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

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.

Recent progress in processing and properties of ZnO

Bottom-gate-type thin film transistors using ZnO as an active channel layer (ZnO–TFT) have been constructed. The ZnO layers were deposited using pulsed laser deposition at 450 °C at an oxygen pressure of 3 m Torr, and the material that was formed had a background carrier concentration of less than 5×1016 cm−3. A double layer gate insulator consisting of SiO2 and SiNx was effective in suppressing leakage current and enabling the ZnO–TFT to operate successfully. The Ion/Ioff ratio of ZnO–TFTs fabricated on Si wafers was more than 105 and the optical transmittance of ZnO–TFTs fabricated on glass was more than 80%. These results show that it is possible to fabricate a transparent TFT that can even be operated in the presence of visible light.

Transparent thin film transistors using ZnO as an active channel layer and their electrical properties

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

We have investigated ZnO epilayers grown under various Zn/O ratios by plasma-assisted molecular-beam epitaxy. The growth conditions are elucidated by a relationship between growth rate and Zn/O ratios. Surface phase diagrams are obtained by investigation of reflection high-energy electron diffraction. Hexagonal-shaped two-dimensional islands with atomic steps measured by atomic force microscopy are observed on ZnO layers grown under oxygen-rich and stoichiometric flux conditions. On the other hand, ZnO layers grown under Zn-rich conditions exhibit hexagonal pits. The x-ray rocking curve of ZnO samples grown under an oxygen-rich condition is broader than that of ZnO samples grown under stoichiometric or Zn-rich flux conditions, implying a reduction in threading dislocation density. Photoluminescence spectra reveal the strongest intensity from ZnO layers grown under stoichiometric flux conditions, compared with those grown under Zn- and oxygen-rich conditions. The relation between linewidth of the x-ray rocking curve and intensity of photoluminescence suggests that threading dislocations act as nonradiative centers. In conclusion, the Zn/O flux ratio during growth has a strong influence on the quality of ZnO epilayer surfaces, crystal structures, and optical properties.

Investigation of ZnO epilayers grown under various Zn/O ratios by plasma-assisted molecular-beam epitaxy

MBE growth of high-quality ZnO films on epi-GaN

Plasma-assisted molecular-beam epitaxy of ZnO epilayers on MgAl2O4(111) substrates is described. Acid mixed by H2SO4 and H3PO4 was used for the substrate etching, which provides atomically flat surfaces with a regular terrace array. The influence of the substrate on lateral growth and coalescence of ZnO islands during epitaxy is discussed with corresponding morphological and structural properties. The epitaxial relationship between the ZnO epilayer and MgAl2O4 substrate is determined as: ZnO[0110]∥MgAl2O4[112] and ZnO[2110]∥MgAl2O4[110]. X-ray diffraction, photoluminescence, and Hall-effect measurements are reported, which indicate that high-quality ZnO epilayers are obtained by using atomically flat MgAl2O4(111) substrates to suppress columnar growth.

Plasma-assisted molecular-beam epitaxy of ZnO epilayers on atomically flat MgAl2O4(111) substrates

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