Optical properties of the niobium centre in 4H, 6H, and 15R SiC Ivan G. Ivanov 1,a , Andreas Gällström 1,b , Stefano Leone 1 , Olof Kordina 1 , N.T. Son 1 , Anne Henry 1 , Viktor Ivády 1,2 , Adam Gali 2,3 , and Erik Janzén 1 1 Department of Physics, Chemistry and Biology, Linköping University, Sweden 2 Research Institute for Solid State Physics and Optics of the Hungarian Academy of Sciences, Hungary 3 Department of Atomic Physics, Budapest University of Technology and Economics, Hungary a ivani@ifm.liu.se, b andga@ifm.liu.se Keywords: transition metals, niobium, photoluminescence, Zeeman effect. Abstract. A set of lines in the photoluminescence spectra of 4H-, 6H-, and 15R-SiC in the near- infrared are attributed to Nb-related defects on the ground of doping experiments conducted with 4H-SiC. A model based on a an exciton bound at the Nb-centre in an asymmetric split vacancy configuration at a hexagonal site is proposed, which explains the structure of the luminescence spectrum and the observed Zeeman splitting of the lines. Introduction Transition metals are well known to introduce deep levels in semiconductors due to the presence of partially-filled d-shells, but until recently they were thought as occupying a single host-atom vacancy or (in some semiconductors) interstitial site. A recent theoretical study shows that for heavier metals from the second and third column of the periodic table (in particular, Nb) the preferable configuration in SiC is not the substitutional one, but the so-called asymmetric split- vacancy (ASV) configuration at hexagonal site [1]. The Nb-related defect in 4H-SiC has been studied also by electron paramagnetic resonance (EPR) and the experimental results support the incorporation of Nb in the ASV configuration [2]. In the present work we describe the photoluminescence (PL) signature of the Nb-related defects in 4H, 6H, and 15R SiC, and demonstrate that not only the number of PL centres corresponds to the number of non-equivalent hexagonal sites in these polytypes, but also that the observed properties of the PL spectra in 4H- and 6H-SiC can be explained as arising from the recombination of an exciton bound to the Nb defect, with strongly bound electron and loosely bound (effective-mass like) hole. Experimental Details The samples used in this study are conventionally undoped samples grown in a high-temperature chemical-vapour deposition (HTCVD) reactor with susceptor parts of NbC. (The relation of the Nb- line in Fig. 1 to incorporation of Nb was demonstrated in a doping experiment on 4H-SiC described elsewhere [3].) The PL spectra are excited either using the multiline UV emission of an Ar + ion laser (351 – 364 nm), or a wavelength of ~750 nm from a Ti-sapphire laser [the latter laser was utilized also as a tuning source for measuring the PL excitation (PLE) spectra)]. Zeeman measurements were performed, although only the Faraday configuration was available. The luminescence was recorded using either a Fourier-transform infrared (FTIR) spectrometer, or a monochromator coupled to a CCD camera, alternatively to an IR-sensitive photomultiplier. Experimental Results, Model and Comparison with Experiment PL Signature of Nb in 4H, 6H and 15R SiC. The PL spectra of the three polytypes shown in Fig. 1 confirm that the number of the observed low-temperature PL lines repeats the number of the inequivalent hexagonal sites (one in 4H and 6H, and two in 15R SiC). Thus, we assume in accord with [1] that the observed line(s) arise from Nb in the ASV configuration at hexagonal site(s), as depicted in the insert of Fig. 1. Furthermore, the temperature dependence of the Nb-related emission Materials Science Forum Vols. 740-742 (2013) pp 405-408 Online available since 2013/Jan/25 at www.scientific.net © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.740-742.405 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 130.236.170.129, Linköping University, Linköping, Sweden-18/02/14,11:27:07)