Junji Yamanaka

High-resolution observations of an amorphous layer and subsurface damage formed by femtosecond laser irradiation of silicon

Using transmission electron microscopy (TEM), we observed the micro- and nanostructures of silicon after irradiation by ∼150fs duration pulses centered at 800nm wavelength. Specimens irradiated with a single pulse of 11J∕cm2 fluence and with five pulses, each with a fluence of 1.3J∕cm2, exhibited various structures which included amorphous phases. The amorphous phases were pure silicon, as was revealed by high-resolution TEM imaging, nanobeam diffraction patterns, high-angle annular dark-field images, conventional diffraction images, and energy-dispersive x-ray spectra. Irradiation with a single pulse of 1.5J∕cm2 produced neither amorphous material nor lattice defects. Single-pulse irradiation at a fluence of 33J∕cm2 and irradiation by four pulses at 11J∕cm2 led to substantial subsurface damage around the center of the laser spot. It is concluded that multiple-pulse irradiation produces crystallographic damage more readily than a single pulse.

Fabrication of high-quality strain-relaxed thin SiGe layers on ion-implanted Si substrates

We fabricated high-quality strain-relaxed thin SiGe layers by Ar ion implantation into Si substrates before epitaxial growth. The surface of 100-nm-thick Si0.8Ge0.2 layers, the relaxation ratio of which was more than 80%, was found to be very smooth, with a rms roughness of 0.34 nm. Cross-sectional transmission electron microscopy analysis confirmed that strain-relieving dislocations were effectively generated due to the ion-implantation-induced defects and confined in the vicinity of the heterointerface, resulting in a dislocation-free SiGe surface. Moreover, in-plane strain-field fluctuation was found to be largely reduced by this ion implantation method.

Surface graphitization of furan-resin-derived carbon

Furan-resin-derived carbon generally produces a glass-like carbon having entangled graphene layers (graphite structure) after high temperature heat-treatments. However, Raman spectroscopy reveals that it produces well-graphitized thin skins on surfaces. The graphitization is promoted on the faces that are formed at lower heat-treatment temperatures. Fractured faces of specimens pre-heat-treated at 1000°C result in well-developed structures in graphitization after re-heat-treatment at 3000°C. It is considered that the surfaces, i.e. free faces, play an important role in the graphitization.

Low nickel germanide contact resistances by carrier activation enhancement techniques for germanium CMOS application

We fabricated and studied nickel germanide (NiGe) contacts on both n- and p-type germanium (Ge) substrates by applying the carrier activation enhancement (CAE) technique. We achieved a high electron concentration of 8.6 × 1019 cm−3 using a P/Sb co-implant and a record-high hole concentration of 8.4 × 1020 cm−3 using a Ge preamorphization implant and a boron implant. We used the circular transfer length method and two-dimensional DC simulation to determine the specific contact resistivity (ρc). Using the CAE technique, we obtained low ρc values of 6.4 × 10−7 Ω cm2 for the NiGe/n+-Ge contact and 4.0 × 10−8 Ω cm2 for the NiGe/p+-Ge contact. Theoretical calculation of ρc shows that, to achieve a ρc of 1 × 10−8 Ω cm2 as required by the International Technology Roadmap for Semiconductors for the year 2015, contacts on p+-Ge need contact process optimization, while contacts on n+-Ge need further CAE improvement and/or Schottky barrier height reduction.

Strained Si n-channel metal-oxide-semiconductor field-effect transistors formed on very thin SiGe relaxed layer fabricated by ion implantation technique

Strained Si n-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) were fabricated on the Si0.83Ge0.17 relaxed thin layer formed by ion implantation technique. Although the SiGe thickness was as small as 100nm, the relaxation was highly enhanced thanks to the pre-ion-implantation into the Si substrate and the strained Si channel with very smooth surface was obtained. A nMOSFET was fabricated in this structure and 100% drive current improvement and 60% mobility enhancement over the control Si MOSFET were achieved. This indicates that the ion implantation technique is very promising for realization of relaxed-SiGe-based devices with very high performances.

