Si quantum dots (Si-QDs) with impurity doped Ge core were self-assembled on thermally-grown SiO(2) from thermal decomposition of alternately pure SiH(4) and GeH(4) diluted with He. delta-doping into the dots were carried out during Ge core formation. Changes in the surface potential caused by electron charging to and discharging from individual Si-QD with and without impurity doped Ge core were studied by using a Kelvin probe technique in atomic force microscopy (AFM). We have demonstrated that the biases applied to a tip for electron injection to and for extraction from dots are reduced appreciably by boron and phosphorus doping, respectively. Such bias reduction is interpreted in terms that a negatively charged state originated from ionized B acceptor(s) results from electron injection to the valence band and that a positively charged state due to ionized P donor(s) is caused by electron extraction from the conduction band. Because of a deep potential well for holes in the Ge core, Si-QDs with B-doped Ge core shows superior retention in positive charged state to coreless B-doped Si-QDs. (c) 2008 Elsevier B.V. All rights reserved.