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  • Abstract：Polycrystalline diamond films and coatings are widely used in optical components, thermal management, and cutting tools. Chemical vapor deposition (CVD) is widely used for their synthesis, while the initial nucleation stage largely determines the continuity and morphology of the diamond layer. Conventional nucleation is achieved by surface seeding with nanodiamond particles. An alternative approach is chemical nucleation, extensively studied for carbon-containing precursors and polymers, whereas nucleation on non-carbon particles remains comparatively unexplored. Here, lithium fluoride (LiF) particles were used as a model non-carbon system to study diamond nucleation and subsequent growth in CH4–H2 microwave plasma CVD. Surface morphology and Raman spectroscopy showed that LiF promotes localized formation of well-faceted diamond crystallites without conventional nanodiamond seeding. Increasing the substrate temperature to 900°C produced a nearly linear increase in nucleation density and enabled the formation of nearly continuous polycrystalline films, whereas further heating to 950°C promoted island-like growth despite increasing growth rate. Methane concentration mainly affected the growth regime: at CH4 contents of 10% and higher, secondary nucleation became pronounced, suppressing crystallite enlargement and promoting the transition from microcrystalline to nanocrystalline diamond. Raman analysis showed that substrate temperature had little effect on the sp3/sp2 ratio, although the diamond Raman line broadened at higher temperatures, whereas elevated methane concentrations increased the contribution of non-diamond carbon phases. These findings suggest that non-carbon chemical nucleation can provide a new perspective on diamond formation during CVD, including the emergence of diamond nuclei under conditions where diamond remains thermodynamically metastable.  

    Ivan A. Tiazhelov, Artem K. Martyanov, Sergey V. Kuznetsov, Sergey S. Savin, Alexander A. Zhivopistsev, Victor G. Ralchenko, Vitaly I. Konov, Vadim S. Sedov

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  • Abstract：Boron and nitrogen co-doped diamond (BNDD) has attracted extensive research attention owing to its promising potential for n-type conductivity. However, boron–nitrogen (B–N)-related complexes in BNDD lead to highly complex optoelectronic properties, which hinders its practical applications. In this study, we present a systematic investigation of the electrical and optical properties of BNDDs with graded nitrogen doping, both in the as-grown state and after high-pressure high-temperature (HPHT) annealing. Nitrogen incorporation is found to sharply increase the resistivity of boron-doped diamond, dominated by enhanced hole scattering from nitrogen-associated defects and impurities. Following HPHT annealing, all BNDDs maintain stable p-type conduction with an elevated hole concentration, which benefits from the combined effects of neutral boron acceptor activation and intrinsic B–N complex dissociation. Strikingly, donor–acceptor pair (DAP) recombination luminescence appears concurrently with a drop in hole concentration in high-nitrogen BNDDs annealed above 2000 °C, directly confirming neutral nitrogen donor activation under HPHT conditions. This work provides a practical strategy for the performance modulation and optimization of BNDD, and reveals the activation effect of HPHT annealing on nitrogen impurities, providing a feasible path toward the realization of n-type diamond.  

    Liangxue Gu, Jiaqi Xia, Shuang Ye, Chuang Wang, Shulong Zhang, Zhonghao Ye, Man Ye, Chengchun Zhao, Yin Hang, Shulin Gu

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  • Abstract：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.  

    Luke Chia Wei Min, Siu Hon Tsang, Apoorva Chaturvedi, Edwin Hang Tong Teo

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  • Abstract：lumina (Al2O3) ceramics with complex geometry are widely used in aerospace thermal components, cutting tools, and catalyst supports because of their excellent thermal stability, corrosion resistance, and wear resistance. However, low ductility problem existed in such brittle material and influences its long-term stable service in extreme environments, especially under load-bearing conditions. Adding carbon material were considered as an effective method to improve mechanical properties of ceramics. In this work, diamond/Al2O3 composites were fabricated using digital light processing (DLP) additive manufacturing technology. Meanwhile, their microstructure, phase composition, and compressive properties were systematically investigated here. After sintering treatment, Al2O3 particles retained a well-defined morphology and unchanged phase composition, which had an average particle size of 6 μm. Diamond particles had an average particle size of 1 μm and transformed obviously from a crystalline structure to an amorphous state, accompanied by characteristic graphitization peaks. From mechanical properties testing results of DLP composites, the specimens exhibited elastic modulus of 331.23 ± 8.69 MPa and compressive strength of 46.19 ± 1.07 MPa. Specially, diamond/Al2O3 composites specimens had an increasing in elongation (27.26 ± 0.17%) compared with pure Al2O3 specimens. This indicated carbonized diamond particles had a positive effect on enhancing the ductility of Al2O3 ceramics. These scientific finding will provided an important reference for investigating additive manufacturing high-performance ceramics.  

