Aluminum nitride on silicon: Role of silicon carbide interlayer and chloride vapor-phase epitaxy technology

A new approach to the deposition of aluminum nitride (AlN) layers with thicknesses ranging within ∼0.1–10 μm on silicon single crystal substrates by hydride-chloride vapor-phase epitaxy (HVPE) has been developed and implemented, which involves the formation of thin (∼100-nm-thick) intermediate silicon carbide (3C-SiC) interlayers. It is established that wavy convex bands with a height of about 40 nm are present on the surface of as-grown AlN layers, which are situated at the boundaries of blocks in the layer structure. It is suggested that the formation of these wavy structures is related to morphological instability that develops due to accelerated growth of AlN at the block boundaries. Experiments show that, at low deposition rates, AlN layers grow according to a layer (quasi-two-dimensional) mechanism, which allows AlN layers characterized by half-widths (FWHM) of the X-ray rocking curves of (0002) reflections about ω_θ = 2100 arc sec to be obtained.     

Chloride vapor-phase epitaxy of gallium nitride on silicon: Structural and luminescent characteristics of epilayers

The structure and luminescent properties of gallium nitride (GaN) epilayers grown by hydride-chloride vapor-phase epitaxy (HVPE) in a hydrogen or argon atmosphere on 2-inch Si(111) substrates with AlN buffer layers have been studied. The replacement of hydrogen atmosphere by argon for the HVPE growth of GaN leads to a decrease in the epilayer surface roughness. The ratio of intensities of the donor-acceptor and exciton bands in the luminescence spectrum decreases with decreasing growth temperature. For the best samples of GaN epilayers, the halfwidth (FWHM) of the X-ray rocking curve for the (0002) reflection was 420 sec of arc, and the FWHM of the band of exciton emission at 77 K was 48 meV.     

Chloride vapor-phase epitaxy of gallium nitride at a reduced source temperature

Gallium nitride (GaN) layers on sapphire substrates have been grown by hydride-chloride vaporphase epitaxy (HVPE) at a Ga-source temperature reduced from 890°C (standard value) to 600°C. All epilayers are transparent and have smooth surfaces. The structural quality of layers was evaluated by measuring the X-ray rocking curve width (FWHM) using a triple-crystal X-ray diffractometer. For the best samples, this quantity amounted to 8 arc min. Analysis of the dependence of the crystal structure quality and photoluminescent properties of GaN layers on the Ga-source temperature showed that a decrease in this parameter to 600°C did not significantly affect the HVPE growth of GaN despite a considerable change in the vapor phase composition (ratio of GaCl and GaCl₃ concentrations).     

Low-pressure vapor-phase epitaxy of silicon on porous silicon

Silicon has been deposited by low-pressure vapor-phase epitaxy (LPVPE) on porous silicon at much lower temperatures than in conventional CVD epitaxy. The epitaxy was performed at 823°C and 0.03 mbar using SiCl₂H₂. After preoxidation to stabilize the porous structure the surface of preoxidized porous silicon was chemically cleaned and finally thermally cleaned at temperatures below 860°C just before the epitaxial step. Epitaxial layers with very good crystalline quality as characterized by RBS channeling spectra and TEM cross sections were obtained. The original microstructure of porous silicon was only slightly modified during the epitaxial process. Full oxidation of the buried porous silicon layer was subsequently possible.     

Analysis of coating interlayer between silicon nitride cutting tools and titanium carbide and titanium nitride coatings

Silicon nitride (Si₃N₄) cutting tools exhibit excellent thermal stability and wear resistance in the high-speed machining of cast irons, but show poor chemical wear resistance in the machining of steel. Conventional chemical vapour deposition (CVD) coating of Si₃N₄ tools has not been very successful because of thermal expansion mismatch between coatings and the substrate. This problem was overcome by developing a CVD process to tailor the interface for titanium carbide (TiC) and titanium nitride (TiN) coatings. Computer modelling of the CVD process was done to predict which phases would form at the interface, and the results compared with analyses of the interface. Three Si₃N₄ compositions were considered, including pure Si₃N₄, Si₃N₄ with a glass phase binder, and Si₃N₄ + TiC composite with a glass phase binder. Results of machining tests on coated tools show that the formation of an interlayer provides superior wear resistance and tool life in the machining of steel as compared to uncoated and conventionally coated Si₃N₄ tools.     

