Growth and annealing study of hydrogen-doped single diamond crystals under high pressure and high temperature

A series of diamond crystals doped with hydrogen is successfully synthesized using LiH as the hydrogen source in a catalyst-carbon system at a pressure of 6.0 GPa and temperature ranging from 1255 C to 1350 C.It is shown that the high temperature plays a key role in the incorporation of hydrogen atoms during diamond crystallization.Fourier transform infrared micro-spectroscopy reveals that most of the hydrogen atoms in the synthesized diamond are incorporated into the crystal structure as sp 3-CH 2-symmetric(2850 cm-1) and sp 3 CH 2-antisymmetric vibrations(2920 cm-1).The intensities of these peaks increase gradually with an increase in the content of the hydrogen source in the catalyst.The incorporation of hydrogen impurity leads to a significant shift towards higher frequencies of the Raman peak from 1332.06 cm-1 to 1333.05 cm-1 and gives rise to some compressive stress in the diamond crystal lattice.Furthermore,hydrogen to carbon bonds are evident in the annealed diamond,indicating that the bonds that remain throughout the annealing process and the vibration frequencies centred at 2850 and 2920 cm-1 have no observable shift.Therefore,we suggest that the sp 3 C-H bond is rather stable in diamond crystals.     

An optical emission spectroscopy study of the plasma generated in the DC HF CVD nucleation of diamond

Optical emission spectroscopy (OES) was used to study the plasma generated by the activation of the gas phase CH₄ + H₂ both by hot filaments and by a plasma discharge (DC HF CVD) during the nucleation of CVD diamond. The effects of nucleation parameters, such as methane concentration and extraction potential, on the plasma chemistry near the surface were investigated. The density of the diamond nucleation and the quality of the diamond films were studied by scanning electron microscopy (SEM) and Raman scattering, respectively. The OES results showed that the methane concentration influenced strongly the intensity ratio of H_β-H_α implying an increase of electron mean energy, as well as CH, CH⁺, C₂. A correlation between the relative increase of CH⁺ and the diamond nucleation density was found, conversely the increase of C₂ contributed to the introduction of defects in the diamond nuclei.     

Magnetic properties of CuCr2Se4 single crystals

The magnetic properties of CuCr₂Se₄ single crystals, which were grown by chemical transport reactions using iodine as a carrier, have been investigated. The magnetic moment at 0 K is found to be 5.07 μ_B per mole. The susceptibility at high temperatures follows a Curie-Weiss law with an asymptotic Curie temperature 430 K and a Curie constant 2.55 emu · $\text{deg}\text{mol}$. The magnetocrystalline anisotropy constants K₁ and K₂ are ? 6.9x 10⁵ and ? 0.9x 10⁵$\text{erg}\text{cm}{}^3$, respectively, at 5.1 K.In order to examine the effects of annealing on the magnetocrystalline anisotropy, the ferromagnetic resonance at room temperature was measured after annealing in vacuum and subsequently in an atmosphere of Se. It is found that the absolute values of K₁ and K₂ decrease after annealing in vacuum and increase to the initial values after annealing in an atmosphere of Se.     

微波化学气相沉积中气压对金刚石薄膜生长速率和质量的影响

研究了微波化学气相沉积中沉积气压对金刚石薄膜生长速率和质量的影响.研究表明，金刚石薄膜的生长速率随沉积气压的提高而增大，生长速率与沉积气压为线性关系.在高沉积气压下生长的金刚石薄膜晶形完整，拉曼谱测量可得到锐利的金刚石相的峰，但电压-电流测量表明，随着制备时沉积气压的提高，金刚石薄膜的暗电流增大，膜的电学质量下降. 关键词： 金刚石薄膜 生长速率 沉积气压     

Influence of photoexcitation on the EPR spectra of Mo5+ in Li2Zn2(MoO4)3: Ce3+,Cu2+ crystals annealed in a CO2 atmosphere

