DI Center Research Articles

Effect of the column design and fabrication method on the reverse recovery characteristics of 1.2 kV SiC-superjunction-MOSFETs

We investigated the effects of the column length and fabrication method of the superjunction (SJ) structure on the reverse recovery characteristics of 1.2 kV silicon carbide (SiC) SJ metal–oxide–semiconductor field-effect transistors (MOSFETs). The voltage slope (dV/dt) and reverse recovery current slope (dir/dt) values during the reverse recovery of the SJ-MOSFETs with the SJ structure formed by Aluminum (Al) ion implantation were comparable to the values of MOSFETs without SJ structure at room temperature (RT), but they increased markedly with temperature. This is caused by the reduced p-column carrier concentration at RT during short recovery transient time due to the deep hole trap (DI center) introduced by ion implantation. At 175 °C, a softer reverse recovery waveform was obtained by shortening the SJ columns. Besides, Al ion implanted SJ-MOSFETs exhibited a weaker current dependence of the reverse recovery charge Qrr at 175 °C indicating a lower carrier injection level compared to the SJ-MOSFETs with the SJ structure formed by trench formation and epitaxial growth. This is caused by the short carrier lifetime of the SJ columns due to the Al ion implantation defects. The short and Al ion implanted SJ columns are effective for soft reverse recovery characteristics in SiC-SJ-MOSFETs.

IBIL Measurement and Optical Simulation of the DI Center in 4H-SiC

In this paper, DI defects are studied via experiments and calculations. The 2 MeV H+ is used to carry on an ion-beam-induced luminescence (IBIL) experiment to measure the in-situ luminescence of untreated and annealed 4H-SiC at 100 K. The results show that the luminescence intensity decreases rapidly with increasing H+ fluence, which means the losses of optical defect centers. In addition, the evident peak at 597 nm (2.07 eV) is the characteristic peak of 4H-SiC, and the weak peak between 400 nm and 450 nm is attributed to the DI optical center. Moreover, the first-principles calculation of 4H-SiC is adopted to discuss the origin of DI defects. The optical transition of the defect SiC(CSi)2 from q = 0 to q = 1 is considered the experimental value of the DI defect center.

Antisite Defect SiC as a Source of the DI Center in 4H‐SiC

DI center, as a widely existed defect in 4H‐SiC, has attracted much attention in recent years, while the origin of it is still unclear. Herein, by comparing first‐principles calculated potential point defects‐related zero‐phonon lines (ZPLs) and nonradiative capture cross‐sections with the DI center‐related values in the experiment, it is proposed that the transition from the bound exciton states about 57 meV below the CBM to the “+1/+2” level of antisite defect SiC is responsible for the DI center in 4H‐SiC. This work describes the temperature and transition energy level dependence of the hole capture cross‐section of antisite defect in 4H‐SiC, which provides an idea for the optimal design of point defects in semiconductor materials.

Chromosome aberrations, micronucleus frequency, and catalase concentration in a population chronically exposed to high levels of radon

