Search

We evaluated the dissociation of isolated gas-phase nucleobase molecules induced by mega electron volt (MeV)-energy ions to gain fundamental insights into the reactions of nucleobases upon fast ion irradiation. We studied five nucleobase molecules—adenine, guanine, cytosine, thymine, and uracil—as gas-phase targets. We compared the fragmentation patterns obtained from carbon ion impacts with those obtained from proton impacts to clarify the effect of heavy ion irradiation. We also compared the results with electron impact and photoionization results. In addition, we identified several delayed fragmentation pathways by analyzing the correlation between fragment pairs generated from singly and doubly charged intermediate ions. To determine the lifetimes of delayed fragmentation from singly charged intermediate ions, we evaluated the detection efficiencies of the microchannel plate detector for the neutral fragment HCN as a function of kinetic energy using a new methodology. As the first demonstration of this method, we estimated the lifetimes of C5H5N5+ generated by 1.2-MeV C+ and 0.5-MeV H+ collisions to be 0.87 ± 0.43 and 0.67 ± 0.09 µs, respectively. These lifetimes were approximately one order of magnitude longer than those of the doubly charged intermediate ion C5H5N52+. [ABSTRACT FROM AUTHOR]

The carrier lifetime control in p-type 4H-SiC epilayers with intentional aluminum (Al) and boron (B) doping is demonstrated as part of work to develop a p-type "recombination-enhancing layer" for n-channel insulated gate bipolar devices fabricated on p-type substrates. The (Al + B)-doped epilayers (Al: 5 × 1017, B: 4 × 1016 cm−3) showed a very short minority carrier lifetime of less than 20 ns at 293 K, resembling that of highly Al-doped epilayers (Al: 1 × 1019 cm−3). Besides, the minority carrier lifetimes in (Al + B)-doped epilayers are stable against post-annealing in Ar and H2 ambient, while that of Al-doped epilayers varied considerably. PiN diodes with a 10 μm-thick (Al + B)-doped buffer layer inserted on p-type substrates showed no evident degradation after a stress test under a pulse current density of 2000 A/cm2. [ABSTRACT FROM AUTHOR]

A mass spectrometric study of secondary ions emitted from droplet surfaces by MeV-energy heavy ion impact was performed to investigate fast-ion-induced molecular reaction processes on liquid surfaces. Herein, a new coincidence technique was developed between secondary ions and scattered projectile ions at a small forward angle. The advantages of this technique were demonstrated by measurement of the collision between 4-MeV C3+ and ethanol droplets. Secondary ion emission probabilities were obtained directly from the coincidence data. Notably, this technique enabled positive fragment ions that had not been identified in previous measurements to be observed by suppressing the strong background originating from gas-phase molecules more than 104-fold. H+, H3O+, C2H5+, and C2H5O+ were found to be produced as major positive fragment ions, in addition to minor fragments H2+, C2H3+, and CH2OH+. Production of these ions suggests that competition between rapid hydrogen ion emission from multiply ionized states and intermolecular proton transfer accompanied by fragmentation through protonated ethanol occurs after fast heavy-ion collisions. Clarification of the positive fragment ions also revealed the characteristic features of negative ions. Negative ions were realized to exhibit higher degrees of fragmentation and reactivity compared with positive ions. Furthermore, the energy loss by forward-scattered ions during droplet penetration was used to evaluate the target thickness at a submicron level. Variations in secondary ion yield, mass distribution, and kinetic energies depending on the penetration length were observed below 1 µm. These results highlight the unknown mechanism of these "submicron effects" observed in secondary ion emission processes as a new phenomenon. [ABSTRACT FROM AUTHOR]

The carrier lifetime is an important parameter for high voltage SiC bipolar devices because its distribution in drift layers affects the device performance. Observation techniques for carrier lifetime, along with the development of carrier lifetime control processes, are important to control carrier lifetime distribution. In this study, we developed a microscopic time-resolved free carrier absorption system that has a variable spot size of excitation light and two different probe light wavelengths (405 and 637 nm). By selecting a relatively small spot size of excitation light and the probe light of shorter wavelength (405 nm), the distribution of carrier lifetime was observed with a high spatial resolution of ∼3 μm. Additionally, by using a relatively large spot size of excitation light and the probe light that leads to stronger free carrier absorption (637 nm), an accurate measurement of carrier lifetime was obtained. The developed system enables the design and development of bipolar SiC devices with carrier lifetime distribution control. [ABSTRACT FROM AUTHOR]

