News Article
Osram improves green laser performance
Refinements to the epitaxial structure of green laser diodes have led to an increase in output power and longer lifetimes.
At the ninth International Conference on Nitride Semiconductors, Uwe Strauss from Osram Opto Semiconductors described the company's efforts to improve the performance of its green laser diodes that are fabricated on the conventional plane of GaN.
Strauss explained that these lasers will target pico-projectors, where they can combine with red and blue equivalents to provide a lighting source with a small footprint that is free from a focusing element.
According to him, the green source in a pico-projector producing a 100 lumen brightness must have: an emission wavelength of 515 nm; an output power of at least 50 mW; a wall plug efficiency of 5 percent or more; and, in both the lateral and vertical directions, a single mode output. If a shorter wavelength source is used – for example, a 505 nm laser – the power output requirements are higher.
In the lab, Osram has exceeded these requirements. It's best lasers produce 70 mW at 522 nm with a wall plug efficiency of 5-6 percent, and have a spectral width of 1.8 nm. What's more, reliability – defined as the time taken for the operating current to increase by 30 percent – is more than 1000 hours.
Strauss offered some insights into how the German outfit has improved the performance of its laser diodes. He explained that when green laser development began, the wafers were plagued by V-shaped defects associated with strain.
To minimise this strain and prevent the formation of V-defects and other imperfections in the crystal lattice, engineers began by fabricating two test structures to understand the extent of this issue. One contained just blue quantum wells - the other had blue wells grown on top of green-emitting ones. The photoluminescence from the latter structure was far weaker, due to defects that were subsequently exposed by transmission electron microscopy. Managing this strain enabled the high output powers realised in the lab.
Engineers have also studied the change in the threshold of the lasers, which is proportional to the square root of the operating time.
One potential culprit behind the higher currents is the diffusion of magnesium atoms from the p-type region to the quantum wells. However, this has been ruled out by secondary ion mass spectrometry. According to Strauss, the most likely reason for the increase in threshold current is the generation of charge defects in the quantum wells.
