Detailed investigations of strain generation and relaxation in Si films grown on thin Si0.78Ge0.22 virtual substrates using Raman spectroscopy are presented. Good virtual substrate relaxation (>90%) is achieved by incorporating C during the initial growth stage. The robustness of the strained layers to relaxation is studied following high temperature rapid thermal annealing typical of CMOS processing (800-1050 °C). The impact of strained layer thickness on thermal stability is also investigated. Strain in layers below the critical thickness did not relax following any thermal treatments. However for layers above the critical thickness the annealing temperature at which the onset of strain relaxation occurred appeared to decrease with increasing layer thickness. Strain in Si layers grown on thin and thick virtual substrates having identical Ge composition and epilayer thickness has been compared. Relaxation through the introduction of defects has been assessed through preferential defect etching in order to verify the trends observed. Raman signals have been analysed by calibrated deconvolution and curve-fitting of the spectra peaks. Raman spectroscopy has also been used to study epitaxial layer thickness and the impact of Ge out-diffusion during processing. Improved device performance and reduced self-heating effects are demonstrated in thin virtual substrate devices when fabricated using strained layers below the critical thickness. The results suggest that thin virtual substrates offer great promise for enhancing the performance of a wide range of strained Si devices.