Technical Insight
Green light for laser diodes?
Researchers at a small, Canadian start-up have unveiled simple test structures that emit incredibly bright green light. Have they filled the green gap, asks Compound Semiconductor.
As organisations worldwide race to commercialise a direct green laser diode to plug the much-coveted green gap, Canadian start-up Meaglow could well reach the finishing line first, and with a truly unique approach.
The company recently developed a series of test InGaN p-n junctions, which in the words of chief scientist, Scott Butcher, “are a hell of a lot brighter than any pn junction I've seen in nitrides before.” So how have Meaglow researchers achieved this?
The company's test structures are grown using a novel technique called migration enhanced afterglow that circumvents the shortcomings of MOCVD and MBE growth. Based on methods of migration enhanced epitaxy developed for MBE, the technique involves saturating a substrate surface with the active metal to ensure high quality crystal growth can take place at the low processing temperature of 550°C. Crucially, this all takes place in a CVD environment, and with a high pressure 2Torr, scalable hollow cathode plasma source.
As Butcher explains, the method works well for several reasons. Low growth temperatures alleviate epiwafer bow on large diameter wafers that typically takes place during high temperature MOCVD growth processes. Meanwhile, maintaining a CVD environment removes the wide area deposition issues that plague MBE.
Factor in the company's high intensity hollow cathode plasma source - that side-steps oxygen contamination associated with other plasma sources - and you have an effective route to fabricating InGaN laser diodes.
“By processing in the CVD environment, we can go to higher pressures than we could with MBE, which also turns out to be very important for high indium content structures,” adds Butcher. “[During deposition] we have a lot more gas collisions with the plasma source, eliminating more of the energetic species that damage the films.”
And the results look good. The researchers recently worked with McGill University, Quebec, to produce InN layers, which as Butcher puts it: “had some of the sharpest, low temperature, PL ever achieved.” They then went onto produce thick - 50 to 250nm - InGaN layers approaching device smoothness, and have constructed simple p-n junction structures to test the electroluminescence.
These structures consisted of 170nm thick, n-type InGaN layers grown on MOCVD p-GaN buffer layers, on sapphire. Part of the p-GaN was masked during growth to provide a step on which to mount an electrode while a second electrode was placed directly onto the InGaN layer.
Butcher has been genuinely shocked by the results. “These structures blow me away they are so bright,” he says. “I'm not sure we understand everything that's happening here and we're still looking at why they are so luminescent.”
What is clear at this stage however, is that growing thick, higher In content GaN layers, seems to reduce - to a certain extent - the strains that typically arise from lattice mismatches between the GaN and InGaN layers. “If we grow our layers directly on sapphire we see segregation, so growing on the MOCVD template is one way around this,” he says. “However, even with this, we still see the effects of strain on the thin GaN layers. But now we find we're able to grow the layers thick enough to ignore this effect.”
So where next for the Canadian start-up? According to Butcher, his team are now collecting electroluminescent spectra for a number of InGaN composition and is also working on fabricating multi-quantum well structures. Devices will also be built with p-type GaN at the top, rather than growing on this layer.
“I think we'll be able to fabricate quantum well [structures] by the end of the year and we'll start seeing some nice bright devices,” says Butcher. “Our technology has produced some the of best results to date for higher indium content InGaN... it has really caught a lot of attention as emission is right in the green gap.”
Image caption:
The yellow emission as shown is from a high carrier concentration sample whose emission is broadened by a Moss-Burstein effect.
