Gan-Based Vertical-Cavity Surface-Emitting Lasers Incorporating Dielectric Distributed Bragg Reﬂectors

applied sciences

Review GaN-based Vertical-Cavity Surface-Emitting Lasers Incorporating Dielectric Distributed Bragg Reﬂectors

Tatsushi Hamaguchi *, Hiroshi Nakajima and Noriyuki Fuutagawa

Compound Semiconductor Development Department, Application Technology Development Division, Sony Corporation, Atsugi, Kanagawa 243-0014, Japan; [email protected] (H.N.); [email protected] (N.F.) * Correspondence: [email protected]

Received: 22 January 2019; Accepted: 18 February 2019; Published: 20 February 2019

Abstract: This paper reviews past research and the current state-of-the-art concerning gallium nitride-based vertical-cavity surface-emitting lasers (GaN-VCSELs) incorporating distributed Bragg reflectors (DBRs). This paper reviews structures developed during the early stages of research into these devices, covering both major categories of GaN-based VCSELs: hybrid-DBR and all-dielectric-DBR. Although both types exhibited satisfactory performance during continuous-wave (CW) operation in conjunction with current injection as early as 2008, GaN-VCSELs have not yet been mass produced for several reasons. These include the difficulty in controlling the thicknesses of nitride semiconductor layers in hybrid-DBR type devices and issues related to the cavity dimensions in all-dielectric-DBR units. Two novel all-dielectric GaN-based VCSEL concepts based on different structures are examined herein. In one, the device incorporates dielectric DBRs at both ends of the cavity, with one DBR embedded in n-type GaN grown using the epitaxial lateral overgrowth technique. The other concept incorporates a curved mirror fabricated on (000-1) GaN. Both designs are intended to mitigate challenges regarding industrial-scale processing that are related to the difficulty in controlling the cavity length, which have thus far prevented practical applications of all-dielectric GaN-based VCSELs.

Keywords: Gallium Nitride; GaN; Vertical-cavity Surface-emitting Lasers; VCSEL

1. Introduction For more than two decades, gallium nitride (GaN) has attracted interest with regard to applications in a wide range of optical devices, such as light-emitting diodes (LEDs) and laser diodes (LDs) [1,2]. Recently, GaN-based vertical-cavity surface-emitting lasers (VCSELs) have received considerable attention as a result of their superior characteristics, including low threshold currents, array formation capability, applicability to high-frequency operation, low manufacturing costs and high efﬁciencies [3–13]. Due to these advantages, GaN-based VCSELs have the potential to replace conventional LEDs and edge emitting LDs as light sources for optical storage, laser printers, projectors, displays, solid state lighting, optical communications, biosensors and many other applications. This review summarizes recent progress in the development of GaN-based VCSELs. In the former sections, an overview of GaAs-based VCSELs is initially provided, followed by a discussion of hybrid DBR GaN-based VCSELs, in which the DBRs are made of semiconductors and dielectric materials. In the latter sections, all-dielectric-DBR VCSELs reported in the early stage of research into such devices are examined, after which the latest progress in other types of all-dielectric-DBR VCSELs based on epitaxial lateral overgrowth [14,15] and curved mirrors [16,17] is summarized.

Appl. Sci. 2019, 9, 733; doi:10.3390/app9040733 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 733 2 of 12

2. GaAs-based VCSELs Gallium-arsenide (GaAs) materials possess properties that are conducive to VCSEL lasing. These materials can be used to fabricate high-reflectivity mirrors and it is thus possible to obtain highly reflective DBRs using arsenide semiconductors via epitaxial growth techniques [18]. One common approach to fabricating GaAs-based VCSELs is to employ AlGaAs materials having two different compositions [19]. Varying the composition of AlGaAs leads to negligible changes in its lattice constant, so that AlGaAs DBR pairs can be grown in large numbers, which is required to achieve a reflectance close to 100%. In addition, GaAs-based materials provide a suitable means of establishing current paths in VCSELs, since AlGaAs DBRs are conductive in both p- and n-type layers. Therefore, a metallic electrode deposited on the outside of each DBR can provide a path for current injection into active regions located between p- and n-type layers. In addition, a layer of an arsenide such as AlAs placed next to the active layers is readily oxidized, thus allowing lateral confinement of the current [20]. The resulting structures give devices capable of concentrating carriers within the limited physical space defined by the active layers so as to enhance population inversion, which is essential for lasing. The third advantage of GaAs-based VCSELs is that a short cavity length can be readily obtained by epitaxial growth. All layers, including both DBRs, p-type and n-type layers and quantum wells, can be grown directly on a GaAs substrate using an epitaxial growth technique such as metal organic chemical vapor deposition (MOCVD).

3. Hybrid-DBR GaN-based VCSELs These approaches do not necessarily work well when fabricating GaN-based VCSELs. The major researching interest in this case is the formation of nitride semiconductor DBRs for n-side mirrors. Lu et al. reported the first demonstration of continuous wave lasing from a GaN-VCSEL, in which the n-side DBR comprised 29 AlN/GaN bilayers grown directly on a GaN substrate by MOCVD [4,5]. In this device, a half-wavelength AlN stack was inserted between every four pairs of AlN/GaN quarter-wavelength stacks to produce the DBR. These AlN stacks suppressed the formation of cracks potentially resulting from the lattice mismatch between GaN and AlN. In contrast, other groups have reported a DBR made of AlInN and GaN. Doping AlN with In reduces the lattice mismatch with GaN to zero [6,7], so this approach is expected to produce crack-free layers. The incorporation of this DBR structure into GaN-based VCSELs has been intensively investigated, primarily by a group at Meijo University [3,7,21] and a significant volume of data regarding threshold current, efficiency and optical output has been reported. This type of GaN-VCSEL has recently demonstrated a remarkably high output power above 10 mW together with a peak wall plug efficiency of 8.9% by Stanley Electronics [22]. However, the small refractive index difference between the two layers is problematic because a large number of layers is required to obtain high reflectivity. It has also been reported that the AlInN growth rate is so low that 12 h are required to grow a DBR with 40 AlInN/GaN DBRs bilayers [7]. The narrow stopband width of epitaxially grown n-side DBRs is another concern with regard to eventual mass production. The lasing wavelength of a VCSEL should ideally be located on the plateaus at which both DBRs exhibit high reflectance, meaning that the lasing mode makes use of the small mirror loss observed upon lasing. However, variations in the fabrication process could disrupt this set of conditions. As an example, a change in the thickness of the DBR layers would shift the position of the stopband. The stopband width for AlGaN/GaN bi-layers is reported to be 19 nm above 90% [7]. Thus, a change in the thickness of a DBR layer having a reflective center at 450 nm by only a small percentage, which is likely to occur in mass production, will shift the reflectance peak by approximately 10 nm. This shift would be sufficient to displace the stopband from the precisely-designed lasing wavelength. Therefore, there will likely be challenges associated with any future mass production of hybrid-DBR GaN-based VCSELs. Appl. Sci. 2019, 9, 733 3 of 12

