Abstract
We have observed optical second harmonic generation in Si1-xGex (x=0.685) crystalline films grown on Si(001) substrates. The substrates are oriented 4 degrees off the [001] direction rotated about the [100] axis. We have measured the SH intensity as a function of the film thickness and the sample rotation angle about the surface normal. The intensity pattern changes as the film thickness is varied from 110 nm to 58 nm. For the film thickness of 110 nm the intensity curve as a function of the rotation angle has three maxima. This angular pattern can be reproduced by a model that includes the effect of interference between the surface and bulk second harmonic amplitudes. For the film thickness of 58 nm the angular pattern has only one maximum. In this case the surface SHG dominates the observed intensity and no interference effect is observed.