Alternatives to aluminum gates for silicon quantum devices: Defects and strain

Gate-defined quantum dots benefit from the use of small grain size metals for gate materials because they aid in shrinking the device dimensions. However, it is not clear what differences arise with respect to process-induced defect densities and inhomogeneous strain. Here, we present measurements of fixed charge, Qf; interface trap density, Dit; the intrinsic film stress, σ; and the coefficient of thermal expansion, α, as a function of forming gas anneal temperature for Al, Ti/Pd, and Ti/Pt gates. We show that Dit is minimized at an anneal temperature of 350  °C for all materials, but Ti/Pd and Ti/Pt have higher Qf and Dit compared to Al. In addition, σ and α increase with anneal temperature for all three metals with α larger than the bulk value. These results indicate that there is a trade-off between minimizing defects and minimizing the impact of strain in quantum device fabrication.