There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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

The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

Atomic layer deposition(ALD), a chemical vapor deposition technique based on sequential self-terminating gas–solid reactions, has for about four decades been applied for manufacturing conformal inorganic material layers with thickness down to the nanometer range. Despite the numerous successful applications of material growth by ALD, many physicochemical processes that control ALD growth are not yet sufficiently understood. To increase understanding of ALD processes, overviews are needed not only of the existing ALD processes and their applications, but also of the knowledge of the surface chemistry of specific ALD processes. This work aims to start the overviews on specific ALD processes by reviewing the experimental information available on the surface chemistry of the trimethylaluminum/water process. This process is generally known as a rather ideal ALD process, and plenty of information is available on its surface chemistry. This in-depth summary of the surface chemistry of one representative ALD process aims also to provide a view on the current status of understanding the surface chemistry of ALD, in general. The review starts by describing the basic characteristics of ALD, discussing the history of ALD—including the question who made the first ALD experiments—and giving an overview of the two-reactant ALD processes investigated to date. Second, the basic concepts related to the surface chemistry of ALD are described from a generic viewpoint applicable to all ALD processes based on compound reactants. This description includes physicochemical requirements for self-terminating reactions,reaction kinetics, typical chemisorption mechanisms, factors causing saturation, reasons for growth of less than a monolayer per cycle, effect of the temperature and number of cycles on the growth per cycle (GPC), and the growth mode. A comparison is made of three models available for estimating the sterically allowed value of GPC in ALD. Third, the experimental information on the surface chemistry in the trimethylaluminum/water ALD process are reviewed using the concepts developed in the second part of this review. The results are reviewed critically, with an aim to combine the information obtained in different types of investigations, such as growth experiments on flat substrates and reaction chemistry investigation on high-surface-area materials. Although the surface chemistry of the trimethylaluminum/water ALD process is rather well understood, systematic investigations of the reaction kinetics and the growth mode on different substrates are still missing. The last part of the review is devoted to discussing issues which may hamper surface chemistry investigations of ALD, such as problematic historical assumptions, nonstandard terminology, and the effect of experimental conditions on the surface chemistry of ALD. I hope that this review can help the newcomer get acquainted with the exciting and challenging field of surface chemistry of ALD and can serve as a useful guide for the specialist towards the fifth decade of ALD research.

Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process

Recent research results pertaining to InN, GaN and AlN are reviewed, focusing on the different growth techniques of Group III-nitride crystals and epitaxial films, heterostructures and devices. The chemical and thermal stability of epitaxial nitride films is discussed in relation to the problems of deposition processes and the advantages for applications in high-power and high-temperature devices. The development of growth methods like metalorganic chemical vapour deposition and plasma-induced molecular beam epitaxy has resulted in remarkable improvements in the structural, optical and electrical properties. New developments in precursor chemistry, plasma-based nitrogen sources, substrates, the growth of nucleation layers and selective growth are covered. Deposition conditions and methods used to grow alloys for optical bandgap and lattice engineering are introduced. The review is concluded with a description of recent Group III-nitride semiconductor devices such as bright blue and white light-emitting diodes, the first blue-emitting laser, high-power transistors, and a discussion of further applications in surface acoustic wave devices and sensors.

https://iopscience.iop.org/article/10.1088/0022-3727/31/20/001/pdf

Growth and applications of Group III-nitrides

Extremely high pressures ($\ensuremath{\sim}10\text{ }\text{ }\mathrm{TPa}$) and temperatures ($5\ifmmode\times\else\texttimes\fi{}{10}^{5}\text{ }\text{ }\mathrm{K}$) have been produced using a single laser pulse (100 nJ, 800 nm, 200 fs) focused inside a sapphire crystal. The laser pulse creates an intensity over ${10}^{14}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$ converting material within the absorbing volume of $\ensuremath{\sim}0.2\text{ }\text{ }\ensuremath{\mu}{\mathrm{m}}^{3}$ into plasma in a few fs. A pressure of $\ensuremath{\sim}10\text{ }\text{ }\mathrm{TPa}$, far exceeding the strength of any material, is created generating strong shock and rarefaction waves. This results in the formation of a nanovoid surrounded by a shell of shock-affected material inside undamaged crystal. Analysis of the size of the void and the shock-affected zone versus the deposited energy shows that the experimental results can be understood on the basis of conservation laws and be modeled by plasma hydrodynamics. Matter subjected to record heating and cooling rates of ${10}^{18}\text{ }\text{ }\mathrm{K}/\mathrm{s}$ can, thus, be studied in a well-controlled laboratory environment.

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Laser-induced microexplosion confined in the bulk of a sapphire crystal: evidence of multimegabar pressures

Nanoscale GaN quantum dots were fabricated on AlxGa1−xN layer surfaces via metalorganic chemical vapor deposition. In order to achieve a self‐assembling dot structure, a two‐dimensional growth mode (step flow) of GaN films on AlxGa1−xN (x=0–0.2) surfaces that is energetically commenced under the conventional growth conditions was intentionally modified into a three‐dimensional mode by using a ‘‘surfactant.’’ The surfactant is believed to inhibit the GaN film from wetting the AlGaN surface due to the change in surface free energy. The resulting morphological structures of GaN dots were found to be sensitive to the doping rate of tetraethyl silane used as a surfactant, the Al content (x) of the AlxGa1−xN layer, and the growth temperature. A very intense photoluminescence emission was observed from the GaN dots embedded in the AlGaN layers.

Self‐assembling GaN quantum dots on AlxGa1−xN surfaces using a surfactant

A new approach toward epitaxial growth of group III nitrides using an "anti-surfactant" is presented. Two unique phenomena, quantum dot formation and dislocation termination, were recognized using this approach. The presence of Si atoms as an anti-surfactant on (Al)GaN surfaces modified the nitride epitaxial growth kinetics. These phenomena appeared to be independent; however, the growth mechanisms indicated a common surface event, which included the formation of a monolayer thick Si–N mask (nano-mask) within the fractional coverage on the surface. The Si–N nano-mask influenced the morphology of the deposited GaN surface, i.e. quantum structures, and also contributed to the termination of threading dislocations in GaN films.

https://iopscience.iop.org/article/10.1143/JJAP.39.L831/pdf

Anti-Surfactant in III-Nitride Epitaxy –Quantum Dot Formation and Dislocation Termination–

Anti-Surfactant in III-Nitride Epitaxy. Quantum Dot Formation and Dislocation Termination.:--- Quantum Dot Formation and Dislocation Termination ---

The photoluminescence emission peak energy of GaN quantum dots was observed to shift to higher energy with decreasing quantum dot size. This effect was found to be a combination of a blueshift from the confinement-induced shift of the electronic levels and a redshift from the increased Coulomb energy induced by a compression of the exciton Bohr radius. From this observation, absolute values of the exciton binding energy as a function of quantum dot size are determined.

Satoru Tanaka

Papers

Laser-induced microexplosion confined in the bulk of a sapphire crystal: evidence of multimegabar pressures

Self‐assembling GaN quantum dots on AlxGa1−xN surfaces using a surfactant

Anti-Surfactant in III-Nitride Epitaxy –Quantum Dot Formation and Dislocation Termination–

Anti-Surfactant in III-Nitride Epitaxy. Quantum Dot Formation and Dislocation Termination.:--- Quantum Dot Formation and Dislocation Termination ---

Observation of confinement-dependent exciton binding energy of GaN quantum dots