Mixed Conduction in Cr-Doped GaAs

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Hall-effect and magnetoresistance measurements have been carried out in GaAs : Cr as functions of magnetic field strength (B = 0–18kG) and temperature (T = 125–420°K). Independent solutions for the mobilities, μn and μp, and the carrier concentrations, n and p, are obtained from the basic mixed-conductivity equations. These quantities, as well as the intrinsic carrier concentration, ni are then calculated as a function of temperature for one sample, and subsequent analysis yields the following values in the range T = 360–420°K: an acceptor (presumably Cr) energy EA = 0.69±0.02eV (from the valence band); the bandgap energy Eg = Eg0 + αT, with Ego = 1.48±0.02eV, α ≅ 3.2 x 10-4eV/°K; μn = 2700±100 cm2/V sec, decreasing slightly with temperature; = 350± 50 cm2/V sec; and an acceptor-to-donor concentration ratio, NA/ND≅8. The electron mobility appears to be limited by neutral impurity scattering, with NA ≅ 2 × 1016cm−3. Several other samples were also investigated but as a function of temperature only (at B = 0). At room temperature both positive (p-type) and negative (n-type) Hall coefficients were observed.