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Radiation detectors have been fabricated using bulk semi-insulating (SI) 4H-SiC crystals and SI and n-type 4H-SiC
epitaxial layers grown by chemical vapor deposition (CVD) on highly doped (0001) 4H-SiC substrates. The devices
have been characterized by optical microscopy, current-voltage (I-V) measurements, thermally stimulated current (TSC)
spectroscopy (94K - 650 K), Hall effect, van der Pauw measurements, and electron beam induced current (EBIC)
technique. Both epitaxial layers exhibited relatively shallow levels related to Al, B, L- and D- centers. Deep level
centers in the n-type epitaxial layer peaked at ~ 400 K (Ea ~ 1.1 eV) and ~ 470 K were correlated with IL2 defect and
1.1 eV center in high purity bulk SI 4H-SiC. The SI epitaxial layer exhibited peak at ~ 290 K (Ea = 0.82 - 0.87 eV) that
was attributed to IL1 center and 3C inclusions, and at ~ 525 K that was related to intrinsic defects and their complexes
with energy levels close to the middle of the band gap. The TSC spectra of the SI epitaxial layer exhibited peaks with
different current polarity which we attributed to the built-in electric field reversal. Results of EBIC and optical
microscopy characterization showed segregation of threading dislocations around comet tail defects in the n-type
epitaxial layers and presence of stacking faults and 3C-SiC inclusions in both epitaxial layers. The I-V characteristics of
the devices on SI epi obtained in wide temperature range (94K - 650 K) exhibited steps at ~ 1 V and ~ 70 V
corresponding to the ultimate trap filling of deep centers peaked at > 500 K and at ~ 250 K (Ea ~ 0.57 eV), & ~ 300 K
(Ea ~ 0.85 eV) respectively. Slow processes of the injected carrier capture on traps resulted in the I-V characteristic with
negative differential resistance (NDR). The high temperature resistivity measurements of bulk SI 4H-SiC sample
revealed resistivity hysteresis that was attributed to the filling of the deep level electron trap centers.
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