Isamu Nashiyama

Intrinsic Defects in Cubic Silicon Carbide

Irradiation of fast particles like 1 MeV electrons and 2 MeV protons was made for single crystalline cubic silicon carbide (3C-SiC) grown epitaxially on Si by chemical vapor deposition in order to introduce point defects in the material. Intrinsic point defects in 3C-SiC have been characterized by electron spin resonance (ESR), positron annihilation spectroscopy (PAS), Hall and photoluminescence (PL) techniques. The structure and annealing behavior of intrinsic defects, e.g. monovacancies at silicon and carbon sublattice sites, are described based on the results obtained by ESR and PAS. The contributions of such point defects to electrical and optical properties of 3C-SiC are discussed using the Hall and PL results, with a brief review of published work.

Electron spin resonance in electron-irradiated 3C-SiC

Electron‐irradiation‐induced defects in epitaxially grown 3C‐SiC crystals have been studied by electron‐spin‐resonance (ESR) measurements. The results indicate the presence of an isotropic ESR center that consists of five lines equally spaced at about 1.5 G and has a g value of 2.0029±0.0001. Isochronal and isothermal annealing of electron‐irradiated 3C‐SiC showed that this center was annealed at three stages (150, 350, and 750 °C) and that the 750 °C stage exhibited first‐order reaction with an activation energy of 2.2±0.3 eV.

Single-event current transients induced by high energy ion microbeams

Focused high-energy ion microbeams were applied to the study of the basic mechanisms of single-event upset. Waveforms of the current transients induced by He-, C-, O- and Fe-ion strikes on silicon diodes were measured by applying extremely low beam currents of an order of 10 fA and a wide-bandwidth digitizing sampling technique. Total collected charges are evaluated from the transient currents as a function of LET (linear energy transfer), bias voltage, and doping level, and are compared with theoretical values calculated using conventional single-event models. It is found that irradiation effects on the total collected charges can be explained by the introduction of displacement atoms calculated using Coulomb potential and the Kinchin-Pease model. >

Radiation induced defects in CVD-grown 3C-SiC

Radiation-induced defects in 3C-SiC epitaxially grown by a chemical vapor deposition method were studied with the electron spin resonance (ESR) technique. A 15-line ESR spectrum was observed in 2-MeV proton and 1-MeV electron irradiated 3C-SiC when the magnetic field was applied parallel to the axis. This spectrum, T1, which has an isotropic g-value of 2.0029+or-0.0001, was interpreted by simultaneous hyperfine interactions of a paramagnetic electron with the surrounding /sup 13/C at four carbon sites and /sup 29/Si at 12 silicon sites. The T1 spectrum appeared to arise from a point defect at a silicon site. The observed hyperfine interactions with neighboring /sup 13/C and /sup 29/Si nuclei are discussed in terms of a simple molecular-orbital treatment of the defect. >

Molybdenum substitutional doping and its effects on phase transition properties in single crystalline vanadium dioxide thin film

Molybdenum (Mo) doped vanadium dioxide thin films were synthesized using a Mo striped vanadium (V) target during pulsed laser ablation process. The film structure was characterized by high resolution x-ray diffraction, x-ray rocking curve and Rutherford backscattering/channeling measurements. The results show that the full width at half magnitude of the x-ray rocking curve is as narrow as 0.0074°, comparable to that of the (0001) sapphire substrate, 0.0042°, in this study. The ratio of the aligned-to-random backscattered yield reaches 5%, implying that the growth is that of the single crystalline epitaxy. The result of angular scans for both V and Mo atomic channelings reveals that Mo atoms successfully take sites of the V sublattice as a substitutional dopant. It has been noted that the degradation of the phase transition properties of the film upon doping is closely related to the conductivity in the semiconductor phase.

