We report measurements of hopping transport in modulation-doped Si field-effect structures with a layer of Ge nanometer-scale dots embedded in proximity with the p-type conductive channel. It is found that the activation energy of hopping conductivity in the impurity band of the doped Si layer changes with increasing quantum dot ͑QD͒ size, passing through a minimum, due to trapping of holes by the QD's. We observed conductivity oscillations with the gate voltage which disappeared in magnetic field. The drain current modulation was attributed to hopping transport of holes through the discrete energy levels of the Ge nanocrystals. Field-effect measurements in structures which contain as many as 10 9 dots enable us to resolve as wellpronounced maxima in GϪV g characteristics the single-electron charging of each dot with up to six holes. The level structure reveals up to three distinct shells which are interpreted as the s-like ground state, the first excited p-like state and the second excited d-like state. We are able to obtain the hole correlation ͑charging͒ energies in the ground and first exited states, the quantization energies and the localization lengths.
The metal-insulator transition (MIT) induced by a magnetic field. in barely metallic and compensated n-type InP has been re-examined. Using new analysis methods, we have determined the magnetic field for which the conductivity change from a metallic behaviour to avariable-range hoppingregime. On themetallicsideoftheMl~, the electricalmnduciivity is found to obey a = a. + mT' down to 60 mK: the zero-temperature conductivity can be described by a scaling law with an exponent U = 1 and there is no evidence for a minimum metallic conductivity.As the MIT is approached, we observe a clear crossover from a T''' to T"' temperature dependenceofthemnductivity, which is related toacompetition between two lengthscales: the correlation length and the interaction length.
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