Simulation of GaAs nanowire annealing

Abstract

The GaAs nanowire (NW) morphology transformation during a high temperature annealing of densely packed wires was analyzed by Monte Carlo simulation. For long densely spaced NWs, the arsenic evaporation with its removal from the system occurs only in the upper NW part at such distance that As2 molecules can overcome due to the channeling between adjacent NWs before reaching vacuum. In the lower NW part, two oppositely directed processes proceed: destruction and restoration of the NW side wall due to the readsorbed material. Thus, the lower part of wire evaporation is close to a quasi-equilibrium process, and the upper part is close to free evaporation into vacuum. An excess of gallium primarily occurs at the top part of the crystal, since it is in the conditions close to a free arsenic evaporation. An excess gallium gathering leads to the Ga droplet formation. Droplets in the middle part of the NW side walls are formed later. A droplet-free region is formed on the sidewall of sufficiently long NWs with a length>345 nm and the distance of 28 nm between the wires after the annealing at T = 890 K. That means that the congruent evaporation temperature varies along the NW axis due to the variation of arsenic pressure, resulting in the congruent evaporation from the lower NW region and incongruent evaporation from the upper wire region. Simulation results predict the dendrite (thin nanowire branching off original wires) growth during a prolonged annealing under incongruent conditions.