Hopping conduction and field effect in Si modulation-doped structures withembedded Ge quantum dots

We report measurements of hopping transport in modulation-doped Si field-ef fect structures with a layer of Ge nanometer-scale dots embedded in proximi ty with the p-type conductive channel. It is found that the activation ener gy of hopping conductivity in the impurity band of the doped Si layer chang es with increasing quantum dot (QD) 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 mod ulation was attributed to hopping transport of holes through the discrete e nergy levels of the Ge nanocrystals. Field-effect measurements in structure s which contain as many as 10(9) dots enable us to resolve as well-pronounc ed maxima in G - V-g characteristic's 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 (charging) energies in the ground and first exited states , the quantization energies and the localization lengths. [S0163-1829(99)08 819-0].