Purpose To deepen our knowledge on the effects of high levels of indoor radon exposure, we assessed the frequencies of unstable and stable chromosome aberrations and micronucleus (MN), as well as the concentration of an endogenous antioxidant (catalase, CAT), in blood samples of individuals chronically exposed to high indoor radon concentrations in Indonesia (Tande-Tande sub-village, Mamuju, West Sulawesi). Moreover, we also investigated the occurrence of a radio-adaptive response (RAR) in Tande-Tande sub-village inhabitants using the G2 MN assay. Materials and Methods The frequencies of dicentric (DC), acentric (AF), ring (R), and translocation (Tr) chromosomes in Tande-Tande inhabitants were compared to those in people living in a reference area with low levels of indoor radon levels (Topoyo village, Indonesia). The number of MN per 1000 binucleated cells (BNC) and CAT concentration per total protein was quantified and compared between groups. Lastly, we irradiated (2 Gy) phytohemagglutinin-stimulated samples in vitro and measured the frequency of MN to verify the occurrence of a RAR in Tande-Tande sub-village inhabitants. Results and Conclusion The frequencies of DC, AF, and Tr did not differ between Tande-Tande inhabitants and control subjects (p = 0.350, 0.521, 0.597). The frequency of MN in Tande-Tande inhabitants was significantly lower than that in the control group (p = 0.006). Similarly, CAT concentration in Tande-Tande inhabitants was also significantly lower than that in the control population (p < 0.001). Significant negative correlations were identified for MN number and CAT concentration versus indoor radon concentration, annual effective dose, or cumulative dose both within groups and when all data were analyzed together. Our findings indicate that, despite the high indoor radon levels, Tande-Tande inhabitants are not under oxidative stress, since this group had lower CAT concentration and MN frequency than those in the control group. The negative correlation between MN frequency and indoor radon concentration, annual effective dose, and cumulative dose suggests the occurrence of an RAR phenomenon in Tande-Tande sub-village inhabitants. This interpretation is also supported by the results of the G2 MN assay, which revealed lower MN frequencies after in vitro irradiation of samples from Tande-Tande sub-village inhabitants than those in samples from the control group (p = 0.0069, for cumulative MN frequency; p = 0.0146, for radiation-induced MN only).

Correlation between cytogenetic biomarkers obtained from DC and CBMN assays caused by low dose radon exposure in smokers

Purpose: Radon is of health concern because they can cause lung cancer when inhaled over many years. Cytogenetic biomarkers proved to be excellent methods to detect and estimate radiation induced DNA damage. Two well established cytogenetic assays are available to detect the DNA damage caused by ionizing radiation namely Dicentric (DC) assay and Cytokinesis-Blocked Micro Nucleus (CBMN) assay. Chromosomal aberrations such as dicentric and acentric fragments can be analyzed in DC assay. Micronuclei and Nucleoplasmic bridges can be analyzed in CBMN assay. The present study focuses on correlation between the aberrations observed in DC and CBMN assay caused by low doses of radon in smokers.Material and methods: Blood samples were collected, after prior consent, from smokers as well as healthy non-smoking individuals and exposed in vitro to radon ranging between 0–5.2 mGy using a simple, portable irradiation assembly designed and tested at authors’ laboratory.Results: A significant increase in frequencies of dicentrics and excess acentric fragments were observed with increasing dose. Interestingly in CBMN assay also, the frequencies of micronuclei and nucleoplasmic bridges were increased with dose. There was a significant correlation between nucleoplasmic bridges and dicentrics for both smokers and non-smokers. Significant correlation between acentric fragments and micronucleus was observed only for non-smokers.Conclusions: By considering the correlation between the two cytogenetic assays, the CBMN assay can be used as an alternative to DC assay for non-smokers. In case of smokers, nucleoplasmic bridges can be used to measure the radiation exposure. This study is first of its kind to correlate the cytogenetic biomarkers obtained from DC and CBMN assay for smokers with very low doses of radon.

Comparison of dicentric dose response curves of 6 MV LINAC X-rays and 60Co γ-rays for biodosimetry application

In vitro Dicentric (DC) dose response curves of 6MV X-rays (3Gy/min) and 60Co γ-rays (0.63Gy/min) were generated and compared (intra-laboratory) to understand their similarities and differences. Human peripheral blood samples exposed to ten different doses (0, 0.05, 0.1, 0.25, 0.5, 1, 2, 3, 4 and 5Gy) of 6MV X-rays and 60Co γ-rays were subjected to Dicentric Chromosome Assay (DCA) and dicentrics, excess acentric fragments (AF) and chromatid breaks (CB) were scored. Totally about 31,553 metaphase spreads were scored for the purpose. Dose response curves of both radiation qualities were almost same except for a 13.8% higher β value for 6MV X-rays. However, blind tests results revealed that both these curves are biologically equivalent and exhibited good dose prediction accuracy for the entire dose range. This demonstrated the feasibility of interchangeable use of these curves in biodosimetry. Consequently it has been suggested that LINAC facilities worldwide can be roped in for biodosimetry capacity augmentation towards managing nuclear emergency situations involving γ-radiation exposures.