Haruka Yamamoto,1 Hideto Tsuchida,1 Takashi Nakamae,1 Seiji Nishida,1 Yuki Sakai,1 Akihito Fujimori,1 Jin Narumoto,1 Yoshihisa Wada,1 Takafumi Yoshida,2 Chiaki Taga,3 Kenji Fukui11Department of Psychiatry, Graduate School of Medical Science, Kyoto Prefectural University of Medicine, Kyoto, Japan; 2Kyoto Cognitive Behavior Therapy Counseling Room, Kyoto, Japan; 3Department of Psychiatry, Kyoto Second Red Cross Hospital, Kyoto, JapanPurpose: Obsessive-compulsive disorder (OCD) patients exhibit a noninhibition response pattern very similar to that observed in schizotypy patients in cognitive tasks. It has been suggested that the reduced cognitive inhibition observed in both schizotypy and OCD may result in the frequent entry into awareness of unacceptable urges and intrusive thoughts. The aim of this study was to investigate the relationship between the severity of obsession or compulsion and schizotypy in OCD.Patients and methods: Sixty subjects (25 males and 35 females) who were OCD outpatients in the University Hospital at the Kyoto Prefectural University of Medicine during the period 2008–2010 were enrolled in the study. Assessments of these patients were made using the Yale–Brown Obsessive Compulsive Scale (Y-BOCS), the Schizotypal Personality Questionnaire (SPQ), the Hamilton Depression Rating Scale (HAM-D), and the Hamilton Anxiety Rating Scale (HAM-A). The Pearson correlation coefficients between Y-BOCS and SPQ scores were calculated. Furthermore, hierarchical multiple linear regression analyses were conducted to assess whether schizotypy predicted the severity of obsession and compulsion.Results: By calculating the Pearson correlation coefficient, it was found that the Y-BOCS obsession score, not the Y-BOCS compulsion score, was correlated with the SPQ total score. Results of the hierarchical multiple linear regression analysis showed that SPQ total score was a significant predictor of the Y-BOCS obsession score, after accounting for control variables (ie, HAM-D and HAM-A).Conclusion: Results of this study showed that the Y-BOCS obsession score, not the Y-BOCS compulsion score, was correlated with the SPQ total score. This finding suggests that OCD patients with an elevated SPQ total score experience a reduction of cognitive inhibition, resulting in the frequent entry into obsession. Future longitudinal studies are recommended to clarify the effect of schizotypy on the clinical course of OCD. Keywords: Y-BOCS, obsession, compulsion, severity

Control of carrier lifetimes in nitrogen (N)-doped n-type 4H-SiC epilayers was attempted by intentional boron (B) doping. Doping concentrations of B were controlled to be within 1015–1018 cm−3 by varying the triethyl boron flow rate during epitaxial growth. Time-resolved photoluminescence measurements for the band edge luminescence of the N + B-doped epilayer showed a fast decay time of less than 30 ns under a low injection level at 300 K, while a slow decay component was observed at elevated temperatures of 423–523 K. To understand the mechanism of carrier capture and recombination in N + B-doped 4H-SiC, excess carrier decay curves were simulated by solving rate equations with a simple model. As an effective "recombination enhancing layer," the 1.6-μm-thick N + B-doped (N: 4 × 1018 cm−3, B: 7 × 1017 cm−3) buffer layer in the PiN diode showed increased intensity of D center-related electroluminescence with increasing injected hole concentrations and prevented the formation of stacking faults from basal plane dislocations in the substrate even at the injection level of 1 × 1016 cm−3 at the interface between the drift and buffer layers. [ABSTRACT FROM AUTHOR]

Wide-ranging control of carrier lifetimes in n-type epilayers by vanadium (V) doping is attempted toward not only developing a buffer layer to prevent the stacking fault expansion but also improving switching loss in 4H-SiC-based bipolar devices. Control of V doping concentrations in lightly and highly nitrogen (N)-doped epilayers was achieved within the range of 1012–1015 cm−3 by changing the input flow rates of vanadium tetrachloride. Photoluminescence (PL) and deep-level transient spectroscopy analyses revealed that incorporated V atoms create the PL bands within the range of 0.8–1.0 eV, and densities of the deep center at the V3+/4+ acceptor level (Ec − 0.97 eV) increase linearly with V doping concentrations. Accordingly, V doping shortens the minority carrier lifetimes in lightly N-doped epilayers from 3 μs to 40 ns as well as lifetimes in highly N-doped epilayers down to 20 ns at 20 °C, achieving intrawafer carrier lifetime uniformities of 3–10% σ/mean. Furthermore, V doping during epitaxial growth exhibited a nonsignificant memory effect and the V-doped epilayers showed high thermal stability against postprocessing by 1700 °C. We also demonstrated PiN diodes with a 2.4 μm-thick N + V-doped buffer layer (N: 1 × 1018 and V: 1 × 1014 cm−3), showing no degradation after a stress test for 1 h under a direct current density of 600 A/cm2. [ABSTRACT FROM AUTHOR]

