Rights: © 2011 American Physical Society (APS). This is the accepted version of the following article: Mäki, J.-M. & Tuomisto, Filip & Varpula, A. & Fisher, D. & Khan, R. U. A. & Martineau, P. M. 2011. Time Dependence of Charge Transfer Processes in Diamond Studied with Positrons. Physical Review Letters. Volume 107, Issue 21. 217403/1-5. ISSN 0031-9007 (printed). DOI: 10.1103/physrevlett.107.217403, which has been published in final form at http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.107.217403.

Defects causing colour in nitrogen-doped chemical vapour-deposited (CVD) diamond can adversely affect the exceptional optical, electronic and spintronic properties of the material. Several techniques were used to study these defects, namely optical absorption spectroscopy, thermoluminescence (TL) and electron paramagnetic resonance (EPR). From our studies, the defects causing colour in nitrogen-doped CVD diamond are clearly not the same as those causing similar colour in natural diamonds. The brown colour arises due to a featureless absorption profile that decreases in intensity with increasing wavelength, and a broad feature at 360 nm (3.49 eV) that scales in intensity with it. Another prominent absorption band, centred at 520 nm (2.39 eV), is ascribed to the neutral nitrogen-vacancy-hydrogen defect. The defects responsible for the brown colour possess acceptor states that are 1.5 eV from the valence band (VB) edge. The brown colour is removed by heat treatment at 1600 degrees C, whereupon new defects possessing shallow (<1 eV) trap states are generated.

We have developed a method called optical transient positron spectroscopy and apply it to study the optically induced carrier trapping and charge transfer processes in natural brown type IIa diamond. By measuring the positron lifetime with continuous and pulsed illumination, we present an estimate of the optical absorption cross section of the vacancy clusters causing the brown color. The vacancy clusters accept electrons from the valence band in the absorption process, giving rise to photoconductivity.

The zero-phonon line (ZPL) at 1.68 eV has been attributed to the negatively charged silicon split-vacancy center in diamond, (Si-V)(-), and has been extensively characterized in the literature. Computational studies have predicted the existence of the neutral charge state of the center, (Si-V)(0), and it has been experimentally observed using electron paramagnetic resonance (EPR). However, the optical spectrum associated with (Si-V)(0) has not yet been conclusively identified. In this paper the 1.31 eV band visible in luminescence and absorption is attributed to (Si-V)(0) using an approach which combines optical absorption and EPR measurements. The intensities of both 1.68 eV and 1.31 eV bands are found to increase in deliberately Si-doped chemical vapor deposition (CVD) grown diamond, and also after electron irradiation and annealing, suggesting the involvement of both Si and a vacancy in the centers. The 1.31 eV ZPL is unambiguously associated to Si by its shift to a lower energy when the dominant Si isotope is changed from Si-28 to Si-29. Charge transfer between (Si-V)(-) and (Si-V)(0) induced via ultraviolet photoexcitation or heating in the dark allows calibration factors relating the integrated absorption coefficient of their respective ZPLs to the defect concentration to be determined. Preferential orientation of (Si-V) 0 centers in CVD diamond grown on {110}-oriented diamond substrates is observed by EPR. The (Si-V)(0) centers are shown to grow predominantly into CVD diamond as complete units, rather than by the migration of mobile vacancies to substitutional Si (Si-S) atoms. Corrections for the preferential alignment of trigonal centers for quantitative analysis of optical spectra are proposed and the effect is used to reveal that the 1.31 eV ZPL arises from a transition between the (3)A(2g) ground state and (3)A(1u) excited state of (Si-V)(0). A simple rate equation model explains the production of (Si-V)(0) upon irradiation and annealing of Si-doped CVD diamond. In as-grown Si-doped diamond the (Si-V) defects only account for a fraction of the total silicon present; the majority being incorporated as SiS. The data show that both Si-S and (Si-V) are effective traps for mobile vacancies.

Optically detected X-ray absorption spectroscopy (ODXAS) and X-ray excited optical luminescence (XEOL) have been applied in parallel to the study of local bonding in luminescent diamonds. Imaging ODXAS and XEOL reveal correlations between lateral variation in colour (transmitted light and luminescence) and local carbon bonding in selected samples showing strong optical absorption contrast. Dark regions of the crystals viewed in transmitted light correlate with a lower total luminescence yield when excited with soft X-rays of photon energy ~280 eV. ODXAS reveals a higher proportion of sp2-bonded carbon species in darker (less luminescent) regions associated with graphitic micro-inclusions.

