D. Tromson

Superconductive B-doped nanocrystalline diamond thin films: Electrical transport and Raman spectra

Electrical transport properties of thin boron doped nanocrystalline diamond films with thicknesses of 60–500nm have been studied. The Raman spectra measured exhibit Fano resonances, characteristic for B concentrations close to the metal-to-insulator transition. Upon increasing the B concentration, the sp2 carbon related Raman resonances vanish. In such boron-doped nanocrystalline diamond films, a positive magnetoresistance could be observed at liquid helium temperatures. The boron doped diamond films show conductivity similar to that of B-doped epitaxial diamond without any significant contribution of the grain boundary transport, leading to the superconductive transition in nanocrystalline diamond at ∼1.66K.

CVD diamond for radiation detection devices

CVD diamond is a remarkable material for the fabrication of radiation detectors. Radiation hardness, chemical resistance and high temperature operation capabilities of diamond explain its use in the fabrication of devices operating in hostile environments such as that encountered in the nuclear industry and in high energy physics. For this purpose, we have investigated the growth of high quality chemically vapour deposited (CVD) polycrystalline diamond as well as specific material and device processing. CVD diamond films were grown using the microwave plasma enhanced technique. Deposition processes were optimised according to the application requirements. This includes the synthesis of films with high sensitivity, with weak optical absorption in the UV-VIS domain or with short carrier lifetime. One inherent problem with diamond is the presence of defect levels altering the detection characteristics: these may be the cause of an observed instability of the device responses. We have found, however, that it was possible to moderate these trends through the fine-tuning of the growth conditions and of the device preparation steps. Films with thicknesses ranging from 5 to 500 μm have been used for detector fabrication. The role of post-growth treatments and the contact formation procedure was also extensively studied, leading to significant improvements of the detector characteristics. We present recent developments studied at CEA for material optimisation towards its use for specific applications, including radiation hard counters; X-ray intensity, shape and beam position monitors; solar blind photo-detectors, and high dose rate gamma-meters.

Diamond as a tool for synchrotron radiation monitoring: beam position, profile, and temporal distribution

Abstract Diamond polycrystalline films have been synthesised using the chemical vapour deposition technique in order to fabricate new types of photodetectors for the characterisation of X-ray light sources as encountered in synchrotron experiments. We present here new diamond-based devices for three different applications, including (i) semitransparent beam position monitors with high position resolution (

Particle and Radiation Detectors Based on Diamond

CVD diamond is a remarkable material for the fabrication of particle and photon radiation detectors. The improvement of the electronic properties of the material has been under intensive investigations and led to the development of a few applications that are addressing specific industrial needs. In particular, we have used diamond layers for industrial applications where it exhibits attractive characteristics as compared with other materials: e.g., radiation and corrosion hardness for α-counters or high gamma-meters at high fluxes; high transparency to low energy X-rays for synchrotron beam line monitoring devices, etc. These specific properties can motivate the use of diamond even though the detection properties remain relatively poor. Indeed, one inherent problem with diamond is the presence of defect levels that are altering the detection characteristics. These are observed in all CVD materials but also in very high quality natural diamonds. They result in unstable responses and carrier losses. Also, it has been observed that high sensitivities may result from the progressive filling of deep levels, e.g. pumping effects, with a detrimental effect on the stability and the response time. Also, the polycrystalline nature is somewhat detrimental as it induces significant non-uniformities of the device response with respect to the position of interaction. We have investigated these features by imaging the response of CVD diamond using a micrometer size focused X-ray beam. The comparison with the grain structure showed that it has a strong influence on the field distribution. We present here recent developments studied at CEA in Saclay for the optimisation of the material with respect to the specific requirements of several applications. They include radiation hard counters; X-ray intensity, shape and beam position monitors, solar blind photodetectors, and high dose rate gamma-meters.

Corrosion hard CVD diamond alpha particle detectors for nuclear liquid source monitoring

Abstract Corrosion hard alpha particle detectors have been fabricated from thin diamond layers grown on silicon using the microwave enhanced chemical vapour deposition technique (CVD). The devices can be directly immersed in radioactive liquid solutions and enable the real time and in situ measurement of their alpha activity. We report here measurements on devices immersed in a plutonium-239 radioactive source diluted in concentrated nitric acid. The tests have shown excellent reliability of the CVD diamond detectors which would be of great benefit in monitoring nuclear fuel assembly reprocessing. We believe that this is the first detection application of CVD diamond which relies on its corrosion hardness.

