Surface Chemistry of III-V Semiconductors After Wet Etching with HCl and H2O2

The introduction of III-V semiconductors in MOSFET technology has been hindered by the lack of good quality III-V oxides. These oxides are a source of surface states at the band edges or within the band gap that are a detriment to electronic performance. The removal of these oxides is typically accomplished by immersion in acid or basic aqueous solutions. The continuous etching of the substrate is achieved by adding an oxidizing agent. After etching, a smooth oxide-free surface is desired. HCl and H₂O₂ mixtures have been used for the etching of III-V materials.

In this work, we perform a comparative study of the etching of the (100) crystal plane of GaAs, InAs, InP, GaSb, and InSb. The selective etching of these materials was studied as a function of H₂O₂ concentration (0.0001-5 M) and at a fixed HCl concentration (0.01 M). The etching rate was determined by profilometry measurements made on wafers patterned with conventional photolithography. The chemical composition of the surfaces after etching was studied with X-ray photoelectron spectroscopy (XPS) and the final roughness was characterized with atomic force microscopy (AFM). The etching rate of GaAs and InAs (same group V atom) showed a volcano-shaped dependence on H₂O₂ concentration (0.01 to 1 M). A maximum etching rate was observed at 0.1 M with a value of 1.1±0.1 nm/s for GaAs and 0.9±0.3 nm/s for InAs. For 1 M H₂O₂, the etching rate decreased to 0.035±0.010 nm/s for GaAs and 0.26±0.13 nm/s for InAs. The etching rate of InP increased when the H₂O₂ concentration was changed from 0.01 to 0.1 M, and showed no significant increase for higher concentrations within the error of the measurements. The etching selectivity of GaAs to InP increased from about 55 at 0.01 M H₂O₂ to 137 at 0.1 M H₂O₂, and then decreased to 4 at 1 M H₂O₂. The etching rate of GaSb and InSb (same group V atom) increased linearly between 0.0001 and 0.001 M H₂O₂, but dropped significantly at higher concentrations. These substrates were visibly oxidized at high H₂O₂ concentrations and HCl etching was unable to keep up with the rate of oxidation. Substrates with the same group V atom exhibited similar etching trends in the range of H₂O₂ and HCl concentrations studied. This suggests that the removal of the group V atom is the rate limiting step in the etching of these materials. Temperature programmed desorption (TPD) of a GaAs sample after immersion in HCl showed that the surface was terminated by As-Cl bonds. These results along with the chemical composition of the samples after etching will be used to propose reaction mechanisms of III-V semiconductors in HCl and H₂O₂ mixtures.