Cathodoluminescence study of h- and c-GaN single crystals grown by a Na or K flux

Luminescence properties of hexagonal (h-) and cubic (c-) GaN freestanding single crystals were studied by means of cathodoluminescence spectroscopy. The h-GaN crystals of about 0.2–2 mm in dimension were synthesized at 750 °C by the reaction of Ga and N₂ in a Na flux, while c-GaN crystals of about 0.3 mm or less in a K flux. The h-GaN showed rather strong band edge emission at room temperature compared with the crystal grown by using NaN₃ as a nitrogen source. At 20 K, the band edge emission of h-GaN was split into four peaks. The main energy peak position was 3.478 eV, which was assigned as the A-free exciton emission. The energy position of the main peak of c-GaN was 3.268 eV. Assuming the binding energies of excitons in h- and c-GaN as 25 and 26 meV, respectively, the energy difference of bandgap between h- and c-GaN is estimated to be 209 meV. Since these crystals are free from strain from the substrates, the peak energies are reliable for the intrinsic GaN crystals. The full widths at half maximum of the emission of c-GaN were much broader than those of h-GaN, suggesting that the cubic phase is much defective compared with the hexagonal one.     

Effect of growth temperature on polytype transition of GaN from zincblende to wurtzite

We have investigated effect of growth temperature on the polytype conversion of cubic GaN (c-GaN) grown on GaAs (001) substrates by MOVPE. It was found that the polytype transition of GaN from zincblende (cubic) to wurtzite (hexagonal) structures is much dependent on the growth temperature. Transmission electron microscopy (TEM) observations demonstrate that the GaN grown layers have the cubic structure (c-GaN) and contain bands of stacking faults (SFs) parallels to {111} planes. For low growth temperatures (∼ 900 °C), XRD results demonstrate that the GaN grown layers with the cubic phase purity higher than 85% were obtained. No different types of single diffraction spots, indicating the incorporation of single-crystal h-GaN, on the selected area diffraction (SAD) pattern was observed. It is also found that a density of SFs decreases with the distance from the interface of c-GaN/GaAs. On the other hand, GaN layers exhibited a transition from cubic to mixed cubic/hexagonal phase under conditions of increasing growth temperature (∼ 960 °C) as determined using TEM-SAD technique with complementary XRD and PL observations. In addition, the optical characteristics of c-GaN layers are shown to be very sensitive to the presence of the single-crystal h-GaN.     

Cathodoluminescence study of h– and c–GaN single crystals grown by a Na or K flux

Luminescence properties of hexagonal (h-) and cubic (c-) GaN freestanding single crystals were studied by means of cathodoluminescence spectroscopy. The h-GaN crystals of about 0.2–2 mm in dimension were synthesized at 750 °C by the reaction of Ga and N2 in a Na flux, while c-GaN crystals of about 0.3 mm or less in a K flux. The h-GaN showed rather strong band edge emission at room temperature compared with the crystal grown by using NaN3 as a nitrogen source. At 20 K, the band edge emission of h-GaN was split into four peaks. The main energy peak position was 3.478 eV, which was assigned as the A-free exciton emission. The energy position of the main peak of c-GaN was 3.268 eV. Assuming the binding energies of excitons in h- and c-GaN as 25 and 26 meV, respectively, the energy difference of bandgap between h-and c-GaN is estimated to be 209 meV. Since these crystals are free from strain from the substrates, the peak energies are reliable for the intrinsic GaN crystals. The full widths at half maximum of the emission of c-GaN were much broader than those of h-GaN, suggesting that the cubic phase is much defective compared with the hexagonal one.     

X-ray diffraction reciprocal space and pole figure characterization of cubic GaN epitaxial layers grown on (0 0 1) GaAs by molecular beam epitaxy

X-ray diffraction reciprocal space maps and pole figures were used to analyse the cubic GaN epitaxial layers grown on (0 0 1) GaAs by r.f. plasma source MBE; the presence of hexagonal phase in cubic GaN layers was detected by high resolution x-ray analysis and the relationships among various crystal axes of cubic and hexagonal phase GaN were analysed with respect to V/III source-supply ratio. As for the growth conditions of the epitaxial layers, the V/III ratio was found to drastically affect the quality of the layers. High-temperature growth under near-stoichiometric conditions was necessary to obtain high quality epitaxial layers. It was found that inclusion of the hexagonal phase in the cubic GaN layers could be less than 0.4%, though previously reported typical c-GaN epitaxial layers included as much as 10–20% hexagonal phase GaN. On the basis of the measurements and analyses of reciprocal space maps and pole figures, it was revealed that the orientation of crystal axes of the hexagonal phase was unique in the present GaN epitaxial layers and they were different from those of previously reported c-GaN epitaxial layers.     

