Structural and magnetic characterizations of Mn2CrO4 and MnCr2O4 films on MgO(001) and SrTiO3(001) substrates by molecular beam epitaxy J. Appl. Phys. 109, 07D714 (2011); 10.1063/1.3545802Structural characteristics and magnetic properties of λ-MnO 2 films grown by plasma-assisted molecular beam epitaxyThe phase and orientation of manganese nitride grown on MgO͑001͒ using molecular beam epitaxy are shown to be controllable by the manganese/nitrogen flux ratio as well as the substrate temperature. The most N-rich phase, phase ͑MnN͒, is obtained at very low Mn/N flux ratio. At increased Mn/N flux ratio, the next most N-rich phase, the phase ͑Mn 3 N 2 ͒, is obtained having its c axis normal to the surface plane. Further increasing the Mn/N flux ratio, the phase ͑Mn 3 N 2 ͒ having its c axis in the surface plane is obtained. Finally, the phase ͑Mn 4 N͒ is obtained at yet higher Mn/N flux ratio. The structural phase variation with Mn/N flux ratio is due to the kinetic control of the surface chemical composition, which determines the energetically most favorable phase. For a given Mn/N flux ratio, the phase is also found to be a function of the substrate temperature, with the less N-rich phase occurring at the higher substrate temperature. The change of phase with temperature is attributed to the change in the chemical composition resulting from the diffusion of N vacancies. Since the magnetic properties of Mn x N y depend on the phase, the Mn/N flux ratio provides a way of directly controlling the magnetic properties. A phase diagram for molecular beam epitaxial growth is presented.

No abstract

The effect of the Ga/N flux ratio on the Mn incorporation, surface morphology, and lattice polarity during growth by rf molecular beam epitaxy of (Ga,Mn)N at a sample temperature of 550 °C is presented. Three regimes of growth, N-rich, metal-rich, and Ga-rich, are clearly distinguished by reflection high-energy electron diffraction and atomic force microscopy. Using energy dispersive x-ray spectroscopy, it is found that Mn incorporation occurs only for N-rich and metal-rich conditions. For these conditions, although x-ray diffraction in third order does not reveal any significant peak splitting or broadening, Rutherford backscattering clearly shows that Mn is not only incorporated but also substitutional on the Ga sites. Hence, we conclude that a MnxGa1−xN alloy is formed (in this case x∼5%), but there is no observable change in the c-axis lattice constant. We also find that the surface morphology is dramatically improved when growth is just slightly metal rich. When growth is highly metal-rich, but not Ga-rich, we find that Ga polarity flips to N polarity. It is concluded that the optimal growth of Ga-polar MnGaN by rf N-plasma molecular beam epitaxy occurs in the slightly metal-rich regime.

An innovative approach for in-situ characterization has been used in this work to investigate the composition, growth mode, morphology and crystalline ordering of the early stages of growth of GaN films grown on sapphire by MOCVD for substrate temperatures in the range of 450°C to 1050°C. We have performed in-situ characterization by Rutherford Backscattering Spectroscopy (RBS), Ion Channeling, X-ray Photoelectron Spectroscopy (XPS), and Low Energy Electron Diffraction. Ex-situ the films have been characterized by Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD) and thickness profilometry. The films have been grown in an in-house designed and build MOCVD reactor that is attached by UHV lines to the analysis facilities. RBS analysis indicated that the films have the correct stoichiometry, have variable thickness and for low substrate temperature completely cover the substrate while for temperatures 850°C and higher islands are formed that may cover as few as 5 percent of the substrate. From Ion Channeling and LEED we have determined the crystallographic phase to be wurtzite. The crystalline quality increases with higher deposition temperature and with thickness. The films are epitaxialy grown with the <0001> crystallographic axis and planes of the GaN films aligned with the sapphire within 0.2 degrees.