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The advantages of depositing AlN–SiC alloy transition layers on SiC substrates before the seeded growth of bulk AlN crystals were examined. The presence of AlN–SiC alloy layers helped to suppress the SiC decomposition by providing vapor sources of silicon and carbon. In addition, cracks in the final AlN crystals decreased from ∼5 × 106/mm2 for those grown directly on SiC substrates to less than 1 × 106/mm2 for those grown on AlN–SiC alloy layers because of the intermediate lattice constants and thermal expansion coefficient of AlN–SiC. X-ray diffraction confirmed the formation of pure single-crystalline AlN upon both AlN–SiC alloys and SiC substrates. X-ray topography (XRT) demonstrated that strains present in the AlN crystals decreased as the AlN grew thicker. However, the XRT for AlN crystals grown directly on SiC substrates was significantly distorted with a high overall defect density compared to those grown on AlN–SiC alloys.
Sublimation growth of AlN was performed on Si-face, 8 ° off-axis 4H-SiC (0001) and 3.5 ° off-axis 6H-SiC (0001) seeds. AlN layers 500 - 900 μm thick and 20 mm in diameter were grown at 1830 °C by consecutive growths and continuous growth. The c-axis growth rate was approximately 8 - 18 μm/hr. On both the 8 ° and 3.5° off-axis SiC substrates, “step” features formed on the AlN surface with uniform terrace width and step density. The step heights and terrace widths increased as the AlN grew thicker. In addition, a single facet of 9 mm × 5 mm formed on the top of the layer grown on the 3.5 ° off-axis SiC. High resolution x-ray diffraction showed the AlN (00.2) 2θ value shifted from pure AlN toward SiC. Approximately 3 – 4 at% of SiC was detected at the surface of the AlN by XPS. Molten KOH/NaOH etching revealed that both samples had Al-polar surface with dislocation densities on the order of 106-107 cm−2. The cross-section etching showed the re-nucleation layer and void defects at the interfaces of the consecutive growths.
A brief overview of the research activities at the Thermionic Energy Conversion (TEC) Center is given. The goal is to achieve direct thermal to electric energy conversion with >20% efficiency and >1W/cm2 power density at a hot side temperature of 300–650C. Thermionic emission in both vacuum and solid-state devices is investigated. In the case of solid-state devices, hot electron filtering using heterostructure barriers is used to increase the thermoelectric power factor. In order to study electron transport above the barriers and lateral momentum conservation in thermionic emission process, the current-voltage characteristic of ballistic transistor structures is investigated. Embedded ErAs nanoparticles and metal/semiconductor multilayers are used to reduce the lattice thermal conductivity. Cross-plane thermoelectric properties and the effective ZT of the thin film are analyzed using the transient Harman technique. Integrated circuit fabrication techniques are used to transfer the n- and p-type thin films on AlN substrates and make power generation modules with hundreds of thin film elements. For vacuum devices, nitrogen-doped diamond and carbon nanotubes are studied for emitters. Sb-doped highly oriented diamond and low electron affinity AlGaN are investigated for collectors. Work functions below 1.6eV and vacuum thermionic power generation at temperatures below 700C have been demonstrated.
Thin layers of GaN have been deposited on 1μm thick MOVPE GaN(0001) thin film substrates using a novel vertical iodine vapor phase epitaxy system. The system features three concentric flow zones that separate the reactant gasses until they reach the substrate. Hydrogen flows through the innermost zone to deliver iodine vapor from an external bubbler to the molten Ga maintained at ∼1050°C and GaI to the substrate; high-purity ammonia flows through the outermost zone; nitrogen flows through the middle zone to prevent reaction between the growth species at the GaI nozzle. GaN growth was found to be a function of time, with decreasing concentration of iodine the likely cause of a decrease in growth rate at longer growth times. The step-and-terrace microstructure of the MOVPE seeds was replaced with a smooth morphology film after the shortest growth experiment. Star-shaped features with hexagonal symmetry grew on the surface with increasing growth time. These features became the tops of hexagonal pyramids; these pyramids grew competitively and dominated the final growth surface. The surface of the films grown for the longest period contained a step-and-terrace microstructure; however, the density of steps of was lower than that on the surface of underlying MOVPE substrate.
