1. Tyler, W.W., J. Phys. Chem. Solids, 8, 1959, 59.
2. Pearton, S.J., Solid State Electronics, 25, 1982, 499.
3. Osip'yan, Y.A., Propenko, V.M. and Tal'yanskii, V.I., JETP Lett., 39, 1984, 149.
4. Grimmeiss, H., Phys. Rev. B, 37, 1988, 6916.
5. Janzen, E., Stedman, R. Grossmann, G. and Grimmeiss, H.G., Phys. Rev. B, 29, 1984, 1907.