Aluminum nitride (AlN) serves as a critical substrate for gallium nitride (GaN) epitaxy due to its favorable material properties, which enable high-performance optoelectronic and electronic devices. The selection of a substrate for GaN epitaxy is crucial because it directly impacts crystal quality, defect density, and device performance. AlN offers advantages over traditional substrates like sapphire and silicon carbide (SiC) in terms of lattice matching, thermal expansion compatibility, and dislocation reduction. This article explores these aspects in detail, providing a comparative analysis with sapphire and SiC substrates.

One of the primary benefits of AlN as a substrate for GaN epitaxy is its close lattice match with GaN. The lattice constants of AlN and GaN are approximately 3.11 Å and 3.19 Å, respectively, resulting in a lattice mismatch of around 2.4%. This relatively small mismatch minimizes strain-induced defects in the epitaxial GaN layer, leading to improved crystal quality. In contrast, sapphire substrates exhibit a much larger lattice mismatch with GaN, approximately 16%, which introduces significant strain and generates high threading dislocation densities (TDDs) in the GaN layer. Silicon carbide substrates offer a better lattice match than sapphire, with a mismatch of about 3.5%, but still fall short of the near-ideal alignment provided by AlN. The reduced lattice mismatch in AlN substrates allows for the growth of GaN films with lower defect densities, enhancing device performance and reliability.

Thermal expansion coefficient matching is another critical factor in substrate selection. AlN has a thermal expansion coefficient of approximately 4.2 × 10⁻⁶ K⁻¹, while GaN’s coefficient is around 5.6 × 10⁻⁶ K⁻¹. Although not perfectly matched, the difference is smaller than that between GaN and sapphire (7.5 × 10⁻⁶ K⁻¹) or SiC (4.5 × 10⁻⁶ K⁻¹). The closer thermal expansion coefficients of AlN and GaN reduce thermal stress during cooling after high-temperature epitaxial growth, minimizing crack formation and wafer bowing. This property is particularly advantageous for high-power and high-frequency devices, where thermal management is essential. Sapphire substrates, despite their widespread use, often lead to thermal stress-related issues due to their larger thermal expansion mismatch with GaN. SiC substrates perform better than sapphire in this regard but still introduce some thermal stress compared to AlN.

Dislocation reduction is a significant challenge in GaN epitaxy, and AlN substrates offer a pathway to mitigate this issue. The high threading dislocation densities in GaN films grown on sapphire substrates, often exceeding 10⁸ cm⁻², degrade device performance by acting as non-radiative recombination centers and leakage paths. AlN substrates enable the growth of GaN with TDDs as low as 10⁶ cm⁻², a substantial improvement over sapphire. This reduction is attributed to the smaller lattice mismatch and the ability of AlN to act as a compliant substrate, accommodating strain more effectively. Silicon carbide substrates also support lower dislocation densities than sapphire, typically in the range of 10⁶ to 10⁷ cm⁻², but AlN’s superior lattice matching provides an edge in achieving the lowest possible defect densities.

The crystalline quality of the AlN substrate itself plays a crucial role in determining the properties of the epitaxial GaN layer. High-quality bulk AlN substrates with low defect densities are essential for optimal GaN growth. However, producing large-area, single-crystal AlN substrates remains challenging due to the high melting point and thermodynamic stability of AlN. Advances in physical vapor transport (PVT) and other bulk growth techniques have improved AlN substrate quality, but cost and scalability remain barriers to widespread adoption. In comparison, sapphire substrates are inexpensive and readily available in large sizes, making them the dominant choice for commercial GaN devices despite their inferior material properties. Silicon carbide substrates, while superior to sapphire in many respects, are costly and face supply chain limitations, restricting their use to high-end applications.

Another advantage of AlN substrates is their high thermal conductivity, approximately 285 W/m·K, which is significantly higher than that of sapphire (35 W/m·K) and comparable to SiC (490 W/m·K). This property is vital for dissipating heat in high-power devices, preventing performance degradation and failure. The combination of high thermal conductivity and close thermal expansion matching makes AlN an excellent choice for high-power GaN-based electronics, such as RF amplifiers and power switches. Sapphire’s poor thermal conductivity limits its use in these applications, while SiC’s superior thermal performance comes at a higher cost.

Despite these advantages, challenges remain in the adoption of AlN substrates for GaN epitaxy. The high cost of high-quality AlN substrates is a significant barrier, particularly for cost-sensitive applications. Additionally, the availability of large-diameter AlN wafers is limited compared to sapphire and SiC. Research efforts are ongoing to develop more cost-effective and scalable AlN substrate production methods, such as hydride vapor phase epitaxy (HVPE) and ammonothermal growth. These advancements could make AlN substrates more accessible and enable their broader use in GaN epitaxy.

In summary, aluminum nitride substrates offer compelling advantages for gallium nitride epitaxy, including close lattice matching, favorable thermal expansion coefficients, and the potential for low dislocation densities. These properties translate to improved device performance, particularly in high-power and high-frequency applications. While sapphire remains the most widely used substrate due to its low cost and availability, and SiC offers a balance of performance and cost, AlN stands out as the optimal choice for applications demanding the highest material quality. Overcoming the challenges associated with AlN substrate production will be key to unlocking its full potential in the GaN device market.