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The carbon vacancy (V C ) has been suggested by different studies to be involved in the Z 1 /Z 2 defect-a carrier lifetime killer in SiC. However, the correlation between the Z 1 /Z 2 deep level with V C is not possible since only the negative carbon vacancy (V − C ) at the hexagonal site, V − C (h), with unclear negative-U behaviors was identified by electron paramagnetic resonance (EPR). Using freestanding n-type 4H -SiC epilayers irradiated with low energy (250 keV) electrons at room temperature to introduce mainly V C and defects in the C sublattice, we observed the strong EPR signals of V − C (h) and another S = 1/2 center. Electron paramagnetic resonance experiments show a negative-U behavior of the two centers and their similar symmetry lowering from C 3v to C 1h at low temperatures. Comparing the 29 Si and 13 C ligand hyperfine constants observed by EPR and first principles calculations, the new center is identified as V − C (k). The negative-U behavior is further confirmed by large scale density functional theory supercell calculations using different charge correction schemes. The results support the identification of the lifetime limiting Z 1 /Z 2 defect to be related to acceptor states of the carbon vacancy.

Two trap-reduction processes, thermal oxidation and C þ implantation followed by Ar annealing, have been discovered, being effective ways for reducing the Z 1=2 center (E C-0.67 eV), which is a lifetime killer in n-type 4H-SiC. In this study, it is shown that new deep levels are generated by the trap-reduction processes in parallel with the reduction of the Z 1=2 center. A comparison of defect behaviors (reduction, generation, and change of the depth profile) for the two trap-reduction processes shows that the reduction of deep levels by thermal oxidation can be explained by an interstitial diffusion model. Prediction of the defect distributions after oxidation was achieved by a numerical calculation based on a diffusion equation, in which interstitials generated at the SiO 2 =SiC interface diffuse to the SiC bulk and occupy vacancies related to the origin of the Z 1=2 center. The prediction based on the proposed analytical model is mostly valid for SiC after oxidation at any temperature, for any oxidation time, and any initial Z 1=2-concentration. Based on the results, the authors experimentally achieved the elimination of the Z 1=2 center to a depth of about 90 lm in the sample with a relatively high initial-Z 1=2-concentration of 10 13 cm À3 by thermal oxidation at 1400 C for 16.5 h. Furthermore, prediction of carrier lifetimes in SiC from the Z 1=2 profiles was realized through calculation based on a diffusion equation, which considers excited-carrier diffusion and recombination in the epilayer, in the substrate, and at the surface. V

The authors investigated deep levels in the whole energy range of bandgap of 4H-SiC, which are generated by low-dose N + , P + , and Al + implantation, by deep level transient spectroscopy ͑DLTS͒. Ne +-implanted samples have been also prepared to investigate the pure implantation damage. In the n-type as-grown material, the Z 1/2 ͑E C − 0.63 eV͒ and EH 6/7 ͑E C − 1.6 eV͒ centers are dominant deep levels. At least, seven peaks ͑IN1, IN3-IN6, IN8, and IN9͒ have emerged by implantation and annealing at 1000°C in the DLTS spectra from all n-type samples, irrespective of the implanted species. After high-temperature annealing at 1700°C, however, most DLTS peaks disappeared, and two peaks, IN3 and IN9, which may be assigned to Z 1/2 and EH 6/7 , respectively, survive with a high concentration over the implanted atom concentration. In the p-type as-grown material, the D ͑E V + 0.40 eV͒ and HK4 ͑E V + 1.4 eV͒ centers are dominant. Two peaks ͑IP1 and IP3͒ have emerged by implantation and annealing at 1000°C, and four traps IP2 ͑E V + 0.39 eV͒, IP4 ͑E V + 0.72 eV͒, IP7 ͑E V + 1.3 eV͒, and IP8 ͑E V + 1.4 eV͒ are dominant after annealing at 1700°C in all p-type samples. The IP2 and IP8 may be assigned to the HS1 and HK4 centers, respectively. The depth analyses have revealed that the major deep levels are generated in the much deeper region than the implant profile.

