Hetero-Epitaxial Diamond Single Crystal Growth on Surface of cBN Single Crystals at High Pressure and High Temperature

We report a new diamond synthesis process in which cubic boron nitride single crystals are used as seeds, FesoNi20 alloy powder is used as catalyst/solvent and natural flake-like graphite is used as the carbon source. The samples are investigated using laser Raman spectra and x-ray diffraction （XRD）. Morphology of the sample is observed by a scanning electron microscope （SEM）. Based on the measurement results, we conclude that diamond single crystals have grown on the cBN crystal seeds under the conditions of high temperature 1230℃ and high pressure 4.8 GPa. This work provides an original method for synthesis of high quality hereto-semiconductor with cBN and diamond single crystals, and paves the way for future development.     

Aluminum nitride substrates for ultraviolet light-emitting diode structures

Currently, wafers of aluminum nitride cut from bulk aluminum nitride crystals (AlN) grown by sublimation are considered promising substrates for obtaining light-emitting diode structures based on nitrides of the third group. In this study, the structural characteristics and electrical properties of AlN, as a prospective substrate material for light-emitting diode heterostructures based on AlGaN/GaN, were investigated. The substrate working surface ((0001) plane, Al-polar) was specifically prepared for epitaxial growth using chemical-mechanical polishing. The surface roughness (“epi-ready”), as estimated by atomic force microscopy, did not exceed 0.3 nm.     

Novel technique for hetero-epitaxial diamond film growth on cubic boron nitride from iron carbide at high temperature and high pressure

Cubic boron nitride (c-BN) crystals about 0.1–0.3 mmin dimension were treated with iron carbide powders (high purity 99%) with size of 80–100 mesh at a high temperature of 1620 K and a high pressure of 5.2 GPa. It was found that hetero-epitaxial diamond films have been grown on the c-BN from iron carbide. The formation of dia-mond films on the cubic boron nitride can be confirmed by laser Raman spectra, face scan of elements and reflective high-energy electron diffraction. It was suggested that diamond films could be epitaxially formed on the c-BN through decomposition of iron carbide. This approach provides a possible and very effective way to realize hetero-epitaxial growth of homogeneous and large-area diamond films on c-BN, which is different from the conventional technique using a chemical vapor deposition method. Received: 20 December 2000 / Accepted: 9 January 2001 / Published online: 28 February 2001     

Investigation of the thermal conductivity of diamond film on aluminum nitride ceramic

Diamond films were successfully synthesized on aluminum nitride(AlN) ceramic substrates by hot-filament chemical vapor deposition (HFCVD) method. The thermal conductivity of the diamond film/aluminum nitride ceramic (DF/AlN) composites was studied by photothermal deflection (PTD) technique. It has reached 2.04 W/cm K, 73% greater than that of AlN ceramic. Compared with the measurement of scanning electron microscopy (SEM) and Raman spectroscopy, the influence of diamond films on the thermal conductivity of the composites was pointed out. The adhesion and the stresses were also studied. The unusual stability and very good adhesion of the diamond film on AlN ceramic substrate obtained is attributed to the formation of aluminum carbide. Received: 24 March 1998 / Accepted: 8 March 1999 / Published online: 5 May 1999     

Impact of surface step heights of 6H-SiC (0 0 0 1) vicinal substrates in heteroepitaxial growth of 2H-AlN

Impact of step height of silicon carbide (SiC) substrates on heteroepitaxial growth of aluminum nitride (AlN) was investigated. Step-and-terrace structures with various step heights, 6 monolayer (ML), 3ML and 1ML, were formed on 6H-SiC (0 0 0 1) vicinal substrates by high-temperature gas etching. 2H-AlN layers were grown on the substrate by plasma-assisted molecular-beam epitaxy (MBE) and then these layers were characterized by atomic-force microscopy (AFM) and X-ray diffraction (XRD). High-quality AlN can be grown on SiC substrates with 6ML- and 3ML-height step, while AlN grown on SiC substrates with 1ML-height step exhibited inferior crystalline quality. A model for high-quality AlN growth on SiC substrates with 3ML-height step is proposed.     

