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Electronic transport and photoconductivity in amorphous carbons have been studied in order to understand their transport, recombination and photoluminescence properties. The role of the density of states and the effect of sp 2 phase nanostructuring are discussed. Localisation effects due to hydrogen bonding are shown.

The work function of tetrahedral amorphous carbon (ta-C) has been measured by Kelvin probe to lie in the range 4–5 eV, irrespective of its sp3 content or nitrogen addition. This implies that the surface barrier to emission is dominant and that emission changes caused by sp3 bonding or nitrogen addition are not directly due to changes in work function. Hydrogen, oxygen, and argon plasma treatments are all found to increase the emission of a-C, but hydrogen and argon treatments are found to reduce the work function while oxygen treatment increases it. Detailed studies of the surface with varying plasma treatment conditions suggest that the changes in emission arise mainly from changes in the surface microstructure, such as the formation of sp2 regions within the sp3 bulk. The need for local field enhancement mechanisms to account for emission over the sizeable barrier is emphasized, which may arise from local chemical nonhomogeneity, or formation of nanometer-size sp2 clusters embedded in an sp3 matrix.

Junction capacitance measurements and voltage-pulse stimulated capacitance transient measurements have been applied to undoped hydrogenated tetrahedral amorphous carbon (ta-C:H)/crystalline-Si (c-Si) heterostructures to deduce the defect densities in as-grown and post-annealed ta-C:H films. Transient capacitance measurements reveal a component corresponding to the emission of carriers out of defect states at the ta-C:H/c-Si interface, and a slower component corresponding to the carrier emission from defect states of interior ta-C:H. The interior defect density of as-grown films was estimated at 1×1018±2×1017cm−3, decreasing to (6.5±2)×1017cm−3 for films annealed at 300°C. A Gaussian defect band was identified with activated hole emission with a 0.35 eV activation energy. Steady state admittance measurements indicate two thermal activation processes and have allowed us to determine a defect density of 1×109±2×108cm−2 at the ta-C:H/c-Si interface.

The photoconductivities of various sp3 bonded amorphous carbons were studied to understand their recombination and photoluminescence processes. At temperatures >200 K, the photoconductivity of each type of carbon does not vary inversely with the paramagnetic defect density, indicating that paramagnetic defects are not the only recombination centres. This effect occurs because carrier demarcation levels lie in the band tails, so both defects and tail states act as recombination centres. The temperature regime

The changes in the photoconductivity of hydrogenated tetrahedral amorphous carbon (ta-C:H) with nitrogen incorporation were studied. Low level nitrogen incorporation improves the photoconductivity, by shifting the Fermi level upwards in the band gap. Films with a photosensitivity of about 200 at room temperature under white light illumination of 35 mW/cm2 were obtained; thus is the highest value so far reported for diamond-like carbons. At high temperatures, photoconductivity is controlled by nonradiative recombination through gap states, whereas at low temperatures it occurs by energy-loss hopping in the band tails. Nitrogen addition does not create extra charge defect recombination centers. Low temperature photoconductivity allows the direct determination of the localization radius of the band tail states. This radius varies from 2–3 A in ta-C:H to 9 A in ta-C. This illustrates how hydrogen can increase state localization and the photoluminescence efficiency in amorphous carbons.

The steady-state photoconductivity of tetrahedral amorphous carbon (ta-C) and hydrogenated ta-C (ta-C:H) has been studied as a function of temperature, light intensity, and photon energy, in order to understand the transport and recombination process in diamond-like carbon. It is found that the levels demarking the recombination states can span only part of the gap, so that the recombination centres can vary from every defect, to some defects, to some tail states, according to conditions.

The photoconductivity of tetrahedral amorphous carbon (ta-C) and hydrogenated tetrahedral amorphous carbon (ta-C:H) has been studied as a function of temperature, photon energy, and light intensity in order to understand the transport and recombination processes. ta-C and ta-C:H are found to be low mobility solids with μτ products of order 10−11–10−12 cm2/V at room temperature because of their relatively high defect densities. Deep defects tend to be the dominant recombination centers, but at high and moderate temperatures only a fraction of these centers or even tail states can act as recombination centers because the carrier demarcation levels do not always span the gap. For excitation by high energy UV photons, a peak in the photoconductivity is found at 200 K, similar to the thermal quenching effect found in a-Si:H, and attributed to competitive recombination between two classes of centers with very different capture cross sections.

