Atomistic Insights into MOCVD Growth of GaAs using a Closed Loop Autonomous System Combining Directed Reinforcement Learning and Kinetic Monte Carlo Simulations
Conventional process optimization of the Metal-Organic Chemical Vapor Deposition (MOCVD) growth parameters to achieve high-quality thin films relies on iterative, resource-intensive experimental runs. While predictive modeling can circumvent this cost and effort, simulating thin-film growth is constrained by the vastly different length and time scales of the deposition process. While continuum (e.g., Computational Fluid Dynamics (CFD)) and first-principles methods (e.g., Molecular Dynamics (MD)) struggle to balance experimental length/time scales and atomistic accuracy, Kinetic Monte Carlo (KMC) bridges this divide by resolving atomistic events whilst remaining tractable at experimental length and time scales. However, the accuracy of KMC relies on kinetic rate parameters that require costly density functional theory (DFT) calculations. In order to circumvent this, we present a framework that calibrates the rate parameters directly against experimental growth data. A forward KMC growth model is coupled in a closed loop with a Monte Carlo Tree Search (MCTS) optimizer that navigates the high-dimensional parameter space, with the search steered by the degree of rate control of each reaction rate parameter. The approach invokes no material-specific assumptions and hence, any deposition process whose forward kinetics can be reduced to a set of rate-controlled events is amenable to the same treatment. We demonstrate it for the MOCVD of GaAs, where the calibrated model captures the characteristic temperature dependence of the measured growth rate. The framework is also able to recover the three well known kinetic regimes characterized by change in the rate determining step from precursor chemisorption to desorption. As a result, the framework converts routinely measured growth data into a mechanistically interpretable model of the deposition process.