureFigure 16. MulticolorMulticolor LED LED structure structure with with a a growth buffer buffer layer.

TM Using its Smart CutTM technology,technology, Soitec Soitec developed a truly relaxed InGaNInGaN pseudo-substratepseudo-substrate (InGaNOS). The InGaNOS layer is deposited betweenbetween the GaN substrate and the InGaN MQW active layer. This This arrangement arrangement can can allevi alleviateate the the compressive compressive strain strain between between the the layers, layers, thereby thereby reducing reducing the latticethe lattice mismatch mismatch and andinternal internal polarization polarization electric electric field field between between the theGaN GaN and and InGaN InGaN with with high high In contentIn content that that exhibit exhibit high high In In doping doping capacity capacity [100,101]. [100,101]. In In 2017, 2017, Even Even et et al. al. tested tested the the performance performance of three InGaNOS types with different different in-plane lattice constants under three various growth conditions (3.190 Å, 3.200 Å, and 3.205 Å), and the results are shownshown inin FigureFigure 1717[ [102].102]. The test results show that the InGaN structure grown on the InGaNOS subs substratetrate can achieve emissions from blue to amber (482 nmnm to to 617 617 nm), nm), and and the IQEthe IQE values values measured measured at 536 nmat 536 and nm 566 nmand are 566 31% nm and are 10%, 31% respectively. and 10%, Therefore,respectively. InGaNOS Therefore, technology InGaNOS can technology effectively can grow effectively LEDs of digrowfferent LEDs colors of ondifferent the same colors substrate. on the same substrate.

