Gamma sensitivity of single-crystal CVD diamond neutron detectors

We have studied the gamma sensitivity of single-crystal CVD diamond neutron detectors using a ²⁵²Cf neutron source placed in a moderator. It has been shown that a major contribution to the count rate of the detectors is made by the gamma rays from the source. We have compared the count rates of a detector with a ¹⁰B boron isotope-based slow-neutron converter and without it. With allowance for the theoretically calculated detection efficiency, the difference between the count rates is consistent with the fraction of slow neutrons measured using a scintillation detector.     

Synthetic single-crystal, homoepitaxially grown, CVD diamond capacitor

Homoepitaxially grown diamond film capacitor has been fabricated and tested for capacitance characteristics. Type IIa diamond was chosen for carbon ion implantation and subsequently microwave plasma CVD diamond was grown. CVD diamond film was separated from the base type IIa diamond substrate by using electrochemical bias. The separated CVD diamond film was annealed at 450°C for 2 h and metallized the film using a d.c. magnetron sputtering technique. The observed capacitance of the fabricated device is of the order of picofarads, close to the expected value of the capacitance. This revised version was published online in July 2006 with corrections to the Cover Date.     

In situ observation of quasimelting of diamond and reversible graphite-diamond phase transformations

Because of technique difficulties in achieving the extreme high-pressure and high-temperature (HPHT) simultaneously, direct observation of the structures of carbon at extreme HPHT conditions has not been possible. Banhart and Ajayan discovered remarkably that carbon onions can act as nanoscopic pressure cells to generate high pressures. By heating carbon onions to approximately 700 degrees C and under electron beam irradiation, the graphite-to-diamond transformation was observed in situ by transmission electron microscopy (TEM). However, the highest achievable temperature in a TEM heating holder is less than 1000 degrees C. Here we report that, by using carbon nanotubes as heaters and carbon onions as high-pressure cells, temperatures higher than 2000 degrees C and pressures higher than 40 GPa were achieved simultaneously in carbon onions. At such HPHT conditions and facilitated by electron beam irradiation, the diamond formed in the carbon onion cores frequently changed its shape, size, orientation, and internal structure and moved like a fluid, implying that it was in a quasimelting state. The fluctuation between the solid phase of diamond and the fluid/amorphous phase of diamond-like carbon, and the changes of the shape, size, and orientation of the solid diamond, were attributed to the dynamic crystallization of diamond crystal from the quasimolten state and the dynamic graphite-diamond phase transformations. Our discovery offers unprecedented opportunities to studying the nanostructures of carbon at extreme conditions in situ and at an atomic scale.     

Crystalline quality and phase purity of CVD diamond films studied by Raman spectroscopy

Two series of diamond films grown—at different temperatures—by two chemical vapor deposition (CVD) methods, i.e., hot filament (HFCVD) and microwave-plasma (MPCVD), were investigated. Raman spectroscopy and scanning electron microscopy were employed to perform a study of both crystalline quality and phase purity of the films grown by the two techniques. It was found that high phase purity can be attained by both methods. However, at high temperatures, the MPCVD technique produced films with higher crystalline quality as compared to those grown by HFCVD. Finally, in order to shed some light into the mechanisms responsible for the lower crystalline quality observed in the HFCVD films, a study based in the phonon confinement model and stress was accomplished.     

High-temperature Raman study in CVD diamond

Measurements of the Raman spectra in chemical vapour deposition (CVD) diamond films at temperatures up to 1200 K are presented. Specifically, the evolution of Raman line position, line width, and intensity were monitored as a function of heating time. The red shifting and the line width broadening of CVD diamond's Raman line with temperature are very similar to that of natural diamond's. However, the detailed temperature dependence of Raman line width depends on the orientation of the CVD diamond crystal and the ambient gas used during thermal treatment. Since the CVD diamond usually exhibited a broader Raman spectra than natural diamond, the evolution of the line width upon heating is thus expected to depend on the origins and the annealing effects of the residual stress. For (111) CVD diamond subjected to annealing in air at 973 K, the line width decreased by more than one wavenumber while the line intensity increased by more than an order of magnitude before it decreased subsequently. In contrast, there is hardly any observable changes of the line width for (100) CVD diamond heated in air at 1173 K. Measurements conducted in He versus in air suggested that the reduction of the non-diamond carbon phase (therefore, the reduction of stress) is likely due to oxidation, which occurs more readily in (111) than in (100).     

