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Review

Advances in Homoepitaxial Mosaic Single-Crystal Diamond: Interface Stress Regulation

by
Rong Rong
1 and
Jie Bai
1,2,*
1
School of Aeronautics, Chongqing Jiaotong University, Chongqing 400074, China
2
Key Laboratory of Low-Grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, Chongqing 400044, China
*
Author to whom correspondence should be addressed.
Crystals 2026, 16(7), 448; https://doi.org/10.3390/cryst16070448
Submission received: 31 May 2026 / Revised: 7 July 2026 / Accepted: 9 July 2026 / Published: 10 July 2026
(This article belongs to the Section Inorganic Crystalline Materials)
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Abstract

Single-crystal diamond is regarded as one of the most promising semiconductor materials for next-generation high-power electronic devices, quantum technologies, and extreme environmental applications, owing to its ultra-wide bandgap, exceptionally high carrier mobility, ultra-high breakdown electric field, and excellent thermal conductivity. However, the lateral dimensions of both natural and synthetic single-crystal diamond are limited, which severely restricts their large-scale industrial application. Mosaic growth, in which multiple small single-crystal seeds are laterally arranged and fused at the interfaces through homoepitaxial growth, offers a promising approach to overcoming the size limitation of seed crystals and producing inch-scale single-crystal wafers. This review systematically covers the entire mosaic growth process, including seed crystal preparation, geometric design, growth parameter optimization, and innovative processing methods. Particular emphasis is placed on the mechanisms of interfacial stress generation, along with characterization techniques and stress control strategies. Finally, future perspectives on the fabrication of large-size, low-stress single-crystal diamond wafers are outlined.

1. Introduction

Single-crystal diamond possesses an ultrawide bandgap (5.47 eV), exceptionally high carrier mobilities (up to 4500 cm2·V−1·s−1 for electrons and 3800 cm2·V−1·s−1 for holes), a remarkably high breakdown electric field (>10 MV·cm−1), and outstanding thermal conductivity (>2000 W·m−1·K−1) [1]. Owing to these exceptional properties, single-crystal diamond (SCD) has been widely recognized as one of the most promising semiconductor materials for high-frequency and high-power electronic devices, deep-ultraviolet photodetectors, quantum information technologies, radiation detectors, and extreme-environment electronics [2,3,4,5,6,7].
Among all wide-bandgap semiconductors, GaN (3.4 eV), SiC (3.3 eV), and Ga2O3 (4.8 eV) exhibit relatively good performance. However, diamond uniquely combines an ultrawide bandgap, extremely high thermal conductivity, and the highest known critical breakdown electric field, making it a highly promising material for high-power and high-frequency electronic devices [2]. Significant progress has been achieved in microwave plasma chemical vapor deposition (MPCVD), crystal growth mechanisms, and industrial-scale diamond synthesis [8,9,10]. Achard et al. comprehensively reviewed the synthesis of quantum-grade diamond by CVD, emphasizing the critical role of defect engineering in achieving long spin coherence times [6]. Ren et al. provided a recent comprehensive overview of homoepitaxial single-crystal diamond growth via MPCVD, covering growth mechanisms, substrate holder design, seed crystal pretreatment, and mosaic splicing [8]. Arnault et al. systematically compared homoepitaxial and heteroepitaxial growth routes, highlighting the respective advantages and limitations of each approach [10]. Nevertheless, the lateral size limitation of available substrates remains a major bottleneck.
Natural diamond crystals are generally too small and contain impurity concentrations unsuitable for electronic device applications, while high-pressure high-temperature (HPHT) synthetic diamonds are typically limited to several millimeters in lateral size [11,12]. Although MPCVD homoepitaxial growth can produce high-purity, low-defect single-crystal diamond, the lateral dimension of the deposited crystal is fundamentally constrained by the size of the seed crystal [8].
To overcome this limitation, two major technological routes have been developed. The first enlarges a single crystal through repeated homoepitaxial lateral growth, as exemplified by repeated lateral enlargement [13,14], high-pressure lateral outward growth [15], and enlarged homoepitaxy on the (110) crystal orientation [16]. Despite continuous improvements, these approaches remain fundamentally constrained by the growth of a single seed crystal and cannot readily meet the requirements for inch-scale wafer production. From the half-inch scale achieved by AIST to the 59% area increase via high-pressure ELO and the approximately 1.6-fold area gain via nitrogen-assisted MPCVD, all single-seed enlargement strategies have demonstrated limited scalability. The maximum lateral dimensions remain at the millimeter-to-half-inch level, and dislocation densities progressively increase with repeated growth cycles [13,14].
An alternative strategy is heteroepitaxial growth on foreign substrates such as Ir/YSZ/Si. Schreck et al. reviewed both homoepitaxial and heteroepitaxial approaches and reported that iridium-mediated heteroepitaxy had produced diamond wafers with pseudo-diameters approaching 92 mm [12]. More recently, Qu et al. demonstrated the growth of two-inch free-standing heteroepitaxial diamond on Ir/YSZ/Si (001) substrates via laser-patterned templates, achieving a dislocation density of approximately 2.2 × 107 cm−2 [17]. However, heteroepitaxial diamond still exhibits threading dislocation densities of approximately 107–109 cm−2, several orders of magnitude higher than the 102–105 cm−2 typically reported for homoepitaxial films [10,12]. This fundamental trade-off between wafer size and crystalline perfection warrants a direct quantitative comparison. Heteroepitaxial growth on Ir has achieved larger wafer dimensions, approaching 92 mm in pseudo-diameter and yielding two-inch free-standing wafers [12,17], but the threading dislocation densities remain in the range of 107–109 cm−2. In contrast, mosaic SCD wafers, though currently limited to two-inch dimensions, have demonstrated dislocation densities as low as approximately 1.0 × 105 cm−2 and residual stress below 0.04 GPa [18,19]—representing defect densities two to four orders of magnitude lower. This inverse relationship between wafer size and crystalline perfection defines distinct application niches: heteroepitaxy offers larger areas for proof-of-concept device development, whereas mosaic growth remains the preferred route for electronic-grade wafers requiring high crystalline perfection and low defect densities.
Owing to these fundamental limitations, mosaic growth has become the preferred route toward electronic-grade, low-defect-density wafers [20,21,22]. In this approach, multiple small single-crystal diamond seeds are placed side by side and overgrown homoepitaxially to heal the interfaces, thereby overcoming the size limitation of a single seed. Nitrogen-assisted lateral overgrowth, for instance, has achieved area gains of up to approximately 1.6-fold from a single seed [23], providing the conceptual foundation for multi-seed mosaic assembly. However, mosaic interfaces inevitably introduce residual stress and crystal defects arising from thermal mismatch during cooling, lattice compression during growth, and structural mismatch from incorporated defects, leading to Raman peak splitting, birefringence fringes, changes in electrical and thermal transport, and interfacial cracking [24,25,26].
Previous reviews have only listed individual construction methods. In contrast, this paper considers mosaic growth as a whole-system engineering chain that encompasses seed cloning and shape design, as well as interface evolution and growth regulation. Three types of interfacial stress arise in this system: thermal expansion mismatch, intrinsic growth stress, and structural mismatch. Corresponding mitigation strategies have been proposed in two directions: suppressing stress generation through process optimization and regulating stress relief through intelligent geometric design. Finally, we propose that future studies should move from passive stress reduction to active strain engineering and explore new approaches to tailoring the structural and functional properties of large-area single-crystal diamond wafers.

