High-resolution photoinduced transient spectroscopy (HRPITS) has been applied to studying electronic properties of point defects associated with the charge compensation in semi-insulating (SI) 6H-SiC substrates. The photocurrent relaxation waveforms were digitally recorded in a wide temperature range of 20 – 700 K and in order to extract the parameters of defect centers, a new computational procedure was implemented. It involves a two-dimensional analysis of the waveforms as a function of time and temperature using the correlation procedure or inverse Laplace algorithm. As a result, a set of waveforms is transformed into the spectral surface visualized in the three-dimensional space as a function of two variables: the temperature (T) and the emission rate (eT). Thus, the processes of thermal emission of charge carriers from defect centers are seen on the spectral surface as the folds, the ridgelines of which give the temperature dependences of emission rate for detected defect centers. Each ridgeline is fitted with the Arrhenius equation and the activation energy Ea, as well as the pre-exponential factor A, related to the capture cross-section, are obtained. The fitting is carried out automatically by means of a neural network (NN) that creates the approximating surface and morphologically matches it to the shape of the fold corresponding to the defect center.
In this work, we have studied defect centers either in vanadium-doped or vanadium-free (undoped) SI 6H-SiC wafers. Vanadium atom (3d34s2) is an amphoteric impurity in SiC acting as a deep acceptor compensating shallow donors related to residual nitrogen atoms, as well as a deep donor compensating shallow acceptors associated with residual boron atoms. The resistivity of the V-doped wafer at 300 K was ~1.0x1010 Ωcm. In this material, five defect centers, labeled as TV1, TV2, TV3, TV4 and TV5, with activation energies of 0.08, 0.28, 0.43, 0.78 and 1.16 eV, respectively, were revealed. The centers TV1 (0.08 eV) and TV2 (0.28 eV) are likely to be attributed to the nitrogen donor and boron acceptor, respectively. On the other hand, the centers TV4 (0.78 eV) and TV5 (1.16 eV) are likely to be related to the vanadium acceptor (electron trap) V3+/4+ and the vanadium donor (hole trap) V5+/4+, respectively. The center TV3 (0.43 eV) can be assigned to a complex involving a vanadium atom and a native defect. The activation energy of dark conductivity equal to 1.26 eV indicates that the Fermi level in the V-doped 6H-SiC is located close to the deep level TV5 (1.16 eV). The resistivity of the undoped 6H-SiC wafer at 300 K was ~ 3.0x107 Ωcm. In this material, twelve defect centers, labeled as TU1-TU12, with activation energies ranging from 0.01 to 0.98 eV were detected. The activation energy of dark conductivity equal to 1.063 eV indicates that the Fermi level in the undoped 6H-SiC is located close to the deep level TU12 (0.98 eV). The origin of very shallow centers TU1 (0.01 eV), TU2 (0.02 eV) and TU3 (0.03 eV) is presently unclear. The experimental data reported so far on defect centers in 6H-SiC are also not sufficient for the identification of centers TU4 (0.09 eV), TU6 (0.11 eV) and TU10 (0.51 eV). However, the shallow centers TU5 (0.08 eV) and TU7 (0.130 eV) can be assigned to residual nitrogen atoms located in the hexagonal and cubic lattice sites, respectively. The center TU8 (0.53 eV) seems to be an electron trap related to the carbon vacancy and the center TU9 (0.63 eV) is presumably also an electron trap associated with the known defect center Z1/Z2 characteristic of both 4H- and 6H-SiC. The atomic configuration of the Z1/Z2 center, however, has not been established yet. On the other hand, the center TU11 (0.65 eV) can be identified with a hole trap assigned to a deep acceptor involving a boron atom and silicon antisite. The center TU12 (0.98 eV), responsible for the material high resistivity, is the predominant acceptor center compensating the shallow nitrogen donors. This center is likely to be attributed to a complex formed by intrinsic point defects including the isolated carbon and silicon vacancies, the Si antisite and the carbon vacancy-carbon antisite pairs.