(-201) Β-Ga2o3 Substrate for Vertical Light Emitting Diodes

hv photonics

Article The Optical Properties of InGaN/GaN Nanorods Fabricated on (-201) β-Ga2O3 Substrate for Vertical Light Emitting Diodes

Jie Zhao 1,2, Weijiang Li 1,2, Lulu Wang 1,2, Xuecheng Wei 1,2, Junxi Wang 1,2 and Tongbo Wei 1,2,*

1 State Key Laboratory of Solid-State Lighting, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China; [email protected] (J.Z.); [email protected] (W.L.); [email protected] (L.W.); [email protected] (X.W.); [email protected] (J.W.) 2 Center of Materials Science and Optoelectronics Engineering, University of the Chinese Academy of Sciences, Beijing 100049, China * Correspondence: [email protected]

Abstract: We fabricated InGaN/GaN nanorod light emitting diode (LED) on (-201) β-Ga2O3 substrate via the SiO2 nanosphere lithography and dry-etching techniques. The InGaN/GaN nanorod LED grown on β-Ga2O3 can effectively suppress quantum conﬁned Stark effect (QCSE) compared to planar LED on account of the strain relaxation. With the enhancement of excitation power density, the photoluminescence (PL) peak shows a large blue-shift for the planar LED, while for the nanorod LED, the peak position shift is small. Furthermore, the simulations also show that the light extraction efﬁciency (LEE) of the nanorod LED is approximately seven times as high as that of the planar

LED. Obviously, the InGaN/GaN/β-Ga2O3 nanorod LED is conducive to improving the optical performance relative to planar LED, and the present work may lay the groundwork for future

development of the GaN-based vertical light emitting diodes (VLEDs) on β-Ga2O3 substrate. Keywords: (-201) β-Ga2O3 substrate; nanorod; quantum conﬁned Stark effect; light extraction

Citation: Zhao, J.; Li, W.; Wang, L.; efﬁciency; vertical light emitting diodes Wei, X.; Wang, J.; Wei, T. The Optical Properties of InGaN/GaN Nanorods

Fabricated on (-201) β-Ga2O3 Substrate for Vertical Light Emitting 1. Introduction Diodes. Photonics 2021, 8, 42. The light emitting diodes (LEDs) have the characteristics of small size, high efficiency, https://doi.org/10.3390/photonics8020042 energy conservation, long lifetime, and so on, which have great attracted attention and interest from both scientific and industrial communities in recent years [1,2]. Generally, GaN Academic Editor: Jing Zhang is an ideal material, and nitride-based devices are heteroepitaxially grown on sapphire, SiC Received: 29 December 2020 or Si substrates. Although heteroepitaxial growth of GaN on sapphire is well established, Accepted: 2 February 2021 sapphire does not represent an adequate candidate because it has insulation characteris- Published: 6 February 2021 tic, and has a 14% lattice mismatch which causes high dislocation density [3]. Another routinely used substrate, SiC, has a much smaller lattice mismatch (3.1%) compared with Publisher’s Note: MDPI stays neutral sapphire [4]. Nevertheless, SiC is expensive and has high optical losses owing to the lack with regard to jurisdictional claims in of transparency [5]. Therefore, it is important to find an alternative substrate which can be published maps and institutional affil- iations. used to fabricate light emitting diode (LED) devices with good lattice match, low dislocation density, appropriate optical and electrical properties, as well as high transparency in visible spectrum regions. As the group of transparent conductive oxides, the β-gallium oxide (β-Ga2O3) has an ultrawide bandgap (Eg ≈ 4.6–4.9 eV) [6–9] and thus exhibits a unique transparency from the Copyright: © 2021 by the authors. visible into the ultraviolet (UV) region. In other words, the transparent nature of β-Ga2O3 Licensee MDPI, Basel, Switzerland. as a substrate provides a wider light-emitting area than conductive SiC or Si substrates. This article is an open access article In addition, monoclinic β-Ga O also has the characteristics of high breakdown electric distributed under the terms and 2 3 conditions of the Creative Commons field (~8 MV/cm) [10], excellent thermal and chemical stability and so on, attracting a lot Attribution (CC BY) license (https:// of attention due to its wide range of potential applications in short wavelength photonics, creativecommons.org/licenses/by/ transparent electronics, power devices and optoelectronic devices. Meanwhile, the lattice 4.0/). mismatch between β-Ga2O3 and GaN is found to be small, is ~4.7% [5,11]. The electrical

