Deep level transient Fourier spectroscopy （DLTFS） measurements are used to characterize the deep impurity levels in n-type 4H-SiC by vanadium ions implantation. Two acceptor levels of vanadium at Ec - 0.81 and Ec - 1.02eV with the electron capture cross section of 7.0 × 10^16 and 6.0 × 10^-16 cm^2 are observed, respectively. Low-temperature photoluminescence measurements in the range of 1.4-3.4eV are also performed on the sample, which reveals the formation of two electron traps at 0.80 and 1. 16eV below the conduction band. These traps indicate that vanadium doping leads to the formation of two deep acceptor levels in 4H-SiC,with the location of 0.8±0.01 and 1. 1 ±0.08eV below the conduction band.
The diffusion behavior of vanadium （V） implanted in SiC is investigated by secondary ion mass spec- trometry. Significant redistribution, especially out-diffusion of vanadium towards the sample surface, is not ob- served after 1650℃ annealing. Higher carrier concentration is obtained due to a lack of compensation of vanadium in the surface region. The electrical characteristics of Ni contacts to V-implanted n-type 4H-SiC are investigated using a linear transmission line method. A specific contact resistance as low as 4.4 × 10^-3Ω · cmA^2 is achieved after annealing at 1050℃ for 10min in gas ambient consisting of 90% N2 and 10% H2 X-ray diffraction analysis shows the formation of Ni2 Si and graphite phase at the interface after annealing. This provides the evidence that the car- bon vacancies,resulting from the out-diffusion of carbon atoms from SiC, contribute to the formation of ohmic contact through the reduction of effective Schottky barrier height for the transport of electrons.
N-wells are created by P＋ ion implantation into Si-faced p-type 4H-SiC epilayer. Ti and Ni are deposited in sequence on the surface of the active regions. Ni2Si is identified as the dominant phase by X-ray diffraction （XRD） analysis after metallization annealing. An amorphous C film at the Ni2 Si/SiC interface is confirmed by an X-ray energy-dispersive spectrometer （XEDS）. The Ni2Si and amorphous C film are etched away selectively,followed by deposition of new metal films without annealing. Measurement of the current-voltage characteristics shows that the contacts are still ohmic after the Ni2 Si and amorphous C film are replaced by new metal films. The sheet resistance Rsh of the implanted layers decreases from 975 to 438f2/D, because carbon vacancies （Vc） appeared during annealing,which act as donors for electrons in SiC.
A model is presented to describe a compensation mechanism for semi-insulating 6H-SiC grown with the intentional doping of vanadium. Because we found nitrogen to be the principal shallow donor impurity in SiC by secondary ion mass spectroscopy （SIMS） measurements, semi-insulating properties in SiC are achieved by compensating the nitrogen donor with the vanadium deep acceptor level. The presence of different vanadium charge states V^3＋ and V^4＋ is detected by electron paramagnetic resonance and optical absorption measurements,which coincides with the results obtained by SIMS measurements. Both optical absorption and low temperature photoluminescence measurements reveal that the vanadium acceptor level is located at 0.62eV below the conduction band in 6H-SiC.