Single crystal CVD synthetic diamond samples grown on substrates close to (001) have been studied using X-ray topography, photoluminescence imaging and Nomarski microscopy. The substrates used for this study were polished up to 15 degrees from (001). Nomarski images of the final growth surface have been compared with X-ray section topographs and photoluminescence images, both sampling a plane close to the surface of a (010) cross-sectional slice. The photoluminescence images provide evidence of the direction of step flow growth which was compared with the surface morphology. Samples grown on substrates polished off-(001) about [010] by greater than 10 degrees exhibit regions possessing both [001] and [101] dislocations as a result of on-axis and off-axis step flow growth respectively. Our previously published model suggests relatively low angle surface inclinations arising from risers are needed to switch dislocation direction from [001] to [101] line direction, and core energies per unit length suggest a minimum riser angle of 10 degrees is required. Samples grown on substrates polished greater than 10 degrees from (001) exhibit dislocations with a [101] line direction consistent with the model. Below this angle the dislocations do not switch line direction. Possible mechanisms influencing dislocation line direction via off-axis growth are discussed.

Received 5 November 2010DOI:https://doi.org/10.1103/PhysRevB.82.249901©2010 American Physical Society

A defect involving both silicon and hydrogen has been characterized using multifrequency electron paramagnetic resonance. The defect, denoted WAR3, was observed in a single-crystal chemical-vapor deposition diamond, which was homoepitaxially grown on a {110}-oriented substrate and doped with isotopically enriched silicon (90% Si-29). The obtained data are explained by a silicon divacancy structure which is decorated by a hydrogen atom and is in the neutral charge state, (Si-V2:H)(0) (S=1/2). The experimentally derived Si-29 and H-1 hyperfine parameters are in agreement with values calculated using the spin-density-functional technique, ruling out a nonplanar structure. Defects are usually randomly oriented such that there is an equal probability for the symmetry axis of the defect to lie along each of the crystallographically equivalent directions. However, the WAR3 defect shows preferential alignment with respect to the {110} growth plane of the sample. Approximately four times as many WAR3 centers were aligned with their mirror planes lying perpendicular to the growth plane, compared to a statistical distribution. This indicates that the majority of WAR3 defects grew in as units rather than by the diffusion and aggregation of constituents. Analysis of the increase in the WAR3 concentration and the decrease in preferential alignment upon annealing the sample at 1400 degrees C shows that WAR3 can also be created post growth.

Homoepitaxial chemical vapour deposition (CVD) on high pressure high temperature (HPHT) synthetic diamond substrates allows production of diamond material with controlled point defect content. In order to minimize the extended defect content, however, it is necessary to minimize the number of substrate extended defects that reach the initial growth surface and the nucleation of dislocations at the interface between the CVD layer and its substrate. X-ray topography has indicated that when type IIa HPHT synthetic substrates are used the density of dislocations nucleating at the interface can be less than 400 cm-2. X-ray topography, photoluminescence imaging and birefringence microscopy of HPHT grown synthetic type IIa diamond clearly show that the extended defect content is growth sector dependent. <111> sectors contain the highest concentration of both stacking faults and dislocations but <100> sectors are relatively free of both. It has been shown that HPHT treatment of such material can significantly reduce the area of stacking faults and cause dislocations to move. This knowledge, coupled with an understanding of how growth sectors develop during HPHT synthesis, has been used to guide selection and processing of substrates suitable for CVD synthesis of material with high crystalline perfection and controlled point defect content.

In this paper we report on the positron lifetime results obtained for brown and colourless natural diamond. Optical effects of the observed vacancy defects in brown, high pressure, high temperature (HPHT) treated colourless and naturally colourless type IIa diamond samples were studied by combining the positron measurement with monochromatic illumination. Brown diamond was found to contain optically active vacancy clusters (40–60 missing atoms) strongly correlated with the optical absorption spectra. The optical activity of these vacancy clusters is manifested by a photo-excitation induced change of charge from neutral to negative. The clusters gradually disappear during the HPHT treatments, and the samples treated at 2500 °C resemble colourless samples optically and show similar positron lifetimes. The results show that the brown colour originates from the vacancy clusters and that their removal by the HPHT treatment causes the loss of coloration.

Single crystal CVD synthetic diamond samples grown on (001) substrates have been studied using X-ray topography, photoluminescence imaging, cathodoluminescence imaging and transmission electron microscopy. X-ray section topographs sampling a plane very close to the surface of a (010) cross-sectional slice show contrast that is closely correlated with photoluminescence image contrast relating to differences in incorporation of point defects on step risers and terraces during growth. The point defect content is too low to influence X-ray topography contrast and therefore the correlation suggests a difference between the dislocation content for terrace and riser growth. Dislocations imaged in cathodoluminescence have been observed to follow paths that are strongly influenced by step flow during growth. Transmission electron micrographs show [001] dislocations with sections of < 110 > 60 degrees or edge dislocations in configurations consistent with formation as a result of step flow growth. It is proposed that a passing riser can cause dislocations to switch from [001] to [101] line direction when this minimises the increase in the energy associated with the dislocation as riser growth proceeds. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim