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

Dilute ferromagnetic oxides having Curie temperatures far in excess of 300 K and exceptionally large ordered moments per transition-metal cation challenge our understanding of magnetism in solids. These materials are high-k dielectrics with degenerate or thermally activated n-type semiconductivity. Conventional super-exchange or double-exchange interactions cannot produce long-range magnetic order at concentrations of magnetic cations of a few percent. We propose that ferromagnetic exchange here, and in dilute ferromagnetic nitrides, is mediated by shallow donor electrons that form bound magnetic polarons, which overlap to create a spin-split impurity band. The Curie temperature in the mean-field approximation varies as (xdelta)(1/2) where x and delta are the concentrations of magnetic cations and donors, respectively. High Curie temperatures arise only when empty minority-spin or majority-spin d states lie at the Fermi level in the impurity band. The magnetic phase diagram includes regions of semiconducting and metallic ferromagnetism, cluster paramagnetism, spin glass and canted antiferromagnetism.

Donor impurity band exchange in dilute ferromagnetic oxides.

Compared with the conventional deposition techniques used for the epitaxial growth of metallic structures on a bulk substrate, wet-chemical synthesis based on the dispersible template offers several advantages, including relatively low cost, high throughput, and the capability to prepare metal nanostructures with controllable size and morphology. Here we demonstrate that the solution-processable two-dimensional MoS(2) nanosheet can be used to direct the epitaxial growth of Pd, Pt and Ag nanostructures at ambient conditions. These nanostructures show the major (111) and (101) orientations on the MoS(2)(001) surface. Importantly, the Pt-MoS(2) hybrid nanomaterials exhibit much higher electrocatalytic activity towards the hydrogen evolution reaction compared with the commercial Pt catalysts with the same Pt loading. We believe that nanosheet-templated epitaxial growth of nanostructures via wet-chemical reaction is a promising strategy towards the facile and high-yield production of novel functional materials.

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Solution-phase epitaxial growth of noble metal nanostructures on dispersible single-layer molybdenum disulfide nanosheets

We review two strategies for growing ZnO nanowires from zinc salts in aqueous and organic solvents. Wire arrays with diameters in the nanoscale regime can be grown in an aqueous solution of zinc nitrate and hexamethylenetetramine. With the addition of poly(ethylenimine), the lengths of the wires have been increased to 25 μm with aspect ratios over 125. Additionally, these arrays were made vertical by nucleating the wires from oriented ZnO nanocrystals. ZnO nanowire bundles have been produced by decomposing zinc acetate in trioctylamine. By the addition of a metal salt to the solution, the ZnO wires can be doped with a range of transition metals. Specifically, ZnO nanowires were homogeneously doped with cobalt and showed a marked deviation from paramagnetic behavior. We conclude by highlighting the use of these solution-grown nanowire arrays in dye-sensitized solar cells. The nanowire cells showed an improvement in the charge collection efficiency over traditional nanoparticle cells.

Solution-grown zinc oxide nanowires.

The observation of ferromagnetism in magnetic ion doped II–VI diluted magnetic semiconductors (DMSs) and oxides, and later in (Ga,Mn)As materials has inspired a great deal of research interest in a field dubbed “spintronics” of late, which could pave the way to exploit spin in addition to charge in semiconductor devices. The main challenge for practical application of the DMS materials is the attainment of a Curie temperature at or preferably above room temperature to be compatible with junction temperatures. Among the studies of transition-metal doped conventional III–V and II–VI semiconductors, transition-metal-doped ZnO and GaN became the most extensively studied topical materials since the prediction by Dietl et al., based on mean field theory, as promising candidates to realize a diluted magnetic material with Curie temperature above room temperature. The underlying assumptions, however, such as transition metal concentrations in excess of 5% and hole concentrations of about 1020 cm−3, have not gotten as much attention. The particular predictions are predicated on the assumption that hole mediated exchange interaction is responsible for magnetic ordering. Among the additional advantages of ZnO-and GaN-based DMSs are that they can be readily incorporated in the existing semiconductor heterostructure systems, where a number of optical and electronic devices have been realized, thus allowing the exploration of the underlying physics and applications based on previously unavailable combinations of quantum structures and magnetism in semiconductors. This review focuses primarily on the recent progress in the theoretical and experimental studies of ZnO- and GaN-based DMSs. One of the desirable outcomes is to obtain carrier mediated magnetism, so that the magnetic properties can be manipulated by charge control, for example through external electrical voltage. We shall first describe the basic theories forwarded for the mechanisms producing ferromagnetic behavior in DMS materials, and then review the theoretical results dealing with ZnO and GaN. The rest of the review is devoted to the structural, optical, and magnetic properties of ZnO- and GaN-based DMS materials reported in the literature. A critical review of the question concerning the origin of ferromagnetism in diluted magnetic semiconductors is given. In a similar vein, limitations and problems for identifying novel ferromagnetic DMS are briefly discussed, followed by challenges and a few examples of potential devices.

