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

Predictions and measurements of the effect of biaxial strain on the properties of epitaxial ferroelectric thin films and superlattices are reviewed. Results for single-layer ferroelectric films of biaxially strained SrTiO3, BaTiO3, and PbTiO3 as well as PbTiO3/SrTiO3 and BaTiO3/SrTiO3 superlattices are described. Theoretical ap- proaches, including first principles, thermodynamic analysis, and phase-field models, are applied to these biaxially strained materials, the assumptions and limitations of each technique are explained, and the predictions are compared. Measurements of the effect of biax- ial strain on the paraelectric-to-ferroelectric transition temperature (TC) are shown, demonstrating the ability of percent-level strains to shift TC by hundreds of degrees in agreement with the predic- tions that predated such experiments. Along the way, important ex- perimental techniques for characterizing the properties of strained ferroelectric thin films and superlattices, as well as appropriate sub- strates on which to grow them, are mentioned.

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Strain Tuning of Ferroelectric Thin Films

Polymers for flexible displays: From material selection to device applications

Functional materials by electrospinning of polymers

Biogenic calcium carbonate forms the inorganic component of seashells, otoliths, and many marine skeletons, and its formation is directed by an ordered template of macromolecules. Classical nucleation theory considers crystal formation to occur from a critical nucleus formed by the assembly of ions from solution. Using cryotransmission electron microscopy, we found that template-directed calcium carbonate formation starts with the formation of prenucleation clusters. Their aggregation leads to the nucleation of amorphous nanoparticles in solution. These nanoparticles assemble at the template and, after reaching a critical size, develop dynamic crystalline domains, one of which is selectively stabilized by the template. Our findings have implications for template-directed mineral formation in biological as well as in synthetic systems.

The initial stages of template-controlled CaCO3 formation revealed by Cryo-TEM

We report on the high-temperature ferromagnetism in Co-doped ZnO films fabricated by the sol–gel method above 350 K. The lattice constant of c axis of wurtzite Zn1−xCoxO follows Vergard’s law for 0<x<0.25. For Zn1−xCoxO with x⩾0.25, a secondary phase is detected. The Zn1−xCoxO exhibits ferromagnetic behavior with a Curie temperature higher than 350 K. By the results of x-ray photoelectron spectroscopy measurement, we assume that Co occupied the Zn site without changing the wurtzite structure. In the case of x=0.2, the coercive field measured by a magnetization–magnetic field hysteresis curve at 350 K was nearly 80 Oe. Additionally, we investigated the electric structure through first-principles pseudopotential plane-wave calculation.

Study of diluted magnetic semiconductor: Co-doped ZnO

We report on the discovery of a room-temperature ferromagnetism in Cr-doped GaN single crystals with a Tc=280 K. The addition of Cr into GaN single crystals grown by the flux method induces the lattice constant increase due to the larger Cr atomic radius. In x-ray photoelectron spectroscopy measurement, Cr 2p3/2 core-level exhibited spectra near 575.7 eV. This binding energy is similar to the reported value of CrN. The coercive field by magnetization–magnetic field (M–H) hysteresis curve at 250 K was 54 Oe. We verified the presence of ferromagnetic transition in the temperature dependence of the electrical resistance measurements. We discuss the ferromagnetic ordering in Cr-doped GaN bulk single crystals excluding the contribution of the substrate crystal structure.

Room-temperature ferromagnetism in Cr-doped GaN single crystals

Copper electrodes with a micromesh/nanomesh structure were fabricated on a polyimide substrate using UV lithography and wet etching to produce flexible transparent conducting electrodes (TCEs). Well-defined mesh electrodes were realized through the use of high-quality Cu thin films. The films were fabricated using radio-frequency (RF) sputtering with a single-crystal Cu target—a simple but innovative approach that overcame the low oxidation resistance of ordinary Cu. Hybrid Cu mesh electrodes were fabricated by adding a capping layer of either ZnO or Al-doped ZnO. The sheet resistance and the transmittance of the electrode with an Al-doped ZnO capping layer were 6.197 ohm/sq and 90.657%, respectively and the figure of merit was 60.502 × 10–3/ohm, which remained relatively unchanged after thermal annealing at 200 °C and 1,000 cycles of bending. This fabrication technique enables the mass production of large-area flexible TCEs and the stability and high performance of Cu mesh hybrid electrodes in harsh environments suggests they have strong potential for application in smart displays and solar cells.

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Cu Mesh for Flexible Transparent Conductive Electrodes

It is well known that ZnO generally has the wurtzite structure. We report the growth of both hexagonal and cubic ZnO on Pt(111)/Ti/SiO2/Si substrate by a solution deposition. The wurtzite structure of ZnO film is enhanced up to the annealing temperature of 600 °C, and disappeared for annealing temperatures above 700 °C. The Pt(111)/Ti/SiO2/Si substrate is reoriented to hexagonal-Pt3Ti(004)/Ti/SiO2/Si when annealed at 700 °C and above due to the Ti out-diffusion and the ZnO thin film grown on the substrate has a cubic structure. The diffusion of Ti was evidenced by Auger electron spectroscopy measurements. From the photoluminescence measurement, the band gap of the wurtzite structure of ZnO film grown by the annealing below 600 °C was 3.37 eV, as is already known, but the cubic ZnO had a band gap of 3.28 eV, which suggests a zinc blende structure. The stability of the zinc blende structure on Zn2TiO4 layer was checked by the calculation of the lattice mismatch using the extended atomic distance mismatch mo...

Structural reconstruction of hexagonal to cubic ZnO films on Pt/Ti/SiO2/Si substrate by annealing

The ferromagnetism of ZnO-based magnetic semiconductors has been investigated by theoretical calculation and many kinds of experiments. In the Cu-doped ZnO system, ferromagnetic ordering by Cu magnetic moments does not exist, but Cu-doped ZnO can have ferromagnetism due to the distortion of the ZnO-based structure by Cu doping. In this study, we investigated the magnetic properties of Cu-doped ZnO and discussed the possible origin of magnetism in this system. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Se-Young Jeong

Papers

Study of diluted magnetic semiconductor: Co-doped ZnO

Room-temperature ferromagnetism in Cr-doped GaN single crystals

Cu Mesh for Flexible Transparent Conductive Electrodes

Structural reconstruction of hexagonal to cubic ZnO films on Pt/Ti/SiO2/Si substrate by annealing

A study of magnetic and optical properties of Cu-doped ZnO