4. All-dielectric-DBR GaN-based VCSELs In addition to nitride semiconductors, it may be possible to fabricate DBR mirrors having wide stopbands with dielectric materials, due to the wide range of refractive indices of these substances. As an example, the two dielectric materials SiO2 and Ta2O5 exhibit a large refractive index difference. Assuming refractive index values of 1.5 and 2.0 for the two dielectric materials and a reflectance peak wavelength of 450 nm, the associated stopband width would be more than 80 nm, which is much wider than the values reported for nitride semiconductor DBRs. Thus, GaN-based VCSELs with dielectric DBRs could avoid some of the issues associated with nitride semiconductor DBRs. In a pragmatic approach reported by Higuchi et al., both DBRs were made of dielectric materials [8], adopting a p-side structure similar to that of Lu et al. [4,5]. The p-side was bonded to a silicon wafer while the n-side was polished to generate a thin nitride plate so as to obtain p-type, active and n-type layers on the silicon wafer. Subsequently, a dielectric DBR acting as the n-side mirrors and other metallic electrodes were deposited. In this process, both mirrors were composed of crack-free dielectric materials possessing wide stopbands. To date, there have been numerous reports of work with this type of GaN-based VCSEL [8–13]. These GaN-VCSEL devices still have associated difficulties regarding control of the cavity length. Typically, the cavity must simultaneously meet two conditions to allow laser operation. The first condition concerns the optical mode loss and Figure1a,b plot the roughly estimated coupling (diffraction) loss and mode spacing as functions of cavity length [16]. Figure1a demonstrates that a shorter cavity is preferable because it leads to a smaller coupling (diffraction) loss after a round trip. Accounting for an estimated gain of up to 2000 cm−1 [23], the amplification resulting from the active layer in the VCSEL is estimated to be approximately 1% for a typical total thickness on the order of 10 nm. Thus, the cavity should preferably be shorter than several microns to limit the diffraction loss to less than 1%. The second condition concerns the longitudinal modes. The wavelength of each longitudinal mode is determined from the cavity length, which must be controlled so that one of the modes overlaps with the gain spectrum of the active region used in the device. As an example, even though the loss in such devices appears to be sufficiently low, in the case of a 5-µm-long cavity, the mode spacing is approximately 7 nm (Figure1b). Consequently, the modes can easily miss the gain spectrum and lasing will not occur unless the cavity length is precisely controlled such that one of the longitudinal modes is located near the top of the gain spectrum of the InGaN quantum wells. It is evident that highly sophisticated technology would be required to control the cavity length to below several microns by polishing. Longer cavities can provide narrower longitudinal mode spacing to ensure that one of the modes appears near the top of the gain spectrum without precise control of the cavity length. For example, a cavity longer than 20 µm gives rise to a longitudinal mode spacing narrower than 2 nm, which is sufficiently narrow to overlap the gain spectrum of typical InGaN quantum wells. However, a cavity of this length often results in a diffraction loss of greater than 1%, which is sufficient to prevent the device from lasing. These requirements for precise control over the cavity length have thus far prevented the mass production of GaN-VCSELs. It is apparent that precise control over the cavity length (to below 5 µm) or reduction of the optical diffraction associated with longer cavities, is required for the robust production of GaN-based VCSELs with all-dielectric DBR configurations. As an alternative to the approach reported by Higuchi et al., in which a lapping process is used to provide a basal plane for dielectric DBR deposition, a thinning technique based on photoelectrochemical (PEC) etching [24] was found to result in an operable GaN-VCSEL with a short cavity of 1.2 µm. If this approach allows precise control of cavity length, it could enable mass production of GaN-VCSELs. However, there has thus far only been limited research into this technique and further work is required to ensure that precise cavity length control is possible, as demonstrated by stability of the lasing wavelength over a wafer. Appl. Sci. 20182019, 98, 733x FOR PEER REVIEW 4 of 12