The influence of recovery annealing in a CO₂ atmosphere at 700°C on the properties of Li₂Zn₂(MoO₄)₃ crystals doped with cerium and copper ions has been studied. The EPR investigation of Li₂Zn₂(MoO₄)₃ crystals annealed in a CO₂ atmosphere has revealed that the annealing leads to the formation of oxygen vacancies in positions adjacent to the oxygen octahedron of lithium, M3, and the oxygen tetrahedron of molybdenum, Mo1. In this case, the charge state of molybdenum becomes Mo⁵⁺ and appears in the EPR spectra in the form of one magnetically nonequivalent position. The analysis of the angular dependence of the EPR spectrum of Mo⁵⁺ made it possible to calculate the spectral parameters g _∥ = 1.862, g _⊥ = 1.933, A _∥ = 71.8 G, and A _⊥ = 34.1 G. The cross relaxation on the hyperfine structure from the molybdenum isotope ⁹⁷Mo is found in the EPR spectra. The photoexcitation of Li₂Zn₂(MoO₄)₃ crystal doped with cerium ions leads to the saturation of the EPR spectrum of Mo⁵⁺ and to the formation of the hyperfine structure from one lithium ion with a hyperfine structure constant of 14 G. For Li₂Zn₂(MoO₄)₃ crystals doped with copper ions, a very weak EPR spectrum of Mo⁵⁺ is observed in the initial crystals. As a result of the photoexcitation, an increase in the intensity of this spectrum by an order of magnitude and manifestation of the EPR spectrum of Cu²⁺ ions take place. It is assumed that such a behavior of the EPR spectra of molybdenum ions in Li₂Zn₂(MoO₄)₃ crystals doped with cerium and copper ions under photoexcitation is caused by different positions of the energy levels of cerium and copper ions with respect to the energy level of the molybdenum ion.     

The Formation of Dimeric Centers of Copper,Iron and Gadolinium in Modified Pb<Subscript>5</Subscript>Ge<Subscript>3</Subscript>O<Subscript>11</Subscript>

It is discovered that the electron paramagnetic resonance (EPR) spectrum of the doubly charged copper centers occurs in single crystals of Pb₅Ge₃O₁₁ doped with gadolinium or iron after annealing in an atmosphere containing chlorine and bromine. Similar annealing of the crystals doped with copper in a chlorine and fluorine atmosphere leads to redistribution of the intensities of the EPR spectra of two types of Cu²⁺ centers. The influence of annealing on the ongoing intensity of spectra of the dimeric triclinic centers Fe³⁺–A, Gd³⁺–A (where A represent Cl^?, Br^?, O^2?, F^?) was the subject of this research. Consideration is given to the mechanisms for changing the charge state and association of copper center with defects.     

Optical properties of nanodiamond layers

Thin ultradisperse diamond (UDD) layers deposited from a water suspension are studied by optical and x-ray photoelectron spectroscopy (XPS). The effective band gap determined by the 10⁴-cm^?1 criterion for ozone-cleaned UDD is 3.5 eV. The broad structureless photoluminescence band (380–520 nm) is associated with radiative recombination through a system of continuously distributed energy levels in the band gap of diamond nanoclusters. The optical absorption of the material at 250–1000 nm originates from absorption on the disordered nanocluster surface containing threefold-coordinated carbon. The surface of UDD clusters subjected to acid cleaning contains nitrogen-oxygen complexes adsorbed in the form of NO ₃ ^? nitrate ions. Annealing in a hydrogen atmosphere results in desorption of the nitrate ions from the cluster surface. The evolution of the oxygen (O1s) and nitrogen (N1s) lines in the XPS spectra under annealing of a UDD layer is studied comprehensively.     

