High Pressure Studies of Impurities in Semiconductors

Abstract

For many years it has been known that those common semiconducting compounds with their lowest conduction band minimum located at the centre of the Brillouin zone (eg GaAs, GaSb, CdTe) have levels resonant with the Γ conduction band in addition to the normal Γ-associated shallow donors (see the review by Paul, 1968). These resonant donor levels can be made to emerge into the forbidden gap by the application of hydrostatic pressure. A sufficiently large pressure will cause a realignment of the bands where the Γ-minimum crosses the L-or X-extrema. The resonant levels emerge into the gap at pressures substantially less than those required to cause band-crossing. Initially this was thought to be simply due to the larger effective masses in the higher extrema with each level being rigidly tied to each critical point in the band structure. This view was supported by the similarity of the pressure coefficients for the activation energies of the impurities observed at high pressures with those found for the band extrema. However, the multiplicity of levels found and the realisation that the positions of many of the levels with respect to the higher order band minima was much greater than the respective effective-mass binding energies showed that the resonant levels are in fact ‘deep’ and are made up of admixtures of states taken from all over the Brillouin zone.