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

Growth and characterization of cubic AlGaN and AlN epilayers by RF-plasma assisted MBE

Residual strain in single crystalline germanium islands, recrystallized by zone melting, on SiO2 substrates has been investigated by Raman microprobe measurements with a spatial resolution of 5 µm. It was found that strain of 3.0×10-3–6.0×10-3, corresponding to local tensile stress of 2.3 to 4.6 kbar, remained in single crystalline germanium islands after zone melting recrystallization.

Residual Strain in Single Crystalline Germanium Islands on Insulator

Infrared and Raman measurements of lattice vibrations in Mg x Cd 1- x Te mixed crystals have been made in the concentration range x =0∼0.6. The modified random-element isodisplacement model has been applied to explain the concentration dependence of long wavelength longitudinal and transverse optic phonon frequencies in the mixed crystal system. From fitting the computed values to the experimental data we have estimated the static dielectric constant e 0 , TO and LO phonons of pure MgTe, and a gap made of Cd in MgTe.

Lattice Vibrations of Mg x Cd 1-x Te Mixed Crystals

We characterized GaN crystals with various dislocation densities by micro-Raman spectroscopy Defects and Strain for the GaN layer were examined through measurements of the Raman shift and the width of the TO phonon bands The broadening of Raman bands in GaN crystals occurred as the dislocation density increased The up-shift of the peak frequencies corresponding to compressive strain was observed for the heteroepitaxial samples grown by MOCVD and MBE The in-plane distribution of defects and strain in epilayers with various dislocation densities was also examined by Raman mapping The Raman maps showed that the examined phonon frequency and band width in samples fluctuated spatially The increase of dislocation density in GaN epilayers induced not only the broadening of Raman bands but also increase of fluctuation (© 2008 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim)

Raman scattering analysis of GaN with various dislocation densities

Damage to silicon implanted with Si++, Au++ and Be++ ions using focused ion beams has been evaluated by a Raman microprobe technique. A quantitative evaluation of damage to ion-implanted silicon was made by comparing the intensities of Raman scattering from implanted and unimplanted areas. The minimum dose at which damage is detectable by Raman measurements is 9×1011 ions/cm2 for Au++, 7.5×1012 ions/cm2 for Si++ and 7.5×1013 ions/cm2 for Be++. From the depth distribution of the damage measured using laser lines of different wavelengths, it is inferred that damage at the near-surface region in Be++ implanted silicon is reduced by self annealing during high-dose-rate implantation and that a channeling effect causes the saturation dose in Be++ implantation to be higher than that in other ion implantations.

https://iopscience.iop.org/article/10.1143/JJAP.26.903/pdf

Shin-ichi Nakashima

Papers

Growth and characterization of cubic AlGaN and AlN epilayers by RF-plasma assisted MBE

Residual Strain in Single Crystalline Germanium Islands on Insulator

Lattice Vibrations of Mg x Cd 1-x Te Mixed Crystals

Raman scattering analysis of GaN with various dislocation densities

Characterization of Silicon Implanted with Focused Ion Beam by Raman Microprobe