Modeling of Al and Ga Droplet Nucleation during Droplet Epitaxy or Droplet Etching

The temperature dependent density of Al and Ga droplets deposited on AlGaAs with molecular beam epitaxy is studied theoretically. Such droplets are important for applications in quantum information technology and can be functionalized e.g., by droplet epitaxy or droplet etching for the self-assembled generation of quantum emitters. After an estimation based on a scaling analysis, the droplet densities are simulated using first a mean-field rate model and second a kinetic Monte Carlo (KMC) simulation basing on an atomistic representation of the mobile adatoms. The modeling of droplet nucleation with a very high surface activity of the adatoms and ultra-low droplet densities down to 5 × 10 [Formula: see text] cm [Formula: see text] is highly demanding in particular for the KMC simulation. Both models consider two material related model parameters, the energy barrier [Formula: see text] for surface diffusion of free adatoms and the energy barrier [Formula: see text] for escape of atoms from droplets. The rate model quantitatively reproduces the droplet densities with [Formula: see text] = 0.19 eV, [Formula: see text] = 1.71 eV for Al droplets and [Formula: see text] = 0.115 eV for Ga droplets. For Ga, the values of [Formula: see text] are temperature dependent indicating the relevance of additional processes. Interestingly, the critical nucleus size depends on deposition time, which conflicts with the assumptions of the scaling model. Using a multiscale KMC algorithm to substantially shorten the computation times, Al droplets up to 460 °C on a 7500 × 7500 simulation field and Ga droplets up to 550 °C are simulated. The results show a very good agreement with the experiments using [Formula: see text] = 0.19 eV, [Formula: see text] = 1.44 eV for Al, and [Formula: see text] = 0.115 eV, [Formula: see text] = 1.24 eV ([Formula: see text] 300 °C) or [Formula: see text] = 1.24 + 0.06 (T[°C] − 300)/100 eV ([Formula: see text] °C) for Ga. The deviating [Formula: see text] is attributed to a re-nucleation effect that is not considered in the mean-field assumption of the rate model.