Masaharu Aoki

Mechanism of Yellow Luminescence in GaN

The mechanism of the yellow luminescence in GaN has been studied. The yellow band is observed in microcrystals synthesized from Ga and NH3 by direct reaction, but is not observed in needle-like crystals grown by sublimation-recrystallization. Doping with carbon emphasizes the yellow band. The characteristic excitation band is observed in C-doped GaN. The yellow band is due to a radiative transition from a shallow donor with a depth of 25 meV to a deep acceptor with a depth of 860 meV. The relation between the characteristic excitation band and the emission band is interpreted by the simple configuration coordinate model. The deep acceptor is thought to be a complex consisting of a gallium vacancy and a carbon atom substituted for a nearest neighbour of the gallium sites.

Epitaxial Growth of Undoped and Mg-Doped GaN

Gallium nitride was epitaxially grown on {0001} and {102} sapphire by the vapor phase reaction of Ga-HCl-NH3-Ar system. The growth rate on {102} sapphire is ~12 µm/hr, which is higher than that on {0001} sapphire by a factor of ~1.5. The crystal on {1102} sapphire has a lower carrier concentration than that of the same thickness on {0001} sapphire by a factor of ~0.5. The lowest carrier concentration and the highest electron mobility of undoped GaN obtained in this study were 1.6×1019 cm-3 and 78 cm2/Vs, respectively, at 300 K. When heavily doped with Mg, the crystal grown changes markedly in growth morphology. The orientation relationships developed in an undoped or lightly Mg-doped state are {0001}GaN//{0001}sapphire and {110}GaN//{102}sapphire, and in a heavily doped state {114}GaN//{0001}sapphire and {110}GaN//{102}sapphire.

Absorption and Reflection of Vapor Grown Single Crystal Platelets of β-Ga2O3

Absorption and reflection spectra of β-Ga2O3 are measured with polarized light in the wavelength region near its absorption edge. The crystals of β-Ga2O3 are grown by using a Ga/HCl/O2/Ar vapor reaction system. The platelike crystals of β-Ga2O3 have (100), (010), and (001) surfaces, and the major surface is (100). The energies of the absorption edge are observed to be 4.90 eV for E//b, 4.54 eV for E//c, and 4.56 eV for E⊥b&c at room temperature. The energies for E//b and for E//c at 77 K are larger than those at room temperature by 40 meV and 220 meV, respectively. Reflection minima are observed at 5.06 eV and 5.30 eV for E//b, and at 4.63 eV and 5.30 eV for E//c at room temperature.

Chemical vapour deposition of thin films of BN onto fused silica and sapphire

Abstract Thin films of BN were prepared by chemical vapour deposition onto fused silica and sapphire using the reaction of BCl 3 with NH 3 . The temperature of the substrate was varied between 600 and 1100°C. Transparent and smooth films of BN were obtained on fused silica and sapphire at substrate temperatures of 1000–1100°C. The growth rate of the film on sapphire was about 1 μm h −1 , and the growth rate on fused silica was about one-half that on sapphire. The films were chemically inert and adherent to the substrate. The absorption of the BN film was measured at room temperature. In the near-UV region, the main absorption peak was at about 6.2 eV and a sharp drop occurred near 5.8 eV. The sharp drop is attributed to the direct band gap. The photoluminescence of the films was measured at room temperature by excitation with light of wavelength 254 nm. A broad emission with a peak near 360 nm was observed.

Solution Growth of ZnS, ZnSe, CdS and Their Mixed Compounds Using Tellurium as a Solvent

Tellurium was found to dissolve ZnS, ZnSe, and CdS to appreciable extents at elevated temperatures. Liquid solubility data for ZnS, ZnSe, and CdS using tellurium as a solvent are presented. Liquid curves lie in orderly array, which indicates that the solubility decreases as the absolute value of the heat of formation of the compound increases. The single crystals of ZnS, ZnSe, CdS and their mixed compounds grown from the tellurium solution were mainly platelike. The concentration of tellurium in the grown crystals was examined using an X-ray microanalyser.

