Role of template layers for heteroepitaxial growth of lanthanum oxide on GaN(0001) via atomic layer deposition

La2O3 has been reported as a good gate dielectric for GaN-based high electron mobility transistor and metal-oxide-semiconductor field effect transistor applications. In this study, atomic layer deposition (ALD) was selected as the deposition technique, and the growth behavior and structure of La2O3 films on GaN(0001) were investigated. Unlike La2O3 deposited by molecular beam epitaxy (MBE) at 450 degrees C that can form a two-dimensional film directly on GaN(0001), island growth mode was observed when a La2O3 film was deposited directly on GaN(0001) at 250 degrees C by ALD using tris(N,N ' -diisopropylformamidinato)-lanthanum as the precursor and H2O as the coreactant. Only with the use of a template layer, a 2nm-thick hexagonal La2O3 film deposited by MBE or a 3nm-thick cubic Er2O3 film deposited by ALD, can a two-dimensional crystalline ALD-La2O3 thin film be formed on GaN(0001). Reflection high-energy electron diffraction, x-ray diffraction, and scanning transmission electron microscopy were conducted to verify the films' crystalline nature and structure. The ALD-La2O3 film retains a cubic structure on ALD-Er2O3 templated-GaN(0001), while it transforms from the cubic phase to a mix of cubic and hexagonal phases on MBE-La2O3 templated-GaN(0001) when the film is thicker than 15nm. The stabilization of a thin cubic La2O3 phase could be related to a difference in the surface energy and the ALD growth temperature.