S. Ves

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

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.

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

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.

Characterization of ion-beam-deposited diamond-like carbon films

Abstract Diamond-like carbon (DLC) films are excellent prospects for a wide range of high-technology applications but their precise structure and properties are not well understood. The purpose of the present work was to use several complementary techniques to characterize the nature, structure and microstructure of DLC films. Thin DLC films were deposited on various substrates in the presence of a Si interlayer (500 A thick) using CH 4 ion-beam deposition at an acceleration energy of 750 eV and a current density of about 2.5 mA cm −2 . The Si interlayer was deposited by either e-beam evaporation or Si evaporation enhanced by Ar + beam bombardment (1 keV). The produced DLC films were featureless, very smooth and of high hardness (2900–3300 kg mm −2 ). Auger electron spectroscopy and electron diffraction showed that the films were mainly amorphous. Their microstracture was characterized by a three-dimensional network structure with a medium-range order of about 25 nm. Fourier transform infrared and Raman spectroscopies showed that the films were mainly composed of sp 3 and sp 2 carbon-bonded hydrogen. The sp 3 sp 2 ratio varied from 3.2 to 4.1 and was found to depend on the nature of the Si bond layer. The results showed that the nucleation of the diamondlike structure was promoted on the Si interlayer that was deposited under Ar + beam bombardment. This effect can be explained by the higher surface roughness produced in this interlayer as suggested by the reflectivity measurements. Spectroscopic ellipsometry revealed that the films had an optical band gap between 1.56–1.64 eV. The present results are consistent with previous proposals suggesting that the DLC structure is composed of small graphitelike clusters (involving fused six-fold rings) that are interconnected by sp 3 -bonded carbon.

PRESSURE DEPENDENCE OF THE LOWEST DIRECT ABSORPTION EDGE OF ZnSe

Abstract The variation of the lowest direct absorption edge (E0) of ZnSe with hydrostatic pressure has been measured at room temperature with a diamond anvil cell for pressures up to the phase transition (13.5±0.2 GPa). The gap varies sublinearly with pressure. However, when the gap is plotted as a function of the relative change of the lattice constant (-Δa/a0) it shows a slight supralinearly in contrast to most III–V tetrahedral semiconductors which retain a slight sublinearly even as a function of (-Δa/a0). The experimental results are compared to theoretical calculations based (a) on the local density approximation combined with the self-consistent band calculations using the LMTO method (LD-LMTO) and (b) on a local empirical pseudopotential. Both calculations predict a sublinear dependence of E0 as a function of (-Δa/a0). The semiempirical Murnaghans equation of state that was calculated with the LD-LMTO method, has also been tested by determining the lattice constant under pressure with an energy dispersive x-ray diffractometer.

A high pressure Raman study of calcium molybdate

Abstract In this work we present a Raman study of CaMoO4 as a function of hydrostatic pressure up softening in the scheelite structure, while in the high pressure phases its slope becomes positive. The v1v3 stretching modes of the MoO2−4 group exhibit drastic changes in their pressure dependencies at both phase transitions. This behavior is connected with the increase in the coordination number of the Mo ion in the high pressure phases. These characteristics are compared with the high pressure behavior of other molybdates and tungstates.

The optical properties of a‐C:H films between 1.5 and 10 eV and the effect of thermal annealing on the film character

The optical properties of amorphous hydrogenated carbon films prepared with various techniques are studied with conventional and synchrotron‐radiation spectroscopic ellipsometry (SE) and the pseudodielectric function 〈e(ω)〉 of diamondlike and graphitelike films is presented in the energy region 1.5–10 eV. Characteristic features of the measured 〈e(ω)〉 and the calculated electron‐energy‐loss (EEL) function are found to serve as useful criteria for the classification of such materials. The results and information obtained by SE are compared to those obtained by EEL and Raman spectroscopy techniques, which are the most widely used for this purpose. Thermal annealing experiments up to 675 °C with in situ monitoring of the 〈e(ω)〉 reveal the undergoing structural changes in the material character from diamondlike into graphitelike during the annealing. The major modifications which turn the material into sp2‐like are found to take place around and above 550 °C. The fundamental gap along with other optical param...

Pressure screening in the interior of primary shells in double-wall carbon nanotubes

The pressure response of double-wall carbon nanotubes has been investigated by means of Raman spectroscopy up to

Lattice dynamical properties of the rare earth aluminum garnets (RE3Al5O12)

10\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}

Temperature and pressure dependence of Raman-active phonons of CaMoO4: an anharmonicity study

. The intensity of the radial breathing modes of the outer tubes decreases rapidly but remain observable up to

Comparative study of thin poly‐Si films grown by ion implantation and annealing with spectroscopic ellipsometry, Raman spectroscopy, and electron microscopy

9\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}