This paper describes the three-dimensional imaging of threading edge dislocations (TEDs), pure c threading screw dislocations (TSDs), and c + a threading mixed dislocations (TMDs) in 4H silicon carbide (4H-SiC) epilayers by two-photon-excited photoluminescence (2PPL), where inclinations of dislocations in epilayers with different offcut angles are investigated. A numerical model based on carrier diffusion explains the mechanism of the dark-contrast 2PPL imaging provided by band edge emission. More than 450 threading dislocations in 4H-SiC epilayers are visualized three-dimensionally and are classified into six -type TEDs, two -type TSDs, and twelve -type TMDs according to the directions of extra half-planes and the chiral geometries (right- or left-handed screw). The 2PPL images reveal that all threading dislocation types incline in the step-flow direction, the extent of which is more prominent for a larger offcut angle. The TEDs also incline in the directions of their extra half-planes. The inclinations of TSDs perpendicular to the step-flow direction are influenced by chirality, while those of TMDs are influenced by both the directions of the extra half-planes and the chirality. The origins of the dislocation inclinations are discussed taking into account the kinetic interactions between the dislocations and the advancing steps on the growing surface. [ABSTRACT FROM AUTHOR]

We investigated the glide velocities of 30° Si-core partial dislocations for the expansion of double-Shockley stacking faults (DSFs) in heavily nitrogen-doped 4H-SiC crystals at high temperatures of approximately 1000 °C. The heavily doped epilayers grown by chemical vapor deposition were successively annealed. The expansion of DSFs in the heavily doped epilayers was tracked by a photoluminescence (PL) imaging technique. From the PL images obtained after each annealing treatment, the glide velocities of the 30° Si-core partial dislocations were estimated. In particular, temperature dependence and nitrogen-concentration dependence of the dislocation velocities were obtained. We also report the influence of the strain energies of the bounding dislocations on the velocities. Based on the experimental results, the quantitative expression of the dislocation glide is discussed. [ABSTRACT FROM AUTHOR]

We investigated the relationship between the dislocation velocity and the injected carrier concentration on the expansion of single Shockley-type stacking faults by monitoring the electroluminescence from 4H-SiC PiN diodes with various anode Al concentrations. The injected carrier concentration was calculated using a device simulation that took into account the measured accumulated charge in the drift layer during diode turn-off. The dislocation velocity was strongly dependent on the injected hole concentration, which represents the excess carrier concentration. The activation energy of the dislocation velocity was quite small (below 0.001 eV between 310 and 386 K) over a fixed range of hole concentrations. The average threshold hole concentration required for the expansion of bar-shaped single Shockley-type stacking faults at the interface between the buffer layer and the substrate was determined to be 1.6-2.5 x 1016cm-3 for diodes with a p-type epitaxial anode with various Al concentrations. [ABSTRACT FROM AUTHOR]

We investigated the structures and expansion behavior of double-Shockley stacking faults (DSFs) formed in heavily nitrogen-doped 4H-SiC during annealing. Heavily doped epilayers prepared as specimens were successively annealed. Various types of DSFs showing different shapes and dislocation contrasts were found in photoluminescence and synchrotron X-ray topography images. Taking account of every possible stacking sequence forming DSFs, the structures of various types of DSFs were determined from observations by plan-view transmission electron microscopy (TEM) and cross-sectional high-angle annular dark-field scanning TEM. We found that a bounding dislocation enclosing a DSF splits into two partial dislocations (PDs), and their Burgers vectors are identical, while the distance of the two PDs depended on their core structures (30° Si-, 30° C- or 90° C-core). We also discussed the contrast rule for the dislocation consisting of two PDs in the synchrotron X-ray topography images and the mobile PDs for the DSF expansion in the epilayers with different nitrogen concentrations. [ABSTRACT FROM AUTHOR]