Ge Nanodroplets Self-Assembly on Focused Ion Beam Patterned Substrates

Abstract

In this chapter we describe an original approach to the formation of Ge nanocrystals (NCs) on Si(001) based on a combination of focused ion beam (FIB) nanopatterning and spontaneous formation of NCs on the patterns. Two processes have been developed, the first on bare Si substrate and the second on SiO₂. In the first case, we investigate the effect of experimental parameters (growth temperature, deposited thickness, holes’ pitch and size) on the NCs’ formation.We show that, depending on the growth temperature, two different mechanisms of NCs formation occur. They are explained by kinetically limited nucleation at low temperature and stress-induced nucleation at high temperature. In the second case, we investigate the formation of Ge NCs on an ultrathin tunnel oxide by crystallization during thermal annealing of an amorphous Ge layer. We show that NCs’ size and density can be controlled by simply adjusting the initial deposited thickness. When using FIB-patterned SiO₂substrate, the formation of Ge NCs is forced inside the patterns because of both thermodynamics and kinetics. This is explained by the reduction of total surface energy. Periodic two-dimensional (2D) arrays of perfectly ordered, highly dense (> 10¹¹/cm²) and ultrasmall (∼ 20 nm) Ge NCs were achieved.