Sadafumi Yoshida

Improvements on the electrical and luminescent properties of reactive molecular beam epitaxially grown GaN films by using AlN‐coated sapphire substrates

The electrical and luminescent properties of the GaN epitaxial films grown on AlN‐coated sapphire by reactive molecular beam epitaxy have been studied. The GaN films on AlN epitaxial films have larger Hall mobilities and show more intense cathodoluminescence peaks at a wavelength of 360 nm than those of the GaN films grown directly on sapphire, which suggests that the crystal qualities of GaN films are improved by use of AlN‐coated sapphire as substrates. The lattice matching and small difference of the thermal expansion coefficients between GaN and AlN are considered to result in the improvements.

Properties of AlxGa1−xN films prepared by reactive molecular beam epitaxy

Single‐crystal films of the solid solution AlxGa1−xN of the entire composition range have been fabricated on sapphire and silicon substrates by reactive molecular beam epitaxy (MBE) at 700 °C. The properties of the films have been studied by the reflection high energy electron diffraction technique, x‐ray diffraction, and electrical and optical measurements. The lattice constant of the film is not a linear function of the composition, and the fundamental absorption edge also shows nonlinear dependence on the composition. The narrow intense peak of the cathodoluminescence concerned with the band to band or shallow impurity band transition, varies from 3.4 to 6 eV with the composition, which suggests the feasibility of AlxGa1−xN films grown by reactive MBE for optical devices in the ultraviolet spectral region.

Epitaxial growth of cubic and hexagonal GaN on GaAs by gas-source molecular-beam epitaxy

GaN epilayers were grown on GaAs substrates by gas‐source molecular‐beam‐epitaxy technique using dimethylhydrazine as a nitrogen source. It was found that cubic GaN grows on GaAs (001) surfaces epitaxially, while hexagonal GaN grows on GaAs (111) surfaces, from the analyses of x‐ray diffraction and reflection high‐energy electron diffraction patterns. Cathodoluminescence measurements suggested that the band‐gap energy of cubic GaN is around 0.37 eV larger than that of hexagonal GaN.

Preparation and structure of carbon film deposited by a mass‐separated C+ ion beam

Carbon films were deposited using mass‐separated C+ ions of 300 and 600 eV. The films have diamond‐like characteristics such as transparency in the visible spectral region with wavelengths longer than about 650 nm and in the infrared, and high electrical resistivity. Transmission electron diffraction analysis shows that the film is amorphous and does not contain graphitically bonded carbon atoms. Kα x‐ray emission spectrum of the carbon in the film agrees well with that of diamond. In the x‐ray photoemission spectrum of the film, no characteristic energy loss due to π plasmon was observed. The atomic density of the film calculated from the energy loss due to the plasma oscillation of valence electrons is 1.7×1023 atoms/cm3, which is in good agreement with that of diamond. These results indicate that the film deposited using C+ ion beam consists of tetrahedraly bonded carbon atoms.

Growth and characterization of cubic GaN

We have grown high-quality cubic GaN epilayers on GaAs and 3C-SiC substrates by molecular beam epitaxy technique using dimethylhydrazine or ammonia/nitrogen plasma as a nitrogen source. An X-ray diffraction peak width of 16 min and a low-temperature photoluminescence peak width of 19 meV were achieved. Various surface reconstruction transitions have been observed for cubic GaN(0 0 1) surfaces, recently. These results, along with previously published studies on cubic nitrides, are summarized, and the current status of the growth and characterization of cubic nitrides including AlN and InN is discussed.

High‐temperature electrical properties of 3C‐SiC epitaxial layers grown by chemical vapor deposition

Electrical properties of 3C‐SiC layers, epitaxially grown on silicon by chemical vapor deposition, have been investigated at the temperatures between room temperature and 850 °C. In this temperature range, the electron mobility changes with temperature as μH∼T−1.2∼−1.4. The weaker temperature dependence of mobility and the larger mobilities compared with other polytypes of SiC suggest that 3C‐SiC is a promising material for devices operated at high temperatures.

Temperature dependence of electrical properties of n‐ and p‐type 3C‐SiC

Electrical properties of unintentionally doped n‐type and Al‐doped p‐type 3C‐SiC layers, epitaxially grown on Si by chemical vapor deposition, have been investigated at temperatures between 10 and 1000 K. Activation energies of Al acceptors and residual donors obtained from the temperature dependence of carrier density are 160 and 18 meV, respectively. 40%–60% of Al acceptors in the p‐type epilayers are compensated, and hole mobility is limited by acoustic phonon scattering above 300 K and by ionized impurity scattering below 250 K.

Monitoring surface stoichiometry with the (2×2) reconstruction during growth of hexagonal‐phase GaN by molecular beam epitaxy

Reflection high‐energy electron diffraction is used to study the stability of the (2×2) reconstruction on the (0001) surface of hexagonal‐phase GaN as a function of growth parameters. The relationship between the critical conditions for existence of the reconstruction, which corresponds to a unique surface stoichiometry, is used to show the interdependency and scalability of growth parameters. A model is proposed to describe the stoichiometric balance of the species arriving on the surface at the critical conditions for observation of the reconstruction. Hall mobility of the GaN epitaxial layers was improved by growing under these conditions.

The origin of persistent photoconductivity and its relationship with yellow luminescence in molecular beam epitaxy grown undoped GaN

The results of persistent photoconductivity (PPC) and photoluminescence measurements made on radio-frequency plasma assisted molecular beam epitaxy grown, undoped, GaN are reported in this work. Hexagonal GaN (h-GaN) epilayers grown on sapphire and cubic GaN (c-GaN) epilayers grown on GaAs and cubic SiC substrates, are employed in this study. Three clear experimental evidences are reported to claim that the commonly seen persistent photoconductivity and yellow luminescence (YL) are related to each other through the same defect. First, PPC is observed only in those samples which show YL. Second, the threshold (the minimum photon energy required) to observe PPC is determined as 1.6±0.2 eV, which is almost at the same energy at which the YL band starts raising. Third, the photocurrent increases monotonically from 1.8 to 2.2 eV, which is consistent with the broad nature of YL band.

Intrinsic Defects in Cubic Silicon Carbide

Irradiation of fast particles like 1 MeV electrons and 2 MeV protons was made for single crystalline cubic silicon carbide (3C-SiC) grown epitaxially on Si by chemical vapor deposition in order to introduce point defects in the material. Intrinsic point defects in 3C-SiC have been characterized by electron spin resonance (ESR), positron annihilation spectroscopy (PAS), Hall and photoluminescence (PL) techniques. The structure and annealing behavior of intrinsic defects, e.g. monovacancies at silicon and carbon sublattice sites, are described based on the results obtained by ESR and PAS. The contributions of such point defects to electrical and optical properties of 3C-SiC are discussed using the Hall and PL results, with a brief review of published work.