Hopping transport through an ensemble of Ge self-assembled quantum dots

We report investigations of the hole transport in arrays of Ge quantum dots buried in Si. Based on measurements of the temperature dependence of the conductance, the charge-transfer mechanism is proposed to be due to variable-range hopping between the dots with the typical hopping energy determined by inter-dot Coulomb interaction. We find that putting a metal plane close to the dot layer causes a crossover from Efros-Shklovskii variable-range hopping conductance to two-dimensional Mott behavior as the temperature is reduced. At the crossover temperature the hopping activation energy is observed to fall off. The experimental results are explained by screening of long-range Coulomb potentials and give evidence for strong electrostatic interaction between dots in the absence of screening. Conductance oscillations with gate voltage resulting from successive loading of holes into the dots are observed in the field-effect structures.