Impact of Surface Treatments on the Transport Properties of Germanium 2DHGs.

Holes in planar germanium (Ge) heterostructures show promise for quantum applications, particularly in superconducting and spin qubits, due to strong spin-orbit interaction, low effective mass, and the absence of valley degeneracies. However, charge traps cause issues such as gate hysteresis and charge noise. This study examines the effect of surface treatments on the accumulation behavior and transport properties of Ge-based two-dimensional hole gases (2DHGs). Oxygen plasma treatment reduces conduction in a setting without applied top gate voltage, improves the mobility, and lowers the percolation density, while hydrofluoric acid (HF) etching provides no benefit. The results suggest that interface traps from the partially oxidized silicon (Si) cap pin the Fermi level and that oxygen plasma reduces the trap density by fully oxidizing the Si cap. Therefore, optimizing surface treatments is crucial for minimizing the charge traps and thereby enhancing the device's performance.