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FigureFigure 17.17. (a–c) are the PL (photoluminescence) spectrasspectras atat 300300 KK of the blue, green, andand amber InGaNInGaN heterostructuresheterostructures grown grown in in InGaNOS InGaNOS under under three three growth growth conditions. conditions. The insetsThe insets in the in lower the rightlower corner right arecorner emission are emission diagrams diagrams of the InGaNOS of the samplesInGaNOS under samples laser excitation,under laser where excitation, (a) blue luminescencewhere (a) blue is observedluminescence for InGaNOS is observed with for a =InGaNOS3.190 Å grown with a at =a 3.190 growth Å grown temperature at a growth and rate temperature of 750 ◦C and and 0.1 rate lm /ofh, b respectively;750 °C and 0.1 ( lm/h,) green respectively; luminescence (b) isgreen observed luminescence for InGaNOS is observed with a for= 3.200 InGaNOS Å grown with at a a= growth3.200 Å temperature and rate of 730 C and 0.1 lm/h, respectively; (c) amber luminescence is observed for grown at a growth temperature◦ and rate of 730 °C and 0.1 lm/h, respectively; (c) amber luminescence InGaNOS with a = 3.205 Å grown at a growth temperature and rate of 720 C and 0.3 lm/h, respectively. is observed for InGaNOS with a = 3.205 Å grown at a growth temperature◦ and rate of 720 °C and 0.3 Reproduced from [102], with permission from American Institute of Physics, 2017. lm/h, respectively. Reproduced from [102], with permission from American Institute of Physics, 2017. In 2020, Dussaigne et al. experimentally proved that the deposition of an InGaN/GaN superlattice In 2020, Dussaigne et al. experimentally proved that the deposition of an InGaN/GaN structure on an InGaNOS pseudo-substrate has the potential to fill surface V defects and further superlattice structure on an InGaNOS pseudo-substrate has the potential to fill surface V defects and improve the crystal quality on InGaNOS [103]. The team used this structure to prepare a red-emitting further improve the crystal quality on InGaNOS [103]. The team used this structure to prepare a red- MQW on InGaNOS and measured its center wavelength at 624 nm with an IQE of 6.5% estimated from emitting MQW on InGaNOS and measured its center wavelength at 624 nm with an IQE of 6.5% the ratio of the PL intensities measured at 20 and 290 K. estimated from the ratio of the PL intensities measured at 20 and 290 K. InGaNOS pseudo-substrate technology can successfully overcome the internal strain of the InGaNOS pseudo-substrate technology can successfully overcome the internal strain of the GaN/InGaN LED structure, allowing the amount of In doping to exceed its own limits and emit longer GaN/InGaN LED structure, allowing the amount of In doping to exceed its own limits and emit wavelength light. With selective area growth technology (SAG), this technology has the potential to longer wavelength light. With selective area growth technology (SAG), this technology has the grow RGB micro-LEDs on the same substrate. potential to grow RGB micro-LEDs on the same substrate. 4.2. Monolithic Wavelength Tunable MQW Micro-LEDs 4.2. Monolithic Wavelength Tunable MQW Micro-LEDs The light wavelength emitted by an LED is mainly controlled by the structure and composition of theThe LED’s light quantum wavelength well. emitted Therefore, by an growing LED is main a QWly structurecontrolled containing by the structure multiple and active composition regions onof the the LED’s same substratequantum andwell. injecting Therefore, carriers growing into a diQWfferent structure active containing layers of themultiple LED structureactive regions in a controllableon the same manner substrate under and externalinjecting conditions carriers into is anotherdifferent method active tolayers realize of the a single-chip LED structure LED thatin a emitscontrollable multi-wavelength manner under light. external conditions is another method to realize a single-chip LED that emitsIn multi-wavelength 2010, Damilano etlight. al. proposed a blue LED structure grown on top of InGaN/GaN MQWs, whichIn can 2010, convert Damilano the emission et al. proposed wavelength a blue from LED blue structure to yellow-green grown on [104 top,105 of ].InGaN/GaN The LED structure MQWs, haswhich two can active convert regions. the emission The first activewavelength region from generates blue blueto yellow-green light through [104,105]. EL and thenThe LED stimulates structure the PLhas of two the active MQW regions. layer, thereby The first realizing active the region conversion generates of blue blue light light to through yellow-green EL and light; then this stimulates region’s overallthe PL structureof the MQW is shown layer, in thereby the left halfrealizing of Figure the 18conversiona. This dual-active-layer of blue light to LED yellow-green structureuses light; metal this organicregion’s chemical overall structure vapor deposition is shown (MOCVD)in the left half technology of Figure to 18a. grow This a 6.6 dual-active-layerµm thick undoped LED GaN structure layer onuses a c-planemetal organic sapphire chemical substrate. vapor Then, deposition 20 periods (MOCVD) of InGaN technology (3.1 nm)/GaN to grow (20 nm) a 6.6 MQW μm thick are deposited undoped andGaN used layer as on the a green-yellowc-plane sapphire light substrate. conversion Then layer., 20 Finally, periods a blueof InGaN LED is(3.1 grown nm)/GaN on MQW (20 nm) as a pumpMQW lightare deposited source. Theand luminousused as the color green-yellow of the LED light depends conversion on the layer. absorption Finally, of a theblue pump LED lightis grown source on byMQW the as yellow-green a pump light light source. conversion The luminous layer, color which of is the the LED IQE depends of the yellow-green on the absorption light of conversion the pump layer.light source The experimental by the yellow-green results show light that conversion the absorption layer, of which the light is the conversion IQE of the layer yellow-green changes with light the pumpconversion light source,layer. The which experimental means that theresults shorter show the th pumpat the wavelength absorption is,of thethe strongerlight conversion the absorption layer capacitychanges ofwith the the light pump conversion light source, layer will which be, and means the closerthat the the shorter emission the wavelength pump wavelength of the LED is, willthe becomestronger to the yellow-green. absorption capacity The right of half the of light Figure conv 18ersiona shows layer the luminescencewill be, and the of thecloser device the whenemission the pumpwavelength light source of the wavelength LED will changesbecome fromto yellow-green. 405 nm to 450 The nm. right half of Figure 18a shows the luminescenceIn 2012, Dawson’s of the device team when demonstrated the pump alight micro-LED source wavelength array made changes of an InGaN from epitaxial405 nm to structure 450 nm. withIn a high 2012, In Dawson’s content up team to 0.4 demonstrated [106]. The micro-LED a micro-LED array array in thismade study of an had InGaN a resolution epitaxial of structure 16 16, with a high In content up to 0.4 [106]. The micro-LED array in this study had a resolution of 16 ×× 16, and the pixel diameter and center spacing were 72 μm and 100 μm, respectively. The authors used a flip-chip design to bond the micro-LED on a CMOS substrate and achieved a programmable dynamic

