Atomistic simulation of amorphous germanium

Electrical doping of Ge is usually performed by ion implantation and subsequent annealing. In many cases ion bombardment leads to formation of an amorphous layer. During annealing the layer recrystallizes by solid-phase epitaxial regrowth. In order to investigate this process by classical molecular dynamics simulations, first of all amorphous Ge with realistic properties must be prepared. This is the subject of the present work. The atomistic simulations use the Stillinger-Weber interatomic potential with a parameter set that yields correct or reasonable structural, thermodynamic and defect properties of diamond-structure Ge. In the first simulation step liquid Ge is prepared. Then, the system is cooled down slowly to 300 K using the method of Luedtke and Landman which was applied to simulate amorphous silicon. Finally, an equilibration at 300 K and zero pressure is performed. The characterization of amorphous Ge obtained in this manner includes density and cohesive energy, radial distribution function and static structure factor, coordination number, bond-angle distribution, distribution of interatomic distances as well as melting temperature and heat of fusion. The simulation results show very good agreement with experimental data and are consistent with previous theoretical investigations.