π bonding versus electronic-defect generation: An examination of band-gap properties in amorphous carbon

Electron spin resonance ͑ESR͒ and Raman spectra measurements are carried out on a-C:H and a-C:H:N films both as grown and implanted with W and Ni ions with doses ranged from 0.5 ϫ10 15 to 1.2ϫ10 16 cm Ϫ2 . The as-grown films have small concentration of paramagnetic centers with a spin density N s of 10 17 cm Ϫ3 . Upon implantation a significant increase in N s of (0.5-22) ϫ1019 cm Ϫ3 centers with g (Si) ϭ2.0055 and g (C) ϭ2.0025 was observed. These defects are ascribed to dangling bonds in the silicon substrate and in the carbon film, respectively. The correlation between variation of N s value with implantation dose and behavior of D and G band position and their intensity ratio in the visible Raman spectra is observed. The effects are attributed to changes in the sp 2 -sp 3 systems and hydrogen loss due to ion induced annealing of the carbon films at high ion doses. The temperature and concentration dependencies of the ESR line shape and linewidth are explained using the mechanism of motional narrowing over the temperature range 4.2-300 K. Low temperature anisotropy of the g value is found in the ESR spectra and is explained as arising from the dipole-dipole interaction in the infinitely thin films.

Section: Introductionmentioning

confidence: 99%

Effects of ion implantation on electron centers in hydrogenated amorphous carbon films

Konchits

Valakh

Shanina

et al. 2003

“…First, we describe the electronic and optical properties of FCVA deposited a-C as a function of the ion energy and relate them to models of electronic structure. [16][17][18][19] We also report on the effect of deposition temperature on the structural characteristics of FCVA deposited a-C films with particular reference to the optical and electronic properties. Related studies have been carried out on a-C prepared by other methods.…”

mentioning

confidence: 99%

Influence of ion energy and substrate temperature on the optical and electronic properties of tetrahedral amorphous carbon (ta-C) films

Chhowalla

Robertson

Chen

et al. 1997

353

127

The properties of amorphous carbon ͑a-C͒ deposited using a filtered cathodic vacuum arc as a function of the ion energy and substrate temperature are reported. The sp 3 fraction was found to strongly depend on the ion energy, giving a highly sp 3 bonded a-C denoted as tetrahedral amorphous carbon ͑ta-C͒ at ion energies around 100 eV. The optical band gap was found to follow similar trends to other diamondlike carbon films, varying almost linearly with sp 2 fraction. The dependence of the electronic properties are discussed in terms of models of the electronic structure of a-C. The structure of ta-C was also strongly dependent on the deposition temperature, changing sharply to sp 2 above a transition temperature, T 1 , of Ϸ200°C. Furthermore, T 1 was found to decrease with increasing ion energy. Most film properties, such as compressive stress and plasmon energy, were correlated to the sp 3 fraction. However, the optical and electrical properties were found to undergo a more gradual transition with the deposition temperature which we attribute to the medium range order of sp 2 sites. We attribute the variation in film properties with the deposition temperature to diffusion of interstitials to the surface above T 1 due to thermal activation, leading to the relaxation of density in context of a growth model.

“…This structural change would occur when the distorted bonds are relaxed during the deposition, because the misorientation between orbitals corresponding to a bent or twisted bond creates paramagnetic defects. 19,20 For example, relaxation of the highly distorted sp 2 clusters results in a decreasing number of unpaired electrons and thus a smaller ESR signal. Relaxation of the twisted sp 2 bonds would also reduce the ESR signal.…”

Section: Resultsmentioning

confidence: 99%

Reduction of the residual compressive stress of tetrahedral amorphous carbon film by Ar background gas during the filtered vacuum arc process

Kim

Lee

et al. 2007

Tetrahedral amorphous carbon (ta-C) film was prepared by the filtered vacuum arc process with Ar background gas. The residual compressive stress of the film decreased significantly as the Ar flow rate increased, whereas negligible change in the mechanical properties was observed. Structure analysis by Raman spectroscopy, near edge x-ray absorption fine structure, and electron spin resonance spectra revealed a close relationship between the residual compressive stress and the bond distortion that invokes paramagnetic defects or unpaired π electrons. These results demonstrate that the stress reduction can occur by relaxation of the distorted or twisted atomic bonds at the same fraction of sp3 hybridized bonds, which enhances the stability of the ta-C coating without deterioration in the advantageous properties. The effect of Ar background gas is discussed in terms of the increased ion population of low kinetic energy in the plasma beam.