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conductivity of β-Ga2O3 can be achieved by doping. In addition, its conductivity can be adjusted on the basis of the demands [12]. Thus, the β-Ga2O3 can serve as a good substrate candidate for the growth of GaN because the β-Ga2O3 substrate combines the beneficial properties (transparency, good lattice match, and conductivity) of sapphire, SiC or Si substrates. Recently, we also carried out some works of GaN layer grown on β-Ga2O3 substrate [13–15]. However, there are still many problems existing in the LEDs based on β-Ga2O3 substrate. One is the lattice mismatch between GaN and β-Ga2O3, still resulting in strong quantum confined Stark effect (QCSE) in multiple quantum wells (MQWs) of LEDs. Another is the low light extraction efficiency (LEE), owing to high refractive index between GaN (n ≈ 2.5) [16] and air (n = 1), as well as high refractive index between β-Ga2O3 (n = 1.68~1.89)[17] and air (n = 1). Thus, most of the light emitted from MQWs is trapped in the devices, leading to the huge light loss [18]. So the LEE of the planar LED is usually low. Generally speaking, nanorod or nanowire LEDs are deemed to a viable solution to unravel long pending issues such as QCSE, inefficient light extraction and so on. Particularly on the issue of QCSE, the changes in the internal field and strain can affect its magnitude, resulting in change of the bandgap. The nanorod or nanowire LEDs can release the internal strain, as well as suppress the lateral transport of carriers in MQWs [19]. Besides, the LEE of the nanorod or nanowire LEDs may also be effectively improved since they avoid total internal reflection and prevent the lateral propagation of light-waveguide effects [20]. Therefore, based on these features, we fabricated the InGaN/GaN nanorod LED on β-Ga2O3 substrate for vertical LED (VLED) by the SiO2 nanosphere lithography and dry- etching techniques. Depending on the Raman measurements, the compressive strain in the planar and nanorod LED can be estimated to be 0.26 GPa and 0.07 GPa, respectively, illustrating the nanorod LED is more close to strain-free state relative to planar LED. Due to the reduction in the QCSE, the scattering and high LEE, the photoluminescence (PL) peak intensity value of the nanorod LED is obviously stronger than that of the planar LED. As the excitation power density of PL increases, emission peak position of the planar LED obviously blue-shifts, while there is slightly shift for the peak position of the nanorod LED. Furthermore, the LEE of the nanorod LED is also effectively improved compared with the planar LED on β-Ga2O3 substrate.

2. Experimental Procedures

Firstly, in order to avoid the β-Ga2O3 substrate being etched by H2, the low-temperature ◦ undoped GaN (LT-GaN) buffer layer was grown at 525 C under an N2 atmosphere on (-201)-oriented monoclinic conductive β-Ga2O3 substrate using a 500 Torr pressure by metal organic chemical vapor deposition (MOCVD), and the thickness was approximately 12 nm. After changing the carrier gas from N2 to H2, a pulse growth procedure was employed, which mainly reduces threading dislocation and improves the crystalline quality of GaN [15]. When the temperature of the substrate was increased to 1020 ◦C, the pulse layer was grown. For the pulse layer, NH3 was inputted by the pulse flow method under circumstance of 200 Torr, and each cycle was 0.3 min. However, it was worth noticing that the Ga source remained normally open, while the NH3 source was only opened in the last 0.2 min in each cycle, and had 200 cycles in total. The temperature was further increased to 1080 ◦C during the deposition of the remainder of the n-type doped GaN (n-GaN) layer. The total thickness of n-GaN layer was approximately 3.6 µm. Figure1a shows the schematic diagram of the LT-GaN buffer layer and n-GaN layer grown on the (-201) β-Ga2O3 substrate. Figure1b shows the atomic force microscopy (AFM) image 2 result for 2 × 2 µm of the epitaxial GaN on β-Ga2O3 substrate. The smooth step-like morphology can be seen, with the root-mean-square (RMS) roughness of approximately 0.187 nm. The Figure1c,d show the X-Ray diffraction (XRD) rocking curves of the epitaxial GaN on β-Ga2O3 substrate. The full width at half maximum (FWHM) of GaN (002) and (102) reflection peaks are approximately 599.4 and 546.6 arcsec, respectively. This means that GaN can be successfully grown on β-Ga2O3 substrate. Then the five periods of Photonics 2021, 8, x FOR PEER REVIEW 3 of 11

