PRELIMINARY STUDIES OF GAAS DEPOSITION ON AU(100), (110), AND (111) SURFACES BY ELECTROCHEMICAL ATOMIC LAYER EPITAXY

The development of a new method for epitaxial growth of compound semiconductors is briefly described: electrochemical atomic layer epitaxy (ECALE). ECALE is based on the successive underpotential deposition (UPD) of atomic layers of different elements to form a compound. Preliminary studies of the ECALE deposition of GaAs were performed in a thin-layer electrochemical cell with a polycrystalline Au electrode. Potentials required for oxidative UPD of As and reductive UPD of Ga were evaluated with different solution concentrations and pH's. Subsequent studies were performed in an ultrahigh vacuum surface analysis instrument interfaced to an electrochemical cell. A gold single crystal supporting the three low-index planes was used in these studies. Auger electron spectrocopy (AES) and coulometry were used for surface composition analysis, while low energy electron diffraction (LEED) was used for structural analysis. Arsenic electrodeposition from HAsO2 solutions resulted in a succession of LEED patterns as the coverage increased on each face. Low coverage structures, corresponding to less than one monolayer of As, were observed only on the (100) and (110) faces: a Au(100)(2 x 2)-As at 1/4 coverage and a Au(110)c(2 x 2)-As at 1/2 coverage. Oxidative UPD of As from AsH3 was observed only on the (100) and (110) faces, resulting in low coverage ordered structures analogous to those formed during deposition from HAsO2 at low As coverages. Ga UPD from Ga2(SO4)3 solutions was observed on all three faces, although it resulted in disordered layers of Ga oxide upon removal of the substrate from solution, due to partial oxidation of the Ga in contact with water in the absence of potential control. UPD of Ga on As-covered surfaces (Au(100)(2 x 2)-As and A(110)c(2 x 2)-As) resulted in stoichiometric coverages of Ga and As on both. Structures displaying (2 x 2) and c(2 x 2) LEED patterns were formed on the (100) and (110) faces, respectively. The stability of GaAs in aqueous solutions was evaluated. Potentials vs. pH plots were calculated for several concentrations of dissolved Ga and As species.