Summary of the International Workshop on Nitride Semiconductors (IWN2000)

The International Workshop on Nitride Semiconductors (IWN2000) was held in Nagoya, Japan, from September 24^th to 27^th 2000, conveniently scheduled just after the 25^th International Conference on the Physics of Semiconductors (ICPS25, Osaka, September 17-22, 2000). The objective of IWN2000 was to provide a forum for eager nitride researchers to present their latest results and achievements, to have in-depth technical discussions and to exchange their knowledge. The various topics taken up covered most of the nitride fields; some concerned with fundamental science, some with emerging applications, as well as some relating to brand-new subjects.

This workshop was organized to complement the International Conference on Nitride Semiconductors (ICNS), one of the major biennial nitride congresses. The 2^nd ICNS was held in 1997 in Tokushima, Japan, the 3^rd in 1999 in Montpellier, France and the 4^th will be held in Boulder U.S., in 2001. The nitride fields are rapidly developing and novel concepts and new results are being reported every day. Therefore, the necessity for a new forum to be held between the biennial ICNS series was building up among nitride researchers. In that time, in 1998, Prof. Gil, chairman of the 3^rd ICNS suggested the possibility of a new forum to Prof. Akasaki, the chairman of the 2^nd ICNS. And then they were talking about the possibility. After Prof. Akasaki came beck to Japan, the Japanese nitride community discussed the situation and decided to organize this new forum, IWN2000.

1.2 Viewing the workshop at the date

The workshop site was the Nagoya Congress Center, which reminded many nitride researchers of the successful Topical Workshop on III-V Nitrides (TWN’95), which had been held there in September 1995. So we just met there again after five years. When we started to organize this workshop, all of us imaged a smaller meeting, so 120-150 papers and 250 participants. Definitely, we failed to estimate the size of workshop. On the workshop the number of participants attained to 500, and 296 papers were described in the workshop.

2. Brief overview of the program

2.1 Comparison with previous conference

One plenary lecture, 8 invited papers, 273 contributed papers, and 14 late-news papers were submitted from 25 different countries and presented at the workshop. The number of contributed papers is compared with those that were submitted to two previous conferences, Topical Workshop on Nitrides (TWN95, 1995, Nagoya Japan) and the 2^nd International Conference on Nitride Semiconductors (ICNS2, 1997, Tokushima Japan), all of which focused exclusively on nitrides and held in Japan. Total number of paper keeps increasing. Two features, the rapid increases of number of papers submitted from Asia/Pacific countries and the number of countries are remarkable. These increases correspond to the spread of nitride technology throughout the world and the start of production of nitride materials and devices in the Asian countries.

2.2 Technical sessions

The workshop started with the plenary lecture by Professor Isamu Akasaki of Meijo University entitled “Renaissance and Progress in Nitride Semiconductors”. Professor Akasaki reviewed his long research history on nitride semiconductors and presented fresh results recently obtained in his laboratory. Eight invited papers and 80 contributed papers presented orally were categorized into the following 17 sessions in which 12 sessions were in parallel in order to accommodate the large number of papers within the 3 days of the workshop. The number in the parentheses is the number of presented papers including invited papers but excluding late-news papers. These topics put together cover almost all the current research fields of nitride semiconductors.

Two special sessions entitled "Optical Processes in InGaN QWs" and "p-Type Contacts" were organized in order to provide more in-depth and focused discussions on these topics. These should entertain much of the audience, as we received many papers which can be categorized within these two fields. Almost 200 papers were presented in the poster sessions on the following topics. 14 late-news papers reporting very recent results were also presented.

Three parallel rump sessions were held in the evening of Sept. 25 for frank discussions on hot and emerging topics.

2.3 Topics of the technical sessions

- In the session of growth and device (MM2), the potential of molecular beam epitaxy (MBE) was clearly shown in the research fields of both optical and electron devices. High efficiency LEDs from UV to Red using InGaN and InN quantum dots (QDs) by MBE were reported by Grandjean (CRHEA/CNRS, France). The room temperature electron mobility exceeding 1000 cm2/Vs in GaN was realized by the precise control of surface stoichiometry in MBE (Heying, UCSB, USA). For the devices, Arakawa ( Univ. of Tokyo, Japan) addressed the effectiveness s of QDs in nitride LDs and Schetzina (NCSU, USA) demonstrated visible-blind UV digital camera using AlGaN pin photodiodes, in which the doping into AlGaN with high Al composition is successfully performed.

- In the session of flow modulation epitaxy (MA2) , this technique was actively applied to grow difficult materials to be grown such as InN and to change and to improve the growth characteristics including lateral growth rate and layer properties.

- Laser lift-off technique to obtain free-standing GaN was reported (ATMI, Samsung, Linkoping U). Successful lift-off of large diameter wafers close to 2 inch was demonstrated for thick GaN on sapphire grown by HVPE.

- Co-doping to obtain p-type, high carrier concentration in GaN was challenged by a novel alternative-supply-technique of TMG and NH3 with TESi and Cp2Mg. The hole density was reportedly more than 1E19/cm3 which was 4-5 times larger than that without this technique (RIKEN).

- The wing tilt on ELO has still been a matter of concern. In-situ tilt monitor was made by SR-beam based x-ray diffraction (UCSB). Either maskless method (North Carolina State U) or special precaution of mask material (SONY) could prevent tilting of overgrown crystal.

