Diamond films grown by plasma chemical vapor deposition techniques display a fairly low resistivity (∼106 Ω cm). Heat treating the films causes an increase in the resistivity by up to six orders of magnitude. The low resistivity of the as-grown films is postulated to be due to hydrogen passivation of traps in the films. Annealing causes dehydrogenation resulting in the electrical activation of deep traps with an attendant increase in the resistivity. This mechanism has been confirmed by an observed reduction of the resistivity of the heat-treated films when they are subjected to a plasma hydrogen treatment.
A series of boron-doped polycrystalline diamond films grown by direct current and microwave plasma deposition was studied with Raman and infrared (IR) absorption spectroscopy. A Fano line shape is observed in the Raman spectra for films with a boron concentration in a narrow range near 1021 cm−3. The appearance of the Fano line shape is correlated with the disappearance of discrete electronic transitions of the boron acceptor observed in the IR spectrum and the shift of the broadened peak to lower energy. The Fano interaction is attributed to a quantum mechanical interference between the Raman phonon (0.165 eV) and transitions from the broadened impurity band to continuum states composed of excited acceptor and valence band states.
Subjecting natural diamond single crystals to the action of atomic hydrogen in a hydrogen plasma is shown to result in the passivation of interband states in the crystal resulting in a marked reduction in the resistivity to about 105 Ω cm from the expected high resistivity of∼1016 Ω cm. When the hydrogenated crystals are heat treated in a neutral ambient, the hydrogen can be expelled from the crystals, restoring the high resistivity. The behavior of natural diamond crystals, with respect to the effects of hydrogen, is shown to be similar to the behavior of diamond thin films synthesized by plasma-enhanced chemical vapor deposition techniques.
The combined electron and hole mobility of a single-crystal type IIa natural diamond and a polycrystalline diamond film deposited by chemical vapor deposition (CVD) were measured using transient photoconductivity as a function of excitation density (1013–1017 cm−3) and temperature (120–410 K). In natural diamond the temperature dependence suggests that the mobility is limited by phonon scattering at low free carrier densities, and by electron-hole scattering at high densities. The combined electron and hole phonon-limited mobility at room temperature is 3000 (±500) cm2/V s. In the CVD film, the mobility at room temperature was estimated to be 50 cm2/V s at low excitation densities. The temperature dependence of the mobility-lifetime product at low excitation densities is different from that of natural diamond, and suggests that charged center scattering, rather than acoustic phonon scattering, is the dominant effect. High densities of nitrogen and dislocations are known to be present in the natural diamond, and these appear to be the dominant recombination sites which limit the carrier lifetime. In the polycrystalline film a variety of structural defects and impurities are believed to exist, but it is unknown which of these dominates the transport and recombination properties.
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