L. M. Brown

Determination of bonding in amorphous carbons by electron energy loss spectroscopy, Raman scattering and X-ray reflectivity

X-ray reflectivity (XRR) and Raman scattering are developed as non-destructive methods to find the density and sp 3 content of unhydrogenated and hydrogenated amorphous carbon films. An empirical relationship is found to derive the sp 3 fraction from visible Raman spectra, while ultraviolet (UV) Raman is able to directly detect sp 3 sites. The sp 3 fraction and density are linearly correlated in amorphous carbon (a-C) and hydrogenated amorphous carbon (a-C:H) films. ” 2000 Elsevier Science B.V. All rights reserved.

Nitrogenated amorphous carbon as a semiconductor

Abstract At present, hydrogenated amorphous carbon (a-C:H) is a poor electronic material primarily due to the excessive density of defect states in the band gap which act as trapping centres. The ability of nitrogen to improve the semiconducting properties of a-C:H is examined. A reduction in the activation energy for electronic conduction in nitrogenated a-C:H (a-C:H:N) films and the approximately constant optical band gap with increasing N content suggest that N influences the bulk electronic properties of a-C:H. Electron spin resonance shows a reduction of the density of gap states in a-C:H:N with increasing N content. Electron energy loss spectroscopy shows the films to be predominantly sp2 bonded with band edge properties which change significantly as a function of the N content. The C:H:N contents of the films were determined by elastic recoil detection analysis and Rutherford backscattering.

Maximized sp3 bonding in carbon nitride phases

Carbon nitride films were deposited using a low pressure, dual ion beam system consisting of a filtered cathodic vacuum arc and a plasma beam source for carbon and nitrogen ions, respectively. This system maintains highly ionized beams even at high nitrogen fluxes, unlike in single beam systems. Film composition and bonding were measured by electron energy loss spectroscopy. Films with nitrogen to carbon atom ratios (N/C) up to 0.5 are produced. The carbon bonding is found to change gradually from sp3 to sp2, rather than sharply above a critical N content, as found previously. This indicates that N atoms form individual C=N bonds rather than causing a reversion of the entire C network to sp2. This allows us to maintain C sp3 bonding to the highest N contents so far achieved.

Cubic boron nitride: Experimental and theoretical energy-loss near-edge structure

A comparison between experimental and theoretical electron Energy Loss Near Edge Structure (ELNES) of B and N K-edges in cubic boron nitride is presented. The electron energy loss spectra of cubic boron nitride particles were measured using a scanning transmission electron microscope. The theoretical calculation of the ELNES was performed within the framework of density functional theory including single particle core-hole effects. It is found that experimental and calculated ELNES of both the B and N K-edges in cubic boron nitride show excellent agreement.

Wear by friction between diamonds studied by electron microscopical techniques

Abstract This paper deals with an experimental study of possible structural changes which occur upon frictional sliding of a diamond stylus on a diamond flat. Reflection electron energy loss spectroscopy in the scanning transmission electron microscope has been applied to determine the structure of a cleaved {111} diamond surface, of a friction track on this surface and of the debris produced during frictional sliding. The structure of the debris appeared to be graphitic and different from the structure of the surface in the track, which was still diamond. As the surface studied was the cleaved {111} face, which is the hardest surface of diamond, it is assumed that the debris mainly consists of material originally belonging to the stylus. So far, no definite conclusion can be drawn about when the transformation of diamond into less dense forms of carbon takes place. The debris acts as a lubricating material which is demonstrated by monitoring the friction coefficient upon removing debris from the stylus. The outcome of this type of research is expected to throw light on the friction and polishing behaviour of different diamond surfaces. With the improving control of the orientational growth of CVD diamond, results will also be relevant to the friction and processing of these polycrystalline diamond surfaces.

Cracks and extrusions caused by persistent slip bands

Cottrell’s account of persistent slip poses a puzzle which has challenged all subsequent research. Persistent slip bands (PSB) endure repeated plastic shear which constantly produces narrower and narrower dipoles, as observed by Veyssière. At the same time, because narrow interstitial dipoles have a larger elastic energy than otherwise identical vacancy ones, shear will eject larger interstitial dipoles to the surface, whilst retaining smaller vacancy ones in the interior, thereby producing excess vacancy loops. This causes a longitudinal tensile stress (the ‘fibre stress’) inside the band. The ejection process lowers the energy by a term linear in the fibre stress, but increases it by a term quadratic in the fibre stress, giving rise to an equilibrium value of the fibre stress, tensile, in order-of-magnitude agreement with observations of extrusions at low temperatures, where only plasticity can occur. The fibre stress produces logarithmic infinities in the surface stress at the edges of the PSB, and thus can be responsible for sharp stage I fatigue cracks. At higher temperatures which allow pipe diffusion and/or volume diffusion, interstitial loops are drawn by the tensile fibre stress into the PSB. Then, as cyclic plasticity attempts to maintain equilibrium, the loops are ejected where the band meets the surface, producing growing extrusions. Such extrusions can grow almost without limit. At low temperatures, the rate of extrusion formation is maximal at one Burgers vector per cycle, but it will be slower than this if it is diffusion limited.

