Shin-ichiro Uekusa

Micrograin structure influence on electrical characteristics of sputtered indium tin oxide films

The relationship between micrograin structures and electrical characteristics of sputtered indium tin oxide (ITO) films was investigated. Micrograin structures were observed by a high resolution scanning electron microscope. Electrical characteristics were evaluated by four point probe resistance measurement and Hall effect measurement. Low resistivity ITO films had domain structures. One domain consisted of many sputter grains having the same orientation. The resistivity decreased with increasing domain size. The domain boundary might cause scattering for conduction electrons. Therefore, larger domain ITO films had a higher Hall mobility. The minimum resistivity was 1.8×10−4 Ω cm, deposited at a sputtering voltage of −250 V and a 250 °C deposition temperature. The electron conduction mechanism in domain structured ITO films was taken into consideration.

Postdeposition Annealing Influence on Sputtered Indium Tin Oxide Film Characteristics

The influence of postdeposition annealing on sputtered indium tin oxide (ITO) film characteristics were investigated. The annealing experiments were carried out in air or vacuum atmosphere. Both air and vacuum annealing decreased the resistivity up to heat treatment of 200° C. Over 300° C treatment, air annealing increased resistivity whereas vacuum annealing decreased it. It was clarified that the resistivity depended on the carrier concentration. The lowest resistivity attained was 1.3×10-4 Ωcm, with film deposited on a 250° C heated substrate and annealed in vacuum atmosphere at 300° C. Transmittance was improved in both air and vacuum annealing. In air and vacuum annealing, crystallinity improved with increasing annealing temperature. The surface topography showed no changes with air or vacuum annealing.

Structural and optical characterization of β-FeSi2 layers on Si formed by ion beam synthesis

Abstract Structural and optical properties have been investigated for surface β-FeSi2 layers on Si(100) and Si(111) formed by ion beam synthesis using 56Fe ion implantations with three different energies (140–50 keV) and subsequent two-step annealing at 600 °C and up to 915 °C. Rutherford backscattering spectrometry analyses have revealed Fe redistribution in the samples after the annealing procedure, which resulting in a Fe-deficient composition in the formed layers. X-ray diffraction experiments confirmed the existence of /gb-FeSi2 by annealing up to 915 °C, whereas the phase transformation from the β to α phase has been induced at 930 °C. In photoluminescence measurements at 2 K, both β-FeSi 2 Si (100) and β-FeSi 2 Si (111) samples, after annealing at 900–915 °C for 2 h, have shown two dominant emissions peaked around 0.836 eV and 0.80 eV, which nearly coincided with previously reported PL emissions from the sample prepared by electron beam deposition. Another β-FeSi 2 Si (100) sample has shown sharp emissions peaked at 0.873 eV and 0.807 eV. Optical absorption measurements at room temperature have revealed the allowed direct bandgap of 0.868–0.885 eV as well as an absorption coefficient of the order of 104 cm−1 near the absorption edge for all samples.

Formation of four new shallow emissions in Mn+ ion‐implanted GaAs grown by molecular beam epitaxy having extremely low concentration of background impurities

Mn+ ions were implanted into ultrapure GaAs layers grown by molecular beam epitaxy. 2 K photoluminescence revealed that in addition to the well‐established Mn‐related deep acceptor emission at ∼1.41 eV, four new shallow emissions denoted by (Mn°, X), ‘‘G’’, ‘‘H’’, and (D, A)2 are formed in the near band edge when the Mn concentration [Mn] exceeds 3×1016 cm−3. Both ‘‘G’’ and ‘‘H’’ exhibit no energy shift with growing [Mn] up to 1×1019 cm−3. In contrast, for shallow acceptor‐ (such as C) doped GaAs with extremely low background concentrations of donor impurities, a series of [g‐g]‐like energy levels, which present strong energy shifting with increasing acceptor concentration, are universally formed. These results show that pairs between deep Mn acceptors do not produce such [g‐g] like energy levels.

Ion‐beam doping of GaAs with low‐energy (100 eV) C+ using combined ion‐beam and molecular‐beam epitaxy

A combined ion‐beam and molecular‐beam‐epitaxy (CIBMBE) system has been developed. This system consists of an ion implanter capable of producing ions in the energy range of 30 eV–30 keV and conventional solid‐source MBE. As a successful application of CIBMBE, low‐energy (100 eV) carbon ion (C+) irradiation during MBE growth of GaAs was carried out at substrate temperatures Tg between 500 and 590 °C. C+‐doped layers were characterized by low‐temperature (2 K) photoluminescence (PL), Raman scattering, and van der Pauw measurements. PL spectra of undoped GaAs grown by CIBMBE revealed that unintentional impurity incorporation into the epilayer is extremely small and precise doping effects are observable. CAs acceptor‐related emissions such as ‘‘g,’’ [g‐g], and [g‐g]β are observed and their spectra are significantly changed with increasing C+ beam current density Ic. PL measurements showed that C atoms were efficiently incorporated during MBE growth by CIBMBE and were optically well activated as an acceptor in...

