We are studying the boron nitride system by using a pulsed excimer laser to ablate from hexagonal BN (hBN) targets to form cubic BN (cBN) films. We are depositing BN films on heated (600°C) silicon (100) surfaces in a flowing (0- 10 sccm) ambient background gas of either NH3 or N2 of varying partial pressure (0–100 m Torr). Infrared (IR) reflection spectroscopy indicates the films have short-range hexagonal order. Some films grown at low laser energy densities have shown the cubic phase in IR transmission. Auger electron spectroscopy (AES) indicates the films are nitrogen deficient, which is linked to changes in the target stoichiometry with increasing laser fluence. Raman spectroscopy on the films shows only a strong background luminescence suggesting a high concentration of defects associated with the nitrogen vacancies. Atomic force microscopy (AFM) of the films shows a surface morphology that roughens as the growth rate increases. In order to improve the film stoichiometry it was necessary to actively enhance the nitrogen content of the films. It was found that bombarding films during growth with ions from an ion gun filled with NH3 gas increased the N/B ratio but did not enhance the cubic phase. RF biasing the substrate gave films which showed both cubic and hexagonal features in IR reflection. High resolution transmission electron microscopy (TEM) confirms the presence of cBN grains of ∼ 200Å size in films grown with an RF bias.

Tetrahedrally coordinated phases of boron nitride (c-BN and w-BN) were produced by reactive sputtering. The structure of the films was investigated by XRD and TEM diffraction, and found to be poly cry stalline. Films with microhardness up to 3500kg/mm2 were deposited but some degradation over time has been observed. A model for the stabilization of the tetrahedral phases over the graphitic one is proposed.

Defects in BN thin films, produced by reactive sputtering, were investigated by Electron Paramagnetic Resonance (EPR) measurements. The EPR signals of films produced with up to 10% N2 in the argon discharge are consistent with films having a cubic structure, and becoming more ordered with nitrogen incorporation in the films. The concentration of spins is in the order of 1019 spins/g and they are attributed to nitrogen vacancies with an electron trapped in. Carbon incorporation changes the EPR signal and increases the concentration of spins significantly. This result is consistent with the notion that carbon doping stabilizes the electron in a nitrogen vacancy.

Electron spin resonance (ESR) spectrum have been used to study undoped cubic boron nitride crystals. The results show that the peak-to-peak width of the ESR signal is 27×10-4 T and the g-value is 2.00248, the spin density N s is 2.8×1015 cm-3, it is considered that the ESR signal is originated from the nitrogen dangling bond. Raman spectrum measurements have been also carried out, the peak positions are at 1054 cm-1 and 1306 cm-1.

The effect of substrate bias on the properties of rf sputtered boron nitride films on Si and GaAs substrate were investigated. IR transmission and reflectivity of films with different substrate bias were measured with Perkin Elmer 983 IR spectroscopy. From the IR reflectivity data, transverse optical mode(TO) and longitudinal optical mode(LO) frequencies were derived by fitting Kramer-Kronig model. Absorption coefficient was determined from IR transmission data. The resultant TO and LO modes showed that substrate bias caused broadening of reststrahlen band of rf sputtered boron nitride. We also tried to dope boron nitride films with silicon by alternate sputtering of BN and Si targets controlling sputtering time of each target followed by annealing. Electrical resistivity was measured over the temperature range between 175 K to 370 K for both intrinsic and Si-doped boron nitride films. Intrinsic rf sputtered boron nitride showed Little change in resistivity (109 Ω cm - 1011 Ω cm ) over the temperature range studied. While Si doped BN showed linear change in resistivity with increasing temperature and its activation energy was about 0.22 eV. The effect of substrate bias was also investigated by monitoring the XPS core level spectra of both Bis and N Is peaks, respectively. Substrate bias caused the shift of both B ls and N ls peak to higher binding energy. The effect of substrate bias on refractive index was also studied.

The submillimeter optical properties of hot-pressed boron nitride with a hexagonal crystal structure were studied at room temperature from approximately 20 cm-1 to 120 cm-1 (500μm -84 μm) with a Fourier Transform Spectrometer. Several grades were studied and probed both parallel and perpendicular to the material's optic axis. The material was found to behave as a negatively uniaxial birefringent crystal. In one case, the birefringence (Δ = ne - no) was quite large with a value of-0.152. The material's absorption properties were also studied. For certain grades, a modest dichroism was observed. The low absorption (α < 1 cm-1) for grade A at frequencies below 38 cm-1 suggests the possibility for millimeter/submillimeter wave applications. Results are compared with data by other researchers on related materials.

This paper describes the thermoelectric properties of amorphous or polycrystalline boron and boron phosphide films prepared using chemical vapor deposition and molecular beam deposition. The temperature dependencies of electrical conductivity of a-B and distorted B13P2 films obey the Mott's rule of loga vs T-1/4, while those of polycrystalline BP films have linear relationship between loga vs T-1. The a-B and B13P2 films have high electric resistivity and show p-type conductors while BP film shows n-type conductor. The estimated thermoelectric figure of merit of boron phosphide film is compatible to that of sintered specimen, but there remains some problems for a-B to alter.

