P. Gonon

Characterization of heavily B‐doped polycrystalline diamond films using Raman spectroscopy and electron spin resonance

Heavily B‐doped polycrystalline diamond films ([B]≳1019 cm−3) are studied by Raman spectroscopy and electron spin resonance. The formation of an impurity band is accompanied by a Fano‐type interference for the one‐phonon scattering. Bands at 1200 and 500 cm−1 are observed in Raman spectroscopy for concentrations above 1020 cm−3. They are related to maxima in the phonon density of states, and are ascribed to disordered regions or crystalline regions of very small size. The concentration of defects associated with the paramagnetic signal observed around g=2.0030 increases drastically above 1021 B cm−3. The Mott insulator‐metal transition is accompanied by the presence of a new paramagnetic signal (g=2.0007 for 2×1020 B cm−3, g=1.9990 for 1021 B cm−3) ascribed to free holes in the impurity band.

Effect of boron incorporation on the “quality” of MPCVD diamond films

We report the morphology and Raman signals of MPCVD (microwave-plasma-assisted chemical vapour deposition) boron-doped diamond films deposited on Si with a B:C ratio in the gas phase ranging from 20 to 10000 ppm. The crystal shape remains cubooctahedral in the whole range of doping, while the growth rate is reduced as B:C increases. Up to B:C = 400 ppm the full width at half-maximum (FWHM) and a very weak 1500 cm−1 component decrease as the B:C ratio increases. From B:C = 1000 ppm, an asymmetric deformation is observed in the Raman spectra and at the same time the FWHM and the 1500 cm−1 component increase with increasing B:C. For the highest B:C ratio of 10000 ppm additional bands appear around 550 and 1220 cm−1.

Electrical conduction and deep levels in polycrystalline diamond films

We have studied the dark conductivity (field, temperature, and frequency dependence), and the photoconductivity in undoped polycrystalline diamond films. Detailed analysis reveals that either of two alternative models can be invoked to explain all the observed features of the dark conductivity. The first model is a Hill‐type hopping conduction involving the presence of discrete acceptor states located at 0.91 eV above the valence band with a density around 1017 cm−3. The second model involves the presence of a band‐tail of acceptor states extending about 1 eV above the valence band. In this case, variable range hopping conduction dominates at low fields with a density of states at the Fermi level around 5×1015 cm−3 eV−1, while space charge limited currents dominate at high fields. The states controlling the dark conductivity give rise to photoconduction with a threshold around 0.85 eV and a peak at 1.1 eV. The shape of the photoconductivity spectrum suggests that lattice relaxation (with a Franck‐Condon s...

Thermally Stimulated Conductivity and Luminescence in Polycrystalline Diamond Films

Thermally stimulated currents (TSC) and thermoluminescence (TL) have been studied in polycrystalline diamond films of different qualities in order to gain information about the deep levels present within the bandgap of this material. The TL has been studied between 300 and 670 K after UV light illumination, and the TSC between 300 and 600 K after various excitations (UV light, from a tungsten ]amp, X-rays). TL and TSC peaks are observed at 470 and 560 K, and depend on the quality of the diamond and on the illumination conditions. At low temperature three TSC peaks appear at 245, 283 and 312 K after illumination with a tungsten lamp. A spectral analysis of the 470 K luminescence and electron emission measurement shows that the defect associated with this signal is an acceptor, and the hole released is trapped on a level 2.6 eV deep. Complementary ESR measurements show that paramagnetic nitrogen centers are not involved in the peak observed at 560 K.

Chemical vapor deposition of B‐doped polycrystalline diamond films: Growth rate and incorporation efficiency of dopants

The growth rate and the incorporation efficiency of dopants have been studied in the case of chemical vapor deposition of B‐doped polycrystalline diamond films. The deposition rate is found to decrease with the addition of diborane in the gas phase. This is correlated with a modification of the plasma chemistry as observed by emission spectroscopy (decrease in the H/H2, CH/H, and C2/H ratios with the addition of diborane). The concentration of boron incorporated in the films is observed to vary with the square of the boron concentration in the gas phase.

Reality of doping by boron implantation of CVD polycrystalline diamond from a comparison of Raman and electrical measurements

Abstract The effectiveness of doping in polycrystalline CVD diamond by 10 13 –10 16 cm −2 boron implantation at 77 K followed by annealing at 800 °C has been studied by Raman scattering and I(V, T) measurements. The amorphization threshold is found to be located around a boron doping of 3 × 10 15 cm −2 . Subsequent annealing of samples implanted with boron doses below and above this threshold results respectively in doped semiconducting diamond and graphite. In the high temperature range, the activation energies (between 0.61 and 0.15 eV) are discussed assuming a highly compensated semiconductor behaviour. In part of the low temperature range, hopping conduction occurs. The compensating centres are suggested to originate from native as well as implantation-induced defects.

