MICROSTRUCTURE IN MOLECULAR-BEAM-EPITAXY-GROWN SI/GE SHORT-PERIOD STRAINED-LAYER SUPERLATTICES

The microstructures of Si12/Ge4 and Si18/Ge6 Short-period strained-layer superlattices (SLS's) on a Si substrate were observed using electron microscopes. These SLS's were fabricated by the phase-locked-epitaxy technique in molecular-beam epitaxy, and were reported to show strong optical transitions [Okumara et al., Jpn. J. Appl. Phys. 28, L1893 (1989); Mater. Sci. Eng. B 9, 245 (1991)]. Flat SLS layers were observed in the region near the Si substrate. After the growth of about ten layers, the cross-sectional image showed gradual wavy layers in the Si18/Ge6 SLS. The layers in the Si12/Ge4 SLS remained relatively flat. A high-resolution micrograph showed flat interfaces for the Ge layers on the Si layers. However, interfaces for the Si layers on the Ge layers were neither flat nor clear. The abruptness at the Si/Ge heterointerfaces is discussed by analyzing the intensity of satellites in the electron-diffraction pattern. Dislocations were observed in the Si substrate at the interface to the superlattice layers, while no dislocations were observed in the SLS. In the diffraction pattern, splitting of fundamental spots due to the difference in lattice parameters between the SLS and the substrate was observed. The swell of the unit cell in the SLS's along a and b axes was recognized to be due to the presence of dislocations in the Si substrate near the interface. From this study the origin of the luminescence is discussed.