The total thickness of n-GaN layer was approximately 3.6 μm. Figure 1a shows the schematic diagram of the LT-GaN buffer layer and n-GaN layer grown on the (−201) β-Ga2O3 substrate. Figure 1b shows the atomic force microscopy (AFM) image result for 2 × 2 μm2 of the epitaxial GaN on β-Ga2O3 substrate. The smooth step-like morphology can be seen, with the root-mean-square (RMS) roughness of approximately 0.187 nm. The Figure 1c,d Photonics 2021, 8, 42 show the X-Ray diffraction (XRD) rocking curves of the epitaxial GaN on β-Ga2O33 sub- of 10 strate. The full width at half maximum (FWHM) of GaN (002) and (102) reflection peaks are approximately 599.4 and 546.6 arcsec, respectively. This means that GaN can be successfully grown on β-Ga2O3 substrate. Then the five periods of InGaN/GaN MQWs and InGaN/GaNthe approximately MQWs 100 and nm the thick approximately p-type doped 100 GaN nm thick (p-GaN) p-type layer doped were GaN epitaxially (p-GaN) grown layer wereon the epitaxially n-GaN layer. grown on the n-GaN layer.

FigureFigure 1.1. ((aa)) TheThe schematicschematic diagramdiagram ofof thethe epitaxialepitaxial GaNGaN on onβ β-Ga-Ga22OO33 substrate.substrate. ( b) The AFM imageimage of thethe epitaxialepitaxial GaNGaN onon ββ-Ga-Ga22O3..substrate.substrate. The The normalized normalized XRD rocking curves ofof thethe epitaxialepitaxial GaNGaN ononβ β-Ga-Ga22OO33 substrate:substrate: (c) (002) reﬂectionreflection peak; (d) (102) reﬂection peak. The schematic illustrations of fabricating GaN-based nanorod LED on the β-Ga2O3 substrate: (e) The planar LED with epitaxial grown on β-Ga O substrate. (f) Transferring self-assembly SiO nanospheres on the 2 3 2 planar LED. (g) Shrinking the diameter of SiO2 nanospheres via CHF3-based ICP etching. (h) Etching GaN via Cl-based ICP etching. (i) Removing SiO2 mask by BOE solution. (j) Removing etching damage by KOH solution.

First of all, the sample was cleaned by the mixed solution of sulfuric acid and hydrogen peroxide (H2SO4:H2O2 = 3:1). Then the nanospheres mixed solution was made via mixing the original solution of the SiO2 nanospheres, deionised water and ethyl alcohol (1:1:1). The nanospheres mixed solution was put into the ultrasonic apparatus and oscillated for Photonics 2021, 8, 42 4 of 10

