Boron nitride was studied as a fiber–matrix interface coating for Nicalon™/SiC composites. The effect of initial O-impurity content within the as-processed BN coatings on the long-term interface stability was investigated at elevated temperatures in flowing oxygen. Two types of Nicalon™/SiC composites were used for this study; one composite had a BN coating with <2% oxygen (low-O BN) and another composite had BN with an oxygen concentration >11% (high-O BN) in the as-processed state. The high-O BN is actually most representative of BN coatings available commercially. The BN coatings in both the high-O and low-O BN containing composites were structurally similar. The samples used here were thinned to <200 μm before oxidation and the final preparation for electron microscopy examination of the interface region was done after the reactions were completed. Thin samples were used to simulate maximum corrosion effects that would occur at the surface of an actual part during service. Ech sample was exposed to flowing oxygen at temperatures as high as 950°C for times up to 400 h. After each oxidation experiment, the BN coatings were examined by TEM to quantify the extent of any reaction which occurred at either the fiber/BN and BN/SiC matrix interfaces. At 950°C for 100 h, there were no interface microstructural changes observed in the low-O BN but there was extensive silica formation at the fiber/BN interfaces in the high-O BN. After 400 h at 950°C, large voids formed at the fiber/BN interface in the high-O BN sample only. Oxygen present within the initial BN coating contributed significantly to the degradation of the interfacial properties of the composite. Several techniques, including transmission electron microscopy (TEM), Auger electron spectroscopy (AES), energy-dispersive spectrometry (EDS), and electron energy-loss spectroscopy (EELS) were used to characterize changes in structure and chemistry of the fiber–matrix interface region and to elucidate and quantify composite degradation mechanisms.

Monocrystalline GaN(0001) films were grown via OMVPE at 950 degrees C on AlN(0001) deposited at 1100 degrees C on alpha(6H)-SiC(0001)(Si) substrates. AlxGa1-xN films (0 less than or equal to x less than or equal to 1) were deposited at 1100 degrees C directly on SiC. X-ray rocking curves for 1.4 mu m GaN(0004) revealed FWHM values of 58 and 151 arcsec for materials simultaneously grown on the on-axis and off-axis SiC, respectively. Silicon donor-doping in highly resistive GaN and AlxGa1-xN (for x less than or equal to 0.4) was achieved for net carrier concentrations ranging from approximately 2X10(17) cm(-3) to 2X10(19) (AlxGa1-xN) or to 1X10(20) (GaN) cm(-3). Mg-doped, p-type GaN was achieved with n(A)-n(D) approximate to 3X10(17) cm(-3), rho approximate to 7 Ohm.cm and mu approximate to 3 cm(2)/V.s.

The optimization of currently available catalysts and the development of new ones requires a detailed understanding of the effects of both microstructure and composition on their function. Previous work has demonstrated that information at the atomic-scale on heterogeneous catalysts can be derived using high resolution transmission electron microscopy (TEM). Under favorable conditions, the morphology of heavy metal catalytic particles can be related to catalytic activity. It has been shown that preoxidation can disrupt the surface of small metal particles causing altered activity and selectivity in reactions such as alkane hydrogenolysis. Metals such as Pt, Rh, or Ru are noble and pick up no more than a monolayer of oxygen when exposed to air during sample preparation for microscopy. This oxygen monolayer is not imaged in the microscope, most likely because the oxygen desorbs during exposure to the high energy electron beam. However, when metals such as Fe, Co, Ni, Pd or Cu are exposed to air, there is a corrosive interaction that alters particle structure quite dramatically.

Thin films of AlN and GaN are deposited primarily via the common forms of organometallic vapor phase epitaxy (OMVPE) and molecular beam epitaxy (MBE). Sapphire is the most common substrate; however, a host of materials have been used with varying degrees of success. Both growth techniques have been employed by the authors to grow AlN and GaN primarily on 6H-SiC(0001). The mismatch in atomic layer stacking sequences along the growth direction produces inversion domain boundaries in the AlN at the SiC steps; this sequence problem may discourage the nucleation of GaN. Films of AlN and GaN grown by MBE at 650°C are textured; monocrystalline films are achieved at 1050°C by this technique and OMVPE. Donor and acceptor doping of GaN has been achieved via MBE without post growth annealing. Acceptor doping in CVD material requires annealing to displace the H from the Mg and eventually remove it from the material. High brightness light emitting diodes are commercially available; however, numerous concerns regarding metal and nitrogen sources, heteroepitaxial nucleation, the role of buffer layers, surface migration rates as a function of temperature, substantial defect densities and their effect on film and device properties, ohmic and rectifying contacts, wet and dry etching and suitable gate and field insulators must and are being addressed.

