Aligned Mesopore Arrays in GaN by Anodic Etching and Photoelectrochemical Surface Etching Mark J. Schwab, † Danti Chen, ‡ Jung Han, ‡ and Lisa D. Pfefferle* ,† † Department of Chemical and Environmental Engineering, Yale University, P.O. Box 208286, New Haven, Connecticut 06520-8286, United States ‡ Department of Electrical Engineering, Yale University, P.O. Box 208267, New Haven, Connecticut 06520-8267, United States ABSTRACT: Aligned mesopore arrays have many potential applications in bulk heterojunction photovoltaics, sensors, and photocatalysts. In this study, vertically aligned mesopores in n- doped GaN thin films were fabricated using an anodic etch procedure in nitric acid. The resulting porous structure remains monocrystalline and is highly conductive. After etching, a low-porosity nucleation layer was observed on the surface, and was subsequently removed with a UV-assisted anodic etch to expose the pores. Removal of the surface layer allows diffusion from the pores, suitable for sensitized photovoltaics and chemical sensors. Extremely fast etching rates of over 2 μm/s were recorded, with indication that hole transport in the epilayer was limiting the reaction rate even in highly doped samples. 1. INTRODUCTION One-dimensional nanostructures of GaN have generated great interest in recent years as substrates for UV, blue, and white LEDs. 1-3 Light extraction is improved by engineering a nanostructured surface, 4,5 and broadband-emitting quantum dots can be easily integrated into a porous nanostructure for white LEDs. 6 This structure also poses advantages for ordered bulk heterojunction photovoltatics due to its low resistivity, high electron affinity, and stability to ionizing radiation 7 and chemical reaction. 8 Additionally, a porous structure allows tuning of the effective index of refraction for photonic devices by manipulating the porosity, 9 and can act as a strain-relaxed substrate for epitaxy. 10-12 We demonstrate large-scale alignment of vertical mesopores in GaN. Mesopore arrays benefit from a high surface-area-to- volume ratio, which is important for such applications as capacitive chemical sensors 13 and bulk heterojunction photo- voltaics. Straight, nonbranching pores are the most beneficial for these applications due to their low barrier for diffusion and minimized tortuosity of mass- and charge-transfer pathways, thereby speeding the response time of sensors and reducing the electrical resistance of excitonic solar cells. Conductive nanopore arrays can also serve as templates for the electro- chemical deposition of metallic nanotubes. 14,15 However, the electrochemical route for the fabrication of these pore arrays typically leaves a low-porosity layer on the surface which acts as a bottleneck for diffusion into the pores. In this work, we discuss the methods and mechanisms behind pore alignment, and demonstrate a method to selectively remove the surface layer, which is often called the nucleation layer because it serves as the initial site for pore formation. Ordered nanopore arrays in silicon have been an active field since the discovery of enhanced luminescence from silicon upon porosification. 16 Backside illumination has shown great effectiveness in forming self-ordered silicon macropore arrays, 17 due to the tendency of the space charge region (SCR) surrounding pore tips to focus mobile charges onto the pore tips, where the curvature and electric field are highest. 17 However, backside illumination is unsuitable for direct band gap materials, including GaN, because the faster rate of recombination limits exciton diffusion lengths to distances far below that required for holes to diffuse from the illuminated backside to the etching front. 17 Anodic etching uses a similar mechanism, in which mobile charges are focused onto growing pore tips, for the creation of aligned pores in direct-gap semiconductors. 17 While many pore morphologies have been achieved in other III-V semiconductors, including vertically aligned pores, 17 the first demonstration of vertical alignment of GaN mesopores was recently given by Han et al. by using electrochemical etching with hydrofluoric acid and a careful choice of anode potential and doping density. 18 Of utmost importance in alignment of etch pores is overlap of the SCRs generated by nearest-neighbor pores. 18,19 Overlap of SCRs inhibits lateral etching by causing the pore walls to be highly resistive, cutting off the charge extraction pathways. 19 In the absence of overlapping SCRs, breakdown of the pore walls causes lateral etching. 19 When the distance between surface pores is much greater than the thickness of the SCR, etching occurs radially from the surface pores. Each surface pore generates a 2-D “domain” of pores beneath the surface that branch radially, giving it an appearance similar to a Voronoi diagram, in which the area is split into a number of domains Received: February 22, 2013 Revised: July 23, 2013 Published: July 24, 2013 Article pubs.acs.org/JPCC © 2013 American Chemical Society 16890 dx.doi.org/10.1021/jp401890d | J. Phys. Chem. C 2013, 117, 16890-16895