Daisy Shearer

The III-V narrowgap semiconductor InSb has properties such as low effective mass, high mobility, and strong spin-orbit coupling which make it an ideal material for spintronics. Recently there have been numerous device schemes that have been proposed for generating spin polarised currents for spin injection in quantum technologies. At the heart of these schemes is the quantum point contact (QPC) which requires nanoscale lithography and gated structures. Here we present a complete fabrication toolkit that employs focused ion beam (FIB) lithography as a flexible direct-write technique for rapid prototyping of QPCs. Two approaches for fabricating QPCs have been explored. The first method involves defining QPCs using direct etch writing of InSb quantum wells. A FIB assisted XeF2 chemistry has been used to form side gates with air gaps. The second technique uses FIB assisted deposition to form a siloxane gate dielectric, as well as direct-write of metalization using platinum to form split top gates. Compatibility of InSb with these processes has been confirmed by post-fabrication measurements of the electronic properties using Shubnikov de Haas and quantum Hall. The versatility of this direct-write technique makes it ideal as a rapid device prototyping tool.

InSb is a III-V narrow-gap semiconductor with properties such as low effective mass, high mobility, and strong spin-orbit coupling making it an ideal material for applications such as spintronics mid-infrared photonics, and nanoelectronics. InSb quantum wells can be made by growing an InSb/InAlSb structure on a Ga substrate using molecular beam epitaxy. However, it is notoriously difficult to fabricate nanodevices from InSb/InAlSb quantum wells due to factors such as its low thermal budget and the production of non-volatile by-products in conventional etching processes, leading to unwanted deposition of material onto the material surface. Current wet and dry etching techniques take a long time and require expensive lithography masks to make new devices, slowing the development of optimised nanodevices.We investigate focused ion beam (FIB) lithography as a "rapid prototyping" fabrication technique to create semiconductor nanodevices from InSb quantum wells. FIB methods have the advantage of being relatively quick and "maskless", making them ideal for use in the research environment as new iterations of device design can be made quickly and different etching chemistries and electrical properties can be tested in-situ. A variety of Xe plasma FIB parameters were tested to optimise the feature resolution and etching quality of milled trenches at low temperatures. The XeF2 gas-assisted etching process was also studied as an alternative to the Cl2 chemistry that is typically employed for dry etching of InSb. Cross-sections and profiles of the trenches indicate that the XeF2 etch yields superior trench smoothness and mills material from the surface at a much higher rate. This method was also less prone to deposition of unwanted material onto the surface of the sample. This high-resolution fabrication method can be used for the rapid development and optimisation of individual nanoscale devices before mass production.