Nanomaterials 2020, 10, 2482 21 of 33

Nanomaterials 2020, 10, x FOR PEER REVIEW 21 of 33 and the pixel diameter and center spacing were 72 µm and 100 µm, respectively. The authors used a imageflip-chip display design through to bond a thecomputer micro-LED and on a a CMOSfield-p substraterogrammable and achieved gate array a programmable (FPGA). The dynamic epitaxial structureimage of display this LED through includes a computer a 1.5 μ andm athick field-programmable GaN buffer layer, gate a array 4 μm (FPGA). thick n-doped The epitaxial GaN structure layer, five- µ µ periodof thisIn0.18 LEDGa0.82 includesN (3 nm)/GaN a 1.5 m thick(10 nm) GaN MQWs buffer layer, (secondary a 4 m thickactive n-doped layer) GaNemitted layer, at five-period 460 nm, five- In Ga N (3 nm)/GaN (10 nm) MQWs (secondary active layer) emitted at 460 nm, five-period period0.18 In0.4Ga0.820.6N (2.5 nm)/GaN (12 nm) MQWs (the main active layer) emitted at 600 nm, and a 210 In Ga N (2.5 nm)/GaN (12 nm) MQWs (the main active layer) emitted at 600 nm, and a 210 nm thick nm thick0.4 p-GaN0.6 layer, as shown in Figure 18b. The secondary active layer with low In content is used p-GaN layer, as shown in Figure 18b. The secondary active layer with low In content is used as a as a carrier storage layer, and a pre-strain relaxation layer is used to improve the radiation efficiency carrier storage layer, and a pre-strain relaxation layer is used to improve the radiation efficiency of the of themain main active active layer. layer. By adjustingBy adjusting the injection the injectio currentn current of the micro-LED, of the micro-LED, the emission the emission wavelength wavelength of the of theLED LED can can be be changed changed from from red red to green, to green, as shown as shown in the in upper the upper part of part Figure of Figure 18c. This 18c. blue This shift blue of shift of thethe wavelength wavelength is is caused caused by the screeningscreening and and band-filling band-filling eff effectsects of QCSEof QCSE on non-polaron non-polar quantum quantum wells.wells. In addition, In addition, to ensure to ensure that that the the pixels pixels of of di differentfferent colorscolors have have uniform uniform brightness, brightness, the dutythe duty cycle cycle of theof driving the driving voltage voltage can can be be changed changed to to modulate modulate thethe averageaverage brightness brightness of eachof each pixel, pixel, as shown as shown in in the lowerthe lower part part of Figure of Figure 18c. 18 c.

Figure 18. (a) The blue LED structure grown on top of InGaN / GaN multiple QWs with light emission Figure 18. (a) The blue LED structure grown on top of InGaN / GaN multiple QWs with light emission under different wavelength pump light sources proposed by Damiano et al. Reproduced from [104], under different wavelength pump light sources proposed by Damiano et al. Reproduced from [104], with permission from American Institute of Physics, 2010; The dual active area micro-LED structure withproposed permission by thefrom Dawson American team In withstitute a high of InPhysics, mole fraction 2010; The up todual 0.4 (activeb), and area its ( cmicro-LED) EL spectrum structure in proposedusing dibyff erentthe Dawson injection team currents with and a high the brightness In mole fraction change ofup the to LED0.4 ( underb), and di ffitserent (c) EL duty spectrum cycles in usingof different the driving injection voltage currents (c). Reproduced and the frombrightness [106], withchange permission of the LED from under Institute different of Electrical duty cycles and of the drivingElectronics voltage Engineers, (c). 2012.Reproduced from [106], with permission from Institute of Electrical and Electronics Engineers, 2012. The multi-active area epitaxial structure micro-LED proposed by Dawson’s team can achieve red lightThe emissionsmulti-active at 0.5 area mA withepitaxial an output structure power ofmicr 0.93o-LED W, green proposed light emissions by Dawson’s at 80 mA withteam 0.5% can duty achieve and 1.03 W output power, and yellow light emissions at 18 mA with a duty cycle of 2% and an output red light emissions at 0.5 mA with an output power of 0.93 W, green light emissions at 80 mA with power of 0.97 W, which surpasses the limitations of monolithic InGaN-based LED multi-color displays. 0.5% duty and 1.03 W output power, and yellow light emissions at 18 mA with a duty cycle of 2% However, because the driving circuit for this structure is too complicated to realize the independent and controlan output of each power pixel, of this 0.97 technology W, which is surpasses currently unable the limitations to achieve of a full-color monolithic micro-LED InGaN-based display, LED multi-colorand the IQEdisplays. and color However, adjustment because range ofthe the driving device needscircuit to for be furtherthis structure optimized. is too In 2016, complicated Chen’s to realizeteam the inserted independent a carrier control blocking of layer each (ICBL) pixel, between this technology multiple QWs is currently in different unable active to regions achieve [107 a]. full- colorThe micro-LED experiments display, showed and that the ICBL IQE can and guide color most adju of thestment carriers range (holes of andthe electrons)device needs into theto be target further optimized.QW where In 2016, they recombine,Chen’s team thereby inserted generating a carrier light blocking of a specific layer wavelength (ICBL) between under the multiple injection QWs of in differentdifferent active controlled regions currents. [107]. The This experiments structure eff ectivelyshowed controls that ICBL the carriercan guide injection most distribution of the carriers of the (holes and entireelectrons) active into region the and target can be QW used where to improve they the re luminouscombine, effitherebyciency andgenerating wavelength light range of ofa thespecific wavelengthmulti-active under QW the structure injection of LEDof different with an adjustablecontrolled wavelength. currents. This structure effectively controls the carrier injection distribution of the entire active region and can be used to improve the luminous efficiency and wavelength range of the multi-active QW structure of LED with an adjustable wavelength.

