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Executive Summary Electronics that operate and control functional systems must currently be protected from extreme environments. Major benefits to system architecture would result if cooling systems for electronic components could be eliminated without compromising system performance (e.g., power, efficiency, speed). The existence of commercially available high-temperature semiconductor devices would provide significant benefits in such areas as: sensors and controls for automobiles and aircraft; high-power switching devices for the electric power industry, electric vehicles, etc.; and control electronics for the nuclear power industry. With the possible exception of light-emitting diodes (LEDs), however, present commercial demand for wide bandgap semiconductor materials is limited. While there are few pressing applications that cannot be achieved without wide bandgap materials, the vast array of applications, and hence, the value, will only be realized once these materials have evolved to such an extent that off-the-shelf devices are available. At the request of the U.S. Department of Defense and the National Aeronautics and Space Administration, the National Materials Advisory Board of the National Research Council convened the Committee on Materials for High-Temperature Semiconductor Devices to assess the national and international efforts to develop high- temperature semiconductors; to identify the technical barriers to their development and manufacture; to deter- mine the criteria for successfully packaging and integrating new high-temperature semiconductors into existing systems; to recommend future research priorities; and to suggest additional, possible applications and advantages. 1 This Executive Summary is divided into two sections. The first section presents general conclusions and recommendations about future research priorities to accelerate the acceptance of high-temperature semi- conductor materials. This section discusses the temperature ranges for the different materials to be used, the competitiveness of U.S. research versus foreign competition, the systems in which high-temperature electronic materials should initially be introduced, and the government/industry/university collaborations required to forward the development of high-temperature semiconductor materials. The second section discusses the barriers to the successful development, manufacture, packaging, and integration of wide bandgap materials into existing systems and presents the key research and development priorities to overcome these barriers. GENERAL CONCLUSIONS AND RECOMMENDATIONS Temperature Ranges Silicon and silicon-on-insulator electronics may be sufficient for some applications for temperatures up to 300 C. Such applications include digital logic, some memory technologies, and some aerated analog and power applications. Silicon-based technology will not be sufficient for many applications operating in the 200- 300 C range, however, such as power-conditioning devices in higher-temperature control systems. These devices will have to be produced from another material system. Based on the evidence presented in this report, silicon-carbide-based devices are currently in the best position to meet this need, particularly e-channel enhan cement-mode metal-oxide semiconductor field elect

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Materials for High-Temperature Semiconductor Devices transistors (MOSFETsJ. However, significant technological barriers, such as micropipes, oxide quality, contacts, metallization, packaging, and reliability evaluation still need to be further addressed. As a result of fundamental limitations, silicon-based technologies will not be useful at temperatures above 300 C. Other materials must be used for these temperature ranges, but the choices are somewhat less clear. Technology based on gallium arsenide (GaAs) might be used for systems operating up to 400 C. Just working at elevated temperatures is not the only concern, however. It is also essential that the devices reliably function over a wide range from very cold (i.e., -20 C) to very hot (i.e., 400 C). Based on the evidence presented in this report, devices based on e-type silicon carbide (SIC) are the only type that currently appear to meet the temperature-range and reliability requirements, but additional development is needed. Eventually, high- temperature electronic technology could be developed for reliable operation even for temperatures above 600 C. U.S. Competitiveness As described in the Preface, considerable international resources are currently being devoted to developing electronic technologies either tailored for or supportive of high-temperature operation. The United States is focusing most of its efforts on high-temperature applications and currently has a slight lead in SiC research. Europe appears to be increasing its effort in wide bandgap materials, especially for power electronics. This research area is synergistic with high-temperature applications because the generation of internal heat is a limiting factor in power devices and can be mitigated by larger bandgap and higher thermal conductivity materials. The dedication of European resources to this area is seen in the founding of the collaborative organization HITEN, which was established in 1992 to coordinate nascent European efforts in high-temperature electronics. Japan is emphasizing the use of wide bandgap materials for opto-electronics and leads in the use of nitrides for light sources. Japan is also becoming interested in power and high-temperature applications. Unfortunately, the closed nature of Japanese industry made it difficult for the committee to determine the true level of interest in wide bandgap materials research. The increased interest in high-power, high-temperature 2 applications is evident in Japan's annual domestic SiC conference, however. The Third Domestic (Japan) SiC Conference convened in Osaka on October 27-28, 1994, with approximately 160 experts in attendance. Contrary to Japan's previous two conferences, there was a greater emphasis at the Osaka conference on high-power, high- temperature applications than on LEDs. The Commonwealth of Independent States had a number of major programs in SiC development, but the current financial difficulties of most of the Common- wealth's institutions are preventing many laboratories from continuing their research. There is a wealth of expertise and information available for leveraging by other countries, however. For instance, the European Community is planning on supporting a SiC growth effort in St. Petersburg (Y.M. Tairov and V.E. Chelnekov, personal communication, 1994~. The committee believes that the U.S. wide bandgap materials research community is currently very competitive in the international research community. To remain competitive in the international research community, the committee recommends that demonstration technologies be pursued to motivate further research and increase interest in high-temperature semiconductor applications. Demonstration Technologies To increase interest and motivate further research in wide bandgap materials, a realistic, inspiring application focus must be found that can make system designers aware of the benefits of high-temperature electronics. A wide bandgap transistor that operates at 150 C will not drive the technology because it will be in direct competition with the more economically efficient silicon technologies. The demonstration technologies must be system circuits (i.e., not an individual device) that can be inserted into essentially nonelectronic systems (e.g., turbine engine, nuclear reactor, chemical refinery, or metallurgical mill) with the goal of measurably increasing system performance. As discussed in Chapter 1, the committee believes that there eventually will be a niche market for semiconductors with temperature capabilities higher than that of silicon, and that this market will be sufficiently large to justify the cost of development. However, this belief is tempered by the recognition that because such

