Electron paramagnetic resonance studies of Nb in 6H-SiC Xuan Thang Trinh a , Andreas Gällström b , Nguyen Tien Son c , Stefano Leone d , Olle Kordina e , and Erik Janzén f Department of Physics, Chemistry and Biology, Linköping University, SE-581 83 Linköping, Sweden a xuatr@ifm.liu.se, b andga@ifm.liu.se, c son@ifm.liu.se, d leonestefano@hotmail.com, e olkor@ifm.liu.se, f erj@ifm.liu.se Keywords: Nb, split-vacancy complex, electron paramagnetic resonance Abstract. Defects in unintentionally Nb-doped 6H-SiC grown by high-temperature chemical vapor deposition were studied by electron paramagnetic resonance (EPR). An EPR spectrum with a hyperfine (hf) structure consisting of ten equal-intensity lines was observed. The hf structure is identified to be due to the hf interaction between an electron spin S=1/2 and a nuclear spin of 93 Nb. The hf interaction due to the interaction three nearest Si neighbors was also observed, suggesting the involvement of the C vacancy (V C ) in the defect. Only one EPR spectrum was observed in 6H- SiC polytype. The obtained spin-Hamiltonian parameters are similar to that of the Nb-related EPR defect in 4H-SiC, suggesting that the EPR center in 6H-SiC is the Nb Si V C complex in the neutral charge state, Nb Si V C 0 . Photoexcitation EPR experiments suggest that the single negative charge state of the Nb Si V C complex is located at ~1.3 eV below the conduction band minimum. Introduction The incorporation of transition metals (TMs) in SiC during crystal growth is common. The 3d TMs such as Ti [1-3], V [4-6], Cr [6-8], Sc [9] and Ni [10] were found to be substitutional impurities at the Si site in 4H- and 6H-SiC. However, only one EPR spectra of Mo (4d TM) [11] and W (5d TM) [12] in 6H-SiC could be observed. In boron nitride, Mn was found to be energetically more favorable in a divacancy site as compared to the B site and form split-vacancy defects [13]. Recent calculations and EPR studies show that Nbpreferentially forms an asymmetric split-vacancy (ASV) defect in 4H-SiC [14,15]. In this work, we used EPR to study Nb in unintentionally Nb-doped 6H-SiC grown by high-temperature chemical vapor deposition (HTCVD). Experiment The samples are unintentionally Nb-doped 6H-SiC substrates grown by HTCVD in a reactor with several parts made of NbC. The Nb concentration is ~3×10 16 cm –3 as determined by secondary ion mass spectrometry. EPR measurements were performed on an X-band Bruker E500 spectrometer equipped with a He-flow cryostat which allowed the regulation of the sample temperature in the range 4-295 K. For optical excitation experiments, a 250 W halogen lamp, a Jobin-Yvon 0.25 m single grating monochromator and appropriate optical filters were used. Result and discussion In darkness, only the SI-1 spectrum [16] was be observed. Under illumination, a new EPR spectrum was observed. Fig. 1 shows EPR spectra measured at 44 K under illumination with light of photon energy of hȞ~2.9 eV for different directions of the magnetic field. For the magnetic field along the c-direction, B|| c, the spectrum shows a clear hyperfine (hf) structure consisting of ten equal-intensity lines which is typical for a hf interaction between an electron spin S=1/2 and a nuclear spin I=9/2 of an element having 100% natural abundance. There are only a few elements in this category: 93 Nb (I=9/2, 100%), 209 Bi (I=9/2, 100%), and two indium isotopes with I=9/2, 113 In