- Blue and green LEDs were fabricated using cubic InGaN/GaN DH structures by MOCVD on GaAs substrates. The EL line width of 20nm was achieved. (Chinese Academy of Science, China) The generation of cubic phase was investigated by XRD and CL techniques for selective area growth of cubic GaN on SiO2-patterned GaAs substrates by MOCVD. (Univ. Tokyo, Japan) Cubic InGaN/GaN MQW structures were fabricated on cubic SiC epilayers by RF-MBE. Excellent structural quality and enhanced emission efficiency of the MQWs were confirmed by XRD satellite peaks and PL selective excitation of well regions, respectively. (Electrotechnical Laboratory, Japan)

- Recent progress of ammonia-MBE technique for III-nitrides was reviewed. Excellent AlGaN/GaN 2DEG properties by ammonia-MBE, such as high mobility more than 1000cm2/Vs, and the fabrication of HFET using the 2DEG and carbon-doped GaN insulating layers were demonstrated. (National Research Council of Canada, Canada) In addition to the keynote speech above, recent drastic quality improvement of III-nitride films grown by RF-MBE was also reported by Sophia University, Japan) and Electrotechnical laboratory, Japan. The importance of polarity control for grown films was emphasized. Several kinds of device fabrication using MBE were shown by P. Chow (SVT associates, USA). Substrate temperature inhomogeneity was pointed out as a problem to be solved.

- Besides the conventional AlGaN/GaN HEMTs, MIS-FETs in which a very thin insulator layer is inserted between the gate metal and AlGaN barrier layer were proposed. The insulator layer was plasma enhanced CVD Si3N4 [WA1-2] or SiO2 [WA1-3]. These MIS-FETs displayed a drastically small gate leakage current and could operated with a gate swing very larger than the conventional HEMTs. The new type of MIS-FETs may be another choice of structure for high power electronic applications.

- PL transients for the C-plane [grown on 6H-SiC (0001)] and M-plane (on-LiAlO2) GaN (5 nm) / Al0.1Ga0.9N (10 nm) MQW structures were investigated. The C-plane sample had a PL decay time of 5.8 ns, while the decay time of the M-plane sample was as small as 0.45 ns at 7 K. It is suggested that no internal electric field due to spontaneous and piezoelectric effects is induced in the M-plane sample along the growth direction. Therefore, the results show the effect of internal electric field on the oscillator strength of excitons, and the possibility of an improved quantum efficiency in the layers grown to non-polar direction. (H.T. Grsahn et al.) Temperature dependence of PL decay was assessed in GaxIn1-xN/GaN 5 QW LED structures (x=0.08, 0.26 and 0.34) grown by MOVPE on c-plane sapphire. In the case of x=0.08, PL decay times decreased with increasing monitoring emission energy at low temperature. Such dependence became less dominant with increasing temperature, and almost no energy dependence was observed at T=300 K. However, for the samples of x=0.26 and x=0.34, emission energy dependence was observed even at T>300 K, suggesting an important role of carrier localization for radiative recombination in In-rich LEDs. (S. Watanabe et al.)

- In the session of TA1, fundamental optical properties in hexagonal GaN and AlGaN/GaN/InGaN heterostructures were discussed. Non-radiative nature of threading dislocations was disclosed. Observation and theoretical analysis of excitonic polaritons in GaN substrate and AlGaN/GaN quantum well were given. Below-gap excitation spectroscopy and transient-grating methods gave a new insight into defects in GaN. Quaternary InGaAlN alloys have a freedom to tune the strain and bandgap discontinuity in Nitride Heterostructures.

- In the session of TA2, InGaN MQWs grown under low pressure, atomospheric pressure, and raised pressure MOVPE are investigated by TEM, optical pumping, and time-resolved spectroscopies. Optical gain, which directly corresponds to the density of states, in AlGaN/GaN/GaInN structures was shown, and excitons were shown to exist for moderately excited QWs. Furthermore, in the poster session optical properties in InGaN and GaN QWs were correlated with microstructures and growth parameters. Fine structure and origin of bound excitons were discussed.

- The characteristics of GaN-based LD were reported by seven groups of Nichia, Sony, Xerox, Samsung, Toyoda Gosei and Osram in WA2 and rump session. Almost groups achieved the reduction of threading dislocation density in the LD using each method such as epitaxial lateral overgrowth (ELO) technique and double buffer, and showed that the reduction should be essential for realizing the high performance of LD including the lifetime. Nichia's group reported the estimated lifetime of 15,000 hours (60C, 30mW) for the LD grown on a free-standing GaN substrate. Sony's group reported the lifetime of more than 200 hours (50C, 30mW) for the LD grown on ELO-GaN.

- Meijo Univ. succeeded to apply the reduction of the threading dislocation by introducing AlN low-temperature-interlayer and lateral growth method to UV LEDs. The efficiency of UV LEDs has been enhanced. (WM1-1) GaN-based p-n junction has been grown on Si substrates by introducing AlAs seed layer. The output power of the blue emission from the LED on Si was dimmer than that of on sapphire substrates by 1/30. The LEDs on Si are attractive in terms of cost. (WM1-4) Resonant cavity blue GaN-based LEDs have been realized mounted on Cu substrate after removing sapphire substrates. The reflection mirrors are composed of two dielectric DBRs. The particular type of LEDs indicates a blue VCSEL. (WM1-6) Solar-blind AlGaN hetero junction photodiodes shows the sharp wavelength selectivity in the range from 250 to 280 nm, which is capable of detecting flames. The p-i-n diodes showed the reverse current of 4-35 pA/mm2 under -5 V bias. The spectral sensitivity from 220 nm to 400 nm has been measured at 500 nW/cm2 and a sharp cutoff at 270 nm could be confirmed. (WM2-2)