Self assembled InAs quantum dots (QDs) are grown by solid source molecular beam epitaxy on (100) and (111) Si substrates using Stranski-Krastanov (SK) growth mode. Reflection high energy diffraction (RHEED) streak patterns confirm that the combination of the atomic hydrogen followed by thermal desorption is an efficient cleaning method. The evolution of size, density and shape of the QDs are ex-situ characterized by atomic-force microscopy (AFM). Different growth parameters such as InAs coverage, growth temperature, In-growth rate and V/III ratio are examined on differently prepared silicon surfaces including ex- and in-situ cleaning procedures. The ex-situ surface preparation consists of a pre-removal of the surface oxide by buffered HF (NH(4)/HF) treatment followed by an in-situ thermal atomic hydrogen (AH) assisted surface cleaning process at 500 degrees C and a final thermal oxide desorption treatment in the growth chamber within a temperature range of 700-900 degrees C. Additional improvement for the cleaning and growth is achieved by exposing the Si surface with Ga at low fluxes. The Ga treatment at a temperature of 560 degrees C for 2 min results in a strong reduction of the lateral size of InAs QDs from 50-70 down to 25-30 nm and a significant enhancement in the homogeneity of the dot size and distribution. The InAs QDs density is strongly increased from 10(8) to 10(11) cm(-2) at V/III ratio in the range of 15-35 (beam equivalent pressure values). InAs QD formations are not observed at temperatures as high as 500 degrees C. Moreover, InAs quantum dashes are observed on [111] orientation and at higher In-growth rate of 0.3 ML/s. (C) 2010 Elsevier B.V. All rights reserved.