Shun Ichiro Tanaka

Optical-capacitance-transient spectroscopy study for deep levels in 4H-SiC epilayer grown by cold wall chemical vapor deposition

Optical cross sections for deep levels in 4H-SiC grown by cold wall chemical vapor deposition (CVD) were investigated by optical-capacitance-transient spectroscopy (O-CTS). By fitting the measured data with the theoretically calculated curve, we obtained optical excitation energy of 1.50 eV for the Z1/2 center. We also obtained its thermal activation energy of 0.61 eV. From these energy values and previously reported the capture barrier and the negative-U property, we drew the configuration coordinate diagram of the Z1/2 center, and then the Frank-Condon shift of 0.96-0.97 eV was estimated. In addition, the EH6/7 center was also characterized by O-CTS measurements. Introduction Crystalline quality of silicon carbide (SiC) has been improved in past decades and thus unipolar devices have already been commercialized. However, the minority carrier lifetime, which affects bipolar device operation, is very short even in high quality SiC crystals. Therefore deep levels, which influence recombination of minority carriers, should be characterized in order to know carrier dynamics in SiC. Usually, the deep level transient spectroscopy (DLTS) is employed for this purpose. The DLTS reveals the activation energy of carriers from deep levels. The optical excitation energy is another important parameter for drawing the configuration coordinate diagrams which graphically shows carrier dynamics at deep levels. In this study we have characterized deep levels in a 4H-SiC epilayer grown by cold wall chemical vapor deposition (CVD) using the optical-capacitance-transient spectroscopy (O-CTS), which observes capacitance transient due to optically excited carriers from deep levels within the depletion region [1]. By performing O-CTS, we obtained optical excitation energies for deep levels in 4H-SiC. Experiments The sample used in this work was an n-type 4H-SiC epilayer grown by horizontal cold wall CVD on a 4H-SiC (0001) Si face substrate with 8° inclination toward <11uf8e520>. The source gases for CVD were silane and propane, and the growth temperature was 1500°C. The source gas ratio (the C/Si ratio) was 1.5. Au was thermally evaporated on the epitaxial layer to form Schottky contacts. The Au contacts were thin enough to be transparent for excitation light, and the contact area was of 1.77 mm. By using the Au contacts, capacitance-voltage (C-V), deep level transient spectroscopy (DLTS) and O-CTS measurements were performed. In the O-CTS measurements, we detected a capacitance transient signal caused by the optical excitation of carriers from deep levels and the signal was converted to an O-CTS spectrum by the rate window scan method. For the excitation light, a 300 W Xe lamp was employed and the light was passed through a monochromater and focused onto the Schottky contact region. The photon flux and the photon energy range of the excitation light are of the order of 10 cms and 1.02-2.38 eV, respectively. Materials Science Forum Online: 2005-05-15 ISSN: 1662-9752, Vols. 483-485, pp 381-384 doi:10.4028/www.scientific.net/MSF.483-485.381

Nanoscale Dynamics at Reactive Wetting Front on SiC

Nanoscale singularity at the reactive wetting front on SiC (0006) was studied using video recorded in situ to clarify the dynamic atomistic behaviours of the brazing and the molten tip spreading on a high-temperature stage of a high-resolution transmission electron microscope. An atomistic process controls the wetting at the front of the spreading film where the classical macroscopic phenomenon never holds true and the singularities are observed in a precursor film. A 0.5-nm-thick precursor film spreading ahead of the main molten alloy on SiC (0006) at 1073 K and continuous spreading of the molten alloy were clearly observed on the SiC (0006) surface with a less than 1-nm-thick amorphous layer. Molten Ti and TiC nanolayers preceded the Ti5Si3 nanolayer at the tip and they traveled continuously at a velocity of 14 nm/sec on the plane perpendicular to SiC (0006). Since Ti atoms in the molten alloy diffuse sufficiently rapidly on the SiC surface to the tip, the formation of these layers may be the rate-determining step of spreading. Discontinuous spreading of the precursor tip on SiC (0006) with a thick amorphous film was observed in contrast to the continuous spreading on SiC with a thin film. This suggests that the spreading of the Ti molten alloy on SiC is also controlled by the dissolution of the amorphous layer.

Nanoparticles of Metastable Copper Nitride Grown by Ar Ion Beam Irradiation

Nanoparticles of metastable copper nitride(Cu3N) have been successfully fabricated from Cu mask using Ar ion ‘transcription method’ which is firstly invented by B.-S. Xu, C. Iwamoto and S.-I. Tanaka in 1996 [1]. The structural and morphological changes with irradiation time are studied by transmission electron microscopy (TEM). The thin film-like crystalline Cu3N which is covered with amorphous or polycrystalline cuprite (Cu2O) layer in the as-received Cu mask plays a role of target. Polycrystalline Cu3N nanoparticles nucleate and grow up to the average size of 15nm after 30 sec-irradiation. Coalesence of 50nm-sized grown Cu3N nanoparticles forms polycrystalline thin film after 2min-irradiation and its growth behavior follows Volmer-Weber mode. As irradiation time increases from 30 sec to 15 min, Cu3N nanoparticles are thought to be grown preferentially along the [111] and [100] directions. Cu2O still remain with Cu3N after 15 min.-irradiation.

