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. ...

/pdf/a-comprehensive-review-of-zno-materials-and-devices-21a3zjadem.pdf

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.

/pdf/fundamentals-of-zinc-oxide-as-a-semiconductor-1s4l8bh009.pdf

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

This paper presents the recent achievements of ZnO/GaN heteroepitaxy. The general controlling method and mechanism for the polarity of heteroepitaxial ZnO and GaN films by interface engineering via Plasma-assisted Molecular beam epitaxy(P-MBE) are introduced in a viewpoint of principle for polarity control. We propose the principle of crystal polarity: Crystal polarity can succeed at the heterointerface when no interface layer is formed, while an interface layer with inversion symmetry is formed, the crystal polarity is inverted at the heterointerfae. The effects of polarity on the interface, surface and bulk structure, and the structural and optical properties of ZnO/GaN epitaxy are also included. The polarity of GaN on ZnO is successfully controlled based on the proposed principle for control of crystal polarity. Additionally, the electronic characteristics such as electron concentration, band-line-up, and C-V characteristics of ZnO/GaN heterointerface are dicussed.

ZnO/GaN Heteroepitaxy

We have measured excitation dependence of photoluminescence (PL) spectra, excitation spectra of PL (PLE) and temporal dependence of the PL intensity in ZnO epitaxial thin films. The ZnO films were grown on a sapphire substrate with a GaN buffer layer to reduce a lattice mismatch. We succeeded in observing a PL due to biexciton for the first time in ZnO epitaxial thin films.

Photoluminescence of Biexciton in ZnO Epitaxial Thin Films

Photoelectrochemical properties of Ga- and N-face GaN grown by hydride vapor phase epitaxy (HVPE) were investigated. The properties were also compared with Ga-face GaN grown by metal-organic vapor phase epitaxy (MOVPE). The flatband potentials were in order of Ga-face GaN grown by MOVPE < N-face GaN < Ga-face GaN. The highest photocurrent density at zero bias was obtained from the N-face GaN. The photocurrent density was over 3 times larger than that of Ga-face GaN.

Improvement of Photoelectrochemical Reaction for Hydrogen Generation from Water using N-face GaN

We have investigated the effect of a thin low-temperature-grown ZnSe (LT-ZnSe) buffer layer in improving ZnSe crystallinity by inserting it between the high-temperature-grown ZnSe epilayer and the GaAs substrate. In particular, the density of stacking fault defects, which is normally observed in ZnSe-based films grown on GaAs substrate, can be well reduced by inserting a thin LT-ZnSe buffer layer. A ZnSe film with stacking fault densities as low as ∼5.4×104/cm2 was obtained by growing on Zn exposed (2×4) As-stabilized surfaces of GaAs buffer layer with LT-ZnSe buffer, in contrast, ∼7×108/cm2 was obtained by directly growing on GaAs substrate, that is, without Zn exposure, LT-ZnSe, and GaAs buffer layers.

Reduction of stacking faults in the ZnSe/GaAs heterostructure with a low-temperature-grown ZnSe buffer layer

Using ion implantated material as a growth source, implant source growth, is an alternative growth method where ion implanted atoms diffuse to the surface and can be used to form nanostructures. By introducing a small amount of a steel based contaminate we were able to grow high quality and high density GaN nanodots on Si(111) which showed strong excitonic UV luminescence. Furthermore by varying the implanted Ga dose and annealing conditions, the size of the nanodots can be controlled. This method for producing GaN nanostructures may be useful for optical or electronic device applications. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Takafumi Yao

Papers

ZnO/GaN Heteroepitaxy

Photoluminescence of Biexciton in ZnO Epitaxial Thin Films

Improvement of Photoelectrochemical Reaction for Hydrogen Generation from Water using N-face GaN

Reduction of stacking faults in the ZnSe/GaAs heterostructure with a low-temperature-grown ZnSe buffer layer

Metal catalyst enhanced growth of high quality and density GaN dots on Si(111) by implant source growth