Possible benefits of tomato juice consumption: a pilot study on irradiated human lymphocytes from healthy donors

BackgroundReactive oxygen species (ROS) mediate much of the DNA damage caused by ionizing radiation. Among carotenoids, lycopene and β-carotene, present in tomato juice, are known to be strong radical scavengers. The aim of the study was to investigate the effect of tomato juice intake on the levels of DNA damage and oxidative stress in human whole blood induced by in vitro exposure to X-rays.MethodsTen healthy adults were asked to drink 190 g of tomato juice, containing 17 mg lycopene and 0.25 mg β-carotene, per day for 3 weeks and then refrain from drinking it for 3 weeks. Peripheral whole blood samples were collected before and after the intake period of tomato juice and after the washout period. The blood samples were exposed in vitro to X-ray doses of 0, 0.1, 0.5, and 2 Gy. Cytogenetic damage was measured using the cytokinesis-block micronucleus (CBMN) assay and the dicentrics (DIC) assay. The level of oxidative stress was determined using serum 8-oxo-7, 8-dihydro-2-deoxyguanosine (8-oxo-dG) and plasma reactive oxygen metabolite-derived compounds (d-ROMs). The concentration of carotenoids in plasma was measured at the three time points.ResultsThe levels of 8-oxo-dG tended to decrease during the intake period and increase during the washout period. A non-significant inverse correlation was noted between the plasma concentration of lycopene plus β-carotene and the level of 8-oxo-dG (P = 0.064). The radiation-induced MN and DIC frequencies increased in a dose-dependent manner, and when compared at the same dose, the MN and DIC frequencies decreased during the intake period compared with those at baseline and then increased during the washout period. The results suggest that continuous tomato juice consumption non-significantly decreases extracellular 8-oxo-dG, d-ROMs, and MN. Tomato juice intake had minimal or no effect on radiation-induced 8-oxo-dG and d-ROMs. For most radiation doses, continuously tomato juice intake lowered the levels of MN and DIC.ConclusionTomato juice consumption may suppress human lymphocyte DNA damage caused by radiation, but further examination is required.Trial registration2014-001 and 2014-R06.

Drug information center as referral service in a South Indian tertiary care hospital

Objective:The objective of this study is to assess the various aspects of drug information services (DISs) provided in the DI center of a tertiary care hospital.Materials and Methods:DI queries received from various departments from April 2013 to May 2017 were included in the study. Various aspects such as year- and department-wise distribution, reason for sending the queries, mode of receipt and reply, time taken for reply, number of visit for bedside examination of patients, and number of references given per query were analyzed. All the results are expressed in numbers and percentages.Results:Fifty-five DI queries were received during the study period. Most of the queries were received from Department of Orthopedics (26, 47.27%), followed by Neurology (4, 7.27%). Most common mode of receipt of queries (41, 74.55%) was by Cross-reference form not case record form followed by phone calls (8, 14.55%) and outpatient department (OPD) case sheet (6, 10.9%). CRF with attached opinion was the most common mode of reply (41, 74.55%) followed by phone calls (7, 12.73%), and OPD case sheets (6, 10.9%). The most common reason for sending queries was antimicrobials-related problem (25, 45.46%), followed by the use of anticoagulants (13, 23.63%). Most of the queries were replied within 24 h (31, 56.36%), followed by 48 h (14, 25.45%). Out of 41 CRF received for in-patients, bedside examination was requested in 23 (56.09%) CRF. There was an increasing trend in the number of queries received every year with more queries received during 2016 (23, 41.82%).Conclusions:DIS if utilized properly can be used as a referral service such as other specialties in a tertiary care hospital.