Interatomic Potentials for Metal/Metal Wetting Systems

Considering the uniqueness of wetting systems consisting of three components, namely, the surface, liquid and liquid/solid interface, it is desirable to construct interatomic potentials following a consistent policy. To investigate the physical meaning of the behavior in terms of the interatomic potentials, the wetting systems are modeled by simple two-body interatomic potentials derived using ab initio molecular orbital calculations for hypothetical clusters representing the above three components. For In and Sn liquid atoms, spreading occurs on a Cu (111) surface, while in contrast, liquid atoms penetrate the substrate and form a surface alloy in the case of a Pd (111) surface.

Simulation of Liquid/Solid Interfacial Structures in Metal/Ceramics Wetting Systems

Wetting can be regarded as a kind of effective nanostructure-forming process. To control the structure, a study on the relationship between atomic interactions and the resultant wetting behaviors is required. To model the wetting system, two sets of interatomic potentials for Metal/MgO(100) systems are derived from first principles calculation results for the simple configurations. A molecular dynamics method is applied to simulate the system and shows that Al atoms wet better than Sn atoms on the MgO substrate. The tendency is consistent with the experimental contact angles. The interfacial structures are different between these two systems.

Reactive Wetting Dynamics on 6H-SiC Surface with Oxide Layer

Nanoscale singularity at the reactive wetting front on the 6H-SiC (0006) surface with amorphous oxide layer was studied using video recorded in situ to clarify the dynamic atomistic behaviors of the brazing and the molten tip spreading on a high-temperature stage of a high-resolution transmission electron microscope. A 0.5-nm-thick precursor film spreading ahead of the main molten alloy on SiC (0006) at 1073 K and continuous spreading of the molten alloy were clearly observed on the SiC (0006) surface with a less than 1-nm-thick amorphous layer. Molten Ti and TiC nanolayers preceded the Ti5Si3 nanolayer at the tip and they traveled continuously at a velocity of 14 nm/sec on the plane perpendicular to SiC (0006). Since Ti atoms in the molten alloy diffuse sufficiently rapidly on the SiC surface to the tip, the formation of these layers may be the rate-determining step of spreading. Discontinuous spreading of the precursor tip on SiC (0006) with a thick amorphous film was observed in contrast to the continuous spreading on SiC with a thin film. This suggests that the spreading of the Ti molten alloy on SiC is also controlled by the dissolution of the amorphous layer.

Stress Corrosion in Mg-Al-Mn Alloy with Residual and Concentrated Stresses

The effects of residual and concentrated stress on a corrosion mode of a thixomolded Mg-Al-Mn alloy (AM-60) in a NaCl aqueous solution were studied in order to elucidate the corrosion mechanism, the time-dependent characteristics of corrosion products and an actual concentrated stress value in a grain (a-Mg) and a grain boundary (Mg 17 Al 12 ) around an artificial notch. The following results were obtained: (1) Stresses in α-Mg and Mg 17 Al 12 phases existing in a thixomolded Mg alloy plate can be measured separately using the X-ray sin 2 ψ method in the Cr-Ka irradiation area of 1.0 mm in diameter. The recommended 2θ 0 values for stress measurement are 140.0 deg for α-Mg (202) and 133.4 deg for Mg 17 Al 12 (660). (2) The measured residual stresses in a AM-60 plate were about 30 MPa compressive in α-Mg and about 30 MPa tensile in Mg 17 Al 12 phase balancing microscopic region. (3) Even in a lapped Mg-alloy a localized micro-cell exists between α-Mg grain in compressive stress state and grain boundary made of Mg 17 Al 12 in tensile stress state. The proposed model explains well the fact that the grain provides Mg 2+ ions for solution whereas grain boundary acts as a nucleation point of the hydrogen gas bubbles. Corrosion in Mg alloy occurs essentially while it is in contact with any aqueous solution. (4) Corrosion mode of AM-60 in 0.9% NaCI aqueous solution was quite different depending on the sort (sign) of the stresses: a fine and dense amorphous hydro-oxide formed in the region of tensile stress whereas a coarse amorphous oxide formed in that of compressive stress. The former reaction product formed in a short time, but the latter grew gradually.

Twist Angle Dependence of Josephson Junction Effect Measured in the [001] Twist Boundary of Bi2Sr2CaCu2Ox Superconductor Bicrystals

We present a study of the twist angle dependence of the Josephson junction effect measured in the [001] twist boundary of Bi 2 Sr 2 CaCu 2 O x (S) superconductor bicrystals. Josephson junctions that have both direct (S/S) and indirect, thin silver layer (S/Ag/S) structures were fabricated by diffusion bonding of the (001) surfaces of two flaky S single crystals. A small junction area measuring 100-200 μm in diameter, was fabricated by Ar ion selective etching of one of the two thin single crystal superconductor S flakes. A 2-100-nm-thick silver layer was deposited on the (001) planes by sputtering prior to the joining of the surfaces to produce an S/Ag/S junction It was clarified that the critical current, I c , of the S/Ag/S junction is a function of temperature with I c =I OH (1-T/T c ) 2 for T≥0.6T c , and I c =I OL (-GT 1/2 ) for T≤0.5 T c . In these junctions, a high I c value was obtained when the [001] twist angle θ was 23°, 28° or 37°. Our results indicated that the Josephson effect is strongly influenced by both the temperature T and the [001] twist angle θ i the S/Sand S/Ag/Sjunctions.