Carbon vacancy in SiC: Electron trap for lifetime control
In short: Silicon carbide (SiC) is an attractive material for high-voltage power devices. High voltages require thick SiC layers and long electron lifetimes. In SiC is the electron lifetime limited by the Z1/Z2 center – a common defect that traps electrons. We have been able to experimentally identify the Z1/Z2 center and correlate it with carbon vacancies. Our results are further supported by calculations, which enable us to identify the Z1/Z2 center as the charged states of the carbon vacancy at two different sites in the SiC crystal. These findings demonstrate that the carbon vacancy is suitable for lifetime control.
Silicon carbide (SiC) is a semiconductor crystal that for long has been considered as an attractive material for high-voltage power devices. Thick layers of SiC must be used to block voltages beyond several kilovolts, for very high-voltage applications such as electric power transmission. In this case, a long electron lifetime is required for effective modulation of the conductivity of the device. For example, a lifetime longer than 5 microseconds is required for good performance of 10 kilovolt SiC diodes. Currently, the electron lifetime in pure SiC is rather short (typically less than ~2 microseconds).
It has been known for a decade that the lifetime in SiC is limited by the so-called Z1/Z2 center–a common defect acting as an efficient trap of electrons. The Z1/Z2 center traps two electrons and changes its charge state, e.g. from neutral to double negative. In a previous study it was suggested that Z1/Z2 is a defect related to a missing carbon-atom – a carbon vacancy. This conclusion was based on the fact that Z1/Z2 can be created by irradiation with electrons energies in the range of 110-250 kilo electronvolts (keV) which can kick carbon atoms out of their natural positions in the crystal, creating vacancy related defects. However, the exact origin of the Z1/Z2 center was not clarified.
In this work, we use electron paramagnetic resonance (EPR) to experimentally identify the Z1/Z2 center. EPR measures the transitions between the electronic energy levels corresponding to different spin polarization (spin up or down) of the defect induced and detected by electromagnetic microwave radiation. The technique provides direct information on the symmetry and chemical identification of defects and its surrounding via hyperfine interactions between the electron spin and nuclear spins of neighboring atoms.
In 4H-type of SiC studied here, there are two inequivalent lattice sites, one called the hexagonal site with the second neighbor arrangement of atoms different from the second so-called quasicubic site. Using SiC layers irradiated by 250 keV, we were able to identify the negatively charged carbon vacancy by EPR. It was also possible to perform EPR and electrical measurements on the same samples, providing a one-to-one correlation between Z1/Z2 and the carbon vacancy in concentration and energy levels. The results are further supported by large scale supercell (576 atoms) calculations, which allowed us to identify the Z1/Z2 center, including high-lying energy levels, to be the double and single negative charge states of the carbon vacancy at the hexagonal and cubic site, respectively. With such energy level arrangement, the carbon vacancy is a suitable defect for carrier lifetime control and also an excellent carrier compensation center to be used for achieving high-purity semi-insulating SiC.
Details of the research are described in Physical Review B 88, 235209 (2013)
- Swedish Research Council VR/Linné Environment.
- Knut and Alice Wallenberg Foundation (KAW).
- National Supercomputer Center in Sweden.
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Last updated: 02/09/14