Photoluminescence of radiation induced defects in 3C‐SiC epitaxially grown on Si

Photoluminescence (PL) has been used to study defects introduced by 1‐MeV‐electron irradiation in cubic silicon carbide (3C‐SiC) films epitaxially grown on Si substrates by means of chemical vapor deposition. A dominant PL line of 1.913 eV observed in 3C‐SiC irradiated with electrons was found to disappear at annealing stages of ≊100 and 700 °C. The annealing stages of the 1.913 eV PL center and its fraction annealed at each annealing stage were in good agreement with those obtained for the T1 electron spin resonance center, which is attributed to isolated vacancies at silicon sublattice sites [H. Itoh, M. Yoshikawa, I. Nashiyama, S. Misawa, H. Okumura, and S. Yoshida, IEEE Trans. Nucl. Sci. NS‐37, 1732 (1990)]. This result indicates that the 1.913 eV PL line arises from silicon vacancies in 3C‐SiC. The characteristics of other PL lines induced in 3C‐SiC epilayers by irradiation are also discussed.

Electron spin resonance study of defects in CVD-grown 3C-SiC irradiated with 2MeV protons

Electron spin resonance (ESR) was used to study defects induced by 2MeV-proton irradiation in cubic silicon carbide (3C-SiC) epitaxially grown on Si substrates by chemical vapor deposition. A new ESR signal labeled T5 was observed at temperatures lower than ≈100 K in Al doped, p-type 3C-SiC epilayers irradiated. The T5 signal has anisotropic g-values of g1 = 2.0020 ± 0.0001, g2 = 2.0007 ± 0.0001,and g3 = 1.9951 ± 0.0001. The principal axes of the g-tensor were found to be along the 〈100〉 directions, indicating that the T5 center has D2 symmetry. Isochronal annealing of the irradiated epilayers showed that the T5 center was annealed at temperatures around 150° C. A tentative model is discussed for the T5 center.

Effects of gamma‐ray irradiation on cubic silicon carbide metal‐oxide‐semiconductor structure

Radiation effects on cubic silicon carbide (3C‐SiC) metal‐oxide‐semiconductor (MOS) structures have been studied with high‐frequency capacitance‐voltage measurements. It was found that interface traps are generated at the 3C‐SiC/SiO2 interface and oxide‐trapped charges are built up in the oxide by 60Co gamma‐ray irradiation. The generation of the interface traps and the oxide‐trapped charges are affected by bias polarity applied to the gate electrode during the irradiation. Absorbed dose dependencies of their generation are discussed comparing with those of Si MOS structures.

Effects of micro-beam induced damage on single-event current measurements

Abstract Radiation damage introduced by the impact of heavy-ion micro-beams has been studied in connection with its effects on single-event transient currents, which are measured by the digitizing sampling method combined with focused ion micro-beams. It is found that the single-event currents decrease with increasing the repetition number of measurements and that there exists a linear relationship between the ion fluence and the reciprocal of the normalized single-event charge. A single-event damage equation is deduced, where the damage constant is proportional to the defect density calculated using either the Kinchin-Pease model or the TRIM code. Experimental techniques are developed to eliminate the effect of radiation damage.

Mechanism of anomalous degradation of silicon solar cells subjected to high-fluence irradiation

We have found anomalous degradation of electrical performance in silicon solar cells designed for space use due to high-fluence irradiation of charged particles, e.g., 1 MeV-electrons of /spl sim/10/sup 17/ e/cm/sup 2/ and 10 MeV-protons of /spl sim/10/sup 14/ p/cm/sup 2/. This anomalous degradation has two typical features, i.e., sudden-drop-down of electrical performances and slight recovery of the short circuit current I/sub SC/ just before the sudden-drop-down, which cannot be explained by a conventional model based on decrease of the minority-carrier life-time. In order to account for these features, we propose a new model, in which decreases of the majority-carrier concentration and the minority-carrier mobility are considered in addition to that of the minority-carrier life-time. The anomalous degradation observed is well represented by this model.