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The effect of plastic deformation on the optical and structural properties of ZnSe crystals has been investigated. The optical properties have been monitored by cathodoluminescence measurements as a function of the deformation degree. Remarkable differences in the defect-related emissions from the most severely deformed areas have been encountered. Deformation of the crystal lattice of ZnSe, associated with slip phenomena, has been studied by means of Electron Backscattered Diffraction and micro-Raman spectroscopy. The relation between the deformation and the optical properties of the ZnSe crystals has been described. [ABSTRACT FROM AUTHOR]

The appearance of segregated wurtzite MgxZn1-xO with low Mg content in thin films with x>0.3 affected by phase separation cannot be reliably probed with crystallographic techniques owing to its embedded nanocrystalline configuration. Here, we show a high-pressure approach which exploits the distinctive behaviors under pressure of wurtzite MgxZn1-xO thin films with different Mg contents to unveil phase segregation for x>0.3. By using ambient conditions, photoluminescence (PL), and with optical absorption and PL under high pressure for x = 0.3, we show that the appearance of a segregated wurtzite phase with a magnesium content of x ~ 0.1 is inherent to the wurtzite and rock-salt phase separation. We also show that the presence of segregated wurtzite phase in oversaturated thin films phase is responsible for the low-energy absorption tail observed above x = 0.3 in our MgxZn1-xO thin films. Our study has also allowed us to extend the concentration dependence of the pressure coefficient of the band gap from the previous limit of x = 0.13 to x ≈ 0.3 obtaining dEg/dP = 29 meV/GPa for wurtzite with x ≈ 0.3 and 25 meV/GPa for the segregated x ≈ 0.09 wurtzite phase. [ABSTRACT FROM AUTHOR]

We report temperature-dependent optical properties of epitaxial CdO thin films grown by metal-organic vapor phase epitaxy on sapphire substrates. Dielectric function [variant_greek_epsilon] = [variant_greek_epsilon]1 + i[variant_greek_epsilon]2 spectra for CdO were extracted from the multilayer modeling of ellipsometric data, using a set of Tauc-Lorentz oscillators from 0.74 to 6.43 eV in the temperature range between 24 and 650 K. Temperature dependence of the energy for the major optical structures in the [variant_greek_epsilon] spectra was analyzed by using Varshni's approximation. Raman scattering (RS) spectroscopy was used to characterize the vibrational properties of CdO from 77 to 500 K. Several RS peaks were observed in the wavenumber range from 100 to 1000 cm-1. Peak positions, accurately determined by a series of Gaussian-Lorentzian mixed line profiles, exhibit a weak linear dependence on temperature. [ABSTRACT FROM AUTHOR]

A detailed optical characterization by means of micro Raman and cathodoluminescence spectroscopy of catalyst-free ZnO nanorods grown by atmospheric-metal organic chemical vapour deposition has been carried out. This characterization has allowed correlating the growth conditions, in particular the precursors partial-pressures and growth time, with the optical properties of nanorods. It has been shown that a high Zn supersaturation can favor the incorporation of nonradiative recombination centers, which can tentatively be associated with ZnI-related defects. Characterization of individual nanorods has evidenced that ZnI-related defects have a tendency to accumulate in the tip part of the nanorods, which present dark cathodoluminescence contrast with respect to the nanorods bottom. The effect of a ZnO buffer layer on the properties of the nanorods has been also investigated, showing that the buffer layer improves the luminescence efficiency of the ZnO nanorods, revealing a significant reduction of the concentration of nonradiative recombination centers. [ABSTRACT FROM AUTHOR]

We have performed Raman-scattering measurements under high hydrostatic pressure on CdO thin films grown by metal-organic vapor phase epitaxy on sapphire substrates. The pressure dependence of the second-order Raman bands is discussed in terms of ab initio lattice-dynamical calculations, which allow us to obtain mode Grüneisen parameters for the zone-center TO and LO modes of CdO. Our experiments and calculations suggest that at low pressures (<4 GPa) the Raman spectra are dominated by second-order modes, while at higher pressures (>4 GPa) the spectra mainly display contributions from disorder-activated first-order modes. [ABSTRACT FROM AUTHOR]

