GaN single crystal growth from a Na-Ga melt

GaN single crystals were prepared in a sealed stainless-steel tube container at 650–840°C for 6–300 h using Ga, NaN₃, and Na as starting materials. A thin GaN layer covered Na-Ga melt surface at the initial stage of reaction between Ga in the Na-Ga melt and N₂ given by the thermal decomposition of NaN₃ around 300°C. In the next stage, pyramidal and prismatic GaN single crystals grew under the layer. Prismatic and platelet crystals also grew from the melt which wetted the tube wall. The reaction rate was enhanced by increasing temperature and by increasing Na content in the melt. The maximum size of pyramidal crystals was about 0.7 mm. The platelet crystals were 1–2 mm in one direction and <0.05 mm thick. The platelet single crystals prepared at 650°C exhibited the sharpest cathodoluminescence peak with the strongest intensity at 362 nm.     

Catalyst-free ZnO nanowires grown on a-plane GaN

ZnO nanowires were grown on a-plane GaN templates by chemical vapor deposition (CVD) without employing a catalyst. The a-plane GaN templates were pre-deposited on an r-plane sapphire substrate by metal-organic CVD. The resulting ZnO nanowires grow in angles off- related to the GaN basal plane. X-ray diffraction (XRD) spectra showed that the ZnO layer was grown with a heteroepitaxial relationship of (110)_ZnO||(110)_GaN. Photoluminescence spectra measured at 17 K exhibited near-band-edge emission at 372 nm with a full width at half maximum of 10 nm. The growth mechanism on a-GaN was the Volmer-Weber (VW) mode and differed from the Stranski-Krastanow (SK) mode observed for growth on c-GaN. This difference results from the higher interfacial free-energy on the a-plane between ZnO and GaN than that on the c-plane orientation.     

GaN:Eu electroluminescent devices grown by interrupted growth epitaxy

In this paper we report on electroluminescent devices fabricated using Eu-doped GaN films grown by interrupted growth epitaxy (IGE). IGE is a combination of conventional molecular beam epitaxy and migration enhanced epitaxy. It consists of a sequence of ON/OFF cycles of the Ga and Eu beams, while the N₂ plasma is kept constant during the entire growth time. IGE growth of GaN:Eu resulted in significant enhancement in the Eu emission intensity at 620.5 nm. The nitridation of the surface that occurs during the OFF cycle appears to be the dominant process producing the enhancement. Thick dielectric devices fabricated on glass substrates using IGE-grown GaN:Eu have resulted in luminance of ∼1000 cd/m² and luminous efficiency of ∼0.15 lm/W.     

Highly c-axis oriented GaN films grown on free-standing diamond substrates for high-power devices

GaN films with highly c-axis preferred orientation are deposited on free-standing thick diamond films by low temperature electron cyclotron resonance plasma enhanced metal organic chemical vapor deposition (ECR-PEMOCVD). The TMGa and N₂ are applied as precursors of Ga and N, respectively. The quality of as-grown GaN films are systematically investigated as a function of deposition temperature by means of X-ray diffraction (XRD) analysis, Hall Effect measurement (HL), room temperature photoluminescence (PL) and atomic force microscopy (AFM). The results show that the dense and uniformed GaN films with highly c-axis preferred orientation are successfully achieved on free-standing diamond substrates under optimized deposition temperature of 400 °C, and the room temperature PL spectra of the optimized GaN film show a intense ultraviolet near band edge emission and a weak yellow luminescence. The obtained GaN/diamond structure has great potential for the development of high-power semiconductor devices due to its excellent heat dissipation nature.     

