A.P.W. Makepeace

On the dilatation of synthetic type Ib diamond by substitutional nitrogen impurity

Sequences of high Bragg-angle (0B = 74°) double-crystal X-ray topographs taken at the SRS (Daresbury, U.K.) have yielded precise measurements of lattice parameter differences between growth sectors of different crystallographic forms in a large undoped synthetic diamond whose type Ib infrared absorption spectrum (principal peak at 1130 cm-1) indicated atomically dispersed nitrogen, singly substituting for carbon, as the only detectable impurity. The plate-shaped specimen, polished parallel to (110), 5.0 x 3.2 mm 2 in area, 0.7 mm thick, possessed an unusually well developed (110) growth sector containing nitrogen impurity concentration of only ca. 10-6, which served as an internal standard of pure-diamond lattice parameter with which lattice parameters of nitrogen-containing growth sectors were compared. The specimen’s suitability for precision diffractometry was checked by comprehensive tests using optical microscope techniques, cathodoluminescence and single-crystal X-ray topography. The double-crystal combination was silicon reference crystal, asymmetric 175 reflection, with diamond specimen symmetrical 440 reflection. The principal measurement was the increase of the lattice parameter, a0, of the (111) growth sector (nitrogen content 88 + 7 parts per 106 atomic) relative to that of the (110) sector: Aa0/ a0 = 1.18 + 0.07 x 10-5. In terms of measured infrared absorption coefficient at 1130 cm-1, this gives Aa0/a0 = (2.95 + 0.27) x 10-6 [p(1130 cm-1)/cm-1], which is believed to hold for growth sectors of all crystallographic forms. Combination with the nitrogen assay findings of Woods, van Wyk & Collins (Phil. Mag. B 62. 589-595 (1990)) provides a direct relation to cN, the fractional atomic concentration of substitutional nitrogen, as Aa0/ a0 = (0.14 + 0.02)cN, which indicates that the effective volume of a single substitutional nitrogen atom in diamond is 1.41 +0.06 times that of the carbon atom it replaces. This substantial dilatation conflicts with several models for the substitutional nitrogen structure.

X-ray topographic studies and measurement of lattice parameter differences within synthetic diamonds grown by the reconstitution technique

A 4x4x1.5 mm3 cuboctahedral diamond and two 0.7 mm thick slabs cut from a truncated octahedral diamond grown by the reconstitution technique were studied in different double-crystal arrangements with both conventional and synchrotron X-ray sources. The back-reflection double crystal topographs of large polished {001} faces intersecting different growth sectors, together with cathodoluminescence patterns, allowed identification of these sectors. A double-crystal arrangement, employing the 1325 quartz reflection matching the symmetrical 004 diamond reflection in CuKα1 radiation, was used for measurement of lattice parameter differences with an accuracy of one and a half parts per million. The simultaneous investigation by means of Lang projection and section topography provided complementary information about the crystallographic defects and internal structures of growth sectors. Observation of the cuboctahedral diamond with a filter of peak transmittance at 430 nm revealed a “Maltese cross” growth feature in the central (001) growth sector, which also affected the birefringence pattern. However, this feature only very slightly affected the double-crystal topographs.

On the Variation of X-ray Diffraction Contrast with Wavelength: A Study with Synchrotron Radiation

Diffraction contrast phenomena on X-ray topographs taken with continuous-spectrum synchrotron radiation have been studied at wavelengths of 0.057, 0.064, 0.071, 0.100, 0.154, 0.206 and 0.250u2005nm. The specimen was a polished plate of natural diamond with surfaces parallel to (110), ½ mm thick. Using the {bar 1}11 reflection and a stored electron beam energy of 1.8u2005GeV all topographs (except that taken with λ = 0.25u2005nm) were harmonic free. The specimen exhibited mixed-habit growth, containing sectors of normal faceted {111} growth and sectors of non-faceted `cuboid growth in which growth-surface orientation was variable and only approximately parallel to {100}. Prior to X-ray topography the specimen had received localized damage from implantation with fluorine ions of 17u2005MeV energy. Features whose variation with wavelength was studied included (1) the relative strengths of integrated reflections from {111} and `cuboid growth sectors, (2) the intensity of `spike disorder diffuse reflections relative to sharp Bragg reflections, (3) contrast from inclusions, polishing striae and fracture damage and (4) lattice bending and diffraction contrast at the sites of fluorine ion implantation. Theoretical predictions of the wavelength variation of the intensity of the diffuse reflection images and of contrast due to resolved defects showed good agreement with the observations.

