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Nuclear-Magnetic-Resonance Study of Heavily Nitrogen-Doped Silicon Carbide

1968; American Institute of Physics; Volume: 172; Issue: 2 Linguagem: Inglês

10.1103/physrev.172.331

1536-6065

Michael N. Alexander,

Glass properties and applications

Samples of heavily nitrogen-doped silicon carbide (SiC: N) have been studied by use of pulse nuclear magnetic resonance (NMR) at a resonance frequency of 8.5 MHz. Measurements were made, for ${\mathrm{Si}}^{29}$ and ${\mathrm{C}}^{13}$, of the Knight shift, the spin-lattice relaxation time ${T}_{1}$, and the free-induction decay time ${{T}_{2}}^{*}$ (which is inversely proportional to the linewidth). The samples of SiC: N studied had nitrogen donor concentrations ${n}_{d}$ estimated to be in the range $1.9\ifmmode\times\else\texttimes\fi{}{10}^{19}\ensuremath{\lesssim}{n}_{d}\ensuremath{\lesssim}6.0\ifmmode\times\else\texttimes\fi{}{10}^{20}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. Samples for which ${n}_{d}<{10}^{20}$ were of the $6H$ polytype, and samples for which ${n}_{d}>{10}^{20}$ were of the cubic polytype. The measurements on the cubic SiC: N samples show a markedly different behavior of the electron interaction with the silicon and carbon sublattices. The ${T}_{1}'\mathrm{s}$ of ${\mathrm{Si}}^{29}$ are an order of magnitude larger than those of ${\mathrm{C}}^{13}$ for these samples. In no case is the ${\mathrm{Si}}^{29}$ Knight shift observable, whereas the ${\mathrm{C}}^{13}$ Knight shift increases with ${n}_{d}$, and is 0.9\ifmmode\pm\else\textpm\fi{}0.1 G (in 7.92 kG) for the most heavily doped sample; the measured ${\mathrm{C}}^{13}$ Knight shifts agree with the ${\mathrm{C}}^{13}$ Knight shifts predicted from ${T}_{1}$, using the Korringa relation. The ${\mathrm{C}}^{13}$ linewidth increases with ${n}_{d}$, suggesting that the linewidth is determined by a distribution of Knight shifts. From the temperature dependence of both the ${\mathrm{Si}}^{29}$ and ${\mathrm{C}}^{13}$ ${T}_{1}'\mathrm{s}$, it is inferred that the electron system is degenerate in the cubic SiC: N samples. The results can be explained in terms of the electron wave functions appropriate to the conduction-band minimum in cubic SiC. Thus it is inferred that the electron degeneracy in these samples is associated with the Fermi energy lying in the conduction band of the host SiC. The NMR properties of the $6H$ SiC: N samples are dominated by nuclear interaction with paramagnetic electrons localized, at liquid-helium temperatures, on donor sites or donor complexes. The temperature dependence of ${T}_{1}$ and of the linewidth indicates that electrons are "frozen" on donor sites and donor complexes at liquid-helium temperatures, and are thermally activated at higher temperatures. The results of the NMR study of $6H$ SiC: N are compared to results from electron-transport measurements; the transport measurements, which were made at temperatures higher than those used in this experiment, indicate that the electron systems of these samples are characterized by the existence of delocalized, mobile electrons.