Improvement of GaN epilayer by gradient layer method with molecular-beam epitaxy

We demonstrated a molecular beam epitaxy method to resolve the dilemma between structural and morphological quality in growth of the GaN epilayer. A gradient buffer layer was grown in such a way that the N/Ga ratio was gradually changed from nitrogen-rich to gallium-rich. The GaN epitaxial layer was then grown on the gradient buffer layer. In the X-ray diffraction analysis of GaN(002) rocking curves, we found that the full width at half-maximum was improved from 531.69″ to 59.43″ for the sample with a gradient buffer layer as compared to a purely gallium-rich grown sample. Atomic force microscopy analysis showed that the root-mean-square roughness of the surface was improved from 18.28 nm to 1.62 nm over an area of 5 × 5 μm² with respect to a purely nitrogen-rich grown sample. Raman scattering showed the presence of a slightly tilted plane in the gradient layer. Furthermore we showed that the gradient layer can also slash the strain force caused by either Ga-rich GaN epitaxial layer or AlN buffer layer.     

Fabrication of GaN nanowires and nanoribbons by a catalyst assisted vapor-liquid-solid process

Wurtzite GaN nanowires and nanoribbons were produced directly from Ga metal by an Au catalyst assisted thermal evaporation process in a flowing ammonia atmosphere. We have investigated the effect of different experimental parameters on the morphology of the nanoforms. The diameters of the nanowires were varied between 20 and 40 nm while the widths of the nanoribbons were within 100-250 nm. Microstructural studies by TEM reveals the role of vapor-liquid-solid process in the nucleation of GaN nanoforms. The evolution of the ribbon like morphology was attributed to the vapor-solid process. Photoluminescence study showed UV-blue emission from the GaN nanostructures. Field emission studies indicated the possibility of these materials to be used as the cathode materials in field emission based devices.     

Structural properties of GaN and related alloys grown by radio-frequency magnetron sputter epitaxy

Single-crystalline layers of GaN and related alloys such as AlGaN and InGaN were grown on Al₂O₃ (0001) substrates by radio-frequency magnetron sputter epitaxy. The crystalline structures of these layers were studied as functions of substrate temperature, N₂ composition ratio in N₂/Ar mixture source gas and gas pressure during the growth. Surface structure of GaN layer depended on Ga/N ratio in flux density, and nitrogen-rich growth condition resulted in pyramid-type facet structure whereas Ga-rich growth produced flat surface. The crystalline quality of GaN layer improved at relatively low N₂ composition ratios, and the GaN layer grown at 30% N₂ condition was transparent and colorless. Al_xGa_1−xN layers with x = 0.06-0.08 and In_xGa_1−xN layers with x = 0.45-0.5, were obtained at 30-40% and 30-50% N₂ composition ratios, respectively. The AlN and InN molar fractions in these layers were considerably different from Al and In molar fractions in starting metal alloys (x = 0.15 in both Al_xGa_1−x and In_xGa_1−x alloys).     

Synthesis and optical properties of single-crystalline GaN nanorods

Single-crystalline GaN nanorods were successfully synthesized on Si(1 1 1) substrates through ammoniating Ga₂O₃/Mo films deposited on the Si(1 1 1) substrate by radio frequency magnetron sputtering technique. The as-synthesized nanorods are confirmed as single-crystalline GaN with wurtzite structure by X-ray diffraction (XRD), selected-area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). Scanning electron microscopy (SEM) displays that the GaN nanorods are straight and smooth with diameters in the range of 100-200 nm and lengths typically up to several micrometers. X-ray photoelectron spectroscopy (XPS) confirms the formation of bonding between Ga and N. The representative photoluminescence spectrum at room temperature exhibits a strong and broad emission band centered at 371.1 nm, attributed to GaN band-edge emission. The growth process of GaN nanorod may be dominated by vapor-solid (VS) mechanism.     

Yb-doped NaBi(WO4)2 fibre single crystals grown by the micro-pulling down technique and emission spectroscopic characterization

NaBi_1−xYb_x(WO₄)₂ fibres single crystals were successfully grown by micro-pulling down technology (MPD). The Yb³⁺-doped NaBi(WO₄)₂ fibres single crystals have been pulled using MPD technique with controlled diameter and stationary stable growth conditions corresponding to flat crystallization interface with meniscus length equal to the fibre radii and pulling rate range [6-48 mm h⁻¹]. We have determined the monophased field of NaBi_1−xYb_x(WO₄)₂ for x ≤ 0.3. The lattices parameters decrease as a function of Yb³⁺ substitution in Bi³⁺ sites. The melt behaviour has been study by DTA/TG analysis. We have found that the stoichiometric compounds NaBi(WO₄)₂ melt congruently at 935 °C. The fibre diameters varied from 0.5 to 1 mm depending on the capillary die diameter, pulling rate and the molten zone temperature. Complementary Yb³⁺ spectroscopic characterization in the NaBi(WO₄)₂ lattice has been done by IR emission measurements under laser pumping at room temperature.     