Figure 1. (a) Calculated coupling (diffraction) losses per round trip for beams having Gaussian lateralFigure profiles1. (a) Calculated with a beam coupling waist from(diffraction) 1 to 10 µ m.losses (b) per Longitudinal round trip mode for beams spacing having calculated Gaussian for the lateral parametersprofiles with n a = beam 2.45, λ waist= 450 from nm and1 to dn/d10 μm.λ = (b−) 0.001Longitudinal [16]. mode spacing calculated for the parameters n = 2.45, λ = 450 nm and dn/dλ = −0.001. 5. All-dielectric-DBR GaN-based VCSELs Fabricated by Epitaxial Lateral Overgrowth 5. All-dielectric-DBRTo mitigate some GaN-based of the challenges VCSELs associatedFabricated withby Epitaxial existing Lateral all-dielectric-DBR Overgrowth GaN-based VCSELs,To mitigate the present some author’s of the group challenges developed associated a GaN-based with VCSELexisting with all-dielectric-DBR a novel structure GaN-based [14,15,25]. ThisVCSELs, device the incorporated present author’s dielectric group DBRs developed at both a ends GaN-based of the cavity, VCSEL with with the a DBR novel on structure one side embedded[14,15,25]. inThis n-type device gallium incorporated nitride grown dielectric by the DBRs epitaxial at both lateral ends overgrowth of the cavity, (ELO) with technique the DBR [26]. on This one type side of GaN-basedembedded in VCSEL n-type has gallium several nitride advantages, grown by including the epitaxial the abilitylateral toovergrowth form highly (ELO) reflective technique. dielectric This DBRtype mirrorsof GaN-based at both VCSEL ends of has the cavityseveral and advantages, precise control including of short the cavity ability lengths to form during highly the reflective epitaxial growthdielectric process. DBR mirrors at both ends of the cavity and precise control of short cavity lengths during the epitaxialFigure2 growthillustrates process. the device introduced in the present section. This unit was fabricated by first formingFigure DBR 2 illustrates islands consisting the device of introduced 14.5 SiN/SiO in2 thebilayers present on section. an n-GaN This substrate, unit was after fabricated which by n-type first GaN seed crystals were grown in the window regions between the DBR islands (where the GaN forming DBR islands consisting of 14.5 SiN/SiO2 bilayers on an n-GaN substrate, after which n-type substrateGaN seed was crystals exposed) were using grown MOCVD. in the window As the MOCVD regions processbetween progressed, the DBR islands these seeds (where grew the in GaN size, engulfingsubstrate was the DBRexposed) islands using and MOCVD. finally forming As the MOCV planarD surfaces process above progressed, them. these MOCVD seeds was grew then in usedsize, toengulfing grow four the quantum DBR islands wells and a p-GaN finally layerforming and plan a contactar surfaces layer, above to a total them. thickness MOCVD of approximatelywas then used 100to grow nm. Afour 30-nm-thick quantum wells indium a p-GaN tin oxide layer (ITO) and layer a cont andact a layer, p-side to DBR a total with thickness 11.5 Ta2 ofO5 approximately/SiO2 bilayers were subsequently deposited on the contact layer and a Ti/Pt/Au n-electrode was overlaid on the 100 nm. A 30-nm-thick indium tin oxide (ITO) layer and a p-side DBR with 11.5 Ta2O5/SiO2 bilayers were subsequently deposited on the contact layer and a Ti/Pt/Au n-electrode was overlaid on the Appl. Sci. 2019, 9, 733 5 of 12

Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 12 bottom of the substrate. A circular current injection region with a diameter of 8 µm was electrically bottomconﬁned of bythe boronsubstrate. ion A implantation circular current [15], injection where implanted region with boron a diameter (>1E19 of cm 8 −μ3m) nullifywas electrically electrical -3 confinedconductivity by ofboron GaN ion around implantation the aperture. , where The cavity implanted region wasboron composed (>1E19of cm the) n-GaN nullify layer electrical grown conductivityover the DBR of islands.GaN around The thicknessthe aperture. of thisThe n-GaNcavity region layer governedwas composed the cavity of the lengthn-GaN of layer the grown device, overwhich the could DBR be islands. controlled The tothickness less than of several this n-GaN microns layer by applyinggoverned athe predetermined cavity length growth of the rate.device, which could be controlled to less than several microns by applying a predetermined growth rate.

FigureFigure 2. 2. SchematicSchematic of of an an all-dielectric-distributed all-dielectric-distributed Bragg Bragg reflectors reﬂectors (DBR) (DBR) gallium gallium nitride nitride based based (GaN)-based(GaN)-based VCSEL VCSEL in in which which the the bottom bottom side side DBR DBR is is encased encased in in n-GaN n-GaN generated generated using using an an epitaxial epitaxial laterallateral overgrowth overgrowth technique. technique.

FigureFigure 3 presents a cross-sectional SEM image of the ELO structure, in which an n-side DBR islandisland was was embedded embedded inside inside an an n-type n-type GaN GaN layer. layer. This This image image demonstrates demonstrates that that the the n-side n-side DBR DBR was was completelycompletely embedded embedded in in the the n-type n-type GaN GaN grown grown by by the the ELO ELO technique. technique. No No cracks cracks or or voids voids are are evident evident inin or or around around the the DBR DBR and a planar surfacesurface thatthat providesprovides the the basal basal plane plane for for the the p-side p-side mirror mirror is is seen seen to tohave have been been formed formed above above the the n-side n-side DBR. DBR. The The distancedistance fromfrom thethe surface of the epitaxial layer layer to to the the interfaceinterface between between the the n-type n-type GaN GaN and and n-side n-side DBR DBR was was determined determined to to be be 4.5 4.5 μµm,m, which which corresponds corresponds to the cavity length for this device. The reflectance spectra generated from the top and bottom DBRs toAppl. the Sci. cavity 2018, length8, x FOR for PEER this REVIEW device. The reflectance spectra generated from the top and bottom DBRs6 of 12 exhibitedexhibited peak peak reflectance reflectance values values of of more more than than 99 99.9%.9% and and stopband stopband widths widths of of 80-97 80-97 nm. nm. These These widths widths are 4-5 times greater than those reported for nitride semiconductor DBRs. are 4-5 times greater than those reported for nitride semiconductor DBRs. Figure 4 plots the current versus light output (I–L) and current versus voltage (I–V) curves for the device, for which the threshold current (Ith) and density (jth) were determined to be 18 mA and 35.8 kA cm-2, respectively. The maximum output power of this device was 1.1 mW. Figure 5 shows the spectra obtained below and above Ith and demonstrates a single longitudinal mode at a wavelength of 453.9 nm above Ith. The obvious threshold in the I–L curve and the fact that only a single longitudinal mode was observed above Ith demonstrate that lasing was achieved. The spacing of the longitudinal modes obtained below Ith was 6.7 nm (Fig. 5), which is corresponding to cavity length of 4.5 μm by calculation for the following parameters: n = 2.45, λ = 450 nm, dn/dλ = −0.001. The obtained cavity length is consistent with the value measured by SEM (Figure. 3) As noted previously in this section, GaN-based nitrides have associated challenges in terms of obtaining DBRs having high reflectivity and a wide stopband and precise control of the fabrication process is required to obtain cavity lengths shorter than 10 μm. The VCSELs introduced in the present section met all these criteria. The DBRs at both ends of the cavity were made of dielectric materials that exhibit high reflectance over a wide spectral band width. It is preferable to determine the cavity length using the precise epitaxial growth afforded by MOCVD rather than to employ a more delicate Figure 3. Cross-sectional scanning electron microscopy (SEM) image of the epitaxial lateral overgrowth processFigure such 3.as Cross-sectionalpolishing, which scanning was used electron in previo microscous studiespy (SEM) [8-13]. image Fig. of 5a the shows epitaxial that thelateral cavity length(ELO) of 4.5 structure, μm gives in whicha diffraction the n-side loss DBR per is round engulfed trip in n-GaNof much grown less usingthan 1% the ELOat a variety process. of The aperture flat surfaceovergrowth of the (ELO) epitaxial structure, layer forms in which a 4.5-mm-long the n-side cavity.DBR is engulfed in n-GaN grown using the ELO sizes. process.The ELO The flatprocess surface that of the is epitprimarilyaxial layer responsible forms a 4.5-mm-long for the cavity.formation of the cavity can be implemented uniformly over a large area of the wafer. Thus, this process enables the fabrication of multiple devices on a single wafer, which makes it very suitable for mass production.