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Ferromagnetism of ZnO and GaN: A Review

Here we report a systematic study of structural, optical, and magnetic measurements on epitaxial Zn0.9Co0.1O films grown on c-plane sapphire single crystal, at various temperatures (500–650°C), using pulsed-laser deposition. The main emphasis in this work has been on the correlation of microstructure with properties, specifically with magnetic properties and the fate of cobalt ions into substitutional sites versus precipitates. The reasons for room-temperature ferromagnetism are explored, and convincingly proved to be one of the inherent properties of the material. Most importantly, the presence of nanoclusters of any magnetic phase was ruled out. This was determined by high-resolution transmission electron microscopy, coupled with electron energy loss spectroscopy and STEM-Z (scanning transmission electron microscopy-atomic number) contrast studies.

Zn0.9Co0.1O-based diluted magnetic semiconducting thin films

We report room temperature ferromagnetism in single crystal Ce1−xCoxO2−δ (x⩽0.05) films deposited on a LaAlO3(001) substrate. Films were grown by a pulsed laser deposition technique and were thoroughly characterized using x-ray diffraction, high-resolution transmission electron microscopy coupled with electron energy loss spectroscopy and scanning transmission electron microscopy-Z contrast, x-ray photoelectron spectroscopy, optical transmission spectroscopy, and magnetic measurements. These films are transparent in the visible regime and exhibit a very high Curie temperature ∼740–875K with a giant magnetic moment. Our results indicate that the ferromagnetic property is intrinsic to the CeO2 system and is not a result of any secondary magnetic phase or cluster formation.

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Ferromagnetism in Co doped CeO2: Observation of a giant magnetic moment with a high Curie temperature

We have investigated the properties of Zn1−xMnxO (x=0.01–0.1) thin films grown on c-plane sapphire single crystals by pulsed laser deposition. The electrical, magnetic, optical, and microstructural properties of these thin films have been characterized systematically, with a primary focus on establishing a correlation between magnetic properties and electrical conductivity. We have shown that this system exhibits ferromagnetism at room temperature when in the conducting as-deposited state. However, upon high temperature annealing in excess oxygen, the samples become insulating and exhibit nonferromagnetic behavior at room temperature. Thus, it is possible to tune ferromagnetism in Zn1−xMnxO diluted magnetic semiconductors by controlling the concentrations of oxygen vacancies and substitutional Mn dopants.

Effect of oxygen annealing on Mn doped ZnO diluted magnetic semiconductors

Here we report on systematic studies of the epitaxial growth and properties of Co and Cu codoped ZnO thin films deposited onto sapphire c-plane single crystals using pulsed-laser deposition. The films display ferromagnetic behavior at room temperature. Detailed atomic scale characterization rules out the presence of clusters and secondary phases as the source of ferromagnetism. Optical measurements and x-ray photoelectron spectroscopy confirm the direct substitution of dopant atoms into Zn lattice sites. At low concentrations of Cu (∼5%) the magnetic moment of Zn1−(0.05+x)Co0.05CuxO materials appears to be additive. At higher concentrations of Cu the net magnetic moment per atom drops off sharply and seems to be relatively insensitive to the Co content. There is a dramatic increase in resistivity of the Co-doped films that accompanies Cu doping. Yet, this change of resistivity does not affect the magnetic moment, suggesting that free carrier mediated mechanism is not a feasible explanation for ferromagnet...

Magnetic, electrical, and microstructural characterization of ZnO thin films codoped with Co and Cu

Here we report systematic studies on the epitaxial growth and properties of Zn1−xVxO[x=0.001-0.2] thin films deposited onto sapphire c-plane single crystals. The thin films were deposited using pulsed laser deposition technique and were found to be epitaxial in nature. X-ray diffraction and high resolution transmission electron microscopy were employed to study the epitaxial relations of Zn1−xVxO with the sapphire substrate and electron energy loss spectroscopy was used to establish the bonding characteristics and oxidation states of vanadium inside the ZnO host. The main emphasis is on the magnetic properties of this system taking into consideration the phase purity and microstructural characteristics of these films. Our results show that the Zn1−xVxO system, with V in zinc substitutional sites, does not exhibit any signature of ferromagnetism, both at room temperature as well as at lower temperatures down to 10 K.

S. Ramachandran

Papers

Zn0.9Co0.1O-based diluted magnetic semiconducting thin films

Ferromagnetism in Co doped CeO2: Observation of a giant magnetic moment with a high Curie temperature

Effect of oxygen annealing on Mn doped ZnO diluted magnetic semiconductors

Magnetic, electrical, and microstructural characterization of ZnO thin films codoped with Co and Cu

Epitaxial growth and properties of Zn1−xVxO diluted magnetic semiconductor thin films