GaN Microdomes for Broadband Omnidirectional

GaN Microdomes for Broadband Omnidirectional Antireflection for Concentrator Photovoltaics Lu Han1, Matthew R. McGoogan1, Tyler A. Piedimonte1, Ian V. Kidd2, Roger H. French2, and Hongping Zhao1, + 1 Department of Electrical Engineering and Computer Science, Case Western Reserve University, Cleveland, OH 44106, USA 2 Department of Material Science and Engineering, Case Western Reserve University, Cleveland, OH 44106, USA + Email: [email protected] ABSTRACT GaN microdomes are studied as a broadband omnidirectional anti-reflection structure for high efficiency multi-junction concentrated photovoltaics. Comprehensive studies of the effect of GaN microdome sizes and shapes on the light collection efficiency were studied. The three dimensional finite difference time domain (3-D FDTD) method was used to calculate the surface reflectance of GaN microdomes as compared to that of the flat surface. Studies indicate significant reduction of the surface reflectance is achievable by properly designing the microdome structures. Formation of the GaN microdomes with the flexibility to tune the size and shape has been demonstrated by using reactive ion etching (RIE) of both GaN and the self-assembled silica monolayer microspheres. Characterizations of the angle-dependence light surface reflectance for both micro-domes and flat surface show the similar trend as the simulation.

1. INTRODUCTION III-nitride (In, Al, Ga-N) semiconductors cover a wide spectral range in solar spectrum from ultraviolet to infrared, which provides a great promise to be used as tandem multi-junction (MJ) cells for the next generation high efficiency concentrated photovoltaics (CPVs) [1, 2]. Due to the higher refractive index of III-nitride semiconductors (~2.5) as compared to that of the air, more than 20% of the normal incidence (θ=0°) light is reflected back into the air. The surface reflectance is even higher when the light incidence angle increases. Thus, the incident photon energy loss due to the reflection and scattering at the interface between semiconductor and free space becomes one of the main issues that limit the total conversion efficiency of solar cells. Current approaches used for surface antireflection include: 1) single layer or multiple layer antireflection coatings (ARCs) [3-5] and 2) sub-wavelength surface topology [6-14]. Single layer ARC is used for moderate suppression of reflectance at normal incidence. The selection of material is a challenge due to the specific requirement of the refractive index of ARC. Multiple layer ARCs are used for broadband antireflection. However, the trade-off between a larger bandwidth and overall reflectivity is still the challenge. In addition, the uniformity of the ARCs highly determines the effectiveness of the antireflection, and it is challenging to achieve highly uniform multiple layer ARCs. The development of the surface topology was essentially triggered by the requirement of omnidirectional broadband antireflection. However, comprehensive theoretical studies and deep understanding on the interaction between incident light and the surface topology is lacking. In addition, a fabrication approach to form desirable surface topology for omnidirectional broadband antireflection with low cost and scalability is urgently needed. In this work, we studied the GaN micro-domes for broadband omnidirectional antireflection for concentrated photovoltaics. Simulation studies on the surface reflection and transmission were based on the three-dimensional finite difference time domain (3D-FDTD) method. Comprehensive studies of the dependence of the surface reflection on the geometrical shapes of the GaN microdomes were performed. The fabrication of the GaN microdomes was based on a low cost, high throughput and scalable self-assembled approach. Reactive ion etching (RIE) was used to etch both GaN and the self-assembled SiO2 microspheres to form GaN microdomes. Characterizations of the surface reflectance indicate significant reduction of the reflection from GaN microdomes as compared to the conventional flat surface. Studies also indicate the geometrical shape of the GaN microdomes have significant effect on the surface reflectance.

Physics, Simulation, and Photonic Engineering of Photovoltaic Devices II, edited by Alexandre Freundlich, Jean-Francois Guillemoles, Proc. of SPIE Vol. 8620, 862016 · © 2013 SPIE CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2002864 Proc. of SPIE Vol. 8620 862016-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/10/2014 Terms of Use: http://spiedl.org/terms

2. NUM MERICAL SIMULATIO S ONS AND RE ESULTS The surfface reflectancce from GaN microdomes m waas calculated bby the 3-D FD DTD method. A As shown in F Fig.1 (a), plane wave was w used as th he incident lig ght source for the calculationn of the reflecction and transsmission for thhe closepacked hexag gonal GaN microdomes as compared to that of the conveentional GaN w with flat surfacce. Surface refflectance and transmisssion of the plane wave sou urce with P polarization andd S polarization are calculaated separatelyy. In the calculation, material m propeerties take into account both the wavelengtth dependence of the refractiive index and material loss. Unit ceells and periodic boundary co onditions are used u to simpliffy the calculattion. The minimum mesh steep in the calculation iss set as 0.25 nm m. The reflectaance monitor po osition is set ass 1.6 times thatt of the microddome height. T The plane wave source wavelength raanges between 300 nm and 12 200 nm, and ligght incidence aangle ranges beetween 0° to 800°, where light incidencce angle of 0° represents r the case c of normall incidence. In the caalculation, the geometrical sh hapes and sizees of the GaN m microdomes arre studied for the surface refflectance studies. Both h GaN micro-h hemispheres (h= =D/2) (as show wn in Fig. 1(b))) and GaN miccro-hemiellipssoid (h≠D/2) (aas shown in Fig. 1(c)) are systematicaally studied as the surface refflection structuures.