Preparation of grass-like GaN nanostructures: Its PL and excellent field emission properties

We here report highly pure and single crystalline grass-like gallium nitride (GaN) nanostructures obtained on silicon substrate via catalyst-assisted CVD route under NH₃ atmosphere inside horizontal tube furnace (HTF) by pre-treating the precursors with aqueous NH₃. The as-obtained GaN nanostructures were characterized by XRD, SEM, EDS, HRTEM and SAED. The field emission (FE) characteristics of grass-like GaN nanostructures exhibited a turn-on field of 7.82 V μm^− 1 and a threshold field of 8.96 V μm^− 1 which are quite reasonable for applications in electron emission devices, field emission displays and vacuum microelectronic devices. Room temperature photoluminescence (PL) measurements of grass-like GaN nanostructures exhibited a strong near-band-edge emission at 368.8 nm (3.36 eV) without any defects related emissions which shows its potential applications in optoelectronics.     

Dislocation reduction in GaN film using Ga-lean GaN buffer layer and migration enhanced epitaxy

A GaN buffer layer grown under Ga-lean conditions by plasma-assisted molecular beam epitaxy (PAMBE) was used to reduce the dislocation density in a GaN film grown on a sapphire substrate. The Ga-lean buffer, with inclined trench walls on its surface, provided an effective way to bend the propagation direction of dislocations, and it reduced the dislocation density through recombination and annihilation processes. As a result, the edge dislocation density in the GaN film was reduced by approximately two orders of magnitude to 2 × 10⁸ cm^− 2. The rough surface of the Ga-lean buffer was recovered using migration enhanced epitaxy (MEE), a process of alternating deposition cycle of Ga atoms and N₂ radicals, during the PAMBE growth. By combining these two methods, a GaN film with high-crystalline-quality and atomically-flat surface can be achieved by PAMBE on a lattice mismatch substrate.     

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).     

Deep level centers in InGaN/GaN heterostructure grown on sapphire and free-standing GaN

Deep level transient spectroscopy has been used to characterize the deep levels in InGaN/GaN grown on sapphire substrate as well as on free-standing GaN. The deep levels at E_c − E_t ∼ 0.17-0.23 eV and E_c − E_t ∼ 0.58-0.62 eV have been detected in our samples which are present in GaN samples reported by others. These two deep levels have been attributed by us to threading dislocations as they exhibit logarithmic capture kinetic behavior and are found to be substantially reduced in its trap concentration (∼ from 10¹⁴ to 10¹² cm^− 2) in GaN grown on free-standing GaN template. Other than the two deep levels, an additional level at E_c − E_t ∼ 0.40-0.42 eV has been identified in both samples, which is believed to be related to In segregation. AFM image shows region of pits formation in InGaN epilayer for sample grown on u-GaN using sapphire substrate while the latter gives a much smoother morphology. From the X-ray diffraction space mapping, the mosaicity of the sample structure for both samples were studied. Dislocations do not play a significant role in the structural properties of InGaN grown on free-standing GaN since the FWHM based on the Δ ω is relatively small (± 0.15°) in the case of InGaN/GaN on free-standing GaN substrate as compared to that on sapphire (± 0.35°). The wider spread in Δω-2θ value for InGaN layer on free-standing GaN also suggested the effect of compositional pulling with increasing InGaN layer thickness.     