Figure 4. I–V and I–L curves generated by an all-dielectric-DBR GaN-based vertical-cavity surface-

emitting laser (VCSEL). Here, Ith was 18 mA and the maximum output power was 1.1 mW, the highest value reported to date for this type of device. The kinks in the I–L curve suggest multimodal operation for the lateral mode, presumably due to the absence of lateral optical confinement.

Figure 5. Spectra obtained from the all-dielectric-DBR GaN-based VCSEL above and below the threshold current (18 mA). Spectrum (a) was obtained at 20 mA and shows a single longitudinal mode at 453.9 nm and a half width at maximum of 0.05 nm. The latter value is equivalent to the resolution Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 12

Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 12

Appl. Sci. 2019, 9, 733 6 of 12

Figure4 plots the current versus light output (I–L) and current versus voltage (I–V) curves for the device, for which the threshold current (Ith) and density (jth) were determined to be 18 mA and 35.8 kA cm−2, respectively. The maximum output power of this device was 1.1 mW. Figure5 shows the spectra obtained below and above Ith and demonstrates a single longitudinal mode at a wavelength of 453.9 nm above Ith. The obvious threshold in the I–L curve and the fact that only a single longitudinal Figure 3. Cross-sectional scanning electron microscopy (SEM) image of the epitaxial lateral mode was observed above Ith demonstrate that lasing was achieved. The spacing of the longitudinal overgrowth (ELO) structure, in which the n-side DBR is engulfed in n-GaN grown using the ELO modesFigure obtained 3. Cross-sectional below Ith was scanning 6.7 nm (Figureelectron5), microsco which ispy corresponding (SEM) image toof cavity the epitaxial length of lateral 4.5 µ m by process. The flat surface of the epitaxial layer forms a 4.5-mm-long cavity. calculationovergrowth for the(ELO) following structure, parameters: in which then n-side= 2.45, DBRλ = is 450 engulfed nm, dn/d in λn-GaN= −0.001. grown The using obtained the ELO cavity lengthprocess. is consistent The flat surface with the of valuethe epit measuredaxial layer byforms SEM a 4.5-mm-long (Figure3). cavity.

Figure 4. I–V and I–L curves generated by an all-dielectric-DBR GaN-based vertical-cavity surface- Figureemitting 4. laser (VCSEL). Here, Ith was 18 mA and the maximum output power was 1.1 mW, the highest Figure 4. I–VI–V and and I–L I–L curves curves generated generated by by an an all-dielectric-DBR all-dielectric-DBR GaN-based GaN-based vertical-cavity vertical-cavity surfacesurface- emittingvalue reported laser (VCSEL). to date for Here, this Itythpewas of 18device. mA and The thekinks maximum in the I–L output curve power suggest was multimodal 1.1 mW, the operation highest emitting laser (VCSEL). Here, Ith was 18 mA and the maximum output power was 1.1 mW, the highest valuefor the reported lateral mode, to date presumably for this type due of device. to the absence The kinks of inlateral the I–L optical curve confinement. suggest multimodal operation value reported to date for this type of device. The kinks in the I–L curve suggest multimodal operation for the lateral mode, presumably due to the absence of lateral optical conﬁnement. for the lateral mode, presumably due to the absence of lateral optical confinement.

Figure 5. Spectra obtained from the all-dielectric-DBR GaN-based VCSEL above and below the thresholdFigure 5. currentSpectra (18 obtained mA). Spectrum from the (a )all-dielectric was obtained-DBR at 20 GaN-based mA and shows VCSEL a single above longitudinal and below mode the atthreshold 453.9 nm current and a (18 half mA). width Spectrum at maximum (a) was of obtained 0.05 nm. at The 20 lattermA and value shows is equivalent a single longitudinal to the resolution mode Figure 5. Spectra obtained from the all-dielectric-DBR GaN-based VCSEL above and below the limitat 453.9 of thenm apparatusand a half usedwidth to at acquire maximum the dataof 0.05 (ADCMT8341). nm. The latter Spectrum value is equivalent (b) was acquired to the resolution at 10 mA threshold current (18 mA). Spectrum (a) was obtained at 20 mA and shows a single longitudinal mode using a different spectrum analyzer (Ocean Optics USB4000) and shows a mode spacing of 6.7 nm. at 453.9 nm and a half width at maximum of 0.05 nm. The latter value is equivalent to the resolution As noted previously in this section, GaN-based nitrides have associated challenges in terms of obtaining DBRs having high reﬂectivity and a wide stopband and precise control of the fabrication process is required to obtain cavity lengths shorter than 10 µm. The VCSELs introduced in the present section met all these criteria. The DBRs at both ends of the cavity were made of dielectric materials that exhibit high reﬂectance over a wide spectral band width. It is preferable to determine the cavity length using the precise epitaxial growth afforded by MOCVD rather than to employ a more delicate process such as polishing, which was used in previous studies [8–13]. Figure5a shows that the cavity length of 4.5 µm gives a diffraction loss per round trip of much less than 1% at a variety of aperture sizes. Appl. Sci. 2019, 9, 733 7 of 12

The ELO process that is primarily responsible for the formation of the cavity can be implemented uniformly over a large area of the wafer. Thus, this process enables the fabrication of multiple devices on a single wafer, which makes it very suitable for mass production.

