Molecular beam epitaxial growth of InSb quantum well heterostructures for applications in topological quantum computing

Abstract

The small effective mass, large Landé g-factor, and strong spin-orbit coupling of InSb make high-quality InSb quantum well (QW) structures particularly appealing for the experimental realization of Majorana bound states (MBS) in the pursuit of topological quantum computing. However, suitable InSb QWs have not yet been realized owing to outstanding challenges in material development. In this thesis, InSb QW heterostructures on GaAs substrates have been developed by molecular beam epitaxy (MBE) and optimized to achieve reduced defect densities, smooth surface morphology, and improved transport properties.

Metamorphic buffers consisted of AlSb or GaSb as the first intermediate buffer and ternary AlInSb as the second-stage buffer to bring the lattice constant of the material structure from that of the GaAs substrate to that of the InSb QW are investigated using a broad range of characterization techniques. The optimization of the interfacial misfit (IMF) growth mode for the GaSb buffers is presented. We further report an effective dislocation filtering by the AlxIn1-xSb/AlyIn1-ySb interlayer buffers. InSb QW structures with a threading dislocation density (TDD) of ~1 × 108 cm-2 have been achieved, tolerable for MBS devices.

Hillock-decorated surface morphologies on different buffers have been studied under various growth parameters and also as a function of GaAs (001) substrate offcut angle. A toy model to demonstrate the offcut-dependent morphological transitions is discussed. The optimal substrate offcut angle for a hillock-free surface was found, which has also been shown to suppress the micro-twins (MT) formation. On GaAs (001) substrates with a 0.55° offcut towards [-110] direction, we have successfully grown hillock-free, MT-free, and atomically smooth InSb QW heterostructures.

The magneto-transport properties of InSb QWs are studied with samples fabricated in either van der Pauw (vdP) or hall bar geometries. The optimization of the doping profile and the study on the effect of the buffer structures are presented for the structures with a standard high-electron-mobility transistor (HEMT) active region. We also present a comprehensive study on the parallel conduction and reproducibility problems accompanying the growth of the InSb QW heterostructures. The development of InSb QW structures in an inverted HEMT design achieved through either modulation doping or a Si-doped back gate is resented. First InSb true surface QW structures are reported and the top gate-ability on which has also been demonstrated with a HfO2 gate dielectric.