Calculation of the electronic structure of carbon films using electron energy loss spectroscopy.

The detailed understanding of the electronic properties of carbon-based materials requires the determination of their electronic structure and more precisely the calculation of their joint density of states (JDOS) and dielectric constant. Low electron energy loss spectroscopy (EELS) provides a continuous spectrum which represents all the excitations of the electrons within the material with energies ranging between zero and about 100 eV. Therefore, EELS is potentially more powerful than conventional optical spectroscopy which has an intrinsic upper information limit of about 6 eV due to absorption of light from the optical components of the system or the ambient. However, when analysing EELS data, the extraction of the single scattered data needed for Kramers Kronig calculations is subject to the deconvolution of the zero loss peak from the raw data. This procedure is particularly critical when attempting to study the near-bandgap region of materials with a bandgap below 1.5 eV. In this paper, we have calculated the electronic properties of three widely studied carbon materials; namely amorphous carbon (a-C), tetrahedral amorphous carbon (ta-C) and C60 fullerite crystal. The JDOS curve starts from zero for energy values below the bandgap and then starts to rise with a rate depending on whether the material has a direct or an indirect bandgap. Extrapolating a fit to the data immediately above the bandgap in the stronger energy loss region was used to get an accurate value for the bandgap energy and to determine whether the bandgap is direct or indirect in character. Particular problems relating to the extraction of the single scattered data for these materials are also addressed. The ta-C and C60 fullerite materials are found to be direct bandgap-like semiconductors having a bandgaps of 2.63 and 1.59eV, respectively. On the other hand, the electronic structure of a-C was unobtainable because it had such a small bandgap that most of the information is contained in the first 1.2 eV of the spectrum, which is a region removed during the zero loss deconvolution.

Transport properties of two-dimensional electron gases containing linear ordering InAs self-assembled quantum dots

We present a study of the anisotropic properties of two-dimensional electron gases formed in GaAs/AlGaAs heterostructures in which InAs self-assembled quantum dots have been inserted into the center of a GaAs quantum well. We observe an anisotropic mobility for the orthogonal [110] and [110] directions. The mobility in the [110] direction was found to be up to approximately twice that in the [110] direction. It is suggested that the interface roughness scattering due to the inserted InAs material could be a cause for the large anisotropies in mobility.

Dual ion plasma-beam sources used to maximise sp3 C–C bonds in carbon nitride

Amorphous carbon nitride films grown by plasma or ion sources never achieve the limit of β-C 3 N 4 , because the nitrogen fraction saturates below 57% and the carbon tends to become sp 2 -bonded at high N content. When a-CN x is grown from a single ion or plasma beam, high nitrogen pressures are needed to promote higher N contents, but this leads to a loss of ionisation. We use a dual ion beam method to grow a-CN x with a filtered cathodic arc (FCVA) to supply carbon and a low pressure, high plasma density electron cyclotron wave resonance (ECWR) source to supply atomic nitrogen ions. The film composition and fraction of unsaturated π * bonding at C and N sites was measured by electron energy loss spectroscopy. We find that the C sp 3 fraction decreases linearly with nitrogen content, rather than showing the sharp fall at N/C = 0.08-0.1 found by others. Thus, we achieve the highest C-C sp 3 content for a-CN x films with N/C > 0.1. We find a rather sharp increase in the fraction of empty N π * states, from 10 to 30% at N/C = 0.2. Whereas previous work suggests that N contents above the critical value of 0.1 induce a transition for all C sites to sp 2 , in our results only those C sites bonded to N revert to sp 2 . The film density was observed to change in a similar way to the density of ta-C films with respect to the C sp 2 fraction. However, strong differences in the optical gap are observed.

Density and sp3 content in diamond-like carbon films by x-ray reflectivity and electron energy loss spectroscopy

Grazing angle x-ray reflectivity (XRR) is used to study density, thickness, internal layering and roughness of a variety of carbon samples, with and without hydrogen and nitrogen. The bulk mass density of optimised tetrahedral amorphous carbon (ta-C) is 3.26 g/cm 2 , for which Electron Energy Loss Spectroscopy (EELS) found a sp 3 fraction of 85%. Combining XRR and EELS we benchmark the dependence of sp 3 fraction on density for hydrogen-free carbons. Hydrogenated ta-C (ta-C:H) deposited by electron cyclotron wave resonance (ECWR) reactor from acetylene gas, has a density of 2.35 g/cm 3 , 75% sp 3 and -30% hydrogen. These data provide a similar validation for density and sp 3 EELS data for hydrogenated DLCs. XRR can also reveal internal layering in films, and indeed less dense layers may be found at the surface or interface of ta-C films, but no such layers are found in ta-C:H films.