Gul, Y; Myronov, M; Holmes, SN; Pepper, M; (2020) Activated and Metallic Conduction in p-DType Modulation-Doped Ge-Sn Devices. Physical Review Applied , 14 (5) , Article 054064. 10.1103/PhysRevApplied.14.054064.

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Abstract

Ge_{1-x}Sn_{x} 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 ∼ 0.1. However, the electrical properties of p-type Ge_{1-x} Sn_{x} 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 4x 10^{11} cm^{-2}. This weakly disordered system with conductivity, sigma ~ epsilon^{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 Ge_{1-x} Sn_{x} 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 Ge_{1-x} Sn_{x} devices is unusual and points to several competing scattering mechanisms in the different experimental regimes.

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