Etching mechanisms in III-V semiconductors: Electrochemical etching of indium phosphide

Anodization of n-InP electrodes was carried out over a range of temperatures and KOH concentrations. Scanning electron microscopy showed <111>A aligned pore growth, with decreasing pore width as the temperature was increased. This variation in pore width is explained in terms of the relative rates of electrochemical reaction and hole diffusion and supports the three-step model proposed earlier. As temperature was increased, both the areal density and width of surface pits decreased, resulting in a large increase in the current density through the pits. This explains a corresponding observed decrease in porous layer thickness: smaller pits sustain mass transport for a shorter time before precipitation of etch products blocks the pores. In galvanostatic experiments, radius of curvature at pore tips increased with current density but pore width decreased. As KOH concentration was increased, both pore width and layer thickness decreased to minima at ∼9 mol dm^-3 after which they again increased. A transition from porous layer formation to planar etching was observed below 2 mol dm^-3 KOH. All of these trends are explained by the three-step model and mass transport effects in surface pits.