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

The formation and structure of a decagonal quasicrystal (D) phase in rapidly solidified Al 70 Pd x Mn 30 − x alloys ( x =10−10) were examined by powder X-ray diffraction and high-resolution electron microscopy. The D phase, coexisting with an icosahedral (I) phase, was found in the range of composition x = 10−15. The structure of the D phase is formed by an inhomogeneous distribution of decagonal symmetric atomic clusters, together with microdomains of its related crystal phases. By annealing at 800 °C, an equilibrium D phase, whose structure involves the packing of decagonal symmetric atomic clusters with definite linkages and a long-range quasiperiodic correlation, was formed as almost a single phase in the range x = 10−13 and coexisting with the I phase at x = 15−20.

Decagonal quasicrystal in rapidly solidified AlPdMn alloys

Electron Channelling Enhanced Microanalysis on Ni-Al-Mn and Al-Mn-Si

As-cast and rapidly solidified alloys with compositions of Al 70− x Ni 25+ x Ru 5 ( x =0, 5, 10 and 15) and Al 70+ y Ni 20− y Ru 10 ( y =0 and 5) were examined by transmission electron microscopy. The results show that the Al–Ni–Ru icosahedral quasicrystal, which belongs to the F-type class, and the decagonal quasicrystal with a 0.4 nm periodicity can coexist in both the as-cast and rapidly solidified states. By means of high-resolution electron microscopy observations, the structural features of the as-cast Al–Ni–Ru icosahedral quasicrystal and the coexisting decagonal quasicrystal, with basic quasiperiodic order, were revealed.

Al–Ni–Ru icosahedral quasicrystal and coexisting decagonal quasicrystals with 0.4 nm periodicity, studied by atomic-resolution electron microscopy observations

Abstracts are not published in this journal

Interfacial structure of Si3N4 whisker reinforced 7064 aluminium composite

The crystal structure of a new phase with an approximate composition of Mg 91 Ce 7 Zn 2 , which is formed as a main phase in an Mg 91 Ce 6 Zn 3 alloy, has been determined by atomic-scale observations of high-resolution transmission electron microscopy (HRTEM) and high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). The structure of this phase can be described as a one-dimensional incommensurate structure with an orthorhombic unit cell of a = 1.03 nm, b = 1.03 nm and c ≒ 3.7 nm, formed by insertion of anti-phase boundaries in the fundamental Mg 12 Ce (Mn 12 Th-type) structure with a tetragonal unit cell of a 0 = 1.03 nm and c 0 = 0.60 nm. The anti-phase boundaries are parallel to the (001) plane of the fundamental tetragonal structure, and an average interval of the boundaries along the [001 ] direction is about 3.1c 0 . Also, a commensurate structure with an interval of 5.5c 0 is observed as a coexisting phase with the incommensurate structure.

One-Dimensional Anti-Phase Structures Based on an Mg12Ce (Mn12Th-Type) Structure in an Mg91Ce6Zn3 Alloy, Studied by High-Resolution Transmission Electron Microscopy and High-Angle Annular Detector Dark-Field Scanning Transmission Electron Microscopy

Kenji Hiraga

Papers

Decagonal quasicrystal in rapidly solidified AlPdMn alloys

Electron Channelling Enhanced Microanalysis on Ni-Al-Mn and Al-Mn-Si

Al–Ni–Ru icosahedral quasicrystal and coexisting decagonal quasicrystals with 0.4 nm periodicity, studied by atomic-resolution electron microscopy observations

Interfacial structure of Si3N4 whisker reinforced 7064 aluminium composite

One-Dimensional Anti-Phase Structures Based on an Mg12Ce (Mn12Th-Type) Structure in an Mg91Ce6Zn3 Alloy, Studied by High-Resolution Transmission Electron Microscopy and High-Angle Annular Detector Dark-Field Scanning Transmission Electron Microscopy