|Ph.D Student||Ashuach Yechezkel|
|Subject||Study of Atomic Intermixing in Short-Period InAs/GaSb|
Superlattices for Infrared Detectors
|Department||Department of Materials Science and Engineering||Supervisor||Professor Emeritus Emil Zolotoyabko|
|Full Thesis text|
InAs/GaSb superlattices are promising candidates for mid- and long-wavelength infrared detectors. Due to quantum confinement of charge carriers in short-period superlattices, the effective band gap can be adjusted by varying the thicknesses of constituent layers, the latter being only few monolayers thick. Atomic intermixing during growth causes interfacial spreading, which in turn results in deterioration of device characteristics. Therefore, careful characterization of interface quality on a monolayer scale is of primary importance towards better device functioning.
In this research, we applied a number of advanced characterization methods in order to extract atomic concentration profiles with sub-nm resolution in close proximity to interfaces. We investigated two sets of MBE-grown superlattices with different layer thicknesses, which were designed, respectively, for mid-wavelength (M-samples) and long-wavelength (L-samples) infrared detection. Most important results were obtained by the aid of high-resolution X-ray diffraction (HRXRD), cross-sectional scanning tunneling microscopy (XSTM), high-angle annular dark field technique in scanning transmission electron microscopy (HAADF-STEM), and local-electrode atom probe tomography (LEAP).
We found that HAADF-STEM permits visualization of the anion-cation dumbbells in individual sub-layers, and is even capable to resolve anions and cations separated by 0.15 nm within a dumbbell. On this basis, highly accurate values for interfacial width (confined in different samples between 0.8 and 1.3 nm) were extracted. XSTM technique allowed us counting of certain kinds of atoms within atomic rows, which directly reflects atomic intermixing in the anion (Sb-As) sub-lattice. In all investigated samples, XSTM analysis showed some residual concentration (on the level of a few atomic percent) of Sb atoms in the middle of the InAs sub-layers, as a result of Sb/As substitutions during growth. This percentage matches well that one extracted from LEAP data. Much lower levels of atomic substitutions were revealed by LEAP for the cation (Ga/In) sub-lattice. Quantitative information on cation intermixing and interface roughness was extracted from full fits of the HRXRD experimental profiles to the calculated ones by using the direct wave summation method. Both anion and cation intermixing levels were found to be significantly lower in the L-samples, in which InAs layers are thicker.
The developed procedures provide solid basis for quantitative analysis of atomic intermixing and interface roughness on a monolayer scale and can be used in the future for optimizing the superlattice growth process.