4.3. Monolithic Wavelength Tunable Nanowire LED The size of a nanowire LED array is at the micron or even submicron level. By horizontally arranging an RGB LED array composed of this nanowire structure, multi-color emissions can be easily generated. Therefore, nanowire LEDs have been widely studied for their possible use in full- color displays [108–111]. Compared with conventional InGaN/GaN quantum-well LEDs, nanowire

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4.3. Monolithic Wavelength Tunable Nanowire LED The size of a nanowire LED array is at the micron or even submicron level. By horizontally arranging an RGB LED array composed of this nanowire structure, multi-color emissions can be easily generated. Therefore, nanowire LEDs have been widely studied for their possible use in full-color displays [108–111]. Compared with conventional InGaN/GaN quantum-well LEDs, nanowire heterostructures have many obvious advantages, including greatly reducing the dislocation density and polarization field in the active region of the device, improving the light extraction efficiency, and offering good compatibility with low-cost, large-area silicon substrates [112–115]. Such nanostructures include dot-in-a-wire nanowires, core/shell nanowires, and nano-column structures. The emission wavelength can be modulated by changing the composition, diameter, and injection current of the nanowires. In 2011, Hong et al. prepared a core/shell GaN nanowire array. By applying different external voltages, the emission wavelength of the nanowire LED can be continuously modulated from red to + blue [116]. The core/shell GaN nanowire structure is grown on an n -GaN/Al2O3 (0001) substrate with a SiO2 growth mask layer that has some submicron hole patterns. The average length, diameter, and adjacent spacing of each nanowire structure are 520 nm, 220 nm, and 550 nm, respectively. All of these parameters can be controlled by changing the lithography design and growth parameters. After the GaN nanorod arrays are grown, nano InxGa1 xN/GaN MQWs are is epitaxially grown on the − entire surface of each GaN nanorod array via selective metal organic vapor phase epitaxy (MOVPE) technology. Subsequently, p-type-doped GaN with Mg is coated on the outside of the MQW to form a capping film. The structure and preparation process are shown in Figure 19a. The emission wavelength of the core/shell GaN nanowire LED is controlled by an external voltage, just like a multi-active QW LED. The lower part of Figure 19a shows a series of EL images under various bias voltages. It can be seen that, with an increase in bias voltage, the color of the emitted light gradually changes from red toNanomaterials blue. 2020, 10, x FOR PEER REVIEW 23 of 33

Figure 19. (a) Shell/core nanowire LED structure and EL images driven by different external Figure 19. (a) Shell/core nanowire LED structure and EL images driven by different external voltages. voltages. Reproduced from [116], with permission from John Wiley and Sons, 2011; (b) InGaN/GaN Reproduced from [116], with permission from John Wiley and Sons, 2011; (b) InGaN/GaN dot-in-a- dot-in-a-wire LED structure. Reproduced from [117], with permission from Institute of Physics, 2011. wire LED structure. Reproduced from [117], with permission from Institute of Physics, 2011. All rights All rights reserved. reserved. In the same year, Nguyen et al. proposed a nanowire LED containing QDs (dot-in-a-wire). In 2020, Liu et al. prepared nanowire LED arrays with sizes as small as 150 nm through selective By changing the composition or size of the QDs in the nanowire structure, efficient emissions can be area growth (SAG) technology [119,120]. The authors used radio frequency plasma-assisted achieved over almost the entire visible wavelength range [117,118]. InGaN/GaN dot-in-a-wire LEDs molecular beam epitaxy (PA-MBE) and a Ti mask to selectively grow nanowire arrays of different are grown directly on Si (111) substrates, and the device heterostructure is composed of a 0.4 µm GaN:Si diameters on a sapphire substrate. The nanowire structure consisted of a 0.44 μm n-GaN layer, a 6- layer, an InGaN/GaN active layer, and a 0.2 µm GaN:Mg layer. This structure is shown in Figure 19b. period InGaN/GaN quantum disk, and a 0.15 μm p-GaN layer, as shown in Figure 20a. The emission There are multiple InGaN/GaN QDs with a diameter of about 3 nm in the active area, and the core of wavelength of a nanowire is related to the diameter of the nanowire. As the diameter decreases, the each QD is InGaN, which is then covered by GaN. The different emission wavelengths are produced emission wavelength moves closer to a red color. This is due to the lateral diffusion of In atoms during the growth of the nanowires. Small diameter nanowires can limit this diffusion, thereby increasing the contribution of In component incorporation. Therefore, by changing the diameter of the nanowire, the emission wavelength of the nanowire LED can be modulated, as shown in Figure 20b. In addition, by increasing the effective area (changing the diameter and length of the nanowire structure), the light extraction efficiency of the device can be significantly improved. This method also offers a controllable emission cone angle, which has great potential in micro-LED full-color displays.