Boron carbide films have been fabricated from pentaborane (B5H9) and methane using plasma enhanced chemical vapor deposition (PECVD). Availability of high quality specimens has facilitated analysis of electronic and optical properties. We have undertaken measurements of photoluminescence spectra and infra-red band-edge absorption. Results show that the bandgap is strongly influenced by the ratio of boron to carbon. There is evidence for the existence of shallow trapping levels.

The formation of submicron crystals of boron carbide (B4C) by rapid carbothermal reduction of intimately mixed carbon-boron oxide precursor powders in an aerosol flow reactor at temperatures above the boiling point of boron oxide is investigated. The employed high heating rates (105 K/s) of the process force release of gaseous boron oxide and suboxides and rupture of the precursor particles resulting in formation of boron carbide molecular clusters that grow to macroscopic particles by coagulation. Consequently, the formation and growth of B4C particles is described by simultaneous interparticle collision and coalescence using a two-dimensional distribution model that traces the evolution of both size and shape characteristics of the particles through their volume and surface area. Here, in addition to the coagulation term, the governing population balance equation includes a coalescence contribution based on B4C sintering law. The predicted evolution of the two-dimensional particle size distribution leads to a direct characterization of morphology as well as the average size and polydispersity of the powders. Furthermore, model predictions of the volume and surface area of boron carbide particles can be directly compared with experimental data of B4C specific surface area and grain size.

A review is given on recent progress in amorphous and microcrystalline silicon-carbide (a-SiC, nc-SiC) semiconductors and their technological applications to optoelectronic functional devices. Firstly, some significant properties in this alloy as a new synthetic material are pointed out with recent advances of thin film technologies, such as plasma CVD, ECR-CVD and ion-beam CVD etc. There exists an energy gap controllability from 1.7eV to 3.6 eV with retaining the valency electron control from n-type through i- to p-type semiconductors. While its conductivity can also be controlled more than ten order of magnitudes, e.g., from 10-9to 102 Scm-1 by controlling the impurity doping and preparation conditions.

Secondly, a series of technical data on the electronic and optoelectronic properties of a-Si, C1−x C1−x and μ-SiC are demonstrated from recent achievements. In the final part of the paper, current state of the art in the field of optoelectronic applications from live technologies on amorphous silicon solar cells. a-SiC visible light LED and EL devices are reviewed. A technological evolution from “microelectronics” to “macroelectronlcs” will be discussed.

A total area conversion efficiency of 11.1% has been achieved for a 1Ocm×1Ocm integrated-type single-junction a-Si solar cell submodule using a high-quality wide-bandgap p-layer doped with B(CH3)3 and other advanced techniques. This is the highest conversion efficiency ever reported for an a-Si solar cell with an area of 100cm2. As for a multi-junction solar cell, 12.1% was obtained for a 1cm2 cell with a high-quality wide-bandgap a-Si i-layer. The layer was fabricated by a hydrogen dilution method at a low substrate temperature for a front active layer of an a-Si/a-Si/a-SiGe stacked solar cell.

For further improvement in conversion efficiency, a wider-bandgap a-SiC was developed using a novel plasma CVD method, called the CPM (Controlled Plasma Magnetron) method. From XPS and IR measurements, the resultant films were found to have high Si-C bond density and low Si-H bond density, p-type a-SiC was fabricated using the post-doping technique, and dark conductivity more than 10-5(Q. cm)-1 was obtained (Eopt3 ≥ 2eV; Eopt2 2.2eV), whereas that of conventional p-type a-SiC is less than 10-6(Ω·cm)-1. These properties are very promising for application to the p-layers of advanced a-Si solar cells.

Amorphous and microcrystalline silicon carbide, undoped and doped, are promising materials as wide band gap semiconductors (Eg > 2 eV). In the present work results on nydrogenated and fluorinated a-SiC and uc-SiC films intrinsic, B or P doped are reported. Energy gap higher than 2 eV are obtained together with electrical dark conductivities in the range 10-12 -10-2 Ω-1cm-1

Polysilane thin films have been grown by the rf glow discharge decomposition of SiH4 at substrate temperatures ranging from -84 to -110°C. The infrared absorption spectra have shown that polysilane chains (SiH2)n are predominantly incorporated in the matrix together with SiH3 which terminates the chain. Also, the infrared absorption band at 2120∼2140 cm-1 and a distinct Raman peak at ∼430 cm-1indicates that fairly long chains (SiH2)n with n>11 are produced. Polysilane prepared at -110°C has an optical bandgap of about 3.1 eV and exhibits a visible luminescence around 2.75 eV at 100 K.

The creation mechanisms of Si and N dangling bond defect centers in amorphous hydrogenated silicon nitride thin films by ultra-violet (UV) illumination are investigated. The creation efficiency and density of Si centers in the N-rich films are independent of illumination temperature, strongly suggesting that the creation mechanism of the spins is electronic in nature, i.e., a charge transfer mechanism. However, our results suggest that the creation of the Si dangling bond in the Si-rich films are different. Last, we find that the creation of the N dangling-bond in N-rich films can be fit to a stretched exponential time dependence, which is characteristic of dispersive charge transport.

