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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities 4 High-temperature, Wideband Gap Materials for High-power Electrical Power Conditioning INTRODUCTION Electronic devices fabricated from high-temperature, wideband gap (WBG) materials offer a number of advantages over corresponding devices fabricated from silicon. These include higher temperature stability; higher chemical stability in extreme environments; higher thermal conductivity, resulting in reduced cooling requirements; and higher breakdown field, which translates into more compact and higher frequency devices. These characteristics are especially desirable in high-power electronic devices such as those used in the power conditioning systems of hybrid electric vehicles. Three types of WBG materials are discussed in this chapter: silicon carbide (SiC), gallium nitride (GaN), and aluminum nitride (AlN). Only SiC was discussed during the data-gathering workshop;1 information on AlN and GaN was obtained from other sources.2 SiC is a polytype material with different possible arrangements of the Si and C atoms in the lattice. Specifically, the variations of SiC considered were 6H-SiC and 4H-SiC (see below). The defect density and the effects of specific defects are presently the most telling metrics for WBG materials. In SiC the defect types and effects (micropipe density, screw-thread density, stacking fault density) have been identified, and efforts are under way to circumvent and eliminate them. The standard semiconductor parameters are also metrics for WBG materials. Specifically, the carrier mobilities as a function of applied electric field, the dielectric breakdown strength, the intrinsic resistivity (minimum impurity density), and the thermal conductivity are of importance in comparing and applying the materials in high-power devices. SILICON CARBIDE SiC is currently the most practical high-temperature WBG material for advanced power electronics. The large band gap (4H-SiC = 3.26 eV) enables operation with device junction temperatures that can exceed 600ºC. The large band gap also enables a very 1 M. Mazzola. 2002. “SiC High-temperature Wideband Gap Materials.” Briefing presented to the Committee on Assessment of Combat Hybrid Power Systems, National Research Council, San Jose, Calif., August 26. 2 S. DenBaars and U. Mishra, University of California, Santa Barbara, private communications, August 19, 2002.
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities high breakdown electric field, as illustrated in Figure 4-1. The high breakdown electric field of SiC, which is 10 times greater than that of silicon, translates into thinner conduction regions at constant doping and thus very low specific conduction resistance, as illustrated in Figure 4-2. FIGURE 4-1 Comparison of SiC and silicon dielectric strength. SOURCE: Courtesy of Michael Mazzola, Mississippi State University. Presently, after many years and millions of dollars of government and private investment, high-quality substrates are commercially available. The commercially available substrates, although not sufficiently defect-free for large area device fabrication, permit homoepitaxial layers to be deposited in which high-power devices, which require low-defect materials, can be fabricated. Furthermore, the high thermal conductivity of SiC (4.9 W/cm-K at 300 K) enables increased power density and thus much more compact or much higher power per unit area than silicon devices. Specifically, increased power with SiC is possible when the difference between the junction temperature (Tj ) and the ambient temperature (Ta) is small, and passive cooling is possible when the difference between Tj and Ta is large.
