Saki Sonoda

Molecular beam epitaxy of wurtzite (Ga,Mn)N films on sapphire(0 0 0 1) showing the ferromagnetic behaviour at room temperature

Wurtzite (Ga,Mn)N films showing ferromagnetic behaviour at room temperature were successfully grown on sapphire(0001) substrates by molecular beam epitaxy using ammonia as nitrogen source. Magnetization measurements were carried out by a superconducting quantum interference device at the temperatures between 1.8K and 300K with magnetic field applied parallel to the film plane up to 7T. The magnetic-field dependence of magnetization of a (Ga,Mn)N film at 300K were ferromagnetic, while a GaN film showed Pauli paramagnetism like behaviour. The Curie temperatures of a (Ga,Mn)N film was estimated as 940K.

Magnetic and transport characteristics on high Curie temperature ferromagnet of Mn-doped GaN

Mn-doped GaN films on sapphire (0001) substrates were grown by molecular beam epitaxy system using ammonia as nitrogen source. The result of magnetization measurement gives Curie temperature as high as 940 K. The field and temperature dependencies of the magnetization show coexistence of ferromagnetic and paramagnetic phases. In addition, the temperature dependencies of electrical resistance and carrier concentration were measured to investigate the relation between the ferromagnetism and transport property. Below about 10 K, a similar anomalous increase of magnetization and resistance is observed.

Room temperature ferromagnetism in Mn-doped γ-Ga2O3 with spinel structure

Mn-doped Ga2O3 (7 cation % of Mn) thin film has been grown on c-cut sapphire substrate using pulsed-laser deposition technique. Electron diffraction analyses by transmission electron microscopy found that the Mn-doped film shows γ phase with spinel structure, which is different from undoped film showing β phase. No secondary phase can be detected. Combination of Mn-L2,3 near edge x-ray absorption experiments with first-principles many-electron calculations unambiguously implies that Mn atoms are located at tetrahedrally coordinated Ga sites with a valence of +2. The doped sample shows ferromagnetism up to 350K.

Essential Change in Crystal Qualities of GaN Films by Controlling Lattice Polarity in Molecular Beam Epitaxy

GaN heteroepitaxial growth on sapphire (0001) substrates was carried out by radio-frequency plasma-assisted molecular beam epitaxy (rf-MBE). A Ga-polarity growth was achieved by using an AlN high-temperature buffer layer. The epilayer polarity was characterized directly by coaxial impact collision ion scattering spectra (CAICISS). It was found that the properties of the GaN films showing Ga-face polarity, including their structural and electrical properties, were dramatically improved compared to those of films with N-face polarity. This important conclusion is considered to be a breakthrough in the realization of high-quality III-nitride films by MBE for device applications.

Characterization of Polarity of Wurtzite GaN Film Grown by Molecular Beam Epitaxy Using NH3

Effects of the initial nitridation of a sapphire(0001) substrate by NH3 on the polarity of GaN{0001} film have been investigated by coaxial impact collision ion scattering spectroscopy. The polarity of ammonia-molecular beam epitaxial (MBE) film grown on the substrate nitrided using NH3 is assigned as (0001). The effect of the initial nitridation of the substrate by NH3 is found to be contrary to that by nitrogen plasma, where the GaN film grown on the nitrided substrate shows the polarity of (0001). The polarity of GaN film grown by rf plasma-assisted MBE on the substrate which is nitrided using NH3 is also (0001). These findings suggest the possibility of polarity control of the grown GaN film by choosing the N source for initial nitridation of the substrate.

Properties of ferromagnetic Ga/sub 1-x/Mn/sub x/N films grown by ammonia-MBE

Using ammonia as nitrogen source for molecular beam epitaxy, the GaN-based diluted magnetic semiconductor Ga/sub 1-x/Mn/sub x/N is successfully grown with Mn concentration up to x/spl sim/6.8% and with p-type conductivity. The films have wurtzite structure with substitutional Mn on Ga site in GaN. Magnetization measurements revealed that Ga/sub 1-x/Mn/sub x/N is ferromagnetic at temperatures higher than room temperature. The ferromagnetic-paramagnetic transition temperature, Tc, depends on the Mn concentration of the film. At low temperatures, the magnetization increases with increasing of magnetic field, implying that a paramagnetic-like phase coexists with ferromagnetic one. Possible explanations will be proposed for the coexistence of two magnetic phases in the grown films.

Local structure around Mn in ferromagnetic GaMnN film studied by X-ray absorption fine structure

The local structure of the Mn in a GaMnN film was investigated using extended X-ray absorption fine structure (EXAFS) analysis. This GaMnN film shows ferromagnetism with an extremely high Curie temperature above 740 K. We carried out EXAFS measurement of the K-edge of the Mn and Ga in this sample using a fluorescence-yield mode. The local structure of the atoms around the Mn is in good agreement with that around Ga, indicating that the Mn atoms substitute for Ga in GaN.

Computer Simulation for Analysis of Lattice Polarity of Wurtzite GaN{0001} Film by Coaxial Impact Collision Ion Scattering Spectroscopy

Signal intensities of coaxial impact collision ion scattering spectra have been computed for c-axis-oriented GaN films at various incident angles in order to analyse the lattice polarity based on the three-dimensional two-atom triple-scattering model. It was found that Ga and N signal intensities show specific incident angle dependences on (0001) and (000) surfaces. The physical image of each characteristic feature in the spectra was also clarified.

Characterization of Polarity of Plasma-Assisted Molecular Beam Epitaxial GaN{0001} Film Using Coaxial Impact Collision Ion Scattering Spectroscopy

Effects of the initial nitridation of a sapphire(0001) substrate and the thickness of a low-temperature GaN buffer layer on the polarity of GaN{0001} films grown by rf plasma-assisted molecular beam epitaxy (MBE) have been investigated by coaxial impact collision ion scattering spectroscopy (CAICISS). The volume ratios of each domain in grown GaN{0001} films, that is, (0001) and (000), were evaluated by making comparisons between the experimental results of the incident angle dependences of Ga and N signal intensities and simulated ones. It was determined that the initial nitridation of a sapphire(0001) substrate increases the volume ratio of the (000) domain in the grown film and, on the contrary, a thick low-temperature GaN buffer layer increases that of the (0001) domain.

Optimization of GaN Growth with Ga-Polarity by Referring to Surface Reconstruction Reflection High-Energy Electron Diffraction Patterns

GaN films with Ga-polarity on (0001) sapphire substrates grown by plasma-assisted molecular beam epitaxy were investigated. The optimization of the growth conditions was performed referring to reflection high-energy electron diffraction reconstruction patterns during the cooling processes. Three kinds of surface reconstruction patterns, named (5×5), (1×2) and (2×2), were observed during the cooling processes. Structural, optical and electrical properties of the GaN films, identified by different reconstruction patterns, were characterized to determine the optimal growth conditions. It was found that high-quality films can be obtained when the films show the (1×2) pattern. Under this condition, we obtained an electron mobility at room temperature of as high as 567 cm2/Vs without using a GaN template.