Photoluminescence and Raman scattering studies of 2 MeV Yb+‐implanted InP as a function of etching depth

In ion‐implanted semiconductors, details of the defects involved and annealing mechanisms which determine the final disorder structure are complicated and difficult to characterize. To investigate the residual defect distribution of the implanted layers, optical experiments, photoluminescence, photoluminescence excitation, and Raman scattering have been performed on 2 MeV Yb+‐implanted InP and subsequently annealed at 750 °C for 15 min as a function of chemical etching depth (d) down to d=4.51×Rp (Rp: projected range =410 nm). Their results were compared with those obtained from Rutherford backscattering spectrometry (RBS) channeling analysis. The above optical experiments showed that two residual defective regions are present at depths of d=0.34–0.80×Rp and about d=2.56×Rp, whereas RBS channeling analysis indicated the existence of the only former region. We assign the type of the two defective regions to ‘‘clamshell’’ defects in the former region and to ‘‘end‐of‐range’’ defects in the latter region.

Optical and crystalline properties of Yb implanted InP

The effect of the annealing temperature on the optical and crystalline properties of 2 MeV Yb+ ion‐implanted InP are systematically studied by photoluminescence (PL), photoluminescence excitation (PLE), selectively excited photoluminescence (SPL), and Raman scattering measurements. PL measurements present that the recrystallization of the implanted layers and the optical activation of Yb3+ begin at 450 °C and 500–550 °C, respectively. It is demonstrated from the Raman experiments that there is a continuous restoration in radiation damages when the as‐implanted samples are annealed at temperatures between 450 °C and 550 °C. However, beyond 550 °C, the crystalline quality near the surface is shown to be degraded. For an explanation of this point, a comparison with optical micrographs is conducted. From the PLE spectra of Yb‐related luminescence, it is concluded that the highest crystalline quality of Yb+ implanted layers could be achieved by annealing at 750 °C for 15 min. In the PLE spectra, a band observe...

Low energy (100 eV) C+ ion doping into GaAs using combined ion beam and molecular beam epitaxial technology

Low‐energy (100 eV) carbon ion (C+) irradiation during molecular beam epitaxy of GaAs was carried out using combined ion beam and molecular beam epitaxy (CIBMBE) technology for the growth temperature (Tg) range between 500 and 590 °C. Carbon incorporation was identified by both low‐temperature (2 K) photoluminescence and Hall effect measurements. In the PL spectra, two well‐established specific emissions, ‘‘g’’ and [g‐g], which are closely related to acceptor impurities, were observed for the above Tg range. The results indicate that carbon was both optically and electrically well activated as an acceptor even at Tg as low as 500 °C. Maximum net hole concentration, ‖NA‐ND‖, as high as 3×1018 cm−3 was obtained with no appreciable radiation damages and undesired impurity contamination.

Transverse second‐order mode oscillations in a twin‐stripe laser with asymmetric injection currents

Transverse second‐order mode oscillations in asymmetrically pumped twin‐stripe lasers are described. Both far‐field patterns and near‐field patterns have two peaks, and a stronger peak appears on the weakly pumped side. The dependence of light intensity in one peak of the far‐field pattern on the current into one stripe shows threshold characteristics. On application of rectangular pulse to one stripe, switching from one peak to the other occurs within 5 ns.

Fabrication of heterostructure p-β-Fe0.95Mn0.05Si2/n-Si diodes by Fe+ and Mn+ co-implantation in Si(100) substrates

We report on the structural and electrical properties of iron silicides in the transformation process from e-FeSi to β-FeSi2 and show the electrical characteristics of heterostructure p-β-Fe0.95Mn0.05Si2/n-Si diodes formed by high-dose Fe+ and Mn+ co-implantation in Si (100). A mixture of polycrystalline e-FeSi and β-FeSi2 with a thickness of 75 nm and the resistivity of ρ=4.9×10−4 Ω·cm was in-situ formed during Fe+-implantation in Si (100) at 350°C. These samples were annealed at Ta=400–1100°C and characterized by Rutherford backscattering spectrometry, van der Pauw and X-ray diffraction. Single β-FeSi2 layers with ρ=0.31 Ω·cm were formed after annealing at Ta =600°C. Although the samples with Ta<600°C exhibited p-type conductivity (hole concentrations of p=5.3–11×1020 cm−3 and hole mobilities of μh=8.7–32 cm2/V/s), the samples with Ta≧600°C presented n-type conductivity (n=4.2–14×1016 cm−3 and μe=220–520 cm2/V/s). The origin of p-type conductivity may be due to contribution of Fe-rich β-FeSi2, while that of the electron carrier could be related to the formation of stoichiometric β-FeSi2, in which the predominant impurity phosphorous atoms remaining in the n-Si substrates could be electrically activated as donors in β-FeSi2 by high-temperature annealing. The I–V and C–V characteristics of the p-β-Fe0.95Mn0.05Si2/n-Si(100) diodes indicated that the impurity distribution of the pn junction is linearly graded, which leads to a high ideality factor of η=4.4.