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2. For work on a-Ge:H see A. J. Lewis, G. A. N. Connell, W. Paul, J. R. Pawlik and R. J. Temkin, inTetrahedrally Coordinated Amorphous Semiconductors, edited by Brodsky, Kirkpatrick and Weaire (AIP No. 20, New York, 1974), p. 27; G. A. N. Connell and J. R. Pawlik, Phys. Rev.B13, 787 (1976);
3. A. J. Lewis, Phys. Rev.B14, 658 (1976);
4. T. D. Moustakas and G. A. N. Connell, J. Appl. Phys.47, 1322 (1976);
5. T. D. Moustakas and W. Paul, Phys. Rev.B16, 1564 (1977).