Manabu Ishimaru

Unexpectedly low thermal conductivity in natural nanostructured bulk Ga2Te3

Here we introduce a promising thermoelectric material: Ga2Te3 with unexpectedly low thermal conductivity, in which certain kinds of superlattice structures naturally form. Two-dimensional vacancy planes with approximately 3.5nm intervals exist in Ga2Te3, scattering phonons efficiently and leading to a very low thermal conductivity, comparable to its calculated minimum. Ga2Te3 and related materials hold great promise in the field of thermoelectrics.

Transmission-electron diffraction by MeV electron pulses

We have developed a compact ultrafast electron diffractometer, consisting of a laser-driven rf photocathode that generates 3.0 MeV probe electron pulses, three-stage lens optics, and a custom-built detector for relativistic electrons. High-quality single-shot transmission electron diffraction has been detected from 180-nm-thick Si single crystals, with an excellent special resolution for diffracted beams; the spot width of 0.02 A−1 is obtained. The pulse width is estimated to be 100 fs in duration. Characteristics of the electron beam and other diffractometer features are discussed.

Electron field emission from GaN nanorod films grown on Si substrates with native silicon oxides

GaN nanorod films have been grown on Si(001) substrates with native silicon oxides by radio-frequency plasma-enhanced molecular beam epitaxy. GaN nanorod films are made up of single-crystalline nanorods with a so-called (0001) fiber-like texture. Each nanorod is elongated along c axis in perpendicular to the substrate surface and has no preferential axis in film plane. Excellent electron field emission characteristics were observed for the fabricated GaN nanorod films with a field emission threshold as low as 1.25V∕μm at a current density of 0.1μA∕cm2 and a field emission current density as high as 2.5mA∕cm2 at an applied field of 2.5V∕μm. These excellent characteristics are attributed to the geometrical configuration of nanorods and their good crystalline quality as well as the low electron affinity of GaN.

Atomistic structures of metastable and amorphous phases in ion-irradiated magnesium aluminate spinel

Ion-beam-induced microstructures in magnesium aluminate (MgAl2O4) spinel have been examined using transmission electron microscopy (TEM). Irradiations were performed at cryogenic temperature (~120 K) on MgAl2O4 spinel single-crystal surfaces with (111) orientation, using 180 keV neon (Ne+) ions to ion fluences ranging from 1016 to 1017 Ne+ cm-2. Cross-sectional TEM observations indicated that the MgAl2O4 spinel transforms first into a metastable crystalline phase and then into an amorphous phase under these irradiation conditions. On the basis of selected-area electron diffraction and high-resolution TEM, we concluded that Ne-ion-beam irradiation induces an ordered spinel-to-disordered rock-salt-like structural phase transformation. Atomistic structures of amorphous MgAl2O4 were also examined on the basis of atomic pair distribution functions. We compared the experimentally obtained results with previous theoretically calculated results for the metastable and amorphous phases of MgAl2O4, and discussed the validity of the proposed ion-beam-induced structural changes in MgAl2O4 spinel.

Local structure analysis of Ge-Sb-Te phase change materials using high-resolution electron microscopy and nanobeam diffraction

As-sputtered and melt-quenched amorphous structures together with the laser-induced crystallized structure of Ge-Sb-Te thin films were investigated using high-resolution electron microscopy (HREM) and nanobeam electron diffraction (NBED). Each of the Ge-Sb-Te thin films was embedded in a four-layered stack, which is the same as the layered structure of phase-change optical disks. Cross-sectional HREM revealed crystalline atomic clusters in the melt-quenched amorphous layer at a greater frequency than in the as-sputtered amorphous layer. Autocorrelation function analysis of the HREM images revealed similarity between the structures of atomic ordered regions in the amorphous phase and that of crystalline Sb. Atomic pair-distribution functions derived from halo NBED intensity analysis indicated that the atomic neighbor correlations developed more in the melt-quenched amorphous phase than in the as-sputtered phase. The development of locally ordered regions is considered to be closely related to the differenc...

