TEM Investigation of Epitaxial Growth of Semiconductor Superlattices on Structured Substrates

In exploring new techniques to produce controllable lateral variations in the physical properties of semiconductor superiattices, substrates containing etched patterns were used for epitaxial crystal growth. High resolution TEM was used for the first time to study this kind of structures. It was found that the resultant superlattices exhibit wavy or zigzag layers which register the history of the topographic evolution of the crystal surfaces during growth and reveal useful information on the fundamental mechanism of the epitaxial process. Alternate layers of Al0.3Ga0.7As and GaAs were grown by MBE and OMCVD on (100) GaAs substrates patterned with grooves and mesas along the [011] and [011] directions. The morphology of the superlattices grown on the grooves and mesas was analyzed with TEM cross-sectional imaging. It was found that the evolution of non-planar crystal surfaces depends not only on the source flux and surface affinity, but also on the lateral diffusion of the adsorbed atoms or molecules on the crystal surfaces. Several growth features at the intersections of adjacent facets are illustrated in this work. Implications on the epitaxial process are discussed.