SOME OPTICAL AND ELECTRON-MICROSCOPE COMPARATIVE-STUDIES OF EXCIMER LASER-ASSISTED AND NONASSISTED MOLECULAR-BEAM EPITAXIALLY GROWN THIN GAAS FILMS ON SI

We present results of Raman and Rayleigh scattering, near-band-edge photoluminescence, and electron microscope examination of GaAs thin films (<0.3 μm) grown on Si(100)+4°〈011〉 substrates via molecular-beam epitaxy with part of the 2-in. wafer under KrF excimer laser irradiation. Comparative examination of laser-assisted and nonassisted growth areas on the same wafer reveals differences in the nature and degree of strain and structural defects. It is found that the Rayleigh scattering intensity is higher and the longitudinal-optical mode Raman peak is broader in the non-laser-irradiated region indicating a higher degree of disorder. The luminescence is higher from the laser-irradiated region. These differences between the non-laser- and laser-irradiated regions on the same wafer are partially caused by a difference in the film thickness of the two regions. Near-edge photoluminescence peak position reflects the presence of compressive stress in these thin GaAs films (both in the non-laser- and laser-irradiated areas), indicating incomplete strain relaxation by dislocations even though the thickness is significantly higher than the thermodynamic ground-state estimates of the critical thickness. Transmission electron microscope studies on the laser-irradiated and nonirradiated areas reveal no discernible differences in the density of dislocations in the thin films studied. However, representative samples from both areas cut perpendicular to the tilt axis about which the substrate was tilted away from a perfect (100) surface exhibit an asymmetry in the distribution of the twins among the two {111} variants which have the tilt axis as the common zone axis. This asymmetry has been observed only for the twins extending from the film-substrate interface to the film surface. A correlation between such twins and the substrate step orientation is established.