1. Many, A., Goldstein, Y., and Grover, N. B., Semiconductor Surfaces, North Holland Publishing Co., Amsterdam (1965).
2. Schrieffer, J. R., Semiconductor Surface Physics, p. 55, R. H. Kingston (ed.), University of Pennsylvania Press, Philadelphia (1957).
3. Fang, F. F., and Howard, W. E., Phys. Rev. Letters 16, 797 (1966).
4. Stern, F., and Howard, W. E., Phys. Rev. 163, 816 (1967).
5. von Klitzing, K., Landwehr, G., and Dorda, G., Solid State Comm. 14, 387 (1974).