Substrate Strain Measurements in Mbe Grown Silicon on Sapphire

An x-ray diffraction study is made of the effect of silicon film thickness on the peak broadening associated with the (024) sapphire Bragg reflection peak for MBE—grown SOS material. The predeposition treatment and growth parameters for the samples in the series were identical. The thicknesses ranged from 5500 to 40,000 angstroms and the growth rate was roughly I angstrom/second.SOS films with silicon thicknesses below 5500A did not produce sapphire peak broadening when compared with the corresponding sapphire standard. With reference to standard sapphire reflection peaks, the 5500A film shows slight broadening while a I micron film broadened 14 seconds of arc when compared at FWHM with the sapphire standard. A possible explanation for this is that at overlayers less than 5500A, the film is not sufficiently massive to exert a uniform strain on the sapphire. At 5500A the film attains a critical thickness sufficient to strain the substrate and produce detectable broadening. For thickerfilms the strain would increase proportionally. Results from lattice parameter measurements on the silicon epilayer indicate that the perpendicular lattice parameter relaxes atgreater thicknesses but is still in tension when compared with bulk silicon. One would expect that the strain gradient suggested by these results would be greatest at the silicon/sapphire interface and that the defect concentration at the interface might beaffected by this strained condition.