The radioprotective effect of metformin against cytotoxicity and genotoxicity induced by ionizing radiation in cultured human blood lymphocytes

Metformin is a widely prescribed drug used in the treatment of patients with type 2 diabetes. In this study, the radioprotective effect of metformin was investigated against cytotoxicity and genotoxicity induced by ionizing radiation (IR) in human peripheral blood lymphocytes. Human lymphocytes were treated with metformin at concentrations 10 and 50μM for 2h and irradiated with 6MV X-rays. The radiation antagonistic potential of metformin was assessed by MTT [3-(4,5-dimethyl-2-thiaozolyl)-2,5-diphenyl-2H tetrazolium bromide] assay, chromosomal aberration (CA) analysis, cytokinesis blocked micronucleus (CBMN) assay, and flow cytometry. Observations demonstrated a radiation-dose-dependent decrease in the percentage of cell viability after 24h. It was found that pretreatment with metformin (10 and 50μM) increased the percentage of cell viability. A highly significant dose modifying factor (DMF) 1.35 and 1.42 was observed for 10 and 50μM metformin, respectively. Metformin (10 and 50μM) pretreatment significantly decreased the frequency of dicentrics (DCs), acentric fragments (AFs), rings (RIs), micronuclei (MN), and nucleoplasmic bridges (NPBs) in irradiated human peripheral blood lymphocytes. Also, treatment with metformin (10 and 50μM) without irradiation did not increase the number of MN, NPBs, DCs, AFs, RIs, and did not show a cytostatic effect in the human peripheral blood lymphocytes. On the other hand, metformin treatment (10 and 50μM) 2h prior to irradiation significantly reduced X-radiation-induced apoptotic incidence in human lymphocytes. The present study demonstrates metformin to be an effective radioprotector against DNA damage and apoptosis induced by IR in human lymphocytes. These data have an important application for the protection of lymphocytes from the genetic damage and side-effects induced by radiotherapy in cancer patients.

Chromosomal Aberrations (CA), Sister Chromatid Exchanges (SCE), and High-Frequency Cells (HFC) Frequencies in Peripheral Blood Lymphocytes of Tunisian Gasoline Truck Loaders

ABSTRACT Gasoline constitutes a mixture of chemicals that contain well-known genotoxicants. Thus, chronic occupational exposure to gasoline may be considered to possess genotoxic risk. In this study, the frequencies of total chromosomal aberrations (TCA), aberrant cells (Ab.c.), sister chromatid exchanges (SCE), high-frequency cells (HFC), and high-frequency cell individual (HFI) were investigated in peripheral blood lymphocytes from 17 gasoline-exposed workers (10 smokers and 7 non-smokers) and 22 unexposed reference subjects (12 smokers and 10 non-smokers). The exposed subjects were gasoline truck loaders at a gasoline company from Tunis City, north of Tunisia. The results indicate multiple CA, such as dicentrics (DIC), chromatid breaks (SB), and chromosome breaks (DB). A significant difference was observed in TCA and Ab.c. frequencies between exposed and unexposed groups (p < 0.01). A significant difference was found in frequencies of SCE (p < 0.01) and HFI (p < 0.05) between exposed and unexposed groups. SCE and TCA frequencies of smokers were found to be significantly higher than those of non-smokers in both groups. There was an interaction between gasoline exposure and smoking habit for TCA (p = 0.020), but not for SCE. Our findings indicate that gasoline truck loaders were under risk of significant cytogenetic damage that was enhanced by their smoking habit.

Evaluating the Radioprotective Efficacy of Dendrodoine Analog Against the Formation of Dicentric Aberration Frequency in Cultured Human Peripheral Blood Lymphocytes

The present study was aimed to evaluate the protective efficacy of dendrodoine analog (DA), an aminothiazole derivative against the formation of radiation induced dicentric (DC) aberration frequency on cultured human peripheral blood lymphocytes. DA was chemically synthesized and the product thus obtained was purified using column chromatography packed with silica gel using chloroform as the solvent. The purity status of the final product was assessed employing high performance thin layer chromatography (HPTLC). The radioprotective efficacy of DA against the formation of DC aberration frequency was analyzed by pre-incubating human peripheral blood lymphocytes with the optimum concentration of DA, selected from our previous study, followed by exposure to different doses of radiation. The results indicated that there was a dose dependent increase in the formation of DC aberration frequency in the irradiated groups when compared to DA pre-treated groups which modulated the toxic effects of radiation by means of its effective DNA protective and antioxidant property.