Influence of Ge atoms on mobility and junction properties of thin-film transistors fabricated on solid-phase crystallized poly-SiGe

The transport properties of thin-film transistors (TFTs) fabricated on polycrystalline-SiGe films, which were formed by solid-phase crystallization of amorphous-Si1−xGex (x=0, 0.3, 0.5, and 0.7) films, were studied. The mobility of TFTs increases as the Ge concentration increases from 0% to 30% and the mobility decreases as the Ge concentration exceeds 30%, which agrees with the Ge concentration dependence of the grain size. The generation rates and the effective lifetimes of electron-hole pairs at the p-n junctions of TFTs increase and decrease monotonically with increasing Ge concentration, respectively, which means that the grain boundary defects in p-n junctions increase with increasing Ge concentration.

Synthesis of electrochromic praseodymium-doped vanadium oxide films by molten salt electrolysis

Abstract The praseodymium oxide (PrxOy) and praseodymium-doped vanadium oxide films were synthesized onto the ITO electrode by electrolysis in molten dimethylsulphone (DMSO 2 )–PrCl 3 , DMSO 2 –PrCl 3 –LiNO 3 and DMSO 2 –PrCl 3 –V 2 O 5 system at the temperature about 150 °C. The obtained films were characterized by X-ray diffraction (XRD) and spectroscopic measurements. Moreover, the electrochromic (EC) reactions of the oxide films in solution of propylene carbonate (PC) were investigated. The absorption spectrum of praseodymium oxide films obtained by electrodeposition in DMSO 2 –PrCl 3 systems had 560- and 390-nm peaks due to mixed valence of Pr 3+ and Pr 2+ . And the effect of lithium ion intercalation or deintercalation was shown in color change of oxide films. The cyclic voltammograms (CV) of praseodymium-doped vanadium oxide films obtained by electrodeposition in DMSO 2 –PrCl 3 –V 2 O 5 systems showed the good reversibility of EC reactions.

Effects of deposition rate on the structure and electron density of evaporated BaSi2 films

In order to control the electrical properties of an evaporated BaSi2 film, which is an emerging candidate for the absorber-layer material of earth-abundant thin-film solar cells, we have investigated the effects of deposition rate on the produced phases, microstructure, and carrier density of the thin films grown by thermal evaporation of BaSi2. X-ray diffraction results show that a high substrate temperature is necessary for BaSi2 formation at a high deposition rate, which is discussed from viewpoints of vapor composition and diffusion time. Microstructural characteristics such as grain size of 30–120 nm, oxide particle arrays present around the interface, and partial oxidation at a low substrate temperature are revealed by cross-sectional transmission electron microscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy combined with an energy-dispersive X-ray spectroscopy. With increasing deposition rate, the crystalline quality of BaSi2 is found to improve, as evidenced by a decrease...

Strain-Field Evaluation of Strain-Relaxed Thin SiGe Layers Fabricated by Ion Implantation Method

Strain-relaxed thin SiGe layers grown on ion-implanted Si substrates were evaluated by X-ray diffraction reciprocal space mapping and spatially resolved micro-Raman spectroscopy. It was found that implantation-induced defects effectively facilitated the misfit dislocation generation and multiplication during postannealing after growth, leading to large and homogeneous strain relaxation. While in-plane strain-field fluctuation with crosshatch morphology was clearly observed in the SiGe layer without implantation, a very uniform strain distribution without such morphology was realized in the sample with implantation. These results indicate that the ion implantation method is a very promising approach for realization of high-quality and extremely thin buffer layers.

Post-annealing effects on the surface structure and carrier lifetime of evaporated BaSi2 films

To improve the surface quality for photovoltaic applications, we have investigated the effects of post-annealing on the surface structure and carrier lifetime of evaporated BaSi2 films. Structural characterizations by Raman spectroscopy and X-ray diffraction analysis show that there is an optimum post-annealing duration for fabricating a homogeneous film up to around the surface. By detailed surface analysis by X-ray photoelectron spectroscopy, the existence of a surface oxidation layer consisting of BaCO3 and barium silicate is revealed, and the thickness of the oxidation layer is found to be smallest for the optimum post-annealing duration. These surface structural changes are discussed from a thermodynamic viewpoint. Carrier lifetime is also investigated by the microwave-detected photoconductivity decay method, which shows that the structural change around the surface by post-annealing has negligible effects on carrier lifetime, possibly because the silicate layer covers the BaSi2 surface irrespective of post-annealing duration.