X-ray-based analysis of quantum-grade homoepitaxial single-crystal diamond fabricated by MPCVD

Homoepitaxial single-crystal diamond grown by MPCVD is increasingly recognised as the optimal substrate for NV centre-based quantum sensors. Establishing eligibility for quantum applications requires evaluating four properties that exceed standard semiconductor criteria: crystal orientation, bulk crystallinity, epilayer–substrate misalignment, and surface roughness. X-ray techniques are uniquely suited to this task, being non-destructive, quantitative, requiring minimal preparation, and spanning laboratory and synchrotron facilities bridging wafer-scale screening and device-region certification. This review critically surveys X-ray methods addressing all four properties: high-resolution X-ray diffraction (HRXRD) rocking curve analysis, synchrotron and laboratory topography, reciprocal space mapping (RSM), X-ray reflectometry (XRR), grazing incidence diffraction (GIXRD), pole figure analysis, and Bragg diffraction imaging. Reported HRXRD (004) rocking curve FWHM values reach 0.0027° (9.7 arcsec) for the best CVD films, approaching the theoretical Darwin width. Synchrotron topography enables Burgers vector identification and dislocation density mapping below 10⁵ cm–²; RSM separates lattice tilt from strain; XRR resolves the sub-nanometre roughness required for shallow NV implantation. We examine how MPCVD growth parameters like substrate misorientation, nitrogen addition, temperature, and methane concentration imprint on X-ray signatures, and establish quality thresholds linking X-ray metrics to NV coherence (T₂ up to 2.4 ms in ¹²C-enriched, phosphorus-doped n-type CVD diamond), distinguishing T₂ from T₂*. Critically, X-ray observables are necessary-but-not-sufficient structural proxies: they certify the host lattice, while spin-bath-limited T₂, zero-phonon-line homogeneity, charge-state stability, and charge-collection efficiency require EPR, SIMS/FTIR, photoluminescence, ODMR, and device-level metrology. We present a “ready reckoner” cross-mapping each X-ray observable to the metrics it can and cannot predict and recommend a standardised HRXRD/XRT–RCI/RSM/XRR protocol with defined pass/fail thresholds for community-wide quantum-grade qualification.