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. ...

/pdf/a-comprehensive-review-of-zno-materials-and-devices-21a3zjadem.pdf

A comprehensive review of zno materials and devices

Zinc oxide is a unique material that exhibits semiconducting and piezoelectric dual properties. Using a solid–vapour phase thermal sublimation technique, nanocombs, nanorings, nanohelixes/nanosprings, nanobelts, nanowires and nanocages of ZnO have been synthesized under specific growth conditions. These unique nanostructures unambiguously demonstrate that ZnO probably has the richest family of nanostructures among all materials, both in structures and in properties. The nanostructures could have novel applications in optoelectronics, sensors, transducers and biomedical sciences. This article reviews the various nanostructures of ZnO grown by the solid–vapour phase technique and their corresponding growth mechanisms. The application of ZnO nanobelts as nanosensors, nanocantilevers, field effect transistors and nanoresonators is demonstrated.

/pdf/zinc-oxide-nanostructures-growth-properties-and-applications-8d5hcz3lg8.pdf

Zinc oxide nanostructures: growth, properties and applications

The purpose of this article is to describe the basic physical processes involved in the nucleation and growth of thin films of materials on solid surfaces. In this introduction the three modes of crystal growth which are thought to occur on surfaces in the absence of interdiffusion are described, and the relationships between the thermodynamics of adsorption and the kinetics of crystal growth are explored in general terms. This is followed by a brief review of atomistic nucleation theory, explaining the relations of such theories to experimental observables. In the next three sections, recent experimental examples of these three growth modes are given, which are interpreted where possible in terms of nucleation and growth theory. The last section discusses observations on the shapes of growing crystallites and the relation of such observations to nucleation and surface diffusion processes.

https://iopscience.iop.org/article/10.1088/0034-4885/47/4/002/pdf

Nucleation and growth of thin films

On the basis of these observations, LkH 233 is the more evolved of the two systems, with well-defined cavities swept out by bipolar outflow and bisected by a very dark lane. LkH 198 is a less evolved system, which is only in the early stages of developing bipolar cavities and possesses lower extinction in the apparent disk midplane. The observed circumstellar environments are consistent with the rotationally flattened infall envelopes models developed for T Tauri stars, indicating that the process of envelope collapse has similar phases, despite the large disparities in mass and luminosity between these two classes of young stars. This morphological similarity leads us to infer that the conservation and transport of angular momentum is the dominant physical process for both classes of stars. Alternate formation pathways have been suggested for OB stars that invoke new physical mechanisms, such as magnetohydrodynamic turbulence (31 )o r stellar mergers (32). The Herbig Ae stars studied here appear to be below the mass threshhold at which such effects become important.

https://science.sciencemag.org/content/sci/303/5662/1348.full.pdf

Single-Crystal Nanorings Formed by Epitaxial Self-Coiling of Polar Nanobelts

Industries such as the automotive, aerospace or military, as well as environmental and biological research have promoted the development of ultraviolet (UV) photodetectors capable of operating at high temperatures and in hostile environments. UV-enhanced Si photodiodes are hence giving way to a new generation of UV detectors fabricated from wide-bandgap semiconductors, such as SiC, diamond, III-nitrides, ZnS, ZnO, or ZnSe. This paper provides a general review of latest progresses in wide-bandgap semiconductor photodetectors.

https://iopscience.iop.org/article/10.1088/0268-1242/18/4/201/pdf

Wide-bandgap semiconductor ultraviolet photodetectors

Photoluminescence (PL) experiments were carried out at 300 and 12 K to investigate the electro‐optical properties of Cd1−xZnxTe grown by molecular beam epitaxy on GaAs substrates. The compositional dependence of the band‐gap energy was determined. It has a quadratic dependence on x. The near band edge PL spectra at 12 K show free and bound exciton lines for x=0 and 1 and only broadened bound exciton peaks for other compositions. The bound exciton broadenings are quantitatively explained based on the compositional fluctuations of the cations. The PL line shapes give indications of the high quality of the layers.

Optoelectronic properties of Cd1−xZnxTe films grown by molecular beam epitaxy on GaAs substrates

Here we present the first direct observation of the atomic structure of threading dislocation cores in hexagonal GaN. Using atomic-resolution Z-contrast imaging, dislocations with edge character are found to exhibit an eight-fold ring core. The central column in the core of a pure edge dislocation has the same configuration as one row of dimers on the {10-10} surface. Following recent theoretical work, it is proposed that edge dislocations do not have deep defect states in the band gap, and do not contribute to cathodoluminescence dislocation contrast. On the other hand, both mixed and pure screw dislocations are found to have a full core, and full screw dislocation cores were calculated to have states in the gap.

Direct observation of the core structures of threading dislocations in GaN

Far-infrared magnetoabsorption experiments done in a HgTe-CdTe superlattice are presented. From the results, which are interpreted in terms of interband transitions from valence to conduction subbands, the superlattice band structure has been deduced. These investigations show, in particular, that this superlattice is a quasi zero-energy-gap semiconductor, and yield the first determination of the offset between the HgTe and CdTe valence bands.

Magneto-optical investigations of a novel superlattice: HgTe-CdTe

Monocrystalline multilayers CdTe‐HgTe have been grown for the first time using the molecular beam epitaxy (MBE) technique. A multilayer consisting alternately of CdTe (44 A thick) and HgTe (180 A thick), repeated 100 times, has been grown at 200 °C with a good crystal quality. As for epilayers grown by MBE the crystallinity of the multilayers is improved by a raise in substrate temperature; moreover, the crystal quality is higher for HgTe than for CdTe up to 200 °C. An upper limit of 40 A for the interdiffusion depth between HgTe and CdTe layers has been determined from Auger electron spectroscopy and ion microprobe profiling measurements carried out on a 13 1/2 ‐period multilayer, each period consisting of 150 A for the CdTe layer and 400 A for HgTe layer. We have also observed that no decrease occurs in the peak‐to‐valley ratio Auger signals if the focus of the electron gun is maintained at the center of the ion crater, during all the analysis, using the secondary electron image.

CdTe-HgTe multilayers grown by molecular beam epitaxy

The structural properties of high-quality (0001)ZnO/Al2O3 films grown by plasma-enhanced molecular-beam epitaxy are investigated by x-ray diffraction and transmission electron microscopy. The only defects encountered are threading dislocations with a density of 1010–4×1010 cm−2. Most numerous dislocations are pure-edge dislocations (Burgers vector of 1/3〈1120〉), which accommodate slight in-plane misorientations between subgrains. The oxygen polarity of these films is also established.

Jean-Pierre Faurie

Papers

Optoelectronic properties of Cd1−xZnxTe films grown by molecular beam epitaxy on GaAs substrates

Direct observation of the core structures of threading dislocations in GaN

Magneto-optical investigations of a novel superlattice: HgTe-CdTe

CdTe-HgTe multilayers grown by molecular beam epitaxy

Defect characterization in ZnO layers grown by plasma-enhanced molecular-beam epitaxy on (0001) sapphire substrates