The near edge structure of cubic boron nitride

We compare the near edge structure (NES) of cubic boron nitride (cBN) measured using both electron energy loss spectroscopy (EELS) and X-ray absorption spectroscopy (XAS) with that calculated using three commonly used theoretical approaches. The boron and nitrogen K-edges collected using EELS and XAS from cBN powder were found to be nearly identical. These experimental edges were compared to calculations obtained using an all-electron density functional theory code (WIEN2k), a pseudopotential density functional theory code (CASTEP) and a multiple scattering code (FEFF). All three codes were found to reproduce the major features in the NES for both ionisation edges when a core-hole was included in the calculations. A partial core hole (1/2 of a 1s electron) was found to be essential for correctly reproducing features near the edge threshold in the nitrogen K-edge and to correctly obtain the positions of all main peaks. CASTEP and WIEN2k were found to give almost identical results. These codes were also found to produce NES which most closely matched experiment based on χ² calculations used to qualitatively compare theory and experiment. This work demonstrated that a combined experimental and theoretical approach to the study of NES is a powerful way of investigating bonding and electronic structure in boron nitride and related materials.     

Optical spectra and electron structure of cubic boron nitride

On the basis of the known reflection spectrum, we calculate a complete set of fundamental optical functions for cubic boron nitride (c-BN) in the region of 2–23 eV. The integral spectrum of dielectric permeability is decomposed into 16 elementary components. Three main parameters (maximum energy, half-width, and oscillator force) for each of the components are determined. Using the well-known theoretical calculations for bands of boron nitride as the base we suggest a scheme of the nature of these dielectric permeability components. To whom correspondence should be addressed. Udmurtiya State University, 1, Universitetskaya Str., Izhevsk, 426034, Russia:e-mail: sobolev@matsim.udmurtia.su. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 4, pp. 579–583, July–August, 1999.     

Luminescence centers in cubic boron nitride

Complex and multiband photoluminescence spectra for GB and HBN centers in single crystals of cubic boron nitride (cBN) were recorded in the wavelength ranges 385–400 nm and 365–395 nm and the nature of these centers was studied. The use of models involving resonance vibrations and strongly shifted configuration diagrams for the electronic ground state and excited state made it possible to associate formation of the GB-1 center with the presence of tungsten impurity in cBN. It was established that the HBN band in the 300–350 nm range of the cathodoluminescence spectra of cBN polycrystals, single crystals, and micropowders is associated with luminescence centers present in microinclusions of graphite-like boron nitride (hBN). The nature of the hBN band is tentatively interpreted within the model of recombination of donor and acceptor defects in hBN: respectively nitrogen vacancies and carbon atoms in positions substituting for nitrogen. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 2, pp. 241–246, March–April, 2007.     

Luminescence of samarium-activated cubic boron nitride

Under high pressure conditions, we have obtained samples of samarium-activated cubic boron nitride in the form of micropowders, ceramic samples, and polycrystals having high-intensity discrete photoluminescence in the red region of the spectrum that is stable in the temperature range 6 K to 300 K and is assigned to internal f-f electronic transitions in the Sm³⁺ ions. The materials obtained on the basis of cBN are intended for use as phosphors and light emitters (sources of red light) having thermal and chemical stability. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 1, pp. 88–93, January–February, 2008.     