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities FIGURE 4-2 Comparison of conduction resistance for SiC and silicon. SOURCE: Courtesy of Michael Mazzola, Mississippi State University. Furthermore, the chemical resistance of SiC is such that devices can be deployed in challenging environments. Fabrication is facilitated in that, like silicon, the native oxide is SiO2, which permits normally-off devices based on the metal oxide semiconductor field effect transistor (MOSFET) to be fabricated. However, presently, junction field effect transistors (FETs) are more practical, because defects at the gate insulator-conduction channel interface limit the switching performance of SiC MOSFETs produced for applications. At present, the world’s first commercially available power device based on WBG materials is the SiC Schottky diode (600 V, 4-12 A). In general, the state of the art in SiC devices is operating voltages up to 10 kV and currents up to 100 A, but these parameters have not been demonstrated in the same device. Figure 4-3 illustrates the demonstrated performance of SiC devices. SiC Crystal Orientations The crystal orientation in SiC is very important in that it determines the orientation of the micropipes so that the crystal orientation is a metric for the micropipe density. In SiC, there are several options for orienting the surface of the material with respect to the basic crystal orientation. These orientations, illustrated in Figure 4-4, include the “c” plane surfaces and the “a” plane surfaces. The vast majority of previous SiC growth and wafer fabrication efforts have resulted in “c” plane substrates. As
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities FIGURE 4-3 Comparison of silicon and SiC operating voltage and conduction resistance. SOURCE: Reprinted by permission from J.A. Cooper. 2002. Opportunities and Technical Strategies for Silicon Carbide Device Development. Materials Science Forum 389-393 15-20. Copyright 2002 by Trans Tech Publications, Switzerland. mentioned previously, the c-plane wafers have micropipe defects that are generally perpendicular to the c-plane wafer surface, and the defect density is sufficiently large to prevent the fabrication of large-area devices. Therefore, the use of low defect density, epitaxial growths on the c-plane wafer surface have been pursued for device fabrication. Three possible arrangements of Si and C atoms in the lattice (designated 3C, 6H, and 4H), are shown for the (1120) “a” plane in Figure 4-5. The 4H SiC is the polytype of choice for power, because it has the largest band gap of common types, a relatively high mobility, and a small mobility anisotropy. The c-plane is polar (i.e., has a surface with a “carbon face” or a “silicon face,” while the (1120) face is nonpolar, which is important due to the fact that SiC chemical vapor deposition (CVD) epi is sensitive to polarity (especially doping). In addition, c-plane epi growth requires an off-axis cut to ensure polytype replication while the (1120) a-plane does not.
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities FIGURE 4-4 Silicon carbide crystal and wafer plane orientations. SOURCE: Courtesy of Marek Skorwonski, Carnegie Mellon University. Silicon Carbide Unit Cell. Available at <http://neon.mems.cmu.edu/skowronski/Silicon%20carbide%20unit%20cell.htm>. Accessed August 29, 2002. FIGURE 4-5 SiC polytype for (1120) “a” plane. SOURCE: Courtesy of Marek Skorwonski, Carnegie Mellon University. Silicon Carbide Unit Cell. Available at <http://neon.mems.cmu.edu/skowronski/Silicon%20carbide%20unit%20cell.htm>. Accessed August 29, 2002. The (1120) a-plane material is important in that the direction of micropipe defects are approximately parallel to the surface of the wafer and thus perpendicular to any applied electric field. Furthermore, the micropipes do not propagate into the device-critical epi layer when lying parallel to the surface, which could lead to large-area micropipe-free devices, important for power scaling future devices.
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities The parameters or metrics that are important in WBG materials such as SiC, GaN, and AlN include lattice mismatch for epitaxial growth, dislocations, interface states, thermal interfaces and thermal resistances, coefficient of thermal expansion, and thermal substrate properties. Note that the lattice mismatch between the base wafer surface and the overlying epitaxial layer is a critical feature for growth of low-defect epitaxial layers. Current Status of SiC SiC devices can be divided into two categories: unipolar and bipolar devices. Unipolar devices, such as Schottky diodes, Junction FETs, MOSFETs, and metal semiconductor field effect transistors (MESFETs), exist and work well. The current in single devices is limited by the defect-free wafer area available. Unipolar discrete devices are limited to a few square millimeters in area by the existing materials; current density ratings of 200 A/cm2 limit device current ratings to ~100 A. To improve performance, micropipe and other material defect densities must decline while substrate diameter increases. Alternative wafer crystal orientations have the potential to greatly increase the area available for device fabrication, and thus they have the potential to scale the device current ratings. SiC power PiN rectifiers and switches (thyristors, IGBTs, and gate turn offs, or GTOs) will not be commercially available in the near term due in part to the forward conduction instability caused by spontaneously created stacking faults whose origin can be traced to the base material. Again, alternate wafer orientations have the potential to solve the forward conduction instability problem. MOSFET performance is limited by high channel resistance, which is due to the large interface state densities at the gate insulator–conduction channel interface. Once again, alternative crystal orientations have demonstrated reduced interface defect densities that have the potential to move SiC MOSFETs ahead in this area. Furthermore, bipolar power switching devices such as GTOs and thyristors are currently limited by the complexity of fabrication. Design trade-offs favor bipolar devices over unipolar around 3 kV. But in addition to the area-scaling problem that limits the current ratings of all SiC power devices (traceable to defects in the substrate), bipolar discrete devices with thick drift regions (required for higher voltages) also suffer dynamic forward voltage instabilities whose origins can also be traced to defects in the substrates. In all cases, the area over which the device can conduct current is limited by the defect density to a few square millimeters.
