Photoinduced self-organization of gallium nanowires on a GaN surface

The mechanism of ultraviolet laser ablation of GaN epitaxial films is determined: it is found to be based on the dissociation of GaN molecules to form volatile nitrogen-containing components. The conditions of exposure under which the formation of gallium nanoclusters on the GaN surface are determined. Regimes of epitaxial growth of GaN are found in which parallel microterraces form on the surface of the samples. It is found that when samples with microterraces in the as-grown state are irradiated by high-power ultraviolet radiation, gallium nanowires are formed on the surface. It is proposed to use these phenomena to develop new UV optical lithographic techniques and to fabricate single-electron devices based on GaN. Pis’ma Zh. Tekh. Fiz. 25, 13–18 (May 26, 1999)     

Harmonic surface acoustic waves on gallium nitride thin films

SAW devices operating at the fundamental frequency and the 5th, 7th, 9th, and 11th harmonics have been designed, fabricated, and measured. Devices were fabricated on GaN thin films on sapphire substrates, which were grown via metal organic vapor phase epitaxy (MOVPE). Operating frequencies of 230, 962, 1338, 1720, and 2100 MHz were achieved with devices that had a fundamental wavelength, /spl lambda/(0) = 20 /spl mu/m. Gigahertz operation is realized with relatively large interdigital transducers that do not require complicated submicrometer fabrication techniques. SAW devices fabricated on the GaN/sapphire bilayer have an anisotropic propagation when the wavelength is longer than the GaN film thickness. It is shown that for GaN thin films, where kh(GaN) > 10 (k = 2/spl pi///spl lambda/ and h(GaN) = GaN film thickness), effects of the substrate on the SAW propagation are eliminated. Bulk mode suppression at harmonic operation is also demonstrated.     

High energy Urbach characteristic observed for gallium nitride amorphous surface oxide

We have observed an “above band-gap” Urbach like characteristic for gallium nitride films (at the high energy side of the band-edge). A combination of X-ray diffraction, secondary ion mass spectroscopy and optical transmission measurements were taken for gallium nitride samples of different thickness. From this data we demonstrate that the high energy Urbach like characteristic is related to the presence of an amorphous surface oxide. It is shown to dominate the absorption spectra of thin gallium nitride samples, for which the influence of surface oxidation is strongest.     

Anodic oxidation of gallium nitride

The anodic oxidation of n-type GaN (carrier concentration 4.6 × 10¹⁸ cm^–3) under laboratory illumination at a constant current density of 5 mA cm^–2 in sodium tungstate electrolyte is examined by high resolution microscopy and surface analysis. The GaN, deposited as a thin layer by molecular beam epitaxy, had an initially faceted surface. Anodic oxidation gives rise to local growth of an amorphous Ga₂O₃-based reaction product, often, but not exclusively, located in the vicinity of troughs formed by intersecting facets. At these regions dislocations in the GaN intersect the surface. The product is non-uniform in thickness and morphology, with pore-like features. With prolonged anodic treatment, local oxidation progresses as channels, which eventually reach the base of the GaN layer, leaving a porous skeleton. The formation of a uniform and compact film material on GaN is considered to be impeded by generation of nitrogen from the anodic reaction, with the strength of the Ga–N bonding focusing oxidation on regions of increased impurity, non-stoichiometry or defect concentration.     

Optical nonlinearities of gallium nitride

Luminescence, induced absorption and degenerate four wave mixing experiments are performed on GaN epilayers grown on a sapphire substrate by MOCVD. We measure the nonlinear behaviour of the luminescence spectra near the excitonic resonance, by using an excitation at 4.026 eV from an excimer laser. At low intensities of excitation, spectra show a saturation of the I₂ line due to the finite donor density in the sample. Higher intensities of excitation induce collision process between photo-created particles. Using a dye laser as a pump beam, we measure the induced variation of absorption of a probe beam as a function of the intensity and of the wavelength of the excitation. With increasing intensities of the pump beam, curves show a red-shift of the absorption edge and of the excitonic resonance. Pulsed degenerate four-wave mixing experiments were performed using the third harmonics of a picosecond Nd-YAG laser at 3.492 eV. A characteristic time of 16 ps has been measured, which is independent of the temperature, of the fringe spacing and of the intensity of the pump beams.     

Solid-state gallium NMR characterization of cubic gallium nitride prepared by the reaction of gallium arsenide with ammonia

The reaction of cubic gallium arsenide (GaAs) with ammonia yielded gallium nitride (GaN). Powder X-ray diffraction patterns of the GaN products showed that they are a mixture of c- and w-GaN, while their Ga MAS NMR spectra revealed that they have the other phase of GaN besides c- and w-GaN and the high reaction temperature (≥900 °C) induces nitrogen deficiency in GaN. The peaks at 353 and 347 ppm in the ⁷¹Ga MAS NMR spectra were tentatively assigned to c-GaN and an intermediate of w- and c-GaN in the stacking order, respectively. The observed ⁷¹Ga chemical shifts of GaN, GaP, GaAs and GaSb in cubic phase were well correlated with the reciprocal of their band gaps.     