Characterization of electron irradiated GaN n-p diode

Electron-beam irradiated GaN n⁺-p diodes were characterized by deep level transient spectroscopy (DLTS) and optical responsivity measurements. The GaN n⁺-p diode structures were grown by metal organic chemical vapor deposition technique, and the electron irradiation was done by the energies of 1 MeV and 2 MeV with dose of 1 × 10¹⁶ cm^− 2. In DLTS measurement, the defect states of E_c − 0.36 eV and E_c − 0.44 eV in the electron irradiated diodes appeared newly. The optical responsivity of GaN n⁺-p diode was characterized in ultra-violet region, and then the maximum optical responsivity at 350 nm was decreased after electron-beam irradiation.     

Single crystal growth and structure refinements of CsMxTe2−xO6 (M = Al, Ga, Ge, In) pyrochlores

Single crystals of CsM_xTe_2−xO₆ pyrochlores with M = Al, Ga, Ge, and In have been grown from a TeO₂ flux. Structure refinements from single crystal X-ray diffraction data are reported. These results are used to discuss deviations from ideal stoichiometry that result in electronic conductivity presumably related to mixed valency of tellurium.     

Low temperature solution synthesis and characterization of ZnO nano-flowers

Synthesis of flower-shaped ZnO nanostructures composed of hexagonal ZnO nanorods was achieved by the solution process using zinc acetate dihydrate and sodium hydroxide at very low temperature of 90 °C in 30 min. The individual nanorods are of hexagonal shape with sharp tip, and base diameter of about 300-350 nm. Detailed structural characterizations demonstrate that the synthesized products are single crystalline with the wurtzite hexagonal phase, grown along the [0 0 0 1] direction. The IR spectrum shows the standard peak of zinc oxide at 523 cm⁻¹. Raman scattering exhibits a sharp and strong E₂ mode at 437 cm⁻¹ which further confirms the good crystallinity and wurtzite hexagonal phase of the grown nanostructures. The photoelectron spectroscopic measurement shows the presence of Zn, O, C, zinc acetate and Na. The binding energy ca. 1021.2 eV (Zn 2p_3/2) and 1044.3 eV (Zn 2p_1/2), are found very close to the standard bulk ZnO binding energy values. The O 1s peak is found centered at 531.4 eV with a shoulder at 529.8 eV. Room-temperature photoluminescence (PL) demonstrate a strong and dominated peak at 381 nm with a suppressed and broad green emission at 515 nm, suggests that the flower-shaped ZnO nanostructures have good optical properties with very less structural defects.     

Crystal growth and structure of R2Ir2O7 (R = Pr, Eu) using molten KF

Single crystals of R₂Ir₂O₇ (R = Pr, Eu) have been synthesized using molten KF at 1373 K. The pyrochlore compounds crystallize in a cubic space group (No. 227, origin choice 2), with Z = 8. At room temperature, the lattice parameters are a = 10.3940(4) Å, V = 1122.92(7) Å³ and a = 10.274(3) Å, V = 1084.5(6) Å³ for Pr₂Ir₂O₇ and Eu₂Ir₂O₇, respectively. In this paper, we report the crystal growth of R₂Ir₂O₇ (R = Pr, Eu) and their structure determinations from single crystal X-ray diffraction experiments at temperatures of 110, 115, and 298 K.     

Synthesis of GaN nanoparticles from NH4[Ga(OH)2CO3] under a flow of ammonia gas

In this investigation, we report the synthesis of gallium nitride (GaN) nanoparticles from ammonium-carbonato-dihydroxo-gallate (NH₄[Ga(OH)₂CO₃]) in the flow of NH₃ gas in a temperature range of 500-900 °C. The GaN nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR). The FTIR and XPS revealed that the conversion of NH₄[Ga(OH)₂CO₃] to GaN under a flow of ammonia proceeds stepwise via amorphous gallium oxynitrides (GaO_xN_y) intermediates. Nanosized GaN particles with an average diameter of approximately 20-40 nm were obtained. The results obtained demonstrate that the large-quantity nanosized GaN particles can be synthesized from NH₄[Ga(OH)₂CO₃] powders.