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

Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

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Spintronics: Fundamentals and applications

Organizational Citizenship Behaviors: A Critical Review of the Theoretical and Empirical Literature and Suggestions for Future Research

We test the relationship between shareholder value, stakeholder management, and social issue participation. Building better relations with primary stakeholders like employees, customers, suppliers, and communities could lead to increased shareholder wealth by helping firms develop intangible, valuable assets which can be sources of competitive advantage. On the other hand, using corporate resources for social issues not related to primary stakeholders may not create value for shareholders. We test these propositions with data from S&P 500 firms and find evidence that stakeholder management leads to improved shareholder value, while social issue participation is negatively associated with shareholder value. Copyright © 2001 John Wiley & Sons, Ltd.

http://faculty.wwu.edu/dunnc3/rprnts.shareholdervaluesocialissues.pdf

Shareholder value, stakeholder management, and social issues: what's the bottom line?

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

We have investigated the epitaxial growth of (Sr,Ba)-based oxides on SrSi2-terminated (001) Si using laser-molecular beam epitaxy. Reflection high-energy electron diffraction (RHEED) confirms the formation of a commensurate submonolayer of (Ba,Sr)Si2 when strontium and barium ablation targets are used as the metal sources. Stability of the silicide RHEED peak intensity between laser ablation pulses indicates that the submonolayer SrSi2 coverage is relatively stable during formation despite the required high temperatures. While the subsequent growth of BaO via Ba metal ablation and O2 flux on the SrSi2-terminated surface is favored at low temperatures, the formation of an epitaxial silicate is observed for oxide formation at temperatures above 550 °C. X-ray diffraction confirms that the Ba2SiO4 film formed with elevated temperature oxidation is in-plane aligned. These results suggest that while epitaxial BaO can be realized on SrSi2-terminated Si, the interface may be susceptible to silicate formation upon thermal treatment. These results also demonstrate that laser-molecular beam epitaxy can be effective in achieving submonolayer surface coverage specifically for terminating silicon for oxide epitaxy.

Strontium silicide termination and silicate epitaxy on (001) Si

Two dominant factors limiting efficiency of optical spin detection in ZnO-based materials system are identified from time-resolved optical orientation and magneto-optical studies. The first is related to the fundamental band structure of the materials characterized by a weak spin-orbit interaction. It leads to cancellation of circular polarization from the optical transitions between the conduction band and the A and B valence band states, which would otherwise carry the desired information on spin polarization of carriers. The second limiting factor is shown to be efficient carrier/exciton spin relaxation, i.e., about 45–80ps.

Dominant factors limiting efficiency of optical spin detection in ZnO-based materials

Characterization of the capacitance–voltage characteristics of ultrathin gate dielectrics is discussed. Focus is given to the limitations of two-element models in extracting gate capacitance from measurements of complex impedance. Of particular interest for discussion is the apparent frequency dispersion and bias dependence of accumulation capacitance. Comparison is made to a three-element model that takes into account both gate dielectric conductance and substrate resistance, but requires a measurement of complex impedance at multiple frequencies. The electrical properties of a specific alternative gate dielectric structure, namely a 10 nm Y2O3 film on silicon, are analyzed within the context of the three models.

Capacitance–voltage measurements on ultrathin gate dielectrics

The epitaxial growth and properties of Nd 1+ x Ba 2− x Cu 3 O y (NdBCO) thin films using pulsed-laser deposition are reported Thin films on single crystal LaAlO 3 having T c0 =93 K and J c ( H =0, T =77 K)=3 MA/cm 2 were reproducibly fabricated NdBCO films were subsequently grown on textured Ni substrates on which a YSZ/CeO 2 buffer-layer architecture was previously deposited by the same PLD technique X-ray diffraction data showed a high degree of in-plane and out-of-plane alignment for the different layers of these coated conductors Electron diffraction analysis of the NdBCO film revealed dense morphology and low-angle grain boundaries (≤3°) Direct current transport measurements yielded a J c of about 300 kA/cm 2 at zero field and 77 K J c dependence on the field intensity and orientation revealed enhanced pinning at high fields with respect to YBCO and NdBCO films on LaAlO 3 

Transport and structural characterization of epitaxial Nd1+xBa2−xCu3Oy thin films grown on LaAlO3 and Ni metal substrates by pulsed-laser deposition

We report on the fabrication of UV LEDs based on a p-n junction p-ZnMgO/n-ZnO/n-ZnMgO double heterostructure. Pulsed-laser deposition was used to grow the complete heterostructure on c -plane sapphire templates. The LEDs were patterned by simple wet etching. Band-edge electroluminescence emission most likely associated with ZnO excitonic transitions was observed at room temperature. However, the devices show sensitivity to the presence of hydrogen in the measurement ambient due to formation of a surface conduction layer. The results show the potential of ZnO-based materials for UV emitters of potentially lower cost and with comparable or higher emission intensity than AlGaN/GaN devices provided adequate surface passivation techniques are developed.

David P. Norton

Papers

Strontium silicide termination and silicate epitaxy on (001) Si

Dominant factors limiting efficiency of optical spin detection in ZnO-based materials

Capacitance–voltage measurements on ultrathin gate dielectrics

Transport and structural characterization of epitaxial Nd1+xBa2−xCu3Oy thin films grown on LaAlO3 and Ni metal substrates by pulsed-laser deposition

Materials and Process Development for ZnMgO/ZnO Light-Emitting Diodes