Micro- and Nano-Integration in the Production of GaAs and Ga2O3 Nanowire Arrays by Top-Down Design

In this paper, a strategy is proposed based on the microstructuring of GaAs substrates by photolithography combined with nanostructuring by electrochemical etching for the purposes of obtaining GaAs nanowire domains in selected regions of the substrate. The micropatterning is based on previously obtained knowledge about the mechanisms of pore growth in GaAs substrates during anodization. According to previous findings, crystallographically oriented pores, or “crysto pores,” grow along specific crystallographic directions within the GaAs substrates, with preferential propagation along the <111>B direction. Taking advantage of this feature, it is proposed to pattern the (111)B surface by photolithography and to, subsequently, apply anodization in an HNO3 electrolyte. It is shown that the areas of the GaAs substrate under the photoresist mask are protected against porosification due to the growth of pores perpendicular to the surface of the substrates in such a configuration. Pores overlapping under adjusted electrochemical etching conditions results in the formation of GaAs nanowire arrays in the substrate regions not covered by photoresist. Thermal annealing conditions in an argon atmosphere with a low oxygen concentration were developed for the selective oxidation of GaAs nanowires, thus producing a wide-bandgap Ga2O3 nanowire pattern on the GaAs substrate. It is shown that the morphology of nanowires can be controlled by adjusting the electrochemical parameters. Smooth-walled nanowire arrays were obtained under specific conditions, while perforated and wall-modulated nanowires were formed when crystallographic pores intersected at a higher applied anodizing potential.