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

Optical Properties of Bulk Single-Crystal Diamonds at 80-1200 K by Vibrational Spectroscopic Methods

Bulk diamonds show great potential for optical applications such as for use in infrared (IR) windows and temperature sensors. The development of optical-grade bulk diamond synthesis techniques has facilitated its extreme applications. Here, two kinds of bulk single-crystal diamonds, a high-pressure and high-temperature (HPHT) diamond and a chemical vapor deposition (CVD) diamond, were evaluated by Raman spectroscopy and Fourier Transform Infra-Red (FTIR) spectroscopy at a range of temperatures from 80 to 1200 K. The results showed that there was no obvious difference between the HPHT diamond and the CVD diamond in terms of XRD and Raman spectroscopy at 300-1200 K. The measured nitrogen content was ~270 and ~0.89 ppm for the HPHT diamond and the CVD diamond, respectively. The moderate nitrogen impurities did not significantly affect the temperature dependence of Raman spectra for temperature-sensing applications. However, the nitrogen impurities greatly influence FTIR spectroscopy and optical transmittance. The CVD diamond showed higher transmittance, up to 71% with only a ~6% drop at temperatures as high as 873 K. This study shows that CVD bulk diamonds can be used for IR windows under harsh environments.

Diamond channel-cut crystals for high-heat-load beam-multiplexing narrow-band X-ray monochromators

Next-generation high-brilliance X-ray photon sources call for new X-ray optics. Here we demonstrate the possibility of using monolithic diamond channel-cut crystals as high-heat-load beam-multiplexing narrow-band mechanically stable X-ray monochromators with high-power X-ray beams at cutting-edge high-repetition-rate X-ray free-electron laser (XFEL) facilities. The diamond channel-cut crystals fabricated and characterized in these studies are designed as two-bounce Bragg reflection monochromators directing 14.4 or 12.4 keV X-rays within a 15 meV bandwidth to ⁵⁷Fe or ⁴⁵Sc nuclear resonant scattering experiments, respectively. The crystal design allows out-of-band X-rays transmitted with minimal losses to alternative simultaneous experiments. Only ≲2% of the incident ∼100 W X-ray beam is absorbed in the 50 µm-thick first diamond crystal reflector, ensuring that the monochromator crystal is highly stable. Other X-ray optics applications of diamond channel-cut crystals are anticipated.

Structure Investigations of Islands with Atomic-Scale Boron-Carbon Bilayers in Heavily Boron-Doped Diamond Single Crystal: Origin of Stepwise Tensile Stress

The detailed studies of the surface structure of synthetic boron-doped diamond single crystals using both conventional X-ray and synchrotron nano- and microbeam diffraction, as well as atomic force microscopy and micro-Raman spectroscopy, were carried out to clarify the recently discovered features in them. The arbitrary shaped islands towering above the (111) diamond surface are formed at the final stage of the crystal growth. Their lateral dimensions are from several to tens of microns and their height is from 0.5 to 3 μm. The highly nonequilibrium conditions of crystal growth enhance the boron solubility and, therefore, lead to an increase of the boron concentrations in the islands on the surface up to 10²² cm^-3, eventually generating significant stresses in them. The stress in the islands is found to be the volumetric tensile stress. This conclusion is based on the stepwise shift of the diamond Raman peak toward lower frequencies from 1328 to 1300 cm^-1 in various islands and on the observation of the shift of three low-intensity reflections at 2-theta Bragg angles of 41.468°, 41.940° and 42.413° in the X-ray diffractogram to the left relative to the (111) diamond reflection at 2theta = 43.93°. We believe that the origin of the stepwise tensile stress is a discrete change in the distances between boron-carbon layers with the step of 6.18 Å. This supposition explains also the stepwise (step of 5 cm^-1) behavior of the diamond Raman peak shift. Two approaches based on the combined application of Raman scattering and X-ray diffraction data allowed determination of the values of stresses both in lateral and normal directions. The maximum tensile stress in the direction normal to the surface reaches 63.6 GPa, close to the fracture limit of diamond, equal to 90 GPa along the [111] crystallographic direction. The presented experimental results unambiguously confirm our previously proposed structural model of the boron-doped diamond containing two-dimensional boron-carbon nanosheets and bilayers.

Towards high-quality nitrogen-doped diamond single crystals for X-ray optics

In this manuscript, characterization of single-crystalline (111) plates prepared from type-Ib diamonds with a nitrogen content of 100-150 ppm by means of high-resolution rocking-curve imaging (RCI) is reported. Contrary to common opinion regarding the intrinsically poor diffraction quality of type-I diamonds, RCI showed the presence of nearly defect-free areas of several millimetres squared in the central part of the diamond plates. The observed broadening of the rocking curves is a result of the cutting and polishing processes, causing strains around the edges of the plates and rare defects. An improvement of the preparation technique will thus allow single-crystalline diamond plates to be made for Laue and Bragg monochromators and beam splitters from type-Ib material with areas large enough to be used as optical elements at fourth-generation synchrotron facilities.

