Polytypes impurities

Hartman, J. S., Richardson, M. F., Sherriff, B. J., and Winsborrow, B. G., Magic angle spinning NMR studies of silicon carbide Polytypes, impurities, and highly inefficient spin-lattice relaxation, J. Am. Chem. Soc., 109, 6059 (1987). 

Semiconducting Properties. Sihcon carbide is a semiconductor it has a conductivity between that of metals and insulators or dielectrics (4,13,46,47). Because of the thermal stabiUty of its electronic stmcture, sihcon carbide has been studied for uses at high (>500° C) temperature. The Hall mobihty in sihcon carbide is a function of polytype (48,49), temperature (41,42,45—50), impurity, and concentration (49). In n-ty e crystals, activation energy for ioniza tion of nitrogen impurity varies with polytype (50,51). 

Newberry, R.J.J. 1979b. Polytypism in molybdenite (I) a non-equilibrium impurity-induced phenomenon. American Mineralogist, 64, 758-767. 

A very striking and beautiful feature of polytypism is the behavior of impurity atoms. In Figure 1.6, it may be seen that the sites are not equivalent in the hexagonal polytypes 6H-SiC and 4H-SiC. The difference is in the second-nearest neighbors. 

Commercial carbide is prepared by reacting quartz sand and carbon with a little NaCI added to aid purification. This reaction begins near 1S00°C with the reduction of Si02 to Si, which subsequently reacts with excess carbon to give the cubic form. The product, however, may be impure as well as a mixture of several polytypes when higher T is used. 

The hexagonal polytypes are obtained as mixtures, although impurities encourage the formation of certain structures e.g., boron produces the 6H structure, A1 gives 4H and La encourages the formation of 2H structures. 

Virtually all minerals contain defects. In addition to point defects (e.g., vacancies that exist in a thermodynamically determined equilibrium number, impurities etc ), macroscopic minerals contain line defects (dislocations), and planar defects such as stacking foults, antiphase boundaries and twins. Intergrown layers of different structure or composition, and polytypic disorder also may be present. 

The form that silicon carbide takes depends on many factors including thermal history, impurity type and level, and environment. The p form is generally felt to be the stable phase at low temperatures, whereas the a form is the high-temperature form. There are many exceptions to the rule, as the conversion to a from /3 and the converse have been reported. The stability and transformations of the various polytypes vary among themselves and constitute a subject that is too broad for this effort. The basic a and p descriptors will be used for the remainder of this section. 

The technique of Raman scattering (RS) to study vibrational spectra in the numerous polytypes of SiC will be described. An explanation of the various notations used to describe the stacking sequences in these polytypes will then be given. Section C discusses the various optical phonons studied by RS and the concept of a common phonon spectrum for all polytypes will be introduced. Raman studies are also used to assess crystalline structure and quality of epitaxial layers of SiC on Si and SiC substrates. Section D outlines several other excitations of interest, e.g. polaritons, plasmons, and electronic RS, as well as impurity and defect recognition in irradiated and ion implanted material. 

PL spectra assigned to nitrogen donors in the SiC system were first reported for the 6H polytype [6]. The observed PL spectrum, consisting of about 50 lines, was attributed to exciton recombination after capture by neutral nitrogen impurity, resulting in a four-particle... 

TABLE 3 Ionization energy (eV) of some acceptor impurities in the most common SiC polytypes. 

Impurity trace analyses have shown that transition metals such as titanium and vanadium are pervasive impurities in Lely and/or modified-Lely grown SiC polytypes [90]. Since transition metals are unstable and have multiple charge states they can be electrically active deep levels. 

In the last few years Schneider and co-workers have performed a number of experiments on various SiC polytypes which exhibit a characteristic infrared emission in the 1.3 to 1.5 pm spectral range [98]. They have assigned this emission band to vanadium impurities substituting the various silicon sites in the lattice. In their extensive work they found three charge states of vanadium which act as an electrically amphoteric deep level in SiC. They also suggest that vanadium may have an important role in the minority-carrier lifetime in SiC-based optoelectronic devices [98,99], Recently, trace amounts of vanadium impurities have been detected in 3C-SiC grown by the modified-Lely technique [100]. 