Monocrystalline GaN(0001) thin films were grown at 950 °Con AlN(0001) buffer layers previously deposited at 1100 °C on α(6H)-SiC(0001)si substrates via metallorganic chemical vapor deposition (MOCVD). Films of AlxGa1 − xN (0 ≤ x ≤ 1) were grown directly on the same SiC surface at 1100 °C. X-ray rocking curves for the GaN(0004) reflection for 1.4 μm films revealed FWHM values of 58 and 151 arc sec for materials grown on on-axis and offaxis substrates, respectively. Cathodoluminescence exhibited strong near band-edge emission for all materials. Controlled n- type Si-doping in GaN and AlxGa1 − xN (for x ≤ 0.4) was achieved with net carrier concentrations ranging from approximately 2 × 1017 cm− 3 to 2 × 1019 (AlxGa1 − xN) or to 1 × 1020 (GaN) cm− 3. Mg-doped, p-type GaN and AlxGa1 − xN (for x ≤ 0.13) was achieved with nA − nD ≈ 3 × 1017 cm− 3.

The mechanical reliability of ceramic matrix composites (CMCs) at elevated temperatures in oxidative environments is primarily dependent upon the chemical and structural stability of the fiber/matrix interface. Graphitic carbon coatings have traditionally been used to control the interfacial properties in CMCs, however, their use is limited in high temperature oxidative environments due to the loss of carbon and subsequent oxidation of the fiber and matrix. Thus, BN is being investigated as an alternative interfacial coating since it has comparable room temperature properties to carbon with improved oxidation resistance. The stability of BN interfaces in SiC/SiC composites is being investigated at elevated temperatures in either flowing oxygen or environments containing water vapor. The effect of several factors on BN stability, including crystallographic structure, extent of BN crystallization, and impurity content, are being evaluated.Nicalon™ fiber preforms were coated with ≈ 0.4 μm of BN by CVD using BCl3, NH3, and H2 at 1373 K. The coated preforms were densified using a forced-flow chemical vapor infiltration (FCVI) technique developed at ORNL.

Amorphous, hexagonal and cubic phases of BN were grown via ion beam assisted deposition on Si(1 0 0) substrates. Gas-source molecular beam epitaxy of the III–V nitrides is reviewed. Sapphire(0 0 0 1) is the most commonly employed substrate with 6H-SiC(0 0 0 1), ZnO(1 1 1) and Si(1 1 1) also being used primarily for the growth of wurtzite GaN(0 0 0 1) in tandem with previously deposited GaN(0 0 0 1) or AlN(0 0 0 1) buffer layers. Silicon(0 0 1), GaAs(0 0 1), GaP(0 0 1) and 3C-SiC(0 0 1) have been employed for growth of cubic (zincblende) β-GaN(0 0 1). The precursor materials are evaporated metals and reactive N species produced either via ECR or RF plasma decomposition of N2 or from ammonia. However, point defect damage from the plasma-derived species has resulted in a steady increase in the number of investigators now using ammonia. The growth temperatures for wurtzite GaN have increased from 650 ± 50°C to 800 ± 50°C to enhance the surface mobility of the reactants and, in turn, the efficiency of decomposition of ammonia and the microstructure and the growth rate of the films. Doping has been achieved primarily with Si (donor) and Mg (acceptor); the latter has been activated without post-growth annealing. Simple heterostructures, a p-n junction LED and a modulation-doped field-effect transistor have been achieved using GSMBE-grown material.