20 min to further mix. Next, adding a large amount of water and 50 µL hydrochloric acid to the quartz container. The glass slide was placed in the edge of the container at a certain inclined angel, and nanospheres mixed solution was slowly dripped on the glass slide via the pipetting gun, until a monolayer SiO2 film was formed on the surface of the water in the container. Following that, a drop of the sodium dodecyl benzene sulfonate (SDBS) solution was added into the water, resulting in more ordered and compact arrangement ◦ of the SiO2 nanospheres. The sample was placed into the water at about 45 angle and slowly moved to the bottom of the monolayer SiO2 film along the horizontal direction. Then the sample was slowly pulled up from bottom to top with hand or puller. In this way, a highly ordered self-assembled monolayer of SiO2 nanospheres was dip-coated on the sample. Finally, the sample was placed in a place without air flow. Figure1e–j show the fabrication flow diagrams of the nanorod LED using the SiO 2 nanosphere lithography and dry-etching techniques. In the first place, in accordance with the above method, a highly ordered self-assembled monolayer of SiO2 nanospheres with the diameter of 660 nm was dip-coated on the planar LED with epitaxial grown on β-Ga2O3 substrate. Subsequently, the diameter of SiO2 nanospheres was shrunk to approximately 580 nm on the sample via CHF3-based inductively coupled plasma (ICP) dry-etching. Because the GaN was etched using Cl-based gas and the etching ratio (Si/GaN) was small, these SiO2 nanospheres could be used as mask and protect the top p-GaN layer. Then the sample was etched down to the n-GaN layer via Cl-based ICP etching, with the etching depth of approximately 1.4 µm. Next, the SiO2 mask was removed via the buffer oxide etchant (BOE) solution and the nanorod LED was obtained. Finally, the nanorod LED was treated in the potassium hydroxide (KOH) solution.

3. Results and Discussion The Figure2a–d show the scanning electron microscopy (SEM) images of nanorod LED on (-201) β-Ga2O3 substrate before and after removing SiO2 nanospheres. It can be seen that the nanorod arrays are well arranged in an orderly manner. The top diameter is approximately 580 nm, and there is enough space between the nanorods. For the sake of removing ICP-induced etching damages on the sidewall surface of the nanorods, the sample is dipped into KOH solution with the concentration of 5% for 2 min. In order to verify whether the strain of nanorod LED on β-Ga2O3 substrate is released or not, the Raman measurements of the GaN/β-Ga2O3 planar and nanorod LED are revealed, respectively, with a range of 450–750 cm−1 [21,22]. The results are shown in Figure3a,b. The observed peaks are mainly A 1(TO), E1(TO), E2(high) and A1(LO) mode peaks of GaN. In the general case, the E2(high) phonon mode of GaN is sensitive to strain within the material [23]. Therefore, strain state in GaN epitaxial layer can be characterized by the peak position of the E2(high) phonon mode. Here, we clearly observe that the peak position of the E2(high) phonon mode within the GaN/β-Ga2O3 planar and nanorod LED. The peak positions of the E2(high) phonon mode within the GaN/β-Ga2O3 planar and nanorod LED are located at approximately 569.1 cm−1 and 568.3 cm−1, respectively. We can conclude that there clearly exist compressive strain because the peak positions of the planar and nanorod LED exhibit a red-shift of 1.1 cm−1 and 0.3 cm−1, respectively, relative to the bulk GaN (568 cm−1)[24]. The strain σ in the GaN ﬁlm can be calculated via the following formula: σ = ∆ω/k. In this equation, ∆ω is the Raman shift from the E2(high) phonon mode of the strain-free GaN ﬁlm, and k is the Raman strain factor. Here, k = 4.2 cm−1/GPa is adopted [25]. The strain σ in the GaN epitaxial layer of the planar and nanorod LED can be estimated to be 0.26 GPa and 0.07 GPa, respectively. It also means that the planar LED is in a state of highly compressive strain. Therefore, the nanorod LED is more close to strain-free state, indicating the alleviated QCSE. Photonics 2021, 8, x FOR PEER REVIEW 5 of 11 Photonics 2021, 8, 42 5 of 10

Figure 2. The SEM images of nanorod LED on β-Ga O substrate: the surface morphology of the (a) top view and (b) 25◦ Figure 2. The SEM images of nanorod LED on β-Ga22O3 substrate: the surface morphology of the (a) top view and (b) 25° Photonics 2021, 8, x FOR PEER REVIEW ◦6 of 11 tilted angle for thethe viewview beforebefore removingremoving thethe SiOSiO22 nanospheres. The The surface morphology of of the ( c) top view and (d) 2525°

tilted angleangle for the view after removing thethe SiOSiO22 nanospheres.nanospheres.

