Shoichiro Tanigawa

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.

Annealing studies of low‐temperature‐grown GaAs:Be

The isochronal and isothermal annealing characteristics of acceptor‐doped GaAs:Be grown at low substrate temperatures (300 °C) by molecular‐beam epitaxy (LTMBE) have been studied. The Be was introduced in a range of concentrations from 1016 to 1019 cm−3. Electrical measurements of as‐grown material up to the highest Be concentration of 1019 cm−3 show that no free holes are contributed to the valence band even though Raman spectroscopy of the Be local vibrational mode indicates that the majority of the Be impurities occupy substitutional sites. It is proposed that Be acceptors are rendered inactive by the high concentration of AsGa‐related native donor defects present in LTMBE material. The concentration of AsGa‐related defects in the neutral charge state was estimated from infrared absorption measurements to be as high as 3×1019 cm−3. A distinct annealing stage at 500 °C, similar to that found in irradiation‐damaged and plastically deformed GaAs, marks a rapid decrease in the concentration of AsGa‐related...

Low temperature buffer growth for modulation doped SiGe/Ge/SiGe heterostructures with high hole mobility

Characterization of low-temperature (LT) Si and SiGe on the LT-Si grown by molecular beam epitaxy was carried out. Positron annihilation spectroscopy showed that the size of point defects in LT-Si grown at 4008C was much larger than divacancies. These vacancy clusters were considered to play an important role in strain relaxation of SiGe layers on the LT-Si. Although Si0.7Ge0.3 layer grown on the LTSi had excellent qualities compared with the graded buffer Si12xGex layer, it was found to degrade when Ge content was increased up to x , 0:5 but be improved again above this content. The surface morphology also changed at this Ge content from crosshatch pattern to smooth surface with increasing Ge content. p-type modulation doped structures with pure-Ge channels were fabricated on the LT-buffers and the transport properties were found to significantly depend on the surface morphology. The reason that the rough surface gave rise to rather high mobility was discussed in the terms of roughness scattering. q 2000 Published by Elsevier Science S.A. All rights reserved.

Stretching of slow positron pulses generated with an electron linac

A facility for generating a high intensity slow positron beam using an electron linear accelerator has been constructed. A conversion efficiency of 6×10−7 slow positrons per incident electron has been obtained for 75 MeV electrons. Storage and stretching of pulsed slow positrons have been successfully carried out with a Penning trap.

Transition and relaxation processes of polyethylene, polypropylene, and polystyrene studied by positron annihilation

Transition and relaxation processes of polyethylene (PE), polypropylene (PP), and polystyrene (PS) were studied by the positron annihilation technique. From measurements of lifetime spectra of positrons as a function of temperature, the lifetime of ortho-positronium, τ3, and its intensity, I3, were found to increase above 260 K for PP. This fact was attributed to a cooperative motion of large segments of molecules above the glass transition temperature, Tg. For PE, above Tg (140 K), the value of τ3 increased, but the temperature coefficient of I3 was negative below 230 K. From this fact, for PE, the molecular motions that cause the glass transition were associated with a rearrangement of molecules by local motions such as kink motions. The discrepancy between the results for PE and PP was attributed to the presence of methyl groups in PP and the resultant suppression of the local motions. For PS (Tg = 340 K), the molecular motions were found to start above 260 K, but those were suppressed by an interphenyl correlation. Detailed annihilation characteristics of positrons in polymers were also discussed.

Evidence for the passivation effect in (NH4)2Sx‐treated GaAs observed by slow positrons

We applied slow positrons to the as‐etched GaAs and/or the (NH4)2Sx‐treated GaAs. The results show that a thin oxide film is easily formed on the surface of as‐etched GaAs as soon as the etched surface is exposed to air for several minutes before the measurement. On the other hand, the monolayer of chemisorbed sulfur atoms in the (NH4)2 Sx‐treated GaAs is effective in protecting the clean surface from the adsorption of oxygen atoms. The mean diffusion length of positrons is not affected by the conditions of the surface treatment. This implies that the centers for the trapping of a positron are not created below the free surface by those treatments.

Impurity effect on the creation of Ga vacancies in a Si‐doped layer grown on Be‐doped GaAs by molecular‐beam epitaxy

The impurity effects on the creation of Ga vacancies in Si‐doped GaAs grown on a Be‐doped epilayer by molecular‐beam epitaxy were investigated through slow positron beam measurements. Doping of Si impurities enhances the creation of Ga vacancies in GaAs. The experimental results support the theoretical prediction of the creation of Ga vacancies in terms of the change in the Fermi‐level position by the Si doping into GaAs, and also suggest that a Si atom diffuses in GaAs as a neutral complex of SiGa‐VGa rather than that of SiGa‐SiAs. The change in the S parameter distribution at the interface between Si and Be‐doped regions is explained by the Be carryforward phenomenon which occurs during the growth of Si‐doped GaAs on a Be‐doped epilayer.

Positron Lifetime Study of Neutron-Irradiated Molybdenum

Annealing behavior of fast-neutron-irradiated molybdenum was studied by means of positron lifetime technique. It was found that Stage III annealing can be mainly identified as the vacancy migration process from the detailed analyses of data. The void growth after successive high temperature annealings was clearly detected through the changes of positron lifetime parameters. An attempt to analyse the size distribution of voids from positron lifetime spectra was presented, and discussions on the evaluation of void concentration from positron data are also given.

A Study of Vacancy-Type Defects in B+-Implanted SiO2/Si by a Slow Positron Beam

Variable-energy (0~30 keV) positron beam studies have been carried out on 80 keV B+-implanted SiO2(43 nm)/Si specimens. Doppler broadening profiles of the positron annihilation as a function of the incident positron energy were shown to be quite sensitive for the detection of vacancy-type defects introduced by B+-implantation. The average depth of the defected regions was found to shift towards the surface of the specimen with increasing the dose of B+ ions. This effect is attributed to the accumulation of vacancy-type defects at the SiO2/Si interface. Dominant defect species were identified as vacancy clusters by their annealing stage.

Positron Study of Electron Momentum Density and Fermi Surface in Titanium and Zirconium

The three dimensional electron-positron momentum densities have been obtained on Ti and Zr from measurements of two dimensional angular correlation of positron annihilation radiation followed by an image reconstruction technique based on direct Fourier transformation. Augmented-plane wave band structure calculations have been carried out and the results are compared with the experiments. Agreement between the experiment and the theory leads to a conclusion that both Ti and Zr have electron surface sheets which are centered at H and hole surface sheets which are running along the \(\varGamma\)-A axis.