Photoreflectance of strained Si 1-xGe x epilayers (0.07≤ x≥0.26) and comparison with spectroscopic ellipsometry

Abstract

Photoreflectance (PR) spectra have been obtained from a range of strained Si 1-xGe x epilayers, 0.065≤ x≤0.255, grown on Si(100) substrates. Fitting third-derivative Lorentzian lineshapes to the spectra yields critical point (CP) transition energies in excellent agreement with those previously obtained using spectroscopic ellipsometry (SE). Although this justifies the use of Aspnes' low-field approximation in evaluating CP energies, a change in the relative oscillator amplitude observed from the PR in the composition range 0.06 < x<0.09 is not seen by SE, and may be due to a polarisation dependence of PR on the interband effective mass of the E 1 + Δ 1 CP transition. Comparison is made between the PR spectra and those obtained as the numerical third derivative of the real and imaginary parts of the dielectric function spectra. The PR spectra could be reproduced from these third derivatives by assuming energy-independent Seraphin terms as fit parameters. Results suggest a large contribution to the PR spectra is from modulation of the imaginary part of the dielectric function. Where the spectral dependence of the Serpahin coefficients in the bulk SiGe alloy is taken into account, PR spectra from samples with low Ge fraction could only be reproduced from the third derivatives of the dielectric function spectra by including Coulomb effects in the simulations. For higher Ge fractions, where epilayers become thinner, the PR spectra could not be reproduced in this way. Modification of the Seraphin terms due to interference effects and/or non-uniform field effects in the alloy layers is proposed as a possible explanation.