Nmj Conway

Defect and disorder reduction by annealing in hydrogenated tetrahedral amorphous carbon

Hydrogenated tetrahedral amorphous carbon (ta-C:H) is a form of diamond-like carbon with a high sp3 content (>60%), grown here using a plasma beam source. Information on the behaviour of hydrogen upon annealing is obtained from effusion measurements, which show that hydrogen does not effuse significantly at temperatures less than 500°C in films grown using methane and 700°C in films grown using acetylene. Raman measurements show no significant structural changes at temperatures up to 300°C. At higher temperatures, corresponding to the onset of effusion, the Raman spectra show a clustering of the sp2 phase. The density of states of ta-C:H is directly measured using scanning tunnelling spectroscopy. The measured gradients of the conduction and valence band tails increase up to 300°C, confirming the occurrence of band tail sharpening. Examination of the photoluminescence background in the Raman spectra shows an increase in photoluminescence intensity with decreasing defect density, providing evidence that paramagnetic defects are the dominant non-radiative recombination centres in ta-C:H.

Reduction in defect density by annealing in hydrogenated tetrahedral amorphous carbon

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.

Ultraviolet Raman characterisation of diamond-like carbon films

Diamond-like carbon (DLC) films with a range of sp(3) fractions have been examined by ultraviolet (UV) spectroscopy with 244 nm excitation. The UV Raman spectra of both hydrogenated and unhydrogenated films show the G peak near 1600 cm(-1) and a broad peak around 1200 cm(-1), which is attributed to C-C stretching vibrations of sp(3) sites. It is found that the ratio of the intensities of the C peak and the 1200 cm(-1) peak follows a universal dependence with the sp(3) content in both hydrogenated and unhydrogenated DLCs. This can be used for non-destructive evaluation of the sp(3) content of any DLC film.

Photoconductivity and electronic transport in tetrahedral amorphous carbon and hydrogenated tetrahedral amorphous carbon

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.

Electronic properties and doping of hydrogenated tetrahedral amorphous carbon films

Abstract Hydrogenated tetrahedral amorphous carbon (ta-C:H) films were deposited over a range of ion energies using a plasma beam source. The optimum ion energy for maximum sp3 content was found to be 92 eV per carbon ion, and films were grown at this energy using both acetylene and methane as the source gas. The conductivity of the acetylene films was found to be 3.9 × 10−8S cm−1. Upon addition of nitrogen, the conductivity increased, while the optical band gap remained constant up to a nitrogen partial pressure of 10−5 mbar, which suggested n-type doping. At higher partial pressures, the band gap rapidly decreased, ta-C:H/Si heterojunction measurements using both p and n-type silicon showed that the films were n-type in character. However, there was found to be a large amount of nitrogen present in the acetylene gas, so a truly undoped film could not be produced. Thus, further films were made using high-purity methane as the source gas. The undoped films were then found to be p-type and could be doped n-type using nitrogen. Intrinsic behaviour was observed at a nitrogen partial pressure of 4 × 10−6 mbar.

Photoconductivity of nitrogen-modified hydrogenated tetrahedral amorphous carbon

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.

Defect band distributions in hydrogenated tetrahedral amorphous carbon/crystalline silicon heterostructures

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.

Photoconductivity and recombination in diamond-like carbon

Abstract 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.