F. Fontaine

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.

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...

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.

Characterization of defects in boron implanted chemically vapour deposited diamond films by electron paramagnetic resonance and cathodoluminescence

Abstract The same electron paramagnetic resonance active defects (C interactive dangling bonds), with similar behaviour with respect to B dose before (20 centres per B ion, amorphization threshold of 10 15 cm −2 ) and after annealing at 800 °C (concentration reduced by 95%), are created by B implantation in chemically vapour deposited polycrystalline films and type IIa natural crystals of diamond. A broad 2.4 eV cathodoluminescence band (substitutional B), appears only in polycrystalline films. Its intensity relative to the 2.92 eV band increases with the B dose and decreases after annealing.

Conduction mechanisms in boron implanted diamond films

Abstract The relationship between variable range hopping (VRH) below 300 K and the defects created during 1013–1016 cm−2 boron ion implantation at 77 K in as-implanted diamond films and films annealed at 800 °C is investigated with the help of electron spin resonance (ESR) and optical measurements. The onset Dp = 2 × 1015 cm−2 of the VRH conduction is attributed to a percolation threshold, and is lower than the amorphization dose Da = 4 × 1015 cm−2. From ESR and optical absorption, implantation creates sp3 carbon dangling bonds (paramagnetic and which act as hopping centres) below Dp and sp2 bonds (which cause metallic conduction and optical absorption) above Da. The hopping conduction takes place between localized levels from sp3 carbon dangling bonds, but only a small proportion of the sp3 defects participates in this conduction.

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.

Metal/insulator/semiconductor tunnel diodes formed by the oxidation of polycrystalline diamond films

Polycrystalline diamond films have been annealed under O2 at 600 °C, or have been dipped in a H2SO4/CrO3 solution. Both treatments result in the formation of a thin electrically insulating layer at the top of the films. Subsequent metallization results in the formation of a metal/insulator/diamond tunnel diode with a potential barrier for holes of 0.85 eV, and with a Fermi level localized at about 0.45 eV above the diamond valence band.

Influence of annealing on the resistance of polycrystalline chemically vapour deposited diamond films: a surface chemical effect

Abstract The I ( V ) and I ( T ) characteristics of diamond films after various treatments are reported and analysed. The ohmic behaviour of contacts on as-grown films originates from a superficial conductive layer making an ohmic contact with diamond. Annealing of the films at 600°C under argon or vacuum keeps this ohmic behaviour. In contrast, annealing under oxygen, or nitrogen, or air, induces a non-linear, very high contact resistance. In this case the electrical properties are ascribed to a metal-insulator-semiconducting-diamond structure, with a depletion layer and a potential barrier height of 0.85 eV between the diamond band and the Fermi level at the diamond-insulator interface. The superficial insulating layer results from oxidation of the as-grown porous superficial conductive layer, which is in agreement with the oxygen-carbon reactions reported for these experimental conditions.

Photoconductivity associated with deep levels in polycrystalline diamond films

Abstract The spectral dependence of the photoconductivity in polycrystalline diamond films has been studied between 0·5 and 2·5 eV. Several structures are observed (a threshold at 0·85 eV, peaks at 1·1 and 1·4eV, a broad band at 1·9eV and weak peak at 2·4 eV), indicative of the presence of deep levels in these films. The peak at 1·1 eV is ascribed to the photoionization of acceptor states located at about 1 eV above the valence band with some evidence of electron-phonon interaction at the sites.