Graphoepitaxy of High‐Quality GaN Layers on Graphene/6H–SiC

The implementation of graphene layers in gallium nitride (GaN) heterostructure growth can solve self‐heating problems in nitride‐based high‐power electronic and light‐emitting optoelectronic devices. In the present study, high‐quality GaN layers are grown on patterned graphene layers and 6H–SiC by metalorganic chemical vapor deposition. A periodic pattern of graphene layers is fabricated on 6H–SiC by using polymethyl methacrylate deposition and electron beam lithography, followed by etching using an Ar/O₂ gas atmosphere. Prior to GaN growth, an AlN buffer layer and an Al_0.2Ga_0.8N transition layer are deposited. The atomic structures of the interfaces between the 6H–SiC and graphene, as well as between the graphene and AlN, are studied using scanning transmission electron microscopy. Phase separation of the Al_0.2Ga_0.8N transition layer into an AlN and GaN superlattice is observed. Above the continuous graphene layers, polycrystalline defective GaN is rapidly overgrown by better quality single‐crystalline GaN from the etched regions. The lateral overgrowth of GaN results in the presence of a low density of dislocations (≈10⁹ cm⁻²) and inversion domains and the formation of a smooth GaN surface.     

The growth of GaN films by alternate source gas supply hot-mesh CVD method

Gallium nitride (GaN) films and Aluminium nitride (AlN) layers were deposited on SiC/Si (111) substrates by an alternating source gas supply or an intermittent supply of a source gas such as ammonia (NH₃), trimethylgallium (TMG) or trimethylaluminum (TMA) in a hot-mesh chemical vapor deposition (CVD) apparatus. The AlN layer was deposited as a buffer layer using NH₃ and TMA on a SiC layer grown by carbonization on Si substrates using propane (C₃H₈). GaN films were grown on an AlN layer by a reaction between NH_x radicals generated on a ruthenium (Ru) coated tungsten (W)-mesh and TMG molecules. An alternating source gas supply or an intermittent supply of one of the source gases during the film growth are expected to be effective for the suppression of gas phase reactions and for the enhancement of precursor migration on the substrate surface. By the intermittent supply of alkylmetal gas only during the growth of the AlN layer, the defect generation in the GaN films was reduced. GaN film growth by intermittent supply on an AlN buffer layer, however, did not lead to the improvement of the film quality.     

Photocatalytic activity of gallium nitride for producing hydrogen from water under light irradiation

Photocatalytic activity of powdered GaN for decomposing water into hydrogen under light irradiation was investigated. It was found that GaN has activity for producing hydrogen from water containing electron donors (Na₂S-Na₂SO₃, CH₃OH) without a loading of a noble metal; the obtained H₂ yield was ca. 150 μmol for a 200 h-Xe lamp (300 W) irradiation. The activity was increased by adding NaOH in the reaction solution due to the photo-etching of GaN by NaOH and concomitant removal of the native oxide from the surface, as confirmed by XPS, XRD, and PL measurements. The band energy scheme for GaN suggests that the conduction band edge of GaN is positioned at 0.5 V higher than the redox potential of H⁺/H₂. This large energy difference (overpotential) makes the photocatalytic H₂ evolution over GaN from water possible.     

Growth of GaN on SiC/Si substrates using AlN buffer layer by hot-mesh CVD

GaN films were grown on SiC/Si (111) substrates by hot-mesh chemical vapor deposition (CVD) using ammonia (NH₃) and trimetylgallium (TMG) under low V/III source gas ratio (NH₃/TMG = 80). The SiC layer was grown by a carbonization process on the Si substrates using propane (C₃H₈). The AlN layer was deposited as a buffer layer using NH₃ and trimetylaluminum (TMA). GaN films were formed and grown by the reaction between NH_x radicals, generated on a tungsten hot mesh, and the TMG molecules. The GaN films with the AlN buffer layer showed better crystallinity and stronger near-band-edge emission compared to those without the AlN layer.     

Study of Densification Process and Microstructure for Highly Dense Doped MgB2 Bulk Samples

In order to give a contribution in the subject of MgB₂ processing technique to obtain low cost high density bulk samples with improved superconducting properties, studies on dilatometry and on the microstructures of in situ and ex situ MgB₂ bulk samples prepared in the presence of carbon compounds are presented in this paper. The ex situ densification was performed by isostatic pressure followed by high temperature treatment in air, which promoted high density samples compared to the common magnesium diboride in situ samples. It was observed that each carbon compound addition (hydrocarbon??C₈H₁₈, SiC and silicon oil??SiC₂H₆O), corresponds to one microstructure, which can be noticed even after the ex situ process for sintering the pellets. Among the compounds, the SiC showed to present the most irregular structure for the in situ samples. However, this microstructure showed to change completely and reaches higher density (90?%) very rapidly when heat treated in the ex situ process. The X ray diffraction (XRD) analyses also revealed that the presence of spurious phases??like Mg₂Si, Mg₃B₂O₆, MgO and others??for the ex situ samples treated in air at high temperature.     