Solid phase epitaxy of amorphous silicon carbide: Ion fluence dependence

We have investigated the effect of radiation damage and impurity concentration on solid phase epitaxial growth of amorphous silicon carbide (SiC) as well as microstructures of recrystallized layer using transmission electron microscopy. Single crystals of 6H-SiC with (0001) orientation were irradiated with 150keV Xe ions to fluences of 1015 and 1016∕cm2, followed by annealing at 890°C. Full epitaxial recrystallization took place in a specimen implanted with 1015 Xe ions, while retardation of recrystallization was observed in a specimen implanted with 1016∕cm2 Xe ions. Atomic pair-distribution function analyses and energy dispersive x-ray spectroscopy results suggested that the retardation of recrystallization of the 1016Xe∕cm2 implanted sample is attributed to the difference in amorphous structures between the 1015 and 1016Xe∕cm2 implanted samples, i.e., more chemically disordered atomistic structure and higher Xe impurity concentration in the 1016Xe∕cm2 implanted sample.

Atomistic simulations of structural relaxation processes in amorphous silicon

Structural relaxation processes in amorphous silicon (a-Si) have been examined by molecular-dynamics (MD) simulations using the Tersoff interatomic potential. The a-Si networks generated by rapid quenching from liquid Si were annealed. Structural changes due to the relaxation of a-Si networks were observed. The present MD simulations reproduce well experimental measurements of changes in radial distribution functions, static structure factors, bond angle distributions, and phonon densities of states due to structural relaxation.

Damage and microstructure evolution in GaN under Au ion irradiation

Damage and microstructure evolution in gallium nitride (GaN) under Au+ ion irradiation has been investigated using complementary electron microscopy, secondary ion mass spectrometry and ion-beam analysis techniques. Epitaxially-grown GaN layers (2??m thick) have been irradiated by 2.0?MeV Au ions to 1.0 ? 1015 and 1.4 ? 1015?cm?2 at 155?K and to 7.3 ? 1015?cm?2 at 200?K. The irradiation-induced damage has been analysed by Rutherford backscattering spectroscopy in a channelling direction (RBS/C). For a better determination of the ion-induced disorder profile, an iterative procedure and a Monte Carlo code (McChasy) are combined to analyse the ion channelling spectra. With increasing irradiation dose, separated amorphous layers develop from the sample surface and near the damage peak region. Formation of large nitrogen bubbles with sizes up to 70?nm is observed in the buried amorphous layer, while the surface layer contains small bubbles with a diameter of a few nanometres due to significant nitrogen loss from the surface. Volume expansion from 3% to 25% in the irradiated region is suggested by cross-sectional transmission electron microscope and RBS/C measurement. The anomalous shape of the Au distributions under three irradiations indicates out-diffusion of Au towards the sample surface. The results from the complementary techniques suggest that nitrogen is retained in the damaged GaN where the crystallinity is preserved. Once the amorphous state is reached in the surface region, GaN starts to decompose and nitrogen escapes from the surface. Furthermore, experimental results show considerable errors in both the disorder profile and the ion range predicted by the Stopping and Range of Ions in Matter code, indicating a significant overestimation of electronic stopping powers of Au ions in GaN.

Direct observations of thermally induced structural changes in amorphous silicon carbide

Thermally induced structural relaxation in amorphous silicon carbide (SiC) has been examined by means of in situ transmission electron microscopy (TEM). The amorphous SiC was prepared by high-energy ion beam irradiation into a single crystalline 4H-SiC substrate. Cross-sectional TEM observations and electron energy-loss spectroscopy measurements revealed that thermal annealing induces a remarkable volume reduction, so-called densification, of amorphous SiC. From radial distribution function analyses using electron diffraction, notable changes associated with structural relaxation were observed in chemical short-range order. It was confirmed that the structural changes observed by the in situ TEM study agree qualitatively with those of the bulk material. On the basis of the alteration of chemical short-range order, we discuss the origin of thermally induced densification in amorphous SiC.

High-temperature thermoelectric properties of Cu2Ga4Te7 with defect zinc-blende structure

Here we show the high-temperature thermoelectric (TE) properties of Cu2Ga4Te7 with the defect zinc-blende structure in which one-seventh of the cation sites are structural vacancies. Cu2Ga4Te7 exhibited relatively low electrical resistivity (ρ) and thermal conductivity (κ) and moderate positive Seebeck coefficient (S) at high temperatures, making this compound a promising high-performance p-type TE material. At 940 K, the S, ρ, and κ were +215 μV K−1, 10.1×10−5 Ω m, and 0.67 Wm−1 K−1, respectively, which resulted in the maximum dimensionless figure of merit ZT (=S2T/ρ/κ, where T is the absolute temperature) of 0.64.