J. Rosa

Dominant defect levels in diamond thin films: A photocurrent and electron paramagnetic resonance study

Characteristic features in photocurrent (PC) and electron paramagnetic resonance (EPR) spectra are discussed and attributed to main defects in the gap of optical-quality chemical vapor deposited diamond. A shoulder in the PC spectra with an onset at about 2.2 eV is attributed to the single-substitutional nitrogen defect (EPR P1 resonance at g=2.0024). A second feature in the PC spectra with an onset of about 1.3 eV is observed on “as-grown” samples with a hydrogen terminated surface. The defect level associated with this feature is hydrogen related, and this defect disappears after oxidation of the diamond sample surface. The EPR g=2.0028, which was also suggested to be H-related, is discussed.

Quantitative study of Raman scattering and defect optical absorption in CVD diamond films

Abstract Raman spectra and sub-bandgap optical absorption measured by photothermal deflection spectroscopy (PDS) of CVD diamond films are compared and discussed in terms of the concentration of the non-diamond component. A reasonably good one-to-one correspondence is found, implying the plausibility of the recent estimate of the Raman cross-section for a-C as well as the capability of the absorption spectroscopy to identify the presence of non-diamond phase with a high sensitivity.

Photoionization cross-section of dominant defects in CVD diamond

Abstract Main defects in the gap of free standing optical-quality CVD diamond have been investigated with the help of electron paramagnetic resonance (EPR) and the constant photocurrent method (CPM). The EPR and CPM measurements reveal well-defined substitutional nitrogen defect state in the gap with a photoionization energy Ei=2.2xa0eV. Another set of defect states comes from the presence of hydrogen. CPM shows a significant effect of the hydrogenation, which generates a bulk defect with a photoionization energy Ei=1.2xa0eV in the gap of CVD diamond. Annealing studies after hydrogenation show that this defect is not responsible for p-type surface conductivity of diamond, because it is thermally stable even at 930xa0°C but the surface conductivity vanishes at much lower temperatures. The EPR resonances in the g≅2.0028 region are also investigated as a function of hydrogenation and oxidation.

Fourier transform photocurrent spectroscopy of dopants and defects in CVD diamond

Fourier-transform photocurrent spectroscopy (FTPS) was used as a very sensitive spectroscopic method to detect shallow and deep impurities (dopants) in CVD diamond layers. Detailed study of experimental conditions (temperature, frequency, electric field, bias light, surface conditions) was performed. Residual boron contamination was detected in many samples, phosphorus spectra were measured in P doped epitaxial layers. Anomalous (opposite) temperature dependence of the defect level with a threshold approximately 0.9 eV was detected and possible explanation of this effect was discussed.

Investigation of n-doping in CVD diamond using gap states spectroscopy☆

In-situ doping of CVD diamond with N and Li using N2 and a Li-containing organometallic precursor was investigated. Photothermal deflection spectroscopy (PDS) was used to study optical transitions from localized electronic states in the bandgap of CVD diamond resulting from the presence of extrinsic and intrinsic impurities. The advantage of PDS is its sensitivity, which allows very low defect levels to be investigated even in thin diamond films. The optical absorption coefficient α of N-doped films (100 ppm N2 in the gas phase) shows absorption bands characteristic of Ib diamond. When a Li-containing precursor is used, a deep defect state around 1.5 eV, which has not been reported previously, is observed. Optical transitions due to this defect state are superimposed on the characteristic background absorption present in all CVD diamond films.

SUBGAP OPTICAL-ABSORPTION IN CVD DIAMOND FILMS DETERMINED FROM PHOTOTHERMAL DEFLECTION SPECTROSCOPY

Abstract Photothermal deflection spectroscopy (PDS) was applied for the first time to measure the subgap optical absorption, induced by defects in free-standing diamond films. Compared with the optical transmission measurement, PDS is far more sensitive and even a defect concentration below parts per million level can be identified. Moreover, the elastic light scattering influences the measured data significantly less. The optical absorption measurement shows an optical transition which starts at about 1 eV, independently of the sp2 content in the film. This level agrees well with the 1 eV activation energy of the dark conductivity. Additionally, an optical transition starting at about 3 eV is observed for white colourless films.

Fourier-transform photocurrent spectroscopy of defects in CVD diamond layers

Fourier-transform photocurrent spectroscopy has been used for the first time to study defects in nominally undoped CVD diamond layers. The hydrogenated surface of diamond (as grown or post-hydrogenated) has been annealed in air and changes in surface conductivity and photoconductivity have been monitored as a function of temperature and time. The reduction of surface conductivity with annealing led to an enhancement of photosensitivity and enabled us to study the spectral distribution of the photoionization cross section of shallow and deep defects in the gap over several orders of magnitude in the temperature range of 77-400 K. The photo-Hall effect was used for the determination of the sign of photogenerated carriers. An anomalous temperature dependence of the acceptor defect level was detected. A residual boron contamination was observed in one layer.

Characteristic defects in CVD diamond: optical and electron paramagnetic resonance study

Limburgs Univ Ctr, Div Mat Phys, Inst Mat Res, B-3590 Diepenbeek, Belgium. Katholieke Univ Leuven, Lab Halfgeleiderfys, B-3001 Heverlee, Belgium. Acad Sci Czech Republ, Inst Phys, Prague 6, Czech Republic.Nesladek, M, Limburgs Univ Ctr, Div Mat Phys, Inst Mat Res, Univ Campus,Wetenschapspk 1, B-3590 Diepenbeek, Belgium.

Simultaneous characterization of defect states in CVD diamond by PDS, EPR, Raman and photocurrent spectroscopies

Abstract The assessment of the quality of CVD diamond films, a very important issue for the preparation of electronics-quality material, has been approached with the help of Raman, luminescence and photocurrent spectroscopy, photothermal deflection spectroscopy (PDS) and electron paramagnetic resonance (EPR). In heteroepitaxial diamond films deposited on silicon wafers by plasma-enhanced chemical vapour deposition (PECVD) we have observed the following defects: sp2 bonded carbon, carbon dangling bonds, nitrogen and silicon. Raman scattering is sensitive to sp2 bonded carbon, PDS sees total absorption due to sp2 bonded carbon, dangling bonds and nitrogen. Typically, the graphitic inclusions between the grains dominate the PDS spectrum. EPR detects carbon dangling bonds (vacancy-like defect, with possible H involvement) with g=2.0028 and paramagnetic form of nitrogen with g=2.0024. A strong photoluminescence peak at 1.68 eV reflects the Si contamination. Nitrogen-related transitions were detected by photocurrent measurement, with a characteristic threshold at about 2.1 eV, and seen in luminescence. A threshold energy of approximately 1 eV was attributed to the carbon dangling bond defect, which was observed also by EPR. This g=2.0028 signal decreases with an increase in material quality, as determined by Raman spectra.

EPR study of preferential orientation of crystallites in N-doped high quality CVD diamond

The directions of preferential growth of free-standing optical-quality CVD diamond wafers have been investigated with the help of electron paramagnetic resonance (EPR). EPR signals of the well-known P1 centre (substitutional nitrogen) have been used as a probe. A computer simulation of EPR spectra of preferentially oriented polycrystalline material allows the estimation of the real orientation and spatial distribution of crystallographic axes of crystallites in the samples oriented preferentially in the [110] direction.