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

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 4 × 10 11 cm −2. This weakly disordered system with conductivity, σ ∼ 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 lighthole 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.

Electron-Phonon Decoupling in Two Dimensions

“…On the contrary, in systems where the equilibrium conductivity is of the form of either Mott (γ = 1/3) or Efros-Shklovskii (γ = 1/2) hopping, the lattice temperature required for the bistability is much lower and practically not accessible. Therefore, for achieving the electron-phonon decoupling necessary for MBL, materials with an Arrhenius conductivity, such as those recently seen in 33 , are most promising.…”

Electron-phonon decoupling in two dimensions

“…On the contrary, in systems where the equilibrium conductivity is of the form of either Mott or Efros-Shklovskii hopping, the lattice temperature required for the bistability is much lower and practically not accessible. Therefore, for achieving the electron-phonon decoupling necessary for MBL, materials with an Arrhenius conductivity, such as those recently seen in 33 , are most promising.…”

Electron-Phonon Decoupling in Two Dimensions

“…On the contrary, in systems where the equilibrium conductivity is of the form of either Mott (γ = 1/3) or Efros-Shklovskii (γ = 1/2) hopping, the lattice temperature required for the bistability is much lower and practically not accessible. Therefore, for achieving the electron-phonon decoupling necessary for MBL, materials with an Arrhenius conductivity, such as those recently seen in 32 , are most promising.…”