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Executive Summary electronics will be used in new ways there is little immediate demand. The market will grow only in synergy with the availability of components. This suggests that development of high-temperature electronics not be undertaken in isolation. Instead, such development can and should be leveraged from development of other technologies with more immediate applications, thus reducing the costs and risks of both. Three suitable application areas are high-power electronics, nuclear reactor electronics, and opto-electronics. Power switching devices, for example, would be a good demonstration technology for high-temperature semiconductor materials. High-voltage, high-power electronics, while not necessarily used as high-temperature devices, nevertheless need wide bandgap semiconductors because of their superior breakdown voltages and high thermal conductivities. There is already considerable research being pursued in this area because (1) improved high-power switching devices could save an estimated $6 billion in the cost of construction of additional transmission lines; and (2) the smoother, more efficient use of the transmission system would reduce the need for new generating capacity, which the Electric Power Research Institute estimates would be a savings of $50 billion in North America alone over the next 25 years (Spitznagel, 1994~. The pursuit of demonstration technologies would not only increase interest in wide bandgap materials, it would also provide significant test beds for the application of the technology and enhance our understanding of the generic technologies required to further high-temperature-device operation (e.g., materials etching and implantation; degradation modes of metallic gates, contacts, and interconnects at high temperatures; packaging behavior at high temperatures; and accelerated-testing and reliability- testing methodologies to ensure proper functioning). The ability to grow a reasonably defect-free material is not the only requirement for the realization of a successful technology. The development of demonstration technologies would also help identify other factors that must be resolved for high-temperature electronics to be incorporated into existing systems. Funding Strategy The need for new development funds for demonstration technologies and future wide bandgap 3 materials is not necessary in the committee's opinion. Government funding currently exists for long-range research in wide bandgap materials, although additional funding would certainly allow more options to be evaluated within a shorter period of time. Industry has also demonstrated a willingness to commercialize new developments if the projected payback to their investments can occur within the short term (NRC, 1993~. The committee believes that the high-temperature research community should leverage the research funding for wide bandgap materials that is currently being provided by the high-power and optics markets, where no viable alternatives to wide bandgap materials currently exist. Building on the funding for other areas dependent on wide bandgap materials reduces the need for potential users of high-temperature devices to fund the required materials development exclusively and, thus, may render it cost effective. The committee recommends the following strategy for the development of wide bandgap materials: develop precompetitive alliances and integrated programs (national laboratories, universities, and industries) for coordinating research, technical skills, and capabilities to expedite research in the most efficient manner; direct research at a technology demonstrator that has definite applications (i.e., is a product) and addresses the usually neglected areas of packaging, assembly, testing, and reliability (e.g., high-power switches; integrated motor control; power phase shifter); concurrently develop materials, design, testing, and packaging; and build and test the demonstration component on a cost-share basis that encourages teaming, ensures adequate funds, and requires periodic deliveries. The committee believes that the founding of a newsletter that provides a summary of published worldwide developments in high-temperature semi- conductor research would assist the establishment, development, and maintenance of (1) a fundamental long- term materials effort, (2) an infrastructure within the industry, (3) a group to monitor international development, and (4) a U.S. information group for highlighting advances.