In order to verify whether the strain of nanorod LED on β-Ga2O3 substrate is released or not, the Raman measurements of the GaN/β-Ga2O3 planar and nanorod LED are revealed, respectively, with a range of 450–750 cm−1 [21,22]. The results are shown in Figure 3a,b. The observed peaks are mainly A1(TO), E1(TO), E2(high) and A1(LO) mode peaks of GaN. In the general case, the E2(high) phonon mode of GaN is sensitive to strain within the material [23]. Therefore, strain state in GaN epitaxial layer can be characterized by the peak position of the E2(high) phonon mode. Here, we clearly observe that the peak position of the E2(high) phonon mode within the GaN/β-Ga2O3 planar and nanorod LED. The peak positions of the E2(high) phonon mode within the GaN/β-Ga2O3 planar and nanorod LED are located at approximately 569.1 cm−1 and 568.3 cm−1, respectively. We can conclude that there clearly exists compressive strain because the peak positions of the planar and nanorod LED exhibit a red-shift of 1.1 cm−1 and 0.3 cm−1, respectively, relative to the bulk GaN (568 cm−1) [24]. The strain σ in the GaN film can be calculated via the following formula: σ = Δω/k. In this equation, Δω is the Raman shift from the E2(high) phonon mode of the strain-free GaN film, and k is the Raman strain factor. Here, k = 4.2 cm−1/GPa is adopted [25]. The strain σ in the GaN epitaxial layer of the planar and nanorod LED can be estimated to be 0.26 GPa and 0.07 GPa, respectively. It also means that the planar LED is in a state of highly compressive strain. Therefore, the nanorod LED is more close to Figure 3. β a b Figure 3.The The normalized normalizedstrain-free Raman Raman state, spectrum indicatingspectrum of of the the GaN/alleviated GaN/β-Ga-Ga2 2QCSE.O3 ((a)) planar planar and and ( (b)) nanorod nanorod LED. LED.

Figure4a shows the PL spectrum of the GaN/ β-Ga2O3 planar and nanorod LED under Figure 4a shows the PL spectrum of the GaN/β-Ga2O3 planar and nanorod LED under the room temperature, and can be expressed by logarithmic coordinates. It can be seen that, the room temperature, and can be expressed by logarithmic coordinates. It can be seen under the same excitation condition, the peak positions of the planar and nanorod LED that, under the same excitation condition, the peak positions of the planar and nanorod are located at 424.2 and 422.8 nm, respectively. Obviously, the peak wavelength position LED are located at 424.2 and 422.8 nm, respectively. Obviously, the peak wavelength position of nanorod LED shows blue-shift of about 1.4 nm relative to planar LED. It proves that the strain is released at a certain extent, indicating that the QCSE can be relieved. The reduced QCSE enables the relieved band bending, the increased wave function overlap of electrons and holes, and therefore increases internal quantum efficiency (IQE) [26]. It can be noticed that PL peak intensity value for the nanorod LED is higher than that of the planar LED. When the nanorod LED is excited by PL, the light from the excitation source can be scattered between adjoining nanorods, leading to the increased the probability of extracted photons. In addition, the nanorod LED shows high LEE, which enables more light generated from the active region and results in the strong PL peak intensity value. Therefore, the enhancement of the PL peak intensity value for the nanorod LED is considered as a consequence the combined effects of the reduction in the QCSE, the scattering and high LEE.

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of nanorod LED shows blue-shift of about 1.4 nm relative to planar LED. It proves that the strain is released at a certain extent, indicating that the QCSE can be relieved. The reduced QCSE enables the relieved band bending, the increased wave function overlap of electrons and holes, and therefore increases internal quantum efﬁciency (IQE) [26]. It can be noticed that PL peak intensity value for the nanorod LED is higher than that of the planar LED. When the nanorod LED is excited by PL, the light from the excitation source can be scattered between adjoining nanorods, leading to the increased the probability of extracted photons. In addition, the nanorod LED shows high LEE, which enables more light generated from the active region and results in the strong PL peak intensity value. Therefore, the enhancement of the PL peak intensity value for the nanorod LED Photonics 2021, 8, x FOR PEERis considered REVIEW as a consequence the combined effects of the reduction7 of in 11 the QCSE, the

scattering and high LEE.