Figure 20. (a) A nanowire structure with different diameters and (b) its normalized EL spectrum under different diameters. Reproduced from [120], with permission from Nano Letters, 2016.

Nanowire LEDs have the growth-dependent characteristics depending on achieving tunable emission wavelength. By selectively growing nanowire structures with different growth parameters on the same substrate to form a nanowire LED array, a full-color display can be realized. Compared

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Nanomaterials 2020, 10, 2482 23 of 33 by changes to the In composition in the InGaN/GaN QD layer. During the growth process of the active region of QDs, by using a relatively low substrate temperature or a high In/Ga flux ratio, a high In composition nanowire structure (up to about 50%) can be achieved, thereby achieving orange/red emissions. The active area of this nanowire structure is in the middle of the nanowire, far away from the growth interface. Therefore, the effect of the strain caused by lattice mismatches on the active region is reduced, and the formation of dislocations is also minimized. In addition, using relatively small (about 3 nm) InGaN QDs can significantly increase the IQE of the LED. Compared with conventional plane InGaN/GaN QW LEDs, due to their efficient lateral stress relaxation, InGaN/GaN dot-in-a-wire Figure 19. (a) Shell/core nanowire LED structure and EL images driven by different external voltages. nanowire LEDs still offer high efficiency in high current density injection. In addition, they also Reproduced from [116], with permission from John Wiley and Sons, 2011; (b) InGaN/GaN dot-in-a- have excellent volt–ampere characteristics, in which cascade resistance (approximately 20–50 Ω) and wire LED structure. Reproduced from [117], with permission from Institute of Physics, 2011. All rights leakage currents are very small under a relatively large reverse bias (Under the reverse voltage of 4 V, reserved. − the leakage voltage is about 0 A). In 2020, Liu et al. prepared prepared nanowire LED arrays with sizes as small as 150 nm through selective area growthgrowth (SAG) (SAG) technology technology [119 ,[119,120].120]. The authorsThe authors used radio used frequency radio frequency plasma-assisted plasma-assisted molecular beammolecular epitaxy beam (PA-MBE) epitaxy and (PA-MBE) a Ti mask an tod a selectively Ti mask to grow selectively nanowire grow arrays nanowire of different arrays diameters of different on a sapphirediameters substrate. on a sapphire The nanowire substrate. structure The nanowire consisted structure of a 0.44 consistedµm n-GaN of layer,a 0.44 a μ 6-periodm n-GaN InGaN layer,/GaN a 6- quantumperiod InGaN/GaN disk, and quantum a 0.15 µm disk, p-GaN and layer,a 0.15 asμm shown p-GaN in layer, Figure as 20showna. The in Figure emission 20a. wavelength The emission of awavelength nanowire isof relateda nanowire to the is related diameter to the of thediamet nanowire.er of the Asnanowire. the diameter As the decreases, diameter decreases, the emission the wavelengthemission wavelength moves closer moves to closer a red to color. a red This color. is This due is to due the to lateral the lateral diffusion diffusion of In of atoms In atoms during during the growththe growth of the of nanowires.the nanowires. Small Small diameter diameter nanowires nanowi canres can limit limit this this diff usion,diffusion, thereby thereby increasing increasing the contributionthe contribution of In of componentIn component incorporation. incorporation. Therefore, Therefore, by by changing changing the the diameter diameter of of thethe nanowire,nanowire, the emission wavelength ofof the nanowire LEDLED can be modulated, as shown in Figure 20b.20b. In In addition, addition, by increasingincreasing thethe e ffeffectiveective area area (changing (changing the the diameter diameter and and length length of the of nanowirethe nanowire structure), structure), the light the extractionlight extraction efficiency efficiency of the deviceof the candevice be significantly can be significantly improved. improved. This method This also method offers aalso controllable offers a emissioncontrollable cone emission angle, which cone angle, has great which potential has great in micro-LEDpotential in full-color micro-LED displays. full-color displays.

Figure 20.20. ((aa)) AA nanowire nanowire structure structure with with diff differenterent diameters diameters and and (b) its (b normalized) its normalized EL spectrum EL spectrum under diunderfferent different diameters. diameters. Reproduced Reproduced from [120 from], with [120] permission, with permission from Nano from Letters, Nano Letters, 2016. 2016.