2. Homoepitaxial Growth: Progress and Limitations

Microwave plasma chemical vapor deposition (MPCVD) has emerged as the predominant approach for homoepitaxial growth of single-crystal diamond, owing to its ability to avoid electrode contamination, generate high-density plasma, and precisely control growth conditions [27,28,29]. In early pioneering work, Mokuno et al. demonstrated that a closed substrate holder maintained a smooth surface morphology after 24 cycles of high-rate repeated growth, successfully expanding a 5 × 5 mm2 seed crystal into a bulk single crystal measuring 10 mm in thickness [14]. The same group subsequently combined a lift-off technique with lateral growth, further enlarging the crystal dimensions to half an inch, corresponding to 12.6 × 13.3 × 3.7 mm3 [13]. More recently, high-pressure MPCVD operated at 165 Torr enabled epitaxial lateral overgrowth from 10 × 10 mm2 to 12.56 × 12.59 mm2, representing an area increase of approximately 59% [15]. Meanwhile, nitrogen-assisted MPCVD was found to enhance the lateral growth rate on the (001) plane, expanding a 5.5 × 5.5 mm2 seed crystal to a diameter of 7.6 mm with an area gain of approximately 1.6-fold, as illustrated in Figure 1 [23]. Liang et al. further demonstrated that under high-pressure conditions of 300 Torr, growth rates as high as 165 µm/h could be achieved with nitrogen addition while preserving good crystal quality [15]. In another study, researchers expanded a 5 × 5 mm2 (110)-oriented substrate to 7.03 × 8.12 mm2, corresponding to an area increase of approximately 75% [16]. Additionally, optimization of closed substrate holder designs increased the achievable single-crystal diamond size from 7 × 7 mm2 to 8.6 × 8.6 mm2 [30]. Beyond conventional (100)-oriented growth, Widmann et al. demonstrated high-quality homoepitaxial growth on (111)-oriented single-crystal diamond substrates, thereby providing an alternative crystallographic platform for diverse device applications [31].
Representative achievements in single-seed homoepitaxial expansion are summarized in Table 1. As shown, early work by Mokuno et al. at AIST achieved a half-inch scale (12.6 × 13.3 mm2) through repeated high-rate growth and lift-off techniques [13,14]. Subsequent efforts have yielded incremental improvements: Yang et al. reported an approximately 1.6-fold area increase via nitrogen-assisted MPCVD, reaching a lateral dimension of 7.6 mm [23]; Kumar et al. achieved a 59% area increase through high-pressure ELO at 165 Torr, attaining 12.56 × 12.59 mm2 [15]; Zhang et al. demonstrated a 75% area increase on the (110) plane, reaching 7.03 × 8.12 mm2 [16]; and Zou Guangtian’s team at Jilin University optimized closed substrate holder designs to achieve an 8.6 × 8.6 mm2 crystal with a 51% area increase [30]. Despite these advances, all homoepitaxial expansion strategies remain fundamentally confined to a single seed crystal. The lateral extension inherently represents only a limited upscaling of the original seed dimensions and cannot achieve an order-of-magnitude breakthrough. Furthermore, repeated growth cycles and ion implantation exfoliation progressively increase the dislocation density within the seed crystal [32], leading to prolonged growth durations and high equipment occupancy that render these methods unsuitable for industrial-scale mass production. These inherent limitations have directly motivated the development of mosaic growth, an approach in which multiple high-quality, small-sized seed crystals are physically joined laterally and fused at the interfaces through homoepitaxial growth. This strategy effectively breaks through the size ceiling imposed by a single seed crystal, thereby enabling the fabrication of inch-scale single-crystal wafers.

3. Advances in Single-Crystal Diamond Mosaic Growth

The concept of mosaic growth was first proposed by Janssen and Giling in 1995 [33]. By arranging two polished HPHT substrates side by side for CVD growth, they demonstrated the feasibility of this approach, although the presence of interfacial gaps and cracks indicated the occurrence of detrimental tensile stress. Subsequent investigations into interface evolution [34] and the fabrication of large-area SCD layers [35] established the core challenge: achieving perfect crystallographic healing at the interface. Ren et al. have reviewed the development trajectory from this early demonstration to the recent realization of 2-inch wafers, noting that interfacial stress and dislocation accumulation remain unresolved issues [8]. Over the past three decades, these challenges have been systematically addressed through advances in seed crystal preparation, geometric design, and growth control.

3.1. Preparation and Pretreatment of Seed Crystals

Seed crystal quality constitutes the material foundation of mosaic growth. HPHT-synthesized seeds, though widely available and cost-effective, contain metallic catalyst inclusions and nitrogen impurities that act as stress concentrators during CVD growth [36]. In contrast, CVD homoepitaxial seeds exhibit significantly lower dislocation densities. Zhao et al. demonstrated that epitaxial layers grown on CVD IIa-type substrates possess approximately half the dislocation density of those grown on HPHT Ib-type substrates [37]. As recently summarized in the comprehensive review by Ren et al., intrinsic defects present in the seed crystal cannot be eliminated during subsequent epitaxial growth; instead, these pre-existing flaws propagate directly into the deposited layer, underscoring that seed selection is as critical as the growth process itself [8]. Therefore, the selection of high-quality seeds, preferably CVD IIa-type or HPHT IIa-type with low dislocation densities, is essential. By employing a lift-off process to exfoliate multiple cloned seed crystals from the same high-quality CVD crystal, identical crystallographic orientation and low defect density can be simultaneously achieved [38,39]. Mokuno et al. reported that CVD seed plates prepared via lift-off exhibited dislocation densities as low as 400 cm−2, markedly outperforming commercial HPHT seeds [39]. Consequently, cloned CVD seeds have become the mainstream choice for electronic-grade wafer fabrication.
Surface quality directly determines the effectiveness of interfacial healing. Laser-cut side faces exhibit heat-affected layers, microcracks, and residual amorphous carbon, which serve as non-epitaxial nucleation sites and dislocation sources during subsequent CVD growth [36]. Systematic surface treatment, broadly categorized into polishing and cleaning or etching, is therefore indispensable. Polishing in this context encompasses two distinct categories depending on the stage of processing. Pre-growth polishing of seed crystals aims to achieve an atomically flat surface to facilitate epitaxial healing at mosaic interfaces, with atomic force microscopy serving as the preferred characterization technique. Post-growth polishing of final wafers must meet stringent roughness requirements of ≤0.2–0.3 nm RMS for microelectronic device fabrication, where X-ray reflectometry provides a more comprehensive assessment by simultaneously determining surface roughness and the thickness of the near-surface damaged layer [40].
Mechanical polishing rapidly removes rough surfaces but introduces subsurface damage. Chemical mechanical polishing achieves lower-damage finishing through the synergistic action of oxidants and abrasives. Plasma-assisted polishing, a recently developed damage-free technique, employs a soft quartz glass plate to remove diamond surfaces following plasma-induced modification. Liu et al. first applied plasma-assisted polishing to a 20 mm square mosaic SCD substrate, attaining a material removal rate of 13.3 μm/h and surface roughness below 0.5 nm Sq [41]. As shown in Figure 2, plasma-assisted polishing completely removed the approximately 100 μm surface waviness generated during CVD growth (Figure 2a,b), produced an AFM-measured surface roughness of only 0.361 nm Sq (Figure 2c), and maintained nearly identical Raman peak positions and full width at half maximum values before and after polishing (Figure 2d), confirming that this technique introduces virtually no residual stress. Other emerging techniques, including medium-vacuum UV-assisted polishing with a material removal rate of up to 8.0 μm/h [42], UV photocatalysis-assisted chemical mechanical polishing achieving a surface roughness as low as 0.079 nm Sa [43], and mechanochemical synergistic polishing with a material removal rate of up to 8.186 μm/h [44], have demonstrated excellent performance, although their effectiveness on mosaic substrates awaits further verification.
Table 2 summarizes the representative polishing techniques for single-crystal diamond surface preparation and compares their key performance metrics. Mechanical polishing offers rapid material removal but introduces severe subsurface damage, including scratches and an amorphous layer, with surface roughness exceeding 1 nm [40]. Chemical mechanical polishing achieves lower-damage finishing through the synergistic action of oxidants and abrasives, yielding a surface roughness of 0.3–0.5 nm and low subsurface damage, albeit at a relatively low material removal rate of approximately 300 nm/h [45]. Plasma-assisted polishing, a damage-free technique, attains a material removal rate of 13.3 μm/h and a surface roughness of 0.4–0.5 nm Sq without introducing subsurface damage [41]. Medium-vacuum UV-assisted polishing similarly achieves damage-free processing with a material removal rate of 8.0 μm/h and surface roughness below 0.5 nm [42]. UV photocatalytic-assisted CMP produces the finest surface finish, with a surface roughness as low as 0.079 nm Sa and a very thin damaged layer of only 0.66 nm, though its material removal rate is moderate at 1168 nm/h [43]. Mechanochemical synergistic wheel polishing offers a high material removal rate of 8.186 μm/h with a surface roughness of 0.721 nm Sa and a very thin amorphous layer [44].
Cleaning and etching treatments expose atomically clean surfaces by removing chemical contaminants and non-diamond phases. Naamoun et al. systematically investigated H2/O2 plasma etching and revealed that pit density progressively decreases with etching time as polishing damage is removed [45]. Ivanov et al. examined hydrogen plasma etching behavior on (100) SCD [46]. Achard et al. combined H2/O2 plasma etching with either chemo-mechanical polishing or inductively coupled plasma reactive ion etching to significantly reduce dislocation density in thick CVD epitaxial layers [47]. Reactive ion etching processes using Ar/O2/CF4 gas mixtures have also been demonstrated to effectively remove polishing-induced subsurface damage, with material removal of 6–10 μm being sufficient to eliminate the damaged layer. More recently, Wei et al. proposed a laser-cutting strategy combined with microwave hydrogen plasma etching that eliminates the time-consuming mechanical polishing step. In their approach, hydrogen plasma removed graphitic carbon residues within 10 min, and after 20 h of homoepitaxial growth, the etched surface displayed ordered, parallel step-flow morphology superior to that on conventionally polished substrates, together with higher crystal quality and lower residual stress [48]. Wet cleaning, typically involving boiling in concentrated H2SO4 and HNO3 at a 3:1 ratio followed by ultrasonic cleaning in deionized water and organic solvents, remains an indispensable complement. For front-end seed preparation, Sakamoto et al. employed ultrashort-pulse laser slicing of {100} facets to provide high-quality starting surfaces for mosaic growth [49]. Dislocations and impurities introduced from etch pits at the epitaxial growth resumption of diamond have been systematically investigated, revealing that etch-pit formation can serve as both a source of dislocation multiplication and a pathway for impurity incorporation, depending on the etching conditions and subsequent regrowth parameters [50].

