Yuji Kagamitani

Ammonothermal Epitaxy of Thick GaN Film Using NH4Cl Mineralizer

Single-crystalline GaN films of ≤ 110 µm in thickness have been fabricated on hydride vapor phase-grown (0001) GaN substrates by employing a relatively low-pressure (≤170 MPa) ammonothermal growth method with NH4Cl as mineralizer. Metallic Ga and polycrystalline GaN has been the chosen precursor. An average growth speed on the (0001) face of the substrate of ≥5 µm/day was observed when using metallic Ga and about 7 µm/day for GaN. The maximum growth speed of 27.5 µm/day was achieved for the film grown at a supersaturation from a combined Ga/GaN precursor. The surface morphology is not affected by the nature of the precursor.

Hydrothermal method grown large-sized zinc oxide single crystal as fast scintillator for future extreme ultraviolet lithography

The scintillation properties of a hydrothermal method grown zinc oxide (ZnO) crystal are evaluated for extreme ultraviolet (EUV) laser excitation at 13.9nm wavelength. The exciton emission lifetime at around 380nm is determined to be 1.1ns, almost identical to ultraviolet laser excitation cases. This fast response time is sufficiently short for characterizing EUV lithography light sources having a few nanoseconds duration. The availability of large size ZnO crystal up to 3in. is quite attractive for future lithography and imaging applications.

Temperature effect of ammonium halogenides as mineralizers on the phase stability of gallium nitride synthesized under acidic ammonothermal conditions

The temperature effect of the ammonium halogenide mineralizers NH4X (X = Cl, Br, I) on the phase stability of GaN synthesized under supercritical ammonothermal conditions in the temperature range 360–550 °C has been investigated. Hexagonal GaN (h-GaN) and cubic GaN (c-GaN) were crystallized. Oxygen impurities force the formation of gallium oxide at low temperatures. The tendency to form c-GaN increases from X = Cl to Br to I. Decreasing temperature supports this trend. Single-phase h-GaN can be grown from X = Cl at ≥470 °C, Br at ≥500 °C and I at ≥550 °C. Mixed mineralizers of type X = Cl + Br and Cl + I are useful to improve both the yield and the temperature stability range for h-GaN. The size of h-GaN crystals decreases from X = Cl to Br to I. The use of a h-GaN substrate has a phase-stabilizing effect and lowers the temperature stability range for overgrown h-GaN films. The choice of precursor will have an impact on the a and c lattice parameters of self-nucleated h-GaN.

Temperature dependence of scintillation properties for a hydrothermal-method-grown zinc oxide crystal evaluated by nickel-like silver laser pulses

The scintillation properties of a hydrothermal-method-grown zinc oxide (ZnO) crystal in the extreme ultraviolet region are evaluated using a nickel-like silver laser with an emission wavelength of 13.9 nm. The temperature dependence of the scintillation properties of the ZnO emission at a wavelength of ~380 nm is investigated. The emission exhibits a broad peak at 386 nm with a full width at half-maximum (FWHM) of 15 nm at room temperature (298 K). Decreasing the ZnO crystal temperature to 25 K causes the peak position to be blueshifted by 12 nm while the FWHM becomes narrower by 9 nm as compared with the room temperature case.

Ammonothermal growth of polar and non-polar bulk GaN crystal

SCAATTM has been developed as a novel ammonothermal method which enables to obtain strain free, high quality and large size bulk gallium nitride (GaN) crystals under high pressure and high temperature super-critical ammonia. One of the unique features of this technique is relatively high growth rate of more than a few hundred micrometers per day toward polar and non-polar axis with excellent crystalline quality. The morphology, X-ray rocking curve, etch pit density and electric properties are presented.

Fabrication and luminescence properties of single-crystalline, homoepitaxial zinc oxide films doped with tri- and tetravalent cations prepared by liquid phase epitaxy

Single-crystalline, homoepitaxial ZnO films doped with tri- and tetravalent cations were fabricated by liquid phase epitaxy (LPE) from low-temperature lithium chloride solution. The LPE is applied as a fast-screening tool to obtain mechanically undisturbed (0001) surfaces of the undoped and In3+ and Ge4+-doped ZnO films. Time-resolved photoluminescence characteristics upon the excitation by a femtosecond laser pulses are studied. Characteristics of the films are discussed and particularly the effect of In3+ and Ge4+ on the luminescence decay kinetics is examined. We find the double-exponential decay at room temperature consisting of an ultra fast and a slower component with decay times τ around 30–60 and 250–800 ps, respectively. High-intensity emission due to donor–acceptor pair recombination peaking around 420 nm at room temperature is obtained from the Li+, In3+ co-doped ZnO film, which shows the inverse power law decay at longer times.