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities Improving the Performance of SiC Devices A critical need for the improvement of the performance of SiC devices is to improve material quality and substrate diameter. An important research priority is to exploit the advantages of the 4H-SiC (1120) a-plane crystal orientation. This material orientation alters the orientation of micropipes and limits their negative effects due to the fact that the micropipe orientation is perpendicular to any applied electric field and current conduction, and because micropipe propagation into device-critical epi layers is eliminated. The (1120) a-plane material has also been shown to reduce defects at the SiC-SiO2 interface, which makes SiC MOSFETs and IGBTs feasible. Another need is for improvement in the science and technology of implantation, implantation activation, and metal-semiconductor metallurgy in WBG devices and materials. Special priority should be given to improving ohmic contact fabrication processes. Presently, n-type contacts can be fabricated with 10−4 Ω-cm2 to 10−6 Ω-cm2; however, contact stability of the preferred nickel-silicide metallurgy requires better science, and better technologies are needed for n-type contacts on subdegenerately doped semiconductors (which are hard to fabricate by implantation). Fabrication methods for low-resistance p-type contacts for use in bipolar devices, such as GTOs and IGBTs, are well behind that of n-type contacts. The science and technology of dopant implantation and implant activation processes must be improved, since diffusion in SiC is impractical and implantation is necessary for selective doping. Very high temperature annealing is necessary to set deposition. Energy switching devices, such as those needed in pulsed power weapons systems, have fundamentally different needs that are not optimally met by technology used in conventional continuous power delivery. The high voltages and high peak currents of these applications further stress the existing ohmic contact technology. Solutions beyond conventional methods are required that better integrate the device and package contacting strategy. Finally, the development of packaging materials and systems to deploy high power density electronic devices is a major—if not the major—obstacle to effectively utilizing the performance advantages of WBG materials in compact high-power systems. SiC is a high-temperature material that can operate at high temperatures, high current densities, and high voltages. Therefore, the package must also accommodate the same parameters while simultaneously providing high rates of heat removal. Fabricating such a package for SiC (in particular) is facilitated because of the wide commercial availability of polycrystalline SiC in several forms for mechanical applications. Specifically, high-resistivity, poly SiC with the same coefficient of thermal expansion (CTE) that can be processed using existing etching techniques is available in large dimensions. The combination of high resistivity and matched CTE in poly SiC makes it an ideal packaging material. The fabrication of the package using poly SiC makes possible the fabrication of microchannel and pin-fin heat exchanger geometries directly in material.
