The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

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A comprehensive review of zno materials and devices

Bulk nanostructured materials from severe plastic deformation

Physical metallurgy of Ti–Ni-based shape memory alloys

In the past ten years we have witnessed a revival of, and subsequent rapid expansion in, the research on zinc oxide (ZnO) as a semiconductor. Being initially considered as a substrate for GaN and related alloys, the availability of high-quality large bulk single crystals, the strong luminescence demonstrated in optically pumped lasers and the prospects of gaining control over its electrical conductivity have led a large number of groups to turn their research for electronic and photonic devices to ZnO in its own right. The high electron mobility, high thermal conductivity, wide and direct band gap and large exciton binding energy make ZnO suitable for a wide range of devices, including transparent thin-film transistors, photodetectors, light-emitting diodes and laser diodes that operate in the blue and ultraviolet region of the spectrum. In spite of the recent rapid developments, controlling the electrical conductivity of ZnO has remained a major challenge. While a number of research groups have reported achieving p-type ZnO, there are still problems concerning the reproducibility of the results and the stability of the p-type conductivity. Even the cause of the commonly observed unintentional n-type conductivity in as-grown ZnO is still under debate. One approach to address these issues consists of growing high-quality single crystalline bulk and thin films in which the concentrations of impurities and intrinsic defects are controlled. In this review we discuss the status of ZnO as a semiconductor. We first discuss the growth of bulk and epitaxial films, growth conditions and their influence on the incorporation of native defects and impurities. We then present the theory of doping and native defects in ZnO based on density-functional calculations, discussing the stability and electronic structure of native point defects and impurities and their influence on the electrical conductivity and optical properties of ZnO. We pay special attention to the possible causes of the unintentional n-type conductivity, emphasize the role of impurities, critically review the current status of p-type doping and address possible routes to controlling the electrical conductivity in ZnO. Finally, we discuss band-gap engineering using MgZnO and CdZnO alloys.

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Fundamentals of zinc oxide as a semiconductor

Wurtzitic ZnO is a wide-bandgap (3.437 eV at 2 K) semiconductor which has many applications, such as piezoelectric transducers, varistors, phosphors, and transparent conducting films. Most of these applications require only polycrystalline material; however, recent successes in producing large-area single crystals have opened up the possibility of producing blue and UV light emitters, and high-temperature, high-power transistors. The main advantages of ZnO as a light emitter are its large exciton binding energy (60 meV), and the existence of well-developed bulk and epitaxial growth processes; for electronic applications, its attractiveness lies in having high breakdown strength and high saturation velocity. Optical UV lasing, at both low and high temperatures, has already been demonstrated, although efficient electrical lasing must await the further development of good, p-type material. ZnO is also much more resistant to radiation damage than are other common semiconductor materials, such as Si, GaAs, CdS, and even GaN; thus, it should be useful for space applications.

Recent Advances in ZnO Materials and Devices

An ordered arrangement of carbon and copper atoms in the new compound (C 0.4 Cu 0.6 )Sr 2 (Y 0.86 Sr 0.14 )Cu 2 O 7 was investigated by means of electron diffraction and high-resolution electron microscopy. Electron diffraction patters show diffuse superlattice reflections corresponding to a compositional modulation with periods in the range from 2 a to 3 a along the a -axis and 2 c along the c -axis. From observed high-resolution images, it is concluded that the modulated arrangement of carbon and copper atoms is responsible for the diffuse superlattice reflections. It is proposed that this modulated structure consists of two kinds of superstructure units having dimensions of 2 a × b ×2 c and 3 a × b ×2 c . There are two different atomic arrangements in the 3 a × b ×2 c units.

Ordered arrangement of carbon and copper atoms in (C0.4Cu0.6)Sr2(Y0.86Sr0.14)Cu2O7

X-ray diffraction study on as-grown and quenched La 192 Sr 008 CuO 4- y single-crystals from 827 K or 1373 K was carried out The symmetry of the quenched samples was kept tetragonal 14/mmm-type An “apical-oxygen” located at 4e(00 z ) site with z =01806(5) is sitting closer to a copper at 2a(000) site in the quenched sample from 1373 K, by contrast with to an apical-oxygen at z =01823(3) in the as-grown sample Neutron single-crystal diffraction study done in the temperature range from 7 to 150 K revealed some increase in the copper/apical-oxygen distance of a CuO 6 octahedron

X-ray and neutron single-crystal diffraction study on La1.92Sr0.08CuO4−y

Microstructures of soft magnetic Fe 73.5 CuNb 3 Si 13.5 B 9 alloys, which were rapidly solidified and then annealed at various temperatures, have been investigated by transmission electron microscopy. The as-quenched alloy shows a homogeneous amorphous strurture without any microcrystalline regions. In the alloys annealed at around 550°C, which show the best soft magnetic properties, bcc crystalline grains with a global shape, about 20 nm in diameters, and amorphous regions surrounding them, were observed

Microstructure of Fe–Cu–Nb–Si–B Soft Magnetic Alloys Studied by Transmission Electron Microscopy

Tetragonal-to-orthorhombic phase transition at about 410 K is found by a X-ray powder diffraction study on the title oxide synthesized in a lamp-image floating zone furnace. The crystal structures of tetragonal and orthorhombic forms of the oxide are determined carefully by X-ray and neutron single crystal diffraction measurements. The space symmetries of the tetragonal and the orthorhombic forms are consistent with the I4/mmm-type (No. 139) and the Pmcb-type (No. 55), respectively. The chemical composition of the title oxide is estimated from the diffraction data as La 2.000(4) CoO 3.86(4) at room temperature.

Tetragonal-Orthorhombic Phase Transition of La2CoO4-x

Al65Ru15Cu20 icosahedral quasicrystals, which were prepared by conventional melting and melt-quenching methods, have been examined by electron and X-ray diffraction. Superlattice reflections interpreted by face-centred six-dimensional hypercubic lattices, which are observed in the conventionally solidified quasicrystal, are considerably weaker or non-existent in the rapidly solidified material. The weak superlattice reflections become stronger upon annealing at 750°C for 30min. The result shows that atomic ordering is suppressed by rapid cooling and the disordered quasicrystal produced transforms to the ordered one at about 750°C

Kenji Hiraga

Papers

Ordered arrangement of carbon and copper atoms in (C0.4Cu0.6)Sr2(Y0.86Sr0.14)Cu2O7

X-ray and neutron single-crystal diffraction study on La1.92Sr0.08CuO4−y

Microstructure of Fe–Cu–Nb–Si–B Soft Magnetic Alloys Studied by Transmission Electron Microscopy

Tetragonal-Orthorhombic Phase Transition of La2CoO4-x

Atomic disordering in an Al–Ru–Cu icosahedral quasicrystal