Two-dimensional localization in GeSn

Localization behaviour is a characteristic feature of thep-type GeSn quantum well (QW) system in a metal-insulator-semiconductor device. The transition to strongly localized behaviour is abrupt with thermally activated conductivity and a high temperature intercept of 0.12 ×e²ħ^-1at a hole carrier density 1.55 × 10¹¹cm^-2. The activation energy for the conductivity in the localized state is 0.40 ± 0.05 meV compared to an activation energy of ∼0.1 meV for conductivity activation to a mobility edge at carrier densities >1.55 × 10¹¹cm^-2. Insulating behaviour can occur from a system that behaves as though it is in a minimum metallic state, albeit at high temperature, or from a conductivity greater than a minimum metallic state behaviour showing that local disorder conditions with local differences in the density of states are important for the onset of localization. In the presence of a high magnetic field, thermally activated conductivity is present down to Landau level filling factor <1/2but without a magnetic-field-dependent carrier density or a variable range hopping (VRH) transport behaviour developing even with conductivity ≪e²h^-1. In the localized transport regime inp-type doped Ge_0.92Sn_0.08QWs the VRH mechanism is suppressed at temperatures >100 mK and this makes this two-dimensional system ideal for future many body localization studies in disordered hole gases that can be thermally isolated from a temperature reservoir.