    Tingting Li, Jiong Zhao, Jing Li, Yangli Xu, Zeling Yang, Xiaopeng Jiang, Luying Chen, Guangyao Han, Zihan Wang, Xuanyang Cao

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  • Abstract：Reliable n-type doping, defined here as dopant incorporation that provides a shallow donor ionization energy, sufficient electron activation at room temperature (approximately 300 K), structural stability, and compatibility with chemical vapor deposition/microwave plasma chemical vapor deposition (CVD/MPCVD) growth, remains a central obstacle to the implementation of diamond electronics. In this work, density functional theory calculations are used to examine the formation, stability, and electronic properties of BN2 defect complexes in diamond, with emphasis on feasible defect reaction pathways rather than experimental synthesis. Substitutional B–N complexes were evaluated in 64-, 216-, and 512-atom supercells, and the effect of residual strain was examined over the range from −0.10% to +0.10%. The BN2 complex exhibits a donor ionization energy of 0.132 eV based on charge-state total-energy analysis, consistent with its potential to support electron activation at approximately 300 K. Precursor-referenced reaction-energy analysis indicates that nitrogen-related precursor defects lower the energetic cost of BNx formation, while dissociation reaction-energy calculations show that BN and BN2 are thermodynamically more resistant to the considered neutral decomposition channels than nitrogen-rich BN3 and BN4 complexes. Strain modulates local B–N and B–C bonding and changes the dissociation energetics of BN2. The most favorable neutral route to BN2 formation is the reaction between BN and substitutional N defects, with a reaction energy of −1.04 eV in the 216-atom model. These results identify thermodynamically plausible routes for incorporating BN2 into diamond and provide guidance for precursor-, strain-, and temperature-controlled CVD/MPCVD growth of n-type diamond.  

    Zhang Dongliang, Hanyang Xu, Xiang Sun, Zhiyin Gan, Sheng Liu

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  • Abstract：Suppressing the formation of dark features during diamond growth has been a key research focus in the chemical vapor deposition (CVD) of high-quality optical polycrystalline diamond. In this study, the formation mechanism and suppression effect of dark features in diamond under different silicon wafer processing and etching conditions were analyzed. Characterization results from X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy indicated that the formation of dark features was primarily associated with the quality of crystals, uniformity of grain size, and whether the crystals exhibiting columnar growth. Reducing the silicon substrate scratching duration from 10 min to 1 min, combined with oxygen etching during growth, significantly reduced the concentration of dark features. The shortened scratching duration promoted uniform columnar diamond growth, therefore suppressing the formation of dark features. Meanwhile, the etching of amorphous carbon and graphite by oxygen during the growth process further suppressed the formation of dark features. As a result, by reducing the substrate scratching duration to 1 min and employing growth conditions with gas flow rates of 500 sccm H2 + 30 sccm CH4 + 2.5 sccm O2 at a temperature of approximately 970 °C, the area fraction of dark features on the surface of the diamond optical window was reduced from 4.44% to 0.22%. Its optical transmittance reached 70.8% at a wavelength of 10.6 μm with an absorption coefficient of 0.081 cm−1, and 66.6% at 1.064 μm with an absorption coefficient of 0.246 cm−1.  

    Dongyang Yan, Shuai Xu, Manyu Xia, Yipu Qu, Yutong Pang, Hui Liu, Shihao Kang, Yueli Li, Xueyang Bai, Yangfan Pan, Huijie Liu, Jiaqi Lu

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  • Hiroshi Amano

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  • Abstract：The heteroepitaxial growth of large-area thick single-crystal diamond is strongly limited by stress accumulation and crack formation during prolonged deposition. Laser-patterned templates have been demonstrated as an effective approach for obtaining crack-free free­standing diamond layers. In this work, the influence of laser grooves on the growth behavior of diamond films grown on Ir/YSZ/Si (001) substrates by microwave plasma chemical vapor deposition (MPCVD) is systematically investigated. Laser grooves introduced into a thin diamond seed layer generate a spatially non-uniform growth template. During the early growth stage, polycrystalline diamond forms inside the grooves, whereas the surrounding regions maintain stable (001)-oriented epitaxial growth. As deposition proceeds, the faster vertical growth of single-crystal diamond progressively overgrows the groove regions, leading to gradual groove filling and closure, ultimately forming a laterally continuous epitaxial diamond film. The buried grooves with polycrystalline diamond act as mechanically compliant zones that facilitate stress redistribution during thick-film growth, thereby suppressing crack formation and enabling the fabrication of millimeter-scale freestanding diamond layers with high crystalline quality. This work clarifies how laser-patterned templates regulate growth evolution and stress accom­modation during diamond heteroepitaxy, providing insights for the controlled growth of large-area thick single-crystal diamond films on highly mismatched substrates.  

    Pengfei Qu, Peng Jin, Xu Han, Zhaorun Dong, Zhanguo Wang

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