Two-dimensional localization in GeSn

Localization behaviour is a characteristic feature of the p-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 x e(2) h(-1) at a hole carrier density 1.55 x 10(11) cm(-2). The activation energy for the conductivity in the localized state is 0.40 +/- 0.05 meV compared to an activation energy of similar to 0.1 meV for conductivity activation to a mobility edge at carrier densities >1.55 x 10(11) cm(-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 <1/2 but without a magnetic-field-dependent carrier density or a variable range hopping (VRH) transport behaviour developing even with conductivity << e(2) h(-1). In the localized transport regime in p-type doped Ge0.92Sn0.08 QWs 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.