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

ZnS nanostructures: From synthesis to applications

Advances in the accuracy and efficiency of first-principles electronic structure calculations have allowed detailed studies of the energetics of materials under high pressures. At the same time, improvements in the resolution of powder x-ray diffraction experiments and more sophisticated methods of data analysis have revealed the existence of many new and unexpected high-pressure phases. The most complete set of theoretical and experimental data obtained to date is for the group-IVA elements and the group-IIIA--VA and IIB--VIA compounds. Here the authors review the currently known structures and high-pressure behavior of these materials and the theoretical work that has been done on them. The capabilities of modern first-principles methods are illustrated by a full comparison with the experimental data.

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High-pressure phases of group-IV, III–V, and II–VI compounds

In the present work, the tribological mechanism of diamond-like carbon (DLC) films was investigated. DLC films were coated on metallic substrates (M50 steel, Ti-6Al-4V alloy and AISI 440C steel) by methane ion-beam deposition. Pin-on-disc experiments showed that the DLC films possess excellent wear resistance and exhibit low values of friction coefficient (ƒ

A study of the wear mechanism of diamond-like carbon films

A method is described for calculation of the optical constants (the refractive index, extinction coefficient, and absorption coefficient) of some III‐V binaries (GaP, GaAs, GaSb, InP, InAs, and InSb), ternaries (AlxGa1−xAs), and quaternaries (In1−xGaxAsyP1−y) in the entire range of photon energies (0–6.0 eV). The imaginary part of the dielectric function [e2(ω)] is derived first from the joint density‐of‐states functions at energies of various critical points (CPs) in the Brillouin zone; then its real part [e1(ω)] is obtained analytically using the Kramers–Kronig relation. The indirect band‐gap transitions are also assumed to provide a gradually increasing e2 spectrum expressed by a power law of (ℏω−EIDg)2, where ℏω is the photon energy and EIDg is the indirect band‐gap energy. The optical dispersion relations are expressed in terms of these model dielectric functions. The present model reveals distinct structures in the optical constants at energies of the E0, E0+Δ0 [three‐dimensional (3‐D) M0 CP], E1, E...

Optical dispersion relations for GaP, GaAs, GaSb, InP, InAs, InSb, AlxGa1−xAs, and In1−xGaxAsyP1−y

The Raman spectra of tetragonal TlIn${\mathrm{Se}}_{2}$ and monoclinic TlGa${\mathrm{S}}_{2}$, TlGa${\mathrm{Se}}_{2}$, and TlIn${\mathrm{S}}_{2}$ have been investigated as a function of pressure at constant temperatures (300 and 110 K, TlIn${\mathrm{Se}}_{2}$ only at 110 K). Contrary to earlier assignment the structure of the monoclinic compounds can be described in the space group $\frac{C2}{c}$. The appearance and disappearance of Raman modes with increasing pressure are due to a crossing of bands with ${A}_{g}$ and ${B}_{g}$ symmetry and do not indicate the occurrence of a structural phase transition. In TlIn${\mathrm{S}}_{2}$ we observe modes with a negative pressure dependence indicating an impending phase transition at pressures above the maximum pressure of our measurements. The dependence of $(\frac{1}{\ensuremath{\nu}})(\frac{d\ensuremath{\nu}}{\mathrm{dp}})$ on $\ensuremath{\nu}$ of the observed Raman bands in these compounds reveals that a hierarchy of bonding forces is present.

High-pressure Raman study of the ternary chalcogenides TlGaS2, TlGaSe2, TlInS2, and TlInSe2

We have measured the effect of pressure on the energy of the direct-optical-absorption edge (${\mathit{E}}_{0}$ gap, ${\mathrm{\ensuremath{\Gamma}}}_{15}^{\mathit{v}}$\ensuremath{\rightarrow}${\mathrm{\ensuremath{\Gamma}}}_{1}^{\mathit{c}}$) of cubic ZnS, covering the full stability range (0\char21{}15 GPa) of the tetrahedral phase. The ${\mathit{E}}_{0}$ gap exhibits a sublinear increase under pressure, with the corresponding (linear) gap deformation potential being -5.0(2) eV. X-ray diffraction shows the high-pressure phase of ZnS to have the NaCl-type structure for pressures up to at least 27 GPa. In contrast to earlier reports, this phase is not metallic but shows a broad optical-absorption edge with onset near 2 eV, which is characteristic of an indirect-gap material. Experimental results for the equation of state and band-gap deformation potential are compared with ab initio calculations based on local-density theory and the relativistic linear-muffin-tin-orbital method.

Cubic ZnS under pressure: Optical-absorption edge, phase transition, and calculated equation of state

High-quality ZnS, ZnSe, and ZnTe epitaxial films were grown on (001)-GaAs-substrates by molecular beam epitaxy. The 1s-exciton peak energy positions have been determined by absorption measurements from 2 K up to about room temperature. For ZnS and ZnSe additional high-temperature 1s-exciton energy data were obtained by reflectance measurements performed from 300 up to about 550 K. These complete E1s(T) data sets are fitted using a recently developed analytical model. The high-temperature slopes of the individual E1s(T) curves and the effective phonon temperatures of ZnS, ZnSe, and ZnTe are found to scale almost linearly with the corresponding zero-temperature energy gaps and the Debye temperatures, respectively. Various ad hoc formulas of Varshni type, which have been invoked in recent articles for numerical simulations of restricted E1s(T) data sets for cubic ZnS, are discussed.

Temperature dependence of exciton peak energies in ZnS, ZnSe, and ZnTe epitaxial films

Pressure dependence of the optical properties and the band structure of the copper and silver halides

The effect of Mg, Si, N, and O ion implantation (with doses in the range 5×1013–1×1018 cm−2), in epitaxially grown GaN samples has been studied using Raman spectroscopy It is found that implantation increases the static disorder and activates modes that were not allowed in the as-grown material More specifically it causes the appearance of three additional Raman peaks at 300, 420, and 670 cm−1 It is found that the position of these peaks does not depend on the type of the implant and thus they do not correspond to local vibrational modes They are attributed to disorder activated Raman scattering (300 cm−1) and/or to implantation induced N and Ga vacancies or interstitials (420 and 670 cm−1) Finally, ion implantation causes a marginal increase of the build-in hydrostatic stress

S. Ves

Papers

High-pressure Raman study of the ternary chalcogenides TlGaS2, TlGaSe2, TlInS2, and TlInSe2

Cubic ZnS under pressure: Optical-absorption edge, phase transition, and calculated equation of state

Temperature dependence of exciton peak energies in ZnS, ZnSe, and ZnTe epitaxial films

Pressure dependence of the optical properties and the band structure of the copper and silver halides

Raman study of Mg, Si, O, and N implanted GaN