Makoto Shiojiri

Structure and growth of ZnO smoke particles prepared by gas evaporation technique

ZnO smoke particles prepared by burning zinc metal in a gas mixture of 80% Ar and 20% O2 at a total pressure of 100 Torr were studied by electron microscopy. The ZnO particles were formed by the oxidation of evaporated zinc vapor in the gas phase. It was found that the particles less than 20 nm in size have the zinc-blende structure. They became tetrapod-like crystals by fast growing along four 〈111〉 directions perpendicular to the {111} or Zn faces. Then, wurtzite crystals formed on their {111} faces by introduced stacking faults, and they grew along their easy growth [0001] directions, and became well-known “fourlings” composed of four needle crystals.

Structure and formation mechanism of V defects in multiple InGaN/GaN quantum well layers

A variety of different transmission electron microscopy techniques, and particularly high-angle annular dark-field scanning transmission electron microscopy, has been used to reveal that V defects or inverted hexagonal pyramid defects in multiple InGaN∕GaN quantum well (QW) layers nucleate on threading dislocations that cross the InGaN QW. The defects have thin walls lying parallel to {101¯1} with the InGaN∕GaN QW structure. A formation mechanism for the V defects is proposed taking into account the growth kinetics of GaN and the segregation of In atoms in the strain field around the cores of the threading dislocations.

Structure and growth mechanism of tetrapod-like ZnO particles

Tetrapod-like ZnO particles having four legs shorter than a micrometre have been observed by transmission electron microscopy and electron diffractometry. The particles are composed of six or more wurtzite-type elements. Four of them make legs and others form (1122) twins with the legs. Particles composed of only four wurtzite-type legs are also observed to form three (1124) twins among them. We propose a growth model that the tetrapod-like ZnO particle grows from wurtzite-type multiple twins induced in a zincblende-type nucleus, taking account of the combination of the slips caused at the phase transformation.

Growth of CdS smoke particles prepared by evaporation in inert gases

Cds particles, which were prepared in He, Ne, Ar, and Xe gases at pressures of 5–150 Torr, were studied by electron microscopy. Crystals with wurtzite structure grew in single hexagonal plates or prisms or in a more complicated form composed of three or more hexagonal prisms. The mean size of the particles was larger if they were prepared in a heavier gas, and increased almost linearly with gas pressure up to a saturation pressure which was lower for heavier gasses. The particle size distributions in the smokes and the temperature distributions in the chamber are presented for various atmospheres, and on these bases the growth of the particles is discussed.

Electron Microscopic Studies of Structure and Crystallization of Amorphous Metal Oxide Films

High-resolution transmission electron microscopy using axial beam illumination was carried out on vacuum-deposited WO3, TiO2, and MoO3 films. In as-deposited WO3 films thinner than 100 A and films heated at 350°C for 2~5 h which gave rise to amorphous haloes in electron diffraction, crossed fringe-like structure images of the order of 10~20 A was observed. This means that the amorphous films consisted of micro-crystallites in which the W–O6 octahedra were arrayed like crystalline WO3. When the films were heated at 350°C for 10 h, they grew to crystallites of WO3 as large as 100 A. On the other hand, in WO3 films thicker than about 200 A, crystals of a few microns grew after shorter heat treatment. Similar results were obtained for the TiO2 and MoO3 films. The differnce in crystallization between the very thin films and the thicker films is interpreted by considering the heat generated during crystallization and its dispersion.

Amplified spontaneous emission from ZnO in n-ZnO/ZnO nanodots–SiO2 composite/p-AlGaN heterojunction light-emitting diodes