6. All-dielectric-DBR GaN-based VCSELs Using an Atomically Smooth Monolithic Curved Mirror As discussed in Section4, one approach to overcoming potential problems associated with all-dielectric DBR VCSELs is to introduce lateral optical confinement, which allows the use of relatively long cavities (greater than 10 µm) with no diffraction loss. There have been a few studies on this topic regarding GaN-VCSELs. Hashemi et al. simulated the effect of lateral optical confinement by step-like lateral structures on the internal losses of GaN-VCSELs [27]. In addition, Hayashi et al. experimentally demonstrated one of those structures and found that the multi-lateral-mode of the device provided evidence for lateral optical guiding [21]. Kuramoto et al. reported improved slope efficiency in the case of a device incorporating a mesa structure [22], which confirmed the effect of lateral optical confinement of the cavity. However, those step-like lateral structures also induced scattering losses. For this reason, the authors of the present review proposed a GaN-VCSEL having a cavity with a graded lateral structure instead of a mesa structure. Resonators cladded with curved and plane mirrors are known to form stable cavities without diffraction and scattering losses, in association with the generation of a beam waist on the plane mirror [28]. Thus, the introduction of curved mirrors on one side of the VCSEL may allow for longer cavities that would help to relax the strict processing requirements during polishing that have prevented GaN-VCSELs from being mass produced. The first experimental result based on this concept was reported following work with GaN-VCSELs by Park et al. [29], who investigated the optical pumping of devices having 50-µm-long cavities with plane and curved end mirrors. InGaN quantum wells were grown epitaxially on the (0001) plane of a sapphire template (t = 50 µm) to fabricate these devices and a monolithic curved mirror was generated on the (000-1) plane of the template using ball-up resin patterns as sacrificial masks during the reactive ion etching of the sapphire. However, this study only demonstrated laser operation in conjunction with optical pumping. The author’s group investigated lasing as a result of current injection into VCSELs with a similar structure, in which InGaN wells were grown epitaxially on (0001) GaN substrates and a curved mirror was fabricated on (000-1) GaN rather than on sapphire substrates (see Figure6)[ 16]. These devices were fabricated by growing three InGaN/GaN MQWs via MOCVD, a p-GaN layer doped with Mg (approximately 1 × 1019 cm−3) and a contact layer doped with Mg (approximately 1 × 1020 cm−3), to a total thickness of 105 nm, on a (0001) GaN substrate. A 20-nm-thick ITO layer and a p-side DBR with 11.5 Ta2O5/SiO2 bilayers were deposited on the contact layer. A hole was etched next to the aperture, reaching the n-GaN layer and two Ti/Pt/Au electrodes were deposited so that they made contact with the ITO layer and the exposed n-GaN, respectively, to form a current path. A circular current injection region was electrically confined by boron implantation, leaving a current aperture with a diameter of 3 µm that was in contact with the ITO and one Ti/Pt/Au electrode to form an emitter. The remaining processing steps during fabrication of the curved mirror were similar to those used in Parks’ research. That is, ball-up resin patterns were employed as sacrificial masks during reactive ion etching of the GaN substrate. Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 12

Appl.Appl. Sci.Sci. 20192018,, 98,, 733x FOR PEER REVIEW 88 ofof 1212

Figure 6. Schematic of a GaN-VCSEL having a curved mirror . Figure 6. Schematic of a GaN-VCSEL having a curved mirror [16]. Figures 7(a) and (b) show laser confocal microscopy images of a lenslet fabricated on the (000-1) Figure7a,b show laser confocal microscopy images of a lenslet fabricated on the (000-1) plane of plane of a GaN substrateFigure fabricated 6. Schematic through of a GaN-VCSEL these procedures. having a The curved top mirror and bird’s-eye . views in Fig. a GaN substrate fabricated through these procedures. The top and bird’s-eye views in Figure7a both 7(a) both demonstrate the smooth droplet shape that was formed. In the cross-sectional analysis (Fig. demonstrateFigures 7(a) the smoothand (b) show droplet laser shape confocal that was microscopy formed. images In the cross-sectionalof a lenslet fabricated analysis on (Figure the (000-1)7b), 7(b)), an ideal parabolic curve has been overlaid on the experimentally determined cross-sections. An anplane ideal of parabolic a GaN substrate curve has fabricated been overlaid through on the these experimentally procedures. determinedThe top and cross-sections. bird’s-eye views An atomicin Fig. atomic force microscopy (AFM) image of the top of the lenslet is shown in Fig. 7c. The root mean force7(a) both microscopy demonstrate (AFM) the image smooth of thedroplet top ofshape the lensletthat was is formed. shown in In Figure the cross-sectional7c. The root mean analysis square (Fig. square roughness of this surface was determined to be 0.2 nm, which is comparable to the value roughness7(b)), an ideal of this parabolic surface curve was determinedhas been overlaid to be 0.2 on nm, the experimentally which is comparable determined to the valuecross-sections. reported forAn reported for a plane mirror that allowed lasing from a GaN-VCSEL in a prior study . This image also aatomic plane mirrorforce microscopy that allowed (AFM) lasing image from of a GaN-VCSELthe top of the in lenslet a prior is study shown [30 in]. ThisFig. image7c. The also root clearly mean clearly shows atomic steps at the top of the lenslet (inset to Fig. 7(c)), indicating the minimal showssquare atomicroughness steps of at this the topsurface of the was lenslet determined (inset to to Figure be 0.27c), nm, indicating which is the comparable minimal roughness to the value of roughness of this surface. Fig. 7(d) presents a transmission electron microscopy (TEM) image of the thisreported surface. for Figurea plane7d mirror presents that a transmissionallowed lasing electron from a microscopy GaN-VCSEL (TEM) in a imageprior study of the . cross-sectionThis image also of cross-section of the curved mirror in this device. Each DBR layer is seen to bend smoothly across the theclearly curved shows mirror atomic in this steps device. at Eachthe top DBR of layer the islensle seent to(inset bend to smoothly Fig. 7(c)), across indicating the top ofthe the minimal lenslet top of the lenslet formed on the GaN, forming a perfect curved mirror. formedroughness on theof this GaN, surface. forming Fig. a perfect7(d) presents curved a mirror.transmission electron microscopy (TEM) image of the cross-section of the curved mirror in this device. Each DBR layer is seen to bend smoothly across the top of the lenslet formed on the GaN, forming a perfect curved mirror.

