Characterizing composition modulations in InAs/AlAs short-period superlattices

The formation of quantum wires has much interest due to their novel electronic properties which may lead to enhanced optoelectronic device performance and greater photovoltaic efficiencies. One method of forming these structures is through spontaneous lateral modulation found during the epitaxial growth of III/V alloys. In this paper, we report and summarize our investigations on the formation of lateral modulation in the molecular-beam epitaxy grown InAlAs/InP(001) system. This system was grown as a short-period superlattice where n monolayers of InAs are deposited followed by m monolayers of AlAs (with n and m similar to 2) and this sequence is repeated to grow a low strain InAlAs ternary alloy on InP(001) that exhibits lateral modulation. Films were grown under a variety of conditions (growth temperature, effective alloy composition, superlattice period, and growth rate). These films have been extensively analyzed using x-ray diffraction, atomic force microscopy, and transmission electron microscopy (TEM) and microcharacterization, in addition to photon-based spectroscopes. Here we present results of several microstructural characterizations using a wide range of TEM-based techniques, and compare them to results from the other methods to obtain a unified understanding of composition modulation. Two strong points consistently emerge:(1) The lateral modulation wavelength is insensitive to growth temperature and effective alloy composition, but the strength of the lateral modulation is greatest near an effective alloy composition of In0.46Al0.54As, which corresponds to a slightly tensile global strain with respect to InP. (2) The composition variation for the strongly modulated films is as much as 0.38 InAs mole fraction. in addition, for these strongly modulated films, the modulation wave is asymmetric showing strongly peaked, narrower InAs-rich regions separated by flat AlAs-rich regions. We discuss these results and their possible implications in addition to detailing the techniques used to obtain them. [S0163-1829(99)10743-4].