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

First-principles calculations have evolved from mere aids in explaining and supporting experiments to powerful tools for predicting new materials and their properties. In the first part of this review we describe the state-of-the-art computational methodology for calculating the structure and energetics of point defects and impurities in semiconductors. We will pay particular attention to computational aspects which are unique to defects or impurities, such as how to deal with charge states and how to describe and interpret transition levels. In the second part of the review we will illustrate these capabilities with examples for defects and impurities in nitride semiconductors. Point defects have traditionally been considered to play a major role in wide-band-gap semiconductors, and first-principles calculations have been particularly helpful in elucidating the issues. Specifically, calculations have shown that the unintentional n-type conductivity that has often been observed in as-grown GaN cannot be a...

First-principles calculations for defects and impurities: Applications to III-nitrides

Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The p...

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Luminescence properties of defects in GaN

Titanium dioxide (TiO2 ) nanocrystals are prepared by a hydrolysis process of tetrabutyl titanate. Nanocrystal samples with various sizes of 6.8-27.9 nm are obtained after annealing from 100 to 650 °C. The crystal structures and the average particle sizes are examined using x-ray diffraction. Raman scattering was employed to investigate the evolution of the anatase phase in the nanocrystals during annealing. Phonon confinement and non-stoichiometry effects are responsible for the blueshift and broadening of the lowest-frequency Eg Raman mode. The influence of interfacial vibrations on the Raman linewidth is also discussed.

https://iopscience.iop.org/article/10.1088/0022-3727/33/8/305/pdf

Raman scattering study on anatase TiO2 nanocrystals

https://hal.archives-ouvertes.fr/hal-00120432/document

Raman Spectroscopy of Nanomaterials: How Spectra Relate to Disorder, Particle Size and Mechanical Properties

The charge-density-wave ~CDW! phase transition in h-Mo4O11 is studied by femtosecond time-resolved reflection. We observed coherent optical phonons in the reflectivity signal in both the normal and the CDW states. Below the transition temperature Tc1 , six vibrational modes were clearly detected. Four of them, centered at 64, 78, 85, and 98 cm, were enhanced below Tc1 . We conclude that the generation of these coherent phonon modes are closely related to the CDW state. @S0163-1829~98!50140-3#

Coherent phonons from the CDW state in h-Mo 4 O 11

The Colloidal Band in KCl Containing NaCl

Lattice Vibrations in MgTe-CdTe Mixed Crystals

The photoluminescence spectra and its excitation spectra of the donor-acceptor pairs are observed in ZnTe crystals doped with Li and As in the region below the bandgap energy. The relaxation of electrons and holes into the first excited state of d-a paris is studied for the three excitation processes: (1) bound-to-bound transitions, (2) bound-to-free transitions, and (3) free-to-free transitions. It is concluded that most of the electrons and holes at the excited states of each impurity level are relaxed rapidly into their ground states before the occurrence of the recombination involving the excited states. For the excitation process (2), conduction electrons are preferentially trapped by positively charged pairs. The redistribution of bound holes by hopping is suggested to explain the broad d– a emission band observed for the bound-to-free excitation for very distant pairs.



Le spectre de photoluminescence et ses spectres d'excitation des pairs donneur-accepteur ont ete observes dans les cristaux de ZnTe dopes avec Li et As dans la region situee au-dessous de l'energie de la bande interdite. La relaxation des electrons et des trous dans le premier etat excite des paires d–a a ete etudiee pour les trois processus d'excitation: 1) transitions etat lie–etat lie; 2) transitions etat lie–etat libre; et 3) transitions etat libre–etat libre. On a conclu que la plupart des electrons et des trous dans les etats excites de chaque niveau d'impurete sont rapidement relaxes dans leurs etats fondamentaux avant la formation de la recombinaison impliquant les etats excites. Pour le processus d'excitation (2), les electrons de conduction sont de preference pieges par des paires chargees positivement. La redistribution des trous lies par saut est proposee pour expliquer la large bande d'emission d–a observee pour l'excitation etat lie etat libre pour des paires tres distantes.

Shin-ichi Nakashima

Papers

Coherent phonons from the CDW state in h-Mo 4 O 11

The Colloidal Band in KCl Containing NaCl

Lattice Vibrations in MgTe-CdTe Mixed Crystals

Excitation and recombination of donor-acceptor pairs in ZnTe

Electronic Properties of Doped SiC at Elevated Temperatures Studied by Raman Scattering