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States in the Gap of Improved a-Ge:H Studied by Photomodulation Spectroscopy

Published online by Cambridge University Press:  21 February 2011

L. Chen
Affiliation:
Division of Engineering and Department of Physics, Brown University, Providence, RI02912
J. Tauc
Affiliation:
Division of Engineering and Department of Physics, Brown University, Providence, RI02912
D. Pang
Affiliation:
Division of Applied Sciences, Harvard University, Cambridge, MA 02138
W. A. Turner
Affiliation:
Division of Applied Sciences, Harvard University, Cambridge, MA 02138
W. Paul
Affiliation:
Division of Applied Sciences, Harvard University, Cambridge, MA 02138
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Abstract

The photomodulation spectra of a-Ge:H of average photoelectronic quality(ημπ = 1 × 10-10cm2/V) and of improved quality (ημπ = 3 × 10-7cm2/V), produced under different plasma conditions in an r.f. diode reactor by glow discharge, were measured at 80K and are analyzed in analogy with earlier studies of a-Si:H. The spectra of the poorer material are dominated by transitions between dangling bond states and the conduction and valence bands. By contrast, the spectra of the better material require contributions of transitions from the band tail states, indicating that the reduced defect density has resulted in pump-beam induced quasi-Fermi levels reaching near the conduction and valence band edges. A very acceptable fit between plausible density-of-states distributions and the experimental spectra has been found.

Type
Research Article
Copyright
Copyright © Materials Research Society 1991

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References

REFERENCES

1. See, for example, Li, Y.M., Turner, W.A., Lee, C. and Paul, W., Proc. Materials Research Society Spring Meeting, San Diego, Vol. 149 (MRL, Pittsburgh, 1989) p. 187, and other papers in the same volume.Google Scholar
2. See for example, Turner, W.A., Jones, S.J., Pang, D., Bateman, B.F., Chen, J.H., Li, Y.M., Marques, F.C., Wetsel, A.E., Wickboldt, P., Paul, W., Bodart, J., Norberg, R.E., El Zawawi, I. and Theye, M.L., J. Appl. Phys. 62, 7430 (1990). Similar improved glow-discharge material has been described byCrossRefGoogle Scholar
Karg, F.H., Bohm, H. and Pierz, K., J. Non-Cryst. Solids 114, 447 (1989), and byCrossRefGoogle Scholar
Godet, C., El Zawawi, I., Theye, M.L. Gauthier, M. and Stoquert, J.P., Solid State Commun. 74, 721 (1990).CrossRefGoogle Scholar
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5. Chen, L., Tauc, J., Lee, J.-K. and Schiff, E.A., Phys. Rev. B43, May 15, 1991.Google Scholar
6. Zhou, T.X., PhD. Thesis, Brown University (1989).Google Scholar
7. Stoddart, H.A., Vardeny, Z. and Tauc, J., Phys. Rev. B38, 1362 (1988).CrossRefGoogle Scholar
8. Vardeny, Z., Zhou, T.X., Stoddart, H.A. and Tauc, J., Sol. State Commun. 65, 1049 (1988).CrossRefGoogle Scholar
9. From this work one can not decide whether one has PM transitions involving the tail of the c.b. or v.b tail or both. In a-Si:H there is evidence [10] that the low temperature PA transitions are associated with the v.b‥ We arbitrarily assumed that this is the case also for a-Ge:H.Google Scholar
10. Morigaki, K. and Kondo, M., Review of Solid State Science 4, 663 (1990).Google Scholar
11. Simmons, J.G. and Taylor, G.W., Phys. Rev. B4, 502 (1971).CrossRefGoogle Scholar

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