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

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 (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 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 well-pronounced maxima in $G-V_{\mathrm{g}}$ 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 (charging) energies in the ground and first exited states, the quantization energies and the localization lengths.