Bulk aluminum nitride (AlN) crystal growth is critical for applications requiring high thermal conductivity, wide bandgap, and piezoelectric properties, such as in high-power electronics, deep-ultraviolet optoelectronics, and acoustic wave devices. The two primary methods for producing bulk AlN crystals are sublimation-recondensation and hydride vapor phase epitaxy (HVPE). Each technique has distinct advantages and challenges, particularly concerning temperature control, crucible selection, and defect management.

Sublimation-recondensation, also known as physical vapor transport (PVT), is the most established method for bulk AlN growth. The process involves heating AlN source material to temperatures exceeding 2000°C in an inert or nitrogen-rich atmosphere, causing sublimation into gaseous aluminum and nitrogen species. These species then recondense on a cooler seed crystal, forming a bulk AlN single crystal. The growth occurs in a crucible, typically made of tungsten or tantalum due to their high melting points and chemical stability at extreme temperatures. However, crucible reactions with the AlN source can introduce impurities such as tungsten or carbon, degrading crystal quality.

A major challenge in sublimation-recondensation is maintaining a stable temperature gradient to ensure controlled growth rates and minimize thermal stress. Uneven temperature distribution can lead to cracking, dislocations, and polycrystalline formation. Additionally, nitrogen vacancies are common defects due to the high equilibrium vapor pressure of nitrogen at elevated temperatures. To mitigate this, growth is often performed under high nitrogen pressures or with nitrogen overpressure systems. Despite these challenges, PVT-grown AlN crystals can achieve dislocation densities below 10^4 cm^-2 when optimized, making them suitable for high-performance substrates.

Hydride vapor phase epitaxy (HVPE) is another prominent technique for bulk AlN growth, offering higher growth rates compared to sublimation-recondensation. In HVPE, aluminum chloride (AlCl3) gas, formed by reacting HCl with molten aluminum, is transported to a growth zone where it reacts with ammonia (NH3) to deposit AlN on a seed crystal. The process occurs at lower temperatures (1000–1200°C) than PVT, reducing thermal stress and energy consumption. However, HVPE requires precise control of gas-phase chemistry to prevent parasitic reactions and ensure stoichiometric AlN deposition.

One advantage of HVPE is the ability to grow thick AlN layers with reduced dislocation densities through epitaxial lateral overgrowth techniques. However, the method faces challenges related to gas-phase nucleation, which can lead to polycrystalline growth if not carefully managed. Furthermore, the corrosive nature of HCl and NH3 demands reactor materials resistant to chemical degradation, such as quartz or specialized ceramics. Despite these difficulties, HVPE-grown AlN exhibits excellent crystalline quality, with dislocation densities comparable to those achieved by PVT.

Comparing these methods with other bulk nitride growth techniques reveals distinct trade-offs. For instance, ammonothermal growth, used for gallium nitride (GaN), involves dissolving AlN in supercritical ammonia with mineralizers at moderate temperatures (400–600°C). While this technique can produce high-quality crystals with low dislocation densities, the slow growth rates and difficulties in scaling hinder its adoption for AlN. Similarly, flux growth methods, which employ metallic solutions to lower AlN synthesis temperatures, struggle with impurity incorporation and limited crystal size.

In contrast to GaN, AlN’s higher melting point and thermodynamic stability make sublimation-recondensation more viable than melt growth techniques, which are impractical due to the extreme temperatures required. The lack of a native AlN substrate for homoepitaxy further complicates bulk growth, necessitating heteroepitaxial approaches on foreign substrates like sapphire or SiC, which introduce lattice mismatch-induced defects.

Efforts to improve bulk AlN growth focus on optimizing temperature gradients, reducing impurity uptake, and enhancing seed crystal quality. Advanced crucible designs with protective coatings, such as tungsten carbide or refractory nitrides, help minimize contamination. In-situ monitoring techniques, including pyrometry and optical spectroscopy, enable better control of growth conditions. Additionally, post-growth thermal annealing in nitrogen atmospheres can reduce point defects and improve crystal uniformity.

While sublimation-recondensation remains the dominant method for commercial bulk AlN production, HVPE offers a promising alternative for applications requiring rapid growth and high purity. Future advancements may involve hybrid approaches combining PVT and HVPE to leverage the benefits of both techniques. The development of large-diameter, low-defect AlN substrates will be crucial for advancing high-power and high-frequency electronic devices.

In summary, bulk AlN crystal growth is a complex process requiring precise control over temperature, chemistry, and defect formation. Sublimation-recondensation and HVPE each present unique advantages and challenges, with ongoing research focused on improving crystal quality and scalability. Compared to other nitride growth techniques, AlN’s thermodynamic properties demand specialized approaches, underscoring the need for continued innovation in bulk crystal synthesis.