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).     

Improvement of quality and strain relaxation of GaN epilayer grown on SiC substrate by in situ SiNx interlayer

Improved structural quality and tensile stress releasing were realized in GaN thin films grown on 6H–SiC by metal organic chemical vapor deposition using an in situ porous SiN_x interlayer. The SiN_x was formed in situ in the growth chamber by simultaneous flow of diluted silane and ammonia, leading to the formation of a randomly distributed mask layer and induced lateral overgrowth similar to conventional epitaxial lateral overgrowth of GaN. The full width at half maximum (FWHM) of X-ray diffraction peaks decreases dramatically by the SiN_x interlayer, indicating an improved crystalline quality. Also, it was found that the biaxial tensile stress in the GaN film was significantly reduced by in situ SiN_x interlayer from Raman spectra. Low temperature photoluminescence spectra exhibited a narrower FWHM by the SiN_x interlayer.     

Calculation of residual thermal stress in GaN epitaxial layers grown on technologically important substrates

A detailed investigation of residual thermal stress and misfit strain in GaN epitaxial layers grown on technologically important substrates is performed. The thermal stress is low when GaN is grown on AlN, SiC and Si, and relatively higher when Al₂O₃ substrate is used. The stress is compressive for AlN and Al₂O₃ and tensile for Si and SiC substrates. Residual thermal stress analysis was also performed for three layer heterostructures of GaN/AlN/6H-SiC and GaN/AlN/Al₂O₃. The stress remains the same when a sapphire substrate is used with or without an AlN buffer layer but reduces by an order of magnitude when a 6H-SiC substrate is used with an AlN buffer layer.     

New nanocrystalline powder substrates for nitrides layer epitaxy

GaN and related III-V nitride materials have been applied for fabrication of electronic and optical devices. The most important factor limiting the mass production of devices based on III-V nitride materials is the high cost of substrates and the elaborate growth techniques. The lack of large, bulk GaN substrates causes that the epitaxial layer of nitrides must be grown on heteroepitaxial substrates. The most widely applied are monocrystalline sapphire, SiC and silicon substrates; but the question of cheap and available substrates for nitrides growth is still open.In this paper, authors present some results of the growth of nitrides layer by the metal-organic vapor-phase epitaxy (MOVPE) technique on new nanocrystalline powder substrates (compressed Al₂O₃+SiC). The influence of substrate composition (the amount of SiC powder) on the properties of the GaN layer are presented. Also the impact of the conditions of epitaxial process on properties of the nitride layers are discussed.     

Structural changes of hot-wire CVD silicon carbide thin films induced by gas flow rates

Silicon carbide (SiC) thin films were prepared by hot-wire chemical vapor deposition in a CH₄ gas flow rate of 1 sccm, and the influence of the gas flow rates of SiH₄ and H₂ gases on the film structure and properties were investigated. In the case of a H₂ gas flow rate below 100 sccm, the SiC:H films obtained in SiH₄ gas flow rates of 3 and 4 sccm were amorphous. On the other hand, when the H₂ gas flow rate was above 150 sccm, SiH₄ gas flow rates of 4 and 3 sccm resulted in a Si-crystallite-embedded amorphous SiC:H film and a nanocrystalline cubic SiC film, respectively. It was found that gas flow rates were important parameters for controlling film structure.     

Characterization of polycrystalline SiC films grown by HW-CVD using silicon tetrafluoride

SiC films were synthesized by hot-wire chemical vapor deposition using a tungsten filament and a gas mixture of SiF₄ and CH₄. The etching of the substrate instead of the film growth occurred on the samples prepared using only source gases without H₂ dilution. The atomic or molecular hydrogen was believed to control the density of radicals containing F in a gas phase or on a growth surface. Polycrystalline 3C-SiC(111) films were successfully obtained at substrate temperatures lower than 500 °C by using H₂ dilution. The growth mode limited by source-gas supply was found to be important to obtain polycrystalline SiC films. The SiC film grown at higher deposition pressure was amorphous and contained no Si-H_x bonds but 6% fluorine. In SiF₄/CH₄/H₂ system, the radicals containing F are considered to play very important roles in the reactions both on a growth surface and in a gas-phase.     