Charge conversion of uranium ions in CaF2

Single crystals of CaF₂ doped with uranium ions exhibiting a light red colour have been grown in an inert atmosphere. ESR spectra belonging to U³⁺ centers unstable at room temperature could be observed only after UV-irradiation at liquid nitrogen temperature. The formation and annealing of these centers as well as the optical absorption date indicate that a new charge conversion mechanism is effective in such crystals.     

Raman spectroscopy of isotopically pure (<Superscript>12</Superscript>C, <Superscript>13</Superscript>C) and isotopically mixed (<Superscript>12.5</Superscript>C) diamond single crystals at ultrahigh pressures

The Raman scattering by isotopically pure ¹²C and ¹³C diamond single crystals and by isotopically mixed ^12.5C diamond single crystals is studied at a high accuracy. The studies are performed over a wide pressure range up to 73 GPa using helium as a hydrostatic pressure-transferring medium. It is found that the quantum effects, which determine the difference between the ratio of the Raman scattering frequencies in the ¹²C and ¹³C diamonds and the classical ratio (1.0408), increase to 30 GPa and then decrease. Thus, inversion in the sign of the quantum contribution to the physical properties of diamond during compression is detected. Our data suggest that the maximum possible difference between the bulk moduli of the ¹²C and ¹³C diamonds is 0.15%. The investigation of the isotopically mixed ^12.5C diamond shows that the effective mass, which determines the Raman frequency, decreases during compression from 12.38 au at normal pressure to 12.33 au at 73 GPa.     

Concentration quenching in Yb:YAG

The concentration quenching in Yb:YAG crystals with high Yb³⁺ doping level was demonstrated, and studied as well. The color center and lattice distortion which originated from Yb²⁺ are the main reasons for producing the concentration quenching in unannealed Yb:YAG crystals, and after annealing at 1600°C in oxygen atmosphere for 24 h, Yb²⁺ vanished. Trace impurity ions are also responsible for this phenomenon.     

40Ar+诱导无定形碳到金刚石纳米晶相变的研究

通过60 keV的⁴⁰Ar⁺辐照无定形碳靶合成了大量尺寸不同的金刚石纳米颗粒.高分辨透射电子显微镜配合能量色散X射线谱和电子衍射以及Raman谱分析的结果表明，这些嵌于具有扰动石墨结构薄膜中的纳米金刚石颗粒，其成核率很高(约为10¹³/cm²)，而且可以生长到较大的尺寸，有的甚至可以达到微米量级.对其相转变过程也进行了初步探讨. 关键词： 离子束 金刚石纳米晶 相变 透射电子显微镜     

Influence of annealing treatment on as-grown Ib-type diamond crystal at a high temperature and high pressure

In this paper,we report on the influence of annealing treatment on as-grown Ib-type diamond crystal under high pressure and high temperature in a china-type cubic anvil high-pressure apparatus.Experiments are carried out at a pressure of 7.0 GPa and temperatures ranging from 1700 C to 1900 C for 1 h.Annealing treatment of the diamond crystal shows that the aggregation rate constant of nitrogen atoms in the as-grown Ib-type diamond crystal strongly depends on diamond morphology and annealing temperature.The aggregation rate constant of nitrogen remarkably increases with the increase of annealing temperature and its value in octahedral diamond is much higher than that in cubic diamond annealed at the same temperature.The colour of octahedral diamond crystal is obviously reduced from yellow to nearly colorless after annealing treatment for 1 h at 1900 C,which is induced by nitrogen aggregation in a diamond lattice.The extent of nitrogen aggregation in an annealed diamond could approach approximately 98% indicated from the infrared absorption spectra.The micro-Raman spectrum reveals that the annealing treatment can improve the crystalline quality of Ib-type diamond characterized by a half width at full maximum at first order Raman peak,and therefore the annealed diamond crystals exhibit nearly the same properties as the natural IaA-type diamond stones of high quality in the Raman measurements.     

