Silicon Carbide Technology:Fundamental SiC Material Properties.

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Introduction

Silicon carbide (SiC)-based semiconductor electronic devices and circuits are presently being developed for use in high-temperature, high-power, and high-radiation conditions under which conventional semi- conductors cannot adequately perform. Silicon carbide’s ability to function under such extreme condi- tions is expected to enable significant improvements to a far-ranging variety of applications and systems. These range from greatly improved high-voltage switching for energy savings in public electric power distribution and electric motor drives to more powerful microwave electronics for radar and communi- cations to sensors and controls for cleaner-burning more fuel-efficient jet aircraft and automobile engines [1–7]. In the particular area of power devices, theoretical appraisals have indicated that SiC power MOSFET’s and diode rectifiers would operate over higher voltage and temperature ranges, have superior switching characteristics, and yet have die sizes nearly 20 times smaller than correspondingly rated silicon-based devices [8]. However, these tremendous theoretical advantages have yet to be widely realized in commercially available SiC devices, primarily owing to the fact that SiC’s relatively immature crystal growth and device fabrication technologies are not yet sufficiently developed to the degree required for reliable incorporation into most electronic systems.

This chapter briefly surveys the SiC semiconductor electronics technology. In particular, the differences (both good and bad) between SiC electronics technology and the well-known silicon VLSI technology are highlighted. Projected performance benefits of SiC electronics are highlighted for several large-scale applications. Key crystal growth and device-fabrication issues that presently limit the performance and capability of high-temperature and high-power SiC electronics are identified.

Fundamental SiC Material Properties
5.2.1 SiC Crystallography: Important Polytypes and Definitions Silicon carbide occurs in many different crystal structures, called polytypes. A more comprehensive introduction to SiC crystallography and polytypism can be found in Reference 9. Despite the fact that all SiC polytypes chemically consist of 50% carbon atoms covalently bonded with 50% silicon atoms, each SiC polytype has its own distinct set of electrical semiconductor properties. While there are over 100 known polytypes of SiC, only a few are commonly grown in a reproducible form acceptable for use as an electronic semiconductor. The most common polytypes of SiC presently being developed for electronics are 3C-SiC, 4H-SiC, and 6H-SiC. The atomic crystal structure of the two most common polytypes is shown in the schematic cross section in Figure 5.1. As discussed much more thoroughly in References 9 and 10, the different polytypes of SiC are actually composed of different stacking sequences of Si–C bilayers (also called Si–C double layers), where each single Si–C bilayer is denoted by the dotted boxes in Figure 5.1. Each atom within a bilayer has three covalent chemical bonds with other atoms in the same (its own) bilayer, and only one bond to an atom in an adjacent bilayer. Figure 5.1a shows the bilayer of the stacking sequence of 4H-SiC polytype, which requires four Si–C bilayers to define the unit cell repeat distance along the c-axis stacking direction (denoted by <0 0 0 1> Miller indices). Similarly, the 6H-SiC polytype illustrated in Figure 5.1b repeats its stacking sequence every six bilayers throughout the crystal along the stacking direction. The <1100> direction depicted in Figure 5.1 is often referred to as one of (along with <1120>) the a-axis directions. SiC is a polar semiconductor across the c-axis, in that one surface normal to the c-axis is terminated with silicon atoms while the opposite normal c-axis surface is terminated with carbon atoms. As shown in Figure 5.1a, these surfaces are typically referred to as “silicon face” and “carbon face” surfaces, respectively. Atoms along the left-or right-side edge of Figure 5.1a would reside on (<1100>) “a-face” crystal surface plane normal to the <1100> direction. 3C-SiC, also referred to as β-SiC, is the only form of SiC with a cubic crystal lattice structure. The noncubic polytypes of SiC are sometimes ambiguously referred to as α-SiC. 4H-SiC and 6H-SiC are only two of the many

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possible SiC polytypes with hexagonal crystal structure. Similarly, 15R-SiC is the most common of the many possible SiC polytypes with a rhombohedral crystal structure.

SiC Semiconductor Electrical Properties

Owing to the differing arrangement of Si and C atoms within the SiC crystal lattice, each SiC polytype exhibits unique fundamental electrical and optical properties. Some of the more important semicon- ductor electrical properties of the 3C, 4H, and 6H SiC polytypes are given in Table 5.1. Much more detailed electrical properties can be found in References 11–13 and references therein. Even within a given polytype, some important electrical properties are nonisotropic, in that they are strong functions of crystallographic direction of current flow and applied electric field (for example, electron mobility for 6H-SiC). Dopant impurities in SiC can incorporate into energetically inequivalent sites. While all dopant ionization energies associated with various dopant incorporation sites should normally be considered for utmost accuracy, Table 5.1 lists only the shallowest reported ionization energies of each impurity.

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For comparison, Table 5.1 also includes comparable properties of silicon, GaAs, and GaN. Because silicon is the semiconductor employed in most commercial solid-state electronics, it is the standard against which other semiconductor materials must be evaluated. To varying degrees the major SiC polytypes exhibit advantages and disadvantages in basic material properties compared to silicon. The most beneficial inherent material superiorities of SiC over silicon listed in Table 5.1 are its exceptionally high breakdown electric field, wide bandgap energy, high thermal conductivity, and high carrier satura- tion velocity. The electrical device performance benefits that each of these properties enables are discussed in the next section, as are system-level benefits enabled by improved SiC devices.

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