Growth and stability of CVD codeposited nanocomposite SiCTiCC

SiCTiCC nanocomposites were chemically vapor deposited ontoa carbon substrate under atmospheric pressure at 1223 K using the TiCl₄SiH₂Cl₂C₄H₁₀H₂ gas system. By TEM and HREM, codeposits were found to consist of long primary needles growing perpendicular to the substrate surface in the 〈220〉 direction, which are connected to smaller secondary needles. Needles are composed of nonocrystals 10 nm wide and about 50 nm long, slightly disoriented from each other. TiC was at the center of the needles and SiC on both sides. The needles were embedded in a poorly crystallized matrix rich in SiC.     

AFM studies of microindented GaN and InGaN

The microhardness characteristics of GaN and GaN/InGaN films epitaxially grown on (0001) sapphire have been investigated using Vickers and Knoop indenters. The variation of H_V and H_K follows a reverse type of indentation size effect (reverse ISE). The microhardness results have also been analyzed using Meyer's law, Hays-Kendall approach and Proportional specimen resistance (PSR) model. The effect of N⁺ implantation on the microhardness of GaN has also been studied. The implanted sample is more resistant to plastic penetration than the unimplanted one and it is found that implantation enhances the surface hardness. Detailed AFM studies around the indented regions of the GaN and GaN/InGaN give the nature and behavior of the deformation on the surface.     

Effect of hydrogen on the structure of high-rate deposited SiC on Si by atmospheric pressure plasma chemical vapor deposition using high-power-density condition

Using the atmospheric pressure plasma chemical vapor deposition (AP-PCVD) technique, SiC films were fabricated from the gas mixture of He, H₂, SiH₄ and CH₄ on silicon substrates. High-power-density condition was adopted to sufficiently activate the reactive gas molecules in the plasma. The structure, composition and crystallinity of the films were investigated as functions of the H₂ concentration in the gas mixture and substrate temperature. It was shown that increase in H₂ concentration in the plasma atmosphere reduced the growth temperature of polycrystalline SiC film. As a result, polycrystalline 3C-SiC film of which grain size was of the order of 10 nm could be grown at a substrate temperature of 820 K with a deposition rate of approximately 6.7 nm/s. It was suggested that atomic hydrogen generated with addition of H₂ in the gas mixture considerably affects not only the reaction process at the film-growing surface but also the form of precursors in the atmospheric pressure plasma. The results indicated the possibility of realizing the columnar growth of large 3C-SiC grains on Si substrate when the H₂ concentration and the VHF power were simultaneously increased in the AP-PCVD process.     

The study of Pt Schottky contact on porous GaN for hydrogen sensing

This article reports the studies of Pt Schottky contact on porous n-type GaN for hydrogen sensing. A simpler and improved electroless etching method has been developed to generate porous GaN in which high uniformity of the porous area could be achieved. Hydrogen sensor was subsequently fabricated by depositing Pt Schottky contacts onto the porous GaN sample. For comparative study, a standard hydrogen sensor was also prepared by depositing Pt Schottky contacts on the as-grown sample. Hydrogen detection was carried out at room temperature and 100 °C. This Pt/porous GaN sensor exhibited a significant change of current upon exposure to 2% H₂ in N₂ gas as compared to the standard Pt/GaN sensor. Morphological studies by scanning electron microscopy revealed that Pt contact deposited on porous GaN having a very rough surface morphology with pores distributed all over the contact layer. Therefore, the increase of current could be attributed to the unique microstructure at porous Pt/porous GaN interface which allowed higher accumulation of hydrogen and eventually led to stronger effect of the H-induced dipole layer.     

Stress generation and relaxation in (Al,Ga)N/6<Emphasis Type="Italic">H</Emphasis>-SiC heterostructure grown by plasma-assisted molecular-beam epitaxy

In situ stress generation and relaxation in Al_0.25Ga_0.75N/GaN/AlN heterostructure with an overall thickness exceeding 3 μm in the process of its growth on a 6H-SiC substrate by low-temperature plasma-assisted molecular-beam epitaxy at substrate temperatures ranging from 690 to 740°C was studied. At room temperature, AlN and GaN layers revealed residual compressive stresses of–2.3 and–0.1 GPa, respectively. This made it possible to avoid cracking during postgrowth cooling of the structure.     