Homoepitaxial growth of GaN single crystals using gallium hydride

The growth of gallium nitride (GaN) single crystals was performed using gallium hydride (GaH_x) as a Ga source. In this study, a GaN film with a smooth surface was obtained by homoepitaxial growth on a GaN film commercially produced by the Metal Organic Chemical Vapor Deposition (MOCVD-GaN). Photoluminescence spectrum of grown film revealed that GaN film obtained in this study shows excellent optical property. An increase in the growth rate was achieved with the amount of GaH_x (x = 1, 2, 3) supplied to the growth portion. The amount of GaH_x produced by a reaction between Ga and H₂ was increased with the residence time of H₂ in a Ga melt. The dependence of the growth rate and surface morphology on the growth condition was examined using Scanning Electron Microscopy (SEM).     

Determination of the scattering lengths of gallium isotopes by neutron interferometry with PNO-apparatus in JRR-3M

For carrying out experiments in the field of the so-called precise neutron optics (PNO), we have implemented special multi-purpose apparatus called the “PNO-apparatus” at JRR-3M. Making use of an Si triple-Laue (LLL) neutron interferometer with the PNO-apparatus, we successfully determined the coherent neutron scattering lengths of gallium isotopes, ⁶⁹Ga and ⁷¹Ga. The results are 8.053±0.013 fm for ⁶⁹Ga and 6.170±0.011 fm for ⁷¹Ga, respectively.     

Investigation on the role of indium in the removal of metallic gallium from soft and hard sputtered GaN (0001) surfaces

Cleaning of GaN by argon sputtering and subsequent annealing introduces metallic gallium on the GaN surface. Once formed, this metallic gallium can be difficult to remove. It has a strong influence on the Fermi level position in the band gap and poses a problem for subsequent epitaxial growth on the surface. We present a method of removing metallic gallium from moderately damaged GaN surfaces by deposition of indium and formation of an In-Ga alloy that can be desorbed by annealing at ~ 550 °C. After the In-Ga alloy has desorbed, photoemission spectra show that the Ga3d bulk component becomes narrower indicating a smoother and more homogeneous surface. This is also reflected in a sharper low energy electron diffraction pattern. On heavily damaged GaN surfaces, caused by hard sputtering, larger amount of metallic gallium forms after annealing at 600 °C. This gallium readily alloys with deposited indium, but the alloy does not desorb until a temperature of 840 °C is reached and even then, traces of both indium and metallic gallium could be found on the surface.     

Radical-beam gettering epitaxy of GaN layers on nitrogen-ion-implanted GaAs substrates

Thin GaN films have been grown on N⁺-ion-implanted single-crystal GaAs(111) substrates by radical-beam gettering epitaxy, and their structural perfection has been assessed by high-resolution x-ray diffraction. At growth temperatures from 770 to 970 K, the layers consist of hexagonal GaN and have mirror-smooth surfaces. Nitrogen-ion implantation into the substrate favors the formation of a sharp film/substrate interface owing to radiation-enhanced gallium diffusion. Analysis of the GaN/GaAs structures by Auger electron and x-ray photoelectron spectroscopies in combination with depth profiling indicates that the GaN layer is enriched in gallium. The N: Ga atomic ratio in the films is 0.98: 1, which is attributable to radiation-enhanced gallium diffusion.     

NEXAFS and XPS study of GaN formation on ion-bombarded GaAs surfaces

Interaction of low-energy nitrogen ions (0.3-2 keV N₂⁺) with GaAs (100) surfaces has been studied by X-ray photoemission spectroscopy (XPS) around N 1s and Ga 3d core-levels and near-edge X-ray absorption fine structure (NEXAFS) around the N K-edge, using synchrotron radiation. At the lowest bombardment energy, nitrogen forms bonds with both Ga and As, while Ga-N bonds form preferentially at higher energies. Thermal annealing at temperatures above 350 °C promotes formation of GaN on the surface, but it is insufficient to remove disorder introduced by ion implantation. We have identified nitrogen interstitials and anti-sites in NEXAFS spectra, while interstitial molecular nitrogen provides a clear signature in both XPS and NEXAFS. The close similarity between NEXAFS spectra from thin GaN films and ion-bombarded GaAs samples supports our proposition about formation of thin GaN films on ion-bombarded GaAs.     