Linear Electrooptic Properties of ZnTe at 10.6 Microns

The low-frequency linear electrooptic coefficient r41T in semi-insulating ZnTe is measured at 10.6 µm at room temperature. The measured value is (3.9±0.2)×10-12m/V, which is only slightly smaller than that in the visible region. This result seems consistent with a lattice theory on the dispersion of r41T in crystals of a zincblende structure [D.P. Akitt et al; IEEE J. Quantum Electronics QE-6 (1970) 496]. The induced birefringence in ZnTe at 10.6 µm is about 30 percent larger than that in GaAs, and about 35 percent smaller than that in CdTe. The figure of merit for the material F1 (that is inversely proportional to the electric power necessary for a lumped modulator) at 10.6 µm is found to be twice as large in ZnTe as in GaAs, which is the material most widely used for the electrooptic light modulation at 10.6 µm.

Temperature Dependence of Photoluminescence from GaN

Photoluminescence intensities, energies, and band shapes of GaN needle crystals are studied as a function of temperature in the range of 4.2–300 K. An emission at 3.472 eV due to a bound exciton localized at a neutral donor site (so-called I2 line) dominates the spectrum at 4.2 K. A donor-acceptor pair recombination band is observed near 3.27 eV. The I2 line quenches with an activation energy of 10±1 meV in the temperature range of 30–80 K, and is not observed at higher temperatures. The activation energy corresponds to the localization energy of the I2-line exciton. The donor-acceptor band quenches at temperatures above 150 K with an activation energy of 160±20 meV, which corresponds to the acceptor binding energy.

Photoluminescenece in P-Doped GaN

The luminescent properties of a blue emission band observed in P-doped GaN have been studied. The blue band is associated with isolated phosphorus atoms. The peak position and the half-width of this band are respectively 2.88±0.01 eV and 375±5 meV at 4.2 K. The peak position shifts monotonically to lower energies as the temperature increases. The temperature dependence of the half-width follows the simple configuration coordinate model. The energy of a local-mode phonon is 50±5 meV. This value agrees with the estimated value on the assumption that a nitrogen atom vibrates against its four nearest gallium atoms neighbours. From the temperature dependence of the emission intensity, the binding energy of a hole to a phosphorus atom is calculated as 280±10 meV.

(Invited) Solution Growth of II–VI Compounds

Crystal growth of Zn and Cd chalcogenides from a variety of solutions has been studied. Solutions investigated included Te, Se, Te–Se mixture and As2Se3. Te corner in the Zn–Se–Te ternary phase diagram has been determined. High quality ZnSe crystals have been grown from Se solution, and also from As2Se3. Phosphorus doped ZnSe was grown from (As2Se3 + P) solution. A shallow acceptor related to phosphorus hasbeenestimated to be (84±4 meV). Thinlayers of ZnSe were grown on ZnS and ZnTe substrates by liquid-phase epitaxial technique utilizing Te or Te–Se as solvents.

Crystal Growth of CuGaS 2 from Te, Te–Cu and Te–Cu–S Solutions

The solution growth of CuGaS2 single crystals using Te, Te-Cu and Te-Cu-S solutions has been studied. Crystallization from Te solution results in only small dark polycrystals. Large single crystals of CuGaS2 are obtained when a prescribed amount of excess Cu is added to Te solution (Te-Cu solution). Furthermore, by adding both excess Cu and excess S to Te solution (Te-Cu-S solution) large single crystals of CuGa2. with good crystallinity are obtained. The crystal structure and lattice constants of the grown crystals are measured by RHEED and X-ray diffraction methods. Applying Vegards law to the lattice constants, we estimate the concentration of Te incorporated in the crystals and find it to be extremely small. The crystallinity of the grown crystals is examined by photoluminescence.