Double-crystal diffractometric and topographic studies of (111) oriented synthetic diamonds

Abstract

Synchrotron x-radiation in a double-crystal geometry has been used for topography and rocking-curve measurements to study imperfections in single-crystal HP/HT synthetic diamonds grown from (111) seeds by the reconstitution technique. The influence that clusters of small opaque particles (sometimes called `pin-point-like inclusions') have upon lattice parameter differences, as well as upon relative lattice tilts within each sample, was of particular interest in this study. Rocking curves measured with a large beam diameter (illuminating entire specimens) gave widths (FWHM) in the range 5 - 30 seconds of arc in a non-dispersive Bragg - Bragg diffraction geometry in which good diamond crystals give and for which the calculated theoretical minimum width is under . Rocking curves were also recorded for small regions of each crystal, using a translatable pin-hole, and these showed that some diamonds had relatively narrow rocking curves in selected areas (of diameter). Epochs of crystal growth were recorded in series of x-ray topographs, taken at intervals of 2.5 seconds of arc. Such series were taken for all four principal azimuths (, 90, 180 and ) of the incident beam by rotating the sample about the normal to the Bragg plane. This procedure yielded important information about lattice parameter distribution and lattice tilt across entire samples. For the samples heavily populated with pin-point-like inclusions it was concluded that there is a possible slight variation in lattice parameter values in some regions of the crystal but most of it falls below the adopted angular step resolution, although better accuracy can be achieved. Relative tilt measurements revealed that there are large misorientations of those parts of the crystal which are close to external outcrops of {111} boundaries. In this case of (111) oriented diamond samples, three separate regions with different tilts can be attributed to the three parts of the crystal close to optically visible {111} growth-sector boundaries.

Goniometric and topographic characterization of synthetic IIa diamonds

“…The sensitivity of the topographs taken at the flank is considered to be less than θ = 0.2 for a tilt of the lattice plane and less than δd/d = cot θ B θ = 2.9 × 10 −6 for a change in the lattice constant, where θ B is the Bragg angle. In the present measurements there was no evidence of growth sector nor growth banding, which had been observed in the x-ray topography of previous reports (Wierzchowski et al 1991, Kowalski et al 1996, Moore et al 1999, Hoszowska et al 2001. This is consistent with the low nitrogen concentration (less than 0.1 ppm) of type IIa diamonds (Toda et al 1997), which is a major reason for fluctuations in the lattice constant.…”

On the role of nitrogen in stiffening the diamond structure

“…The {111} diamond specimen, at a variable Bragg angle close to 44.42 , diffracted the X-rays into a scintillation detector (for the rocking curve) or onto photographic plate (for X-ray topographs), using the 333 reflection. This geometry is convenient because it is nondispersive, as the incident and doubly diffracted X-rays are almost parallel (Kowalski et al, 1996).…”

An automatic optical imaging system for birefringent media

“…However, it is well known th a t it does in fact show a weak straininduced birefringence, associated particularly with growth sector boundaries, dis locations and defects. Figures 4a and 6 show the |sin<5| and orientation images of a synthetic diamond (this sample, labelled HS, was also the subject of an X-ray topography study by Kowalski et al (1996)) looking down the [111] direction. It is immediately apparent th a t there is an approximate three-fold symmetry in the | sin6| image, consistent with the symmetry of this direction in the crystal.…”