Monocrystalline GaN(0001) thin films have been grown at 950 °C on high-temperature, ≈ 100 nm thick, monocrystalline AlN(0001) buffer layers predeposited at 1100 °C on α(6H)−SiC(0001)Si substrates via OMVPE in a cold-wall, vertical, pancake-style reactor. These films were free of low-angle grain boundaries and the associated oriented domain microstructure. The PL spectra of the GaN films deposited on both vicinal and on-axis substrates revealed strong bound excitonic emission with a FWHM value of 4 meV. The near band-edge emission from films on the vicinal substrates was shifted slightly to a lower energy, indicative of films containing residual tensile stresses. A peak attributed to free excitonic emission was also clearly observed in the on-axis spectrum. Undoped films were too resistive for accurate Hall-effect measurements. Controlled n-type, Si-doping in GaN was achieved for net carrier concentrations ranging from approximately 1 × 1017 cm−3 to 1 × 1020 cm−3. Mg-doped, p-type GaN was achieved with nA−nD ≈ 3 × 1017 cm−3, ρ ≈ 7 Ω · cm, and μ ≈ 3 cm2/V · s. Double-crystal x-ray rocking curve measurements for simultaneously deposited 1.4 μm GaN films revealed FWHM values of 58 and 151 arcsec for deposition on on-axis and off-axis 6H−SiC(0001)Si substrates, respectively. The corresponding FWHM values for the AlN buffer layers were approximately 200 and 400 arcsec, respectively.

Monocrystalline GaN(0001) thin films with smooth surfaces were grown at 950 degrees C on AlN(0001) buffer layers previously deposited at 1100 degrees C on alpha(6H)-SiC(0001)Si substrates via MOCVD in a cold-wall, vertical, pancake reactor. AlxGa1-xN films (0 less than or equal to x less than or equal to 1) were grown directly on the same SiC surface at 1100 degrees C. X-ray rocking curves revealed FWHM values of 58 and 151 are sec for GaN grown on the on-axis and vicinal substrates, respectively. The corresponding values for the AlN(0004) buffer layers were approximate to 200 and approximate to 400 are sec. Undoped GaN and AlxGa1-xN films were very resistive. Donor (Si) doping in GaN and AlxGa1-xN (for x less than or equal to 0.4) was achieved between approximate to 2x10(17) cm(-3) and 2x10(19) (AlxGa1-xN) or 1x10(20) (GaN) cm(-3). Acceptor(Mg) doping in GaN was achieved to 3 x 10(17) cm(-3).

Silicon carbide (SiC) and aluminum nitride (AlN) thin films have been grown on 6HSiC(0001) substrates by gas-source molecular beam epitaxy (GSMBE) at 1050°C. Step flow, step bunching and the deposition of 6HSiC occurred at the outset of the exposure of the (1 × 1) vicinal substrate surface to C2H4Si2H6 gas flow ratios of 1, 2 and 10. Subsequent deposition resulted in step flow and continued growth of 6H films or formation and coalescence of 3CSiC islands using the gas flow ratio of one or the ethylene-rich ratios, respectively. The (3 × 3) surface reconstruction observed using the former ratio is believed to enhance the diffusion lengths of the adatoms, which in turn promotes step flow growth. Essentially atomically flat monocrystalline AlN surfaces were obtained using on-axis substrates. Island-like features were observed on the vicinal surface. The coalescence of the latter features at steps gave rise to inversion domain boundaries (IDBs) as a result of the misalignment of the SiC bilayer steps with the AlN bilayers in the growing film. The quality of thicker AlN films is strongly influenced by the concentration of IDBs. Undoped, highly resistive (102 Ω · cm) and Mg-doped, p-type (0.3 Ω · cm) monocrystalline GaN films having a thickness of 0.4–0.5 μm have also been grown via the same technique on AlN buffer layers without post-processing annealing.

Monocrystalline GaN(0001) thin films, void of oriented domain structures and associated low-angle grain boundaries, have been grown via organometallic vapor phase epitaxy (OMVPE) on high-temperature monocrystalline AlN(0001) buffer layers predeposited on vicinal α(6H)–SiC(0001) wafers using TEG, TEA, and ammonia in a cold wall, vertical, pancake-style reactor. The surface morphology was smooth, and the PL spectrum showed strong near-band-edge emission with a full width at half-maximum (FWHM) value of 4 meV. The dislocation density within the first 0.5 μm was ≊1×109 cm−2; it decreased substantially with increasing film thickness. Controlled n-type Si doping of GaN has been achieved for net carrier concentrations ranging from ∼1×1017 to 1×1020 cm−3. Double-crystal XRC measurements indicated a FWHM value of 66 arcsec for the GaN(0004) reflection.