Nanowire LEDs have thethe growth-dependentgrowth-dependent characteristicscharacteristics depending on achievingachieving tunabletunable emission wavelength.wavelength. ByBy selectivelyselectively growing growing nanowire nanowire structures structures with with di ffdifferenterent growth growth parameters parameters on theon the same same substrate substrate to form to form a nanowire a nanowire LED LED array, arra a full-colory, a full-color display display can be can realized. be realized. Compared Compared with the conventional InGaN/GaN QW structure, nanowires offer effective stress relaxation to obtain efficient quantum efficiency, due to their small contact areas and active regions far from the interface, which can reduce the polarization electric field and dislocation density inside the active region. In addition, Nanomaterials 2020, 10, x FOR PEER REVIEW 24 of 33

Nanomaterialswith the conventional2020, 10, 2482 InGaN/GaN QW structure, nanowires offer effective stress relaxation to 24obtain of 33 efficient quantum efficiency, due to their small contact areas and active regions far from the interface, which can reduce the polarization electric field and dislocation density inside the active region. In becauseaddition, of because the extremely of the extremely small size small and resistance size and re ofsistance nanowires, of nanowires, they offer th aey good offer current a good injection current levelinjection and level heat dissipation.and heat dissipation. 4.4. Monolithic Integrated Nano-Ring Micro-LED Array 4.4. Monolithic Integrated Nano-Ring Micro-LED Array In 2018, Sung et al. implemented the conversion of the emission wavelength from green to blue by In 2018, Sung et al. implemented the conversion of the emission wavelength from green to blue preparing a nano-ring (NR) structure on a green LED. After that, the authors added red QDs to the blue by preparing a nano-ring (NR) structure on a green LED. After that, the authors added red QDs to nanoring to prepare a monolithic RGB micro-LED array [121]. The entire monolithic RGB micro-LED the blue nanoring to prepare a monolithic RGB micro-LED array [121]. The entire monolithic RGB device consists of three parts: a normally grown green LED, a blue LED with a nano-ring structure, micro-LED device consists of three parts: a normally grown green LED, a blue LED with a nano-ring and a red LED coated with red QDs on the blue LED, as shown in Figure 21a–c. Nano-ring LEDs can structure, and a red LED coated with red QDs on the blue LED, as shown in Figure 21a–c. Nano-ring achieve a tunable emission wavelength by changing the width of the nano-ring wall. Due to the lattice LEDs can achieve a tunable emission wavelength by changing the width of the nano-ring wall. Due mismatch between InGaN and GaN, there is a large strain inside the active region, which generates an to the lattice mismatch between InGaN and GaN, there is a large strain inside the active region, which excessive piezoelectric polarization electric field. This will reduce the overlap of the wave functions of generates an excessive piezoelectric polarization electric field. This will reduce the overlap of the holes and electrons, so that the PL emission peak of the green nanoring will be significantly blue-shifted. wave functions of holes and electrons, so that the PL emission peak of the green nanoring will be Through the experimental verification in Ref. [122], it is found that the strain in the active region is significantly blue-shifted. Through the experimental verification in Ref. [122], it is found that the related to the wall thickness of the nanoring. Therefore, by reducing the width of the nanoring wall, strain in the active region is related to the wall thickness of the nanoring. Therefore, by reducing the the blue shift of the emission wavelength of the green nanoring can be achieved. The outer and inner width of the nanoring wall, the blue shift of the emission wavelength of the green nanoring can be diameters of the NR prepared by the authors were 900 nm and 700 nm, respectively. The normalized achieved. The outer and inner diameters of the NR prepared by the authors were 900 nm and 700 nm, EL spectrum of the three-color RGB NR-QD-micro-LED is shown in Figure 21d. The peak wavelengths respectively. The normalized EL spectrum of the three-color RGB NR-QD-micro-LED is shown in of RGB are 630 nm, 525 nm, and 467 nm, and the peak EQE of the green and blue sub-pixels are 16% Figure 21d. The peak wavelengths of RGB are 630 nm, 525 nm, and 467 nm, and the peak EQE of the and 15%, respectively. green and blue sub-pixels are 16% and 15%, respectively.

