Growth evolution of Ge quantum dot modulated by the atom bombardment during ion beam sputtering deposition

Abstract

The Ge quantum dots on Si substrate are prepared by ion beam sputtering deposition (IBSD). The growth evolution is observed to experience two stages with Ge coverage (θ) increasing. When θ increases from 6 monolayers (ML) to 10.5 ML, the average base width and height of quantum dots both increase, and the dome shape dots with small aspect ratio values are obtained. As the dots grow up, Ge atoms are also accumulated in the wetting layer, which contributes to the observed quantum dot density increasing mildly during this stage. When θ is in a range from 11.5 ML to 17 ML, vertical growth dominates the dot evolution. Another dome shape quantum dots are prepared with large aspect ratio values. Ge coverage gain results in the dot density increasing rapidly. A wetting layer decomposition process is demonstrated to give significant effect on that. The growth transition occurs as θ increases from 10.5 ML to 11.5 ML, and the dot density is enhanced 6.4 times in this course. So it is concluded that the evolution of Ge quantum dot prepared by IBSD is very different from that deposited on the thermal equilibrium condition. The observed characters of the dot shape and size distribution result from the kinetic behaviors of the surface atoms which are restricted by the thermodynamic limitation. Ge coverage is the one of the most important factors which can change the free energy. On the other hand, the energic sputtered atom bombardment enhances surface diffusion and defers nucleation of three-dimensional islands until the superstrain wetting layer is formed, which can also change the system free energy and the surface atom kinetic behaviors. So the growth evolution of Ge quantum dots prepared by IBSD is related so much with the effect of atom bombardment on the quantum dot growth.