HPHT growth and x-ray characterization of high-quality type IIa diamond

Abstract

The trend in synchrotron radiation (x-rays) is towards higher brilliance. This may lead to avery high power density, of the order of hundreds of watts per square millimetre at the x-rayoptical elements. These elements are, typically, windows, polarizers, filters andmonochromators. The preferred material for Bragg diffracting optical elements at present issilicon, which can be grown to a very high crystal perfection and workable sizeas well as rather easily processed to the required surface quality. This allowsx-ray optical elements to be built with a sufficient degree of lattice perfectionand crystal processing that they may preserve transversal coherence in the x-raybeam. This is important for the new techniques which include phase-sensitiveimaging experiments like holo-tomography, x-ray photon correlation spectroscopy,coherent diffraction imaging and nanofocusing. Diamond has a lower absorptioncoefficient than silicon, a better thermal conductivity and lower thermal expansioncoefficient which would make it the preferred material if the crystal perfection (bulkand surface) could be improved. Synthetic HPHT-grown (high pressure, hightemperature) type Ib material can readily be produced in the necessary sizes of 4–8 mmsquare and with a nitrogen content of typically a few hundred parts per million.This material has applications in the less demanding roles such as phase plates:however, in a coherence-preserving beamline, where all elements must be of thesame high quality, its quality is far from sufficient. Advances in HPHT synthesismethods have allowed the growth of type IIa diamond crystals of the same sizeas type Ib, but with substantially lower nitrogen content. Characterization ofthis high purity type IIa material has been carried out with the result that thecrystalline (bulk) perfection of some of the HPHT-grown materials is approaching thequality required for the more demanding applications such as imaging applicationsand imaging applications with coherence preservation. The targets for furtherdevelopment of the type IIa diamond are size, crystal perfection, as measuredby the techniques of white beam and monochromatic x-ray diffraction imaging(historically called x-ray topography), and also surface quality. Diamond platesextracted from the cubic growth sector furthest from the seed of the new lowstrain material produces no measurable broadening of the x-ray rocking curvewidth. One measures essentially the crystal reflectivity as defined by the intrinsicreflectivity curve (Darwin curve) width of a perfect crystal. In these cases themore sensitive technique of plane wave topography has been used to establisha local upper limit of the strain at the level of an ‘effective misorientation’ of10−7 rad.