Figure 21. (a) Structure of the nano-ring (NR)-QD-micro-LED; (b,c) are the SEM images of the RGB Figure 21. (a) Structure of the nano-ring (NR)-QD-micro-LED; (b,c) are the SEM images of the RGB pixel array and the nanostructure, respectively; (d) is the RGB NR-QD-micro-LED normalized EL pixel array and the nanostructure, respectively; (d) is the RGB NR-QD-micro-LED normalized EL spectrum. Reproduced from [121], with permission from Photonics Research, 2017. spectrum. Reproduced from [121], with permission from Photonics Research, 2017. To further improve the performance of the micro-LED, the authors used atomic layer deposition To further improve the performance of the micro-LED, the authors used atomic layer deposition (ALD) to deposit a 1 nm thick Al2O3 layer on the sidewall of the nanoring structure. Doing so effectively (ALD) to deposit a 1 nm thick Al2O3 layer on the sidewall of the nanoring structure. Doing so reduced the total internal reflection caused by the inner wall of the nano-ring and the non-radiative effectively reduced the total internal reflection caused by the inner wall of the nano-ring and the non- recombination caused by the surface defects of the side wall, which increased the photoluminescence radiative recombination caused by the surface defects of the side wall, which increased the intensity of the nano-ring LED by 143.7%. This process also enabled the QDs to be adjacently coupled photoluminescence intensity of the nano-ring LED by 143.7%. This process also enabled the QDs to with QWMs through non-radiative resonance energy transfer (NRET), thereby improving the color be adjacently coupled with QWMs through non-radiative resonance energy transfer (NRET), thereby conversion efficiency of the QDs. Finally, the authors added a distributed Bragg reflector to the red improving the color conversion efficiency of the QDs. Finally, the authors added a distributed Bragg sub-pixel to increase the reuse of blue photons. reflector to the red sub-pixel to increase the reuse of blue photons. Thus, the red light produced by this method is generated by QD, which will certainly affect the Thus, the red light produced by this method is generated by QD, which will certainly affect the luminous efficiency of the device. However, this wavelength-tunable nanostructure provides a method luminous efficiency of the device. However, this wavelength-tunable nanostructure provides a for growing blue and green LEDs on a single substrate, which reduces the cost and complexity of preparing micro-LED full-color displays.

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4.5. Summary In summary, due to the differences in the growth substrate and composition of LEDs with different emission wavelengths, there is an extremely low miscibility between them. The consequences of this low miscibility are mainly manifested as low quantum efficiency, excessively broad emission spectrum, blue shift of the central emission wavelength, high lateral resistance, and low stability. In addition, the existing growth process is another factor limiting the emergence of single-substrate multi-color LEDs. Since LEDs of different compositions need to be grown at different temperatures or gases, it is also a difficult point to grow one type of LED without destroying other types of LEDs. Furthermore, stable and precise fine-pitch masks are also necessary for growing multi-color micro-LEDs on the substrate. A lot of research has been devoted to the preparation of multi-color LEDs that can grow monolithically. These growth technologies can greatly reduce the cost and difficulty of micro-LEDs and have great significance to realize the full-color display of micro-LEDs in the future. Growing a buffer layer is a feasible solution. By depositing a growth buffer layer between the growth substrate and the active layer, the lattice mismatch between them can be relieved, and the crystallinity of the heterojunction can be improved, allowing LED structures with different In contents to be grown on the same substrate. However, the IQE of the green and red sub-pixels will be much lower than that of the blue sub-pixel, which will affect the uniformity of the full-color display. Therefore, improving the luminous efficiency of long-wavelength LEDs is an important step for the realization of single-substrate multi-color micro-LEDs in the future. Many strategies have been proposed to effectively improve the performance of long-wavelength LEDs, such as using non-polar or semi-polar growth substrates, to relieve the spontaneous polarization electric field inside the LED [123], using different MQW structures to reduce the piezoelectric polarization caused by the strain [124,125], using a quaternary AlGaInN alloy as a barrier, and mutually compensating for the piezoelectric polarization electric field and the spontaneous polarization electric field by adjusting the composition of Al and In [126]. Multi-active area QW LEDs can recombine carriers in different active areas by changing the injection current, thereby emitting lights of different wavelengths. However, excessive In doping will increase the strain in the InGaN / GaN QWs, which will generate a strong internal electric field that forces the separation of the electron-hole wave function and reduces the quantum efficiency of the device. Therefore, the light-emitting wavelength range of the multi-active QW LED cannot cover the entire visible light band. In addition, since the multi-active area LED modulates the light-emitting wavelength via the injecting current, designing a drive circuit that can independently control each pixel is key to applying this technology in displays. The emergence of nanowire-structured LEDs provides another direction for the monolithic growth of multicolor LEDs. However, the structural preparation conditions of LEDs are harsh, and, due to the lack of effective lateral restrictions, the IQEs of such devices may be severely limited by the non-radiative recombination of carriers caused by defects on the surfaces of the nanowires. In addition, as the In content in the nanowire structure increases, the emission wavelength will become difficult to control. Therefore, if nanowire LED displays are to move out of the laboratory in the future, their growth conditions must be controllable, and their growth distribution must be uniform. Multicolor micro-LED monolithic growth technology is still in its exploratory period, but there is no doubt that this technology has great potential. For displays, it is difficult for multi-active QW LEDs to achieve independent control of each sub-pixel, making it impossible to achieve local dimming. Therefore, this technology is more suitable for visible light communications with different wavelengths. Nano-structured LEDs are complex to prepare, and mass production will increase costs. Therefore, although growing multi-color micro-LEDs on the same substrate can greatly reduce the frequency of mass transfers, thereby reducing the cost of transfer and the rate of defective chips. Nano-structured micro-LED displays still need more preparation time before they can appear on the market. But once the multi-color micro-LED arrays are epitaxial, the RGB pixel pitch is fixed, so the multi-color micro-LED arrays cannot be applied to drive substrates of different sizes. And once a pixel Nanomaterials 2020, 10, 2482 26 of 33 is damaged, a single chip cannot be replaced, which also makes subsequent maintenance difficult. However, monolithic integrated multi-color micro-LEDs have appeared on the market. In October 2019, the micro-LED company Plessey successfully grew blue and green native micro-LEDs on the same wafer, and, in December of the same year, Plessey realized the growth of efficient red micro-LEDs on InGaN, which showed the possibility to grow RGB three-color micro-LEDs on the same wafer. Multi-color LEDs grown on a buffer layer may be the focus of monolithic multi-color LEDs in the future.

