Cross–Sectional STM Analysis of MBE–Grown InAs/InAsSb Strained–Layer Superlattices
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We show how cross–sectional scanning tunneling microscopy is used to examine a gallium–free, type–II InAs/InAsSb superlattice and perform compositional analyses of the as–grown structure through isovalent impurity identification. We describe the optimization of cleaving protocols, upgrades to the vacuum system, and standardized lab protocols for minimizing STM non–idealities. These improvements allow acquisition of representative, device–scale STM surveys yielding statistically–significant image ensembles. We describe protocols for identifying surface antimony–for–arsenic substitutions in STM images, which facilitate monolayer–by–monolayer analyses of the antimony fraction across surveyed repeats. Reconstruction of representative bulk composition profiles, based on appropriate approximations to the bulk period, reveal compositional grading across superlattice interfaces consistent with anion segregation. HRXRD simulations based on these profiles provide insights into the discrepancies between intended and observed x–ray spectra. We develop a quantitative, continuum segregation model to fit the observed antimony profiles, and examine the resulting fit parameters to determine what they reveal about segregation and cross–incorporation in InAs/InAsSb superlattices. We show how the model best describing the bulk profile relies on two, spatially distinct segregation sources with an offset close to one monolayer, consistent with either monolayer roughness or substrate vicinality. This model also provides self–consistent period measurements over surveyed sections of the multilayer stack, that agree with bulk period approximations based on sliding window averages, thereby substantiating the occurrence of more than one bulk period in the superlattice. The insights achieved through such detailed analyses of the as–grown structure can be combined with STM and SIMS data pointing to a vertical evolution in the total incorporated antimony per period to obtain x–ray simulations in excellent agreement with the experimental HRXRD spectrum. Finally, we demonstrate how cross–sectional STM may be used to measure local periods in superlattice structures via a novel, reciprocal–space technique analogous to Bragg’s law in x–ray diffraction. The period measurements obtained with this technique are compared with those from sliding–window averages and the continuum segregation–model to validate the accuracy of this new method, and pinpoint period variations within the multilayer stack.
Kanedy, Kara Maxine (2016). Cross–Sectional STM Analysis of MBE–Grown InAs/InAsSb Strained–Layer Superlattices. Doctoral dissertation, Texas A&M University. Available electronically from