EPR and Pulsed ENDOR Study of El6 and Related Defects in 4H-SiC

Abstract

The EI5 and EI6 centers are typical intrinsic defects in radiation-damaged and semi-insulating SiC. So far, the origins of EI5 and EI6 have been identified as positively-charged carbon vacancies (VC) and silicon antisites (SiC), respectively. However, our complete set of Si hyperfine (HF) data changes these identifications. Our EPR data clearly show that both centers can be well described by VC but their locations should be different (cubic sites for EI5 and hexagonal sites for EI6), as recently proposed by other groups. It was also found that both defects have similar high thermal stability over 1000 oC. In addition to EI5 and EI6, we found a new thermally-stable center, labeled HEI1, in n-type radiation-damaged 4H-SiC. Introduction Electron or neutron irradiation to SiC has been widely studied to explore a variety of intrinsic defects (vacancies, antisites, etc.) in this material. These radiation experiments were performed at relatively low temperatures (4 K or room temperature ~ 450 oC). From a technological point of view, defects that have high thermal stability are of crucial importance. Also, the origin of high thermal stability is of great interest to science. Therefore, we investigate thermally-stable defects in SiC by means of electron irradiation at a high temperature (850 oC) and identify their atomic structures by using electron paramagnetic resonance (EPR). In our study, 4H polytype was selected because of its importance for electronic-device applications. In our high-temperature radiation experiments, only selected (i.e., thermally-stable) defects were found in a specimen. There were at least three types of thermally-stable defects; EI5, EI6, and a new center (we label it “HEI1”). Although the identification of EI5 and EI6 had been published [1,2], more recent EPR experiment [3] and theoretical calculation [4] suggested an alternative model that both EI5 and EI6 are VC defects but they are located at cubic and hexagonal sites, respectively [4]. The conclusion has not been obtained yet, because of a lack of HF interaction data. Thus, we here report complete angular dependence of Si HF interactions of EI5 and EI6. Then, we show that EI6 is well compatible with a hexagonal VC rather than a simple SiC. In addition, EPR data of HEI1 are briefly reported. Experimental SiC substrates are commercial 4H-SiC(0001) wafers supplied by Nippon steel corporation. Dopant concentrations are 1×10 and 1×10 cm for the pand n-type wafers, respectively. Electron irradiation was performed with an electron energy of 3 MeV and a dose of 4×10 cm. During the irradiation, substrates were located in a high vacuum and were heated at a constant temperature of 850 oC. Then, they were measured by X-band EPR and pulsed-EPR spectrometers (Bruker E500 and E580 systems, respectively). Pulsed-EPR was used to deconvolute HF structures by means of pulsed electron-nuclear-double-resonance (ENDOR) techniques. Results and Discussions EI5 and EI6 in p-type 4H-SiC. Figure 1(b) shows EPR spectrum of a p-type sample measured for B (magnetic field) || c. As is indicated in the figure, EPR spectrum consists mainly of central lines of EI5 and EI6, and many HF satellites (labeled a to g). Before starting the analyses of this spectrum, we should mention that EI5 (VC) was stable over 1000 °C, although its anneal-out temperature was known to be much lower (450 ~ 600 °C [1,2,4]) than that of EI6 (> 1000 °C [1]). In our anneal study, both EI5 and EI6 were fully stable at 1000 °C, started to be annihilated at 1200 °C, and almost disappeared at 1500 °C. Thus, we have to change our mind with respect to the thermal stability of EI5. Figure 1(a) shows an angular-dependence map when B was rotated from the c axis ([0001]) to the c-normal direction ([112 _ 0]). The HF satellites a, b and c were already reported by Son et al. [1,2]. EN D O R in te ns ity [a .u .]