Atom force microscopy study on HTHP as-grown diamond single crystals

For understanding the mechanism of diamond growth at high temperature–high pressure (HTHP) from a metallic catalyst–graphite system, it is of great interest to perform atomic force microscopy (AFM) experiments, which provide a unique technique different from that of normal optical and electronic microscopy studies, to study the topography of HTHP as-grown diamond single crystals. In the present paper, we report first AFM results on diamond single crystals grown from a Fe-Ni-C system at HTHP to reveal the growth mechanism of diamond single crystals at HTHP. AFM images for as-grown diamond samples show dark etch pits on the (111) surface, indicating dislocations. Some fine particles about 100–300 nm in dimension were directly observed on the (100) diamond surface. These particles are believed to have been formed through transition of graphite to diamond under the effect of the catalyst and to have been transported to the growing diamond surface through a metallic thin film by diffusion. The roughness of the (100) diamond surface is found to be about several tens of nanometers through profile analysis. The diamond growth at HTHP, in a sense, could be considered as a process of unification of these fine diamond particles or of carbon-atom-cluster recombination on the growing diamond crystal surface. Successive growth interlayer steps on the (111) diamond surface were systemically examined. The heights of the growth interlayer steps were measured by sectional analysis. It was shown that the heights of the growth interlayer steps are quite different and range from about 10 to 25 nm. The source of the interlayer steps might be dislocations. The diamond-growth mechanism at HTHP could be indicated by the AFM topography of the fine diamond particles and the train-growth interlayer steps on the as-grown diamond surfaces. Received: 29 March 2001 / Accepted: 20 August 2001 / Published online: 2 October 2001     

Fabrication and Frequency Response Characteristics of AlN-Based Solidly Mounted Resonator

Film bulk acoustic resonator (FBAR) with solidly mounted resonator (SMR)-type is carried out by rf magnetic sputtering. To fabricate SMR-type FBAR, alternative high and low acoustic impedance layers, Mo/Ti multilayer, are adopted as Bragg reflector deposited by dc magnetron sputtering. The influences of sputtering pressure, substrate temperature and sputtering power on the surface roughness of Bragg reflector layer are discussed. From the atom force microscopy (AFM) analysis, the surface roughness of the Bragg reflector is improved remarkably by controlling deposition conditions. Under the appropriate sputtering condition, AlN thin films with highly c-axis-preferred orientation are deposited by rf magnetron sputtering. The performance of fabricated Mo/Ti SMR shows that the electromechanical coupling coefficient is 3.89%, the series and parallel resonant frequencies appear at 2.49 and 2.53GHz, with their quality factors 134.2 and 97.6, respectively.     

n‐type conductivity in sublimation‐grown AlN bulk crystals

AlN crystals grown by physical vapour‐phase transport in the presence of a SiC doping source possess n‐type conductivity. The net donor concentration attains up to mid 10¹⁷ cm^–3. The investigation reveals shallow donors forming an impurity band and acceptor‐like electron traps at about 0.5 eV below the conduction band edge. Thermal electron emission from these traps is responsible for the observed n‐type conductivity. The shallow donors are suggested to be due to Si atoms on Al sites. The majority of them is assumed to be compensated by deep acceptors in the lower half of the band gap. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis of nano and micro crystals of Cd(OH)2 and CdO in the shape of hexagonal sheets and rods

Cd(OH)₂ and CdO nano/micro crystals were synthesized in ethanol-water medium using cadmium foil as a source under solvothermal condition. The experimental parameters such as ratio of ethanol to water, concentration of NaOH and synthesis temperature all play important role in determining the size, shape and crystalline phase of the products. The products were characterized by X-ray diffraction and scanning electron microscopy. Nano/micro crystals of CdO were also achieved by thermal treatment of Cd(OH)₂ crystals in air at different temperatures.     

Effects of Al and Ti/Cu on Synthesis of Type-IIa Diamond Crystals in Ni70Mn25Co5-C System at HPHT

High-quality type-Ⅱa gem diamond crystals are successfully synthesized in a NiToMn25Co5-C system by temperature gradient method （TGM） at about 5.5 GPa and 1560 K. Al and Ti/Cu are used as nitrogen getters respectively. While nitrogen getter Al or Ti/Cu is added into the synthesis system, some inclusions and caves tend to be introduced into the crystals. When Al is added into the solvent alloy, we would hardly gain high-quality type-Ⅱa diamond crystals with nitrogen concentration Nc 〈 1 ppm because of the reversible reaction of Al and N at high pressure and high temperature （HPHT）. Piowever, when Ti/Cu is added into the solvent alloy, high-quality type-Ⅱa diamond crystals with Nc 〈 1 ppm can be grown by decreasing the growth rate of diamonds.     

