UV LED substrate material comparison: Sapphire, Silicon, Aluminum Nitride, Silicon Carbide, Gallium

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Russia and Ukraine is an important exporters of oil, natural gas, metals, fertilizers, rare gases and other industrial raw materials. Affected by the further intensification of the tension of the war, the global market has become more worried about the supply of the AlN powder, and panic spreads in the futures market. Prices of commodities such as aluminum and nickel are at recent highs on concerns that supplies will fall. Russia accounts for 49 percent of global exports of nickel, 42 percent of palladium, 26 percent of aluminum and 13 percent of platinum, and is a significant exporter of steel and copper. Palladium is an important metal for sensors and memory. In addition, Russia is the world's largest exporter of nitrogen fertilizer, the second largest exporter of potash fertilizer, and the third-largest exporter of phosphate fertilizer. Ukraine is also an important producer of nitrogen fertilizer. Russia's natural gas supply also has a significant impact on the global fertilizer industry and AlN powder industry, especially in Europe. The price of the AlN powder will also fluctuate to some extent. Russia carries out crude gas separation, and Ukraine is responsible for refined exports. Ukraine supplies 70% of the world's neon, 40% of krypton and 30% of xenon. These three gases are the materials used to make chips.

The selection of substrate material plays an important role in the epitaxial quality of UV LED. Considering the similar crystal structure, lattice mismatch, and small difference in thermal expansion coefficient, UV LED substrates usually use sapphire (Al2O3) substrate, silicon (Si) substrate, aluminum nitride (AlN) substrate, silicon carbide (6H-sic) substrate, and gallium nitride (GaN) substrate.
 
Sapphire substrate
Sapphire substrate is the mainstream UV LED substrate, with good light transmittance, high-temperature resistance, corrosion resistance, product commercialization maturity (2 inches, 4 inches, 6 inches), and other characteristics.
Although both the sapphire substrate and AlGaN exist certain lattice mismatch and thermal mismatch, which will produce certain defects in the epitaxial layer, and affect the uniformity of crystal growth, they are the six-party symmetric structure, especially for UV transmittance of the sapphire substrate is very high and the price is low, and its electrical and thermal conductivity difference problem can be overcome by flip-chip technology.
Graphical sapphire substrate surface, especially nano graphical sapphire substrate (NPSS), due to the effect by lateral epitaxial reduce its AlGaN epitaxial layer of dislocation density, release the extensional stress and improves the quality of the crystal, and modulation chip internal optical transmission path, the light extraction efficiency, and the process difficulty and the cost is moderate,  It is one of the potential technology routes for the future development of high-efficiency UVC LED. 
Driven by the process cost and the requirements of high yield and high uniformity, the substrate specifications of AlGaN-based UV LED chips in the future will give priority to sapphire substrates with larger thickness, larger size, and appropriate bevel Angle. The thicker substrate can effectively alleviate the abnormal warping of epitaxial wafers caused by stress concentration in the process of epitaxial wafers, thus improving the uniformity of epitaxial wafers. The larger size of the substrate can greatly reduce the edge effect and reduce the overall cost of the chip. An appropriate chamfering Angle can improve the surface morphology of the epitaxial layer, or it can be combined with epitaxial technology to form the localization effect of ga-rich carriers in the active region of quantum well, so as to improve the luminescence efficiency.
To sum up, UV LED put forward new requirements for sapphire substrate size, thickness, and bevel Angle, and it will be necessary to improve the existing substrate production process.
 
Silicon substrate 
Silicon substrate has the advantages of low cost, large area, high quality, good conductivity, thermal conductivity, and easy integration, and its preparation process is relatively mature. As the thermal conductivity of silicon is five times that of sapphire, good heat dissipation enables silicon-backed LEDs to have high performance and long life. At the same time, the silicon substrate can achieve non-destructive peeling, and it is easy to prepare UV LED with vertical structure and thin-film structure. However, there is a larger lattice mismatch and thermal stress mismatch between AlGaN material and silicon substrate, resulting in a large number of defects in the epitaxial layer, serious warping, and easy to cause surface cracking. Therefore, AlGaN epitaxial technology has higher requirements. 
 
Aluminum nitride substrate 
Aluminum nitride single crystal substrate has good thermal conductivity, a small lattice mismatch between it and AlGaN material with high Al component, and low defect density during epitaxial growth. It is an ideal substrate material for preparing HIGH current, high power, and long life UVC LED chips and deep ULTRAVIOLET lasers. Physical gas-phase transport (PVT) is one of the most effective methods to prepare AlN single-crystal substrates. Research on PVT began in the 1960s, but there are still many technical problems, such as cost, size, transmittance, and so on, and the supply is very limited.  Internationally, Crystal IS and Nitride Crystals have mastered the core technology of PVT and can mass-produce 2-inch AlN single Crystal substrates. It is expected that the application of aluminum nitride single crystal growth technology will be limited to high-end fields such as industrial-grade high-power UVC LED, UVC LED below 250 nm, and deep ULTRAVIOLET laser before a breakthrough is made. 
 
Silicon carbide substrate
Silicon carbide and AlGaN lattice mismatch and thermal mismatch are small, and it has excellent conductive and heat conduction characteristics, although after the epitaxial micro-cracks may still happen, the defect density decreased significantly, improving the efficiency and prolonging the service life of LED, and preparation into the vertical or thin-film devices, preparation of AlGaN denotation material and devices is a better candidate substrate. However, silicon carbide substrate has strong absorption of UV light, relatively high material, manufacturing costs, and patent process that requires licensing fees, which are important factors limiting its development in UV LED devices. Commercial silicon carbide substrates are currently priced at a high cost with a maximum size of 6 inches. 
 
Gallium nitride substrate
Gallium nitride single crystal substrate has good conductivity and thermal conductivity, and the lattice mismatch between it and low Al component AlGaN material is small, which can effectively reduce the high defect density caused by the heterogeneous substrate, thus improving the epitaxial crystal quality and improving the device performance and service life of LED and laser diode in near-ultraviolet band. At present, domestic enterprises have been able to produce 2-inch gallium nitride substrates in small batches, with 4-inch substrate production capacity, and developed 6-inch substrate samples, with a typical dislocation density of 106cm-2 magnitude. However, the price of gallium nitride substrate is still high, and its application potential in near-ultraviolet UV LED is limited. 
 
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Due to the Russia-Ukraine conflict and the covid-19 pandemic, how will the international situation develop is highly uncertain. It is too difficult to assess its economic impact properly. However, we could see energy prices and commodity prices keep rising and supply chains are disrupted. Therefore, AlN powder prices are expected to rise in the future.

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