Apigenin ameliorates gamma radiation-induced cytogenetic alterations in cultured human blood lymphocytes

The aim of the present study was to assess the protective effect of apigenin, a dietary flavone, against cytogenetic alterations in human peripheral blood lymphocytes (HPBL) induced by Cobalt-60 radiation (3Gy). Results of MTT [3-(4, 5-dimethyl-2-thiaozolyl)-2,5-diphenyl-2H tetrazolium bromide] assay revealed that 37.2μM of apigenin was found to be non-toxic in HPBL. At this dose (37.2μM) of apigenin, the LD50 radiation dose of HPBL increased from 2.9Gy to 3.4Gy, which resulted in a DMF of 1.17. Apigenin (37.2μM) treatment 1h before irradiation significantly (p<0.05) reduced DNA damage in irradiated HPBL as measured by comet assay (% tail DNA, tail length, tail moment, and olive tail moment). Moreover, apigenin treatment significantly decreased the frequencies of dicentric (DC), acentric fragments (AF), and acentric rings (AR) in irradiated HPBL. Apigenin pretreatment also reduced the radiation-induced CBMN (cytokinesis blocked micronuclei) anomalies such as micronuclei (MNi), nucleoplasmic bridges (NPB) and nuclear buds (NBUD) in HPBL. These results also showed that there was a significant correlation between NPB and DC frequencies and MNi and AF+AR. Treatment with apigenin alone had no significant effect on DNA damage and chromosomal aberrations in HPBL. Thus, the current studies indicate that apigenin protects HPBL from radiation-induced cytogenetic alterations.

Thermal Histories of Defect Centers as Measured by Low Temperature Photoluminescence in n- and p-Type 4H SiC Epilayers Generated by Irradiation with 170 keV or 1 MeV Electrons

In this paper we describe an effort to find correlations between low temperature photoluminescence spectroscopy (LTPL) and deep level transient spectroscopy (DLTS) of electron irradiated samples annealed from 25 °C to 1700 °C in 100 °C steps. We report on thermal histories of defect centers created by 170 keV and 1 MeV electron irradiation, as observed by LTPL only. The DLTS results on "twin" samples are presented in a separate paper. Our results indicate that in n-type 4H SiC there is no correlation between the Z1/Z2 center in DLTS and the L1 peak of the DI center seen in LTPL. In p-type 4H SiC we do not find a correlation between a 350 meV DLTS peak above the valence band and the LTPL L1 peak of the DI center. Consequently, we cannot find evidence for a 350 meV ground state postulated in the “Pseudo–Donor” model [3].

Thermal stability of irradiation-induced point defects in cubic silicon carbide

This work aims specifically at studying the evolution of point defects induced by electron irradiation in the cubic polytype of SiC (3C-SiC) at temperatures ranging from 10 to 1450 K by means of photoluminescence (PL) spectroscopy. We identified a first annealing stage between 200 and 245 K, which probably results from migration of interstitials in the carbon sublattice. Moreover, we confirmed the high thermal stability of defect-related PL signals up to about 1100 K and calculated the activation energies associated with their annihilation. Finally, we studied the effect of a high temperature treatment at 1400 K on the DI center PL intensity in a single-crystal sample irradiated by electrons below the threshold displacement energy of the silicon sublattice. This allows checking the relevance of recent defect models based upon the migration of atoms in the carbon sublattice during the irradiation process. We conclude that the DI center does not involve the silicon vacancy and could be assigned to an isolated silicon antisite SiC.

