Electrochemical nanostructuring of n-GaAs photoelectrodes.

Methods to simultaneously optimize carrier collection and light in-coupling in semiconductors are important for developing low-cost, high-efficiency photovoltaics and photoelectrodes. We anodically etched nanostructures into planar (100) n-GaAs wafers with different bulk minority carrier diffusion lengths L(D). The structures were varied by changing the anodization parameters. A ferrocene/ferrocenium electrolyte provided a conformal rectifying contact to the anodized n-GaAs and enabled the measurement of carrier generation and collection as a function of nanostructure geometry and L(D). Internal quantum efficiency Φ(int) of photoelectrodes varied with nanostructure geometry and L(D). External quantum efficiency Φ(ext) also depended on the reflectance of the nanostructured GaAs-electrolyte interface. Reflectance was minimized using anodization current densities of 100-150 mA cm(-2), which etched subwavelength trigonal prismatic nanostructures ~400 nm in width at their base. For Si-doped n-GaAs with L(D) = 170 nm, peak Φ(ext) of ~75% and Φ(int) of ~85% was achieved using J(anod) = 150 mA cm(-2). The control of both surface nanostructure (to minimize reflection) and pore depth and spacing (to optimize 3D carrier collection) via two-step anodization yielded photoelectrodes with peak Φ(ext) of ~85% and peak Φ(int) of ~95% for Te-doped n-GaAs with a bulk L(D) of only 420 nm. The measured short-circuit current densities for the nanostructured photoelectrodes were up to 2.5 times that of planar controls, demonstrating that appropriate nanostructuring significantly improves carrier collection even for direct bandgap materials with large absorption coefficients like GaAs.