Refining Au/Sb alloyed ohmic contacts in undoped Si/SiGe strained quantum wells

TL;DR

This study improves Au/Sb alloyed ohmic contacts in undoped Si/SiGe quantum wells by understanding surface morphology issues and applying aqua regia treatment, resulting in high-mobility, reliable contacts for quantum devices.

Contribution

It identifies the causes of surface roughness in Au/Sb alloyed contacts and demonstrates a treatment method to produce smoother, more effective ohmic contacts in undoped Si/SiGe heterostructures.

Findings

Surface roughness is due to Au/Si eutectic reaction and threading dislocations.

Aqua regia treatment removes protrusions and improves contact quality.

Achieved high mobility (~10^5 cm^2/Vs) and carrier concentration (~10^11/cm^2).

Abstract

Shallow undoped Si/SiGe quantum wells are the leading platforms for hosting quantum processors based on spin-qubits. The ohmic contacts to the electron gas in these systems are accomplished by ion-implantation technique since the conventional Au/Sb alloyed contacts present a rough surface consisting of sharp islands and pits. These sharp protrusions cause electrical discharge across the gate-dielectric between the ohmic contacts and the accumulation-gates causing device break-down. A clear understanding of the surface morphology, elemental, compositional and electrical characterization of the alloyed region would enable one to engineer a smoother post alloyed surface. In this work, we find that the rough surface morphology is a cumulative effect of the Au/Si eutectic reaction and the threading dislocations inherent in the heterostructure. The structural, elemental, and chemical-state analysis show that the inverted pyramidal pits are resulting from the enhanced Au/Si eutectic reaction at the threading dislocations stemming from the heterostructure interface, while, the sharp protrusions causing accumulation gate-leakage are gold-rich precipitations. The protrusions are removed using an aqua regia treatment prior to the deposition of the gate-oxide and gate electrode. Exploiting a Hall bar device, we analyse the mobility and carrier concentration of the undoped Si/SiGe consisting of Au/Sb alloyed contacts down to 1.5 K. The measured mobility ~10^5 cm^2/Vs and carrier concentration of ~10^11/cm^2are comparable to the reported values on similar high-mobility heterostructures confirming the efficacy of our modified Au/Sb alloy technique in accomplishing high-efficiency contacts to undoped Si/SiGe heterostructures.