Diamond, due to its exceptional hardness, ultra-high thermal conductivity, excellent wide bandgap semiconductor characteristics, and biocompatibility, holds great promise for applications in high-temperature and high-frequency electronic devices, optical windows, and heat spreaders. However, the preparation of large, high-quality single-crystal diamond remains extremely difficult and costly. Therefore, heteroepitaxial growth of diamond films or quasi-single-crystal diamond on non-diamond substrates has become an important technical route.

Key Requirements for Substrate Selection

Choosing an appropriate substrate is crucial for successful epitaxy. Ideally, it should satisfy the following conditions:
1. Lattice Matching – The closer the lattice constants of the substrate and diamond, the fewer the defects in the epitaxial layer.
2. Thermal Expansion Coefficient Matching – To avoid film cracking or warping due to differential contraction during cooling.
3. Chemical Compatibility and Stability – The substrate must remain stable under high temperatures (typically 700–1000°C) and hydrogen plasma environments used for diamond growth, and should not react strongly with carbon.
4. Cost and Availability – The substrate should be available in large sizes with high flatness and reasonable cost.

Detailed Analysis and Comparison of Three Typical Substrates

1. Sapphire Substrate

Sapphire was one of the earliest materials used for diamond epitaxy.

Material Properties:
Chemical formula: α-Al₂O₃
Crystal structure: Trigonal
Thermal expansion coefficient: ~7.5 × 10⁻⁶ /K (along the a-axis)
Thermal conductivity: ~35 W/(m·K) (room temperature)

Advantages:
Mature technology and moderate cost: Widely used in the LED industry, making large (up to 12-inch), high-quality, low-cost sapphire wafers easily available.
Excellent optical properties: Transparent, suitable for diamond devices requiring optical transmission such as UV detectors.
High chemical stability: Extremely stable at high temperatures and does not form carbides with carbon.

Disadvantages:
Severe lattice mismatch: The mismatch with diamond is about 30%, resulting in a high density of disordered grains when grown directly, typically forming polycrystalline diamond films rather than single crystals.
Thermal expansion mismatch: The thermal expansion coefficient of sapphire is much lower than that of diamond (~1.0 × 10⁻⁶ /K), causing tensile stress and microcracks upon cooling.
Lack of nucleation sites: The surface provides few effective nucleation centers for diamond, often requiring aggressive pretreatment (e.g., diamond powder seeding) or bias-enhanced nucleation, both of which introduce many defects.

Main Applications:
Polycrystalline diamond films for cutting tool coatings, heat spreaders, and optical protective layers where crystal quality is not critical.
Not suitable for high-performance electronic devices.

2. Silicon Substrate

Silicon, the cornerstone of the modern microelectronics industry, is also a widely studied substrate for diamond growth.

Material Properties:
Chemical formula: Si
Crystal structure: Diamond cubic
Thermal expansion coefficient: ~2.6 × 10⁻⁶ /K
Thermal conductivity: ~150 W/(m·K) (room temperature)

Advantages:
Extremely low cost and excellent availability: Among all substrates, silicon is the cheapest, largest (up to 12-inch), flattest, and most technologically mature.
Perfect compatibility with existing microelectronic processes: Facilitates integration of diamond-based devices with silicon circuits, which is its biggest advantage.
Identical crystal structure: Both diamond and silicon share the diamond cubic structure, though their lattice constants differ.

Disadvantages:
Large lattice mismatch: About 52% with diamond, leading to high nucleation defect densities. The initial growth layer is typically polycrystalline, requiring advanced methods (e.g., step-flow or bias-assisted growth) to achieve quasi-epitaxy, though crystal quality remains limited.
Severe C–Si interdiffusion: At diamond growth temperatures, carbon reacts with silicon to form a silicon carbide interlayer. This layer can partially relieve stress but also introduces uncontrolled defects.
Thermal expansion mismatch: Silicon expands more than diamond, causing compressive stress and possible film wrinkling or delamination during cooling.

Main Applications:
Polycrystalline or nanocrystalline diamond films for mechanical coatings, MEMS devices, and heat spreaders.
Diamond-on-Si heterojunction electronic devices using quasi-epitaxy are an active research topic, though performance still lags behind true single-crystal diamond devices.

3. Silicon Carbide (SiC) Substrate

Silicon carbide is considered the most promising substrate for epitaxial single-crystal diamond growth.