Twin quintuplet surfaces in CVD diamond

Chemical vapor deposition (CVD) of diamond films and wafers is a well established and flexible technology in which deposition parameters control the deposition rate as well as diamond properties, quality and shape. This article deals with surface morphology and crystallography of wafers, with emphasis on twin blocks and twin quintuples. In the study described here we have determined surface facet crystallography of two diamond wafers groups, one that contains Σ3 twin blocks and the other that has a large population of twin quintuplets. The tool used for the study is a scanning electron microscope (SEM) and the technique to identify surface crystallography involves only accurate tilting and surface observation. A basic knowledge of the twin structure enables accurate determination of surface crystallography.     

Thermal conductivity of CVD diamond films

Diamond films 60 and 170 µm in thickness were grown by PACVD (plasma-assisted chemical vapor deposition) under similar conditions. The thermal diffusivity of these freestanding films was measured between 100 and 300 K using AC calorimetry. Radiation heat loss from the surface was estimated by analyzing both the amplitude and the phase shift of a lock-in amplifier signal. Thermal conductivity was calculated using the specific heat data of natural diamond. At room temperature, the thermal conductivity of the 60 and 170 m films is 9 and 16 W-cm^–1. K^–1 respectively, which is 40–70% that of natural diamond, The temperature dependence of thermal conductivity of the CVD diamond films is similar to that of natural diamond, Phonon scattering processes are considered using the Debye model, The microsize of the grain boundary has a significant effect on the mean free path of phonons at low temperatures. The grain in CVD diamond film is grown as a columnar structure, Thus, the thicker film has the larger mean grain size and the higher thermal conductivity. Scanning electron microscopy (SEM) and Raman spectroscopy were used to study the microstructure of the CVD diamond films. In this experiment, we evaluated the quality of CVD diamond film of the whole sample by measuring the thermal conductivity.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

The toughness of free-standing CVD diamond

A four-point bend test was used to determine the fracture toughness of mechanical grade and di-electric (optical) grade chemical vapour deposited (CVD) diamond. The validity of the test was first confirmed by measuring the toughness of alumina and confirming the results with literature values. The toughnesses of both types of CVD were similar; 8.5 ± 1.0 and 8.3 ± 0.4 MPa respectively. This is higher than the value of 3 4 ± 0 5 MPa measured for good quality natural diamond by Field and Freeman, [1] using an indentation technique. It is suggested that this is primarily due to differences in surface roughness. There were enough samples to make a preliminary study of the effect of temperature and these data are reported.     

n-Type CVD diamond: Epitaxy and doping

Diamond is a semiconductor with superlative properties in terms of operating temperature, breakdown voltage and thermal dissipation for high power electronic applications. The p-type doping is currently controlled but it is not the case for the n-type doping. In spite of the low solubility of substitutional donors bigger than carbon, n-type material can be achieved via impurity doping during the growth of diamond with phosphorus dopant (E_c = 0.6 eV). Theoretical studies also predict arsenic as a shallower dopant (E_c = 0.4 eV). This paper outlines the n-type doping issues with phosphorus and explores the arsenic doping.     

The influence of working gas on CVD diamond quality

The effect of the working gas pressure and its composition on diamond quality and particles size was investigated. The diamond layers were grown in Hot Filament Chemical Vapor Deposition (HF CVD) reactor. A methanol–hydrogen gas mixture was used as the precursor gas. The structure of these films was characterized by scanning electron microscopy (SEM) and micro-Raman spectroscopy. Typically, the diamond's crystallite size decreased with increasing pressure and increasing with methanol concentration. Additionally the admixture of non-diamond (sp²-hybridized carbon) phase also increased with increasing both of deposition pressure and of methanol concentration. It was observed that the deposition pressure has a weaker influence on diamond quality than the methanol concentration.     