The sublimation growth of AlN fibers: transformations in?morphology & fiber direction

The growth of AlN fibers using sublimation method was investigated in the temperature range from 1600 °C to 2000 °C. Large-scale AlN fibers are obtained with diameters from 100 nm to 50 μm and lengths up to several millimeters. The fiber morphology and growth direction are characterized by X-ray diffraction (XRD), field emission scanning electron microscope (SEM), high-resolution transmission electron microscopy (HRTEM), and Raman scattering. The fibers change from wire-like to prism-like in morphology and increase in diameter as rising temperatures, accompanying a transformation in axial direction from [10 ] to [0001]. The transformation in the growth direction is discussed in terms of AlN structure and supersaturation of AlN gas species. These results provide useful information for controlling the growth of large-scale AlN fibers.     

HPHT Synthesis of Different Shape Coarse-Grain Diamond Single Crystals

Synthesis of coarse-grain diamond crystals is studied in a China-type SPD6× 1670T cubic high-pressure apparatus with high exact control system. To synthesize high quality coarse-grain diamond crystals, advanced indirect heat assembly, powder catalyst technology and optimized synthesis craft are used. At last, three kinds of coarse- grain diamond （about 0.85 mm） single crystals with hexahedron, hex-octahedron and octahedron are synthesized successfully under HPHT （about 5.4 GPa, 1300-1450℃）. The growth characters of different shape crystals are discussed. The results and techniques might be useful for the production of coarse-grain diamonds.     

Pulsed-laser-deposited epitaxial aluminum nitride films on (111) Si for surface acoustic-wave applications

Epitaxial (001) aluminum nitride (AlN) thin films on (111) Si substrates are prepared using pulsed-laser deposition. The epitaxial structure of the as-prepared thin films is characterized by checking the X-ray-diffraction θ-2 θ scan and pole-figure, using scanning electron microscopy, infrared radiation (IR) spectroscopy and Raman spectroscopy. The surface acoustic-wave resonance at 345 MHz for a 1.5 μm thick AlN film on a (111) Si substrate is observed using an inter-digital electrode. Received: 18 September 2001 / Accepted: 29 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: liujm@nju.edu.cn     

Excimer laser surface treatment of aluminum alloy in nitrogen

The excimer laser nitriding process reported is developed to enhance the mechanical and chemical properties of aluminum alloys. An excimer laser beam is focused onto the alloy surface in a cell containing 1-bar nitrogen gas. A vapor plasma expands from the surface and a shock wave dissociates and ionizes nitrogen. It is assumed that nitrogen from plasma in contact with the surface penetrates to some depth. Thus it is necessary to work with a sufficient laser fluence to create the plasma, but this fluence must be limited to prevent laser-induced surface roughness. The nitrogen-concentration profiles are determined from Rutherford backscattering spectroscopy and scanning electron microscopy coupled to energy-dispersive X-ray analysis. Crystalline quality is evidenced by an X-ray diffraction technique. Transmission electron microscopy gives the in-depth microstructure. Fretting coefficient measurements exhibit a lowering for some experimental conditions. The polycrystalline nitride layer obtained is several micrometers thick and composed of a pure AlN (columnar microstructure) top layer (200–500 nm thick) standing on an AlN (grains) in alloy diffusion layer. From the heat conduction equation calculation it is shown that a 308-nm laser wavelength would be better to increase the nitride thickness, as it corresponds to a weaker reflectance R value for aluminum. Received: 17 October 2000 / Accepted: 19 October 2000 / Published online: 23 May 2001     

Optical behaviour of VTE treated near stoichiometric LiNbO3 crystals

Near stoichiometric LiNbO₃ crystal wafers of thickness up to 2 mm were prepared by vapour transport equilibration technique (VTE) at various process temperatures. Crystals were characterised by measurement of the UV absorption edge, refractive index, second harmonic generation (SHG) efficiency, and conoscopy pattern analysis. The comparison of VTE treated crystals show that the blue shift in cut off wavelength occurred with the increasing process temperature (i.e. increasing Li/Nb ratio). The refractive indices were found decreasing with increasing process temperature of VTE samples. The SHG efficiency increases in the range of 1.98-2.3 times for the VTE processed samples with respected to congruent crystals. Conoscopy pattern reveals the optical homogeneity of the VTE treated crystal.     