AlN single crystals were grown on AlN/SiC seeds by sublimation of AlN powder in TaC crucibles in a nitrogen atmosphere. The seeds were produced by metallorganic chemical vapor deposition (MOCVD) of AlN on SiC crystals. The influence of growth temperature, growth time and source-to-seed distance on the crystallinity and the crystal growth rate were investigated. Crystals were grown in an RF heated sublimation reactor at growth temperatures ranging from 1800-2000°C, at a pressure of 600 Torr, nitrogen flow-rate of 100 sccm and source-to-seed distances of 10 and 35 mm. At 1870°C and a source-to-seed distance of 35 mm, isolated crystals were observed with few instances of coalescence. At 1930°C, a source-to-seed distance of 10 mm and longer growth times (~30 hrs), crystal coalescence was achieved. Above 1930°C, the decomposition of SiC was evidently affecting the growth morphology and resulted in growth of polycrystalline AlN. After an initial nucleation period, the observed growth rates (10-30 µm/hr) were in close agreement with predictions of a growth model that assumed gas-phase diffusion controlled growth. Optical and electron microscope observations revealed step-flow growth, while X-ray diffraction results showed the single crystal nature of the grown material. Single crystalline AlN was grown over surface areas of 200-300 mm2 and was transparent and essentially colorless.
Gallium nitride (GaN) films were grown on (0001) sapphire substrates at 1050°C by controlled evaporation of gallium (Ga) metal and reaction with ammonia (NH3) at a total reactor pressure of 800 Torr. Pure nitrogen (N2) was flowed directly above the molten Ga source to prevented direct reaction between the molten Ga and ammonia, which causes Ga spattering and GaN crust formation. At the same time, this substantially enhanced the Ga transport to the substrate. A simple mass-transport model based on total reactor pressure, gas flow rates and source temperature was developed and verified. The theoretical calculations and growth rate measurements at different ammonia flow rates and reactor pressures showed that the maximum growth rate was controlled by transport of both Ga species and reactive ammonia to the substrate surface.
Thick GaN layers as well as AlGaN/GaN and AlN/GaN heterostructures grown by metalorganic vapor phase epitaxy have been photoelectrochemically (PEC) etched in various dilute electrolytes, and bandgap-selective etching has been demonstrated in heterostructures. This result is a significant step forward in the fabrication of group III-nitride devices and one-dimensional photonic bandgap (PBG) structures in the deep UV. Based on initial results from thick GaN layers, a method was developed to achieve self-stopping selective etching of thin GaN layers in AlGaN/GaN and AlN/GaN heterostructures. Selective PEC etching requires the use of a suitable light source with photon energies larger than the bandgap of GaN, but smaller than that of AlGaN or AlN, thus enabling selective hole generation in the GaN layers to be etched. Additionally, it is imperative to use an electrolyte that supports PEC etching of GaN without chemically etching AlGaN or AlN.
Bonding of polished, polycrystalline diamond films to silicon was performed in ultra high vacuum at 32 MPa of applied uniaxial stress. The transmission electron microscopy (TEM) investigation revealed that the interface of all bonded samples was non-uniform. An abrupt boundary between the two wafers existed only in some parts of the interface, while other parts contained an amorphous interlayer of up to 40 nm in thickness. Electron energy loss spectroscopy (EELS) revealed that this interlayer consisted of oxygen, carbon and silicon. Based on comparison of the microstructure and chemical composition of the interface formed at different bonding temperatures, we propose a model for the silicon/diamond wafer fusion process.
AlN crystals were grown from the vapor phase in an RF heated AlN sublimation reactor. The studies were performed with the following goals: 1) to optimize the growth rate by investigating mass transfer effects, and 2) to establish a process for epitaxial growth on AlN seeds. A one-dimensional mass transfer model based on equilibrium sublimation and gas-phase diffusion was developed. Model parameters were estimated and the model was validated from growth experiments carried out in a 600 Torr nitrogen atmosphere and temperatures ranging from 2000 to 2400°C. Continuous growth on AlN seed crystals was accomplished as a result of optimizing the initial stage of growth and achieving a delicate balance between the rate of mass transfer and the rate of surface rearrangement. During this experimental study, centimeter-size single crystals of AlN were obtained within the 1.25” diameter boule that was grown at a predicted growth rate of 0.1–0.3 mm/hr, at 500 Torr of nitrogen, short source-to-seed distance, low supersaturation and growth temperatures of 2110–2140°C. Chemical analysis of impurities in the grown AlN boules confirmed a very low oxygen contamination of 100 ppm wt. Cathodoluminescence studies showed well defined near band edge emission peak located slightly above 6 eV.