Femtosecond-laser-driven photoelectron-gun for time-resolved cathodoluminescence measurement of GaN

A rear-excitation femtosecond-laser-driven photoelectron gun (PE-gun) is developed for measuring time-resolved cathodoluminescence (TRCL) spectrum of wide bandgap materials and structures such as semiconductors and phosphors. The maximum quantum efficiency of a 20-nm-thick Au photocathode excited using a frequency-tripled Al(2)O(3):Ti laser under a rear-excitation configuration is 3.6×10(-6), which is a reasonable value for a PE-gun. When the distance between the front edge of the PE-gun and the observation point is 10 mm, the narrowest electron-beam (e-beam) diameter is 19 μm, which corresponds to one tenth of the laser-beam diameter and is comparable to the initial e-beam diameter of a typical W hair-pin filament of thermionic electron-gun. From the results of TRCL measurements on the freestanding GaN grown by the ammonothermal method and a GaN homoepitaxial film grown by metalorganic vapor phase epitaxy, overall response time for the present TRCL system is estimated to be 8 ps. The value is the same as that of time-resolved photoluminescence measurement using the same excitation laser pulses, meaning that the time-resolution is simply limited by the streak-camera, not by the PE-gun performance. The result of numerical simulation on the temporal e-beam broadening caused by the space-charge-effect suggests that the present PE-gun can be used as a pulsed e-beam source for spatio-time-resolved cathodoluminescence, when equipped in a scanning electron microscope.

Annealing Effect in Terbium–Scandium–Aluminum Garnet Single Crystal

Terbium–scandium–aluminum garnet (TSAG) single crystal grown by the Czochralski method was annealed in a reducing or an oxidizing atmosphere. The oxidation state of terbium cations was studied using transmission spectra and magnetic susceptibility. By annealing in the reducing atmosphere, Tb4+ was reduced to Tb3+. In the case of annealing in the oxidizing atmosphere, some of Tb3+ ions were oxidized to Tb4+. Verdet constant and extinction ratios were measured after annealing in both atmospheres. The highest Verdet constant was obtained for the TSAG annealed in the reducing atmosphere (for λ=649.1 nm V=8.580×10-3 degOe-1cm-1). The extinction ratio of the sample annealed in the oxidizing atmosphere showed the highest value of 40.35 dB.

Time-Resolved Photoluminescence of a Two-Dimensional Electron Gas in an Al0.2Ga0.8N/GaN Heterostructure Fabricated on Ammonothermal GaN Substrates

A characteristic photoluminescence signal is identified for a two-dimensional electron gas (2DEG) confined at an Al0.2Ga0.8N/GaN heterointerface fabricated on an ammonothermal GaN (AT-GaN) substrate. The use of a gas-phase synthesized NH4Cl acidic mineralizer reduced oxygen contamination in AT-GaN by two orders of magnitude, and metalorganic vapor phase epitaxy of atomically smooth, coherent AlGaN/GaN heterostructures was realized. The emission originating from the 2DEG is interpreted using self-consistent Schrodinger–Poisson calculation, taking the interfacial immobile charge due to polarization discontinuity into account. The initial decay time at low temperature was close to that of the bulk free excitons, reflecting the lifetime of photoexcited holes.

Acidic ammonothermal growth of GaN crystals using GaN powder as a nutrient

Ammonothermal crystal growth of gallium nitride (GaN) was realized for the first time using GaN powder as a nutrient with growth rates of about 61 μm per day on both Ga and N faces. The GaN powder was synthesized by ammonolysis of Ga metal in the presence of NH4I. The crystal quality of the as-grown GaN wafers was characterized by scanning electron microscope (SEM), X-ray rocking curve (XRC) and photoluminescence (PL) measurements. We examined the effect of different nutrients, including polycrystalline GaN, Ga metal, and GaN powder, on the acidic ammonothermal crystal growth of GaN using an NH4I-based mineralizer. Our results suggest that growth rate and crystal quality of GaN depend largely on the type of nutrient. Polycrystalline GaN as a nutrient can afford high growth rates of up to 150 and 237 μm per day on Ga and N faces, respectively. Growth rates up of to 33 μm per day were achieved using Ga metal as a nutrient. However, SEM, XRC, and PL measurements of GaN crystals grown using different nutrients indicate that Ga metal and GaN powder nutrients can provide better crystal quality of GaN in terms of surface morphology and crystal uniformity than polycrystalline GaN.