There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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

Room-temperature ultraviolet lasing in semiconductor nanowire arrays has been demonstrated The self-organized, oriented zinc oxide nanowires grown on sapphire substrates were synthesized with a simple vapor transport and condensation process These wide band-gap semiconductor nanowires form natural laser cavities with diameters varying from 20 to 150 nanometers and lengths up to 10 micrometers Under optical excitation, surface-emitting lasing action was observed at 385 nanometers, with an emission linewidth less than 03 nanometer The chemical flexibility and the one-dimensionality of the nanowires make them ideal miniaturized laser light sources These short-wavelength nanolasers could have myriad applications, including optical computing, information storage, and microanalysis

Room-temperature ultraviolet nanowire nanolasers

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

The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

Hard x-ray valence band photoemission spectroscopy (PES) is realized using high-energy and high-brilliance synchrotron radiation. High-energy (∼6 keV) excitation results in larger probing depths of photoelectrons compared to conventional PES, and enables a study of intrinsic electronic property of materials in actual device structures much less influenced by surface condition. With this technique, requirements for surface preparation are greatly reduced, if not eliminated. It is a nondestructive tool to determine electronic structure from surface to genuine bulk as shown by a study on SiO2/Si(100). Electronic structure modification related to the ferromagnetism in the diluted magnetic semiconductor Ga0.96Mn0.04N is also observed.

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A probe of intrinsic valence band electronic structure: Hard x-ray photoemission

Structural and magnetic properties of Mn3O4 films grown on MgO(001) substrates by plasma-assisted MBE

Following the previous work on formation of quantum-dot structures in ultrathin ZnSe/ZnS quantum wells, we investigate the temperature dependence of the photoluminescence spectra in a temperature range from 20 to 100 K The quantum-dot structure exhibits an intense but broadened photoluminescence line at low temperature However, the emission quenches rapidly when temperature rises to around 60 K, suggesting the existence of defect-related states either in the ZnSe well layer or in the vicinity of ZnSe/ZnS interfaces It was found that the thermal activation energy that characterizes the quench process is strongly dependent on the energy position inside the broadened emission lines, which is interpreted as originating from the different extent of lateral quantum confinement of each individual quantum dot

Temperature dependence of the photoluminescence of ZnSe/ZnS quantum-dot structures

Omnidirectional (omni-) reflector and transmission filter optimized at a wavelength of 1.55 μm have been realized using Si/SiO2 one-dimensional photonic crystals (1D PCs). Photonic band structures in the PCs with and without a defect layer have been also analyzed. The 1D PCs fabricated have very large omnidirectional photonic band gaps (omni-PBGs), and their measured reflectance spectra are in very good agreement with the calculated results. The omni-PBG in a six-pair Si/SiO2 1D PC (no defect layer, a filling factor η=0.406) exists in a normalized frequency range of 0.223–0.297. Introducing a defect layer generates a defect state in the PBG, and it behaves like electronic impurity levels in the gap with a change in thickness of the defect layer (dD). A six-pair Si/SiO2 1D PC with the structure parameters of η=4.060, dH (Si thickness)=95.3 nm, dL (SiO2 thickness)=234.9 nm, and dD=2.336⋅dH could act as a 1.55 μm-transmission filter for normal incidence light.

Si-based omnidirectional reflector and transmission filter optimized at a wavelength of 1.55 μm

We have investigated the characteristic of the dislocations in the ZnO layers grown on c sapphire by the plasma-assisted molecular beam epitaxy under the different Zn∕O flux ratios. The ZnO layers were characterized by the transmission electron microscopy (TEM) and the high-resolution x-ray diffraction (HRXRD). The TEM and HRXRD experiments revealed that the major threading dislocations (TDs) in the ZnO layers are the edge dislocations running along the c axis with Burgers vector of 1∕3⟨11–20⟩. The TD densities are determined to be 6.9×109, 2.8×109, and 2.7×109cm−2, for O-rich, stoichiometric, and Zn-rich grown ZnO, respectively. Different from the O-rich grown ZnO where the dislocations run along the c-axis, several dislocations in the stoichiometric and the Zn-rich grown ZnO are inclined to 20°∼30° from the c-axis. By considering the slip system in the wurtzite-structure ZnO, the glide planes of the dislocations are close to (10-10) for the O-rich grown ZnO and close to (10-11) for the stoichiometric an...

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Takafumi Yao

Papers

A probe of intrinsic valence band electronic structure: Hard x-ray photoemission

Structural and magnetic properties of Mn3O4 films grown on MgO(001) substrates by plasma-assisted MBE

Temperature dependence of the photoluminescence of ZnSe/ZnS quantum-dot structures

Si-based omnidirectional reflector and transmission filter optimized at a wavelength of 1.55 μm

Characteristics of dislocations in ZnO layers grown by plasma-assisted molecular beam epitaxy under different Zn∕O flux ratios