In-plane and out-of-plane mass transport during metal-assisted chemical etching of GaAs
We have demonstrated the dependence of the metal-assisted chemical etching of GaAs on catalyst thickness. For ultra-thin (3–10 nm) Au catalysts, we found that the etch rate was significantly enhanced, an unexpected phenomenon in light of the conventional mechanism. Numerous pinholes in the metal catalyst are postulated to enable out-of-plane mass transport of reactants and products across the catalyst-covered GaAs. When this process is dominant, the GaAs etch rate is facilitated and an anisotropic profile is formed. With thicker (>15 nm) Au catalysts, the conventionally known in-plane mass transport becomes dominant and lowers the etch rate with an isotropic profile. To our knowledge, this is the first report that experimentally verifies the vertical mass transport during metal-assisted chemical etching of semiconductors. Metal-assisted chemical etching of GaAs with controlled metal catalyst thickness suggests that this technique is more attractive and useful for a wide range of practical applications.