Effect of Carbon Source with Different Graphitization Degrees on the Synthesis of Diamond

Using three kinds of graphites with different graphitization degrees as carbon source and Fe-Ni alloy powder as catalyst, the synthesis of diamond crystals is performed in a cubic anvil high-pressure and high-temperature apparatus （SPD-6 × 1200）. Diamond crystals with perfect hexoctahedron shape are successfully synthesized at pressure from 5.0 to 5.5GPa and at temperature from 1570 to 1770K. The synthetic conditions, nucleation, morphology, inclusion and granularity of diamond crystals are studied. The temperature and pressure increase with the increase of the graphitization degree of graphite. The quantity of nucleation and granularity ofdiamonds decreases with the increase of graphitization degree of graphite under the same synthesis conditions. Moreover, according to the results of the M6ssbauer spectrum, the composition of inclusions is mainly Fe3 C and Fe-Ni alloy phases in diamond crystals synthesized with three kinds of graphites.     

Characterization of hetero-interfaces between group III nitrides formed by PLD and various substrates

We have grown group III nitride epitaxial films on various substrates by pulsed laser deposition and investigated structural properties of the surfaces and the hetero-interfaces using grazing incidence angle X-ray reflectivity (GIXR) and atomic force microscopy (AFM). It has been found that hetero-interfaces between PLD AlN and conventional substrates such as Al₂O₃ and Si are quite abrupt (about 0.5 nm) probably due to a less reactive growth ambience. However, we observed a thin interfacial layer (less than 10 nm) at the hetero-interface between AlN and (Mn,Zn)Fe₂O₄. The surface roughness of AlN is mainly determined by the extent of the lattice mismatch. It has been also found that the roughness at the hetero-interface between GaN and AlN formed by PLD coincides with the surface roughness of the AlN layer.     

Morphology controlled solvothermal synthesis of Cd(OH)2 and CdO micro/nanocrystals on Cd foil

Cadmium hydroxide (Cd(OH)₂) and cadmium oxide (CdO) nano and micro crystals were synthesized in ethanol-water medium using cadmium foil both as a source and substrate under solvothermal condition. Different concentrations of ammonium hydroxide, hydrazine hydrate, sodium hydroxide and potassium hydroxide were added to study the structural and morphological variations in the products. Synthesis was carried out at different temperatures to study the growth stages of the nano/microstructures. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The as-prepared Cd(OH)₂ products were transformed to CdO by thermal treatment in air. The possible growth mechanism for the formation of different morphologies at different basic medium has been proposed. The optical absorption measurement was carried out to determine the values of the band gap of CdO.     

Fabrication of aluminum nitride heterostructures on Si (1 1 1) substrate by plasma-assisted MBE

The aluminum nitride (AlN) heterostructures were successfully grown on silicon substrate by plasma-assisted molecular beam epitaxy. The surface morphology and structural and optical properties of the sample have been investigated by reflection high electron diffraction, scanning electron microscopy, field emission scanning electron microscopy (FESEM), energy dispersive X-ray, high-resolution X-ray diffraction (HR-XRD), Raman spectroscopy, and photoluminescence, respectively. HR-XRD measurement showed that the sample has a typical diffraction pattern of hexagonal AlN/GaN/AlN heterostructures. The Schottky characteristic of Pt contact on AlN/GaN/AlN heterostructures was also investigated under different annealing temperatures in nitrogen ambient. The temperature dependence and structural evolution of the Schottky barrier height (SBH) of Pt contacts were studied using the current–voltage measurement and FESEM, respectively. Our findings presented that the SBH of the samples change with different annealing temperatures.     

From anti‐Stokes photoluminescence to resonant Raman scattering in GaN single crystals and GaN‐based heterostructures

The progress on anti‐Stokes photoluminescence and Stokes and anti‐Stokes Raman scattering in GaN single crystals and GaN/AlN heterostructures is reviewed. Anti‐Stokes photoluminescence investigated in the past was primarily attributed to two‐photon absorption, three‐photon absorption, and phonon‐assisted absorption. On the other hand, anti‐Stokes Raman scattering was used to determine electron‐phonon scattering time and decay time constant for longitudinal‐optical phonons. In a typical high electron mobility transistor based on GaN/AlN heterostructures, strong resonances were reached for first‐order and second‐order Raman scattering processes. Therefore, both Stokes and anti‐Stokes Raman intensities were dramatically enhanced. The feasibility of laser cooling of a nitride structure has been demonstrated. Anti‐Stokes photoluminescence and Raman scattering have potential applications in upconversion lasers and laser cooling of nitride ultrafast electronic and optoelectronic devices.