Thermally Stimulated Investigations on Diamond Based Radiation Detectors

Recent improvements in the chemical vapour deposition (CVD) technique have enabled the fabrication of detectors for particle as well as photon detection applications. However, according to the material quality, it has been shown that inherent defect concentration may induce significant deterioration of the detection signal. In order to gain better understanding of these effects, in this study we have investigated the presence of traps using thermally stimulated current (TSC) technique for CVD as well as for natural diamond. In the 100 to 550 K temperature range, TSC revealed six peaks or shoulders for CVD diamond material grown under different methane concentration. Four trapping levels at 0.3, 0.6, 0.87 and about 1.45 eV could be identified. For natural diamond, significant differences in the TSC spectra have been observed for two natural diamonds, used as radiation detectors. The correlation between the presence of trapping levels and the detection properties is discussed. Five energy levels, at 0.3, 0.45, 0.68, 0.78, and 1.37 eV could be identified in natural diamonds, whereas four of them were found to be associated with the poor radiation detection capabilities. Further, the effect of annealing treatments was studied for natural diamonds. The results of TSC investigations gave evidence of a trapping level that could be associated with an improvement of the detection properties.

Geometrical non-uniformities in the sensitivity of polycrystalline diamond radiation detectors

Chemical vapour deposited (CVD) diamond is a remarkable material for the fabrication of photon and particle detectors. However, little is known about the perturbations induced by the polycrystalline nature of this material. For this purpose, we have used a micrometer size X-ray beam generated from a synchrotron light source to induce photocurrents in a CVD diamond-based detector. By comparing the measured currents in the device as the beam interaction position is moved on the sample with the topographical image of the surface observed using a scanning electron microscope (SEM), a significant non-uniformity has been observed that could be correlated with the grain structure.

Imaging of the sensitivity in detector grade polycrystalline diamonds using micro-focused X-ray beams

We have used a micrometer size X-ray beam generated from a synchrotron light source to measure the map of sensitivity in polycrystalline diamond films. Devices probed consist of sandwich structures in order to maintain the electric field uniform in the device. This study focuses on high-grade (detector) commercially available CVD material, with high charge transport properties and big grain size. As the X-ray beam is scanned over the device surface, the response current is measured and plotted on 2D images. The sensitivity exhibits very strong variations that are associated with the grain boundaries within the material. By varying the electric field, it appears that the signal does not increase linearly with the bias, revealing that carrier velocity saturation can be locally reached in some grains. Also, by varying the X-ray energy, the penetration depth could be varied in order to probe the material preferentially either in the volume or in the near-surface regions.

Study of deep defects in polycrystalline CVD diamond from thermally stimulated current and below-gap photocurrent experiments

Abstract We study the relaxations of the below-gap photocurrent (BGPC) following UV illumination in polycrystalline chemical vapour deposition diamond at different wavelengths (300 nm⩽λ⩽657 nm) and the implication on subsequent thermally stimulated current (TSC) experiments. The high temperature peak usually observed in TSC curves is shown to be intimately related to the BGPC relaxation. Results suggest that optical de-trapping of carriers previously trapped during UV illumination occurs during BGPC relaxation, with an onset at 2.3–2.5 eV photon energies. This is much more than the values of activation energies attributed to the defect level responsible for the TSC peak. Results are discussed in the framework of existing models of defect distributions.

Superconductivity and low temperature electrical transport in B-doped CVD nanocrystalline diamond

Abstract In a recent X–ray absorption study in boron doped diamond, Nakamura et al. have seen a wellisolated narrow boron impurity band in non-superconducting samples and an additional narrow band at the chemical potential in a superconducting sample. We interpret the beautiful spectra as evidence forupper Hubbard band of a Mott insulating impurity band and an additional metallic ‘mid–gap band’ of a conducting ‘self–doped’ Mott insulator. This supports the basic framework of a recent theory of the present author of strongly correlated impurity band superconductivity (impurity band resonating valence bond, IBRVB theory) in a template of a wide-gap insulator, with no direct involvement of valence band states.