Growth, luminescence and magnetic properties of GaN:Er semiconductor thin films grown by molecular beam epitaxy

We report on the growth, surface, luminescence and magnetic properties of 180 nm thick Er-doped GaN thin films grown by molecular beam cpitaxy (MBE) on c-sapphire substrates with no buffer layer and with different Er concentrations. In situ reflection high-energy electron diffraction (RHEED) patterns revealed crystalline and uniform growth of the films. The x-ray diffraction (XRD) pattern showed c-axis-oriented growth. Atomic force microscopy (AFM) analysis showed enhancement of surface morphology and smoothness with increasing Er doping, which could be due to minimization of surface defects because of the gettering effect of the rare earth. Scanning area-dependent surface morphology analysis showed a power law dependence indicating the fractal nature of the surface, which is confirmed by the observation of a non-integer D (fractal dimension) value. X-ray photoluminescence spectroscopy (XPS) revealed the formation of a GaN: Er phase and ruled out the presence of Ga and Er metallic and native oxide phases. The semi-quantitative elemental composition of the films was determined using N 1s, Ga 2p(3/2) and Er 4d photoemission lines. The Er concentration was estimated from the x-ray photoelectron spectra and found to be between 3.0 and 9.0 at.% (similar to 10(21) atoms cm(-3)). Photoluminescence (PL) and cathodoluminescence (CL) studies showed visible emission and concentration quenching of Er3+ ions in agreement with reported results. Excitation of the Er3+ ion might be affected by charge trapping due to Er-doping-induced defect complexes. The magnetic measurements carried out by a superconducting quantum interference device (SQUID) showed a ferromagnetic-paramagnetic phase transition at low temperature, contrary to the reported room temperature ferromagnetism in metalorganic chemical vapor deposition (MOCVD)-grown GaN: Er thick films of 550 nm.