Studies of the DX Centre in Heavily Doped n+GaAs
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, SiGa. 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.
KeywordsCarrier Concentration Pressure Coefficient Donor Level Free Carrier Concentration Conduction Band Minimum
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