Magnesium outdiffusion from porous silicon carbide substrates during autodoping of gallium nitride epilayers

We have studied the outdiffusion of magnesium from silicon carbide substrates during autodoping of gallium nitride epilayers. The autodoping effect was observed in the case of porous substrates obtained by surface anodization of 6H-SiC wafers. It is established that the magnesium distribution profiles can be controlled by post-growth annealing. The fact of doping is confirmed by the results of photoluminescence measurements at 77 K.     

Effect of frequency on high-temperature fatigue crack growth in a silicon carbide reinforced silicon nitride composite

A detailed study on a silicon nitride reinforced with silicon carbide whiskers, Si₃N₄SiC_W, has been undertaken at elevated temperature during static and dynamic loading at increasing K and ΔK respectively. It is shown that cyclic sub-critical crack growth rates are lower than static crack growth rates. The increased crack growth rate during static far field loading is attributed to the stress relaxation of the inter-granular glass phase which allows time-dependent processes to occur ahead of the crack tip which lead to enhanced sub-critical crack growth rates. During cyclic fatigue the glass phase has insufficient time to relax and glassy ligaments are able to bridge the crack wake thereby shielding the crack tip from the full force of the applied load. Also, at particular temperatures, bridging between the surfaces of the crack wake by the inter-granular glass phase results in increased strength and fatigue retardation. The extent of ‘crack wake healing’ is shown to be time and temperature dependent. The viscosity of the glass phase is directly related to the temperature and the bonding force associated with glass phase bridging is observed to reduce with increasing temperature. The results from a previous study at room temperature are compared to those found during this investigation.     

Preparation of silicon carbide whiskers from silicon nitride

Silicon carbide whiskers have been prepared by sintering silicon nitride powder in a graphite reactor at 1800°C under a nitrogen atmosphere. The whiskers differ in morphology: tubular needles, hollow faceted fibers with a square cross section, and solid fibers with a triangular cross section. The average diameter of the needles is 0.5?5 μm, and that of the faceted fibers is up to 20 μm. The fibers range in length up to several millimeters. Such silicon carbide whiskers can be used as reinforcing agents for structural ceramics based on nonoxide materials.     

Effect of frequency on ambient temperature fatigue crack growth in a silicon carbide reinforced silicon nitride composite

A detailed study on a silicon nitride reinforced with silicon carbide whiskers has been undertaken on room temperature fatigue during static and dynamic loading at constant ΔK. It is shown that sub-critical crack growth rates are lower when the material experiences sustained far field loading than during cyclic far field loading. The increased crack growth rate during cyclic loading is attributed to a wedging effect within the crack wake causing an increase in the tensile stress and resultant increased micro-cracking ahead of the crack tip. This additional micro-structural damage leads to enhanced sub-critical crack growth rates during cyclic loading. The asperities that are responsible for the wedging effect are attributed to the isolation of small portions of material due to branching of small cracks and by degradation of the bridging SiC whiskers and Si₃N₄ grains within the crack wake.     

Bulk GaN layers grown on oxidized silicon by vapor-phase epitaxy in a hydride-chloride system

Bulk GaN layers with a thickness of up to 1.5 mm and a lateral size of 50 mm were grown by hydride-chloride vapor-phase epitaxy (HVPE). The best samples show a half-width (FWHM) of the X-ray rocking curve of ω_θ=27′, a charge carrier density of ∼8 × 10¹⁹ cm⁻³, and a mobility of ∼50 cm²/(V s). The photoluminescence spectra of the obtained epitaxial GaN layers exhibit an edge emission band at 348 eV. The HVPE layers are characterized by a high mechanical strength: H _V = 14 GPa.     

Silicon carbide platelet-reinforced silicon nitride composites

Different grades of high aspect ratio SiC platelets were used to reinforce Si₃N₄. Dispersion of additives (4 wt % Y₂0₃ and 3 wt % Al₂0₃) was achieved by ball milling in ethanol using alumina balls, while dispersion of platelets was done by ball milling using plastic balls. Consolidation of the composites was carried out by uniaxial hot pressing. A slight decrease in flexural strength was measured, while significant increases in elastic properties, fracture toughness and Weibull modulus were noted. Microstructure and crack-propagation studies as well as reinforcement mechanisms are presented.     