Pressure dependence of morphology and phase composition of SiC films deposited by microwave plasma chemical vapor deposition on cemented carbide substrates

SiC films were deposited on cemented carbide substrates by employing microwave plasma chemical vapor deposition method using tetramethylsilane (Si(CH₃)₄) diluted in H₂ as the precursor. Scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and scratching technique were used to characterize morphology, composition, phases present and adhesion of the films. Experimental results show that the deposition pressure has great influence on morphologies and phase composition of the films. In sequence, SiC films with a cauliflower-like microstructure, granular films with terrace-featured SiC particles coexisting with Co₂Si compound and clusters of nanometer SiC nanoplatelets appear as a function of the deposition pressure. In terms of plasma density and substrate temperature, this sequential appearance of microstructures of SiC films was explained. Adhesion tests showed that among the three types of films studied, the films with the terrace-featured SiC particles have relatively higher adhesion. Such knowledge will be of importance when the SiC films are used as interlayer between diamond films and cemented carbide substrates.     

Influence of N2 flux on the improvement of highly c-oriented GaN films on diamond substrates

High-quality GaN films are deposited on freestanding thick diamond films by electron cyclotron resonance plasma-enhanced metal organic chemical vapour deposition (ECR-PEMOCVD). Trimethyl gallium (TMGa) and N₂ are applied as precursors and different N₂ fluxes are used to achieve high-quality GaN films. The influence of N₂ flux on the properties of GaN films is systematically investigated by X-ray diffraction analysis (XRD), reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM) and Hall effect measurement (HL). The results show that the high-quality GaN films with small surface roughness of 4.5 nm and high c-orientation are successfully achieved at the optimized N₂ flux of 90 sccm. The most significant improvements in morphological, structural, and optical properties of GaN films are obtained by using a proper N₂ flux.     

Preparation of SiC ultrafine particles from SiH2Cl2 and C2H4 gas mixtures by a CO2 laser

Preparation of SiC ultrafine particles from SiH₂Cl₂-C₂H₄ mixtures by a CO₂ laser was investigated. The powders with specific surface area in the 8–150 m² g^–1 range were obtained by irradiating SiH₂Cl₂-C₂H₄ gas mixtures with a CO₂ laser at atmospheric pressure. X-ray diffraction of the products showed that silicon, SiC and free carbon were produced and the composition of the powders depended on the C₂H₄/SiH₂Cl₂ ratio. The reaction flame temperature changed from less than 1273 K to more than 3073 K with the laser power density and C₂H₄/SiH₂Cl₂ ratio. When SiH₂Cl₂ was irradiated with the CO₂ laser, the reaction temperature was less than 1273 K and silicon particles were formed. When the SiH₂Cl₂-C₂H₄ mixture was irradiated with a CO₂ laser, the reaction temperature was low (<1273 K) at low power density and low C₂H₄/SiH₂Cl₂ ratio, but it increased rapidly to around 3000 K at high laser power density and high C₂H₄/SiH₂Cl₂ ratio (>0.3). SiC was formed at both high and low reaction flame temperatures. It was considered that the rapid increase in the reaction flame temperature was caused by the initiation of exothermic reactions and the increase in laser absorption which was caused mainly by carbon particle formation. Hysteresis was observed between the reaction flame temperature and the power density of the laser beam. It was found that SiH₂Cl₂ underwent a disproportionation reaction on irradiation with the CO₂ laser, and silicon and SiC particles were formed through the various products of the disproportionation reaction. In particular, at low reaction flame temperature, the reactive species, such as SiH₄ and SiH₃Cl, produced by the disproportionation of SiH₂Cl₂ were considered to play an important role in the formation of silicon and SiC particles.     

Improvements of epitaxial quality and stress state of GaN grown on SiC by in situ SiN<Subscript>x</Subscript> interlayer

In this study, 4.5 μm thick GaN films with graded Al_xGa_1?xN/AlN buffer and SiN_x interlayer were prepared on 6H–SiC substrates by metal–organic chemical vapor deposition. To determine the effects of SiN_x interlayer on epitaxial quality and stress state of GaN films, a series of comparative experiments were carried out by changing the deposition time and the insert location of SiN_x interlayer. By optimizing growth conditions of SiN_x interlayer, the full width at half maximum values of \( (0002) \) and \( (10\bar{1}2) \) rocking curves of GaN films were improved to 142 and 170 arcsec, respectively. A crack-free GaN film with a small root-mean-squared roughness of 0.21 ± 0.02 nm was achieved. Simultaneously, the reduction in threading dislocation density of GaN films was confirmed by using wet etching method. In addition, stress values in GaN films were investigated by Raman and low-temperature photoluminescence spectra, which indicated that the lower tensile stress in GaN film, the higher the film’s crystallinity.