Laser deposition of gallium nitride films

Insulating c-oriented hexagonal epitaxial gallium nitride (GaN) films have been obtained by means of pulsed laser sputtering of a gallium target in nonactivated nitrogen atmosphere. The GaN films were deposited onto (0001)-oriented sapphire substrates either directly or above a ZnO buffer layer. The laser-deposited films exhibit edge photoluminescence at 370 nm.     

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.     

Homogeneous dispersion of gallium nitride nanoparticles in a boron nitride matrix by nitridation with urea

A Gallium Nitride (GaN) dispersed boron nitride (BN) nanocomposite powder was synthesized by heating a mixture of gallium nitrate, boric acid, and urea in a hydrogen atmosphere. Before heat treatment, crystalline phases of urea, boric acid, and gallium nitrate were recognized, but an amorphous material was produced by heat treatment at 400 degrees C, and then was transformed into GaN and turbostratic BN (t-BN) by further heat treatment at 800 degrees C. TEM obsevations of this composite powder revealed that single nanosized GaN particles were homogeneously dispersed in a BN matrix. Homogeneous dispersion of GaN nanoparticles was thought to be attained by simultaneously nitriding gallium nitrate and boric acid to GaN and BN with urea.     

Vaporization reactions of manganese gallium sulfide

The vaporization of manganese gallium sulfide was investigated in the temperature range 500–975°. Manganese gallium sulfide was found to vaporize incongruently by the reaction:

$\text{3}\text{MnGa}{}^2\text{S}{}^4\text{(s) =}\text{Mn}{}^3\text{Ga}{}^2\text{S}{}^6\text{(s) + 2}\text{Ga}{}^2\text{S}\text{(g) + 2}\text{S}{}^2\text{(g)}$

and

$\text{Mn}{}^3\text{Ga}{}^2\text{S}{}^6\text{(s) = 3}\text{MnS(s)}\text{+}\text{Ga}{}^2\text{S}\text{(g) +}\text{S}{}^2\text{(g)}$

Approximate vapor pressures were measured by the Knudsen effusion technique. Lattice parameters, density, and chemical analysis of the new ternary compound Mn₃Ga₂S₆ are reported.     

Optical properties of nanocrystalline gallium nitride films

Nanocrystalline GaN films with different crystallite sizes were deposited onto quartz and NaCl substrates by magnetron sputtering of a GaN target in argon plasma. All the films showed predominant hexagonal phase. The band gap values were always found to be higher than that of the bulk. This blue shift in band gap could be attributed to the quantum confinement effect. The optical absorption in these films could be explained by the combined effects of phonon and inhomogeneity broadening along with optical loss due to light scattering at the nanocrystallites. Band edge luminescence is absent in these GaN nanocrystalline films. The line shapes of the photoluminescence (PL) spectra are asymmetric and broad. The film deposited at lower substrate temperature showed broader PL peak. It may be observed that no significant energy shift in the peak positions was observed with reduction in crystallite size but the intensity of the peak decreased for films with the reduction in crystallite size. Below band gap emission observed in this study may also originate due to the presence of polarization-induced electric field present in wurtzite GaN deposited here.     

The absorption spectra of gallium nitride crystals doped with erbium ions

We have measured the absorption spectra of Er³⁺ ions introduced by diffusion into bulk GaN crystals grown by vapor phase epitaxy in a chloride system. The wavelength interval of 518–527 nm corresponding to the region of the ⁴ I _15/2→² H _11/2 transition was studied in detail at 293, 77, and 2 K. At 2 K, the spectrum of this transition displays six lines, which correspond to the theoretically possible number of sublevels of the ² H _11/2 state of Er³⁺ ion occurring in a noncubic crystal field. The positions of levels in the ² H _11/2 multiplet correspond to 2.360, 2.361, 2.365, 2.369, 2.379, and 2.386 eV. Both the number and narrow width of lines observed at 2 K indicate that Er³⁺ ions predominantly occupy the same regular position in GaN crystals. Most probably, erbium ions occupy gallium sites in the crystal lattice.     

