Dielectric Functions of a-Si_1-xGe_x:H versus Ge Content, Temperature, and Processing: Advances in Optical Function Parameterization

Abstract

We have applied an advanced model to analyze the dielectric functions ε = ε₁ + iε₂ of amorphous silicon-germanium alloys (a-Si_1-xGe_x: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 T_m 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 T_m 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.