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Toyoda Gosei makes 3-inch non-polar GaN

Inexpensive sapphire substrates etched with stripes allow the Japanese chipmaker to produce what it says are the highest output power hetero-epitaxial non-polar GaN LEDs.

Commercial LEDs that avoid the handicap of piezoelectric polarization are now a step closer, after Toyoda Gosei produced 3-inch diameter non-polar GaN films.

The Japanese LED maker deposited non-polar GaN, which eliminates internal polarization fields that pull apart and spatially separate electrons and holes, across whole 3-inch sapphire substrates by MOCVD.

The company then made functioning emitters to exploit any resulting increase in electron and hole recombination rate, light emission, and internal quantum efficiency.

Writing in two papers published online in Applied Physics Express in March 2009, Toyoda Gosei researchers deposited m-plane GaN on striped a-plane sapphire substrates.

This approach counters the expense associated with native non-polar substrates which have been used most commonly to date in research to avoid the effects of internal polarization fields.

“The a-plane sapphire substrates are basically the same price as c-plane,” Ryoichi George Tohmon, a member of the Toyoda Gosei team, told compoundsemiconductor.net. “The fabrication of such GaN layers might trigger the mass production of non-polar optical devices.”

Tohmon and his colleagues produced 346 µm x 346 µm LED chips on the material. These emitted 461 nm light with 3 mW output and 5 percent external quantum efficiency (EQE). The figures "“ corresponding to the peak efficiency "“ were obtained at 3.5 V forward voltage and 20 mA driving current.

The Toyoda Gosei group concedes that this output value is much lower than that obtained with more expensive free-standing non-polar GaN substrates.

“To our knowledge, however, this milliwatt-scale output power in the blue region is the highest value obtained for non-polar LEDs grown on hetero-epitaxial GaN layers,” they write.

The team etched the stripes in the a-plane sapphire substrates along the m-plane axis using conventional photolithography and dry etch techniques. The stripes had a period of 3 µm, with 1.5 µm wide, 1 µm deep etched channels.

Although Tohmon indicated that the stripe pattern was crucial in producing non-polar GaN films, he described the reason that it does so as “confidential information”.

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