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Ambipolar Phototransport (μτe = μτh) Observed as an Intrinsic Property of a-SiGe:H

Published online by Cambridge University Press:  15 February 2011

Paul Wickboldt
Affiliation:
Lawrence Livermore National Laboratory, Livermore, CA 94550
Dawen Pang
Affiliation:
Division of Applied Sciences, Harvard University, Cambridge, MA 02138
William Paul
Affiliation:
Division of Applied Sciences, Harvard University, Cambridge, MA 02138
Joseph H. Chen
Affiliation:
Department of Physics, Boston College, Chestnut Hill, Boston, MA 02167
Chih-Chiang Chen
Affiliation:
Department of Physics, University of Oregon, Eugene, OR 97403
J. David Cohen
Affiliation:
Department of Physics, University of Oregon, Eugene, OR 97403
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Abstract

A study is presented of a series of high quality PECVD a-Si0.33Ge0.67 films, produced by cathodic deposition, in which small concentrations of PH3, B2H6 or air impurities were added during deposition. The quantum efficiency-mobility-lifetime product (ημτ) increases, and the ambipolar diffusion length (Lamb) decreases monotonically with dopant concentration for both PH3 and B2H6. This result is strong evidence that for these films neither photocarrier is dominant (μτe= μτh) at zero doping. This result is very different from what has been typically observed by other researchers, that the electron is the dominant photocarrier for undoped a-SiGe:H.

Drive level capacitance (DLC) measurements of these alloys show an unusual behavior of being temperature-independent, and the dark conductivity activation energy is maximum for zero doping. It is proposed that all of these unusual properties are due to the unusually low impurity concentration of these films, and that these properties are, in fact, the intrinsic properties for a-SiGe:H alloys. To verify this, films were prepared with a calibrated and controlled air leak introduced during deposition. As the air leak was increased, the film properties changed to typical behavior. Even for air concentrations as low as 2 ppm (gas phase), the transport measurements showed changes consistent with a shift in the Fermi level toward the conduction band.

Type
Research Article
Copyright
Copyright © Materials Research Society 1997

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References

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Ambipolar Phototransport (μτe = μτh) Observed as an Intrinsic Property of a-SiGe:H
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Ambipolar Phototransport (μτe = μτh) Observed as an Intrinsic Property of a-SiGe:H
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