Kerma rate effects on thermoluminescent response of CVD diamond

The thermoluminescence (TL) response of chemical vapour deposition diamond is studied after X-irradiation (45 kV) at room temperature for various air kerma rates ranging from 0.7 to 100 Gy.min(-1). For a given air kerma, the TL signal as a function of air kerma rate exhibits large variations and is higher for low air kerma rates than for higher ones. In order to explain these results, the simplest energy-level scheme has been considered, that is, with only one electron trap and one recombination centre. Kinetic equations are numerically solved. Calculations are made for various electron-hole generation rates and show that the trap filling as a function of electron-hole generation rate follows, actually, the same evolution as the one of experimental TL response. The kerma rate effect on TL response is explained by the competitivity between the recombination and the trapping processes during irradiation.     

Uniform coating of CVD diamond on metallic wire substrates

Diamond was coated onto wire substrates of various transition metals (Mo, W or Ti) of 0.5 mm diameter by the microwave plasma CVD method from a gas mixture of the CO–H₂ system. The CVD conditions for a uniform diamond coating were microwave power, 750–1100 W; total pressure, 2000 Pa; total flow rate, 200 ml min-1; CO concentration, 5 vol%; treatment time, 5 h. The wire substrates were mounted vertically or horizontally on a pyrophyllite susceptor, which was placed parallel to the irradiation direction of microwave power. Homogeneous and fine-grained diamond film was prepared on the whole surface of horizontal W wire substrate with a wire height of 2 mm from the susceptor. To obtain a dense diamond coating, the height has to be as low as possible in the plasma region, where the plasma density is higher at lower substrate temperature. Low pressure and high microwave power were suited for fine-grained coating. Diamond deposition rate was found to be more dependent on pressure than substrate temperature. As the pressure increased, a glassy carbon film was formed instead of diamond. This revised version was published online in November 2006 with corrections to the Cover Date.     

CVD金刚石膜的场发射机制

利用热灯丝化学气相沉积方法在光滑的钼上沉积了金刚石膜,用扫描电子显微镜和Raman谱对金刚石膜进行了分析。结果表明金刚石膜是由许多金刚石晶粒组成,晶粒间界主要是石墨相,并且在膜内有许多缺陷。金刚石膜的场发射结果表明高浓度CH4形成的金刚石膜场发射阈位电场较低浓度CH4形成的金刚石为低。这意味着杂质（如石墨）和缺陷（悬挂键）极大地影响了膜的场发射性能。根据以上结果,提出了一种CVD金刚石膜的场发射机制即膜内的缺陷增强膜内的电场,石墨增大电子的隧穿系数以增强CVD金刚石膜的场发射。     

Improvements in CVD diamond properties for radiotherapy dosimetry

The goal of this work was to compare the behaviour of a chemical vapour deposited (CVD) diamond sample, grown at the University of Florence using a local procedure, with that of a commercial CVD diamond. The comparison was performed exposing both systems to 25 MV photons and measuring the current response during irradiation. Properties of dosimetric interest such as stability of response, dose rate dependence and rise time were investigated. After a preliminary study, which evidenced better performances of the commercial device with respect to the local CVD diamond, the latter was irradiated with a high fluence of fast neutrons. As a result of the neutron treatment, the quality of the CVD home-made diamond has been improved to match with that of the commercial dosemeter.     

Hardness,indentation fracture toughness and compositional formula of X-phase sialon

The hardness and indentation fracture toughness,K _IC of sintered X-phase sialon, produced by simultaneous carbothermal reduction and nitriding of kaolinitic clays, are determined. By comparing the values obtained for the sialon with those for a commercially pure mullite, it is suggested that X-phase sialon is a material with potential for similar applications to those in which mullite is currently used. On the basis of the oxygen/nitrogen ratios of the sialon determined by the inert gas fusion method, the analysis of the energy of the dispersed X-rays (EDAX) from thinned samples in the transmission electron microscope (TEM) and the aluminium/silicon ratios, also determined by the TEM EDAX method, a modified compositional formula is proposed for X-phase.     