Luminescence of thulium-activated cubic boron nitride

Under high pressure and temperature conditions, we have obtained samples of thulium-activated cubic boron nitride in the form of micropowders, ceramics, and polycrystals activated by thulium in the presence of aluminum. We studied the cathodoluminescence (CL), photoluminescence (PL), and photoluminescence excitation spectra of the samples. In the luminescence spectra we observe structured bands with maxima at ∼370, ∼475, ∼660, and ∼ 800 nm, assigned to electronic transitions in the triply charged thulium ions. We have established that the most efficient method for excitation of “blue” luminescence at ∼475 nm for thulium ions in cBN is excitation by an electron beam. The cBN samples synthesized in the presence of Al have photoluminescence spectra with a more complex structure compared with samples not containing Al, with the band of dominant intensity at about 660 nm. Hypothetically, this is a consequence of incorporation of thulium ions into the crystalline phases cBN and AlN, which are equally likely to be formed during synthesis. The observed photoluminescence spectrum of the indicated samples is the superposition of the photoluminescence spectra of the Tm³⁺ ions located in the crystal fields of cBN and AlN of different symmetries. The presence in the photoluminescence excitation spectra (at 450, 490, and 660 nm) of structure, with features at wavelengths shorter than the excited photoluminescence, suggests a nonresonant mechanism for its excitation. We have established that luminescence of Tm³⁺ ions is less intense than for other rare earth elements incorporated into cubic boron nitride. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 4, pp. 547–555, July–August, 2008.     

Theoretical study of the influence of vacancies on the electronic structure of a hexagonal boron nitride monolayer

The influence of boron and nitrogen vacancies and divacancies on the electronic structure of a hexagonal boron nitride h-BN monolayer is studied. In the presence of vacancies in the structure, the introduced states appear in the forbidden band. The position of an introduced state with respect to the upper occupied level and the lower vacant level depends on deformation. Calculations show that, depending on the defect type and the magnitude of the applied deformation, the introduced state can be both localized and not localized on atoms surrounding the defect. When the state is localized in the system, the inhomogeneous distribution of the spin density is observed, resulting in the appearance of the magnetic moment in the system.     

立方氮化硼薄膜的最新研究进展

立方氮化硼(cBN)作为一种在自然界中并不存在的人造材料具有优异的理化特性. 在超硬刀具、高温电子器件和光学保护膜等领域有着广泛的应用前景,已经成为材料科学的研究热点之一. 但是气相生长高质量cBN薄膜仍然还有许多难点需要攻克. 在综述近几年cBN薄膜研究所取得的一些突破性进展后,结合研究现状提出今后可能的主要研究方向. 关键词： 立方氮化硼薄膜 压缩应力 异质外延 掺杂     

Surface properties of cubic boron nitride thin films

Studying the surface properties of cubic boron nitride (c-BN) thin films is very important to making it clear that its formation mechanism and application. In this paper, c-BN thin films were deposited on Si substrates by radio frequency sputter. The influence of working gas pressure on the formation of cBN thin film was studied. The surface of c-BN films was analyzed by X-ray photoelectron spectroscopy (XPS), and the results showed that the surface of c-BN thin films contained C and O elements besides B and N. Value of N/B of c-BN thin films that contained cubic phase of boron nitride was very close to 1. The calculation based on XPS showed that the thickness of hexagonal boron nitride (h-BN) on the surface of c-BN films is approximately 0.8 nm.     

Melting of cubic boron nitride at extreme pressures

Due to its large pressure range of stability and inert nature, cubic boron nitride has been proposed as a potential pressure standard for high pressure experiments. It is extremely refractive upon compression, although its melting temperature is not known beyond 10 GPa. We apply first-principles molecular dynamics to evaluate the thermodynamics of zincblende structured (cubic) and liquid boron nitride at extreme temperatures and pressures, and compute the melting curve up to 1 TPa by integration of the Clapeyron equation. The resulting equations of state reveal that liquid boron nitride becomes denser than the solid phase at pressures of around 0.5 TPa. This is expressed as a turnover in the melting curve, which reaches a maximum at 510 GPa and 6550 ± 700 K. The origin of this density crossover is explained in terms of the underlying liquid structure, which diverges from that of the zincblende structured solid as the phases are compressed.     

