Semiconductor nanowires provide a fruitful ground for exploring quantum coherence effects . Their intrinsic 1D confinement and the ability to be grown from a wide variety materials using vapor-liquid-solid (VLS)  growth make them especially attractive for quantum information processing. To be successful at this purpose requires high-quality single-crystalline nanowires with finely tuned diameters and lengths. For this reason we grow nanowires in-house, enabling a tight feedback between synthesis and transport measurements. To grow nanowires, we operate two home built metal-organic chemical vapor deposition reactors, one of which is shown in panel (a). The reactors feature a load-lock and a high vacuum chamber capable of reaching pressures of 10-10 Torr between growth runs, guaranteeing a pristine environment free of contaminants.
Panel (b) illustrates the VLS growth mechanism. We insert a substrate with gold catalyst particles into the growth chamber. We then heat the substrate while introducing a vapor of metal-organic chemicals. These decompose on the growth substrate leaving free metal atoms, which alloy with the gold catalyst particle. Upon saturation, the catalyst nucleates epitaxial growth of a nanowire. With this mechanism we can achieve a high degree of control over the dimensions of the nanowire by controlling the diameters of the seed particles and the growth time.
The first of our growth systems is dedicated to growing InAs nanowires, which are used to form spin-orbit qubits. Panel (c) shows a scanning electron micrograph of a growth substrate. Here we patterned a regular array of gold catalyst particles using electron-beam lithography. We are thus able to control the wire spacing increasing the homogeneity of nanowires across the substrate. A transmission electron micrograph in panel (d) shows the single-crystalline nature of the nanowires.
Our second growth system is dedicated to the growth of bismuth-based topological insulator compounds Bi2Se3, Bi2Te3, etc. These compounds have recently been shown to exhibit electronic surface states with extraordinary properties. Nanostructures with high surface-to-volume ratio are studied here to make the surface properties dominate. We synthesize nanostructures using a metal-organic chemical vapor deposition process similar to that of InAs. Although the deposition processes of these materials are not as well understood as more conventional semiconductors such as InAs, we have leveraged similarities with InAs to develop novel syntheses of topological insulator nanostructures.
Panel (e) shows a SEM image of a gold catalyst particle at the top of a Bi2Te3 nanowire, showing that a VLS process is at work. We have also demonstrated in Bi2Se3 that a "root-growth" mechanism, in which the catalyst particle is at the base, can dominate under some growth conditions. Unlike InAs, the Bi-based topological insulators have a highly layered structure, which leads to wires with a square or rectangular cross section. The wires shown in (e) have a rectangular cross section with dimensions of approximately 100 nm x 30 nm.References
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