Long-range Coulomb interaction in arrays of self-assembled quantum dots

An array of 3x10(7) Ge self-assembled quantum dots is embedded into the act ive channel of a Si metaloxide-field-effect transistor. Conductance oscilla tions with a gate voltage resulting from a successive lending of holes into the dots are observed. Based on measurements of the temperature dependence of the conductance maxima, the charge-transfer mechanism in the channel is identified as being due to variable-range hopping between the dots, with t he typical hopping energy determined by interdot Coulomb interaction. The c haracteristic spatial dimension of the hole wave functions as well as the c harging energies of the dots are determined from the conductance data. The effect of the proximity of a bulk conductor on hopping transport is studied . We find that putting a metal plane close to the dot layer causes a crosso ver from Efros-Shklovskii variable-range hopping conductance to two-dimensi onal Mott behavior as the temperature is reduced. At the crossover temperat ure the hopping activation energy is observed to fall off. The metal plane is shown not to affect the conductance of samples which show Mott hopping. In the Efros-Shklovskii hopping regime, the conductance prefactor was found to be similar or equal to e(2)/h, and the conductance to scale with the te mperature. In the fully screened limit, the universal behavior of the prefa ctor is destroyed, and it begins to depend on the localization length. The experimental results are explained by a screening of long-range Coulomb pot entials, and provide evidence for strong electron-electron interaction betw een dots in the absence of screening.