Rearrangement of the Ultrasmooth Surface of La3Ga5SiO14 Crystals at Heating

The morphology and phase composition of the surface of La₃Ga₅SiO₁₄ (langasite) crystals at annealing in a temperature range 1000–1200°C have been studied using electron and atomic force microscopy. It has been shown that trigonal lanthanum oxide (La₂O₃) crystals with sizes to 3–4 μm, as well as a microstructure with sizes to 50 μm with gallium excess, with the approximate composition of 15 mol % La₂O₃, 65 mol % Ga₂O₃, and 20 mol % SiO₂ are formed on the surface of langasite crystals annealed in air at temperatures above 1100°C. Possible reasons for thermal destruction of the compound can be a significant rearrangement of the disordered crystal structure of langasite caused by the interaction with air oxygen and under the intense surface diffusion of atoms of the crystal, as well as the incongruent character of melting of the La₃Ga₅SiO₁₄ compound. The revealed thermal destruction of the surface of langasite crystals should be taken into account when using this material to fabricate piezoelectric elements for operation at high temperatures.

     

Electrical properties of LiNbO3 crystals reduced in a hydrogen atmosphere

The temperature dependence of the electrical conductivity and pyroelectric coefficient of lithium niobate crystals reduced in a hydrogen atmosphere has been studied. It has been established that the activation energy of dark electrical conduction in these crystals in the temperature range 288?C350 K differs from the corresponding values for crystals reduced in vacuum and is equal to 0.68 ± 0.02 eV. It has been shown that the annealing of LiNbO₃ crystals in a hydrogen atmosphere hardly affects their pyroelectric properties. The mechanism of electrical conduction of LiNbO₃ crystals reduced in a hydrogen-containing atmosphere has been discussed.     

Thermal annealing behavior of luminescence of Gd3+-doped PbWO4 single crystal in air atmosphere

The results of photoluminescence measurements on as-grown PbWO₄:Gd³⁺ show that Gd³⁺ acts as a sensitizer of luminescence in the PbWO₄ lattice. Effects of sequential annealing in air atmosphere on luminescence of PbWO₄:Gd³⁺ were investigated by means of X-ray excited luminescence and photoluminescence. The annealing experiments indicate that the essence of the annealing in air atmosphere is the exchange of oxygen components between PbWO₄:Gd³⁺ crystal and the environment to improve the defects in the lattice. Annealing at a temperature of about 640 °C could further enhance the luminescence of PbWO₄ and depress it at a higher temperature. However, PbWO₄:Y³⁺ and PbWO₄:La³⁺ crystals do not show any luminescence enhancement in the same annealing process. A tentative mechanism of luminescence enhancement due to annealing is suggested. PACS 74.62.Dh; 81.40.Tv; 78.55.-m; 82.80.Pv; 78.70.En     

EPR study of charge compensation of chromium centers in the strontium titanate crystal

Chromium centers and their charge compensation in two single strontium titanate crystals, i.e., SrTiO₃: Cr (0.05 at %) and Sr_0.9995TiO₃: Cr_0.0005 grown with strontium deficiency, have been studied by the electron paramagnetic resonance method. The crystals have been investigated both immediately after growth and after oxidation and reduction procedures. Oxidation and reduction are performed by crystal annealing in a corresponding gas atmosphere at high temperature. Chromium centers associated with oxygen vacancy (Cr³⁺-V _O) are detected in the reduced crystals. It is shown that strontium vacancies are formed in the crystal grown with strontium deficiency, which leads to a lowering of the tetragonal symmetry of (Cr³⁺-V _O) and Cr⁵⁺ centers to the orthorhombic symmetry. Possible compensation mechanisms for charges of various chromium centers are considered.     