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities OTHER WIDEBAND GAP, HIGH-POWER ELECTRONIC MATERIALS: GaN AND AlN Both SiC and GaN materials are superior to the omnipresent silicon because of their larger band gap and the cumulative superiority of a number of other parameters. The larger band gap results in higher bulk material dielectric strength (higher voltage per unit thickness), which in turn leads to more compact and higher frequency devices. In addition, the peak drift velocity of the electrons in GaN can be double that of Si and GaAs (107 cm/s) at much higher electric fields. The drift velocity of electrons in SiC continues to rise as the electric field increases above 200 kV/cm, and is also nearly double that of silicon. These characteristics result in a figure of merit that is shown in Table 4-1. The microwave performance for wideband gap materials is also compared in Table 4-2. TABLE 4-1 Wideband Gap Materials Figure of Merit Material Combined Figure of Merit Silicon 1 GaAs 7.36 6H-SiC 393 4H-SiC 404 GaN 404 SOURCE: Courtesy of M.S. Shur, Rensselaer Polytechnic University, Troy, New York. TABLE 4-2 Microwave Performance of Wideband Gap Materials Device Gate Length (micron) Power (W/mm) fT (GHz) fmax (GHz) 4H-SiC MESFET 0.45 3 @ 1.8 GHz 42 6H-SiC MESFET 2.8 @ 1.8 GHz 16.2 32 SiC SIT 0.5 1.36@ 0.6 GHz 6H-SiC Lateral JFET 0.3 1.3 @ 0.85 GHz 7.3 9.2 GaN 3.3 @ 1.8 GHz SOURCE: Courtesy of M.S. Shur, Rensselaer Polytechnic University, Troy, New York. Current Status of GaN and AlN At present there are approximately 50 research groups working on GaN and AlN. The research is focused on electro-optical applications of GaN for high-efficiency light emitting diodes (LEDs), high-temperature FETs, and continuous-wave blue lasers. For example, 15 companies are working on developing a blue laser, but only the Nichia company has a blue laser that can operate for 10,000 hours. The now-common LED traffic signal lights and rear auto tail lights are expanding markets for GaN LEDs, and interior lighting applications are also being pursued. The target for luminous efficiency in an LED is 120 lumens/watt, which can be compared to the luminous efficiency of a
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities fluorescent tube of 80 lumens/watt. Nichia is the current leader in LED efficiencies, with 11 percent for blue, 10 percent for green, and 5 percent for white LEDs. Another group of R&D efforts are focused on high-power, high-temperature electronics for application in energy-efficient power switching and operation in harsh (high-temperature) environments. For example, NEC is developing a 100 watt continuous-wave, RF (>10 GHz), high electron mobility transistor (HEMT) power amplifier. Improving the Performance of GaN and AlN Devices As with SiC, the problems currently being addressed in GaN and AlN materials center on large defect densities and the lack of understanding of their effects. As with SiC, the present approach is to build devices in epitaxial layers. Essentially, a successful or workable GaN substrate technology does not exist. Current surface defect densities are greater than 108 cm−2; this is acceptable for LEDs, but not for laser diodes and other devices. The current goal is to reduce the defect density below 105 cm−2. The progress in this area is very slow due to the fact that thin film deposition on GaN is very difficult. Metal Organic Chemical Vapor Deposition (MOCVD) systems are currently being used for epitaxial depositions while Molecular Beam Epitaxy (MBE) and Hydride Vapor Phase Epitaxy (HVPE) systems are being developed. Progress in GaN-like materials is also being made using Lateral Epitaxial Overgrowth (LEO), which uses a SiO2 layer to block surface defects in the epitaxial growth. Two-flow MOCVD epitaxial growth results in the highest-quality epitaxial films. It appears that the problems related to growing defect-free GaN and AlN materials are limiting the ability to fabricate high-quality devices, and specifically high-power devices that are of interest to the CHPS program. Substrates, and especially low defect density substrates, are currently not available. In fact, except for LEDs, only devices that can be fabricated in an epitaxial layer are available. This situation limits the volume and area of a single device and thus the power-handling capability. Thus, GaN- and AlN-type materials and power devices will not be available within the next five years unless a major breakthrough occurs. SUMMARY A summary of the technical challenges and opportunities for improvement of devices based on WBG materials is given in Table 4-3.
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities TABLE 4-3 Technical Challenges, Performance Metrics, and Research Priorities Associated with the Application of WBG Materials to Combat Hybrid Power Systems System/Component Technical Challenge Performance Metric R&D Priorities Bulk SiC Improvement of material quality and substrate diameter Low defect density Processing to exploit advantages of 4H-SiC (1120) a-plane crystal orientation Metal-semiconductor contacts Improve ohmic contact fabrication processes Contact stability under extreme conditions Improvement of science and technology of implantation, implantation activation, and metal-semiconductor metallurgy in wideband gap devices and materials Device packaging Development of packaging that can accommodate the high-temperature, high-power characteristics of wideband gap devices while providing high rates of heat removal Stability, heat removal rate For SiC devices, development of processes for high-resistivity poly SiC with a matched coefficient of thermal expansion Bulk GaN and AlN Improvement of substrate material quality Low defect density Fundamental processing research to control defects in bulk GaN and AlN
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