Deep levels by proton and electron irradiation in 4H-SiC J. Appl. Phys. 98, 053706 (2005);Vacancies and deep levels in electron-irradiated 6H SiC epilayers studied by positron annihilation and deep level transient spectroscopyThe Z 1=2 center in n-type 4H-SiC epilayers-a dominant deep level limiting the carrier lifetime-has been investigated. Using capacitance versus voltage (C-V) measurements and deep level transient spectroscopy (DLTS), we show that the Z 1=2 center is responsible for the carrier compensation in n-type 4H-SiC epilayers irradiated by low-energy (250 keV) electrons. The concentration of the Z 1=2 defect obtained by C-V and DLTS correlates well with that of the carbon vacancy (V C ) determined by electron paramagnetic resonance, suggesting that the Z 1=2 deep level originates from V C . V C 2013 American Institute of Physics. [http://dx.

A longest carrier lifetime of 33.2 µs was achieved by eliminating the Z1/2 center via thermal oxidation at 1400 °C for 48 h and subsequent surface passivation with a nitrided oxide on a 220-µm-thick n-type 4H-SiC epilayer. By deep-level elimination, photoluminescence (PL) in the infrared region (wavelength: 700–950 nm) was remarkably enhanced at locations of threading dislocations. A threading screw dislocation exhibited much stronger infrared PL than a threading edge dislocation. The present results indicate that carrier recombination at extended defects becomes pronounced through the elimination of the Z1/2 center in the epilayers.

Temperature dependent recombination dynamics in InP/ZnS colloidal nanocrystals Appl. Phys. Lett. 101, 091910 (2012) Intrinsic defect in BiNbO4: A density functional theory study J. Appl. Phys. 112, 043706 (2012) The CuInSe2-CuIn3Se5 defect compound interface: Electronic structure and band alignment Appl. Phys. Lett. 101, 062108 (2012) Investigation of deep-level defects in conductive polymer on n-type 4H-and 6H-silicon carbide substrates using I-V and deep level transient spectroscopy techniquesMajor deep levels observed in as-grown and irradiated n-type 4H-SiC and 6H-SiC epilayers have been investigated. After low-energy electron irradiation, by which only carbon atoms are displaced, five traps, EH1 ͑E C − 0.36 eV͒, Z 1 / Z 2 ͑E C − 0.65 eV͒, EH3 ͑E C − 0.79 eV͒, EH5 ͑E C − 1.0 eV͒, and EH6/7 ͑E C − 1.48 eV͒, were detected in 4H-SiC and four traps, E 1 / E 2 ͑E C − 0.45 eV͒, RD 5 ͑E C − 0.57 eV͒, ES ͑E C − 0.80 eV͒, and R ͑E C − 1.25 eV͒, were detected in 6H-SiC. The Z 1 / Z 2 , EH6/7 centers in 4H-SiC and the E 1 / E 2 , R centers in 6H-SiC exhibit common features as follows: their generation rates by the e − -irradiation were almost the same each other, their concentrations were not changed by heat treatments up to 1500°C, and they showed very similar annealing behaviors at elevated temperatures. Furthermore, these defect centers were almost eliminated by thermal oxidation. Taking account of the observed results and the energy positions, the authors suggest that the Z 1 / Z 2 center in 4H-SiC corresponds to the E 1 / E 2 center in 6H-SiC, and the EH6/7 center in 4H-SiC to the R center in 6H-SiC, respectively. Since the concentrations of these four centers are almost the same for as-grown, electron-irradiated, annealed, and oxidized samples, these centers will contain a common intrinsic defect, most likely carbon vacancy. The authors also observed similar correspondence for other thermally unstable traps in 4H-SiC and 6H-SiC.

Diffraction microscopy with iterative phase retrieval using a 20 kV electron beam was carried out to explore the possibility of high-resolution imaging for radiation-sensitive materials. Fine, homogeneous, and isolated multiwall carbon nanotubes ͑MWCNTs͒ were used as specimens. To avoid lens aberrations, the diffraction patterns were recorded without a postspecimen lens. One-and two-dimensional iterative phase retrievals were executed. Images reconstructed from the diffraction pattern alone showed a characteristic structure of MWCNTs with the finest feature corresponding to a carbon wall spacing of 0.34 nm.