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Figure 1:: (a) Schematic of the 3-D FDTD TD simulation off surface reflectaance of close-paccked hexagonal GaN microdom mes; (b) GaN microdomes m with h hemispherical geometrical sha ape; (c) GaN miccrodomes with hhemi-ellipsoid geeometrical shapee.

Figure 2 plots the su urface reflectio on comparison n as a functioon of the lighht incidence angle for GaN N micro hemispheres with differentt diameter (D) and the conveentional GaN fflat surface forr both P [Fig. 2(a)] and S [F Fig. 2(b)] polarizationss. The incident light waveleng gth is fixed at 500 nm. The ssurface reflectaance of GaN m micro hemispheeres with diameters off 100 nm, 200 nm, 300 nm, 500 nm, 750 nm and 1000 nm were calcuulated. From tthe results, thee surface reflection strrongly dependss on the size of o the micro hemispheres. Thhe P polarizattion reflection form the convventional GaN flat surrface shows th he Brewster an ngle at around d 70°, as show wn in Fig. 2(a)). From Fig. 22(b), the generral trend indicates thatt the surface reeflection increaases as the ligh ht incidence anggle increases. A As compared tto the GaN flatt surface, the use of GaaN micro hemiispheres lead to t significant reduction of thee reflection at different inciddence angles foor both P and S polarizzations. To furth her study the effect of the geeometrical shap pe of GaN miccrodomes on th the surface refl flection, calculaations of the surface reflection r weree performed fo or GaN micro odomes with fi fixed diameter and varied heeights (H). Heere, GaN microdomes with diameterr (D) of 1000 nm n and heightss (H) of 375 nnm, 500 nm, 8775 nm, 1250 nnm, 1500 nm, 22000 nm and 3000 nm m were studied and compared to the conventtional GaN flatt surface. The iincident wavellength was fixeed at 500 nm. Surface reflections of both b P and S polarizations p were w calculatedd and comparedd as shown in F Figs. 3(a) and 3(b). By tuning the heeight of the GaN N microdomess, the surface reeflection is furt rther reduced as compared to the case of GaaN micro hemispheres (H=D/2). Fro om Fig.3, as th he height of th he GaN mciroddomes increasses, the surfacee reflection is reduced nce angle for both b P and S polarizations. For the GaN N mcirodomes with D=1000 nm and further at different inciden H=3000 nm, the surface reeflection of 50 00 nm incidentt wavelength iss reduced to below 3% for a wide incidennce angle range. Thus, the design off microdome sttructures with controllable diiameter and heeight lead to ssignificant reduuction of gles for concen ntrated photovooltaic applicatiion. surface reflecction with widee incidence ang

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Figure 3: Simulation of the reflection of GaN microdomes with hemiellipsoid structures with incident light wavelength of 500 nm as a function of the incidence angle for both (a) P polarization and (b) S polarization. The GaN microdome hemiellipsoid structure diameter is D=1000 nm with different heights of 375, 500, 875, 1250, 1500, 2000 and 3000 nm, respectively. The simulations are compared to conventional GaN with flat surface.

3. EXPERIMENTAL FABRICATION AND CHARACTERIZATIONS In this work, the GaN microdomes are fabricated based on a self-assembled low cost, scalable, and controllable approach. Figure 4 shows the fabrication flow chart of forming the GaN microdomes, which includes the following steps: (a) surface hydrophilic treatment for GaN; (b) SiO2 microsphere monolayer deposition via dip-coating method; (c) reactive ion etching of both SiO2 microsphere and GaN to form GaN microdomes; and (d) hydrofluoric acid wet etching of the sample to remove the residue of SiO2. Here, the GaN wafers were grown on sapphire substrate with thickness of 5 μm, which were provided by Kyma Technologies. We utilized the ultraviolet (UV) ozone system for the GaN surface treatment. Our experiments have