Annealing effects on electrical and optical properties of ZnO thin films synthesized by the electrochemical method

Microstructural and electrical properties of potentiostatically electrodeposited ZnO thin films from an aqueous bath were investigated after annealing at different temperatures in Ar and 5% H₂/Ar atmospheres. It is confirmed that the bandgap energy of ZnO thin films decreased with annealing from 3.42 to 3.27-3.29 eV by calculating the wavelength of the absorption region. The annealing at temperatures as low as 200 °C decreased the sheet resistance of ZnO thin films because of the extinction of Zn(OH)₂ in the atmosphere. In addition, the sheet resistance of ZnO thin films decreased by annealing in a 5% H₂ atmosphere, which caused an increase of carrier concentration by hydrogen reduction.     

Influence of annealing on the concentration profiles of boron implantations in silicon

The influence of annealing on the concentration profiles of boron implanted into silicon with does of 10¹⁴ ions/cm² up to 10¹⁶ ions/cm² and an energy of 70 keV was studied. The concentration profiles were measured with Secondary Ion Mass Spectrometry (SIMS). The broadening of the concentration profiles during annealing can be described as a superposition of effects resulting from a relatively immobile and a mobile boron fraction. The properties of the immobile boron fraction were studied by measuring the influence of a boron implantation on the distribution of a homogeneous boron background dope. From these experiments it was concluded that the immobile boron fraction consists of boron precipitates. The properties of the mobile fraction were studied from concentration profiles that were obtained after annealing during different periods at the same temperature. It was found that during the initial stage of the annealing process a fast broadening of the profile occurs; this was assumed to be due to an interstitial type boron diffusion. After prolonged annealing the much slower substitutional type diffusion prevails, due to trapping of the interstitial boron atoms by vacancies. The reliability of the SIMS method, as applied to profile measurements, was checked for the high boron doses used in this investigation. Excessive boron precipitates, obtained after annealing of a high dose, such as 10¹⁶ ions/cm² at about 1000°C, appear to give some increase of the ion yield.     

籽晶尺寸对宝石级金刚石单晶生长的影响

本文在国产六面顶压机上,在5.6 GPa, 1250—1450℃的高压高温条件下,分别选用边长0.8, 1.5和2.2 mm三种尺寸的籽晶,系统开展了Ib型宝石级金刚石单晶的生长研究.文中系统考察了籽晶尺寸对宝石级金刚石单晶生长的影响.首先,考察了籽晶尺寸变化对宝石级金刚石单晶裂晶问题带来的影响.研究得到了籽晶尺寸变大,裂晶出现概率增加的晶体生长规律.其次,在25 h的生长时间内,考察了上述三种尺寸籽晶生长金刚石单晶时,生长时间与单晶极限生长速度的关系.得到了选用大尺寸籽晶,可以提高优质单晶合成效率、降低合成成本的研究结论.借助扫描电子显微镜和光学显微镜,对三种尺寸籽晶生长金刚石单晶的表面形貌进行了标定.最后,傅里叶微区红外测试,对三种尺寸籽晶生长宝石级金刚石单晶的N杂质含量进行了表征.研究得到了选用大尺寸籽晶实现快速生长金刚石的同时,晶体的N杂质含量会随之升高的晶体生长规律.     

Cooperative down-conversion of UV light in disordered scheelitelike Yb-doped NaGd(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript> and NaLa(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals

Concentration series of disordered scheelitelike Yb:NaGd(MoO₄)₂ and Yb:NaLa(MoO₄)₂ single crystals are grown by the Czochralski method. The actual concentrations of Yb³⁺ ions in the crystals are determined by optical-absorption spectroscopy. The luminescence of Yb³⁺ ions in these crystals in the region of 1 μm is studied under UV and IR excitation. In the case of UV excitation, this luminescence appears as a result of nonradiative excited state energy transfer from donor centers of unknown nature to ytterbium. The character of the concentration dependence of Yb³⁺ luminescence indicates that the energy transfer at high Yb concentrations occurs with active participation of a cooperative mechanism, according to which the excitation energy of one donor center is transferred simultaneously to two Yb³⁺ ions. In other words, the quantum yield of this transfer exceeds unity, which can be used to increase the efficiency of crystalline silicon (c-Si) solar cells.