CVD金刚石薄膜在钨丝和碳纤维上的制备

用CVD方法在钨丝和碳纤维上沉积金刚石薄膜,约含有10000根纤维的纤维束经特殊处理后分离为单根,生长条件使碳纤维中心在CVD生长结束后仍保持固态,CVD金刚石在纤维上生长的平均激活能为93.15KJ/mol。SEM照片给出了纤维涂层在不同生长条件下的表面和中心的形貌。纤维内部的石墨碳和外部的金刚石层有很大的不同。钨丝的断裂强度和杨氏模量的测量表明,具有金刚石涂层的钨丝的断裂强度为0.567GPa,非常接近不具有金刚石涂层的钨丝的断裂强度,但具有金刚石涂层的钨丝的断裂应变为4.8%,比没有金刚石涂层的钨丝的新裂应变7%要小得多,这表明了金刚石涂层可以减少钨丝的断裂应力,提高其机械性能。     

Spectroscopic studies of hydrogen related defects in CVD diamond

Thin diamond films prepared by the hot filament chemical vapour deposition (HFCVD) method at various deposition pressures have been characterized using a variety of spectroscopic techniques. Interpretation of the spectral details have provided useful information about the nature of the films. Deposition pressure appears to affect the quality of the diamond films which is reflected in terms of the position and width of the characteristic Raman peak of diamond. Raman spectra of the films prepared at low deposition pressures showed the presence of a sharp peak at ∼1332 cm⁻¹ characteristic of theT _2g mode of diamond. The study of the effect of deposition pressure on the diamond growth, shows that in the range between 20 torr and 60 torr, there is little effect on the width and the shift of the 1332 cm⁻¹ Raman peak. However, at higher pressures the peak showed a blue shift and was considerably broadened. These studies indicate the development of strain in the lattice due to the introduction of unetched hydride layer, at higher deposition pressures, as well as distortions in the lattice leading to partial lifting of the degeneracy of theT _2g mode. A broad band corresponding to the non-diamond phase (which exists at the grain boundaries, interface or as inclusions inside the grain), which can be attributed to the effect of hydrogen impurity creeping into the lattice at higher deposition pressures is also observed. SEM and XRD patterns have confirmed the dominance of diamond phase in these films.     

An assessment of radiotherapy dosimeters based on CVD grown diamond

Diamond is potentially a very suitable material for use as a dosimeter for radiotherapy. Its radiation hardness, the near tissue equivalence and chemical inertness are some of the characteristics of diamond, which make it well suited for its application as a dosimeter. Recent advances in the synthesis of diamond by chemical vapour deposition (CVD) technology have resulted in the improvement in the quality of material and increased its suitability for radiotherapy applications. We report in this paper, the response of prototype dosimeters based on two different types (CVD1 and CVD2) of CVD diamond to X-rays. The diamond devices were assessed for sensitivity, dependence of response on dose and dose rate, and compared with a Scanditronix silicon photon diode and a PTW natural diamond dosimeter. The diamond devices of CVD1 type showed an initial increase in response with dose, which saturates after ≈6 Gy. The diamond devices of CVD2 type had a response at low fields (<1162.8 V/cm) that was linear with dose and dose rate. At high fields (>1162.8 V/cm), the CVD2-type devices showed polarisation and dose-rate dependence. The sensitivity of the CVD diamond devices varied between 82 and 1300 nC/Gy depending upon the sample type and the applied voltage. The sensitivity of CVD diamond devices was significantly higher than that of natural diamond and silicon dosimeters. The results suggest that CVD diamond devices can be fabricated for successful use in radiotherapy applications.     

CVD金刚石膜中黑色缺陷的存在形式

采用光学显微镜在透射光、反射光和侧光模式下研究了多晶CVD金刚石厚膜中黑色缺陷的存在形式,利用X射线光电子能谱、拉曼光谱研究了黑色缺陷的组成成分。结果表明:晶界处的裂隙或者孔洞和晶粒内部的结晶缺陷是金刚石膜内部黑色缺陷的存在形式之一。