SiC wafers with an RMS roughness of 1.5 nm were bonded in a dedicated ultrahigh vacuum bonding chamber. Successful fusion of wafers was observed at temperatures as low as 800°C under a uniaxial mechanical stress of 20 MPa. Cross-section transmission electron microscopy (XTEM) of a specimen bonded at 1100°C revealed parts of the interface where wafers were in intimate contact, while other parts contained an up to 3 nm thick amorphous carbon interlayer. The bonded SiC retained its high crystalline quality; no extended defects emanating from the interface were observed within the sampling region. Electrical measurements showed that the azimuthal orientation of the bonded couple significantly influences the electrical character of the junction.
Bulk AlN crystal growth by Physical Vapor Transport (PVT) is studied both experimentally and numerically. This paper presents the analysis of heat and mass transport mechanisms in closed and partially open crucible geometries. The heat transfer in the growth system used at North Carolina State University (NCSU) is simulated. The computed temperature profiles are used in the analysis of mass transport in the growth cell to gain understanding of the effect of species exchange between the crucible and environment on the AlN growth rate. The model predictions are in reasonable agreement with observations.
The energy distribution of electrons transported through an intrinsic AlN film was directly measured as a function of the applied electric field. Following the transport, electrons were extracted into vacuum through a semitransparent Au electrode and their energy distribution was measured using an electron spectrometer. The electron energy distribution featured kinetic energies higher than that of completely thermalized electrons. Transport through 80 nm thick layers indicated the onset of quasi-ballistic transport. This was evidenced by symmetric energy distributions centered at energies above the conduction band minimum for fields greater than 530 kV/cm. Drifted Fermi-Dirac energy distributions were fitted to the measured energy distributions, with the energy scale referenced to the bottom of the AlN conduction band. The drift energy and the carrier temperature were obtained as fitting parameters. Overshoots as high as five times the saturation velocity were observed and a transient length of less than 80 nm was deduced. In addition, the velocity-field characteristic was derived from these observations. This is the first experimental demonstration of this kind of transport in AlN.
The energy distribution of electrons transported through an intrinsic AlN film was directly measured as a function of the applied field, and AlN film thickness. Following the transport, electrons were extracted into vacuum through a semitransparent Au electrode and their energy distribution was measured using an electron spectrometer. Transport through films thicker than 95 nm and applied field between 200 kV/cm -350 kV/cm occurred as steady-state hot electron transport represented by a Maxwellian energy distribution, with a corresponding carrier temperature. At higher fields (470 kV/cm), intervalley scattering was evidenced by a multi-component energy distribution with a second peak at the energy position of the first satellite valley. Electron transport through films thinner than 95 nm demonstrated velocity overshoot under fields greater than 550 kV/cm. This was evidenced by a symmetric energy distribution centered at an energy above the conduction band minimum. This indicated that the drift component of the electron velocity was on the order of the “thermal” component. A transient length of less than 80 nm was deduced from these observations.
Two field-emission states of single-walled carbon nanotubes were identified according to their respective emission current levels. The state yielding increased emission current was attributed to the presence of adsorbates on the nanotubes as confirmed by electron emission measurements at different background pressures. Application of high electric fields induced large emission currents and a transition between the two states. During this transition, a current drop to 10% of the original value was observed. Under a constant applied electric field, the current took around 1000 s to recover its original level at a background pressure of 10-10 Torr, while it took half that time at 10-6 Torr. For the high current state, field-emitted electrons originated from states located up to 1 eV below the Fermi level, as was determined by field-emission energy distribution measurements. This suggested that adsorbates introduced a resonant state on the surface that enhanced the tunneling probability of electrons. The adsorbed states are removed at high applied electric fields, presumably due to ohmic heating caused by large emission currents. This adsorption/desorption process is completely reversible.