Raman scattering of light in doped cubic boron nitride

The Raman spectra of single crystals of cubic boron nitride (cBN) doped with beryllium and also synthesized in the absence of dopants are investigated. It is shown that the wide bands at 215 and 535 cm^–1 can be registered in the Raman spectra of the ( ) faces of undoped cBN crystals and that the Be impurity penetrates into the cBN crystals predominantly in the direction of these faces. In the spectra of the investigated cBN samples doped with Be, the contour of the TO and LO phonon lines does not take the Fano shape, in contrast to diamond, which can point to an insufficient concentration of the dopant for attaining the Fano interference or to the absence of it in cBN. The wide bands have been investigated on the assumption that they are boson-like and result from small-size disorders in both undoped and doped cBN samples with the sp ³ bond deformed by Be.Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 71, No. 6, pp. 803–809, November–December, 2004.This revised version was published online in April 2005 with a corrected cover date.     

Ultraviolet and infrared spectra of cubic boron nitride

Ultraviolet absorption spectra of cubic boron nitride crystals indicate a minimum value for the band gap of 6.4 ± 0.5 eV. This value is in good agreement with a previously reported maximum value of 6.0 ± 0.5 eV obtained experimentally from soft X-ray spectra. Infrared absorption spectra differ significantly from previously published data.     

第一性原理计算研究立方氮化硼空位的电学和光学特性

对立方氮化硼的空位进行了基于密度泛函理论框架下的第一性原理平面波超软赝势方法的研究. 通过对总能量、能带结构、态密度及电子密度分布图的分析发现, B空位相比起N空位更加稳定. 并且空位仅影响最近邻原子的电子分布, 空位浓度的增加使禁带宽度逐渐变窄. 从复介电函数和光学吸收谱分析中发现, 随着空位浓度的增加, 立方氮化硼在深紫外区的吸收逐渐减弱. 并且B空位还导致在可见光区域出现明显的吸收带. 关键词： 立方氮化硼 空位 第一性原理 电光学特性     

Superhard superconducting materials based on diamond and cubic boron nitride

Superhard superconducting samples with a critical temperature of T_c = 12.6 K are obtained when synthetic diamond powders that were preliminarily coated with a niobium film are sintered at a pressure of 7.7 GPa and a temperature of 1973 K. Superhard superconductors with T_c = 9.3 K are obtained when diamond and molybdenum powders are sintered at a pressure of 7.7 GPa and a temperature of 2173 K. Superconducting samples with T_c = 36.1–37.5 K have been obtained in the systems diamond-MgB₂ and cubic boron nitride-MgB₂.     

Structure and luminescence of gadolinium-doped cubic boron nitride powder

The structural characteristics and chemical, morphological, and optical properties of cBN and cBN:Gd micropowders are studied by x-ray diffraction, energy-dispersive electron probe microanalysis (x-ray spectral microanalysis), and photoluminescence techniques. Cubic boron nitride (cBN) micropowders were synthesized at high pressures and temperatures from hexagonal boron nitride (hBN) micropowder and Li₃N catalyst. cBN:Gd micropowders were synthesized from mixtures of hBN, Li₃N, and GdF₃ micropowders. A lattice parameter of a~3.615 ? is calculated for both types of powder (cBN and cBN:Gd). The photoluminescence spectra of the cBN:Gd powder are found to contain emission lines attributable to intracenter optical transitions of Gd³⁺ ions.     

Luminescence investigations of cubic boron nitride doped with beryllium

The cathodoluminescence and photoluminescence spectra of cubic boron nitride doped with beryllium under high-pressure and high-temperature conditions are investigated. It is revealed that, upon doping of cubic boron nitride with beryllium, the cathodoluminescence spectra exhibit a broad stable band. An increase in the impurity content leads to a shift in the position of the maximum of this band toward the short-wavelength range from ～315 to ～250 nm and to a change in the crystal color from dark yellow to blue. The structure, the intensity, and the position of the band at the maximum are studied as a function of the temperature of the cathodoluminescence measurement. The nature of the band is tentatively interpreted in the framework of the model of recombination at defects of the donor and acceptor types. It is assumed that several overlapping subbands which are associated with differently charged acceptor levels of beryllium are located in the vicinity of the valence band in the electronic structure of the doped cubic boron nitride. It is found that the photoluminescence spectra of single crystals of the doped cubic boron nitride contain three previously unknown zero-phonon lines at energies of 2.135, 2.270, and 2.600 eV.