Metalorganic molecular beam epitaxy of GaN and Al(Ga)N on GaAs(001) studied using laser reflectometry and reflectance anisotropy spectroscopy

We report the growth of GaN and AlGaN films on GaAs (0 0 1) substrates in the temperature range 400–800 °C by metalorganic molecular beam epitaxy. An r.f. plasma nitrogen source was used in conjunction with triethylgallium and ethyl-dimethylamine-alane group III sources. Growth was initiated using either a low temperature AlN buffer layer or a graded arsenide-nitride buffer layer. The growth was monitored in real time using in-situ laser reflectometry. The temperature dependence of growth rates for the nitride layers are compared with their arsenide analogs. The relative growth rate of gallium nitride/gallium arsenide from triethylgallium was found to be in the range 54–60%, the Ga incorporation rates are closely comparable when the higher density of GaN is taken into account. The range of growth temperatures for gallium nitride extends to higher temperatures compared with gallium arsenide probably due to lower evaporation rates of Ga bound to the nitride surface. Reflection anisotropy spectra indicate that atomic nitrogen readily reacts with the GaAs (0 0 1)-c (4 × 4) As-stabilized surface at temperatures as low as 400 °C but without the gross faceting that has been observed at higher temperatures.     

Growth of GaN films on nitrided GaAs substrates using hot-wire CVD

Epitaxial growth of cubic-type gallium nitride (c-GaN) by hot-wire CVD on GaAs(100) substrates was investigated. Prior to the epitaxial growth, a nitridation layer was formed using ammonia plasma generated by electron cyclotron resonance (ECR). It was found that the crystal phase of the epitaxial layer was predominantly determined by that of the nitrided layer. The best nitridation condition using ECR plasma for the growth of the GaN films with preponderant cubic-type structure was obtained.     

A computational study of ion-implanted beryllium diffusion in gallium arsenide

The diffusion of implanted beryllium in gallium arsenide at 100 keV for doses of 1 × 10¹³ and 1 × 10¹⁴ cm⁻² during post-implant RTA were studied and simulated at temperatures of 700–900 °C for 1–4 min. The observed Be diffusion profiles, obtained by the SIMS technique, can be satisfactorily explained in terms of a “kick-out” model of the substitutional-interstitial diffusion mechanism, involving singly ionized Be and doubly ionized Ga interstitial species. The generation of the excess Ga interstitials, according to the “plus-one” approach, and its annihilation in the local Ga interstitial sink region were taken into account. The corresponding coupled partial differential equations of the relevant diffusion model were solved numerically with proper initial and boundary conditions using the computational algorithms based on finite-difference approximations.     

Synthesis of gallosilicates — MTW- type structure zeolites: Evidence of Ga-substituted T atoms

Gallosilicates with the ZSM-12 structure have been synthesized from the reaction mixture of sodium silicate, gallium nitrate, and methyl triethylammonium bromide at 353 K in an autoclave. XRD results associated with an i.r. study of the lattice vibrations indicated that highly crystalline materials with the ZSM-12 structure were obtained. ⁷¹Ga MAS n.m.r. studies along with ²⁹Si measurements showed that Ga is in tetrahedral positions and the Si/Ga atomic ratio = 70. T.p.d. of ammonia adsorbed on H-GaZSM-12 clearly showed the acid nature of the material, which supports strongly the presence of Ga³⁺ in the zeolite framework.     

Structural,optical, and electrical characteristics of 70 Mev Si<Superscript>5+</Superscript> ion irradiation-induced nanoclusters of gallium nitride

We report the formation of nanoclusters on the surface of gallium nitride (GaN) epilayers due to irradiation with 70 MeV Si ions with the fluences of 1 × 10¹² ions/cm² at the liquid nitrogen temperature (77 K). GaN epilayers were grown using a metal organic chemical vapor deposition system. Omega scan rocking curves of (002) and (101) plane reflection shows irradiation-induced broadening. Atomic force microscopy imagery revealed the formation of nanoclusters on the surface of the irradiated samples. X-ray photoelectron spectroscopy confirms that the surface features are composed of GaN. The effects of ion-beam-produced lattice defects on the surface, electrical, and optical properties of GaN were studied and possible mechanisms responsible for the formation of nanoclusters during irradiation have been discussed.     

Characterization of wurtzite indium nitride synthesized from indium oxide by In-115 MAS NMR spectroscopy

Wurtzite indium nitride (w-InN) powders synthesized from the reaction of indium oxide (In₂O₃) with ammonia were characterized by ¹¹⁵In magic-angle spinning (MAS) NMR spectroscopy and nitrogen analyzer. The powders were not a single phase of w-InN but a mixture of w-InN and In-incorporated w-InN. The incorporation of In metal in InN lattice due to thermal decomposition caused the ¹¹⁵In MAS NMR peak of w-InN to be downfield shifted and might be responsible for the increase in the band gap of w-InN.     