New Type of Defects Explored by Theory: Silicon Interstitial Clusters in SiC

We have studied the small clusters of silicon and carbon interstitials by ab initio supercell calculations in 4H-SiC. We found that silicon interstitials can form stable and electrically active complexes with each other or with a carbon interstitial. Local vibration modes and ionization energies were also calculated in order to help the identification of the defects. We propose that silicon interstitials can emit from these clusters at relatively high temperatures, which may play an important role in the formation of the DI center.

Antisite defect centers in 3C-SiC produced by ion irradiation

We have studied the ion-induced defects in a semiconductor (3C-SiC) using an empirical molecular dynamics calculation at temperature below 2000K. The structures of produced defects were analysed by the Pixel Mapping method, focusing on the antisite related defects. In addition to the DI center that had been ascribed to be due to single antisite pair (CSi–SiC) aligning in <111> directions, we observed antisite clusters composed of 3–5 antisites. As a model cluster, SiC(CSi)4 had been predicted by quasistatic calculation, which was consistent with the lower formation energy of CSi than SiC. This implied inversely SiC and CSi(SiC)4 can be more stable if once they were formed under energetic ion impacts. In fact, we observed a clear trend of growth of CSi(SiC)n approaching to CSi(SiC)4 under 5keV boron impacts above annealing temperature.

Theoretical study of small silicon clusters in4H−SiC

We have studied the small clusters of silicon and carbon interstitials by ab initio supercell calculations in 4H-SiC. We found that silicon interstitials can form stable and electrically active complexes with each other or with a carbon interstitial. Local vibration modes and ionization energies were also calculated in order to help the identification of the defects. We propose that silicon interstitials can emit from these clusters at relatively high temperatures, which may play an important role in the formation of the DI center. © 2007 The American Physical Society.

Anti-site pair in SiC: a model of the DI center

The DI low-temperature photoluminescence center is a well-known defect stable up to 1700°C annealing in SiC, still its structure is not known after decades of study. Combining experimental and theoretical studies in this paper we will show that the properties of an anti-site pair can reproduce the measured one-electron level position and local vibration modes of the DI center and the model is consistent with other experimental findings as well. We give theoretical values of the hyperfine constants of the anti-site pair in its paramagnetic state as a means to confirm our model.

Defects in SiC

Recent results from studies of shallow donors, pseudodonors, and deep level defects in SiC are presented. The selection rules for transitions between the electronic levels of shallow donors in 4H–SiC in the dipole approximation are derived and the ionization energy for the N donor at hexagonal site is determined. Optical and electrical studies of the DI center reveal the pseudodonor nature of this defect. Defects in high-purity semi-insulating (SI) SiC substrates including the carbon vacancy (VC), silicon vacancy (VSi), and (VC−CSi) pair are studied. The annealing behavior of these defects and their role in carrier compensation in SI 4H–SiC are discussed.

Comparison of chronic exposures received by radiation workers using different biological end‐points with the doses recorded by TLD

The frequency of different biological end-points such as translocation, dicentrics (DC) and micronuclei (MN) was studied in 14 radiation workers and 21 non-radiation workers. The average frequencies for different types of aberrations were significantly higher in radiation workers compared to those of respective aberrations in non-radiation workers. Out of 14 radiation workers, eight subjects showed a dose above the detection limit as per translocation and seven subjects as per DC frequency and no patient showed a dose above the detection limit as per MN frequency. Regression analysis carried out between the recorded doses according to Thermo Luminescence Dosimeter (TLD) and the dose estimated as per translocation frequency gave a correlation coefficient of 0.32, whereas that obtained with TLD dose and the dose estimated as per DC was 0.81. When the correlation was made between the TLD dose, which was above 0.15 Gy (the detection limit for translocation), and the dose estimated as per translocation frequency in these subjects, a correlation coefficient of 0.98 was found. A similar analysis between the TLD dose above 0.5 Gy (the detection limit for DC) and the dose estimated as per DC frequency in these subjects, a correlation coefficient of 0.26 resulted. This paper discusses the reasons for the poor correlation obtained between TLD dose and dose estimated as per DC and MN frequency.