Figure 7. (a) Laser scanning confocal microscope images and (b) cross-sectional profiles of lenslets fabricatedFigure 7. ( ona) theLaser (000-1) scanning plane confocal of GaN wafers. microscope In (b ),images multiple and profiles (b) cross-sectional acquired from profiles specimens of lenslets having differentfabricated diameters on the (000-1) are overlaid, plane of while GaN the wafers. red dashed In (b),line multiple is an idealprofiles parabolic acquired curve from fitted specimens to one ofhaving the plots. different (c) AFM diameters image are of theoverlaid, top of while a lenslet the fabricatedred dashed on line the is (000-1) an ideal plane parabolic of a GaN curve substrate. fitted to TheFigure inset 7. is(a a) Laser high-resolution scanning confocal image of microscope a 1 µm × 1 images um area and that (b shows) cross-sectional atomic steps profiles on the of surface.lenslets (fabricatedd) Cross-sectional on the (000-1) TEM image plane of of the GaN curved wafers. mirror In of(b the), multiple device used profiles for anacquired optical pumpingfrom specimens test. having different diameters are overlaid, while the red dashed line is an ideal parabolic curve fitted to Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 12

one of the plots. (c) AFM image of the top of a lenslet fabricated on the (000-1) plane of a GaN Appl. Sci.substrate. 2018, 8, xThe FOR inset PEER is REVIEWa high-resolution image of a 1 μm × 1 um area that shows atomic steps on the9 of 12 surface. (d) Cross-sectional TEM image of the curved mirror of the device used for an optical pumping onetest. of the plots. (c) AFM image of the top of a lenslet fabricated on the (000-1) plane of a GaN substrate. The inset is a high-resolution image of a 1 μm × 1 um area that shows atomic steps on the Appl. Sci.surface.Figure2019, 98(d, 733shows) Cross-sectional the current-voltage TEM image of (I-V) the curved and current-optical mirror of the device output used (I-L)for an curves optical pumpingfor the GaN-9 of 12 test. VCSEL device. The I-L curve exhibits a clear threshold at an injected current of 0.25 mA ( Jth = 3.5 kA/cm2 for a current aperture diameter of 3 μm) during CW injection. Figure 9 provides the emission Figure8 8 showsshows the current-voltage current-voltage (I-V) (I-V) and and current-optical current-optical output output (I-L) (I-L) curves curves for the for GaN- the spectrum for the device above the threshold current, which exhibits a dominant peak at a wavelength GaN-VCSELVCSEL device. device. The I-L The curve I-L exhibits curve exhibits a clear athre clearshold threshold at an injected at an injectedcurrent of current 0.25 mA of ( 0.25 Jth = mA 3.5 of 445.3 nm. The2 width of this spectrum is equivalent to the resolution limit of the analyzer (0.1 nm). (JkA/cmth = 3.52 for kA/cm a currentfor aperture a current diameter aperture of diameter 3 μm) during of 3 µ CWm) duringinjection. CW Figure injection. 9 provides Figure the9 provides emission The emission spectrum contains multiple peaks and the primary peaks are spaced at intervals of 1.27 thespectrum emission for the spectrum device forabove the the device threshold above current, the threshold which current,exhibits a which dominant exhibits peak a at dominant a wavelength peak nm, as indicated by the filled triangles. This interval corresponds to the mode spacing calculated for atof a445.3 wavelength nm. The ofwidth 445.3 of nm. this Thespectrum width is of equivalent this spectrum to the is resolution equivalent limit to the of resolutionthe analyzer limit (0.1 of nm). the a cavity length of 27 μm when using n = 2.45, λ = 445.3 nm and dn/dλ = -0.001 . This result analyzerThe emission (0.1 nm). spectrum The emission contains spectrum multiple containspeaks and multiple the primary peaks peaks and the are primary spaced peaks at intervals are spaced of 1.27 at demonstrates that the lasing occurred with multi-longitudinal modes. The dimensional parameters intervalsnm, as indicated of 1.27 nm, by the as indicated filled triangles. by the ﬁlledThis inte triangles.rval corresponds This interval to correspondsthe mode spacing to the modecalculated spacing for for this device were determined to be L = 27 μm using scanning electron microscopy (data not shown) calculateda cavity length for a cavityof 27 lengthμm when of 27 usingµm when n = 2.45, using λn == 445.3 2.45, λnm= 445.3and nmdn/d andλ = dn/d-0.001λ = . −This0.001 result [16]. and R = 29.1 μm using confocal laser microscopy. According to classic Gaussian optics theory , this Thisdemonstrates result demonstrates that the lasing that occurred the lasing with occurred multi-longitudinal with multi-longitudinal modes. The modes. dimensional The dimensional parameters cavity had an associated resonant mode with a beam waist (4σ) of 1.3 μm on the planar mirror. This parametersfor this device for were this determined device were to determined be L = 27 μm to using be L scanning = 27 µm electron using scanning microscopy electron (data microscopynot shown) value is smaller than the current injection diameter of 3 μm. (dataand R not = 29.1 shown) μm using and R confocal = 29.1 µ laserm using microscopy. confocal According laser microscopy. to classic According Gaussian tooptics classic theory Gaussian , this opticscavity theoryhad an [associated16], this cavity resonant had anmode associated with a beam resonant waist mode (4σ) withof 1.3 a μ beamm on waistthe planar (4σ) ofmirror. 1.3 µm This on thevalue planar is smaller mirror. than This the value current is smaller injection than diameter the current of 3 injectionμm. diameter of 3 µm.

Figure 8. I-V and I-L curves obtained from a GaN-VCSEL with a 3 μm aperture at room temperature in response to continuous current injection. The inset shows an enlarged view of the data around the Figurethreshold 8. I-V current. and I-L curves obtained from a GaN-VCSEL with a 3 µm aperture at room temperature inFigure response 8. I-V to and continuous I-L curves current obtained injection. from a The GaN-VCSEL inset shows with an a enlarged 3 μm aperture view of at the room data temperature around the thresholdin response current. to continuous current injection. The inset shows an enlarged view of the data around the threshold current.

Figure 9. Emission spectrum generated by a GaN-VCSEL with a threshold current of 0.25 mA in response to a current injection of 0.30 mA.