Epitaxy of gallium nitride in semi-polar direction on silicon

The idea of a new method for growing gallium nitride (GaN) epilayers on (100)-oriented silicon substrates is disclosed. It has been experimentally established that the formation of a special oriented thin (600 nm) buffer layer of aluminum nitride (AlN) by hydride-chloride vapor-phase epitaxy (HVPE) makes possible the growth of GaN in semi-polar direction. For the best epilayers obtained by this method, the X-ray rocking curve half-width is ω_θ(0004) = 30 arcmin. The photoluminescence spectra of GaN films measured at 77 K exhibit both exciton and donor-acceptor recombination bands.     

Specific features of gallium nitride selective epitaxy in round windows

Specific features of gallium nitride (GaN) selective growth by MOVPE in round windows has been studied. It is established that a decrease in the flux of ammonia results in an increase in the lateral size of crystallites, while the minimum studied fluxes lead to a decrease in the height of crystallites and the formation of (0001) and { 1[`1]00 }\left\{ {1\bar 100} \right\} facets in addition to the usual { 1[`1]01 }\left\{ {1\bar 101} \right\} facets. The phenomenon of growth suppression was observed for a fraction of crystallites at the onset of their coalescence. In addition, selective growth was suppressed near large nonmasked surface regions.     

Joining of silicon nitride to silicon nitride and to Invar alloy using an aluminium interlayer

Si₃N₄ has been bonded to Si₃N₄ and to the Invar alloy using an aluminium interlayer at temperatures above the melting point of aluminium. Reaction was hardly observed at the interface between Si₃N₄ and aluminium up to 1223 K. The highest strength of the Si₃N₄-Al-Si₃N₄ joints was beyond 500 M Pa. In the Si₃N₄-Al-Invar joint, two main intermetallic compound layers were formed at the AI-Invar interface. The strength of the joints was between 150 and 200 MPa. It is expected that the aluminium layer and the reaction layer with the fine cracks growing perpendicular to the interface play an important role to compensate for the thermal expansion mismatch.     

On the epitaxy of aluminum nitride on silicon substrates in a chloride-hydride process

The technological conditions under which the silicon surface interacts with vapor-phase reactants present in a chloride-hydride system for the epitaxial growth of aluminum nitride are determined. The method of electron channeling patterns is used to show that the growth of single-crystal layers of AlN on silicon substrates in the chloride-hydride system is hindered by the interaction of the silicon with NH₃ in the presence of HCl at T⩽800°C, with the formation of an amorphous layer of Si₃N₄. To obtain a high-quality texture it is important that prior to deposition of the AlN layers the silicon substrates be held in an NH₃ atmosphere in order to form a dense layer of Si₃N₄. Single-crystal growth of AlN can be achieved in a chloride-hydride system of chemical deposition from the vapor phase at a reduced pressure, since the deposition temperature is then substantially lower (down to 550°C) and the chemical interaction with the substrate is hindered. Pis’ma Zh. Tekh. Fiz. 24, 52–57 (October 26, 1998)     

Hydrothermal corrosion of nitride and carbide of silicon

Silicon nitride (Si₃N₄) and carbide (SiC) are getting applied to engine parts in Japan. We have to estimate the life of these parts whenever we use them for engine applications. After burning of fuel in the air we get water vapour. Therefore, reactions between Si₃N₄ and/or SiC and water vapour are important for application for engine parts. The author presents a review on this topic.     

Effect of microstructure on the high temperature strength of nitride bonded silicon carbide composite

Four compositions of nitride bonded SiC were fabricated with varying particle size of SiC of ∼ 9.67, ∼ 13.79, ∼ 60 μ and their mixture with Si of ∼ 4.83 μ particle size. The green density and hence the open porosity of the shapes were varied between 1.83 to 2.09 g/cc and 33.3 to 26.8 vol.%, respectively. The effect of these parameters on room temperature and high temperature strength of the composite up to 1300°C in ambient condition were studied. The high temperature flexural strength of the composite of all compositions increased at 1200 and 1300°C because of oxidation of Si₃N₄ phase and blunting crack front. Formation of Si₃N₄ whisker was also observed. The strength of the mixture composition was maximum.