Nitrides: Strategies To Boost Performance
Presentations at the twelfth International Conference on Nitride Semiconductors offered an insight into strategies for improving the performance of UV LEDs, lasers and various forms of transistor by Jean-Yves Duboz from CRHEA-CNRS
There is no doubt that a material system that has been used to manufacture countless devices for more than two decades is a success. But that does not mean that the devices have been commoditised, or that the material system is fully understood.
That's the state of affairs for GaN-based devices. While myriad LEDs and lasers have been shipped, netting billions of dollars, many questions remain that cover many bases. These questions are related to optoelectronic, RF and power devices, and to material properties and growth mechanisms.
Anyone wishing to hear the latest views on any of these matters should have attended this year's International Conference on Nitride Semiconductors, held in the historic city of Strasbourg during several rather wet days at the end of July. If they made the trip, they would have had the chance to talk to more than 800 delegates from all over the globe that met together in the Palais des Congrès et de la Musique. Many made the trip from Asia, which accounted for 43 percent of attendees, including 20 percent from Japan, while Europe contributed 42 percent and the US 12 percent. Chris Van de Walle from UCSB provided a beautiful opening talk to this biannual meeting with a didactic overview of GaN material and its specific properties.
The west-coast academic championed the need for a proper reference for calculating the spontaneous polarization in nitrides "“ it is a key feature of these materials, and one that plays a critical role in device behaviour. Van de Walle also discussed droop, the decline in LED efficiency at high drive currents. He stressed the importance of the Auger effect, which he believes has been underestimated by many authors. In addition, he explained how non-radiative recombination on point defects can be enhanced by the excited states of defect levels.
Echoing some of these themes in the final plenary talk on the closing day, Slatko Sitar from North Carolina State University highlighted the importance of point defects in AlGaN. These imperfections increase non-radiative recombination, leading to a reduction of UV LED efficiency. According to Sitar, it is the growth mode that governs the population of these point defects, which can take the form of either intentional or residual doping. Sitar also explained why intermediate AlGaN phases appear during the growth of AlGaN epilayers on AlN substrates. He pointed out that this low dislocation system allows the observation of a kinetically driven phase separation on the surface, which occurs due to strain. The extent of the separation depends on the temperature, growth rate and off-cut angle of the substrate.
During the conference many other groups reported results rela ted to these additional AlGaN phases. However, in most cases they arose in more dislocated systems, where surface kinetics are obscured by the effect of dislocations.
One of the highlights of the material session was the presentation by Bastien Bonef from UCSB, who showed both the beauty and the limitation of atom probe tomography. He argued that contrary to popular belief, atom probe tomography cannot be quantitative, unless it involves the use of a reference sample. Bonef also pointed out that the exact "“ and largely unknown "“ shape of the sample directly impacts the topology of the reconstructed profile.
Another talk in this session, given by Al Balushi from Pennsylvania State University, revealed that it is possible to form a two-dimensional layer of GaN between a SiC substrate and a graphene layer obtained thereon with a migration-enhanced encapsulation method. This is a triumph for GaN, as it can now replicate what has been accomplished with its BN cousin. It is not yet clear what applications may benefit from two-dimensional GaN, but its very high bandgap of 4.8 eV suggests that it has great promise in the UV. By undertaking precise transmission electron microscopy measurements, Balushi and co-workers observed how gallium atoms intercalate between graphene and the SiC, before reacting with ammonia to form a thin GaN layer with R3m symmetry.
Improvements in the visible...
Although academics dominated the conference, there were plenty of presentations from those in industry. They included plenary speaker Guillaume Arthuis, President of BBRight, a French-based developer of laser projection technologies. Arthuis argued that lasers can revolutionise movie projection by lowering the power consumption compared to filtered xenon lamps. What's more, lasers can simplify 3D projection, and by separating the light source from the digital light projector, they can yield simpler, cheaper, and more reliable systems in movie theatres. However, the downside of laser projection is speckle, stemming from the high degree of coherence of the laser. Addressing this issue is not easy "“ so far the best solution, which is far from ideal, is to shake the screen.