Nanostructured GaN nucleation layer for light-emitting diodes

This paper addresses the formation of nanostructured gallium nitride nucleation (NL) or initial layer (IL), which is necessary to obtain a smooth surface morphology and reduce defects in h-GaN layers for light-emitting diodes and lasers. From detailed X-ray and HR-TEM studies, researchers determined that this layer consists of nanostructured grains with average grain size of 25 nm, which are separated by small-angle grain boundaries (with misorientation approximately 1 degrees), known as subgrain boundaries. Thus NL is considered to be single-crystal layer with mosaicity of about 1 degrees. These nc grains are mostly faulted cubic GaN (c-GaN) and a small fraction of unfaulted c-GaN. This unfaulted Zinc-blende c-GaN, which is considered a nonequilibrium phase, often appears as embedded or occluded within the faulted c-GaN. The NL layer contained in-plane tensile strain, presumably arising from defects due to island coalescence during Volmer-Weber growth. The 10L X-ray scans showed c-GaN fraction to be over 63% and the rest h-GaN. The NL layer grows epitaxially with the (0001) sapphire substrate by domain matching epitaxy, and this epitaxial relationship is remarkably maintained when c-GaN converts into h-GaN during high-temperature growth.     

Electrical and optical properties of silicon-doped gallium nitride polycrystalline films

Si-doped GaN films in polycrystalline form were deposited on quartz substrates at deposition temperatures ranging from 300–623 K using r.f. sputtering technique. Electrical, optical and microstructural properties were studied for these films. It was observed that films deposited at room temperature contained mainly hexagonal gallium nitride (h-GaN) while films deposited at 623 K were predominantly cubic (c-GaN) in nature. The films deposited at intermediate temperatures were found to contain both the hexagonal and cubic phases of GaN. Studies on the variation of conductivity with temperature indicated Mott’s hopping for films containing c-GaN while Efros and Shklovskii (E-S) hopping within the Coulomb gap was found to dominate the carrier transport mechanism in the films containing h-GaN. A crossover from Mott’s hopping to E-S hopping in the ‘soft’ Coulomb gap was noticed with lowering of temperature for films containing mixed phases of GaN. The relative intensity of the PL peak at ∼2·73 eV to that for peak at ∼3·11 eV appearing due to transitions from deep donor to valence band or shallow acceptors decreased significantly at higher temperature. Variation of band gap showed a bowing behaviour with the amount of cubic phase present in the films.     

Growth of c-GaN films on GaAs(100) using hot-wire CVD

Cubic gallium nitride (GaN) films were grown on nitrided layers of GaAs(100) by hot-wire chemical vapor deposition. The nitrided layer was also formed by NH_x radicals generated on a tungsten hot-wire surface. Nitridation conditions for the growth of GaN with a cubic-type structure were investigated. As a result, GaN film with a preponderant cubic phase was grown on the GaAs surface layer nitrided at a substrate temperature of 550 °C, a filament temperature of 1200 °C and an ammonia (NH₃) pressure of 1 Torr.     

Phase separation in oxygen deficient gallium oxide films grown by pulsed-laser deposition

The oxygen pressure and the substrate temperature during pulsed-laser deposition play a major role on the nature and properties of gallium oxide films. At moderate substrate temperature (673 K) and under high vacuum (10⁻⁷ mbar) a nanocomposite film composed of Ga metallic clusters embedded in a stoichiometric Ga₂O₃ matrix may be obtained without postdeposition annealing. The growth of such films is due to a phase separation of largely oxygen deficient metastable gallium oxide films Ga₂O_x (x = 2.3) into the most stable phases (Ga and Ga₂O₃) and occurs for particular growth conditions. The composition and the surface morphology of films as well as their electrical behaviour are interpreted according to the effects of the parameters governing this phase separation (oxygen deficiency and temperature). It is suggested that the initial step in the disproportionation reaction is the formation of stoichiometric Ga₂O₃ nanocrystallites in the metastable sub-oxide Ga₂O_x phase. The crystallization of such nanosize particles is governed by the local distribution of oxygen and gallium species impinging the substrate during the growth and allowing nucleation centre with the Ga₂O₃ composition.     

Preparation of single phase gallium nitride from single crystal gallium arsenide

An analysis has been conducted on the final products obtained in attempts to prepare single phase gallium nitride from single crystal gallium arsenide. When the intermediate oxide phase was nitrided in pure ammonia it was found that (i) the lowest temperature at which rate of conversion of-Ga₂O₃ to GaN became significant was in the range 600 to 700°C, (ii) over the temperature range 700 to 1000°C GaN was found to be the only crystalline phase present, (iii) above 1100°C-Ga₂O₃ was the main constituent. In comparison, when the oxide phase was nitrided in a 50% NH₃-50% N₂ atmosphere it was found that (i) the lowest temperature at which conversion to GaN occurred lay between 700 and 750°C, (ii) there was only a narrow range of temperatures, 750 to 870°C, in which the final products were found to contain GaN as the only crystalline phase, (iii) samples nitrided above 870°C exhibited both GaN and-Ga₂O₃ phases, the proportion of-Ga₂O₃ increasing with increasing temperature.