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MBE Legend Highlights Nitride Challenges

Plenary speaker at Photonics West advocates use of non-polar substrates, but cautions on high defect levels in epilayers grown on native GaN.

By Richard Stevenson in San Jose, California
Klaus Ploog says that the community developing nitride semiconductors still has tough problems to solve, despite III-nitrides being “good for everything".

The ground-breaking MBE researcher extolled the versatility of nitrides in a packed plenary talk at Photonics West, in San Jose, CA on 27 January.

However, he emphasized droop, the decline in efficacy as LED current is ramped, as a major limitation on the full exploitation of nitrides. The best devices deliver efficacies of well over 100 lm/W at low currents, but this falls substantially at the current densities employed for solid-state lighting. The cause of droop is highly controversial.

The former professor of Germany s Paul Drude Institute for Solid State Electronics outlined a handful of potential explanations, including non-radiative carrier loss and electron leakage.

He went on to say that internal electric fields resulting from strong polarization in nitrides can hamper optoelectronic devices performance by separating the electrons and holes in the quantum well.

Turning to deposition on non-polar planes - an approach that Ploog pioneered with nitride growth on LiAlO2 at the start of this decade - avoids this.

Crystal growth researchers are directing a great deal of research effort into producing GaN boules, which could lead to epitaxy on native substrates. Many in the nitride community have been wishing for GaN substrates for many years, but Ploog believes that this platform may not live up to its expectations.

He urged for more research into growth mechanisms on GaN, and pointed out that some nitride epilayers deposited on this platform have had a higher defect density than their underlying substrates.

Forget your blues
Ploog reminded the audience that one of the key attributes of the nitride family of materials is its wide bandgap range, which covers 0.7-6.2 eV. This enables the production of optoelectronic devices spanning the infrared to the deep ultraviolet.

Yet few other wavelengths have matched the success of blue emitters, and there are more difficulties in exploiting other areas of the spectrum.

One problem is the green gap, the decline in internal quantum efficiency as the emission wavelength of LEDs with InGaN quantum wells is increased.

Ploog went on to highlight two successes with dilute nitrides - Quanlight s production of red-emitting LEDs, and the development of lasers on silicon by the German start-up NAsP III-V.

He also believes that multi-junction solar cells can exploit dilute nitrides as one of their constituent layers.

Publications relating to InN have rocketed in last few years, and Ploog went on to outline some of the major challenges with this nitride. These include electron accumulation, which produces n-type material and complicates the fabrication of a p-n junction.

These areas of investigation and more will keep industrial and academic researchers occupied for some time to come, Ploog assured. “Worldwide R&D in III-nitrides will thrive for another decade," he concluded.

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