Gallium nitride (GaN) bulk substrate fabrication is critical for high-performance electronic and optoelectronic devices. The quality of the substrate directly impacts device efficiency, reliability, and power handling. Two primary methods dominate GaN bulk substrate production: ammonothermal growth and hydride vapor phase epitaxy (HVPE). Each technique has distinct advantages and limitations, influencing their suitability for different applications. Additionally, foreign substrates such as sapphire and silicon carbide (SiC) remain widely used due to cost and scalability considerations, despite inherent lattice and thermal mismatch challenges.

Ammonothermal growth is a high-pressure, high-temperature crystal growth technique inspired by hydrothermal methods used for quartz synthesis. In this process, GaN is dissolved in supercritical ammonia with mineralizers, such as alkali metals, facilitating nutrient transport and crystal growth under controlled conditions. The method produces high-quality, low-defect-density GaN crystals with excellent uniformity. Dislocation densities as low as 10^4 cm^-2 have been reported, making ammonothermal GaN ideal for high-power and high-frequency devices. However, the slow growth rate and high equipment costs limit its widespread adoption. The technique’s scalability remains a challenge, though ongoing research aims to optimize mineralizer chemistry and reactor designs to improve yield.

HVPE is another leading method for GaN bulk substrate fabrication, offering faster growth rates compared to ammonothermal techniques. In HVPE, gallium chloride (GaCl) reacts with ammonia (NH3) at elevated temperatures, depositing GaN on a seed crystal. Growth rates exceeding 100 µm/h are achievable, enabling the production of thick, freestanding GaN substrates. HVPE-grown GaN typically exhibits dislocation densities in the range of 10^6 cm^-2, higher than ammonothermal material but still suitable for many device applications. The primary advantage of HVPE is its ability to produce large-area wafers, which is critical for commercial semiconductor manufacturing. However, challenges such as strain-induced cracking and impurity incorporation require careful process control to ensure substrate quality.

Comparing these methods, ammonothermal growth excels in crystal quality, making it preferable for demanding applications like laser diodes and high-electron-mobility transistors (HEMTs). In contrast, HVPE offers a balance between quality and throughput, serving as a practical solution for LEDs and RF devices. The choice between the two depends on specific performance requirements and cost constraints.

Foreign substrates like sapphire and SiC are commonly used as alternatives to native GaN due to their availability and lower cost. Sapphire, despite its large lattice mismatch (~16%) with GaN, remains popular for optoelectronic devices such as blue LEDs. Its transparency and thermal stability are advantageous, but the high defect density from heteroepitaxy limits performance in high-power applications. Techniques like patterned sapphire substrates (PSS) and epitaxial lateral overgrowth (ELOG) help mitigate dislocation propagation, yet bulk GaN still outperforms sapphire-based wafers in critical metrics.

SiC presents a closer lattice match (~3.5%) to GaN and superior thermal conductivity, making it a preferred choice for high-power electronics. Devices grown on SiC substrates demonstrate lower thermal resistance and higher breakdown voltages compared to those on sapphire. However, SiC’s high cost and limited wafer sizes hinder its widespread use. Additionally, the residual strain from thermal expansion mismatch can affect device reliability over time. Despite these drawbacks, SiC remains a leading substrate for RF and power applications where performance outweighs cost considerations.

The impact of substrate choice on device performance is significant. Bulk GaN substrates, whether grown via ammonothermal or HVPE methods, enable devices with lower defect densities, higher breakdown voltages, and improved thermal management compared to foreign substrates. For instance, GaN-on-GaN HEMTs exhibit lower dynamic on-resistance and higher power densities than their sapphire or SiC counterparts. Similarly, laser diodes on native substrates achieve longer lifetimes and higher efficiencies due to reduced non-radiative recombination.

In optoelectronics, the reduced threading dislocation density in bulk GaN leads to higher internal quantum efficiency in LEDs and laser diodes. The absence of strain-related piezoelectric fields further enhances radiative recombination rates, resulting in brighter and more efficient devices. For power electronics, the superior thermal conductivity of bulk GaN minimizes self-heating effects, enabling higher current densities and reliability.

Despite these advantages, cost remains a barrier to the widespread adoption of bulk GaN substrates. Foreign substrates continue to dominate the market due to their mature manufacturing processes and lower prices. However, as demand for high-performance GaN devices grows in sectors like electric vehicles, 5G communications, and renewable energy, investments in bulk GaN production are expected to increase. Advances in growth techniques, such as hybrid approaches combining HVPE with ammonothermal polishing, may further bridge the gap between quality and affordability.

In summary, ammonothermal growth and HVPE are the leading methods for GaN bulk substrate fabrication, each offering distinct trade-offs in quality and scalability. While foreign substrates like sapphire and SiC remain prevalent due to economic factors, native GaN substrates provide superior device performance. The ongoing development of bulk GaN technologies will play a pivotal role in enabling next-generation semiconductor devices.