Spectroscopic ellipsometry of Si1-xGex multi-quantum wells

Spectroscopic Ellipsometry (SE) has been used to characterise a range of strained Si1-xGex/Si multi-quantum well (MQW) samples (x almost-equal-to 0.2) using recently acquired strained si1-xGex reference dielectric function spectra. SE results for thickness and composition compared well with those from double crystal X-ray diffraction (DCXRD) and cross-sectional transmission electron microscopy (XTEM) for well widths above 50 angstrom. For thinner wells quantum confinement and/or interface effects may be important, invalidating the use of 'bulk' strained Si1-xGex reference dielectric function spectra. Models and procedures appropriate for fitting simulated SE spectra to those measured on MQW samples are discussed in detail. Improvements are seen in fits to some experimental spectra by assuming the period thickness (barrier + well) varies with depth in the MQW.