Shallow and Undoped Germanium Quantum Wells: A Playground for Spin and Hybrid Quantum Technology

Buried-channel semiconductor heterostructures are an archetype material platform for the fabrication of gated semiconductor quantum devices. Sharp confinement potential is obtained by positioning the channel near the surface; however, nearby surface states degrade the electrical properties of the starting material. Here, a 2D hole gas of high mobility (5 x 10(5) cm(2)V(-1)S(-1)) is demonstrated in a very shallow strained germanium (Ge) channel, which is located only 22 nm below the surface. The top-gate of a dopant-less field effect transistor controls the channel carrier density confined in an undoped Ge/SiGe heterostructure with reduced background contamination, sharp interfaces, and high uniformity. The high mobility leads to mean free paths approximate to 6 mu m, setting new benchmarks for holes in shallow field effect transistors. The high mobility, along with a percolation density of 1.2 x 10(11)cm(-2), light effective mass (0.09m(e)), and high effective g-factor (up to 9.2) highlight the potential of undoped Ge/SiGe as low-disorder material platform for hybrid quantum technologies.