Kakuya Iwata

Uniaxial locked epitaxy of ZnO on the a face of sapphire

High-quality, c-oriented ZnO epitaxial films have been grown on the a surface using molecular-beam epitaxy. The use of a-oriented sapphire eliminates rotational domains and related structural defects which have limited the use of ZnO in electronic applications. The ZnO epitaxial layers are uniquely oriented with the ZnO/sapphire orientational relationship [0001]‖[1120] and 〈1120〉‖[0001]. This unique orientation is a consequence of the anisotropy of the a-sapphire surface in conjunction with a strong correlation along a single direction leading to the term uniaxial locked epitaxy. High-resolution x-ray diffraction measurements show an increase in x-ray lateral coherence length from several tens of nanometers to >0.7 μm for growth of c-oriented ZnO on the a surface as opposed to the c surface of sapphire.

Interactions between gallium and nitrogen dopants in ZnO films grown by radical-source molecular-beam epitaxy

It has been recently predicted that the co-doping of an acceptor (nitrogen) and a donor (aluminum, gallium, indium) in a 2:1 ratio will dope ZnO p-type due to a reduction in the Madelung energy making the nitrogen acceptor energy level more shallow. We have been growing gallium and nitrogen co-doped ZnO films by radical-source molecular-beam epitaxy by use of oxygen and nitrogen radicals supplied via rf radical source cells. Diode-like current–voltage characteristics and donor acceptor pair-like photoluminescence emission were observed for a Ga and N doped ZnO film grown on an undoped ZnO buffer layer. However, Hall measurements revealed that the conductivity was n-type. Formation of a non-ZnO phase in the sample was confirmed by secondary ion mass spectroscopy and x-ray diffraction measurements. Zn and Zn+O secondary ion intensities fell sharply by two orders of magnitude in going from the undoped ZnO layer to the highly co-doped ZnO. X-ray diffraction measurements indicated the formation of ZnGa2O4.

Band-gap modified Al-doped Zn1−xMgxO transparent conducting films deposited by pulsed laser deposition

Al-doped Zn1−xMgxO films have been deposited on glass substrates at a substrate temperature of 200°C by a pulsed laser deposition system. A resistivity of 3×10−4Ωcm was obtained at x=0.06. Film resistivity was found to increase with further increases in Mg composition. The maximum band gap of films with a resistivity ρ⩽1×10−3Ωcm was found to be 3.97eV, demonstrating band-gap engineering possibilities in the range of Eg=3.5–3.97eV with a resistivity ρ⩽1×10−3Ωcm. The average transmittance of the films was higher than 90% in the wavelength region λ=400–800nm, a range suitable for transparent conducting film applications.

Improved External Efficiency InGaN-Based Light-Emitting Diodes with Transparent Conductive Ga-Doped ZnO as p-Electrodes

Transparent conductive Ga-doped ZnO (ZnO:Ga) was fabricated to serve as p-contacts of InGaN-based light-emitting diodes (LEDs) using molecular-beam epitaxy. As-grown ZnO:Ga films typically have resistivities of ?=2-4?10-4 ??cm, and over 80% transparency in the near UV and visible wavelength ranges. The current-voltage characteristics between as-grown ZnO:Ga contacts and p-GaN layers were ohmic. The brightness of LEDs fabricated with ZnO:Ga p-contacts was nearly double compared to LEDs with conventional Ni/Au p-contacts. We obtained the external efficiency as high as 20.8% in the case of the near UV LED. The forward voltage at 20 mA was found not to increase even after the lamp LED with ZnO:Ga were kept for 80 h in high humidity and high temperature environments.

Growth of Undoped ZnO Films with Improved Electrical Properties by Radical Source Molecular Beam Epitaxy

High-quality undoped ZnO epitaxial films with mobilities as high as 120 cm2V-1s-1 and carrier concentrations as low as 7.6 ×1016 cm-3 have been grown on (1120) a-sapphire substrates using low-temperature buffer layers, a slow substrate cooling process and a modified oxygen radical cell. Pole figure measurements reveal that a-plane sapphire substrates are effective for the elimination of 30° rotation domains, which usually appear in the case of ZnO growth on c-sapphire. The low-temperature buffer layers allow high-temperature growth, because initial ZnO growth does not occur with high initial growth temperature. The use of slow substrate cooling prevents the deterioration of the electrical properties of the ZnO films. Use of quartz insulators in the oxygen radical cell eliminates aluminum contamination, which is a serious problem when using conventional alumina insulators.

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.

Degenerate layers in epitaxial ZnO films grown on sapphire substrates

ZnO films were grown on low-temperature (LT) buffer layers on sapphire a-plane (11–20) substrates by radical source molecular-beam epitaxy. The LT buffer layers were found to effect the electrical properties of subsequently grown undoped ZnO films, and their presence was found to be indispensable for the growth of films with low carrier concentrations and high mobilities. Temperature-dependent Hall measurements showed the existence of a degenerate region related to the LT buffer layers. It was found that the effects of degenerate layers could be reduced by using annealing treatments and nitrogen doping of the LT buffer layers. The dominant residual donor energy of 110 meV was found to be different than previously reported. The carrier concentration of a ZnO film fabricated using a nitrogen-doped buffer layer was 7.5×1016 cm−3 with a mobility of 132 cm2/V s at room temperature.

New III-V Compound Semiconductors TlInGaP for 0.9 μm to over 10 μm Wavelength Range Laser Diodes and Their First Successful Growth

New III–V compound semiconductors TlInGaP (thallium indium gallium phosphide) lattice-matched to InP are proposed for 0.9 µm to over 10 µm wavelength range laser diodes and their first successful growth is reported by gas-source molecular beam epitaxy. A type-I band lineup and a larger band discontinuity in the conduction band than in the valence band are expected for their heterostructures. They also have the potential to exhibit a temperature-independent band-gap energy (wavelength), which is promising for the fabrication of lasers that can be used in wavelength division multiplexing (WDM) optical fiber communication. Grown layers exhibit (2×4) surface reconstruction and have mirror-like surfaces. Phase separation is not observed in this material system by X-ray diffraction measurement.

GaN-Rich Side of GaNAs Grown by Gas Source Molecular Beam Epitaxy

A large variation in wavelength from the ultraviolet to longer than 2 µm could be achieved in the GaN-rich side of the GaN1-xAsx alloy due to the large bowing of bandgap energy. Layers of GaN1-xAsx are grown on (0001) sapphire substrates by electron cyclotron resonance molecular beam epitaxy (ECR-MBE) using an ion-removed ECR radical cell after the growth of GaN buffer layers. During the growth of GaN1-xAsx layers, a streaky reflection high-energy electron diffraction (RHEED) pattern was observed. The excitonic photoluminescence (PL) peak from the GaN-rich side of the GaN1-xAsx layer shows a large red shift as the As content changes. When an As content of up to x=0.009 is attained, a bandgap bowing parameter of 19.6 eV is experimentally obtained. Such a large value of the bowing parameter is promising for applications to optical devices operating over wide range of wavelength.