3.2. Geometry and Process Design for Mosaic Growth

The crystallographic orientation of side surfaces directly governs interfacial healing quality. Zhao et al. systematically compared square and rectangular holes with {100} and {110} side orientations for epitaxial lateral overgrowth. They found that rectangular holes with {100} side surfaces reduced the dislocation density from 6 × 106 cm−2 to 3 × 105 cm−2, representing a 20-fold decrease, and lowered the tensile stress by approximately 0.14 GPa, confirming the pronounced advantage of the {100} orientation [51].
Crystallographic orientation matching is the primary determinant of interfacial quality. The Yamada team at AIST in Japan pioneered the cloning seed crystal approach, in which multiple seeds with identical orientations are obtained through repeated ion implantation and lift-off cycles, enabling the successful fabrication of 1-inch [52] and 2-inch (40 × 60 mm2) mosaic wafers [53]. Prior to the achievement of high-quality 2-inch mosaic SCD by the Zou Guangtian team, Yamada et al. had demonstrated the fabrication of a 2-inch (40 × 60 mm2) mosaic wafer using 24 cloned SCD plates, but cracking along boundaries remained a challenge when the off-direction was parallel or perpendicular to the edges. The wafer quality also exhibited non-uniformity, with Raman peak shifts of approximately 0.5 cm−1 between the central and edge regions, corresponding to an internal stress difference of about 0.17 GPa [53]. Academician Zou Guangtian’s team at Jilin University achieved a major breakthrough in step-flow control, demonstrating that similar step-flow directions enable excellent interfacial healing, whereas mismatched directions induce overgrowth and stress concentration [18]. As shown in Figure 3, the optical microscopy image of the central mosaic region (Figure 3a) and enlarged boundary images (Figure 3b–e) clearly demonstrate that consistent step-flow directions produce smooth interfacial fusion, while misalignment leads to overgrowth and stress concentration. Raman FWHM mapping and internal stress distribution (Figure 3f,g) confirm a residual stress as low as approximately 0.04 GPa. Extending this principle to a 2-inch (50 × 50 mm2) scale, they achieved a residual stress as low as approximately 0.04 GPa and a dislocation density of about 1.0 × 105 cm−2 [19], representing the first demonstration of high-quality 2-inch mosaic single-crystal diamond worldwide.
The mosaic interface angle is another critical geometric parameter. A 60° interface angle yields the smoothest junction with a residual stress of only 0.42 GPa. This is attributable to the inclined interface altering the meeting pattern of lateral growth fronts and preventing direct collision perpendicular to the growth surface [54]. This finding is consistent with the characterization of interfaces in mosaic CVD diamond crystals, which demonstrated that inclined interfaces promote more favorable stress distributions compared to vertical junctions. Furthermore, recent work on seamless mosaic diamonds by precise control of crystallographic orientations followed by surface polishing has demonstrated that orientation deviations as small as θ < 1° enable nearly seamless coalescence with Raman FWHM of approximately 2.83 cm−1 and thermal conductivity exceeding 2100 W/mK [55]. Inter-seed gap control is equally important. Zhao et al. demonstrated that a thickness difference of ≤50 μm maintains a smooth, defect-free interface, whereas a difference of 100 μm introduces distinct defects [51]. A Chinese patent further proposes selecting seeds with similar step-flow directions and limiting the deviation angle to 0–10° to reduce defect density at the mosaic interface [56]. Optimizing single-crystal diamond mosaic growth through seed thickness variation and pre-growth treatment has also been systematically investigated, revealing that thickness differences ≤ 50 μm and appropriate pre-growth cleaning protocols are essential for achieving seamless interfacial fusion [57].
Significant progress has been made in geometric optimization, including the use of {100} side orientations, 60° interface angles, step-flow alignment, and inter-seed gap control. However, the underlying mechanisms governing dislocation multiplication and stress evolution at mosaic interfaces remain incompletely understood. Moreover, most reported geometric designs have been validated under specific growth conditions and reactor configurations, which raises questions about their transferability to different MPCVD systems and wafer scales. Furthermore, the interplay between geometric parameters and growth kinetics, such as the coupled effects of interface angle and CH4 concentration on step-flow dynamics, has not yet received systematic investigation. Future work should prioritize the development of in situ characterization techniques capable of tracking real-time stress evolution during growth, thereby enabling direct correlation between geometric design choices and the dynamic response of the coalescing interface.

