III-V Nitrides were the subject Symposium N of the 1996 Fall MRS Meeting, held in Boston at the beginning of December. The symposium was chaired by Profs. Moustakas, Akasaki, Monemar and Dr. Ponce. The program they assembled was first rate, and their labors have earned the deep appreciation of the entire Nitride research community!
The meeting was led off by Shuji Nakamura's review of his recent results. The Nichia blue laser effort has acheived 35 hours of CW room temperature lasing, and the trend line indicates that it should surpass the best reported II-VI laser lifetime imminently. Dr. Nakamura also showed the white LED's now being commercialized by Nichia. They consist of UV emitting LED's packaged with a phosphor to produce white light. Nichia will apparently sell you a flashlight with a white LED. The violet laser makes a terrible pointer, since the wavelength is so short that your eyes can't really see it, but the little laser box that Dr. Nakamura carried around during the coffe breaks and poster sessions was still impressive.
A recurrent theme of the meeting was segregation in InGaN. Shigefusa Chichibu of Science University of Tokyo showed TEM images from Nichia material that seemed to show pseudo periodic contrast in InGaN quantum wells highly suggestive of some sort of spinodal decomposition. (InGaN is known to have a large miscibility gap, and should thus be unstable to unmixing. Spinodal decomposition is an unmixing phenomenon in which periodic composition waves are kinetically favored. Perhaps an expert can contribute a more detailed explanation.) The speculation was that quantum dot phenomena resulting from segregation are responsible for InGaN being such a good light emitter.
A related phenomenon, called "composition pulling" was reported by Prof. Hiramatsu of Nagoya U. InGaN thinner than some critical value was shown to have a composition which better matched the lattice constant of the underlying substrate. Similar phenomena have been reported in other systems, for example Allovon et al. J. Electr. Mater. 18, 505, 1989.
In the MBE area, people are now starting to investigate the detailed mechanisms of growth. Guy Feullet from Electrotechnical Lab presented a "phase diagram" of the 2x surface reconstruction. Ruediger Held of University of Minnesota presented some fascinating work on Desorption Mass Spectroscopy and RHEED Oscillations in GaN growth with ammonia. Winning the award pushing MBE to be like MOCVD was Mark Johnson of NC State, who reported high growth rates with the new EPI plasma source at substrate temperatures up to 1000°C.
The Polish GaN crystals continue to get bigger. Perhaps even more exciting is the reemergence of AlN bulk crystal growth, which has been dormant since Slack's work in the 70's. Cengiz Balkas of NC State reported work on some new crystals.
Papers presented on optical and structural characterization seemed to be summarized by one audience member who said: "Your results are very interesting, but the thing to remember is they are true for your samples." Given this proviso, it seems to me that things are progressing quite nicely. In the microstructure area, there now seems to be agreement that inversion domain boundaries are an important defect and people are finally paying attention to the polarity of GaN. In the optical characterization area, it seems that the effects of strain and zincblende inclusions are finally being sorted out. The origin of yellow luminescence does not seem as settled.
On the last day, Steve's DenBaars and Binari showed us that GaN FET's are an application we shouldn't overlook, although a number of problems remain to be solved.
Needless to say, there were lots of other presentations worthy of note, but I didn't get to see everything. I have structured this as a discussion so that people can contribute summaries of stuff that I missed or didn't quite get. In particular, I haven'tdone justice to the MOCVD work, the doping stuff or the processing talks.
This discussion is moderated by E. S. Hellman.
© 1996 The Materials Research Society