Material Properties:
Chemical formula: SiC
Crystal structure: Various polytypes; the most common are 4H-SiC and 3C-SiC. 3C-SiC shares the cubic structure with diamond.
Thermal expansion coefficient: ~4.5 × 10⁻⁶ /K (4H-SiC, along the a-axis)
Thermal conductivity: ~350–490 W/(m·K) (depending on polytype) — the highest among common substrates.

Advantages:
Smallest lattice mismatch: Among all common substrates, SiC has the lowest lattice mismatch with diamond (~22% for 3C-SiC and as low as ~1.3% for 3C-SiC(001)), depending on crystal face and polytype. This is the key to achieving high-quality epitaxy.
Excellent thermal properties: High thermal conductivity makes SiC an outstanding heat spreader, forming a “super heat dissipation” combination with diamond.
Chemical and structural compatibility: Being a carbide itself, it avoids the interdiffusion issues seen with Si substrates. Proper surface treatments (e.g., hydrogen etching) can generate a graphite-like layer on SiC that facilitates diamond nucleation.
Good thermal expansion match: Its expansion coefficient lies between that of silicon and diamond, resulting in minimal thermal stress.

Disadvantages:
High cost: Producing large, high-quality single-crystal SiC wafers is difficult and expensive, significantly limiting large-scale use.
Limited wafer size: Commercial SiC wafers are mainly available up to 6 inches (8-inch under development), smaller than silicon wafers.
Complex surface preparation: Requires precise control of surface termination and reconstruction to achieve diamond-compatible surfaces.

Main Applications:
Epitaxial growth of high-quality single-crystal diamond for high-performance diamond-based electronic devices (e.g., HEMTs, diodes) and quantum devices.
Advanced heat dissipation applications.


Picture: Diamond on SiC

Summary Comparison Table

Property	Sapphire	Silicon	Silicon Carbide
Lattice Mismatch	Very large (~30%)	Very large (~52%)	Small (~22% or less)
Thermal Expansion Match	Poor (diamond under tensile stress)	Poor (diamond under compressive stress)	Good (minimal stress)
Chemical Stability	Excellent	Poor (forms SiC layer)	Excellent
Thermal Conductivity	Low	Moderate	Very high
Substrate Cost	Moderate	Very low	Very high
Availability & Size	Excellent (up to 12”)	Excellent (up to 12”)	Moderate (mainly 6”)
Process Compatibility	Fair	Perfect	Good (wide-bandgap semiconductor processes)
Achievable Diamond Quality	Polycrystalline	Poly-/nano-/quasi-single-crystal	Single-crystal / high-quality epitaxial
Main Applications	Optical coatings, tool coatings	MEMS, heat spreaders, exploratory electronics	High-performance electronics, quantum devices, top-tier heat spreaders

Conclusions and Outlook

Sapphire remains valuable for optical and polycrystalline diamond coatings where crystal quality is less critical, due to its cost and optical transparency.
Silicon offers unmatched cost-effectiveness and CMOS compatibility, making it the most promising platform for large-scale applications in MEMS, heat dissipation, and low-cost sensors. Current research focuses on improving diamond grain size and orientation uniformity despite the severe mismatch.
Silicon Carbide (SiC) is undoubtedly the substrate of choice for pursuing ultimate performance. Its superior lattice and thermal compatibility make it ideal for next-generation high-frequency, high-power diamond electronic and quantum devices. As SiC costs decrease and wafer sizes increase, its application range will expand further.
In summary, selecting among these substrates represents a classic performance–cost–application trade-off. There is no absolute “best” substrate—only the “most suitable” one for a specific scenario. Future breakthroughs may come from new buffer layer technologies or advanced heterogeneous integration methods that overcome current substrate limitations.

Products from JXT Technology Co.,Ltd.

JXT Technology Co.,Ltd. provides ultra-thick substrates of various materials for heteroepitaxial diamond growth:
Silicon wafers: 2 / 3 / 4 / 6 / 8 / 12 inches, thickness 1–10 mm, single-side or double-side polished
Sapphire wafers: 2 / 3 / 4 / 6 / 8 / 12 inches, thickness 1–10 mm, single-side or double-side polished
Silicon carbide wafers: 2 / 3 / 4 / 6 / 8 / 12 inches, thickness 1–10 mm, single-side or double-side polished