Structural Characterization of Multilayered Epitaxial Films: a Study of Substrate Orientation Influences

The structural quality of epitaxial multilayered semiconductor films grown using molecular beam epitaxy (MBE) on substrates oriented in various directions, including (311), (511) and vicinal (100), has been investigated. Measurements were performed using a high resolution five-crystal x-ray diffractometer and transmission electron microscopy (TEM). Theoretical calculations of both x-ray and TEM diffraction for single layer and superlattice structures have been performed to explain experimental results. In the study of nearly lattice-matched GaAs/AlAs superlattices grown at 600^circC, good crystallinity can be achieved in both (311) and (100) samples. The relative intensities of the satellites have been compared with theoretical results to study the interface abruptness. The experimental results show that the (311) superlattices reveal a superior interface abruptness. The meandering of surface steps is proposed as a mechanism to interpret inferior interface profiles in (100) superlattices. Samples of GaAs epilayers grown on (311) and (511) Si are prepared to study the influence of substrate orientation in large lattice mismatch systems. Investigation of these samples reveals that the dislocation density is not significantly affected by substrate orientation. This indicates that there is a limitation on the strain accommodated by misfit dislocations, and thus the reduction of the dislocation density is restricted. No evidence of antiphase boundaries (APBs) is found in these polar-on-nonpolar growths. It is believed that the appearance of both single and double dangling bond sites on the (m11) surface could guide the deposited atoms to the proper lattice sites and thus suppress the formation of APBs. Since APBs could be suppressed by self-elimination, polar-on-nonpolar growth is further studied in Ga _{1-x}Al_{x} As/Ge superlattices. Again, no evidence of APBs is observed in the (m11) samples. Further, stacking faults and/or microtwins found in this system further indicate that the specific characteristics of the materials and growth conditions employed have significant influence on the structural quality of the epitaxial films.