3.3. Parameter Control in Mosaic Growth

Maintaining uniform temperature and gas flow distributions across the entire growth area is the primary engineering challenge for large-area mosaic growth. Substrate holder design plays a critical role in regulating the temperature and flow fields, with two main configurations currently employed: open and closed types [14,58]. Open structures enable higher growth rates but exhibit pronounced edge polycrystallization, whereas closed structures provide a more uniform temperature field and effectively suppress non-epitaxial crystallization. As noted by Ren et al., Mokuno et al. demonstrated that the closed-type structure maintained a smooth surface after 24 growth cycles, while the open-type developed central cracks after only 5 cycles [8,14]. Yamada et al. used simulations to reveal the influence of seed crystal placement depth on gas flow direction and step-flow uniformity [59]. Asmussen et al. at Michigan State University were the first to show that a 915 MHz MPCVD reactor could deposit hundreds of single-crystal diamond samples simultaneously on a 300 mm-diameter substrate holder [58]. Liang et al. later expanded on this idea to reach a production rate of approximately 100 g·day−1 at the Carnegie Institution, demonstrating that high-frequency MPCVD systems hold good prospects for large-scale diamond production. To further suppress edge polycrystallization, Nad et al. proposed a stepped substrate holder design with a precisely adjusted groove depth to achieve uniform temperature across the seed top surface. Building on this, Meng et al. introduced an inclined sidewall structure, obtaining a 1.94 mm thick epitaxial layer in a single 246 h growth run, with an approximately 31% increase in lateral area [60].
Methane concentration is a core parameter governing growth rate and crystal quality. The gas-phase chemistry and surface reaction mechanisms underlying CVD diamond growth, including the interplay between atomic hydrogen, methyl radicals, and other hydrocarbon species in determining growth rate, morphology, and defect incorporation, have been comprehensively reviewed in the literature [8,27,29]. Zhang et al. systematically investigated the effect of CH4 concentration on SCD surface morphology [61]. As shown in Figure 4, increasing CH4 concentration from 6.0% to 8.0% caused the surface to evolve from a smooth step-flow morphology to a rough surface covered with polycrystalline and non-epitaxial crystallites. The lowest RMS roughness of approximately 0.573 nm was obtained at 6.4% CH4, while the surface deteriorated significantly at 8.0% CH4 [61]. Achard et al. have shown that nitrogen is a deep donor in diamond with an activation energy of approximately 1.7 eV, and its incorporation efficiency is relatively low, generally in the range of 10−5 to 10−3 [6]. Various defect complexes can form in the incorporated nitrogen during CVD growth, such as substitutional nitrogen, nitrogen-vacancy centers, and nitrogen-vacancy-hydrogen complexes. In their ELO experiments, Zhao et al. employed a 4% CH4/H2 ratio, yielding a lateral-to-vertical growth rate ratio λ of 1.1 [51]. When λ exceeds 1, lateral growth dominates and dislocations tend to bend and propagate laterally, substantially reducing the dislocation density within the ELO region. Oxygen addition introduces OH radicals that selectively etch non-diamond phases [62], though excessive O2 suppresses the growth rate. Ren et al. have summarized the optimized process window: a CH4/H2 ratio of approximately 3%, an oxygen flow rate of around 1.5 sccm, and a growth rate of approximately 15 μm h−1 for maintaining high crystal quality while suppressing non-diamond carbon formation [8]. Teraji used a high-oxygen growth environment to suppress nitrogen incorporation to below 1 ppb and increased the 12C isotopic purity to 99.998% to produce ultra-high-purity single-crystal diamond for quantum and electronic applications [27]. Nitrogen addition can increase the growth rate several-fold [63]; Tallaire et al. further examined the effects of growth parameters and low-level nitrogen addition on thick-film diamond quality [64,65]. Microwave power density and chamber pressure jointly determine the plasma state. Achard et al. demonstrated that increasing power density from 65 to 125 W/cm3 markedly elevates the concentration of surface-active species [66], whereas raising chamber pressure contracts the plasma volume, thereby limiting the scale of multi-seed simultaneous mosaic growth [67]. Lloret et al. further investigated the influence of methane concentration on MPCVD overgrowth of 100-oriented etched diamond substrates, revealing that methane concentration critically affects the surface morphology and defect density during regrowth on pre-etched surfaces [68].

3.4. Innovative Methods for Mosaic Growth

Epitaxial lateral overgrowth (ELO) is one of the most effective strategies for reducing dislocation density at mosaic growth interfaces. Its principle relies on geometric structures that force dislocations to bend or terminate rather than propagate vertically [51]. Tang et al. achieved smooth lateral overgrowth using an Ir/SiO2/Ir periodic stripe mask [69], while Li et al. proposed a two-step ELO process that further reduces dislocation density [70]. As shown in Figure 5, the schematic of the two-step ELO process (Figure 5a) and the corresponding FE-SEM images of etch pits on the first and second ELO layers (Figure 5b,c) demonstrate a pronounced decrease in etch-pit density after the second growth step. Hole-type ELO, fabricated by laser cutting, avoids metal contamination: Tallaire et al. first demonstrated that macroscopic holes on HPHT substrates reduce dislocation density in laterally grown regions by over an order of magnitude [71], and Zhao et al. subsequently identified rectangular {100} holes as the optimal configuration [51]. However, the periodic nature of the mask/hole structures employed in ELO can introduce long-range periodic strain modulations in the overgrown layer, as the alternating regions of high and low dislocation density create a spatially varying stress field. As illustrated in Figure 5, the etch-pit density in the second ELO layer decreases substantially compared with the first, but residual strain fields may still propagate through the overgrown material. Careful optimization of mask geometry and spacing is therefore essential to minimize these effects.
It should be noted that the periodic nature of the mask/hole structures employed in ELO can introduce alternating regions of high and low dislocation density, thereby generating long-range periodic strain modulations in the overgrown layer. Although the amplitude of such strain modulations is typically modest (on the order of <0.1 GPa), they may still compromise device-level uniformity requirements. Minimizing these effects therefore necessitates systematic optimization of mask geometric parameters, including periodicity and duty cycle, in conjunction with growth condition adjustments.
Shu et al. employed confocal Raman stress mapping to reveal a mixed tensile–compressive stress distribution (maximum ~0.6 GPa) at mosaic interfaces, with the stress-affected zone expanding from ~40 to ~250 μm as the film thickness increases [72]. Beyond specific experimental demonstrations, the broader role of defect engineering in mosaic SCD has been systematically analyzed in the review by Arnault et al., who emphasized that the stressed zone at coalescence boundaries can extend to distances of the order of 100 µm from the junction, with threading dislocation bundles reaching densities below 1010 cm−2 in the most severely affected regions [10].
In complex-shaped mosaic growth, Shu et al. achieved three-dimensional CVD diamond growth with a high-defect zone confined to only ~20 μm at the vertical–horizontal junction [73]. The “sandwich”-structure method employs double-sided epitaxial growth to fill inter-seed gaps, offering the advantages of dual-side filling and symmetrical stress distribution [74]. Wang et al. introduced SiC nanowires as a flexible interlayer, leveraging nanowire deformability to provide additional degrees of freedom for stress relief—effectively transforming rigid mosaic growth into a flexible connection [75]. Additionally, metal-assisted termination techniques involving heavy tungsten doping suppress dislocation propagation [76,77]; Wang et al. further demonstrated that in situ tungsten incorporation effectively regulates dislocation density [78]. As shown in Figure 6, SEM images of etched surfaces under different tungsten doping conditions (Figure 6a–d) and the corresponding variation in dislocation density (Figure 6e) confirm a marked decrease with increasing tungsten incorporation. Arnault et al. have also noted that tungsten-containing buffer layers represent one of the most promising approaches for mitigating the deleterious effects of coalescence boundaries on device performance [10].

4. Sources and Evolution of Interfacial Stress

Residual stress is inevitably introduced at mosaic growth interfaces regardless of the process employed, causing Raman peak splitting, interfacial cracking, and abrupt changes in electrical and thermal transport properties [24,26]. A systematic understanding of stress origins, together with effective characterization and control strategies, is therefore essential for advancing mosaic growth toward practical application.

