Undoped Semi-Insulating GaAs of Very Low Residual Acceptor Concentration

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Abstract
We have studied undoped GaAs of shallow acceptor concentrations close to and below 5·1014cm-3. After ingot annealing with slow cooling rate this material shows resistivity values below 107 Ωcm, spanning several orders of magnitude. By an additional annealing procedure these values can be raised to some 107 Ωcm. DLTS results are presented indicating that the electrical characteristics in the ingot annealed state are governed by uncompensated deep donors shallower than EL2, mainly EL6 near E c -0.35 eV. After the additional annealing a drastic decrease in EL6 concentration is observed, which explains the rise in resistivity. This is confirmed by theoretical calculation of the resistivity in dependence of the EL2 and EL6 to net acceptor compensation ratio, which agrees quantitatively with the experimental values.