Ge/Si quantum dot nanostructures grown with low-energy ion beam-assisted epitaxy

Scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED) experiments were performed to study growth modes induced by hyperthermal Ge+ ion action during molecular beam epitaxy (MBE) of Ge on Si(100). The continuous and pulsed ion beams were used. These studies have shown that ion beam bombardment during heteroepitaxy leads to decrease in critical film thickness for transition from two-dimensional (2D) to three-dimensional (3D) growth modes, enhancement of 3D island density, and narrowing of island size distribution, as compared with conventional MBE experiments. Moreover, it was found that ion beam assists the transition from hut- to dome-shaped Ge islands on Si(100). The crystal perfection of Ge/Si structures with Ge islands embedded in Si was analyzed by Rutherford backscattering/channeling technique (RBS) and transmission electron microscopy (TEM). The studies of Si/Ge/Si(100) structures indicated defect-free Ge nanopaticles and Si layers for the initial stage of heteroepitaxy (five monolayers of Ge) in pulsed ion beam action growth mode at 350 °C. Continuous ion beam irradiation was found to induce dislocations around Ge clusters. The results of kinetic Monte Carlo (KMC) simulation have shown that two mechanisms of ion beam action can be responsible for stimulation of 2D-3D transition: (1) surface defect generation by ion impacts, and (2) enhancement of surface diffusion.