5. Conclusions Ultimately, this article reviewed the various technologies and solutions proposed over the past ten years for micro-LEDs to achieve full color and summarized the characteristics and defects of these various technologies. Integrating micro-LEDs of different colors and materials onto the same driving substrate through transfer and bonding technology is the main method for preparing micro-LED full-color displays for the market. This article outlined the use of full-color micro-LED displays prepared by this method and introduced the mass transfer technology used in micro-LED chips. However, with the improvement of the mass transfer yield index and the demand for high-resolution high-end displays, it is difficult for current mass transfer technology to achieve high-efficiency and high-precision transmission, which has aggravated the production costs and maintenance costs of micro-LED displays and stagnated their marketization. Therefore, before the transfer yields and costs of mass transfer technologies are improved, full-color micro-LED displays with color conversion layers will dominate the entire market. Color conversion technology has developed rapidly in recent years. With this technology, LEDs of various colors do not need to be grown on different wafers, as well as to carry out transfer bonding. One only needs to prepare a color conversion medium in a color conversion layer and use blue or UV LED for pump excitation to achieve multi-color light emission. QDs are the most popular color conversion medium, and their preparation process is gradually mature. For example, Sharp used QDs to prepare a 1053 pixels per inch (PPI) full-color micro-LED display prototype. However, the conversion efficiency of this color conversion material is not perfect, and it is easily degraded by various external factors during light excitation. These defects are the key research directions for using color conversion technology to realize micro-LED displays in the future. Growing RGB three-color LEDs on the same wafer or substrate is currently somewhat impractical, but this is the ultimate goal for micro-LED full-color displays. At the end of this article, the potential LED technologies for growing different colors directly on a single chip were introduced. These technologies have unlimited potential and can overcome the limitations of transfer technology and color conversion technology. Among them, nanowire LEDs offer good strain relaxation due to their special structures, giving them great prospects in multi-wavelength emissions. However, their harsh growth conditions and processes make them suitable only for the laboratory; there is still work needed before we can grow multi-color LEDs directly on the same wafer. In the past ten years, micro-LED technology has developed rapidly, but full-color displays using micro-LEDs are still in their infancy. However, their development is optimistic. Display technology plays an extremely important role in today’s society. Various emerging products have accompanied an endless demand for high-resolution, high-contrast, and high-efficiency micro-LED displays. Since Apple acquired Luxvue in 2014 and began to develop micro-LEDs, major electronics giants have successively entered the market. This is a good sign because the introduction of the market provides many possibilities for the development of micro-LEDs. More and more universities and enterprises have begun to participate in the research and preparation of micro-LEDs. We consider that the purpose of mass transfer technology research is to improve the efficiency and accuracy of transfer, which is the only way to reduce the cost of micro-LED displays. And developing the full-color display of micro-LED is the key to applying micro-LED to the display field. We believe that the two mountains of micro-LED display in the next decade will be crossed, and micro-LED will lead the field by virtue of its outstanding performance in energy consumption, color, resolution, and life. From small displays, Nanomaterials 2020, 10, 2482 27 of 33 such as smart watches, head-mounted displays, and smart glasses, to micro-LED TVs, smart phones, and projectors, micro-LED technology has quietly arrived as the next generation of display technology.

Author Contributions: Conceptualization, writing, review and supervision J.M., investigation, writing and editing Y.W., writing, review and supervision P.S., writing, review L.Z., writing, review B.X. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Basic Research Program of Shenzhen, grant number (JCYJ20170412171744267) and the Shenzhen general research fund (JCYJ20190813172405231). Conflicts of Interest: The authors declare no conflict of interest.

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