Jh Edgar

New technique for sublimation growth of AlN single crystals

Single crystalline platelets of aluminum nitride (AlN) were successfully grown by a new technique. It consists of (1) depositing an AlN buffer layer on a SiC substrate by metal organic chemical vapor deposition (MOCVD) below 1100°C, (2) forming an (AlN) x (SiC) 1−x alloy film on the AlN film by condensing vapors sublimated at a temperature of 1800°C from a source mixture of AlN-SiC powders, followed by (3) condensing vapors sublimated from a pure AlN source (at 1800°C). The necessity of the first two steps for the successful AlN sublimation growth on SiC substrate was illustrated by the initial nucleation studies of alloys on SiC substrates with and without MOCVD AlN buffer layers: an AlN MOCVD buffer layer leads to continuous, single grain growth mode; The (AlN) x (SiC) 1−x alloy film reduces the crack density because its thermal expansion coefficient is intermediate between SiC and AlN. X-ray diffraction (XRD) and Raman spectroscopy studies indicated the high quality of the AlN single crystal.

The Durability of Various Crucible Materials for Aluminum Nitride Crystal Growth by Sublimation

Producing high purity aluminum nitride crystals by the sublimation-recondensation technique is difficult due to the inherently reactive crystal growth environment, normally at temperature in excess of 2100 °C. The durability of the furnace fixture materials (crucibles, retorts, etc.) at such a high temperature remains a critical problem. In the present study, the suitability of several refractory materials for AlN crystal growth is investigated, including tantalum carbide, niobium carbide, tungsten, graphite, and hot-pressed boron nitride. The thermal and chemical properties and performance of these materials in inert gas, as well as under AlN crystal growth conditions are discussed. TaC and NbC are the most stable crucible materials with very low elemental vapor pressures in the crystal growth system. Compared with refractory material coated graphite crucibles, HPBN crucible is better for AlN self-seeded growth, as crystals tend to nucleate in thin colorless platelets with low dislocation density.

Defect structures in B12As2 epitaxial layers grown on (0001) 6H-SiC

A detailed analysis of the microstructure in B12As2 epitaxial layers grown by chemical-vapor deposition on (0001) 6H-SiC substrates is presented. Synchrotron white beam x-ray topography enabled macroscopic characterization of the substrate/epilayer ensembles and revealed the presence of a quite homogeneous solid solution of twin and matrix epilayer domains forming a submicron mosaic structure. The basic epitaxial relationship was found to be (0001)B12As2⟨112¯0⟩B12As2∥(0001)6H-SiC⟨112¯0⟩6H-SiC and the twin relationship comprised a 180° (or equivalently 60°) rotation about [0001]B12As2 in agreement with previous reports. Cross-sectional high resolution transmission electron microscopy revealed the presence of a ∼200u2002nm thick disordered transition layer which was shown to be created by the coalescence of a mosaic of translationally and rotationally variant domains nucleated at various types of nucleation sites available on the (0001) 6H-SiC surface. In this transition layer, competition between the growth of...

Photoluminescence investigation of the indirect band gap and shallow impurities in icosahedral B12As2

The indirect band gap of icosahedral B12As2 (IBA) has been determined by variable temperature photoluminescence measurements (8u2009K-294u2009K) on solution-grown bulk samples. In addition, evidence of three shallow acceptor levels and one shallow donor level is reported. The low-temperature spectra were characterized by broad and intense deep defect emission, donor-acceptor pair (DAP) bands, and exciton recombination. The appearance of DAP emission verifies the incorporation of a donor in IBA, which has not been reported previously. The temperature dependence of the free exciton (FE) intensity reflected a FE binding energy of 45u2009meV. The variation of the FE peak position with temperature was fitted with both Varshni and Passler models to determine an expression for the temperature dependence of the indirect band gap. The resulting low and room temperature band gaps are Eg(0)u2009=u20093.470u2009eV and Eg(294u2009K)u2009=u20093.373u2009eV, respectively. The latter is not consistent with previous reports of the room temperature band gap, 3.2...

Thermal conductivity and Seebeck coefficients of icosahedral boron arsenide films on silicon carbide

The thermal conductivity of icosahedral boron arsenide (B12As2) films grown on (0001) 6H–SiC substrates by chemical vapor deposition was studied by the 3ω technique. The room temperature thermal conductivity decreased from 27.0 to 15.3 W/mu2009K as the growth temperature was decreased from 1450 to 1275u2009°C. This is mainly attributed to the differences in the impurity concentration and microstructure, determined from secondary ion mass spectrometry and high resolution transmission electron microscopy, respectively. Callaway’s theory was applied to calculate the temperature-dependent thermal conductivity, and the results are in good agreement with the experimental data. Seebeck coefficients were determined as 107u2002μV/K and 136u2002μV/K for samples grown at 1350u2009°C with AsH3/B2H6 flow ratio equals to 1:1 and 3:5, respectively.

Electronic excitations in B12As2 and their temperature dependence by vacuum ultraviolet ellipsometry

The dielectric response function of epitaxial B(12)As(2) films on 4H-SiC was determined at room temperature and at 10 K in the spectral region of 3.6-9.8 eV, i.e., in the vacuum ultraviolet (VUV) spectral region, by synchrotron ellipsometry. The experimental dielectric function was simulated with the critical point parabolic band model. The parameters of the dispersive structures were derived by numerical fitting of the experimental data to the proposed model. New high energy optical transitions are resolved at 5.95, 7.8 and 8.82 eV and their lineshape and origin are discussed. The temperature dependence of the critical point energies and transition strengths was determined, and the excitonic effect is considered.

An investigation of phonon decay in B12As2 by Raman scattering spectroscopy

Raman scattering spectroscopy was used to study the spectral linewidth and frequency of characteristic phonon modes of boron arsenide (B12As2) as a function of temperature between 7 and 680 K. A combination of two- and three-phonon decay processes was found to dominate the dynamics of the intra-icosahedral phonon modes at 624, 682, and 742u2002cm−1, whilst two-phonon decay appears to be favored for the As–As stretch mode at 310u2002cm−1 and for the 506u2002cm−1 mode. Changes in frequency with temperature predominantly contain contributions from lattice expansion for all modes except the 506u2002cm−1 Eg mode, where contributions from phonon damping play a more significant role.