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...

/pdf/luminescence-properties-of-defects-in-gan-4hjs241x7f.pdf

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

Phase-dependent noise in femtosecond pump-probe experiments on Bi and GaAs

The resonant Raman spectra of 4H-SiC, 6H-SiC, and 15R-SiC were measured from visible excitation to ultraviolet excitation. A selective enhancement of the intensities of the folded longitudinal phonon modes relative to the nonfolded longitudinal optical phonon mode was found. To explain this, we used a bond-Raman-polarizability model with a one-dimensional lattice dynamics model. It uses bond Raman polarizabilities of three bond arrangements, which we assumed to have different excitation energy dependences. By using this model, we successfully reproduced the excitation energy dependence of the relative intensities of the folded longitudinal phonon modes. The bond Raman polarizabilities and force constants used in our calculations are common for all polytypes. We found that the bond Raman polarizabilities of the three bond arrangements had different resonance energies.

/pdf/selective-resonance-effect-of-the-folded-longitudinal-phonon-1nr6wxc3gj.pdf

Selective resonance effect of the folded longitudinal phonon modes in the Raman spectra of SiC

The structure of silica glass implanted with O+, B+ and Ag+ at an acceleration energy of the order of MeV has been investigated by infrared reflection and Raman scattering spectroscopies It is found that the Si–O–Si bond angle decreases with ion implantation Three-fold siloxane rings are formed with ion implantation It is deduced that compaction of silica glass caused by ion implantation is attributed to the decrease in the Si–O–Si bond angle and the formation of small siloxane rings The modification of the silica glass structure caused by ion implantation is similar to that caused by neutron irradiation

Surface Structure of Ion-Implanted Silica Glass

We have developed a high-throughput deep-ultraviolet (DUV) Raman microspectrometer with excitation from a continuous wave (cw) laser operated at 244 nm that enables us to characterize thin surface layers of wide-gap semiconductors. This spectrometer system consists of a filter spectrometer for the rejection of stray light and a high-dispersion spectrograph combined with a liquid nitrogen cooled charge-coupled device (CCD) detector and extends the low-frequency limit of the observable spectral range down to 170 cm-1. In the microscope we use a Cassegrain reflective objective for the collection of the scattered light and an off-axis mirror for introduction of the excitation laser light. DUV Raman spectroscopy has been applied for studying wide-gap semiconductors including SiC and AlGaN epitaxial films and shallow implanted layers of these materials. Raman spectra of various crystals have also been measured for examining the performance of this system. Resonance enhancement of Raman bands has been observed for several semiconductors, and the results are discussed.

Shin-ichi Nakashima

Papers

Phase-dependent noise in femtosecond pump-probe experiments on Bi and GaAs

Selective resonance effect of the folded longitudinal phonon modes in the Raman spectra of SiC

Surface Structure of Ion-Implanted Silica Glass

Deep-ultraviolet Raman microspectroscopy: characterization of wide-gap semiconductors.

Optical properties of (CdTe)1‐x(CdSe)x and (CdTe)1‐x(cds)x systems