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.
Arthius will have been pleased to hear talks at the meeting describing recent improvements in visible semiconductor laser performance. Masahiro Murayama from Sony Corporation announced that the company's green lasers can now deliver a 1 W CW output at 530 nm, under a drive current of 1 A. This laser has a wall plug efficiency of 17 percent, and an estimated lifetime of over 20,000 hours. Another pioneer of powerful green lasers is Osram Opto Semiconductors. Spokesman for that company, Harald König, told delegates that its green 517 nm lasers can now produce 120 mW at 200 mA, with a wall plug efficiency of 11 percent. Increase the wavelength to 532 nm and wall plug efficiency falls to 6.5 percent.
Other developments in visible nitride emitters included impressive results on green VCSELs by a team including Xin Zhang, who is affiliated to Xiamen University. Zhang described a surface-emitting device that produced CW, room-temperature emission at 560 nm with a threshold of 780 A cm-2. Meanwhile, Czesław Skierbiszewsk, from TopGaN and the Institute of High Pressure Physics at the Polish Academy of Sciences, revealed the use of tunnel junctions in MBE-grown blue lasers. These devices demonstrated a slope efficiency of 0.85 W/A. Erin Young from UCSB is pursuing the same goal of improving the laser slope efficiency by introducing a tunnel junction in her blue edge-emitting lasers and VCSELs. With this modification she obtained a seven-fold increase in the power produced by the VCSEL.
Moving further to the red, Yasufumi Fujiwara from Osaka University revealed that the addition of europium to the nitride quantum well enables a shift in emission to longer wavelengths. He and his co-workers fabricated red LEDs that produce 1.2 mW at 621 nm under a drive current of 20 mW. These results correspond to an external quantum efficiency of 3 percent. Higher efficiencies are possible, suggested Fujiwara, by optimising the incorporation site of europium so that it enhances energy transfer between this element and the GaN lattice.
An even more exotic approach to increasing the emission wavelength of nitride materials is to incorporate antimony into the active region during MBE growth. Zetian Mi, now at Michigan University, is leading a team exploring this approach. He told delegates that incorporating just 1 percent of antimony can shift emission to 600 nm. He explained that while changes to indium content merely impact the conduction band, the addition of antimony pushes up the valence band. Confirmation of these findings could unlock the door to new designs of nitride optoelectronic devices.
... and the UV
The performance of nitride devices is also improving in the UV. For LEDs operating in this spectral domain, many groups are exploring the trade-off between a good light extraction, by employing a p-AlGaN contact layer, and realising good carrier injection with a p-GaN contact layer. Both options lead to a similar wall plug efficiency: it is about 10 percent at 275 nm. RIKEN chief scientist Hideki Hirayama showed that this wall plug efficiency can be achieved with an AlGaN top contact, leading to an external quantum efficiency of 20 percent. With the alternative approach of the GaN contact, results reported in recent academic literature show that although the external quantum efficiency is just 10 percent, the lower bias results in a similar wall plug efficiency.
Going to even shorter wavelengths causes efficiency to plummet. According to Leo Schowalter, Chief Technology Officer at Crystal IS, efficiency halves for every shortening of wavelength by 5 nm. This rule of thumb is backed up by his report of an external quantum efficiency of about 0.3 percent at 239 nm, for a 1.8 mW LED, and the claim of a 4 percent external quantum efficiency by Akira Hirano from UV Cratftory, for a 260 nm LED.
Modifications to the standard device architectures are producing exciting results in the UV. Zetian Mi spoke about the use of AlN nanowires, which realise extraordinarily high levels of p-type doping, thanks to strain relaxation. Using this approach, his group has produced 239 nm lasers and 207 nm LEDs. Adding a tunnel-junction is another modification that shows promise. Siddharth Rajan from The Ohio State University revealed the successes of his group with this approach, including an on-wafer wall plug efficiency of 1 percent for a 287 nm device operated at 12 V. An even more novel approach, described by Thomas Wunderer from PARC of Pan Alto, CA, is to replace electrical injection with electron-beam pumping. This approach enabled lasing action at 387 nm; and also emission at 246 nm, with a power of 230 mW, using pumping with a 4.5 mA current and a 12 kV accelerating voltage. For the shorter wavelength, wall plug efficiency is 0.4 percent.
Progress in LEDs and lasers continues to be driven by a combination of experimental efforts and theoretical work "“ together they can lead to new insights. At this year's ICNS, Aurélien David, Senior Principal Scientist at Soraa, gave new insights into the physics of LEDs within the framework of the well-known ABC model, which describes the various recombination mechanisms for the device. David explained that the A, B and C coefficients all depend on current, and revealed that with this modification it is possible to produce a very good fit to experimental data. Meanwhile, Gerhard Klimeck, an academic at Purdue University, explained that it is possible to correctly describe the transport in LEDs with a full quantum model that is based on a non-equilibrium Green function.
