The use of hydrostatic pressure and alloying to introduce deep levels in the forbidden gap of InSb and GaAS

Abstract

In both GaAs and InSb common residual contaminants introduce both shallow donor states and other levels normally resonant with the conduction band but taking their predominant character from higher conduction band minima. These states can be made to emerge into the forbidden gap by applying hydrostatic pressure. Judged by the measured pressure coefficients two sets of levels are closely tied to the L- and X-conduction band minima. At low temperatures the X-associated states show local relaxation effects and metastability (i.e., D(X) behaviour). Following on these experiments with bulk material, pressure experiments were extended to spike-doped GaAs both in single layer and in superlattice form. These experiments were in collaboration with A. Zrenner and F. Koch of the Technical University at Munich. Shubnikov-de Haas peaks can be observed above 3T. Fourier analysis of these shows that up to six sub-bands can be occupied. The application of hydrostatic pressure of up to 20kbar dramatically increases the mobility of the electrons in the lowest sub-band by as much as an order of magnitude with smaller increases occurring the higher sub-bands. The increases in mobility arise from the diffusion of the silicon donors during growth. Localised donor states associated with the L-conduction band drop below the Fermi energy in the centre of diffused distribution of donors on the application of pressure and become neutralised. This reduction of ionised scattering centres affects the most confined electron states and thus the mobility of the lowest sub-band increases very substantially.