Electron field emission from amorphous carbon is found to depend on the clustering of the sp2 phase. The size of the sp2 phase is varied by thermal annealing and it dominates the effect of other parameters, such as chemical composition, surface termination, sp3 content, or conductivity. The optimum size of the sp2 phase is determined by Raman spectroscopy and is of the order of 1 nm. The field emission originates from the sp2 regions and is facilitated by the large field enhancement from more conductive sp2 clusters in an insulating sp3 matrix.

Junction capacitance measurements performed on tetrahedrally bonded amorphous carbon (ta-C)/crystalline-Si (c-Si) heterostructures are used to deduce the defect density in thin ta-C films. Using the drive level capacitance profiling technique, we determine a homogeneous defect density of 6±1.5×1017 cm−3 in the undoped (p-type) ta-C films and one order of magnitude larger in the nitrogen-doped (n-type) films. Admittance measurements reveal defect states at the ta-C/c-Si interface with a density exceeding 2×1012 cm−2.

The possibility of achieving nanometer-scale sensitivity in a surface-enhanced Raman (SERS) experiment while using larger-sized probes (0.1--1 \ensuremath{\mu}m) is investigated. The application targeted is carbon film transformation under high-energy beam irradiation, and, primarily, the transformation of amorphous carbon into nanocrystalline graphite. The carbon film covers nanometer-size Ag particles which enhance the signal from zones of material adjacent to them. This geometry gives access to the film/substrate interface, and in this way it complements scanning near-field techniques which have similar spatial sensitivity but are mainly surface sensitive. The SERS effect has been studied as a function of the Ag nanoparticle size, carbon film thickness, and excitation wavelength. A selective enhancement of the Raman cross section of the D band of amorphous carbons was observed. The dielectric properties of the carbon film, when used as an overlayer, strongly affect the SERS enhancement, so that changes in the dielectric function upon irradiation can be used to produce local enhancement contrast, and to establish an identification procedure for material transformation.

We have applied junction capacitance and transient photocapacitance measurements to undoped tetrahedral amorphous carbon (ta-C)/silicon carbide (SiC) heterostructures to deduce defect densities and defect distributions in ta-C. The junction capacitance measurements show two thermally activated processes. One can be related to the activation of carriers out of defects at the ta-C/SiC interface while the other one with an activation energy of 0.36eV is an intrinsic property of the ta-C. The defect density at the ta-C/SiC interface is estimated to be roughly 9 ± 2 × 109 cm−2. The transient photocapacitance measurements have allowed us to observe the broader band tail of ta-C, giving a value (Urbach energy) of 230meV.

Tetrahedrally bonded amorphous carbon (ta-C) is a predominately sp3-bonded semiconductor with a band gap of order 2 eV. It can be doped n-type using nitrogen but no successful p-type doping has been reported until now. On the other hand, it has recently been shown that the incorporation of boron reduces the intrinsic compressive stress of ta-C, while still maintaining its high fraction of sp3 sites. This paper reports a detailed study of the electrical properties of boron-doped ta-C (ta-C:B). The ta-C:B films are deposited in a filtered cathodic vacuum arc system using a pressed cathode of graphite and boron powder. The composition and structure of the films are examined by electron energy loss spectroscopy. We find that the room temperature conductivity of the films increases by five orders of magnitude with a boron concentration from 0 to 8%. The conductivity activation energy decreases for the same boron concentration, while the E04 gap remains constant. N-type silicon/ta-C:B heterojunctions show a rectifying behaviour as a function of the boron concentration of the films. The films show photo-conductivity. The combined results indicate p-type doping of ta-C.

We report the results of junction capacitance measurements on thin tetrahedral amorphous carbon (ta-C) films to deduce their defect densities. We find defect densities in the range 3 - 7 × 1017 cm−3 in the undoped ta-C films, and roughly an order of magnitude larger in the nitrogen doped (n-type) films. In some cases fairly uniform defect profiles were obtained covering a thickness of a couple of hundreds angstroems. We also observed a thermal activation process of carriers from defect states at the ta-C/c-Si interface with an activation energy in the range of 0.4eV to 0.5eV.

Electronic applications of diamond-like carbon have been limited by its relatively high disorder and defect density. We find that the density of paramagnetic defects in hydrogenated tetrahedral amorphous carbon and the Urbach slope of the optical absorption edge can be reduced by annealing at 300 °C, with little effect on the optical gap. This leads to a reduction in the dark conductivity and an increase in the photosensitivity. The effect is attributed to the migration of hydrogen through the C–C network, to allow better passivation of dangling bonds and a modification of the more weakly bonded sp2 clusters with narrower local band gaps.