Studies of stacking-fault contrast by synchrotron X-ray section topography

The periodicities, profiles and contrast of stacking-fault fringes observed in high-resolution synchrotron X-ray section topographs have been comprehensively studied with controlled changes of diffraction parameters such as wavelength and polarization mode, which allowed observations under diffraction conditions inaccessible with conventional sources of characteristic radiation. Comparisons with simulated patterns and computed fringe profiles are presented. The fringe patterns were generated by a single intrinsic stacking fault of area 1.3u2005mm2 contained in a polished parallelepiped specimen of natural diamond unusually free from lattice defects producing long-range strains. The principal X-ray wavelengths employed were 0.08, 0.10, 0.12 and 0.15u2005nm, providing values of μ0t0 ranging from 0.25 to 1.5 (μ0 = normal photoelectric absorption coefficient, t0 = specimen thickness). Fringe orders up to about 100 were observed at the longer wavelengths. Controlled fractional changes of Pendellosung interference order were obtained by wavelength fine-tuning (Δλ ≃ 10−3 nm). Matching of simulations with the resultant patterns is discussed in detail. Fringe patterns were recorded with pure σ, pure π and mixed polarization modes. Change of polarization mode was employed in order to vary the ratio of anomalous to normal absorption without having to change other diffraction variables. Particular attention was devoted to exploring the range of diffraction conditions under which fault vector sign could be experimentally determined from the sign of `first-fringe contrast, and to examining how this range can be extended with the aid of comparison with pattern simulations.

Combining optical microscopy, X-ray microradiography and X-ray topography in the study of individual CVD diamond crystallites

Abstract Procedures are described for matching optical microscopic images of individual crystallites in a polycrystalline diamond film with their corresponding radiographic and diffraction topographic images. Choosing appropriate diffraction geometry and using well-collimated X-ray illumination, such as is provided by synchrotron radiation sources, the variation of lattice perfection within the crystallite volume can be investigated, from its root outwards towards its terminating facets. Misorientations within individual crystallites can be measured down to a milliradian or less by analysis of their X-ray topographic images.

Synchrotron X-ray microdiffraction measurements of lattice tilts within CVD diamond crystallites terminated by cathodoluminescent blue-cross-quartered cube facets

Abstract A [001]-textured microwave plasma CVD diamond film contains crystallites with large square (001) facets, edge lengths up to ∼100xa0μm, that have attracted attention because of an unusual highly symmetric step pattern on these (001) facets. This pattern consists of arrays of steps (heights in the 10–20-nm range) facing inwards from the facet corners towards the facet centre, thereby dividing the facet surface into quadrants with a rough mirror symmetry in (110) and (1 1 0); and these dividing mirror planes are also loci of blue–violet cathodoluminescent emission, the “Blue-Cross”. The present work has applied synchrotron X-ray microradiography as a high-contrast imaging technique for displaying size and habit distribution in the crystallite population, and microdiffraction and topography for measuring misorientations within chosen individual crystallites. Studies of the microscopically observed step patterns, surface tilt profiles and X-ray diffraction pattern fine structure are detailed for two crystallites, one being in the largest size bracket, facet diameter 91xa0μm, and the other of a more common size, facet diameter 53xa0μm. X-ray diffraction data corroborated and amplified findings made in previous work by electron channelling patterns. The X-ray diffraction patterns strikingly demonstrated that the crystal lattice under the (001) facet has a fourfold split, the normals to the (001) lattice planes in each quadrant being tilted outward towards the facet corner cube direction of the quadrant concerned. Approximate values of the mean outward tilt of the quadrants, relative to the mean [001] axis of the crystallite, were 0.85° in the larger crystallite and 0.4° in the smaller crystallite.