Small Bragg-plane slope errors revealed in synthetic diamond crystals

Wavefront-preserving X-ray diamond crystal optics are essential for numerous applications in X-ray science. Perfect crystals with flat Bragg planes are a prerequisite for wavefront preservation in Bragg diffraction. However, this condition is difficult to realize in practice because of inevitable crystal imperfections. Here, X-ray rocking curve imaging is used to study the smallest achievable Bragg-plane slope errors in the best presently available synthetic diamond crystals and how they compare with those of perfect silicon crystals. It is shown that the smallest specific slope errors in the best diamond crystals are about 0.08 (3) µrad mm^-2. These errors are only 50% larger than the 0.05 (2) µrad mm^-2 specific slope errors measured in perfect silicon crystals. High-temperature annealing at 1450°C of almost flawless diamond crystals reduces the slope errors very close to those of silicon. Further investigations are in progress to establish the wavefront-preservation properties of these crystals.

X-ray Based in Situ Investigation of Silicon Growth Mechanism Dynamics—Application to Grain and Defect Formation

To control the final grain structure and the density of structural crystalline defects in silicon (Si) ingots is still a main issue for Si used in photovoltaic solar cells. It concerns both innovative and conventional fabrication processes. Due to the dynamic essence of the phenomena and to the coupling of mechanisms at different scales, the post-mortem study of the solidified ingots gives limited results. In the past years, we developed an original system named GaTSBI for Growth at high Temperature observed by Synchrotron Beam Imaging, to investigate in situ the mechanisms involved during solidification. X-ray radiography and X-ray Bragg diffraction imaging (topography) are combined and implemented together with the running of a high temperature (up to 2073 K) solidification furnace. The experiments are conducted at the European Synchrotron Radiation Facility (ESRF). Both imaging techniques provide in situ and real time information during growth on the morphology and kinetics of the solid/liquid (S/L) interface, as well as on the deformation of the crystal structure and on the dynamics of structural defects including dislocations. Essential features of twinning, grain nucleation, competition, strain building, and dislocations during Si solidification are characterized and allow a deeper understanding of the fundamental mechanisms of its growth.

Mapping of individual dislocations with dark-field X-ray microscopy

This article presents an X-ray microscopy approach for mapping deeply embedded dislocations in three dimensions using a monochromatic beam with a low divergence. Magnified images are acquired by inserting an X-ray objective lens in the diffracted beam. The strain fields close to the core of dislocations give rise to scattering at angles where weak beam conditions are obtained. Analytical expressions are derived for the image contrast. While the use of the objective implies an integration over two directions in reciprocal space, scanning an aperture in the back focal plane of the microscope allows a reciprocal-space resolution of ΔQ/Q < 5 × 10−5 in all directions, ultimately enabling high-precision mapping of lattice strain and tilt. The approach is demonstrated on three types of samples: a multi-scale study of a large diamond crystal in transmission, magnified section topography on a 140 µm-thick SrTiO3 sample and a reflection study of misfit dislocations in a 120 nm-thick BiFeO3 film epitaxially grown on a thick substrate. With optimal contrast, the half-widths at half-maximum of the dislocation lines are 200 nm.

Large-surface-area diamond (111) crystal plates for applications in high-heat-load wavefront-preserving X-ray crystal optics

Fabrication and results of high-resolution X-ray topography characterization of diamond single-crystal plates with large surface area (10 mm × 10 mm) and (111) crystal surface orientation for applications in high-heat-load X-ray crystal optics are reported. The plates were fabricated by laser-cutting of the (111) facets of diamond crystals grown using high-pressure high-temperature methods. The intrinsic crystal quality of a selected 3 mm × 7 mm crystal region of one of the studied samples was found to be suitable for applications in wavefront-preserving high-heat-load crystal optics. Wavefront characterization was performed using sequential X-ray diffraction topography in the pseudo plane wave configuration and data analysis using rocking-curve topography. The variations of the rocking-curve width and peak position measured with a spatial resolution of 13 µm × 13 µm over the selected region were found to be less than 1 µrad.

