Phase Control of Crystalline Ga2O3 Films by Plasma-Enhanced Atomic Layer Deposition

Ga2O3 has emerged as a promising material for next-generation power electronics. Beyond the most stable and studied beta phase, metastable alpha-, epsilon-, and kappa-Ga2O3 have unique characteristics such as larger bandgaps, potential alloying for dopant and band engineering, and polarization, all of which can be leveraged in electronic device applications. Plasma-enhanced atomic layer deposition (PEALD) is a conformal, energy-enhanced synthesis method with many advantages including reduced growth temperatures, access to metastable phases, and improved crystallinity. In this study, PEALD was employed to deposit highly resistive, crystalline Ga2O3 films from 265 to 475 degrees C on c-plane sapphire substrates. Crystallinity, atypical at these low growth temperatures, was presumably due to the high flux of energetic ions to the growth surface independent of other growth parameters. Phase selectivity of beta, alpha, epsilon(kappa)-Ga2O3 was demonstrated as a function of plasma gas composition, gas flow and pressure during the plasma pulse, as well as growth temperature. Factors such as atomic oxygen generation and the flux of energetic ions were found to have a significant impact on the ability to attain metastable phases. Optimum films of each phase were fully characterized to determine the feasibility of PEALD Ga2O3 films. While both high-quality, single-phase beta- and alpha-Ga2O3 films were achieved, epsilon-Ga2O3 films were not able to be completely isolated and even under the best conditions contained components of beta- and kappa-Ga2O3 as identified by transmission electron microscopy. Trends suggest that this could be a limitation of the underlying substrate or reactor configuration.