The energy distribution of electrons that were transported through a thin intrinsic AlN film was directly measured as a function of the applied field. The measurements were realized by extracting the electrons into vacuum through a semitransparent Au contact and measuring their energy using an electron spectrometer. At moderate applied fields (350 kV/cm), the energy distribution followed a Maxwellian model corresponding to an electron temperature of 2700 K and a drift component below the spectrometer resolution. At higher fields, intervalley scattering was evidenced by the presence of a second peak at 0.7 eV. This coincides well with the energy position of the L-M valley in AlN. To the best of our knowledge, these are the first measurements that offer direct evidence of intervalley scattering in any solid system.
Free-standing single crystals of bulk GaN were grown via unseeded vapor phase transport at 1130°C on hexagonal BN surfaces via direct reaction of Ga with ammonia. The number of nucleation events was reduced and the crystal size increased by introducing the ammonia at high temperatures. The resulting crystals were either needles or platelets depending on the process variables employed. Low V/III ratios achieved via ammonia flow rates ≤ 75sccm and/or ammonia total pressures ≤ 430Torr favored lateral growth. The average lateral growth rate for the platelets was ∼50μm/hr; the average vertical growth rate for the needles was ∼500μm/hr. Growth rates in all other directions for each of these two morphologies were very low. Seeded growth of both needle and platelet crystals was also achieved; however, the growth rate decreased at longer times and higher pressures due to reaction with H2 from the increased decomposition of ammonia. Nitrogen dilution suppressed this decomposition. A 2mm × 1.5mm GaN crystal was grown with minimal decomposition in a 66.7%NH3 and 33.3%N2 gas mixture.
It has been observed that diamond deposition by flat flame chemical vapor deposition is achieved over a very narrow range of reactant composition. We demonstrate that this diamond deposition window is strongly determined by the nature of the substrate material. Furthermore, once a continuous diamond film is formed, the window appears to be independent of the original material. Substrates examined include silicon, glass, titanium, tungsten, nickel, and molybdenum. The dependence of growth rate, morphology, and quality on reactant composition has been quantified using scanning electron microscopy, Raman spectroscopy, and secondary ion mass spectroscopy (SIMS). It was found that the highest quality diamond was grown at conditions where diamond does not nucleate on ultrasonically scratched silicon. Thus, the production of high quality diamond on silicon by combustion synthesis requires different conditions for nucleation and growth.
The growth of coalesced, highly oriented diamond films has been achieved on nickel substrates using a multistep process that consisted of (i) seeding the Ni surface with 0.5 μm diamond powder, (ii) annealing at 1100 °C in a hydrogen atmosphere, and (iii) growth at 900 °C in a mixture of hydrogen and 0.5% methane. Auger depth profile analysis of a sample quenched after the annealing stage showed the presence of significant amounts of carbon (6 at. %) close to the substrate surface and about 3 at.% deeper in the substrate. The loss of carbon into the substrate resulted in relatively low nucleation density. The addition of methane into the gas phase during the annealing stage proved very effective in compensating for the diffusion. An addition of 0.5% methane in the gas phase produced optimum results, as the nucleation density, orientation of diamond particles, and uniformity were substantially improved. Substrates nucleated under these conditions were grown out into coalesced, 30 μm thick films. Both (100) and (111) oriented films showed a high degree of orientation and Raman spectra obtained from these orientations showed intense and narrow diamond signature peaks with FWHM's of 5 and 8 cm-1, respectively.
Field emission from wide bandgap materials was investigated through voltage dependent field emission energy distribution (V-FEED) analysis. As compared to classical I-V characterization, V-FEED analysis can provide additional, detailed information about the origin of and the mechanism responsible for the field emission of electrons. The V-FEED technique consists of measuring the energy distribution of field emitted electrons collected at various extraction voltages. By measuring changes in the energy of the field emission peak at different voltages, data can be extrapolated to flat-band condition to determine the energy of the band from which the electron emission originated. In this study, field emission from cubic boron nitride (c-BN) coated and diamond coated tip-shaped Mo emitters was examined. For the nominally intrinsic wide bandgap coating materials studied, a linear voltage drop across the wide bandgap material, usually on the order of 1% of extraction voltage was observed and explained by field induced band-bending. For the intrinsic c-BN and diamond samples studied, the electron emission originated from the conduction band minimum at the wide bandgap material/vacuum interface.