S. Gonda

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.

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.

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.

Schottky barrier diodes on 3C‐SiC

Schottky barrier contacts have been made on n‐type 3C‐SiC epitaxially grown by chemical vapor deposition, and their characteristics were studied by the capacitance and photoresponse measurements. By evaporating Au onto chemically etched surfaces of 3C‐SiC, good quality Schottky barrier junctions have been obtained. The barrier height determined by the capacitance measurements is 1.15 (±0.15) eV, while the height by the photoresponse measurements is 1.11 (±0.03) eV. These values are about a half of the energy band gap Eg at room temperature.

Determination of the conduction‐band discontinuities of GaAs/AlxGa1−xAs interfaces by capacitance‐voltage measurements

The relation between the conduction‐band discontinuity ΔEc and the Al composition x of refined GaAs/AsxGa1−xAs (x<0.42) heterointerfaces was determined by means of capacitance‐voltage measurements. The resulting relation, ΔEc=0.67ΔEg, is different from Dingle’s rule. The interface charge density σ of a series of the samples was also investigated in relation to the reliability of the determination of ΔEc. It was found that σ less than 1×1011/cm2 is required to determine ΔEc with the precision of ±10 meV.

Infrared study of amorphous B1−xCx films

Amorphous boron carbide (a‐B1−xCx) is believed to have an icosahedron‐based random network. In this paper, vibrational properties of a‐B1−xCx films are studied by IR and Raman spectra, placing particular emphasis on the interpretation of the most prominent 1100‐cm−1 band associated with the B–C bond. The 1100‐cm−1 band appears in both IR and Raman spectra, and the frequency variation and the intensity as a function of the C content are examined, together with evaluation of the absolute absorption coefficients. Within the framework of the impurity‐induced vibration theory, the 1100‐cm−1 band is characterized. The estimation of the force constants by the observed frequencies leads to a conclusion that the vibration can be classified as an extrinsic and preferably a local mode. This vibration is well described as the stretching mode of a localized two‐center bond between the B and C atoms. In this sense, the C atom in amorphous B1−xCx is not regarded as a network constituent. The frequency shift with the C c...

Gas source molecular beam epitaxy growth of GaN on C-, A-, R- and M-plane sapphire and silica glass substrates

GaN layers are grown on C-, A-, R- and M-plane sapphire substrates by gas source molecular beam epitaxy (MBE). The c-axis of GaN is perpendicular to the surface plane and photoluminescence spectra exhibit strong and sharp (full width at half maximum≤39 meV at 77 K) excitonic emission without deep level emission for all cases. GaN layers grown on silica glass substrates also have the c-axis perpendicular to the surface, although they are poly-crystalline. They exhibit an n-type conduction with an electron concentration of 7×10-16 cm-3 and a mobility of 23 cm2/Vs. They also exhibit strong photoluminescence comparable to that of GaN grown on sapphire substrates, although showing a wide spectral half width (245 meV at 77 K). GaN layers grown on glass substrate is considered promising for fabrication of large area and low cost light emitting devices and solar cells.

High Quality GaN Growth on (0001) Sapphire by Ion-Removed Electron Cyclotron Resonance Molecular Beam Epitaxy and First Observation of (2×2) and (4×4) Reflection High Energy Electron Diffraction Patterns

GaN layers are grown on (0001) sapphire substrate by electron cyclotron resonance molecular beam epitaxy (ECR-MBE) using an ECR plasma cell with ion removal magnets on the cell top for the nitrogen source. The efficiency of the ion removal magnets in this ECR plasma cell is 99%. High-quality GaN layers are obtained. In particular, (2×2) and (4×4) RHEED (reflection high-energy electron diffraction) patterns are observed during GaN growth and during cooling after growth, respectively, indicating a flat and smooth surface of GaN. These results show the superiority of the ion-removed ECR plasma cell.

Optical properties of GaN layers grown on C-, A-, R-, and M-plane sapphire substrates by gas source molecular beam epitaxy

GaN layers were grown on C-, A-, R-, and M-plane sapphire substrates by the electron cyclotron resonance–molecular beam epitaxy technique. We addressed a combined utilization of Raman spectroscopy, photoluminescence (PL) and reflectance measurements to investigate the optical properties of these high-quality GaN layers. First order optical phonons of A1, E1, and E2 symmetries were observed in the Raman spectra and the peaks are indicative of the wurtzite crystal structure. All three intrinsic exciton transitions arising from A, B, and C interband transitions were observed in reflectance measurements. The PL spectra were dominated by A and B free exciton transitions and the recombination of an exciton bound to a neutral donor. The experimental data clearly revealed a thickness-dependent change of the biaxial strain in the GaN layers grown on (0001) C-plane sapphire. The residual strain induced in these layers was found to have a strong influence in determining the energies of the excitonic transitions. Res...