Highly strained GaAs layers have been grown by molecular beam epitaxy
on (100)-, (111)- or (112)-oriented Si or CaF2 substrates. The origin,
sign, bisotropic nature, value, homogeneity and relaxation of the bui
lt-in strain have been studied by Raman spectroscopy. Specific measure
ments have been performed by using selection rules and line shape anal
ysis over a wide temperature range.