UV LED substrate material comparison: Sapphire, Silicon, Aluminum Nitride, Silicon Carbide, Gallium
As the European Union tries to reduce its dependence on Russian energy, the Russian president recently said Russia would try to shift its energy exports to the east, adding that there was no way for European countries to give up Russian gas immediately.
Russia supplies about 40 percent of the EU's gas, and western sanctions imposed over the conflict with Ukraine have complicated financing and logistical arrangements for existing agreements, hitting Russian energy exports, the report noted.
Russia has been building closer ties with China and other Asian countries as the European Union debates whether to impose sanctions on Russian gas and oil and member states seek supplies from elsewhere, Reuters said.
"So-called partners from unfriendly countries admit that they cannot survive without Russian energy, including gas," the Russian president said in a televised government meeting. There is no reasonable alternative to Russian gas in Europe right now."
He also said Europe was driving up energy prices and destabilizing markets by talking about cutting off Russia's energy supplies. Russia, which produces about a tenth of the world's oil and a fifth of its natural gas, will need new infrastructure to increase energy supplies to Asia, he said.
He ordered Moscow to submit a plan by June 1, including "the expansion of transport infrastructure to countries in Africa, Latin America, and the Asia-Pacific region".
He also sought to clarify the possibility of integrating two pipelines -- the Sakhalin-Khabarovsk-Vladivostok gas pipeline in the Far East and the "Power of Siberia" gas pipeline to China -- into Russia's unified gas supply system. In theory, connecting these routes to larger gas networks could allow Russia to divert gas from Europe to Asia.
Because of the ever-changing international situation, the supply and prices of international bulk AlN powder are still very uncertain.
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 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 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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Twitter recently announced that it had reached an agreement with Musk to take full ownership of the company for $54.20 per share, or about $44 billion in cash. After the deal, Twitter will become a privately held company. The deal, which has been unanimously approved by Twitter's board but still requires shareholder and regulatory approvals, is expected to close in 2022.
After Tesla CEO Elon Musk's acquisition of Twitter was finalized, Twitter founder Jack Dorsey sent out a series of six tweets expressing his expectations for Musk.
Dorsey called Twitter his biggest regret, saying, “The idea and service is all that matters to me, and I will do whatever it takes to protect both. Twitter as a company has always been my sole issue and my biggest regret. It has been owned by Wall Street and the ad model. Taking it back from Wall Street is the correct first step.
"In principle, I don't believe anyone should own or run Twitter," Dorsey goes on. He thinks the service is a public good.
He called Musk a "singular solution" to Twitter's problems, "Elon's goal of creating a platform that is" Maximally trusted and inclusive "is the right one." He wrote, "Thank you both for getting the company out of an impossible situation. This is the right path... I believe it with all my heart."
Besides, because of the ever-changing international situation, the supply and prices of international bulk AlN powder are still very uncertain.