Raman study of diamond films deposited by MPCVD: effect of the substrate position

Abstract Diamond is deposited from 100 sccm H 2 /0.5 sccm CH 4 or 100 sccm H 2 /4 sccm CH 4 /2 sccm O 2 by microwave plasma assisted chemical vapour deposition, either in the middle of the plasma ball or in a remote position at the level of the bottom wall of the wave guide. When H 2 /CH 4 mixtures are used, from Raman spectroscopy the parasitic phases concentration and the full width at half maximum (FWHM) of the diamond line are seen to decrease from the middle to the bottom position. In the bottom position, the diamond crystallite characteristics are further improved by using the mixture containing oxygen: no parasitic phases can be detected, and the FWHM remains between 3 and 4 cm −1 between 680 °C and 880 °C. The concentration of luminescent centers (from the Raman backgrounds) also decreases for a lower deposition temperature, O 2 in the gas, and a remote substrate position, but has a minor effect on the FWHM. These centers are ascribed to defects in the bulk of the crystallites while the FWHM and non-diamond Raman signal are both ascribed to parasitic phases on the crystallite surfaces. Coherent phenomenological models are proposed for the origins of the decrease of the FWHM and of the concentration of the luminescent centers with the previous deposition conditions.

Spectral response of the photoconductivity of polycrystalline chemically vapor deposited diamond films

Abstract Photoconductivity and photovoltaic signals of diamond films were measured at 300 K between 0.5 eV and 2 eV. We obtained bands around 0.5, 1.08, and 1.38 eV, and a plateau around 1.9 eV, which are ascribed to transitions from the valence band to localized states in the gap. The concentration of defects involved in the 1.08 eV band decreases after annealing at 600 °C. Different polarization offsets are needed to cancel these bands. This difference is ascribed to the occurrence of different defects according to the orientation of the crystallites under the contact. Space-charge zones are assigned to diamond in contact with an intermediate layer under the contact.

The influence of oxygen, in gas mixtures and various substrate positions, on the broad cathodoluminescence bands of MPCVD diamond films

Abstract One or two broad cathodoluminescence bands (2.88 and 1.9 eV) are often seen in CVD diamond thin films. In this work, we have looked for their fit using a sum of Gaussian curves, and then for a relationship between the deposition conditions and the occurrence of these Gaussian bands in an attempt to understand their origin. The films have been prepared with various deposition temperatures (between 630 and 880°C), gas mixtures (0.5% CH 4 in H 2 , or 4% CH 4 and 2% O 2 in H 2 ) and substrate positions (in the middle or below the plasma ball). Without oxygen, apart from the Gaussian band at 2.88 eV, two additional broader Gaussian bands at around 1.9 and 2.4 eV appear with increasing intensities relative to the 2.88 eV band as the deposition temperature decreases. With oxygen, besides the Gaussian band at 2.88 eV, only a weak band around 2.4 eV is obtained over the entire temperature range with approximately the same shape. Many of the broad bands found in natural and synthetic diamonds are found in these CVD films. These bands show a dependence on the preparation parameters which is consistent with their previous physical description.

Temperature dependence of particle–particle interactions in electrorheological fluids

We report on the temperature dependence of particle–particle interactions in electrorheological (ER) fluids for the temperature range 20–100 °C. The attraction force between polyamide spheres immersed in silicone oil is measured as a function of temperature. The force–temperature characteristic shows a broad maximum around 40 °C, corresponding to an increase of about 30% compared to the force measured at room temperature. In view of these results we proposed that the temperature dependence of the shear stress in ER fluids is directly related to the variation of the local particle–particle attraction forces. Data are discussed in light of models which were proposed in the literature to describe particle–particle interactions. At high electric fields “conduction models” could explain the observed temperature dependence through the variations of the oil breakdown field with temperature. However, limitations of such models are also clearly evidenced by data obtained at low electric fields.We report on the temperature dependence of particle–particle interactions in electrorheological (ER) fluids for the temperature range 20–100 °C. The attraction force between polyamide spheres immersed in silicone oil is measured as a function of temperature. The force–temperature characteristic shows a broad maximum around 40 °C, corresponding to an increase of about 30% compared to the force measured at room temperature. In view of these results we proposed that the temperature dependence of the shear stress in ER fluids is directly related to the variation of the local particle–particle attraction forces. Data are discussed in light of models which were proposed in the literature to describe particle–particle interactions. At high electric fields “conduction models” could explain the observed temperature dependence through the variations of the oil breakdown field with temperature. However, limitations of such models are also clearly evidenced by data obtained at low electric fields.