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

This review covers important advances in recent years in the physics of thin-film ferroelectric oxides, the strongest emphasis being on those aspects particular to ferroelectrics in thin-film form. The authors introduce the current state of development in the application of ferroelectric thin films for electronic devices and discuss the physics relevant for the performance and failure of these devices. Following this the review covers the enormous progress that has been made in the first-principles computational approach to understanding ferroelectrics. The authors then discuss in detail the important role that strain plays in determining the properties of epitaxial thin ferroelectric films. Finally, this review ends with a look at the emerging possibilities for nanoscale ferroelectrics, with particular emphasis on ferroelectrics in nonconventional nanoscale geometries.

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Physics of thin-film ferroelectric oxides

Controlled self-organization of nanoparticles can lead to new materials. The colloidal crystallization of non-spherical nanocrystals is a reaction channel in many crystallization reactions. With additives, self-organization can be stopped at an intermediary step-a mesocrystal-in which the primary units can still be identified. Mesocrystals were observed for various systems as kinetically metastable species or as intermediates in a crystallization reaction leading to single crystals with typical defects and inclusions. The control forces and mechanism of mesocrystal formation are largely unknown, but several mesocrystal properties are known. Mesocrystals are exiting examples of nonclassical crystallization, which does not proceed through ion-by-ion attachment, but by a modular nanobuilding-block route. This path makes crystallization more independent of ion products or molecular solubility, it occurs without pH or osmotic pressure changes, and opens new strategies for crystal morphogenesis.

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Mesocrystals: Inorganic superstructures made by highly parallel crystallization and controlled alignment

Transparent conductors as solar energy materials: A panoramic review

Ultrathin metal films and particles on oxide surfaces: structural, electronic and chemisorptive properties

The surface chemical states of the perovskite-type compounds, strontium doped lanthanum cobalt oxides (La1−xSrxCoO3), have been investigated using X-ray photoelectron spectroscopy (XPS). Catalytic oxidations of both methane and CO have also been investigated using flow methods. The chemical composition of the surface of La1−xSrxCoO3 was very different from that in the bulk, which was measured by X-ray fluorescence spectroscopy (XRFS). The catalytic activity of La1−xSrxCoO3 increased with an increase in the quantity of cobalt atoms on the surface.

Surface characterization and catalytic properties of La1−x -Sr x CoO3

We have prepared a diluted system of Co3O4 nanocrystals dispersed in nonmagnetic SiO2 by heating an MCM-41 molecular sieve soaked in 0.075 mol/L CoCl2 aqueous solution in flowing oxygen at 300 °C for 3 h. Transmission electron microscopy showed that fine particles of about 3 nm exhibiting the (220) and (331) lattice fringes of the Co3O4 crystal distribute randomly in amorphous SiO2. Magnetic measurements using a SQUID magnetometer suggested the presence of quantum resonant spin tunneling of intraparticle superparamagnetic moments below the blocking temperature. The maximum points in the ac susceptibility shifted toward higher temperature from ≈3 to ≈5 K with increasing the frequency from ≈10-2 to ≈103 Hz. There were no divergent peaks above 1.7 K in the nonlinear susceptibility. The field dependent magnetization from 0 to ≈20 kG showed a small hysteresis loop at 2 K, whereas there were no hysteresis loops at both 5 and 10 K. The diluted Co3O4 nanocrystals is a new and ideally suitable material for the stu...

Intraparticle Magnetic Properties of Co3O4 Nanocrystals

Thin films of the Bi-Sr-Ca-Cu-O system were prepared by sequential deposition of Bi, SrCu, CaCu and SrCu oxides using multitarget sputtering. It was confirmed that the c-axis spacing of lattice planes could be controlled by the variation of the CaCu oxide layer. Also, thin films of the Bi system with the insertion of four Cu-O2 planes were synthesized. The Tcs of the films were found to be 80 K, 110 K and 90 K for the Bi system with two, three and four Cu-O2 planes, respectively.

https://iopscience.iop.org/article/10.1143/JJAP.27.L1883/pdf

Formation of Superconducting Bi–Sr–Ca–Cu–O Thin Films with Controlled c -Axis Lattice Spacings by Multitarget Sputtering

Ti 2p and Ti 3p X-ray photoelectron spectra (XPS) were taken for TiO2, SrTiO3 and BaTiO3 single crystals fractured insitu in an electron spectrometer. The inelastic scattering energy loss parts were removed from the observed Ti core level XPS by a deconvolution method using O 1s spectra as a response function. The Ti 2p spectrum for TiO2 has four maxima and it indicates that the final states of the photoemission had quasi continuous energy states. The Ti 2p spectra for SrTiO3 and BaTiO3 had only three maxima. The Ti 3p spectra of the three samples had satellite peaks with energy separation of about 14 eV from the main peaks. The intensity at the higher binding energy side of the main peaks continuously decreased with the relative binding energy from 0 to 8 eV. The spectral features of the Ti 2p and Ti 3p spectra are explained by the charge transfer model after the photoemission.

Ti 2p and Ti 3p X-ray photoelectron spectra for TiO2, SrTiO3 and BaTiO3

Enhancement of the conductivity of zinc oxide through doping with hydrogen atoms was examined by using ion implantation of highly resistive thin films deposited by rf magnetron sputtering at room temperature. With a doping of 1×1017 atoms cm−2, the conductivity after annealing at 200 °C in an N2 atmosphere at 1 atm rose from the initial 1×10−7 Ω−1 cm−1 to 5.5×102 Ω−1 cm−1.

Shigemi Kohiki

Papers

Surface characterization and catalytic properties of La1−x -Sr x CoO3

Intraparticle Magnetic Properties of Co3O4 Nanocrystals

Formation of Superconducting Bi–Sr–Ca–Cu–O Thin Films with Controlled c -Axis Lattice Spacings by Multitarget Sputtering

Ti 2p and Ti 3p X-ray photoelectron spectra for TiO2, SrTiO3 and BaTiO3

Enhanced conductivity of zinc oxide thin films by ion implantation of hydrogen atoms