Photoluminescence Spectra of SiC Polytypes

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 1 Principal phonon energies (meV) in six polytypes derived from the luminescence spectrum (from [26]). The notation (TA) for transverse acoustic, (LA) for longitudinal acoustic, (TO) for transverse optical, and (LO) for longitudinal optical phonons is accurate for the cubic polytype, but not for the hexagonal and rhombohedral polytypes. For details see Datareview 2.2. 

Low temperature absorption measurements performed in p-type 3C-, 4H-, 6H- and 15R-SiC doped with boron and 6H-SiC doped with Al show extrinsic absorption features which have been attributed to excitons bound to neutral acceptors [69-72]. Electroabsorption measurements performed in boron-doped 6H polytype show that the observed effective microfield is considerably stronger than the expected values generated by the substitutional boron acceptor, suggesting that the excitons may be bound to dipole complexes, which may include the N-donor [73]. 

In the DAP recombination mechanism electrons bound at neutral donors recombine radiatively with holes bound at neutral acceptors. If an isolated neutral donor and acceptor are separated by a reasonably small distance r, the electron and hole can recombine with emission of a photon of energy E(r) given by [75]  

Choyke and Patrick reported a well resolved sharp line spectra between 2.10 and 2.35 eV in Al-doped Lely-grown crystals of cubic SiC [77]. They attributed these sharp lines to the recombination of close DAPs, where the N replaces C and Al replaces Si (Type II spectrum). These lines were resolved up to shell m = 80, and an extrapolation to infinite separation yielded an accurate value of the minimum photon energy, Emin = 2.0934 eV. If we use the value of Eg = 2.403 eV, at 4.2 K [77], we obtain from Eqn (2)  

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Photoluminescence spectra of SiC polytypes C ACCEPTORS AND DONOR-ACCEPTOR PAIRS... 

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. 

L Patrick, WJ Choyke. Photoluminescence of Ti in four SiC polytypes. Phys Rev B10 5091, 1974. J Schneider, HD Muller, K Maier, W Wilkenning, F Fuchs, A Domen, S Leibenzeder, R Stein. Infrared spectra and electron spin resonance of vanadium deep level impurities in silicon carbide. Appl Phys Lett 56 1184, 1990. 

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