Monocrystalline GaN(0001) thin films have been grown at 950°C on high-temperature, 100 nm thick, monocrystalline AlN(0001) buffer layers previously deposited at 1100°C on α(6H)-SiC(0001)Si substrates via MOCVD in a cold-wall, vertical, pancake-style reactor. AlxGa1−xN films (0≤x≤1) were grown directly on the same SiC surface at 1100°C. Abrupt heterojunctions among the alloy composition were demonstrated. All films possessed a smooth surface morphology and were free of low-angle grain boundaries and associated oriented domain microstructures. Double-crystal x-ray rocking curve measurements for the GaN(0004) reflection for simultaneously deposited 1.4 μm films revealed FWHM values of 58 and 151 arcsec for materials grown on on-axis and off-axis material, respectively. The corresponding values for the AlN(0004) buffer layers were ≈ 200 and ≈400 arc sec, respectively. A similar relationship was found for the alloys for 0≤x≤0.2. The PL spectra of the GaN films deposited on both vicinal and on-axis substrates revealed strong bound exciton emission with a FWHM value of 4 meV. The spectra of these films on the vicinal substrates were shifted to a lower energy, indicative of films containing residual tensile stresses. A peak believed to be associated with free excitonic emission was also observed in each on-axis spectrum. Rutherford backscattering, Auger depth profiling and energy dispersive analysis were used to determine the AlN/GaN ratios in the alloys. Cathodoluminescence of solutions with x<0.5 exhibited strong near band edge emission with a FWHM as low as 31 meV. The band gaps were determined via spectral ellipsometry. Undoped GaN and AlxGa1−xN films were too resistive for accurate Hall-effect measurements. Controlled n-type Si-doping in GaN and AlxGa1−xN (for x≤0.4) was achieved for net carrier concentrations ranging from approximately 2 × 1017 cm−3 to 2 × 1019 (AlxGa1−xN) or to 1 × 1020 (GaN) cm−3. Mg-doped, p-type GaN was achieved with nA−nD ≈ 3 × 1017cm−3, ρ ≈ 7 Ω·cm and μ ≈ 3 cm2/V·s.

Organometallic vapor phase epitaxy (OMVPE) and molecular beam epitaxy (MBE) are the most common methods for the growth of thin films of A1N and GaN. Sapphire is the most common substrate; however, a host of materials have been used with varying degrees of success. Both growth techniques have been employed by the authors to grow AIN, GaN and AlxGa1−xN thin films primarily on 6H-SiC(0001). The mismatch in atomic layer stacking sequences along the growth direction produces double positioning boundaries in A1N and the alloys at the SiC steps; this sequence problem appears to discourage the two-dimensional nucleation of GaN. Films of these materials grown by MBE at 650°C are textured; monocrystalline films are achieved between 850°C (pure GaN) and 1050°C (pure A1N) by this technique and OMVPE. Donor and acceptor doping of GaN has been achieved via MBE without post growth annealing. Acceptor doping in CVD material requires annealing to displace the H from the Mg and eventually remove it from the material. High brightness light emitting diodes are commercially available; however, numerous concerns regarding metal and nitrogen sources, heteroepitaxial nucleation, the role of buffer layers, surface migration rates as a function of temperature, substantial defect densities and their effect on film and device properties, ohmic and rectifying contacts, wet and dry etching and suitable gate and field insulators must and are being addressed. Selected issues surrounding the growth of these materials with particular examples drawn from the authors' research are presented herein.

