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

Raman scattering spectra from LO-phonon-plasmon coupled modes in n-type hexagonal GaN epitaxial layers have been measured in detail for different carrier densities in n=10{sup 17}-10{sup 18} cm{sup -3}. Both the upper and lower branches of the coupled modes were clearly observed, showing striking changes in lineshape and peak frequency with the carrier density. A spectral lineshape analysis shows that n-type GaN belongs to a system of under-damped plasmon satisfying {omega}{sub P}{tau}>or{approx}1, where {omega}{sub P} and {tau} are the plasmon frequency and scattering time, respectively. This is in contrast with other wide band-gap semiconductors such as SiC, which shows clearly the character of over-damped plasmon, {omega}{sub P}{tau}<1. A measurement of spatial distribution of carrier density in epitaxial layers is also presented for an application of the coupled-mode analysis. (orig.) 9 refs.

Raman Microprobe Measurement of Under-Damped LO-Phonon-Plasmon Coupled Mode in n-Type GaN

Raman scattering from 2H-NbS2 and intercalated NbS2

Femtosecond time-resolved reflectance and Raman scattering studies have been made on GaAs epitaxial layers grown at temperatures between 200 and 300° C and subsequently annealed. A subpicosecond carrier lifetime (∼0.25 ps) has been measured for a sample grown at 250° C and annealed at 600° C. Raman measurements using a back scattering geometry show a strong TO phonon band for samples grown at 200° C and 250° C, while it is absent for samples grown at 275 and 300° C. The band width of the LO band increases with decreasing growth temperature. A phonon band corresponding to As precipitates is also observed at 200 cm-1 for samples grown at 200 and 250° C. A strong correlation is found between the measured carrier lifetime and Raman profile.

https://iopscience.iop.org/article/10.1143/JJAP.33.4807/pdf

Spectroscopic Characterization of Low-Temperature Grown GaAs Epitaxial Films

Raman spectra from p-type GaN have been systematically studied in the hole density range of 5×1016–1×1018 cm−3. Contrary to the case of n-type samples, spectral profiles of the LO-phonon-plasmon coupled mode in p-type show no remarkable change with the hole density. Thus, precise evaluation of electrical transport parameters such as carrier density and mobility from the coupled mode profile is difficult. However, a continuum band has been observed in the low-frequency range of the spectra, becoming intense with the increase of the hole density. This band has been attributed to the inter-valence-band transition of holes, and the intensity can be used as a good measure of the hole density.

Electronic properties in p-type gan studied by raman scattering

Raman spectroscopy has been applied to the evaluation of strains in ZnSe films grown by molecular beam epitaxy on GaAs. The observed variation of the ZnSe LO phonon frequency with thickness is explained well in terms of the conversion from the elastic strain due to accommodation of the lattice mismatch to the thermal strain due to the difference in thermal expansion coefficients of the substrate and epitaxial film. The strains estimated from the Raman measurements are consistent with the results obtained from X-ray analyses by other researchers.

Shin-ichi Nakashima

Papers

Raman Microprobe Measurement of Under-Damped LO-Phonon-Plasmon Coupled Mode in n-Type GaN

Raman scattering from 2H-NbS2 and intercalated NbS2

Spectroscopic Characterization of Low-Temperature Grown GaAs Epitaxial Films

Electronic properties in p-type gan studied by raman scattering

Raman Scattering Measurements of Strains in ZnSe Epitaxial Films on GaAs