Dielectric Functions of a-Si 1-x Ge x :H versus Ge Content, Temperature, and Processing: Advances in Optical Function Pa
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0910-A10-01
Dielectric Functions of a-Si1-xGex:H versus Ge Content, Temperature, and Processing: Advances in Optical Function Parameterization Nikolas J. Podraza1, Christopher R. Wronski2, Mark W. Horn2, and Robert W. Collins1 1 Department of Physics and Astronomy, University of Toledo, Toledo, OH, 43606 2 Materials Research Institute, The Pennsylvania State University, University Park, PA, 16802 ABSTRACT We have applied an advanced model to analyze the dielectric functions ε = ε1 + iε2 of amorphous silicon-germanium alloys (a-Si1-xGex:H) (i) as a function of alloy content x 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 ε 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 x 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 ε 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. INTRODUCTION Parameterizations of the optical functions of semiconductors have three important applications [1]. First, they can be used in performance simulations of optoelectronic devices that involve photon collection -- solar cells being an important example. In this case, the effects of deviations in the optical band gaps for the component i-layers on the measured quantum efficiency can be predicted in order to assess fabrication tolerances. Second, parameterizations can be applied in fitting transmittance, reflectance, ellipsometric, or optical quantum efficiency spectra acquired on multilayer stacks that include one or more of the layers. By using parameter databases, one can extract wavelength-independent parameters directly such as thicknesses, compositions (or preferably optical gaps), and even temperature that directly assist in evaluating the processing and material properties of interest in devices. Finally, parameterizations can provide succinct information useful for correlating with other properties for indirect assessment of the
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