Reversible Electrical-Properties of LEC GaAs

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Undoped, low-pressure, liquid-encapsulated Czochralski GaAs can be reversibly changed from conducting (ρ ∼ 1Ω-cm) to semi-insulating (ρ ∼ 107Ω-cm) by either slow or fast cooling, respectively, after a 5 hr, 950° C soak in an evacuated quartz ampoule. The semi-insulating wafers are very uniform and lead to tight threshold-voltage control in direct-implant MESFET’s. We have studied crystals in both states by temperature-dependent Hall effect, photoluminescence, IR absorption, mass spectroscopy, and DLTS. It is shown that donor and acceptor concentrations are typically more than an order of magnitude greater than the C and Si concentrations, which are both less than 3 × 1014 cm−3. The EL2 concentration remains relatively constant at about 1.0 ×1016 cm−3. Thus, the normal EL2-Si-C compensation model does not apply. The most likely explanation for the reversibility involves a delicate balance between native-defect donors and acceptors in equilibrium at 950° C, but with the donors dominating after a slow cool, and the acceptors after a fast cool. A consistent model includes a dominant donor at Ec 0.13eV, probably VAs – AsGa, and a dominant acceptor at Ev + 0.07eV, probably VGa GaAs. In this model, vacancy motion is very important during the slow cool. Such processes must be strongly considered in the growth of bulk, high-purity GaAs.



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