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

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

Point defects and impurities strongly affect the physical properties of materials and have a decisive impact on their performance in applications. First-principles calculations have emerged as a powerful approach that complements experiments and can serve as a predictive tool in the identification and characterization of defects. The theoretical modeling of point defects in crystalline materials by means of electronic-structure calculations, with an emphasis on approaches based on density functional theory (DFT), is reviewed. A general thermodynamic formalism is laid down to investigate the physical properties of point defects independent of the materials class (semiconductors, insulators, and metals), indicating how the relevant thermodynamic quantities, such as formation energy, entropy, and excess volume, can be obtained from electronic structure calculations. Practical aspects such as the supercell approach and efficient strategies to extrapolate to the isolated-defect or dilute limit are discussed. Recent advances in tractable approximations to the exchange-correlation functional ($\mathrm{DFT}+U$, hybrid functionals) and approaches beyond DFT are highlighted. These advances have largely removed the long-standing uncertainty of defect formation energies in semiconductors and insulators due to the failure of standard DFT to reproduce band gaps. Two case studies illustrate how such calculations provide new insight into the physics and role of point defects in real materials.

First-principles calculations for point defects in solids

We present a review of current research on the optical properties of ZnO nanostructures. We provide a brief introduction to different fabrication methods for various ZnO nanostructures and some general guidelines on how fabrication parameters (temperature, vapor-phase versus solution-phase deposition, etc.) affect their properties. A detailed discussion of photoluminescence, both in the UV region and in the visible spectral range, is provided. In addition, different gain (excitonic versus electron hole plasma) and feedback (random lasing versus individual nanostructures functioning as Fabry-Perot resonators) mechanisms for achieving stimulated emission are described. The factors affecting the achievement of stimulated emission are discussed, and the results of time-resolved studies of stimulated emission are summarized. Then, results of nonlinear optical studies, such as second-harmonic generation, are presented. Optical properties of doped ZnO nanostructures are also discussed, along with a concluding outlook for research into the optical properties of ZnO.

Optical properties of ZnO nanostructures.

Leu-M

We have investigated the structural and optical properties of Ga-doped ZnO films grown on GaN templates by plasma-assisted molecular-beam epitaxy. The carrier concentration in Ga-doped ZnO films can be controlled from 1.33×1018/cm3 to 1.13×1020/cm3. Despite high Ga incorporation, the linewidth of (0002) ω-rocking curves of Ga-doped ZnO films still lies in the range from 5 to 15 arc min. Photoluminescence (PL) spectra of Ga-doped ZnO films show dominant near-bandedge emission with negligibly weak deep-level emission, independent of carrier concentration. The PL spectrum exhibits a new emission line at 3.358 eV, which corresponds to exciton emission bound to a Ga donor. To avoid degradation of the PL intensity, the maximum dopability of Ga in ZnO is determined to be around 2.6×1019/cm3.

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Ga-Doped ZnO Films Grown on GaN Templates by Plasma-Assisted Molecular-Beam Epitaxy

By introducing a thin MgO buffer, layer-by-layer growth of ZnO epilayers on Al2O3(0001) substrates is achieved by plasma-assisted molecular beam epitaxy. The MgO buffer is very effective on the improvement of surface morphology during the initial growth stage, which eventually leads to an atomically flat surface. As a result, (3×3) surface reconstruction of ZnO is observed and reflection high-energy electron diffraction intensity oscillations are recorded. Structural analysis indicates that the twin defect with a 30° in-plane crystal orientation misaligned is completely eliminated, while the total dislocation density is reduced. Free exciton emissions at 3.3774 eV (XA) and 3.383 eV (XB) are observed in photoluminescence at 4.2 K further indicating the high quality of the resulting ZnO epilayers.

Layer-by-layer growth of ZnO epilayer on Al2O3(0001) by using a MgO buffer layer

We report the results of an experimental investigation on lasing mechanisms in optically pumped ZnO epilayers at room temperature. High-quality ZnO epilayers grown on sapphire by plasma-assisted molecular-beam epitaxy employing an MgO buffer were used. Free exciton emissions and their phonon replicas dominate the photoluminescence from low excited samples. Inelastic exciton–exciton scattering contributes to the mechanism of stimulated emission mainly at intermediate excitation. By using the variable stripe length method, we measured the near threshold optical gain spectrum of the ZnO epilayers. Different from the interband transition governed mechanisms, exciton–exciton scattering gives rise to a nearly symmetric gain spectrum with the peak at 3.17 eV. The electron-hole plasma emerges to contribute to the optical gain when excitation exceeds 220 kW/cm2.

Stimulated emission and optical gain in ZnO epilayers grown by plasma-assisted molecular-beam epitaxy with buffers

The authors fabricated GaN-based light-emitting diodes (LEDs) on two different GaN templates with the same LED structure. One on thin GaN template (∼2μm) with high dislocation density [low (109cm−2)] grown by metal-organic vapor-phase epitaxy (sample A) and the other on thick GaN template (∼20μm) with comparatively low dislocation density [high (108cm−2)] by hydride vapor-phase epitaxy (sample B). In order to understand the mechanism of leakage current in LEDs, the correlation between current-voltage characteristics and etch pit density of LEDs was studied.

Origin of forward leakage current in GaN-based light-emitting devices

Porous nanowire-structured cupric oxide (CuO) film is synthesized by deposition of Cu on porous single-walled carbon nanotube (SWNT) substrate followed by a thermal oxidation process. Oxidation is done in air at a temperature range of 300–800 °C to oxidize the Cu while removing the SWNT template. The oxidation temperature determines the stoichiometry of the CuO formed, and thus the electrical property. The structures and electrical properties of the synthesized materials are investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), and current–voltage measurements. Sensing property is examined using hydrogen gas. Gas sensing mechanism and the advantages of nanowire structure as a sensor are also discussed. The best response and recovery results are observed with the CuO nanowires oxidized at 400 °C at a working temperature of 250 °C.

Soon-Ku Hong

Papers

Ga-Doped ZnO Films Grown on GaN Templates by Plasma-Assisted Molecular-Beam Epitaxy

Layer-by-layer growth of ZnO epilayer on Al2O3(0001) by using a MgO buffer layer

Stimulated emission and optical gain in ZnO epilayers grown by plasma-assisted molecular-beam epitaxy with buffers

Origin of forward leakage current in GaN-based light-emitting devices

Synthesis of porous CuO nanowires and its application to hydrogen detection