AC-Hopping Conductance of Self-Organized Ge/Si Quantum Dot Arrays

TL;DR

This study investigates the AC conductance mechanism in dense Ge/Si quantum dot arrays using surface acoustic wave attenuation, revealing a combination of diffusion and hopping conduction at low temperatures and magnetic fields.

Contribution

It provides experimental evidence for the AC conduction mechanism involving diffusion and hopping in self-organized Ge/Si quantum dot arrays, analyzed through SAW attenuation measurements.

Findings

AC conduction involves diffusion at the mobility edge and hopping between localized states.

SAW attenuation depends on magnetic field, temperature, and frequency, indicating complex conduction processes.

Magnetic field influences the SAW attenuation in a manner consistent with existing theoretical models.

Abstract

Dense ($n=4 \times 10^{11}$ cm$^{-2}$) arrays of Ge quantum dots in Si host were studied using attenuation of surface acoustic waves (SAWs) propagating along the surface of a piezoelectric crystal located near the sample. The SAW magneto-attenuation coefficient, $\Delta\Gamma=\Gamma(\omega, H)-\Gamma(\omega, 0)$, and change of velocity of SAW, $\Delta V /V=(V(H)-V(0)) / V(0)$, were measured in the temperature interval $T$ = 1.5-4.2 K as a function of magnetic field $H$ up to 6 T for the waves in the frequency range $f$ = 30-300 MHz. Basing on the dependences of $\Delta\Gamma$ on $H$, $T$ and $\omega$, as well as on its sign, we believe that the AC conduction mechanism is a combination of diffusion at the mobility edge with hopping between localized states at the Fermi level. The measured magnetic field dependence of the SAW attenuation is discussed basing on existing theoretical concepts.