1. R.C. Chittick, J.H. Alexander and H.F. Sterling, J. Electrochem. Soc. 116: 77 (1969).
2. N.F. Mott, Adv. Phys. 16:49 (1967). N.F. Mott, Philos. Mag. 19:835 (1969).
3. W.E. Spear and P.G. LeComber, Solid State Commun. 17:1193 (1975). W.E. Spear and P.G. LeComber, Philos. Mag. 33: 935 (1976).
4. M.H. Brodsky and R.S. Title, Phys. Rev. Lett. 23:581 (1969).
5. A. Triska, D. Dennison, H. Fritzsche, Bull.Am.Phys.Soc. 20: 392 (1975).