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.     

Hardness of single-crystal CVD diamond and phase transformations in it on indentation

Hardness of single-crystal CVD diamond placed into a shell of a diamond composite thermostable material produced at high pressures and temperatures as well as structural variations in diamond during indentation have been studied.     

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.     

Broad band photoluminescence studies of diamond layers grown by hot-filament CVD

Photoluminescence and Raman spectroscopy were employed to investigate the broad band luminescence in thin diamond films grown on a silicon substrate by the HF CVD technique. The broad band luminescence with a maximum emission at 1.8–2 eV observed for CVD diamonds is characteristic for amorphous carbon with sp²-hybridized carbon bonds. As was shown by the Raman spectroscopy our diamond layer contained certain amounts of amorphous carbon phase and diamond nanocrystals which were the source of an additional energy state within the diamond energy gap. The experimental results precluded the possibility of broad band luminescence being due to the electron–lattice interaction. The amorphous carbon and diamond nanocrystals admixture in polycrystalline diamond layer introduced a defect state in the energy gap not in the form of point defects but rather in the form of a line or extended defects. In consequence these extended defects were responsible for the broad PL spectrum in the CVD diamond films.     

Electrical characterization of CVD diamond thin films grown on silicon substrates

Diamond thin films grown on high resistivity, 100 oriented silicon substrates by the hot filament chemical vapor deposition (HFCVD) method have been characterized by four-point probe and current-voltage (through film) techniques. The resistivities of the as-grown, chemically etched and annealed samples lie in the range of 10² Ω cm to 10⁸ Ω cm. The Raman measurements on these samples indicate sp³ bonding with a sharp peak at 1332 cm⁻¹. The surface morphology as determined by scanning electron microscope shows polycrystalline films with (100) or (111) faceted structures with average grain size of ≈2.5 μm. The through film current-voltage characteristics obtained via indium contacts on these diamond films showed either rectifying or ohmic behavior. The difference in Schottky and ohmic behavior is explained on the basis of the high or low sheet resistivities measured by four-point probe technique. 5% methane to hydrogen concentration during film growth resulted in poor surface morphology, absence of sp³ bonds, and low resistivity.     

Electrochemical studies of nano-structured diamond thin-film electrodes grown by microwave plasma CVD

In this paper, we report the investigation of the electrochemical properties of nano-structured diamond thin-film electrodes on porous silicon (PSi) synthesized by microwave plasma chemical vapor deposition (MPCVD). For the application, boron-doped and undoped diamond thin film has been performed and fabricated into an electrode device, and its microstructure, electrical and chemical properties have been studied. In order to enlarge the surface area of diamond electrodes, a negative bias was applied to the MPCVD process to deposit diamond thin film in a nano-structured form, so that its surface remained rough and nano-fine structured. Diamond thin films were analyzed by Raman spectroscopy and SEM. The morphology of boron-doped diamond thin films on PSi reveals nano-rods in the shape of diamond crystallites. Their electrochemical properties were evaluated by performing cyclic voltammetry (CV) measurement in inorganic K₄[Fe(CN)₆] in a K₂HPO₄ buffer solution. Boron-doped diamond thin film on PSi has demonstrated good electrochemical properties, with a larger redoxidation current of CV, due to its rough surface, which provides a more active electrochemical interface.     

CVD diamond layers for electrochemistry

Diamond electrodes of different morphologies and qualities were manufactured by hot filament chemical deposition (HF CVD) techniques by changing the parameters of diamond growth process. The estimation of diamond quality and identification of different carbon phases was performed by Raman spectroscopy measurements. The effect of diamond quality and amorphous carbon phase content on the electrochemical response of an obtained diamond electrode in 0.5 M H₂SO₄ as supporting electrolyte was investigated by cyclic voltammetry with [Fe(CN)₆]^4?/3? as a redox probe. The kinetic parameters such as catalytic reaction rate constant k₀ and electron transfer coefficient α were determined. The obtained results show that the analytical performance of undoped diamond electrodes can be implemented just by the change of diamond layers quality.     

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.     

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).     

Porous silicon capping by CVD diamond

In the last few years important research efforts were put concerning the optical properties of porous silicon (PS). Some interesting devices can be made using this new material (ex: L.E.D.s and micro cavities). However, a set of unsolved problems is keeping some distance between PS and its applications. Among these problems are the chemical instability and mechanical weakness.In this work, we attempted a CVD diamond protective layer deposition on PS. The optical properties of PS were preserved, as confirmed by photoluminescence (PL). The diamond film adhesion was checked by microhardness testing showing no crack propagation with a load of 200 gf. Nanoindentation measurements with 0.1 gf on the resulting surface showed a ten fold increase in hardness with respect to the uncoated material.     

CVD金刚石紫外探测器

CVD金刚石紫外探测器有极强的辐射硬度及耐腐蚀性,在宽禁带半导体紫外探测器中占有重要地位.本文主要对金刚石紫外探测器的发展进展、探测机理、电极模式及应用领域做了简要回顾.     

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.     

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.     

Optical bleaching, TSL and OSL features of CVD diamond

Luminescence and optical features of chemical vapour deposition (CVD) diamond have been studied in view of the potential application of this material in ionising radiation dosimetry field. For this purpose, thermally stimulated luminescence (TSL) and optically stimulated luminescence (OSL) techniques have been used. A large amount of work has emphasised the excellent dosimetric properties of CVD diamond. Nevertheless, TSL measurements showed that after irradiation, this material is extremely sensitive to ambient light and the stored dose information is drastically affected by optical bleaching. From OSL analysis, it follows that both types of processes (TSL and OSL) were characterised by the same excitation and emission spectra and that optical bleaching originated from a broad stimulation band lying from visible to near infrared with a continuous character.     

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。     

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.     

Growth, processing and properties of CVD diamond for optical applications

In recent years there has been an increased need for optical materials for use in adverse chemical, thermal, abrasive, and/or radiation environments. Diamond is a natural candidate for many of these applications because of its radiation hardness, superb resistance to chemical attack and abrasive wear, high thermal conductivity, and low absorption coefficient throughout the visible and much of the infrared. The use of synthetic (high pressure-high temperature) and natural diamond in optical components has been limited by the size and shapesof available crystals, and the inability to coat optical elements. The chemical vapor deposition (CVD) of polycrystalline diamond does not suffer the same limitations, and is therefore the focus of an expanding worldwide research effort. CVD diamond is not without its own shortcomings, however, and in this paper a status report is given on the obstacles and current research related to using CVD diamond as an optical material. Natural diamond's relevant physical properties and the optical applications envisioned for CVD diamond are also discussed.     

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.