Sharp Laboratories of Europe, based on the Oxford Science Park, has demonstrated what it claims is the first room-temperature continuous-wave (CW) violet laser diode fabricated by molecular beam epitaxy (MBE).
“We are the only group who has demonstrated CW violet by MBE,” Jon Heffernan, director of advanced optical devices at Sharp told Electronics Weekly. “For a long time, people thought MBE could not produce the [required] quality. CW operation proves the quality.”
All other violet CW lasers, including some close to production, are made by metal-organic chemical vapour deposition (MOCVD).
“We believe MBE has more accurate control, particularly with dopants,” said Heffernan, “although many MOCVD processors would probably argue with that.”
The devices were fabricated in a gas-source MBE system using ammonia as a source of nitrogen and elemental sources for gallium, indium and aluminium.
The lab produced a pulsed version of the diode on a sapphire substrate in January 2004 (EW 28/01/04), and has since been working towards CW operation.
No major structural changes were made, although details like super lattices were introduced. “We made significant improvements, reducing threshold current by a factor of four. Once we were at that level of threshold current, we expected CW on sapphire,” said Heffernan.
Unfortunately, Sharp did not get it. “We didn’t because fab issues with sapphire limit performance. Without an awful lot more process development, we were not going to get CW,” he said.
To dodge that further wave of development, Sharp moved its improved structure to free-standing GaN substrates.
Compared with sapphire, GaN has better thermal conductivity and fewer imperfection-inducing dislocations. It is also electrically conductive, allowing one electrode to be moved to the back of the substrate and, added Heffernan, “you can cleave GaN to get very clean facets”.
The device remains experimental, with only three minutes of CW operation before self-destruction. Reduced power consumption will increase lifetime, so a further reduction on threshold current is sought as well as a cut in operating voltage from improved p-doping which will cut parasitic resistance.
Sharp has access to both MBE and MOCVD processes. MBE is an ultra-high vacuum epitaxial growth process, whereas MOCVD is a chemical process operating at atmospheric pressures.
Violet lasers are needed for next-generation DVD systems including Blu-ray.