03 Jun 2009
Commercial viability of green diodes beckons with devices that boast lifetimes of hundreds of hours.
Nichia has produced individual 515 nm continuous-wave laser diodes, crashing past the 500 nm mark that was previously the longest wavelength reached by electrically driven InGaN lasers.
The Japanese GaN pioneer did not provide reliability data for the 515 nm devices, but it has also produced 510–513 nm diodes that operate for 500 hours without a change in their electrical characteristics.
Beyond 500 hours, the lasers' performance steadily degraded. The Nichia researchers estimated their overall lifetime, which is defined as ending when operating current increases by 30% from its initial value, to be 5000 hours.
In an Applied Physics Express paper, published 22 May, the team developing the lasers reports a separate confinement heterostructure diode "almost the same as" a design dating back to 2001.
The device was deposited on a free-standing c-plane GaN substrate, with optimized MOCVD processes critical to producing the high-indium-content material required to access green wavelengths.
"We improved the growth condition of epitaxial layers, especially multiquantum-well layers," the Nichia team writes. "The crystal quality was improved even in indium-rich InGaN active layers."
However, the Japanese company did not reveal any further details of these optimization efforts. When compoundsemiconductor.net asked expert Nichia GaN developer Takashi Mukai, an author on the paper, for more details on this and his company's commercial plans for green lasers, he declined to comment.
When Nichia previously produced 488 nm InGaN diodes, poor crystal quality resulted in nonradiative regions that showed up as dark areas under photoluminescence analysis. Mukai and colleagues write that they have now eliminated these regions.
At room temperature, threshold current and current density for the 515 nm diodes were 53 mA and 4.4 kA/cm2 respectively. The devices hit 5 mW optical output at 88 mA operating current and 5.5 V operating voltage.
Also starting at room temperature, the 510–513 nm diodes' wavelength increased by 0.022 nm/K, making them less temperature-sensitive than both Osram's and Rohm's recent 500 nm emitters.
• This article first appeared on compoundsemiconductor.net.
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