Boron nitride (BN) thin films have been grown on the (100) surfaces of Si, diamond, Ni and Cu via ion beam assisted deposition (IBAD) using electron beam evaporation of B in tandem with N and Ar ion bombardment within the ranges of substrate temperature and ion flux of 200-700°C and 0.20–0.30 mA/Cm2, respectively. Fourier-transform infrared spectroscopy (FTIR) and high resolution transmission electron microscopy (HRTEM) revealed a growth sequence of amorphous (a-BN), hexagonal (h-BN) and cubic (c-BN) layers on Si and diamond under most conditions. This sequence is attributed primarily to increasing biaxial compressive stress with film thickness due to interstitial Ar incorporation observed via Rutherford backscattering spectroscopy (RBS). The effect of deposition conditions, specifically substrate temperature and bombardment intensity, on the film growth was studied. Increasing the substrate temperature above 400°C led to the onset of the cubic phase at a greater film thickness, while increased ion flux led to earlier growth of this phase. These results may be explained by the relaxation of intrinsic stress in the films at higher temperatures due to increased adatom mobility and to increased intrinsic stress in the films resulting from increased ion bombardment, respectively. Lower temperatures led to mixed phase growth. A minimum substrate temperature (200-300°C) is required for nucleation and growth of single phase c-BN by this technique. A combination of h-BN and c-BN was deposited on Ni; only h-BN was obtained on Cu substrates.

The morphology and interface chemistry occurring during the initial deposition of BN, AlN and GaN films via metal evaporation and N2 decomposition under UHV conditions have been determined. FTIR spectroscopy and TEM revealed the consecutive deposition of an initial 20Å layer of a-BN, 20-60Å of oriented h-BN, and a final layer of polycrystalline c-BN. This sequence is attributed primarily to increasing intrinsic compressive stress in the films. XPS analysis revealed the growth of GaN on sapphire to occur via the Stranski-Krastanov mode; growth on SiC showed characteristics of three-dimensional growth. AlN grew layer-by-layer on both substrates. Vicinal 6H-SiC(0001) substrate surfaces contain closely spaced, single bilayer steps. During deposition of Si and C at 1050°C, 6H layers initially form and step bunching occurs. The latter phenomenon results in more widely spaced steps, the nucleation of 3C-SiC both on the new terraces and at the larger steps and formation of double position boundaries. The C/Si ratio in the gaseous reactants also affects the occurrence of these three phenomena.

Fabrication of optoelectronic devices from III-N materials, operable in the blue and ultraviolet regions of the spectrum, has been a goal of many groups since the first infrared and red devices were commercially produced. Commercially viable blue-light-emitting diodes have now been achieved in these materials, and numerous investigators are now entering tills field. Microelectronic devices are also of considerable interest.While much of the original research on GaN and InN centered around films with the wurtzite structure, the zincblende structure nitrides have recently been receiving increasing interest because of their potential inherent advantages, e.g. increased carrier mobility. Recent results from several groups around the world have successfully addressed sonic of the traditional problems associated with growth of these materials.Chemical vapor deposition (CVD) has been a traditional growth technique for nitride growth and continues to be very successful. However, many researchers, including those growing III-V nitrides, have turned to plamsa-enhanced molecular beam epitaxy (MBE) for its important advantages in purity and in situ, analytical capabilities. This paper reviews the recent developments in MBE growth of cubic boron nitride, aluminum nitride and the two polytypes of gallium nitride.

Boron nitride (BN) thin films have been grown on [100] oriented single crystal Si, diamond, Cu and Ni substrates by ion beam assisted deposition using electron beam evaporation of boron together with simultaneous bombardment by nitrogen and argon ions. Characterization by Fourier-transform infrared spectroscopy and high-resolution transmission electron microscopy showed that the films on Si and diamond consisted of initial noncubic (amorphous and hexagonal BN) layers, followed by the growth of cubic BN (c-BN). This growth sequence was attributed primarily to increasing compressive intrinsic stress with increased film thickness. Increasing the substrate temperature above 400 °C led to the onset of c-BN at a greater film thickness while increased ion flux resulted in earlier growth of this phase. These results may be explained by the relaxation of the intrinsic stress in the films at higher temperatures due to increased adatom mobility and to increased intrinsic stress in the films resulting from increased ion bombardment. Lower temperatures led to mixed phase growth. A minimum substrate temperature (200–300 °C) is required for nucleation and growth of single phase c-BN by this technique. It is believed that the interstitial Ar observed in Rutherford backscattering spectrometry studies is primarily responsible for the stress generation in the films. A combination of h-BN and c-BN was deposited on Ni; only h-BN was obtained on Cu substrates.