SiC polytyp

Fig. 8 13C and 29Si MAS-NMR of two SiC polytypes. Left panel-, see text for assignments. Right panel-. Experimental and calculated ultra-slow MAS-NMR spectra at ultra-high field of 21.1 T. Modified and reprinted with permission from [133]. Copyright 2009 by the American Chemical Society... 

Equiatomic tetrahedral structure types. (Carborundum structure types). To this group pertain the diamond-type structure, the wurtzite (h) and the sphalerite (c) types, and the large family of SiC polytypes (such as he, hcc, hccc, hcchc,. .. (hcc)5(hccc)(hcc)5hc. .. (hchcc)17(hcc)2,. .. (hcc)43hc...). 

Furthermore, the stacking order has been identified as that of the 3C-SiC polytype and, according to the stndy by Stahlbnsh, an explanation to the recombinative behavior of the stacking fanlt is that the 3C-SiC, having a lower bandgap than 4F1-SiC, acts as a qnantnm well, thereby enhancing the recombination [63]. It is a very serious materials issue that must be solved prior to the realization of commercial bipolar devices. 

In this chapter, we review the current status of doping of SiC by ion implantation. Section 4.2 examines as-implanted depth profiles with respect to the influence of channeling, ion mass, ion energy, implantation temperature, fluence, flux, and SiC-polytype. Experiments and simulations are compared and the validity of different simulation codes is discussed. Section 4.3 deals with postimplant annealing and reviews different annealing concepts. The influence of diffusion (equilibrium and nonequilibrium) on dopant profiles is discussed, as well as a comprehensive review of defect evolution and electrical activation. Section 4.4 offers conclusions and discusses technology barriers and suggestions for future work. 

In this section we will review the current knowledge of ion damage accumulation and chemical profiles in as-implanted single-crystal SiC. We will investigate the effects of the postimplant annealing process in Section 4.3. Results for the 4H- and 6H-SiC polytypes will also be presented. 

The structure of presolar silicon carbide grains can provide information about the conditions of formation. Crystalline silicon carbide is known to form about 100 different polytypes, including cubic, hexagonal, and rhombohedral structures. Presolar silicon carbide exists in only two of these, a cubic (fi-SiC) polytype and a hexagonal (a-SiC) polytype (Daulton et al.,... 

Perhaps the more important questions raised by Rule 5, as Burdett and McLarnan (1984) point out, concern the extent to which it really is borne out by observation. For example, Baur et al. (1983) have developed a numerical index for the degree of parsimony in a crystal structure and have shown that, using this measure, many crystal structures are not parsimonious but lavish in their use of different local environments. Also, the dominance of short-range forces is by no means obvious when ordered structures with extremely large unit cells are observed (e.g., a c dimension of 1500 A in some SiC polytypes Shaffer, 1969). The explanation of such structures poses problems for electrostatic as well as covalent models. 

Figure 9.14. A. C MAS NMR spectra of hexagonal SiC polytypes, from Hartman et al. (1987). The spectrum of the cubic (zincblende) structure was not detected by these authors under a wide range of conditions. B. Angultu" dependence of the C NMR lines of single-crystal 6H polytype of SiC. Curve (a) corresponds to the resonance at 21.9 ppm in this crystal, curve (b) corresponds to the resonance at 17.2 ppm and curve (c) to the resonance at 25.4 ppm. From Richardson etal. (1992). Both figures used by permission of the American Chemical Society.

C-SiC is the sole cubic polytype among the many SiC polytypes. It has the highest electron mobility of the SiC polytypes. Its band structure is indirect. Most of the commercial applications of SiC are with 4H-SiC and 6HSiC. 3C-SiC has a potential shallow-level donor (n-type dopant) with nitrogen (activation energy of 0.06-0. leV). 

In the LDA, Adolph and Bechstedt [157,158] adopted the approach of Aspnes [116] with a plane-wave-pseudopotential method to determine the dynamic x of the usual IB V semiconductors as well as of SiC polytypes. They emphasized (i) the difficulty to obtain converged Brillouin zone integration and (ii) the relatively good quality of the scissors operator for including quasiparticle effects (from a comparison with the GW approximation, which takes into account wave-vector- and band-dependent shifts). Another implementation of the SOS x —2 ffi, ffi) expressions at the independent-particle level was carried out by Raskheev et al. [159] by using the linearized muffin-tin orbital (LMTO) method in the atomic sphere approximation. They considered... 

Kackell P., Wenzien B. and Bechstedt R, Influence of Atomic Relaxations on the Structural Properties of SiC Polytypes from Ab-Initio Calculations, Phys. Rev. B50, 17 037 (1994). Kaxiras E. and Duesbery M. S., Free Energies of Generalized Stacking Faults in Si and Implications for the Brittle-Ductile Transition, Phys. Rev. Lett. 70, 3752 (1993). 

The primitive cells of the n H and 3n R SiC poly types contain n formula (Si-C) units, and the unit cell of the polytypes along the c-axis is n times larger than that of the basic 3 C SiC polytype. The BZ of the corresponding polytype is thus reduced in the T — L direction by a factor f/n [70]. One then speaks of folded BZ and some of the folded acoustical phonons with non-zero frequencies at the zone centre are IR- and Raman-active. Their absorptions, with lines as sharp as 0.03 cm-1 at LHeT have been reported for the 6H and 15R SiC polytypes ([77], and references therein). 

For the 4/f-SiC polytype, a detailed study of the donor level classification and selection rules for an EM donor at the hexagonal (h) site has been given by Ivanov et al. [114]. It has been applied to the N donor, for which 10 electronic lines between 38 and 56 meV have been reported by different groups, with an ionization energy Eh(N) of 61.4meV ([114] and references therein). This value of Eh N) contrasts with the value obtained for the ionization energy Ec(N) at the cubic site, which rises to 125.5 meV ([113]. 

Fig. B.5. Unit cell of the 6//-SiC polytype showing the ABCACB stalking sequence and the different sites (see text)...

Optical absorption and refractive index of SiC TABLE 3 Refractive index of SiC polytypes. 

The interband transitions in SiC polytypes give rise to the characteristic colours of green (6H), yellow (15R) and green-yellow (4H). Cubic SiC exhibits a pale yellow colour when undoped and greenish yellow in the doped state. The variations of refractive index with wavelength for the 3C, 2H, 4H, 6H and p-SiC polytypes are listed. 

TABLE 1 Seven of the most simple SiC polytypes with four notations (R = Ramsdell notation, J = Jagodzinski notation, Z = Zhadanov notation). They are listed by increasing percent hexagonality. 

TABLE 3 Symmetry and frequency of phonon modes for 4H-, 6H- and 15R-SiC polytypes, x = q/qmax is the phonon reduced momentum. N.O. stands for not observed. 

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