The success of Nichia Chemical in demonstrating blue GaN LEDs has generated a lot of excitement both in Japan and in the United States (Nakamura 1994). Many Japanese companies the JTEC panel visited, including Toshiba and Matsushita, are beginning efforts in this materials system. Some are still continuing previous efforts in the II-VIs. In Japan good products seem to find their way to market very rapidly. For example, the committee observed very bright Nichia blue LEDs on sale in shops in Tokyo's Akihabara district.
In the United States there is also a significant amount of new work, mainly in universities (e.g., Illinois, Northwestern, North Carolina State, University of Texas at Austin, and the University of California at Santa Barbara), but notably also in such companies as Hewlett-Packard, Xerox, SDL, APA Optics, and Honeywell. Hewlett-Packard appears ready to launch a major activity in this direction.
Lasing has been reported in GaN in a relatively crude optically pumped structure (Yang et al. 1995), but most work has been with LEDs. Many applications call for LEDs more than for lasers; however, in the important area of optical recording, lasers are clearly required. This application alone justifies the work being done to develop lasers in this system.
In the future the panel expects to see numerous reports of improved efficiency and lifetime of LEDs as well as lasers in the GaN-AlN-InN system. The major issue is one of finding or generating a lattice matched substrate. Surprisingly, the large number of defects accompanying the growth of GaN on Al 2O 3 do not seem to destroy its luminescence properties (Lester et al. 1995). However, it is widely assumed that a better substrate or technique for growing layers on mismatched substrates will be key to making practical device-quality material. Japan appears to have a slight lead because of the Nichia work, but in terms of general research results, the United States is somewhat closer. In fact, it seems that the United States is moving about as fast as Japan in developing this materials system. Nevertheless, moving the research into products still may be a hurdle for the United States.
In the ZnSe-based II-VI materials there are continuing efforts at a number of Japanese companies, which are routinely reporting progress. For example, Sony has recently reported 1 hour CW operation of its lasers (Ishibashi 1994). Toshiba, Matsushita, and Sharp are also concentrating on making practical devices from the II-VI-based materials. In the United States, North Carolina State and Eagle-Pitcher have demonstrated blue and green LEDs grown on ZnSe substrates with probably the best performances to date. They report 650-hour lifetimes and high brightness (Yu et al. 1994).
In the II-VIs it is well understood that defects must be eliminated to create long-lived devices. Most efforts focus on finding techniques to remove the defects associated with growth on lattice mismatched GaAs substrates or techniques to improve upon the ZnSe lattice matched substrates mentioned above. The United States appears to be slightly behind Japan in the II-VI area, but U.S. and Japanese efforts are relatively close. As in other cases, being able to move research results into marketable products is a key issue.
Work on organics falls into three general areas: passive waveguides, electro-optic waveguide modulators, and electroluminescent display materials. In all cases, the hope is to create a lower-cost and more manufacturable technology. Also, it is believed that organic device structures can be created on a wide variety of substrates without much regard to lattice matching or surface topology. In the waveguide areas, significant research efforts exist at Matsushita, Fujitsu, NEC, and NTT. Although still at the basic research level, they seem to aim at both passive optical interconnects (incorporating such elements as splitters and couplers) and active modulator structures (for high-speed switching and signal encoding applications). Low-loss waveguides have been measured fairly recently (Matsuura et al. 1993; Langhoff, Stokich, and Heistand 1993). There is also some work on nonlinear effects in NEC and Toyota. In the United States, there are analogous efforts at Honeywell, General Electric, and a number of universities. All things considered, it would appear that Japanese companies are closer to marketing products than U.S. companies.
In the electroluminescent display area, Sanyo, Idemitsu, and Matsushita are active in developing light-emitting polymers that may find use in a variety of display applications (Yu, Pakbaz, and Heeger 1994; Yang et al. 1995). In the United States, a few university efforts exist, and a new start-up company, UNIAX Corporation, is actively developing product concepts in light-emitting polymers. Table 5.2 summarizes the primary Japanese companies involved in the materials work.
New Optoelectronic Materials Efforts In Japan