K.W. Nugent

Systematic variation of the Raman spectra of DLC films as a function of sp2:sp3 composition

Abstract We report the Raman spectra of a set of hydrogen-free diamond-like carbon (DLC) films prepared under conditions that result in different types of carbon bonding and for which the sp 2 :sp 3 content was independently determined by electron energy loss spectroscopy. The films were prepared using a mass selected C + ion beam deposition system covering the ion energy range 10 eV to 20 keV. Previous measurements have shown that the sp 2 component of the films decreases as the ion energy is increased from about 10 to 300 eV. For films made with ion energies in excess of 1 keV, the sp 2 component increases with increasing ion energy. The Raman spectra of these films show a broad asymmetric peak which narrows with decreasing sp 2 content. The spectra were fitted with a single skewed lorentzian peak described by the Breit-Wigner-Fano lineshape. The data presented may serve as a reference for DLC characterization. In particular, the parameters from the fits show a strong dependence on the sp 2 component of the films which may be used to identify unambiguously (hydrogen-free) DLC films of low sp 2 content (i.e. very “diamond-like” films).

The Raman spectrum of nanocrystalline diamond

Abstract Nanometre sized diamond powder has been purified by centrifugation to remove contamination from sp2 bonded carbon. The purified powder has been characterized using electron energy loss spectroscopy (EELS) and Raman spectroscopy. The EELS spectra confirmed the absence of sp2 bonded carbon and showed strong contributions from surface plasmons. Strong relatively sharp peaks are observed in the Raman spectra at 500, 1140, 1132 and 1630 cm −1 . By comparing the Raman spectra of the nanodiamond clusters with that of amorphized diamond and with calculations of the vibrational density of states we are able to suggest the origin of features in the vibrational spectrum from nanocrystalline diamond.

Direct quantitative detection of the sp3 bonding in diamond-like carbon films using ultraviolet and visible Raman spectroscopy

The bonding in a series of unhydrogenated amorphous carbon films has been analyzed quantitatively using Raman spectroscopy excited by laser light in both the visible and ultraviolet regions of the spectrum. The asymmetry of the peak near 1550 cm−1 in the visible Raman spectra is correlated with the percentage of sp3 bonding in the films. The ultraviolet Raman spectra exhibit two broad Raman peaks at 1650 and 1100 cm−1, due to sp2 and sp3 vibrational modes, respectively. The former is a resonance feature associated with a large proportion of paired sp2 sites, while the latter is a weighted phonon density-of-states for the distorted random network of sp3 sites. The position and relative intensity of the two peaks are shown to be strongly correlated with the percentage of sp3 sites in the films, providing a reliable measure of sp3 bonding which is both semiquantitative and nondestructive.

Temperature dependence of Raman scattering in single crystal GaN films

Micro-Raman scattering from single crystal GaN films, both free-standing and attached to Al2O3 substrates, was performed over the temperature range from 78 to 800 K. These measurements reveal that the Raman phonon frequency decreases and the linewidth broadens with increasing temperature. This temperature dependence is well described by an empirical relationship which has proved to be effective for other semiconductors. The experiments also demonstrate that the strain from Al2O3 substrates compresses the epitaxial GaN in the c-axis direction.

The Raman spectrum of amorphous diamond

Abstract We present the Raman spectrum of an amorphous, fully sp 3 -bonded carbon network. The reduced Raman spectrum agrees closely with the calculated density of states of diamond. The results have been obtained from nanoclusters produced deep inside a single crystal diamond irradiated with MeV He ions. The deep implantation creates amorphous sp 3 bonded C clusters along the ion tracks, within a largely intact diamond matrix. The matrix maintains the clusters under high pressure, preventing the relaxation to sp 2 bonded structures. Sharp peaks associated with defect structures unique to MeV ion implantation are observed at 1422, 1447, 1467, 1496, 1540, 1563, 1631, 1649, 1683 and 1726 cm −1 . We also observe a shoulder in the reduced Raman spectrum at ca 1120 cm −1 which we tentatively attribute to surface phonon modes. The results provide the Raman signature that might be expected from tetrahedrally bonded amorphous carbon films with no graphite-like amorphous components.