The research described above confirmed the appearance of a near field image and the beam waist matched the theoretically predicted shape, demonstrating that the resonant mode was confined in the Appl. Sci. 2019, 9, 733 10 of 12 lateral direction, as predicted by Gaussian optics. Because a small aperture of 1 to 2 µm in conjunction with a cavity longer than 20 µm should result in a large diffraction loss (up to approximately 100%) per round trip (see Figure2a), the previously discussed small threshold current strongly suggests the occurrence of lateral optical confinement in this type of GaN-based VCSEL.

7. Conclusions This review described a variety of possible GaN-based VCSEL structures. The initial section concentrated on early prototypes, of which there were two main types: hybrid-DBR and all-dielectric-DBR. Although both types exhibit lasing during CW operation in conjunction with current injection, there are challenges associated with mass production. Specifically, it is difficult to control the thicknesses of each nitride semiconductor layer in the DBRs in hybrid-DBR types and the size of the cavity itself in all-dielectric-DBR devices. The latter part of this document reviewed two types of novel all-dielectric GaN-based VCSELs having different structures. One such device incorporates dielectric DBRs at either end of the cavity, with one side of each DBR embedded in n-type GaN grown using ELO. The other type incorporates a curved mirror fabricated on (000-1) GaN. These designs are intended to eliminate the requirement for strict control of the cavity length and to address the absence of lateral optical confinement, both of which have prevented the practical use of all-dielectric GaN-based VCSELs. As discussed throughout this paper, a variety of structures have been reported for GaN-based VCSELs during the research history spanning approximately 10 years. As a result of this short period of time, as of 2018, hybrid-DBR GaN-based VCSELs have been developed that exhibit wall plug efficiencies close to 10%, while all-dielectric-DBR devices have shown threshold currents much lower than 1 mA. The authors believe that additional research activity will improve the performance of GaN-based VCSELs and allow their practical use in the near future.

Author Contributions: T.H. has written the manuscript following advices given by N.F. and H.N. Funding: This research received no external funding. Acknowledgments: The authors are grateful to the following colleagues who have worked on the development of GaN-based VCSELs at the Sony Corporation: Izumi, Murayama, Mitomo, Satou, Fujii, Watanabe, Jyoukawa, Ito, Kobayashi, Ohara, Kawanishi, Koda, Narui and Yanashima. The authors thank Takeuchi, for his helpful advice during the writing of this paper and Fujioka, for providing the opportunity to produce this review. Conﬂicts of Interest: The contents of this paper are based on the authors’ knowledge acquired though research activity as employees of the Sony Corporation.

References

1. Akasaki, I.; Amano, H. Breakthroughs in Improving Crystal Quality of GaN and Invention of the p–n Junction Blue-Light-Emitting Diode. J. Appl. Phys. 2006, 45, 12. [CrossRef] 2. Nakamura, S.; Senoh, M.; Nagahama, S.; Iwasa, N.; Yamada, T.; Matsushita, T.; Sugimoto, Y.; Kiyoku, H. Room-temperature continuous-wave operation of InGaN multi-quantum-well structure laser diodes. Appl. Phys. Lett. 1996, 69, 4056. [CrossRef] 3. Takeuchi, T.; Kamiyama, S.; Iwaya, M.; Akasaki, I. GaN-based vertical-cavity surface-emitting lasers with AlInN/GaN distributed Bragg reflectors. Rep. Prog. Phys. 2019, 82, 1. [CrossRef][PubMed] 4. Lu, T.C.; Kao, C.C.; Kuo, H.C.; Huang, G.S.; Wang, S.C. CW lasing of current injection blue GaN-based vertical cavity surface emitting laser. Appl. Phys. Lett. 2008, 92, 141102. [CrossRef] 5. Lu, T.C.; Chen, S.W.; Wu, T.T.; Tu, P.M.; Chen, C.K.; Chen, C.H.; Li, Z.Y.; Kuo, H.C.; Wang, S.C. Continuous wave operation of current injected GaN vertical cavity surface emitting lasers at room temperature. Appl. Phys. Lett. 2010, 97, 071114. [CrossRef] 6. Carlin, J.F.; Dorsaz, J.; Feltin, E.; Butte, R.; Grandjean, N.; Ilegems, M. Crack-free fully epitaxial nitride microcavity using highly reflective AlInNGaN Bragg mirrors. Appl. Phys. Lett. 2005, 86, 031107. [CrossRef] 7. Kozuka, Y.; Ikeyama, K.; Yasuda, T.; Takeuchi, T.; Kamiyama, S.; Iwaya, M.; Akasaki, I. Growths of AlInN Single Layers and Distributed Bragg Reflectors for VCSELs. MRS Proc. 2015, 1736.[CrossRef] Appl. Sci. 2019, 9, 733 11 of 12