Optical microscopic, synchrotron X-ray topographic and reticulographic study of homoepitaxial CVD diamond

Abstract Surface topography and crystal-lattice perfection of homoepitaxial layers deposited by microwave plasma CVD on (0xa00xa01) and near-(0xa00xa01) facets polished on HPHT synthetic diamond are described. Optical micrographic techniques included birefringence, Nomarski and 2-beam interference. The synchrotron X-ray experiments comprised Laue topography plus a recently developed sensitive misorientation-measuring technique, reticulography. Two special circumstances enhanced information yield from the experiments. First, the substrate crystal was unusually strain-free and had a very low dislocation content. Second, epilayer growth had taken place in two stages, depositing thicknesses of 10xa0μm and 30–34xa0μm, respectively. This double deposition complicated the observations, but added features of scientific and practical interest. Epilayer cracking finally present had occurred almost entirely before the second growth stage. With assistance from quantitative data provided by reticulography, the X-ray diffraction properties of the substrate and epilayers are analysed. Lattice misorientations on the untreated lower surface of the substrate were only ∼1 arcsec except close to growth-sector boundaries and dislocation outcrops. The final epilayer growth surface above areas where cracking in the first epilayer was absent or sparse exhibited near-perfect-crystal diffraction behaviour.

Synchrotron spike topography of natural Type IA diamond

Abstract Natural Type Ia diamond contains impurity platelet precipitates of only a few atoms thick and of diameters typically 10–100 nm, lying on {100} planes. Such platelets produce 〈100〉 spikes in diffraction associated with some reciprocal lattice points. They are very weak and long exposures are needed to record them. Hitherto, X-ray topographic studies of reciprocal lattice spikes in diamond have concentrated upon the strong 111 reflexion, which is surrounded by six spikes; but here we have studied with powerful synchrotron radiation the two spikes emanating from the weaker 331 reflexion. This has the advantage that only one spike at a time, rather than three, is intersected by the Ewald sphere; and consequently, only one image is recorded, rather than three (possibly overlapping) ones. We have discovered that the intensity of these spikes follows an inverse 3 2 power law with distance from the 331 reciprocal lattice point, whereas an inverse square law had been expected. Associated with platelets are voidites. These are {111} faceted defects of low electron density; and in certain diamonds, sheets of voidites replace platelets. Spikes in the 〈111〉 directions from the 111 reciprocal lattice point appear to arise from voidite sheets, and we have observed weak 〈111〉 spikes emanating from the 331 reciprocal lattice point.

Morphological and synchrotron X-ray microdiffraction studies of large columnar CVD diamond crystallites

Optical microscopic and goniometric measurements were combined with microradiography, diffraction-pattern analysis and topography to study a 2u2005mm thick [001]-texture CVD (chemical vapour deposition) diamond film that had developed a coarse-grained structure composed of separate columnar crystallites. Individual columns were capped by large (001) facets, with widths up to 0.5u2005mm, and which were smooth but not flat, whereas the column sides were morphologically irregular. The refractive deviation of X-rays transmitted through the crystallites was exploited for delineating facet edges, thereby facilitating the controlled positioning of small-cross-section X-ray beams used for recording diffraction patterns from selected volumes in two representative crystallites. Their structure consisted of a [001]-axial core column surrounded by columns in twin orientation with respect to the core. The diamond volume directly below the (001) facets was free from low-angle boundaries, and no dislocation outcrops on the facets were detected. Significant elastic deformation of this volume was only present close to the facet periphery, where misorientations reached a few milliradians. Lattice imperfection was high in the twins, with ∼1° misorientations.

X-Ray Studies of Synthetic Radiation-Counting Diamonds

Synthetic diamonds with a nitrogen content less than 100 ppm may be used as radiation dosemeters in a conduction counting mode, and are especially useful in medical applications. Crystal imperfections, revealed by x-ray diffraction topography, were found to affect counting performance. The best quality diamond gave the highest photocurrent (500 nA at 50 V mm-1 and 2.75 Gy min-1). Diamonds containing dislocations had lower photocurrents but had the advantage of shorter settling times (seconds rather than minutes). Placing contacts on two opposite cube (100) faces gave a higher photocurrent than on a pair of octahedral (111) faces. Higher photocurrents were also achieved when the majority of dislocations were perpendicular rather than parallel, to the electric field. Some recommendations for selecting synthetic diamonds for dosemeters are given.