4.1. Sources and Multiscale Characterization of Interfacial Stress

Accurate characterization is a prerequisite for understanding the origins and evolution of interfacial stress. Researchers have developed multiscale methods ranging from atomic to macroscopic scales to systematically investigate stress generation mechanisms.
Thermal mismatch stress originates from the temperature gradient that persists as the mosaic wafer cools from CVD growth temperatures (~900–1000 °C) to room temperature, with slight crystallographic orientation deviations between adjacent seeds inducing differential contraction during cooling [26]. As shown in Figure 7, Chen et al. performed Raman mapping on a mosaic SCD interface region. The confocal laser-scanning images of the as-grown mosaic sample after 24 h and 48 h growth (Figure 7a,b) with red arrows indicating off-axis directions, combined with Raman FWHM mapping (Figure 7c), diamond peak shift mapping (Figure 7d), and internal stress distribution (Figure 7e), reveal that the FWHM broadens from ~3.0 cm−1 in the bulk to ~8.0 cm−1 at the interface, while the diamond peak shifts from 1332.37 to 1331.45 cm−1 [79]. The resulting internal stress distribution forms a pronounced local stress concentration zone spanning ~200–400 μm around the interface, directly confirming that non-negligible residual stress exists at mosaic growth interfaces and expands progressively with increasing epitaxial layer thickness.
Intrinsic growth stress arises from the mutual compression of adjacent seed crystals when their lateral growth fronts meet, as well as from dislocations and stacking faults introduced under non-equilibrium conditions. Chen et al. observed that the off-axis directions of substrates 1–4 are mutually inconsistent (Figure 7a), causing adjacent growth fronts to advance with different step-flow directions, collide, and induce localized lattice distortion and stress concentration [79]. Fischer et al. proposed the “effective dislocation climb” mechanism to explain this phenomenon: under non-equilibrium growth, non-conservative dislocation motion leads to insufficient lattice relaxation and residual macroscopic stress [80]. When the lateral-to-vertical growth rate ratio λ exceeds 1, the dislocation density in the ELO region decreases by a factor of 20 and tensile stress drops by ~0.14 GPa [51]. Furthermore, impurity atoms (N, B, Si) introduced during CVD growth induce local lattice distortion owing to atomic radius mismatch with carbon [81]; first-principles calculations by Lu et al. indicate that such local stress can reach the gigapascal order [82].
Structural mismatch stress results from the formation of non-diamond phases (e.g., amorphous carbon, graphite) at mosaic growth interfaces and their specific volume difference relative to diamond. In the ELO process, low-density phases persisting in laser-damaged zones at hole edges generate significant localized stress under constrained conditions. Using confocal Raman imaging, Ichikawa et al. directly observed the mirror-symmetric tensile–compressive strain field distribution around single dislocations [83]. Although this microscopic stress field contributes negligibly to macroscopic stress, it markedly affects carrier transport and optical properties in the vicinity of dislocations.
Among characterization techniques, micro-Raman spectroscopy is the most widely used non-destructive method, based on the stress-induced frequency shift in the first-order diamond Raman peak (1332.5 cm−1): compressive stress produces a blue shift, tensile stress a red shift. For (001)-oriented diamond under the biaxial stress approximation, σ (GPa) = −0.61·Δω (cm−1). Confocal Raman imaging enables two- and three-dimensional stress mapping; Ichikawa et al. employed this technique to directly observe the mirror-symmetric strain distribution around single dislocations [83]. Zhao et al. demonstrated that under optimized ELO conditions, the Raman peak position was 1332.08 cm−1 with an FWHM of 1.84–2.09 cm−1, corresponding to a tensile stress reduction of ~0.14 GPa [51]. Shu et al. systematically applied confocal Raman stress mapping to obtain full-field stress distributions across mosaic interfaces [72]. Guo et al. compared the Raman FWHM of SCD films grown under different atmospheres, providing a comparative benchmark for evaluating crystal quality at mosaic growth interfaces [84].
X-ray topography (XRT), in particular, enables full-wafer visualization of dislocations, point defects, block boundaries, and long-range strain fields with micron-scale spatial resolution. When combined with 2D detectors, X-ray diffraction and topography methods can comprehensively determine both the magnitude and spatial distribution of mechanical deformations across the entire crystal. Complementary techniques include X-ray diffraction (XRD), which characterizes lattice bending and misorientation via rocking curve FWHM analysis [85]; birefringence and photoelastic imaging, which leverage the stress-induced birefringence effect in diamond to enable real-time full-field stress visualization [86]; transmission electron microscopy (TEM), which provides direct atomic-scale imaging of dislocation cores, stacking faults, and interface structures, offering fundamental insights into the mechanisms of defect-mediated stress relaxation [87]—Tallaire et al. have systematically identified the main types of dislocations in CVD-grown single-crystal diamond [88]; and cathodoluminescence spectroscopy, which provides nanoscale spatially resolved stress–defect correlation analysis [89]. More recently, X-ray microfluorescence microscopy has begun to show some promise in observing the distribution of trace impurities that are often the cause of local stress concentration.
Residual stress at the mosaic interface will change the structure and reduce heat conduction; therefore, it is relatively detrimental to high-power electronic devices. Stress-induced defects, threading dislocations and grain boundaries act as effective phonon-scattering centers to reduce the effective thermal conductivity of single-crystal diamond. Molecular dynamics simulations have shown that the thermal conductivity in the area of severe stress concentration may be reduced by about 30–50 per cent, and thus, the local heat-spreading ability near the mosaic interface is approximately one order of magnitude lower than that in the surrounding bulk crystal [90]. Arnault et al. have also shown that defect-rich areas near mosaic junctions have a relatively high thermal resistance and are significantly reduced in-plane heat conduction [10]. Recently, with the development of time-domain thermoreflectance (TDTR) and Raman thermometry, both local thermal conductivity and stress distribution at mosaic interfaces can be mapped simultaneously. However, the quantitative relationship among the size of the interfacial stress, defect density and degradation of thermal conductivity remains unknown. The development of such structure–stress–thermal transport correlations will be an important direction for future research on mosaic-grown single-crystal diamond.
Residual stress and micro-crystal defects caused by mosaic growth can also change the structure, electrical properties, optical properties and thermal properties of single-crystal diamond wafers. The main types of defects are threading dislocations (with edge, 45° mixed, and 60° mixed character) [88], stacking faults, twins and impurity-related point defects; all of them are concentrated near mosaic interfaces where adjacent growth fronts meet. Threading dislocations, typically originating from the seed crystal or the mosaic interface, propagate vertically through the epitaxial layer and act as carrier scattering centers and non-radiative recombination sites. Stacking faults and twins, commonly observed at mosaic junctions where misoriented growth fronts coalesce, introduce localized electronic states within the bandgap. Nitrogen-related defects, prevalent in HPHT seeds and incorporated during CVD growth under nitrogen-rich conditions, can serve as both unintentional dopants and stress concentrators due to lattice expansion around substitutional nitrogen sites. No single characterization technique can identify all kinds of defects; therefore, many are used in combination: selective chemical etching followed by SEM imaging for etch-pit density quantification; cathodoluminescence spectroscopy for identifying specific defect-related luminescence signatures [89]; and high-resolution X-ray diffraction for assessing global crystalline quality through rocking curve analysis [85].

4.2. Stress Control via Process Optimization

The first dimension of stress control addresses the growth process itself, aiming to reduce stress generation and accumulation at the source through parameter optimization. MPCVD growth parameters directly govern intrinsic growth stress: when λ > 1, lateral growth dominates and tensile stress in the ELO region is substantially reduced [51]. By finely tuning methane concentration, chamber pressure, and temperature, the λ value can be optimized while maintaining growth efficiency, thereby achieving synergistic control of stress and growth rate. Sedov et al. quantitatively analyzed the effect of CH4 concentration on the sp2/sp3 carbon ratio in MPCVD diamond [91]. As shown in Figure 8, the monotonic increase in the sp2/sp3 ratio with increasing CH4 concentration (νc) at substrate temperatures of 800 °C (Figure 8a) and 900 °C (Figure 8b) directly demonstrates that excessive methane promotes non-diamond carbon phase formation, leading to higher defect density and elevated intrinsic growth stress. The addition of an appropriate amount of O2 selectively etches non-diamond phases, thereby reducing defect-induced intrinsic stress [62]; pulsed growth, in turn, interrupts continuous stress accumulation through alternating growth and etching cycles [92].
The cooling process from the high growth temperature to room temperature is the primary stage in which thermal stress is generated. Controlling the cooling rate and introducing annealing temperature plateaus provides sufficient atomic and dislocation mobility, enabling stress relaxation through diffusion and climb. Wuxi Xinlei Precision Technology Co., Ltd. proposed an epitaxial healing method for mosaic growth seams based on in situ high-temperature annealing strengthening [93]. By promoting sufficient atomic diffusion across the interfaces to eliminate residual stress, this approach brings the electrical, thermal, and mechanical properties of the healed interface close to those of bulk single crystals.