The microstructure, electrical, and optical properties of in situ phosphorus doped hydrogenated macrocrystalline silicon (μc-Si:H:P) films are strongly affected by deposition parameters and subsequent thermal processes. μc-Si:H:P films with thickness ranging Trom 200 to lOOOÅ have been deposited on Si and SiO2 substrates. The μc-Si:H:P film is best deposited at 200°C in terms of structure, and H and P content. In this film, microcrystallites arc embedded in an amorphous matrix and have a volume fraction of ≃85%. The band-gap of the film is 1.8 eV. In this paper, the materials and processing issues of μc-Si:H:P deposited in a parallel plate RF plasma and in a downstream RF plasma chemical deposition reactor have been studied using HRTEM, SIMS, electrical, and optical measurements.

Hydrogenated microcrystalline silicon with oxygen(mc-Si:O:H) is grown using radio frequency glow discharge method. Oxygen is introduced during growth by varying it's partial pressure in the growth chamber. The crystalline volume fraction ‘f’ and the crystallite size ‘δ’ are found to vary with the oxygen content. Results indicate that oxygen can etch the silicon surface when present in low amount while it forms a-SiO2-x with increasing contents. Optical absorption studies in the range of 2 to 3 eV suggest that the absorption coefficient ‘α’ lies in between the values of c-Si and a-Si:H.being closer to a-Si:H. The Hall mobility measurements for these samples indicate that for optimum oxygen contents the mobility as high as 35 cm2 V-1 sec-1 can be obtained. Results on I-V characteristics for p-i-n structure are presented.

A variety of applications are identified for heteroepitaxial structures of wide gap I-III-VI2 and II-IV-V2 semiconductors, and are assessed in comparison with ternary III-V alloys and other wide gap materials. Non-linear optical applications of the I-III-VI2 and II-IV-V2 compound heterostructures are discussed, which require the growth of thick epitaxial layers imposing stringent requirements on the conditions of heteroepitaxy. In particular, recent results concerning the MOCVD growth of ZnSixGe1-xP2 alloys lattice-matching Si or GaP substrates are reviewed. Also, heterostructures of CuzAg1-zGaS2 alloys that lattice-match Si, Ge, GaP or GaAs substrates are considered in the context of optoelectronic devices operating in the blue wavelength regime. Since under the conditions of MOCVD, metastable alloys of the II-IV-V2 compounds and group IV elements are realized, II-IV-V2 alloys may also serve as interlayers in the integration of silicon and germanium with exactly lattice-matched tetrahedrally coordinated compound semiconductors, e.g. ZnSixGe1-xP2.

A new method for the direct observation of two-dimensional gas flow patterns in a CVD reactor has been developed by combining a laser scanning technique with generating micron-sized TiO2 particles. With this specially developed technology, the size of generated TiO2 particles are quite uniform, and of high density by the use of hydrolysis of Ti-alkoxide in the ceramic honeycomb at the top inlet of the model chamber. In this system, vertical cross sections of the gas flow patterns can be visualized by illuminated TiO2 particles in a He-Ne laser light sheet. Using this technique, detailed gas flow patterns can be clearly identified in the reaction chamber. Changes in the gas flow patterns with the various growth conditions, such as gas flow rate and pressure, have been measured. In this presentation, GaAs thin film growth by the MOCVD method will be reported as an example.

This gas flow visualization method could be a useful tool to identify the mechanism of CVD reactions to give better understanding about carrier gas transport and thin film growth for wide band gap semiconductors such as GaN, a-SiC, SiNx, etc.

Vanadium pentoxide (vanadia) is a wide band gap semiconductor. Its layered orthorhombic structure consists of alternating sublayers of V+O atoms and O atoms (vanadyl O) alone aligned perpendicular to the b-axis. This unique structure makes vanadia a useful host for alkali atom intercalation for electrochromic applications, and therefore, an understanding of its optical properties is important. Here, we study the optical absorption characteristics of vanadia in the incident photon energy range E=2.5–6.0 eV (λ=490–200 nm). The material is in the form of 0.1μm thick films sputter deposited in Ne/O2 discharges. Two types of films were studied: single-oriented films with the b-axis perpendicular to the substrate, and amorphous films with an oxygen deficiency. The optical absorption coefficient, α(E), was determined and interpreted in terms of the structure of the V 3d conduction band. Amorphous, O-deficient vanadia were examined for room temperature aging and were found to oxidize and increase in transmittance in the photon energy range studied.

Reactive sputter deposition is a widely-used process for growing films of high melting point materials near room temperature and desirable metastable structures not attainable in material grown under conditions of thermodynamic equilibrium. Both categories include wide band-gap metal oxides. A first step towards reproducible growth is to develop a “phase map” for the metal-oxygen system of interest. The map graphically relates independent sputter deposition process parameters, the growth environment, and the metallurgical phase(s) formed in the film. This paper shows how phase maps are constructed and used to observe general trends in oxide phase formation sequence, with examples from the Nb-O, Y-O, and Zr-O systems.