Resonant Raman Scattering at Indium-Arsenide Interfaces.

An investigation of semiconductor surface space charge regions at InAs surfaces in air and in metal-oxide -semiconductor (MOS) junctions was carried out, using resonant Raman scattering at the E(,1) energy gap of InAs by longitudinal optical (LO) phonons and their coupled modes with charge carrier excitations. Backscattering data were obtained using laser excitation frequencies in the vicinity of the E(,1) gap where LO phonon scattering mechanisms, i.e., "allowed" deformation potential and electro-optic scattering mechanisms, and "forbidden" electric field-induced and wave-vector dependent scattering mechanisms, are strongly enhanced. Polarization selection rules were used, in the case of (100) surfaces, to distinguish the allowed and forbidden scattering mechanisms and, in ths case of (111) surfaces, to observe their interference. The results are briefly summarized in the following. The (100) surfaces of n-InAs in air are accumulated. Allowed scattering spectra for (100) surfaces of high density n -InAs exhibit broad structures which correspond to scattering by coupled LO phonon-bulk plasmon modes having a wide range of wave-vectors. The absence of the broad structures in the forbidden scattering spectra indicates that the wave -vector dependent forbidden scatterings by coupled LO phonon -bulk plasmon modes are weak. The A(111) and B(111) surfaces of n-InAs in air are depleted. The(' ). difference in character of the A(111) and the B(111) surfaces is(' ). ascertained from the nature and strength of interference between the "allowed" and surface space charge field-induced "forbidden" scatterings. We have established, among other things, the fields at A surfaces of n- and p-InAs in air point in the same direction, but opposite to the fields at B surfaces. In unbiased MOS junctions, the n-InAs surfaces are strongly accumulated and the p-InAs surfaces are strongly inverted. Therefore, the surface of InAs in air and the interface of InAs with pyrolytic SiO(,2) are quite different. From the InAs surfaces in zero and positive gate bias MOS junctions, where surface electrons form a quasi -two-dimensional plasma, we have observed, for the first time, scattering via LO phonon scattering mechanisms by coupled LO phonon-collective inter-subband excitations. From our data, we provide a value for the static dielectric response from the inter-subband transitions, with which future theoretical investigation of the subband structures at the InAs surfaces can have a comparison. Therefore, in this investigation of InAs surfaces in air and in MOS junctions, resonant Raman scattering at the E(,1) gap by modes which have nonzero macroscopic electric fields has shown its great effectiveness in obtaining information about semiconductor surface space charge regions.