By separating a 2-pm-thick molecular-beam-epitaxial GaAs layer grown at 200 "C from its 650~pm-thick substrate, we have been able to obtain accurate Hall-effect and conductivity data as functions of annealing temperature from 300 to 600 "C. At a measurement temperature of 300 K, analysis confirms that hopping conduction is much stronger than band conduction for all annealing temperatures. However, at higher measurement temperatures (up to 500 K), the band conduction becomes comparable, and a detailed analysis yields the donor and acceptor concentrations and the donor activation energy. Also, an independent absorption study yields the total and charged AsGa concentrations. Comparisons of all of these quantities as a function of annealing temperature T, show a new feature of the annealing dynamics, namely, that the dominant acceptor (probably Vo, related) strongly decreases and then increases as TA is increased from 350 to 450 "C. Above 450 "C, ND, NA , and [Asod all decrease, as is known from previous studies. 306
Absorption measurements at 1.1 and 1.2 μm were used along with the known electron and hole photoionization cross sections for EL2 to determine deep donor (EL2-like) and acceptor concentrations ND=9.9×1019 and NA=7.9×1018 cm−3, respectively, in a 2-μm-thick molecular-beam epitaxial GaAs layer grown at 200 °C on a 2-in.-diam semi-insulating wafer. Both lateral and depth uniformities of ND over the wafer were excellent as was also the case for the conductivity. Band conduction was negligible compared to hopping conduction at 296 K as evidenced by the lack of a measurable Hall coefficient.
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