4.3. Stress Control via Geometric Design

The second dimension of stress control addresses interfacial stress through geometric design, guiding stress release along predetermined paths. Shandong University proposed fabricating a SiO2 mask at the mosaic growth seam after the first growth step, followed by a second epitaxial growth [94]. The laterally overgrowing epitaxial layer effectively “bypasses” the defects beneath the mask, preventing defect and stress concentration zones at the seam from propagating vertically into subsequent epitaxial layers. Owing to the low fracture toughness of diamond (~5–6 MPa·m1/2), it is highly sensitive to edge stress concentration; chamfering or rounding seed crystal edges can effectively disperse mechanical stress [95]. Furthermore, rectangular holes (aspect ratio > 1) outperform square holes in reducing stress and dislocation density, and a 60° mosaic growth angle is more advantageous than vertical joining for lowering residual stress [54].
Beyond these “passive” stress relief strategies, strain engineering offers a novel “active” perspective on interfacial stress control. Using density functional theory (DFT), Lu et al. systematically investigated the effects of biaxial strain on the ionization energies of B- and P-doped diamond, finding that a 3% biaxial tensile strain reduced the B-doped ionization energy from 0.33 eV to 0.12 eV (a 64% reduction) and the P-doped ionization energy from 0.56 eV to 0.35 eV (a 37% reduction) [82]. Jing et al. demonstrated that centimeter-scale polycrystalline diamond films can withstand elastic tensile strains of up to ~4% without fracture, providing a material foundation for the experimental realization of strain engineering [96]. Although these studies focused primarily on doping regulation, the intrinsic “strain–electronic structure” correlation they revealed offers important insights for stress control at mosaic growth interfaces: if the strain state can be actively tuned through interface design to a range beneficial to device performance, “stress” can be transformed into a “functional control tool.”

4.4. Theoretical Modeling of Interfacial Stress

Prior to the development of phase-field and machine-learning-based simulations, fluid-based plasma modeling had already provided valuable insights into the growth uniformity of large-area mosaic wafers. Yamada et al. numerically simulated the distributions of atomic hydrogen and methyl radicals over a 2-inch substrate and found that the substrate temperature uniformity played a critical role in maintaining a consistent growth rate across the entire area [59]. Their results highlighted that the edge temperature drop limited the growth rate uniformity at the wafer periphery, a finding that remains relevant for scaling beyond 2-inch dimensions.
The phase-field method offers unique advantages in describing anisotropic crystal growth and interface morphology evolution. Liu et al. proposed an improved anisotropic interfacial energy coefficient function and successfully simulated the faceted growth process of diamond; the simulation results revealed that the total free energy of diamond grains reaches a minimum when the reconstructed surface interfacial energy lies in the range of 6.7–7.2 J/m2 [97]. Molecular dynamics simulations based on machine-learned interatomic potentials have revealed the atomic-scale structural transformation mechanism of incoherent twin boundaries under stress [90]. As shown in Figure 9, MD snapshots (Figure 9(a1–a7)) capture the transition from an ordered diamond structure to a disordered sp2-rich interface phase under shear loading, while the relative proportion of C–C bonds at the grain boundary interface (Figure 9(b1–b6)) indicates bond-breaking and bond-forming stages, with the interfacial thermal conductance dropping by ~80% during this transformation—a finding that provides important insights into the coupled effects of stress, defects, and thermal transport. Molecular dynamics simulations have also been applied to investigate the influence of interfacial microdefects on thermal conductivity, where the presence of vacancies simultaneously provides potential sites for stress relaxation [98]. Beyond providing atomic-scale insights, these theoretical simulations offer directly testable hypotheses for experimental process optimization. The MD simulations by Lu et al. revealed a critical transition from an ordered diamond structure to a disordered sp2-rich interface phase under shear loading, accompanied by an ~80% drop in interfacial thermal conductance [90]. This finding suggests a clear, actionable strategy: avoiding growth conditions that drive incoherent twin boundaries toward this sp2-rich transition state could simultaneously prevent severe stress accumulation and mitigate thermal transport degradation at mosaic interfaces. Experimentally, this translates to optimizing the CH4/H2 ratio and substrate temperature to kinetically suppress this detrimental transition—a hypothesis that can be validated through in situ Raman monitoring during growth or ex situ TEM characterization of interfaces grown under varying parameter windows [87]. The phase-field simulations of Liu et al. further complement this by providing quantitative guidance on the optimal interfacial energy range (6.7–7.2 J/m2) for minimizing grain boundary free energy, offering a thermodynamic target for experimental geometric design [97].
Table 3 shows the main stress-regulating strategies reported for mosaic-growth interfaces, as well as their corresponding mechanisms, representative implementation methods, and typical effects on residual stress, interface quality and crystal performance.

5. Summary and Prospects

This review has traced the full engineering chain of single-crystal diamond mosaic growth, from seed preparation to geometric design, parameter control, and stress management. Mosaic growth is not merely a technical remedy for the size limitation of single seeds. Rather, it represents a paradigm shift that decouples wafer dimensions from seed dimensions through the assembly of cloned, crystallographically matched plates. The central challenge is fundamentally mechanical rather than chemical: interface quality is governed by the generation, accumulation, and mitigation of stress arising from thermal mismatch, intrinsic growth compression, and structural incompatibility. Current stress regulation strategies have followed two largely parallel tracks, namely process optimization and geometric design, which have rarely been integrated. Meanwhile, the emerging concept of strain engineering offers a transformative third path that reframes stress not as a defect to be eliminated but as a functional degree of freedom for modulating electronic properties.
Looking forward, the transition from laboratory demonstration to industrial manufacturing hinges on three interconnected bottlenecks: scalable seed cloning with sufficient throughput, closed-loop process control integrating in situ monitoring with machine learning, and the systematic challenges of scaling from 2-inch to 4-inch wafers without compromising uniformity or stress management. Beyond these engineering hurdles, the integration of strain engineering with interface design and the development of structure-stress correlations represent the most promising frontiers for transforming mosaic SCD from a materials science achievement into a practical platform for next-generation electronics. The foundational science is largely established. The engineering challenges, however, are substantial, and their resolution will determine whether mosaic growth fulfills its potential as the enabling technology for large-area single-crystal diamond devices.

Author Contributions

Conceptualization, J.B.; investigation, R.R.; writing—original draft preparation, R.R.; writing—review and editing, R.R. and J.B.; supervision, J.B. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (Grant No. 52302037), the China Postdoctoral Science Foundation (Grant No. 2023M730389), and Research and Innovation Program for Graduate Students in Chongqing (Grant No. CYS25583).