When talking about nitrides, one does not necessarily think about quantum optics. However, nitrides are good candidates for fabricating true single-photon emitters working at room temperature. Good results have been obtained by Yasuhiko Arakawa's group from the University of Tokyo. Spokesman for this team, Mark Holmes, detailed quasi perfect, single-photon emitters (g2(0)=0.02). These were obtained by working at a low temperature (10K), and by exploiting the quantum-dot like fluctuations in GaN quantum wells.
Further highlights from this session that covered optical devices included: the use of LEDs in high bandwidth communication and positioning, described in a talk by Phil Dawson from the University of Manchester; a presentation from Qian Sun from Sinano detailing a CW, room-temperature laser grown on silicon, as well as photonic circuits "“ including an emitter, guides and detector "“ made with this material system; and in a similar vein, the talk by Fabrice Semond from the University Côte d'Azur, describing an optically pumped microdisk laser grown on silicon that emits from 275 nm to 470 nm.
Another advance, reported by Moti Katz from Soreq NRC, Israel, was a transition at 1.8 μm associated with strong coupling. This is commonly observed with interband transitions, but in this case it resulted from intersubband transitions, observed in both transmission and in the photocurrent from a quantum cascade detector.
Power devices
Although GaN power devices are not generating the sales of their LED cousins today, revenues are rising, and the potential for further growth is very promising. One of the leaders of device production is Infineon Technologies, and at ICNS-12 Thomas Detzel, the firm's Senior Manager for GaN Technology Development, closed the conference with a historical and technical comparison of the capabilities of GaN and silicon for power applications. In his plenary talk, he argued that the fundamental advantage brought by the nitrides over silicon is the combination of a lower on-state resistance (RDSON) and a negligible recovery charge. Thanks to these attributes, GaN devices can go faster while consuming less power. That does not mean, however, that silicon devices are doomed. Detzel believes that it will take some time before GaN can take significant market share in the power arena, and he expects GaN to coexist with silicon for a long time, rather than replacing the incumbent.
Within the electronic session, the focus was on power electronics, and in particular established horizontal FETs and emerging vertical transistors, plus Schottky diodes. However, there were also reports on RF devices, including those with vertical structures. At the gathering in Strasbourg, researchers described the development of devices with classical and quantum transport. Examples of the former include finFETs pursued by Tomás Palacios' group at MIT and fin MOSFETs fabricated by Maher Tahhan and co-workers at UCSB, while the latter includes the tunneling hot electron transistor, described by Siddharth Rajan from The Ohio State University.
The talks in this session revealed two trends in nitride electronics: a move to more vertical devices; and the use of nanotechnology to provide better control of the electric field in the gate region. A group pioneering the latter is that of Elison Matioli and co-workers from EPFL. Using slanted tri-gate structures, they realised breakdown voltages of up to 1.8 kV.
Another move within the power electronics community is to grow GaN devices on a native substrate. Tohru Oka from Toyoda Gosei described efforts in this direction, including the fabrication of a Schottky diode with a lateral field plate that had a breakdown voltage, at 1.8 mΩ cm-2, of 770 V. The recovery time for this diode is just 50 ns, a value far less than that for equivalents made from silicon and SiC.
Oka revealed that the field played a crucial role in realising a high breakdown voltage in the MOSFET "“ its addition propelled the breakdown voltage from 775 V to 1600 V. However, reaching an on-resistance of just 1.8 mΩ cm-2 required a slight sacrifice to this figure. Oka and co-workers have formed switching circuits formed from the FETs and GaN Schottky diodes with a switching time of just 20 ns, and switching energies for turn-on and turn-off of 12 μJ and 90 μJ, respectively.
Those attending ICNS-12 will be aware of the move to higher frequencies for applications and circuits. GaN technology could serve here: Rüdiger Quay from Fraunhofer IAF and Kozo Makiyama from Fujitsu both argued that point-to-point communication at 84 GHz could benefit from this wide bandgap technology. However, José Jimenez, a Fellow of Device Physics at Qorvo, warned that GaN still suffers from a lack a linearity and a lack of reproducibility in this characteristic. In his opinion, based on existing performance, GaN has limited capability to replace GaAs.
Another highlight of the electronic session was the presentation by Grace Xing from Cornell University. Xing reported convincing results on resonant tunneling diodes grown by MBE on GaN substrates. She observed clear, stable and repeatable negative differential resistance at current densities ranging from a few to 180 kA cm-2. This indicates that there is tunnel transport through fundamental and excited states in a thin GaN well in between two AlN barriers. This negative differential resistance led to oscillations at 300 MHz when the diode was inserted into a resonant circuit. In addition, impact ionization was observed, likely due to high-energy ballistic electrons crossing the outer GaN region of the structure.
More reports on GaN transistors and other devices will be given at the next ICNS, held in Seattle in July 2019, and chaired by Alan Doolittle from Georgia Tech. For researchers that can't wait that long to hear and discuss developments in the nitrides, one option is the International Workshop on Nitride Semiconductors, which will be held next year in Kanazawa, Japan, in mid-November.