ESR has been used to characterize a number of impurity-related and structural defects in several polytypes of SiC. Most centres observed in SiC can be described by a simple spin-Hamiltonian ... 

Nitrogen, which is a residual shallow donor in all polytypes, is the most extensively studied impurity in both Lely grown crystals and CVD grown films. Other impurities which have been investigated include acceptors and transition metals. Radiation-induced structural defects have been studied in 3C and 6H polytypes, initially in Lely grown crystals and more recently in CVD grown films. 

Clearly, the most important impurity in all polytypes of SiC is nitrogen, which appears to primarily substitute for carbon and act as a shallow donor. There is some dispute over... 

Electron spin resonance reveals the unpaired electrons associated with impurities or structural defects and can be used to identify the lattice site positions of these features. Nitrogen is shown to substitute for carbon and acts as a shallow donor. The various ESR triplets due to nitrogen in several SiC polytypes give information on the lattice sites occupied. For the acceptor boron, ESR shows it to occupy Si sites only, in disagreement with DAP photoluminescence measurements which show only boron on carbon sites. It may be that boron substitutes on both sites and the two techniques have sensitivity for only one particular lattice site. The aluminium acceptor is not observed in ESR but gallium has been noted in one report. Transition metals, Ti and V, have been identified by ESR both isolated on Si sites and in Ti-N complexes. Several charged vacancy defects have been assigned from ESR spectra in irradiated samples. 

Hall, and C-V, four-point probe and spreading resistance measurements to some extent provide a measure of the net impurity concentration of dopants in SiC ([ND - NA] or (Na - Nd]). In addition, Hall measurements provide a method for obtaining the mobility of the net carriers. These measurements have been applied to both n- and p-type SiC and its various polytypes. In this Datareview, we will report on the mobilities for most of the SiC polytypes under various growth conditions. 

Values for both the hole and electron mobilities and carrier densities in various SiC polytypes are listed. Ionized and neutral impurity, acoustic phonon, piezoelectric and polar optical phonon scattering mechanisms are all found in SiC. In general, mobilities have increased and carrier concentrations decreased with time, reflecting the improvement in crystal quality whether bulk or epitaxially-grown material is considered. 

The ionisation energies of the electronically active impurities have been determined primarily by photoluminescence techniques and Hall measurements. Ionisation energy levels of such impurities as nitrogen and some of the group III elements (aluminium, gallium, boron) in 3C-, 4H-, 6H- and 15R-SiC polytypes are compiled in TABLE 2. Nitrogen gives relatively shallow donor levels. In contrast, other p-type dopants have deep-level acceptor states. 

Most impurity and defect states in SiC can be considered as deep levels. Both capacitance and admittance spectroscopy provide data on these deep levels which can act as donor or acceptor traps. Bulk 6H-SiC contains intrinsic defects which are thermally stable, up to 1700 °C. In epitaxial films of 6H-SiC a deep acceptor level is seen in boron-implanted samples but not when other impurities are implanted. Other centres, acting as electron traps, are also seen in p-n junction and Schottky barrier structures. Irradiation of 6H-SiC produces 6 deep levels, reducing to 2 after annealing. Only limited studies have been carried out on the 3C-SiC polytype, in the form of epitaxial films on silicon substrates. No levels were seen in thick films but electron traps were seen in thin n-type films and a hole trap (structural defect) was found to be a mobility killer. Neutron irradiation produces defects most of which can be removed by annealing. Two levels were found in Al-implanted 4H-SiC. 

In the commercial development of Si devices, diffusion is an important semiconductor fabrication process. This process does not play a major role (except in the case of the sublimation growth of SiC discussed in Chapter 8) in the development of SiC, because the diffusion coefficients for the most part are negligible at temperatures below approximately 1800 °C. As a result of this commercial insignificance, the diffusion process in SiC and its various polytypes has not received a great deal of scientific attention and diffusion data are incomplete. It does, however, appear that the solubility of impurities and their diffusive mobilities in different SiC polytypes are very similar. 

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