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Nitrogen-assisted lateral overgrowth of homoepitaxial single-crystal diamond. Optical photographs showing the evolution of sample N180 during MPCVD growth. The crystal expands from an initial 5.5 × 5.5 mm2 square seed (0 h) to a 7.6 mm polygonal crystal after 20 h, corresponding to an area increase of approximately 1.6 times [23].
Figure 1. Nitrogen-assisted lateral overgrowth of homoepitaxial single-crystal diamond. Optical photographs showing the evolution of sample N180 during MPCVD growth. The crystal expands from an initial 5.5 × 5.5 mm2 square seed (0 h) to a 7.6 mm polygonal crystal after 20 h, corresponding to an area increase of approximately 1.6 times [23].
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Figure 2. Plasma-assisted polishing (PAP) of a 20 mm square mosaic SCD substrate. (a,b) SWLI overview images before and after PAP, showing the complete flattening of the ~100 μm CVD-induced waviness. (c) AFM image after PAP, showing a sub-nanometer smooth surface (Sq = 0.361 nm). (d) Statistical comparison of Raman peak position and FWHM between unpolished and PAP-polished areas. No significant differences are observed, confirming that PAP introduces virtually no residual stress. Reproduced under the CC BY 4.0 license from Ref. [41].
Figure 2. Plasma-assisted polishing (PAP) of a 20 mm square mosaic SCD substrate. (a,b) SWLI overview images before and after PAP, showing the complete flattening of the ~100 μm CVD-induced waviness. (c) AFM image after PAP, showing a sub-nanometer smooth surface (Sq = 0.361 nm). (d) Statistical comparison of Raman peak position and FWHM between unpolished and PAP-polished areas. No significant differences are observed, confirming that PAP introduces virtually no residual stress. Reproduced under the CC BY 4.0 license from Ref. [41].
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Figure 3. Step-flow modulation and its effect on mosaic SCD interface quality. (a) OM image of the central region of the mosaic sample with four boundaries marked. (b–e) Enlarged images of boundaries ①–④, with red lines and arrows indicating step-flow directions: consistent directions produce smooth fusion, while inconsistent directions cause overgrowth and stress concentration. (f,g) FWHM mapping of first-order Raman peaks and internal stress distribution over a 2 × 2 mm2 area covering the four boundary intersections, revealing a residual stress as low as ~0.04 GPa. Reproduced under the CC BY 4.0 license from Ref. [18]. Copyright 2023, Taylor & Francis.
Figure 3. Step-flow modulation and its effect on mosaic SCD interface quality. (a) OM image of the central region of the mosaic sample with four boundaries marked. (b–e) Enlarged images of boundaries ①–④, with red lines and arrows indicating step-flow directions: consistent directions produce smooth fusion, while inconsistent directions cause overgrowth and stress concentration. (f,g) FWHM mapping of first-order Raman peaks and internal stress distribution over a 2 × 2 mm2 area covering the four boundary intersections, revealing a residual stress as low as ~0.04 GPa. Reproduced under the CC BY 4.0 license from Ref. [18]. Copyright 2023, Taylor & Francis.
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Figure 4. Effect of methane concentration on the surface morphology of CVD-grown single-crystal diamond. Optical microscopy images of SCD surfaces grown under different CH4 concentrations: (a) 6.0%; (b) 6.2%; (c) 6.4%; (d) 7.0%; (e) 8.0%. The smoothest surface with ordered step-flow is obtained at 6.4% CH4 (~0.573 nm RMS), whereas 8.0% CH4 produces abundant non-epitaxial crystallites and polycrystalline regions. Reproduced under the CC BY 4.0 license from Ref. [61]. Copyright 2021, MDPI.
Figure 4. Effect of methane concentration on the surface morphology of CVD-grown single-crystal diamond. Optical microscopy images of SCD surfaces grown under different CH4 concentrations: (a) 6.0%; (b) 6.2%; (c) 6.4%; (d) 7.0%; (e) 8.0%. The smoothest surface with ordered step-flow is obtained at 6.4% CH4 (~0.573 nm RMS), whereas 8.0% CH4 produces abundant non-epitaxial crystallites and polycrystalline regions. Reproduced under the CC BY 4.0 license from Ref. [61]. Copyright 2021, MDPI.
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Figure 5. Two-step epitaxial lateral overgrowth (ELO) for dislocation reduction. (a) Schematic of the two-step ELO process. (b,c) FE-SEM images of etch-pits on the first and second ELO layers after plasma etching, showing a pronounced decrease in etch-pit density. Reproduced under the CC BY 4.0 license from Ref. [70]. Copyright 2017, MDPI.
Figure 5. Two-step epitaxial lateral overgrowth (ELO) for dislocation reduction. (a) Schematic of the two-step ELO process. (b,c) FE-SEM images of etch-pits on the first and second ELO layers after plasma etching, showing a pronounced decrease in etch-pit density. Reproduced under the CC BY 4.0 license from Ref. [70]. Copyright 2017, MDPI.
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Figure 6. Reduction in threading dislocations in single-crystal diamond by in situ tungsten incorporation. (a–d) SEM images of etched surfaces under different tungsten doping conditions. The arrow in (d) points to the S13 sample with a double-layer tungsten-incorporated structure. (e) Corresponding variation in dislocation density, demonstrating a marked decrease with increasing tungsten incorporation. Reproduced under the CC BY 4.0 license from Ref. [78]. Copyright 2022, MDPI.
Figure 6. Reduction in threading dislocations in single-crystal diamond by in situ tungsten incorporation. (a–d) SEM images of etched surfaces under different tungsten doping conditions. The arrow in (d) points to the S13 sample with a double-layer tungsten-incorporated structure. (e) Corresponding variation in dislocation density, demonstrating a marked decrease with increasing tungsten incorporation. Reproduced under the CC BY 4.0 license from Ref. [78]. Copyright 2022, MDPI.
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Figure 7. Stress distribution at the interface junction of CVD mosaic single-crystal diamond. (a,b) Confocal laser-scanning images of the as-grown mosaic sample after 24 h and 48 h growth, respectively, with red arrows in (a) indicating off-axis directions of the original substrates. The red box in (b) marks the 3 × 10 mm2 Raman mapping region, which corresponds to the scanning area shown in (e). (c) Raman FWHM mapping of the junction region. (d) Raman sp3 diamond peak shift mapping (1332.37–1331.45 cm−1). (e) Internal stress distribution calculated from the peak shift, and the red box indicates the same Raman mapping region as in (b). Reproduced under the CC BY 4.0 license from Ref. [79]. Copyright 2020, MDPI.
Figure 7. Stress distribution at the interface junction of CVD mosaic single-crystal diamond. (a,b) Confocal laser-scanning images of the as-grown mosaic sample after 24 h and 48 h growth, respectively, with red arrows in (a) indicating off-axis directions of the original substrates. The red box in (b) marks the 3 × 10 mm2 Raman mapping region, which corresponds to the scanning area shown in (e). (c) Raman FWHM mapping of the junction region. (d) Raman sp3 diamond peak shift mapping (1332.37–1331.45 cm−1). (e) Internal stress distribution calculated from the peak shift, and the red box indicates the same Raman mapping region as in (b). Reproduced under the CC BY 4.0 license from Ref. [79]. Copyright 2020, MDPI.
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Figure 8. Effect of methane concentration on the sp2/sp3 carbon ratio in MPCVD diamond. Ratio of sp2 to sp3 carbon bonds as a function of methane concentration (νc) at substrate temperatures of 800 °C (a) and 900 °C (b). The monotonic increase in the sp2/sp3 ratio with increasing νc quantitatively demonstrates that excessive methane promotes non-diamond carbon phase formation, leading to higher defect density and increased intrinsic growth stress. The inset shows a representative Raman spectrum deconvolution for the sample grown at νc = 5% and Ts = 900 °C. Reproduced under the CC BY 4.0 license from Ref. [91]. Copyright 2023, MDPI.
