Effect of C/B Sequential Implantation on the B Acceptors in 4H-SiC
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Effect of C/B Sequential Implantation on the B Acceptors in 4H-SiC Yoshitaka Nakano, Rajesh Kumar Malhan#, Tetsu Kachi and Hiroshi Tadano Toyota Central Research and Development Laboratories, Inc., Nagakute, Aichi 480-1192, Japan # Denso Corporation, Komenoki, Nissin, Aichi 470-0111, Japan ABSTRACT We have systematically investigated the effect of C/B sequential implantation on the B-related acceptors and deep levels in 4H-SiC using thermal admittance spectroscopy. With increasing concentration of co-implanted C, the density of deep levels was found to start decreasing and to be completely suppressed for the C and B ratio of 1:1. Moreover, the density and ionization energy of B acceptors were seen to start increasing and lowering, respectively, with increasing C concentration. However, we found that excess C-content leads to the formation of a new complex defect. The C-V results also support the expected increase in the total hole concentration with increasing concentration of the co-implanted C-atoms, which is followed by a decrease in the concentration under C-rich condition. This is in reasonable agreements with the behavior of the B acceptors and deep defect levels. Therefore, the concentration of co-implanted C-atoms is considered to be very sensitive to the formation of the B acceptor levels.
INTRODUCTION Boron (B) is one of the promising group-IIIA acceptor impurities for SiC and forms shallow acceptor level (~300 meV) and deep level (650 ± 50 meV) [1,2]. For the design of electronic devices, specially from selective area doping point of view, the light B-atom can be implanted deeper into SiC for a given implantation energy and causes less damage in SiC crystal compared to Al or Ga atoms. The B-atom forms shallow acceptor levels when residing at Si-lattice sites [3]. Magnetic resonance studies by Adrian et al. [4] confirmed that the shallow acceptor level consists of B-atom residing at Si-lattice site. The microscopic structure of B-related deep defect level, known as D-center, is still an open issue and needs to be addressed theoretically as well as experimentally. Therefore, it is important to understand the conditions under which deep levels are formed. The formation of deep defects can lead to the degradation of a device’s electrical characteristics over the long terms. Therefore, a suitable process is needed to suppress these D-centers for the practical use of B-atoms as a p-type impurity of SiC. Considering that B has middle values between Si and C both in atomic covalent radius and electronegativity, and also that both Al and B form shallow acceptor levels in Si, it is expected that B substituted on a Si-lattice site in SiC will have low formation energy and will form a shallow level. This implies that a suitable process is needed to increase the probability for B-atom to occupy the favored Si-lattice site for high electrical H5.32.1
activation of B atoms. The B-atom on a Si-lattice site is favorable under the C-rich condition to achieve high electrical activation, based on a site-competition effect
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