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Japan Toyota Synthetic Preparation of 3-Inch Non-Polar GaN
Japanese chip makers use inexpensive, etched striped sapphire substrates to produce devices that are said to be the highest output heterojunction epitaxial non-polar GaN LEDs. Since Toyota synthesized a 3-inch non-polar GaN LED, it is getting closer and closer to commercial LEDs that are not affected by piezoelectric polarization. Japanese LED manufacturers use MOCVD to deposit non-polar LEDs on 3-inch sapphire substrates, eliminating the effects of internal polarization and avoiding electron-hole recombination. Next, Toyota Synthetic began to produce functional emitters, developing a device that improved both in electron-hole recombination, light emission, and internal quantum efficiency. In an Applied Physics Express paper published in March, Toyota Synthetic researchers deposited m-plane GaN on etched striped a-plane sapphire substrates. The substrate prepared by this method is less expensive than the intrinsic non-polar substrate. Today, intrinsic non-polar materials are often used in research work, eliminating the effects of internal polarization fields. Ryoichi George Tohmon is a member of the Toyota Synthetic Group. "The price of a sapphire substrate is basically the same as that of the c-plane. The successful preparation of these GaN layers may lead to mass production of non-polar optoelectronic devices." Tohmon and His colleagues used this non-polar material to prepare a 346 × 346 μm LED chip with an emission wavelength of 461 nm, an output power of 3 mW, and an EQE of 5%. A value related to the peak efficiency can be obtained when the driving current is 20 mA and the forward voltage is 3.5 V. Toyota Synthetic recognizes that the output power is much lower than a more expensive self-supporting non-polar GaN substrate. They mentioned in the paper that this non-polar blue LED grown on a heteroepitaxial GaN layer has an output power of milliwatts. Using conventional lithography and dry etching techniques, the Tohohm team etched stripes along the m-plane axis on a sapphire substrate with a spacing of 3 μm, a width of 1.5 μm, and a etched channel depth of 1 μm. Although Tohmon mentions in the text, the strip pattern is critical in the preparation of non-polar GaN films. But he did not explain the detailed reasons.