Figure 4. (a) TheFigure PL spectrum4. (a) The PL ofspectrum the GaN/ of the βGaN/-Gaβ‐2GaO32Oplanar3 planar and nanorod nanorod LED LED in logarithmic in logarithmic coordinates. coordinates. The PL spectrum The PL spectrum of the GaN/β‐Ga2O3 (b) nanorod and (c) planar LED under the different excitation power densities which increases from of the GaN/β-Ga2O3 (b) nanorod and (c) planar LED under the different excitation power densities which increases from 1% to 100%. (d) The PL peak wavelength position and (e) the PL peak FWHM of the nanorod and planar LED as a function 1% to 100%. (d)of The the excitation PL peak power wavelength density. position and (e) the PL peak FWHM of the nanorod and planar LED as a function of the excitation power density.

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Figure4b,c show the PL spectrum of the GaN/ β-Ga2O3 nanorod and planar LED under the different excitation power densities which increases from 1% to 100% (The maximum excitation power density value of the light source is approximately 9.55 kW/cm2). The number of photogenic electron-hole pairs increases with the enhancement of excitation power density, resulting in the increased PL peak intensity values of the nanorod and planar LED. The curves of the extracting values of the peak wavelength position and the FWHM under different excitation power densities are shown in Figure4d,e. For the planar LED, the position of the PL peak wavelength obviously shifts to shorter wavelength (from 437.6 to 425.1 nm) with the enhancement of excitation power density, but there is a slightly red-shift (425.9 nm) at 100% excitation power density. For the nanorod LED, the peak wavelength position shows a slight red-shift as increasing the excitation power density from 0 to 1 kW/cm2, which may be caused by the self-heating with the enhancement of excitation power density. The shift of peak wavelength position may be negligible with the increase in excitation power density from 1 to 9.55 kW/cm2. Besides, under the same excitation power density, the peak wavelength position of the nanorod LED is shorter than that of the planar LED, indicating the strain relaxation within the nanorod LED. It can also be seen that as the excitation power density increases, the FWHM of the planar LED and nanorod LED become narrowed, as shown in Figure4e. The FWHM of the planar LED drops markedly from 80.93 to 33.69 nm, while the FWHM of the nanorod LED drops slightly from 27.97 to 21.17 nm. This is mainly because the screening effects of the piezoelectric field is continuously enhanced by increased carriers with the enhancement of excitation power density [27]. According to finite-difference time domain (FDTD) simulation, the nanorod LED can also significantly improve the LEE. Figures5a–c and5d–f show the FDTD simulation results of the planar and nanorod LED on (-201) β-Ga2O3 substrate, respectively. The simulation structure including 2 µm Ga2O3 substrate, 600 nm n-GaN, 100 nm MQWs, 100 nm p-GaN, the nanorods with the diameter of the 580 nm, respectively, as well as the refractive index and the extinction coefficient of the corresponding materials are imported. For the planar LED, most of the light emitted from MQWs is trapped in the epitaxial layer and substrate because photons emitting outside the range of the critical angle are totally reflected at the GaN–air and β-Ga2O3–air interface. On the contrary, for the nanorod LED, emitted light from MQWs is no longer confined to the escape cone. The nanorod LED forms quasi waveguide structure, and the light emitted from the MQWs of one individual nanorod can couple into the vertically directed guided mode [28], or couple into adjoining nanorods to again form vertically directed guided mode, leading to the increased chance of photons escaping from the substrate side. Moreover, the Bragg diffraction induced by the highly ordered nanorod LED also increases the photons extracted from the sidewall in the vertical direction [29]. Still having a bit is, the downsize with the p-GaN leads to the decrease with the absorption of photons, and thereby increases the escaping probability of the photon from the inside of the chip [26]. Therefore, on the basis of the simulation results, the LEE value of the nanorod LED is 61.51%, which is much higher than that of planar LED with the value of 8.89%. Photonics 2021, 8, x FOR PEER REVIEW 9 of 11 Photonics 2021, 8, 42 8 of 10

