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Dielectric Functions of a-Si1-xGex:H versus Ge Content, Temperature, and Processing: Advances in Optical Function Parameterization

Published online by Cambridge University Press:  01 February 2011

Nikolas J. Podraza ,
Christopher R. Wronski ,
Mark W. Horn  and
Robert W. Collins
Nikolas J. Podraza
Affiliation:
[email protected], University of Toledo
Christopher R. Wronski
Affiliation:
[email protected], The Pennsylvania State University, Materials Research Institute, University Park, PA, 16802, United States
Mark W. Horn
Affiliation:
[email protected], The Pennsylvania State University, Materials Research Institute, University Park, PA, 16802, United States
Robert W. Collins
Affiliation:
[email protected], University of Toledo, Department of Physics and Astronomy, Toledo, OH, 43606, United States
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Abstract

We have applied an advanced model to analyze the dielectric functions e = e1 + ie2 of amorphous silicon-germanium alloys (a-Si1-xGex:H) (i) as a function of alloy content × by varying the flow ratio G = [GeH4]/{[SiH4]+[GeH4]} in plasma-enhanced chemical vapor deposition (PECVD), and (ii) for the first time as a function of the measurement temperature Tm by cooling the newly-deposited film. All e spectra (1.5 − 4.5 eV) have been measured by spectroscopic ellipsometry (SE) either in real time during deposition or in situ post-deposition in order to avoid surface contamination. From the resulting extensive database, the optical properties of the alloys can be predicted for any value × and Tm within the ranges of the database. Such a capability is expected to be useful, for example, in real time control of optical gap in the PECVD process and in predicting the quantum efficiency of multijunction a-Si:H-based solar cells versus operating temperature. The effect on the database of other deposition parameters such as the electrode configuration and the H2-dilution ratio R = [H2]/{[SiH4]+ [GeH4]} have also been explored. The latter two studies provide useful insights into materials properties that can be extracted from a single spectroscopic measurement performed in real time during PECVD. For example, the energy width of the resonance in e correlates closely with the precursor surface diffusion characteristics observed throughout growth -- both determined from real time SE. This result indicates that short-range ordering in the film is improved when surface diffusion is promoted.

Keywords

photovoltaicSiGe
Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 910: Symposium A – Amorphous and Polycrystalline Thin-Film Silicon Science and Technology – 2006 , 2006 , 0910-A10-01
Copyright
Copyright © Materials Research Society 2006

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