Why Doesn’t Gllium Nitride (GaN) Epitaxy Grow on a GaN substrate?

The growth of GaN epitaxy on GaN substrate presents a unique challenge, despite the material’s superior properties when compared to silicon. GaN epitaxy offers significant advantages in terms of band gap width, thermal conductivity, and breakdown electric field over silicon-based materials. This makes the adoption of GaN as the backbone for the third generation of semiconductors, which provide enhanced cooling, lower conduction loss, and improved performance under high temperatures and frequencies, a promising and crucial advancement for the photonic and micro-electronic industries.

GaN, as the primary third-generation semiconductor material, especially shines due to its wide applicability and has been regarded as one of the most important materials following silicon. GaN power devices demonstrate superior characteristics compared to current silicon-based devices, such as higher critical electric field strength, lower on-resistance, and faster switching frequencies, leading to improved system efficiency and performance under high operational temperatures.

In the GaN semiconductor value chain, which includes substrate, GaN epitaxy, device design, and manufacturing, the substrate serves as the foundational component. GaN is naturally the most suitable material for serving as the substrate on which GaN epitaxy is grown due to its intrinsic compatibility with a homogenous growth process. This ensures a minimal degree of stress due to disparities in material properties, resulting in the generation of epitaxial layers of superior quality compared to those grown on heterogenous substrates. By using GaN as the substrate, high-quality GaN epistemology can be produced, with internally reduced defect density by a factor of one thousand compared to substrates like sapphire. This contributes to a significant reduction in the junction temperature of LEDs and enables a tenfold enhancement in lumens per unit area.

However, the conventional substrate of GaN devices is not GaN single crystals due to the difficulty associated with their growth. The advancement in GaN single crystal growth has progressed significantly slower than in conventional semiconductor materials. The challenge lies in the cultivation of GaN crystals that are elongated and cost-effective. The first synthesis of GaN occurred in 1932, using ammonia and a pure metal gallium to grow the material. Since then, extensive research has been pursued into GaN single crystal materials, yet challenges remain. GaN’s inability to melt under normal pressure, its decomposition into Ga and nitrogen (N2) at elevated temperatures, and its decompression pressure that reaches 6 gigapascal (GPa) at its melting point of 2,300 degrees Celsius make it difficult for existing growth equipment to accommodate the synthesis of GaN single crystals at such high pressures. Traditional melt growth methods cannot be employed for GaN single crystal growth, thus necessitating the use of heterogenous substrates for epitaxy. In the current state of GaN-based devices, growth is typically performed on substrates such as silicon, silicon carbide, and sapphire, rather than using a homogeneous GaN substrate, hampering the development of GaN epitaxial devices and hindering applications that require a homogeneous substrate-grown device.

Several types of substrates are employed in GaN epitaxy:

1. Sapphire: Sapphire, or α-Al2O3, is the most widespread commercial substrate for LEDs, capturing a significant chunk of the LED market. Its use was heralded for its unique advantages, particularly in the context of GaN epitaxial growth, which produces films with equally low dislocation density as those grown on silicon carbide substrates. Sapphire’s manufacturing involves melt growth, a mature process that enables the production of high-quality single crystals at lower costs and larger sizes, suitable for industrial application. As a result, sapphire is one of the earliest and most prevalent substrates in the LED industry.

2. Silicon Carbide: Silicon carbide (SiC) is a fourth-generation semiconductor material that ranks second in market share for LED substrates, following sapphire. SiC is characterized by its diverse crystal forms, primarily classified into three categories: cubic (3C-SiC), hexagonal (4H-SiC), and rhombohedral (15R-SiC). A majority of SiC crystals are 3C, 4H, and 6H, with the 4H and 6H-SiC types being utilized as substrates for GaN devices.

Silicon carbide is an excellent choice as an LED substrate. Nevertheless, the production of high-quality, sizable SiC single crystals remains challenging, and the material’s layered structure makes it prone to cleavage, which affects its mechanical integrity, potentially introducing surface defects that impact epitaxial layer quality. The cost of a single crystal SiC substrate is approximately several times that of a sapphire substrate of the same size, limiting its widespread application due to its premium pricing.

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3. Single Crystal Silicon: Silicon, being the most extensively used and industrially established semiconductor material, provides a solid foundation for the production of GaN epitaxial substrates. The availability of advanced single crystal silicon growth techniques ensures a cost-effective, large-scale production of high-quality, 6 to 12 inch substrates. This significantly reduces the cost of LEDs and paves the way for the integration of LED chips and integrated circuits through the use of single crystal silicon substrates, driving advancements in miniaturization. Furthermore, compared to sapphire, which is currently the most common LED substrate, silicon-based devices offer advantages in terms of thermal conductivity, electrical conductivity, capability to fabricate vertical structures, and better fit for high power LED fabrication.**