Studies of the DX Centre in Heavily Doped n⁺GaAs

Abstract

The effect of hydrostatic pressure on the electrical properties of MBE-grown GaAs layers heavily doped in Si with Sn is described. It is shown how these measurements provide fundamental information about the DX centre in n GaAs. Shubnikov-de Haas measurements show that as increasing pressure is applied, electrons are trapped out from the Γ conduction band minimum of GaAs into the localised DX states, with an accompanying increase in carrier mobility. Optical illumination at T≲100K causes persistent photoconductivity in which free carriers are released back into the conduction band with an accompanying fall in mobility. The results show that the DX centre produces a resonant donor level between the Γ- and L- conduction band minima at a concentration comparable with the doping level. The energy and occupancy of the DX level are calculated using Fermi-Dirac statistics. For the Si-doped samples, comparison with local vibrational mode measurements indicate that the DX level can be identified with the simple substitional donor, Si_Ga. In the heaviest doped samples (n ≳ 1 × 10¹⁹ cm^-3) we conclude that the DX level is partially occupied at 300 K and atmospheric pressure, thus acting to limit the free carrier concentration. Our results are discussed in terms of other recent work on the DX centre in heavily doped GaAs.