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

The development of novel materials is a fundamental focal point of chemical research; and this interest is mandated by advancements in all areas of industry and technology. A good example of the synergism between scientific discovery and technological development is the electronics industry, where discoveries of new semiconducting materials resulted in the evolution from vacuum tubes to diodes and transistors, and eventually to miniature chips. The progression of this technology led to the development * To whom correspondence should be addressed. B.L.C.: (504) 2801385 (phone); (504) 280-3185 (fax); bcushing@uno.edu (e-mail). C.J.O.: (504)280-6846(phone);(504)280-3185(fax);coconnor@uno.edu (e-mail). 3893 Chem. Rev. 2004, 104, 3893−3946

http://depa.fquim.unam.mx/amyd//archivero/FidelmarLechugaSanabria_4940.pdf

Recent advances in the liquid-phase syntheses of inorganic nanoparticles.

Oxide pyrochlores — A review

Biopolymer-based hydrogels as scaffolds for tissue engineering applications: a review

Synthesis methods for nanosized hydroxyapatite with diverse structures.

Band-gap energies of sol-gel-derived SrTiO3 thin films were studied in terms of annealing temperature and film thickness. The band-gap energies of highly crystallized films were comparable to those of single crystals reported, whereas for poor-crystallized films, their band-gap energy values were much larger than those of single crystals. The larger band-gap energy shift was believed to be mainly due to both quantum size effect and existence of amorphous phase in thin films. The band-gap energies also showed a strong dependence on film thickness. There was a critical film thickness (∼200 nm), above which the films had band-gap energies close to those of crystals or bulks, but below that, the values shifted largely, which can be attributed to the influence of crystallinity of thin films. Such a thickness effect of band-gap energy should be of high interest in optical device applications.

Band-gap energies of sol-gel-derived SrTiO3 thin films

A series of new compounds, Ba 1−x R 2x/3 Nb 2 O 6 (RNd, Eu, Gd, Tb, Dy, Ho and Er) with tetragonal tungsten bronze (TTB) structure were synthesized. For Ba–R–Nb–O system, single phase TTB was obtained when X =0·25. To understand the formation condition of these TTB type compounds, two tolerance factors for TTB, t A1 and t A2 , were proposed against the coordination polyhedra around A1 and A2 site cations, respectively. One of the factors was at least a requisition to stabilize crystal structure for all TTB type compounds. Ba 1− x R 2 x /3 Nb 2 O 6 showed ferroelectric characteristics and the Curie temperature, T c , increased as the decrease of the ionic radius of rare earth cations. These new compounds have potential to be candidates to apply ferroelectric random access memory (FRAM).

Synthesis and dielectric properties of Ba1-xR2x/3Nb2O6(R : Rare earth) with tetragonal tungsten bronze structure

A new concept of the formation of the monolayer of colloidal particles on the air-liquid interface based on a new simple model to consider the relation between the interparticle forces and the monolayer structures was proposed. Colloidal monolayers were formed by spreading monodisperse silica particles coated with alkoxyl chains of different length at the air-benzene interface and picking up the floating clusters or ordered domains with a mica substrate. Polycrystalline, two-dimensional monolayers were formed when the particles were coated with dodecyl chains, while coating with short alkoxyl chains (butyl and decyl chains) resulted in inhomogeneous, porous films. The difference in monolayer structure could be related to the interparticle forces at the air-liquid interface which can be controlled by changing the degree of immersion or the range of the repulsive interaction. It was concluded that the formation of an ordered colloidal monolayer required that the interparticle attraction is sufficiently weak allowing the formation of ordered domains which can deform during drying.

Preparation of Colloidal Monolayers of Alkoxylated Silica Particles at the Air-Liquid Interface

An XPS study of the nucleation and growth behavior of an epitaxial Pb(Zr,Ti)O3/MgO(100) thin film prepared by MOCVD

The relation between the suspension state and the rheological properties, the consolidation, and packing of a very fine (nanosized) BaTiO3 powder has been investigated. The BaTiO3 powder was suspended in a nonaqueous medium by adsorbing fatty acids and a polymeric dispersant, poly(12-hydroxy stearic acid), (PHS), at the BaTiO3/decane interface. Calculated interparticle energies imply that the suspension with PHS adsorbed is colloidally stable, while the suspensions with oleic and octanoic acid can be characterized as weakly and strongly flocculated, respectively. Analysis of settling experiments and rheological measurements at high concentrations confirmed these characteristics. Pressure filtration resulted in nearly identical green body densities in spite of the differences in colloidal properties, but the preliminary sintering experiments and microstructural characterization showed that the strongly flocculated suspension displays a significantly retarded sinterability compared to the colloidally stable and the weakly flocculated suspensions. The absence of a correlation between green density and sintering behavior was explained by considering both the volume taken by the adsorbed fatty acids and the PHS polymer—which can be substantial for nanosized powders—and the state of the suspension. While a decrease in the thickness of adsorbed surfactant or polymer layer will enable a higher particle packing density, such a thin adsorbed layer results in a more strongly flocculated suspension which will resist dense packing. Hence, it is suggested that the green bodies of the colloidally stable and the weakly flocculated suspensions correspond to a relatively homogeneous, but loosely packed, green body microstructure. The strongly flocculated suspension results in a green body with a more inhomogeneous microstructure.

Kazuo Shinozaki

Papers

Band-gap energies of sol-gel-derived SrTiO3 thin films

Synthesis and dielectric properties of Ba1-xR2x/3Nb2O6(R : Rare earth) with tetragonal tungsten bronze structure

Preparation of Colloidal Monolayers of Alkoxylated Silica Particles at the Air-Liquid Interface

An XPS study of the nucleation and growth behavior of an epitaxial Pb(Zr,Ti)O3/MgO(100) thin film prepared by MOCVD

Colloidal Processing of a Very Fine BaTiO3 Powder—Effect of Particle Interactions on the Suspension Properties, Consolidation, and Sintering Behavior