Optical and magneto-optical characterization of heteroepitaxial gallium nitride

We describe some recent work on the characterization of strain effects on excitons and band structure in heteroepitaxial GaN, as well as magnetoluminescence studies of excitons and donors in this material. Epilayer strains were measured directly from wafer curvature, which avoids the need to know the lattice constants of unstrained GaN in advance. The change in strain between room temperature (where it is usually measured) down to low temperature, where detailed optical spectroscopy is usually performed, was estimated and found to be significant. Low temperature photoluminescence and reflectance were used to determine the exciton energies as a function of measured strain, yielding some of the deformation potentials and band structure parameters of the material. Magnetospectroscopy and reflectance were used to identify excited states of the excitons, yielding an exciton binding energy around 26.4 meV. Residual donors and their binding energies were studied from two-electron transitions, which have been identified for the first time by their Zeeman splittings in high magnetic fields. Resonant electronic Raman scattering was also used to study excited states of shallow donors, and yielded improved resolution compared to photoluminescence.     

Epitaxial growth of gallium nitride by ion-beam-assisted evaporation

The epitaxial growth of gallium nitride thin film was obtained on the inclined Si(111) substrates by the process of ion-beam-assisted evaporation (IBAE) at the low temperature of 500 °C. The film composition determined by Rutherford backscattering spectrometry shows that the synthesized film is a stoichiometric nitride. The epitaxial quality of GaN film is enhanced by minimizing the bombardment-induced film damage by decreasing the ion flux. However, the crystallinity of the GaN film becomes very poor when the ion flux is not sufficient to densify the film. The optimum flux ratio of N⁺₂ to Ga and the energy of incident N⁺₂ ions for the epitaxial growth were found to be 3.4 and 50 eV, respectively. The GaN film deposited on the 4 °-inclined Si (111) with respect to substrate surface shows much better crystalline quality compared with that on the 0 ° inclined Si(111) due to many stable nucleation sites. A thin amorphous layer exists at the interface between GaN and Si(111) substrate and acts as a buffer zone enabling the subsequent epitaxial growth of GaN by relaxing the large misfit strain (23%) in the early stage of film growth. The epitaxial GaN film shows an orientational relation with the Si(111) substrate.     

Crystallographic alignment of high-density gallium nitride nanowire arrays

Single-crystalline, one-dimensional semiconductor nanostructures are considered to be one of the critical building blocks for nanoscale optoelectronics. Elucidation of the vapour-liquid-solid growth mechanism has already enabled precise control over nanowire position and size, yet to date, no reports have demonstrated the ability to choose from different crystallographic growth directions of a nanowire array. Control over the nanowire growth direction is extremely desirable, in that anisotropic parameters such as thermal and electrical conductivity, index of refraction, piezoelectric polarization, and bandgap may be used to tune the physical properties of nanowires made from a given material. Here we demonstrate the use of metal-organic chemical vapour deposition (MOCVD) and appropriate substrate selection to control the crystallographic growth directions of high-density arrays of gallium nitride nanowires with distinct geometric and physical properties. Epitaxial growth of wurtzite gallium nitride on (100) gamma-LiAlO(2) and (111) MgO single-crystal substrates resulted in the selective growth of nanowires in the orthogonal [1\[Evec]0] and [001] directions, exhibiting triangular and hexagonal cross-sections and drastically different optical emission. The MOCVD process is entirely compatible with the current GaN thin-film technology, which would lead to easy scale-up and device integration.     

Thermal stability of multilayer contacts on gallium nitride

The results of investigation of a new system of metallization for nonrectifying contacts on n-GaN are presented. The new contact metallization system involves the following sequence of layers: Au(200 nm)-Ti(TiB_x)(100 nm)-Al(20 nm)-Ti(50 nm), where the TiB_x layer plays the role of a diffusion barrier. The contacts with the TiB_x layer retain their layer structure and electrical characteristics upon annealing up to 700°C, whereas the usual Au-Ti-Al-Ti structure exhibits degradation upon rapid thermal annealing at T = 700°C. Further increase in the annealing temperature to 900°C leads to smearing of the layer structure of the Au-TiB_x-Al-Ti-GaN contact. Physical factors responsible for the change in the parameters of such contact systems are considered.     

Thermal doping of rare-earth ions in gallium nitride

Doping of rare-earth ions in epitaxial gallium nitride material has been performed through a thermal diffusion process. The technique involves a brief photolytic etching of the surface followed by heating with a melt of rare-earth salt under reducing conditions. Europium-doped GaN pumped with above gap UV radiation showed strong red emission which was insensitive to a moderately strong magnetic field. The temperature dependence of the intensity of this red emission is also described. Neodymium caused surface pitting, through an unknown chemical mechanism, and consequent enhancement of defect-generated yellow luminescence.     

Double-cross epitaxial overgrowth of nonpolar gallium nitride layers

Epitaxial gallium nitride (GaN) structures have been manufactured by the lateral overgrowth technology, whereby GaN epilayers are grown in stripe windows on a partly masked initial GaN layer. It is established that in addition to the traditional orientation of stripes across the c axis, the process is also possible for the stripes oriented at 45° relative to this axis. In this case, two lateral overgrowth processes in mutually perpendicular directions can be performed, which would significantly reduce the relative area of imperfect material formed over windows in the mask.