Figure 8. Effect of methane concentration on the sp2/sp3 carbon ratio in MPCVD diamond. Ratio of sp2 to sp3 carbon bonds as a function of methane concentration (νc) at substrate temperatures of 800 °C (a) and 900 °C (b). The monotonic increase in the sp2/sp3 ratio with increasing νc quantitatively demonstrates that excessive methane promotes non-diamond carbon phase formation, leading to higher defect density and increased intrinsic growth stress. The inset shows a representative Raman spectrum deconvolution for the sample grown at νc = 5% and Ts = 900 °C. Reproduced under the CC BY 4.0 license from Ref. [91]. Copyright 2023, MDPI.
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Figure 9. Atomic-scale insights into stress-driven grain boundary transition in diamond. (a1–a7) MD snapshots of the structural transformation of incoherent twin boundaries under shear loading, with I–VII marking sequential strain states from the initial structure (I) through the lowest bond-breaking point (IV) to the final transitioned state (VII). (b1–b6) Corresponding local atomic structure evolution, showing bond-breaking (red) and bond-forming (blue) stages. The interfacial thermal conductance drops by ~80% during this transformation. Reproduced under the CC BY 4.0 license from Ref. [90]. Copyright 2025, Wiley-VCH.
Figure 9. Atomic-scale insights into stress-driven grain boundary transition in diamond. (a1–a7) MD snapshots of the structural transformation of incoherent twin boundaries under shear loading, with I–VII marking sequential strain states from the initial structure (I) through the lowest bond-breaking point (IV) to the final transitioned state (VII). (b1–b6) Corresponding local atomic structure evolution, showing bond-breaking (red) and bond-forming (blue) stages. The interfacial thermal conductance drops by ~80% during this transformation. Reproduced under the CC BY 4.0 license from Ref. [90]. Copyright 2025, Wiley-VCH.
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Table 1. Representative achievements in homoepitaxial single-crystal diamond worldwide.
Table 1. Representative achievements in homoepitaxial single-crystal diamond worldwide.
Research TeamYearTechnical ApproachMaximum Lateral SizeKey Limitation
Mokuno et al.
(AIST, Japan)		2005Repeated high-rate growth on a closed substrate holder [14].Thickness: 10 mm
(lateral dimensions only 5 × 5 mm2)		Excellent vertical thickening capability, but lateral dimensions remain limited
Mokuno et al.
(AIST, Japan)		2009Lift-off process combined with lateral growth for diameter expansion [13].12.6 × 13.3 mm2
(half-inch)		Limited lateral expansion (from 9 mm to 12.6 mm)
Zou Guangtian’s team
(Jilin University)		2022Optimization of seed crystal spatial positioning and growth mode using a closed substrate holder [30].8.6 × 8.6 mm2
(~51% area increase)		Improved uniformity, but no independent breakthrough in lateral size achieved
Zhang et al.
(Xidian University)		2025Lateral growth on (110) crystal plane with optimized CH4/O2 ratio [16].7.03 × 8.12 mm2
(~75% area increase)		Size remains at the millimeter scale, still falling short of the inch scale
Yang et al.
(NYCU, Taiwan)		2025Nitrogen-assisted MPCVD enhancing (001) lateral growth [23].7.6 mm
(~1.6-fold area increase)		Limited lateral expansion; absolute size remains at the millimeter scale
Kumar et al. (IIT Madras)2026Lateral outward growth under high-pressure MPCVD conditions (165 Torr) [15].12.56 × 12.59 mm2
(59% area increase)		Limited lateral expansion; no order-of-magnitude breakthrough in absolute size
Table 2. Key performance indicators of different polishing techniques for single-crystal diamond.
Table 2. Key performance indicators of different polishing techniques for single-crystal diamond.
Polishing TechniqueCore MechanismMRRSurface RoughnessSub-Surface Damage
Mechanical Polishing (MP)High-speed cutting with diamond grit [40].->1 nmSevere (scratches, amorphous layer)
Chemical Mechanical Polishing (CMP)Synergistic action of oxidant and abrasive [45].~300 nm/h0.3–0.5 nmLow
Plasma-Assisted
Polishing (PAP)		Ar/O2 plasma irradiation + removal by quartz glass [41].13.3 μm/h0.4–0.5 nm SqNone
Medium-Vacuum
UV-Assisted Polishing		UV irradiation-induced graphitization + removal by quartz glass [42].8.0 μm/h<0.5 nmNone
UV Photocatalytic-Assisted CMPTiO2 + PB/H2O2 ·OH oxidation + abrasive removal [43].1168 nm/h0.079 nm SaVery thin (0.66 nm)
Mechanochemical
Synergistic Wheel Polishing		Active metal-catalyzed graphitization + wheel removal [44].8.186 μm/h0.721 nm SaVery thin amorphous layer
Table 3. Classification and effects of stress regulation strategies at mosaic growth interfaces.
Table 3. Classification and effects of stress regulation strategies at mosaic growth interfaces.
Regulation StrategySpecific MethodRegulation MechanismTypical Effect
Process
optimization		Optimize the lateral-to-vertical growth rate ratio λ (λ > 1)Promote dislocation bending and lateral propagation rather than vertical extension to the surface [51].Dislocation density reduced from 6 × 106 to 3 × 105 cm−2 (20-fold reduction); tensile stress reduced by ~0.14 GPa [51]
Process
optimization		Addition of appropriate amount of O2 + pulsed growthO2 introduces OH radicals that selectively etch non-diamond phases [62]; pulsed mode interrupts continuous stress accumulation through alternating growth–etching cycles [92].Improved crystal quality, reduced Raman FWHM, and decreased intrinsic stress [62,92].
Process
optimization		In situ high-temperature annealingSufficient atomic diffusion across the interface during in situ annealing at elevated temperatures eliminates residual stress [93].Electrical, thermal, and mechanical properties of the healed interface approach those of bulk single crystals [93]
Geometric
design		SiO2 mask blockingAfter the first mosaic growth, a SiO2 mask is deposited at the seam; defects beneath the mask are “bypassed” by the laterally overgrowing epitaxial layer [94].Effectively prevents defects and stress at the seam from propagating vertically into subsequent epitaxial layers [94].
Geometric
design		Seed crystal edge chamferingChamfering or rounding the edges of seed crystals to disperse mechanical stress [95].Reduces the edge stress concentration factor, lowering the risk of edge chipping and cracking [95].
Geometric
design		60° interface angleThe inclined interface alters the manner in which lateral growth fronts meet, preventing direct collision perpendicular to the growth surface [54].The interface is the smoothest, with the lowest residual stress of only 0.42 GPa [54]
Geometric
design		Rectangular holes (aspect ratio > 1)The longer side provides a longer stable extension distance, maintaining an ordered step-flow [51].More effective than square holes in reducing stress and dislocation density [51].
Strain
engineering		3% biaxial tensile strainModulates the band structure of diamond and the electronic states of dopant atoms [82].B-doping ionization energy decreases from 0.33 eV to 0.12 eV (a 64% reduction), and P-doping decreases from 0.56 eV to 0.35 eV (a 37% reduction) [82].
Flexible
connection		SiC nanowire interlayerNanowires penetrate the diamond crystal to form a flexible transition layer; their deformability provides additional degrees of freedom for stress release [75].With a coefficient of thermal expansion (CTE) intermediate between that of diamond and metal, it can effectively mitigate the accumulation of thermal mismatch stress (theoretical prediction) [75].
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Rong, R.; Bai, J. Advances in Homoepitaxial Mosaic Single-Crystal Diamond: Interface Stress Regulation. Crystals 2026, 16, 448. https://doi.org/10.3390/cryst16070448

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Rong R, Bai J. Advances in Homoepitaxial Mosaic Single-Crystal Diamond: Interface Stress Regulation. Crystals. 2026; 16(7):448. https://doi.org/10.3390/cryst16070448

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Rong, Rong, and Jie Bai. 2026. "Advances in Homoepitaxial Mosaic Single-Crystal Diamond: Interface Stress Regulation" Crystals 16, no. 7: 448. https://doi.org/10.3390/cryst16070448

APA Style

Rong, R., & Bai, J. (2026). Advances in Homoepitaxial Mosaic Single-Crystal Diamond: Interface Stress Regulation. Crystals, 16(7), 448. https://doi.org/10.3390/cryst16070448

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