Activated and Metallic Conduction in p-DType Modulation-Doped Ge-Sn Devices

Ge1-xSnx quantum wells can be incorporated into Si-Ge-based structures with low-carrier effective masses, high mobilities, and the possibility of direct band-gap devices with x similar to 0.1. However, the electrical properties of p-type Ge1-xSnx devices are dominated by a thermally activated mobility and metallic behavior. At 30 mK the transport measurements indicate localization with a mobility of 380 cm(2)/Vs, which is thermally activated with a temperature-independent carrier density of 4 x 10(11) cm(-2). This weakly dis ordered system with conductivity, sigma similar to e(2)/h, where e is the fundamental charge and h is Planck's constant, is a result of negatively charged "Sn-vacancy" complex states in the barrier layers that act as hole traps. A measured hole effective mass of 0.090 +/- 0.005m(e) from the Shubnikov-de Haas effect, where m(e) is the free electron mass shows that the valence band is heavy hole dominated and is similar to p-type Ge with the compressive strain playing the role of quenching the spin-orbit coupling and shifting the unoccupied light hole states to higher hole energies. The Ge1-xSnx devices have a high quantum mobility of approximately 36 000 cm(2)/Vs that is not thermally activated. The ratio of transport-to-quantum mobility of approximately 0.01 in Ge1-xSnx devices is unusual and points to several competing scattering mechanisms in the different experimental regimes.