shown that th he hydrophilic surface treatm ment is a cruciaal step for the ffollowing SiO2 microsphere ddip-coating depposition. The depositio on of the self-assembled mo onolayer (SAM M) of SiO2 miccrospheres is based on the diip-coating methhod [1518]. In this sttudy, the suspeension of SiO2 microspheres with diameterr of 1 μm was prepared withh mono-disperssed silica powder. Duee to the solven nt evaporation n, the capillary y force betweeen the particlees leads to thee self-assemblly of the microspheress when the GaN aN substrate is withdrawn fro om the suspenssion. With opttimized SiO2 m microsphere susspension preparation and a substrate withdrawal sp peed, well org ganized SiO2 m microsphere m monolayer is aable to be form med. An example of th he scanning eleectron microsccope (SEM) im mage of the selff-assembled SiO2 microspherre monolayer ddeposited on GaN subsstrate is shown in Fig. 4(b). The reacctive ion etchiing process waas used to sim multaneously eetch both GaN N and SiO2 miicrospheres so that the spherical shaape of SiO2 microspheres m is transferred to the underneatth GaN substrrate. In this stuudy, Cl2 and S SF6 were selected as etching e gases to t selectively etch e GaN and SiO2 microsphheres, respectiively. We utilized the Lam R Research 9400 Etcher to perform thee etching proceess with RF po ower of 500 W and bias volttage of 108 V. The chamber pressure was set as 10 0 mTorr. In ourr studies, we fo ound that the ratio of the Cl2//SF6 flow rate would effectivvely control thee etching rate ratio of GaN and SiO O2, which in turrn determines the aspect rattio of the GaN N microdomes. Figures 4(c) and 4(d) show the SEM M images of GaN G microdom mes with differeent aspect ratio , which were fformed by usinng two differentt etching recipes. Thu us, the GaN microdomes m wiith controllablee aspect ratio could be form med through ccontrolling thee plasma etching cond ditions.

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subst Figure 4:: (a) Process flo ow chart of the fabrication f of GaN G microdomess; SEM images oof (b) SiO2 micrrospheres depossited on GaN using g dip-coating meethod; (c) GaN microdomes m with h low aspect rati tio; and (d) GaN N microdomes wiith high aspect raatio.

a performedd using a J. A A. Woollam thhe Vacuum Ulltraviolet The chaaracterizations of the surfacee reflectance are Variable Angle Spectrosco opic Ellipsomeeter (VUV-VA ASE). VUV-V VASE was used to take variiable angle refflectance measurementts on the sam mples. The insstrument has an MgF2 autoo-retarder andd is fully nitroogen purged tto avoid absorption of VUV light by b ambient ox xygen and watter vapor. The VASE also hhas two light ssources that pass light through a do ouble-chamberr Czerny-Turner monochrom mator to providde wavelengthh selection andd stray-light rrejection. Measuremen nts were taken at angles betw ween 25 and 85 degrees andd energies from m 1.45 to 4.15 eV, which thee energy corresponds to wavelengthss from 299 to 855 8 nm.


Figure 5 plot the measured surface reflectance for the following three samples: (i) conventional GaN with flat surface; (ii) GaN microdomes S1 (D=0.9 μm, H=0.23 μm); and (iii) GaN microdomes S2 (D=1 μm, H=1.5 μm) as a function of the incidence angle and with incidence wavelength λ=500 nm for both P and S polarizations. Measurements were taken with the incidence angle from 250 up to 820. GaN microdome structures show significant reduction of reflectance as compared to that of the conventional flat surface within the whole angle range. In addition, GaN microdomes with higher aspect ratio (for S2) show lower surface reflection as compared to the one with low aspect ratio (for S1). This result agrees well with the simulation results that we performed by using the 3-D FDTD method.

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Figure 5: Measured reflectance of conventional flat GaN surface, GaN microdomes (S1: low aspect ratio; S2: high aspect ratio) as a function of light incidence angle for both (a) P polarization and (b) S polarization (incidence λ=500 nm).

4. SUMMARY In summary, GaN microdomes have been studied as surface antireflection structures for concentrated photovoltaic application. 3-D FDTD method was used to calculate the surface reflectance and transmission as a function of incidence angle and incidence wavelength for both P and S polarized plane wave source. Studies indicate significant reduction of the surface reflection by using the GaN microdomes with optimized diameter and aspect ratio. Experimental demonstration of forming the GaN microdomes with controllable geometrical shape has been demonstrated. By using the self-assembled microsphere lithography method, the uniform long-range GaN microdomes were successfully fabricated. Characterizations of the surface reflectance show good agreement with the simulation results. Further, the GaN microdomes have also been studied for enhancing light extraction efficiency in III-nitride based LEDs [19, 20]. Acknowledgement: The authors acknowledge financial support through start-up funds from Case Western Reserve University, and SDLE Center experimental characterization was funded by the Ohio Third Frontier under Tech 12-004.

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