In MBE Si1−xGex which is grown to thicknesses greater than the critical thickness hc
, the dislocation-related luminescence peaks Dl and D2 have energies which are independent of x up to x ≈ 0.3, and then decrease, as observed in LPE Si1−xGex. In MBE Si1−xGex layers grown to thicknesses less than hc
, post-growth annealing produces dramatic changes in the luminescence, giving spectra as from relaxed alloy, even though the relaxation determined by X-rays is negligible. These results establish photoluminescence as a sensitive diagnostic tool for detecting dislocations in Si1−xGe*.