8. Higuchi, Y.; Omae, K.; Matsumura, H.; Mukai, T. Room-Temperature CW Lasing of a GaN-Based Vertical-Cavity Surface-Emitting Laser by Current Injection. Appl. Phys. Express 2008, 1, 121102. [CrossRef] 9. Omae, K.; Higuchi, Y.; Nakagawa, K.; Matsumura, H.; Mukai, T. Improvement in Lasing Characteristics of GaN-based Vertical-Cavity Surface-Emitting Lasers Fabricated Using a GaN Substrate Kunimichi. Appl. Phys. Express 2009, 2, 052101. [CrossRef] 10. Kasahara, D.; Morita, D.; Kosugi, T.; Nakagawa, K.; Kawamata, J.; Higuchi, Y.; Matsumura, H.; Mukai, T. Demonstration of Blue and Green GaN-Based Vertical-Cavity Surface-Emitting Lasers by Current Injection at Room Temperature. Appl. Phys. Express 2011, 4, 072103. [CrossRef] 11. Kasahara, D.; Marui, H.; Kawamata, J.; Kosugi, T.; Higuchi, Y.; Nagahama, S. Pure green 529 nm room- temperature lasing of GaN-based vertical-cavity surface-emitting Laser by current injection. In Proceedings of the 1st Laser Display Conference, Yokohama, Japan, 26–27 April 2012. 12. Liu, W.J.; Hu, X.L.; Ying, L.Y.; Zhang, J.Y.; Zhang, B.P. Room temperature continuous wave lasing of electrically injected GaN-based vertical cavity surface emitting lasers. Appl. Phys. Lett. 2014, 104, 251116. [CrossRef] 13. Onishi, T.; Imafuji, O.; Nagamatsu, K.; Kawaguchi, M.; Yamanaka, K.; Takigawa, S. Continuous Wave Operation of GaN Vertical Cavity Surface Emitting Lasers at Room Temperature. IEEE J. Quant. Electron. 2012, 48, 1107. [CrossRef] 14. Hamaguchi, T.; Fuutagawa, N.; Izumi, S.; Narui, H. Milliwatt-class GaN-based blue vertical-cavity surface-emitting lasers fabricated by epitaxial lateral overgrowth. Phys. Status Solidi 2016, 213, 1170. [CrossRef] 15. Hamaguchi, T.; Nakajima, H.; Ito, M.; Mitomo, J.; Satou, S.; Fuutagawa, N.; Narui, H. Lateral carrier confinement of GaN-based vertical-cavity surface-emitting diodes using boron ion implantation. Jpn. J. Appl. Phys. 2016, 55, 12. [CrossRef] 16. Hamaguchi, T.; Tanaka, M.; Mitomo, J.; Nakajima, H.; Ito, M.; Ohara, M.; Kobayashi, N.; Fujii, K.; Watanabe, H.; Satou, S.; et al. Lateral optical confinement of GaN- based VCSEL using an atomically smooth monolithic curved mirror. Sci. Rep. 2018, 8, 10350. [CrossRef][PubMed] 17. Hamaguchi, T.; Nakajima, H.; Tanaka, M.; Ito, M.; Ohara, M.; Jyoukawa, T.; Kobayashi, N.; Matou, T.; Hayashi, K.; Watanabe, H.; et al. Sub-milliampere-threshold continuous wave operation of GaN-based vertical-cavity surface-emitting laser with lateral optical confinement by curved mirror. Appl. Phys. Express 2019.[CrossRef] 18. Koyama, F.; Kinoshita, S.; Iga, K. Room temperature CW operation of GaAs vertical cavity surface emitting laser. IEICE Trans. 1988, 71, 1089–1090. 19. Coldren, L.A. Vertical-Cavity Surface-Emitting Lasers; Cambridge University Press: Cambridge, UK, 1999. 20. Choquette, K.D.; Schneider, R.P.; Lear, K.L.; Geib, K.M. Low threshold voltage vertical-cavity lasers fabricated by selective oxidation. Electron. Lett. 1994, 30, 2043. [CrossRef] 21. Hayashi, N.; Ogimoto, J.; Matsui, K.; Furuta, T.; Akagi, T.; Iwayama, S.; Takeuchi, T.; Kamiyama, S.; Iwaya, M.; Akasaki, I. A GaN-Based VCSEL with a Convex Structure for Optical Guiding. Phys. Status Solidi 2018, 215, 1700648. [CrossRef] 22. Kuramoto, M.; Kobayashi, S.; Akagi, T.; Tazawa, K.; Tanaka, K.; Saito, T.; Takeuchi, T. Enhancement of slope

efficiency and output power in GaN-based vertical-cavity surface-emitting lasers with a SiO2-buried lateral index guide Masaru. Appl. Phys. Lett. 2018, 112, 111104. [CrossRef] 23. Witzigmann, B.; Laino, V.; Luisier, M.; Schwarz, U.T.; Feicht, G.; Wegscheider, W.; Engl, K.; Furitsch, M.; Leber, A.; Lell, A.; et al. Microscopic analysis of optical gain in InGaN/GaN quantum wells. Appl. Phys. Lett. 2006, 88, 021104. [CrossRef] 24. Leonard, J.T.; Cohen, D.A.; Yonkee, B.P.; Farrell, R.M.; Margalith, T.; Lee, S.; DenBaars, S.P.; Speck, J.S.; Nakamura, S. Nonpolar III-nitride vertical-cavity surface-emitting lasers incorporating an ion implanted aperture. Appl. Phys. Lett. 2015, 107, 011102. [CrossRef] 25. Izumi, S.; Fuutagawa, N.; Hamaguchi, T.; Murayama, M.; Kuramoto, M.; Narui, H. Room-temperature continuous-wave operation of GaN-based vertical-cavity surface-emitting lasers fabricated using epitaxial lateral overgrowth. Appl. Phys. Express 2015, 8, 062702. [CrossRef] 26. Okur, S.; Shimada, R.; Zhang, F.; Hafiz, S.D.A.; Lee, J.; Avrutin, V.; Morkoç, H.; Franke, A.; Bertram, F.; Christen, J.; Özgür, Ü. GaN-Based Vertical Cavities with All Dielectric Reflectors by Epitaxial Lateral Overgrowth. Jpn. J. Appl. Phys. 2013, 52, 08JH03. [CrossRef] Appl. Sci. 2019, 9, 733 12 of 12

27. Hashemi, E.; Gustavsson, J.; Bengtsson, J.; Stattin, M.; Cosendey, G.; Grandjean, N.; Haglund, Å. Engineering the Lateral Optical Guiding in Gallium Nitride-Based Vertical-Cavity Surface-Emitting Laser Cavities to Reach the Lowest Threshold Gain. Jpn. J. Appl. Phys. 2013, 52, 08JG04. [CrossRef] 28. Kogelnik, H.; Li, T. Laser Beams and Resonators. Appl. Opt. 1966, 10, 1550. [CrossRef][PubMed] 29. Park, S.; Kim, J.; Sakong, H.J.; Lee, S.N.; Chae, S.; Jeong, Y.P.; Cho, G.Y. Room-temperature GaN vertical-cavity surface-emitting laser operation in an extended cavity scheme. Appl. Phys. Lett. 2003, 83, 15. [CrossRef] 30. Liu, W.; Chen, S.; Hu, X.; Liu, Z.; Zhang, J.; Ying, L.; Lv, X.; Akiyama, H.; Cai, Z.; Zhang, B. Low Threshold Lasing of GaN-Based VCSELs With Sub-Nanometer Roughness Polishing. IEEE Photon. Technol. Lett. 2013, 25, 20. [CrossRef]

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).