Abstract
High-quality nano-thick Gd2O3 and Y2O3 epitaxial films have been grown on GaN(0001)h substrate by molecular beam epitaxy (MBE). The epi-layer of R2O3 (where R denotes Gd and Y) still exhibit remarkably uniform thickness and highly structural perfection despite a large lattice mismatch of 15-17% with the substrate.Structural and morphological investigation were carried out by in-situ reflection high energy electron diffraction (RHEED)、synchrotron x-ray diffraction (XRD) and x-ray reflectivity (XRR). The electric properties were studied by measuring their C-V and J-V characteristics.The initial stage of the Gd2O3 and Y2O3 epitaxial growth corresponds to a hexagonal phase with 6-fold symmetry. The hetero-structure follows an epitaxial relationship R2O3(0001)h [11-20]h // GaN(0001)h [11-20]h. With increasing layer thickness, the structure of the R2O3 film changes from hexagonal with (0001)h normal to monoclinic with (-201)m normal and the in-plane alignment with GaN[11-20]h changes from [11-20]h to [020]m. We concluded that approximate 3-4 nm is the critical thickness characterized by in-situ RHEED and ex-situ HRXRD. We utilized layer-like structural feature to nicely account for the phase transformation between the two phases. Overall, the strain filed might be responsible for the thickness dependent hexagonal to monoclinic phase transformation during MBE growth.