Reducing disorder in Ge quantum wells by using thick SiGe barriers

Abstract

We investigate the disorder properties of two-dimensional hole gases in Ge/SiGe heterostructures grown on Ge wafers, using thick SiGe barriers to mitigate the influence of the semiconductor-dielectric interface. Across several heterostructure field effect transistors we measure an average maximum mobility of $(4.4\pm 0.2)\times 10^6\mathrm{c}{\mathrm{m}}^2/Vs$ at a saturation density of $(1.72\pm 0.03)\times 10^{11}\mathrm{c}{\mathrm{m}}^{-2}$, corresponding to a long mean free path of $(30\pm 1)\hat{\mathrm{I}}¼\mathrm{m}$. The highest measured mobility is $4.68\times 10^6\mathrm{c}{\mathrm{m}}^2/Vs$. We identify uniform background impurities and interface roughness as the dominant scattering mechanisms limiting mobility in a representative device, and we evaluate a percolation-induced critical density of $(4.5\pm 0.1)\times 10^9\mathrm{c}{\mathrm{m}}^{-2}$. This low-disorder heterostructure, according to simulations, may support the electrostatic confinement of holes in gate-defined quantum dots.