Diamond nanocrystals formed by direct implantation of fused silica with carbon

We report synthesis of diamond nanocrystals directly from carbon atoms embedded into fused silica by ion implantation followed by thermal annealing. The production of the diamond nanocrystals and other carbon phases is investigated as a function of ion dose, annealing time, and annealing environment. We observe that the diamond nanocrystals are formed only when the samples are annealed in forming gas (4% H in Ar). Transmission electron microscopy studies show that the nanocrystals range in size from 5 to 40 nm, depending on dose, and are embedded at a depth of only 140 nm below the implanted surface, whereas the original implantation depth was 1450 nm. The bonding in these nanocrystals depends strongly on cluster size, with the smaller clusters predominantly aggregating into cubic diamond structure. The larger clusters, on the other hand, consist of other forms of carbon such as i-carbon and n-diamond and tend to be more defective. This leads to a model for the formation of these clusters which is based o...

Cross-sectional Raman microscopy of MeV implanted diamond

Abstract The defect structures created in diamond by MeV He ion implantation are studied using Raman microscopy. The diamond was irradiated edge on, and the micro-Raman spectra were collected in a direction perpendicular to the implantation direction. This ‘cross-sectional’ geometry allowed the Raman spectrum of the damage to be resolved as a function of depth from the ion implanted surface. Both the position and the FWHM of the first order Raman mode at 1332 cm−1 were found to scale linearly with dose; a result readily interpretable as defect induced strain in the implanted layer. The measurements thus provide the depth profile of ion implanted damage in MeV irradiated diamond, and allow a direct test of the accuracy of theoretical models, such as TRIM, for the assessment of damage in ion irradiated diamond.

The characterisation of high-grade synthetic quartz, corundum and spinel using ionoluminescence (IL)

Abstract Ionoluminescence (IL) has been used to characterise synthetic quartz (SiO 2 ), corundum (Al 2 O 3 ) and spinel (MgAl 2 O 4 ) samples at cooled and ambient temperatures. Results are compared with cathodoluminescence (CL) and photoluminescence (PL) data obtained from the same samples. A study of the radiation damage, impurity concentration and temperature effects was undertaken in order to illustrate the use of IL an analytical tool for nuclear microprobes, and to further understand ion-solid interactions.

Polarized Raman spectroscopy of chemically vapor deposited diamond films

Polarized micro‐Raman spectra of chemically vapor deposited diamond films are presented. It is shown that important parameters often extracted from the Raman spectra such as the ratio of the diamond to nondiamond component of the films and the estimation of the level of residual stress depend on the orientation of the diamond crystallites with respect to the polarization of the incident laser beam. The dependence originates from the fact that the Raman scattering from the nondiamond components in the films is almost completely depolarized while the scattering from the diamond components is strongly polarized. The results demonstrate the importance of taking polarization into account when attempting to use Raman spectroscopy in even a semiquantitative fashion for the assessment of the purity, perfection, and stress in chemical vapor deposition diamond films.

CONFOCAL RAMAN STRAIN MAPPING OF ISOLATED SINGLE CVD DIAMOND CRYSTALS

Abstract A number of isolated single CVD (chemical vapour deposition) diamond crystals were mapped using confocal Raman microscopy at a spatial resolution of 1 μm. Both square and triangular faceted crystals were included. It was found that, in general, regions of both types of crystals which corresponded to (111) growth sectors showed an overall shift of the diamond peak to low wavenumber, while the regions which corresponded to (100) growth sectors showed a shift to high wavenumber. This is interpreted as deriving from the presence of high concentrations of defects in the (111) growth sectors resulting in an overall tensile strain in these regions, and a compressive stress where these low density regions border the higher quality diamond from (100) growth sectors.