Diamond drumhead crystals for X-ray optics applications

Thin (<50 µm) and flawless diamond single crystals are essential for the realization of numerous advanced X-ray optical devices at synchrotron radiation and free-electron laser facilities. The fabrication and handling of such ultra-thin components without introducing crystal damage and strain is a challenge. Drumhead crystals, monolithic crystal structures composed of a thin membrane furnished with a surrounding solid collar, are a solution ensuring mechanically stable strain-free mounting of the membranes with efficient thermal transport. Diamond, being one of the hardest and most chemically inert materials, poses significant difficulties in fabrication. Reported here is the successful manufacture of diamond drumhead crystals in the [100] orientation using picosecond laser milling. Subsequent high-temperature treatment appears to be crucial for the membranes to become defect free and unstrained, as revealed by X-ray topography on examples of drumhead crystals with a 26 µm thick (1 mm in diameter) and a 47 µm thick (1.5 × 2.5 mm) membrane.

Demonstration of simultaneous experiments using thin crystal multiplexing at the Linac Coherent Light Source

Multiplexing of the Linac Coherent Light Source beam was demonstrated for hard X-rays by spectral division using a near-perfect diamond thin-crystal monochromator operating in the Bragg geometry. The wavefront and coherence properties of both the reflected and transmitted beams were well preserved, thus allowing simultaneous measurements at two separate instruments. In this report, the structure determination of a prototypical protein was performed using serial femtosecond crystallography simultaneously with a femtosecond time-resolved XANES studies of photoexcited spin transition dynamics in an iron spin-crossover system. The results of both experiments using the multiplexed beams are similar to those obtained separately, using a dedicated beam, with no significant differences in quality.

Optics for coherent X-ray applications

Developments of X-ray optics for full utilization of diffraction-limited storage rings (DLSRs) are presented. The expected performance of DLSRs is introduced using the design parameters of SPring-8 II. To develop optical elements applicable to manipulation of coherent X-rays, advanced technologies on precise processing and metrology were invented. With propagation-based coherent X-rays at the 1 km beamline of SPring-8, a beryllium window fabricated with the physical-vapour-deposition method was found to have ideal speckle-free properties. The elastic emission machining method was utilized for developing reflective mirrors without distortion of the wavefronts. The method was further applied to production of diffraction-limited focusing mirrors generating the smallest spot size in the sub-10 nm regime. To enable production of ultra-intense nanobeams at DLSRs, a low-vibration cooling system for a high-heat-load monochromator and advanced diagnostic systems to characterize X-ray beam properties precisely were developed. Finally, new experimental schemes for combinative nano-analysis and spectroscopy realised with novel X-ray optics are discussed.

All-diamond optical assemblies for a beam-multiplexing X-ray monochromator at the Linac Coherent Light Source

All-diamond optical assemblies holding state-of-the-art type IIa diamond crystals enable the construction of a beam-multiplexing X-ray double-crystal monochromator for hard X-ray free-electron lasers. Details on the design, fabrication and X-ray diffraction characterization of the assemblies are reported.

A double-crystal diamond (111) monochromator recently implemented at the Linac Coherent Light Source (LCLS) enables splitting of the primary X-ray beam into a pink (transmitted) and a monochromatic (reflected) branch. The first monochromator crystal, with a thickness of ∼100 µm, provides sufficient X-ray transmittance to enable simultaneous operation of two beamlines. This article reports the design, fabrication and X-ray characterization of the first and second (300 µm-thick) crystals utilized in the monochromator and the optical assemblies holding these crystals. Each crystal plate has a region of about 5 × 2 mm with low defect concentration, sufficient for use in X-ray optics at the LCLS. The optical assemblies holding the crystals were designed to provide mounting on a rigid substrate and to minimize mounting-induced crystal strain. The induced strain was evaluated using double-crystal X-ray topography and was found to be small over the 5 × 2 mm working regions of the crystals.

Rocking curve measurements revisited

In order to correctly analyse high-resolution rocking curves of high-quality crystals, a special effort is needed to estimate the systematic contributions coming from the experimental setup. This article highlights the main areas that require special analytical treatment and presents results obtained using different approaches to the problem, as well as some typical results for high-quality silicon and diamond crystals.

Performance of a beam-multiplexing diamond crystal monochromator at the Linac Coherent Light Source

A double-crystal diamond monochromator was recently implemented at the Linac Coherent Light Source. It enables splitting pulses generated by the free electron laser in the hard x-ray regime and thus allows the simultaneous operations of two instruments. Both monochromator crystals are High-Pressure High-Temperature grown type-IIa diamond crystal plates with the (111) orientation. The first crystal has a thickness of ~100 μm to allow high reflectivity within the Bragg bandwidth and good transmission for the other wavelengths for downstream use. The second crystal is about 300 μm thick and makes the exit beam of the monochromator parallel to the incoming beam with an offset of 600 mm. Here we present details on the monochromator design and its performance.