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2. E. F. Schubert, B. Ullrich, T. D. Harris and J. E. Cunningham, Phys. Rev.B38, 8305 (1988b).
3. E. F. Schubert, T. D. Harris, J. E. Cunningham and W. Jan, Phys. Rev.B39, 11011 (1989c).
4. For a review, see E. F. Schubert inSemiconductors and Semimetals, ed. by R. K. Willardson and A. C. Beer; Vol. ed. A. C. Gossard (Academic Press, New York, 1990) and J. Vac. Sei. Technol. A May/June (1990).
5. S. J. Bass, J. Cryst. Growth47, 613 (1979).