This study demonstrates amplified spontaneous emission (ASE) of the ultraviolet (UV) electroluminescence (EL) from ZnO at lambda~380 nm in the n-ZnO/ZnO nanodots-SiO(2) composite/p- Al(0.12)Ga(0.88)N heterojunction light-emitting diode. A SiO(2) layer embedded with ZnO nanodots was prepared on the p-type Al(0.12)Ga(0.88)N using spin-on coating of SiO(2) nanoparticles followed by atomic layer deposition (ALD) of ZnO. An n-type Al-doped ZnO layer was deposited upon the ZnO nanodots-SiO(2) composite layer also by the ALD technique. High-resolution transmission electron microscopy (HRTEM) reveals that the ZnO nanodots embedded in the SiO(2) matrix have diameters of 3-8 nm and the wurtzite crystal structure, which allows the transport of carriers through the thick ZnO nanodots-SiO(2) composite layer. The high quality of the n-ZnO layer was manifested by the well crystallized lattice image in the HRTEM picture and the low-threshold optically pumped stimulated emission. The low refractive index of the ZnO nanodots-SiO(2) composite layer results in the increase in the light extraction efficiency from n-ZnO and the internal optical feedback of UV EL into n-ZnO layer. Consequently, significant enhancement of the UV EL intensity and super-linear increase in the EL intensity, as well as the spectral narrowing, with injection current were observed owing to ASE in the n-ZnO layer.

Polarity and inversion twins in ZnSe crystals observed by high-resolution electron microscopy

Abstract Zinc selenide crystals (zinc-blende structure) with (110) surface planes, produced by a solid–solid reaction, have been examined by means of a multi-beam lattice-imaging technique in a 200 kV electron microscope with a pole piece of C s = 1·2 mm, and by simulaton using the multi-slice method. In the experimental images the columns of Zn and fie atoms have been positoned so that the crystallographic polarity, as well as the stacking sequence, can be determined. Inversion twins, the components of which have opposite senses of polar axes across the boundary, have been found parallel to (111). It is concluded that the lattice on one side of the twin boundary is displaced from that on the other side by α/8[110] and that the formal charge at the two types of (111) boundary, positive and negative, is compensated by removing or supplying Zn atoms. It has been observed that layers with a wurtzite-type stacking sequence ere also formed by the introduction of similar growth stacking defects.

Desities of Amorphous Thin Films

The densities of amorphous Ge, C, SiO, Se, and WO3 films prepared by vacuum-evaporation were measured using multiple beam interferometry in combination with an oscillating crystal microbalance. Optically polished quartz crystals were used as both the microbalance sensing elements and the substrates for interferometry. The film densities were found to be independent of thickness. The measurements indicated that the amorphous film densities were 23–31% less than the crystalline bulk densities and 13–14% less than the amorphous or vitreous bulk densities. This density decrease may be attributed to voids, the presence of which is deduced from a microcrystalline model of the amorphous structure confirmed by recent high-resolution electron microscopic observation.

Electron Microscopic Study of Metal Oxide Smoke Particles Prepared by Burning Metals in Ar–O2 Gas

Some oxide smoke particles which were prepared by burning metals in a gas mixture of 80% Ar and 20% O2 were studied by electron microscopy. CaO smoke particles grew in single crystal cubes. In2O3, Sb2O3, Bi2O3, Cu2O, SnO2, CuO, and WO3 smoke particles grew in single-crystal octahedra or sometimes grew in spheres or more complicated polyhedra. Particles with layer structure such as SnO and PbO grew in thin plates. It is shown by scanning electron microscopy that smoke particles grew by coalescence in which two or more single particles joined in definite orientations.

Ultraviolet Electroluminescence From n-ZnO–SiO

Ultraviolet (UV) light-emitting diodes composed of n-ZnO:Al-SiO2-ZnO nanocomposite/p-GaN:Mg heterojunction were fabricated on the (0002) Al2O3 substrate. A SiO2 layer embedded with ZnO nanodots was prepared on the p-type GaN using spin-on coating of SiO2 nanoparticles together with atomic layer deposition (ALD). An n-type Al-doped ZnO layer was deposited also by ALD. The SiO2-ZnO nanocomposite layer accomplishes a role of the current blocking layer and also causes, by its low refractive index, the increase in the light extraction efficiency from n-ZnO. Significant UV electroluminescence from n-ZnO was achieved at a low forward-bias current of 1.8 mA. Strong UV emission arising from impact ionization in GaN, ZnO, and GaN:Mg states was also observed at reverse breakdown bias.