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Materials for High-Temperature Semiconductor Devices MATERIALS-SPECIFIC CONCLUSIONS AND RECOMMENDATIONS The first three parts of this section concentrate on the major wide bandgap materials discussed in this report: SiC, nitrides, and diamond. The final part of this section concerns the generic problems in packaging that will affect the production of all high-temperature electronic devices. Silicon Carbide SiC is an indirect bandgap semiconductor and has enjoyed the longest history and greatest development with regard to both materials growth and device realization. As such, SiC is currently the most advanced of the wide bandgap semiconductor materials and in the best position for near-term commercial application. Its main application will be in high-power, high-temperature, high-frequency, and high-radiation environments. It will not be suitable for blue lasers or ultraviolet light emitters, however, except as a potential substrate material. The three key research efforts for the development of commercially viable SiC devices are: Wafer production: The 1- and 2- inch SiC wafers now in production are rapidly approaching device quality where they might be used for commercial production of devices and circuits with acceptable yield. It could be argued that such small wafers are entirely sufficient for what will be a relatively small market (compared with silicon) with a very high-price premium, and therefore an early investment in larger wafers is not justified. How- ever, the entire commercial infrastructure for electronics manufacture is based on a wafer size of at least 3 inches, and preferably 4 inches, as a minimum. Reconstructing a small-wafer infra- structure that became obsolete over 30 years ago will be both an expense and an obstacle to the introduction of commercial SiC electronics. The committee believes that the development of larger SiC wafers is viewed as the more cost-effective approach to commercial development. Film growth: Chemical vapor deposition molecular-beam epitaxy, and other film-growth 4 technologies and chemistries require refinement to produce epitaxial films with n- and p-type doping ranges from 10~3 to 102 C~3 for nitrogens aluminum, boron, gallium, transition metals, and rare earth elements. Manufacturing processes: Lower-cost device- production methods are required to make the manufacture of SiC devices more competitive with the silicon technologies. c7 , Nitrides Interest in the direct bandgap nitride materials (i.e., gallium nitride, aluminum nitride, aluminum gallium nitride, and indium gallium nitride) has dramatically increased recently because of their optical properties. The materials show great promise and are likely to dominate the visible and ultraviolet opto-electronics market. Nichia's recent bright blue LEDs have already stimulated increased industrial effort (e.g., Hewlett Packard, Spectra Diode Laboratories, Xerox PARC) in materials growth, contact metallurgy and reliability, and device reliability and testing, although the materials have defect densities of greater than 10~/cm2 and the mechanism of photo emission is currently unknown. Heterojunctions in the nitrides also hold promise for higher-speed devices com- pared with SiC. Their applicability for power development and high-frequency devices is unproven at this time, and the technologies for wafer production, doping, and etching are currently less developed than SiC and require more longer-term research before they will be competitive with other electronic materials. However, as development of photonic applications for wide bandgap materials progresses, the opto-electronic market may provide an effective way to leverage the development of these materials for high-temperature-device applications. The committee identified the following three research efforts as being key to the development of nitride devices: Compatible substrates: Better-matched substrates are required for nitride wafer production to be commercially tenable. Wafer production: Growth of quasi-crystalline films of gallium nitride, aluminum gallium nitride, and aluminum nitride should be pursued on substrates such as SiC to gain thermal advantages.

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Executive Summary Doping: Methods for both n- and p-type doping of Group III nitrides are required. Diamond Diamond is a well-understood material, but its use for active electronic device applications is not feasible at this time because of the difficulties associated with its economical growth and doping. While diamond transistors have been designed, fabricated, and tested, their perfor- mance is also orders of magnitude less than that which is expected from the electrical properties intrinsic to dia- mond. The poor performance is thought to result from excessive nitrogen impurities and from as yet not fully explained surface-depletion effects. The current prognosis for diamond is primarily as a protective coating, a thermal management film, and a material for electron-emitting cathodes. Packaging Much more research is required in the area of high- temperature packaging. For high-temperature electronics to be commercially viable arut provide true performance 5 advantages, interconnection and packaging technologies are required that can reliably operate at temperatures up to 600 Cfor 104 hours. To attain these goals, innovative packaging techniques will be required. The three key research efforts for the development of high-temperature packages are: Metallization: Contacts are required in the 10-6 to 10-7 Q/cm2 range that have long-term durability at temperatures up to 600 C. Greater understanding is needed of the long-term effects of high tem- peratures on contact and interconnect metallurgy, degradation and failure modes, reliability, and interfaces. Device reliability and aging testing: Existing methods of accelerated, environmental-life testing of packages must be adapted for high-temperature applications to ensure the accurate assessment of device reliability and aging. Computer-aided design tools: Computer-aided design tools are required that incorporate electrical and mechanical simulation of high- temperature electronic systems.

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