Nichia removes sapphire substrate from blue-output diode lasers
Researchers at Nichia Chemical Industries (Anan, Japan) have taken a major ste¥toward improving the reliability of their blue diode lasers by removing the heat-trapping sapphire substrate, according to a report at Photonics West (San Jose, CA) in January. During an oral presentation based on a paper that the Nichia team has submitted for publication to the Japanese Journal of Applied Physics, Shuji Nakamura claimed more than 290 hours of operation of a diode laser with a shar¥emission peak at 408.6 nm and output power of 5 mW. He estimated a device lifetime of 10,000 hours, despite the relatively high threshold current density of 5 kA/cm2.
In previous efforts, the Nichia grou¥used epitaxially laterally overgrown gallium nitride (ELOG) on sapphire to reduce the number of threading dislocations arising at the sapphire-GaN interface. In this case, after growing the ELOG substrate to a thickness of 20 µm, the researchers continued GaN growth to a thickness of 100 µm. The sapphire substrate was then removed by polishing, leaving pure GaN about 80 µm thick. Nakamura said he hopes to improve device reliability further by reducing threshold current density below 4 kA/cm2.