FigureFigure 5. The 5. FDTDThe FDTD simulation simulation of the of planar the planar LED LED on on(-201) (-201) β-βGa-Ga2O23O substrate:3 substrate: (a ()a T) Thehe simulation simulation structure. structure. (bb)) TheThe far- field distributionfar-field distribution of light ofenergy light energy.. (c) The (c) c Theross cross-sectional-sectional near near-field-field distribution distribution of lightlight energy energy at at X-Z X plane.-Z plane The. The FDTD FDTD simulationsimulation of the of nanorod the nanorod LED LED on on(-201) (-201) β-βGa-Ga2O23O substrate:3 substrate: (d (d)) T Thehe simulation structure. structure. (e )(e The) The far-field far-field distribution distribution of of light energylight energy.. (f) The (f) c Theross cross-sectional-sectional near near-field-field distribution distribution of of light energyenergy at at X-Z X-Z plane. plane. 4. Conclusions 4. Conclusions In summary, we have fabricated GaN/β-Ga2O3 nanorod LED via the SiO2 nanosphere lithographyIn summary, and we dry-etching have fabricated techniques. GaN/ Comparedβ-Ga2O3 tonanorod planar structure,LED via the the SiO strain2 nanosphere can be lithographyrelaxed owing and todry the-etching formation techniques of nanorod. Comp LED.ared The results to planar of the structure, PL measurement the strain show can be relaxedthe increased owing to luminescence the formation intensity of nanorod value for LED. the nanorodThe results LED of compared the PL measurement to that of planar show the LED.increased As the luminescence excitation power intensity density valu increases,e for the peaknanorod wavelength LED compared of the nanorod to that LED of planar slightlyLED. As shifts, the excitation while the PL power peak density wavelength increases, of planar the LED peak greatly wavelength blue-shifts, of the and nanorod the LEDFWHM slightly becomes shifts, narrow while forthe both. PL peak In the wavelength meantime, we of also planar perform LED the greatly FDTD blue simulations-shifts, and to explore the LEE improvement of nanorod LED. The GaN/β-Ga O nanorod LED is the FWHM becomes narrow for both. In the meantime, we also perform2 3 the FDTD simu- expected to pave the way toward reliable and bright vertical-injection GaN-based LEDs. lations to explore the LEE improvement of nanorod LED. The GaN/β-Ga2O3 nanorod LED is expectedAuthor Contributions: to pave the Conceptualization,way toward reliable J.Z., W.L. and and bright T.W.; vertical methodology,-injection J.Z. and GaN W.L.;-based software, LEDs. W.L.; validation, J.Z., W.L. and T.W.; formal analysis, J.Z., W.L. and L.W.; investigation, J.Z. and W.L.; Authorresources, Contributions: X.W., J.W. and Conceptualization, T.W.; writing—original J.Z., draft W.L preparation,. and T.W.; J.Z.; methodology, writing—review J.Z. andand editing, W.L.; software,T.W.; W. supervision,L.; validation, T.W.; J.Z project., W.L. administration, and T.W.; formal X.W., analysis, J.W. and T.W.;J.Z., W funding.L. and acquisition, L.W.; investigation, X.W., J.W. J.Z. andand W.L T.W..; resources, All authors X.W have., J. readW. and and T agreed.W.; writing to the published—original version draft preparation, of the manuscript. J.Z.; writing—review andFunding: editing, TThis.W research.; supervision, was funded W.T by.; project the National administration, Key R&D Program X.W., of J. ChinaW. and (No. T. 2018YFB0406702)W.; funding acquisi-, tion,the X. NationalW., J.W. Naturaland T.W Science. All authors Foundation have of read China and (No. agreed 61974139) to the and published the Beijing version Natural of Science the manuscript.Foundation (No. 4182063). Conflicts of Interest: The authors declare no conflict of interest. Funding: This research was funded by the National Key R&D Program of China (No. 2018YFB0406702), the National Natural Science Foundation of China (No. 61974139) and the Beijing Natural Science Foundation (No. 4182063). Conflicts of Interest: The authors declare no conflict of interest.

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