Semiconductor quantum dots (QDs) are important candidates as light absorbing materials in low cost and high efficiency sensitized solar cells (SCs). We present a combination of structural, chemical, electrical, and optical characterization that provides insight to the photovoltaic efficiencies of devices formed by TiO2 electron conducting oxide network sensitized with CdSe. In devices using colloidal QDs the collection efficiency under short circuit conditions (CESCs) for photoinjected electrons is rather high (∼90%) but the photovoltaic performance is limited by the low loading of QDs into the mesoporous TiO2 structure. On the other hand, chemical bath deposited (CBD) QDSCs exhibit a remarkably high optical density, but only slightly higher short circuit current and efficiency. It is observed that CESC is ∼50% due to the high recombination rates of the closed packed QDs structure. Our results indicate routes for improvement of QDSCs performance by the increase in colloidal QDs loading and the reduction in recombination in QDs grown in situ. [ABSTRACT FROM AUTHOR]

We present a lattice-dynamics study of rocksalt cadmium oxide (CdO). We use Raman scattering to investigate a series of high-quality CdO epilayers grown on sapphire substrates by means of metal-organic vapor phase epitaxy. Two main features are found to dominate the Raman spectra of CdO: a sharp peak at ∼265 cm-1 and a broad feature at ∼390 cm-1. To assign these and other weaker second-order features that appear in the Raman spectra, we have carried out an ab initio calculation of the phonon dispersion in CdO. From the calculated two-phonon density of states (PDOS) we tentatively assign the peak at 265 cm-1 to a 2TA(L) mode. The broadband centered at ∼390 cm-1 seems to reflect the high two-PDOS in the 300–450 cm-1 spectral region. [ABSTRACT FROM AUTHOR]

We report above-bandgap ordinary optical properties of [variant_greek_epsilon]-phase GaSe single crystal. Reference-quality pseudodielectric function <[variant_greek_epsilon](E)>＝<[variant_greek_epsilon]1(E)>+i<[variant_greek_epsilon]2(E)> and pseudorefractive index ＝ +i spectra were measured by spectroscopic ellipsometry from 0.73 to 6.45 eV at room temperature for the light polarization perpendicular to the optic axis (E[accent:_right_hook_over]⊥c⁁). The <[variant_greek_epsilon]> spectrum exhibited several interband-transition critical-point structures. Analysis of second-energy derivatives calculated numerically from the measured data yielded the critical-point energy values. [ABSTRACT FROM AUTHOR]

Photoluminescence and positron annihilation spectroscopy have been used to characterize and identify vacancy-type defects produced in ZnO films grown on sapphire by metal-organic chemical-vapor deposition. The photoluminescence of the samples in the near band edge region has been studied, paying particular attention to the emission at 370.5 nm (3.346 eV). This emission has been correlated to the concentration of Zn vacancies in the films, which has been determined by positron annihilation spectroscopy. [ABSTRACT FROM AUTHOR]

Zn1-xCdxO(1120) films have been grown on (0112) sapphire (r-plane) substrates by metal-organic vapor phase epitaxy. A 800-nm-thick ZnO buffer, deposited prior to the alloy growth, helps to prevent the formation of pure CdO. A maximum uniform Cd incorporation of 8.5 at. % has been determined by Rutherford backscattering spectrometry. Higher Cd contents lead to the coexistence of Zn1-xCdxO alloys of different compositions within the same film. The near band-edge photoluminescence emission shifts gradually to lower energies as Cd is incorporated and reaches 2.93 eV for the highest Cd concentration (8.5 at. %). The lattice deformation, due to Cd incorporation, has been described using a new reference frame in which the lattice distortions are directly related to the a-plane surface structure. Cd introduction does not affect the c lattice parameter but expands the lattice along the two perpendicular directions, [1120] and [1100], resulting in a quadratic volume increase. [ABSTRACT FROM AUTHOR]

CdO(110) layers with a self-organized surface structure have been grown on (1010) sapphire (m Plane) substrates by metal-organic vapor phase epitaxy. The epitaxial relationships between layer and substrate have been determined and a crystallographic model that accounts for the CdO in-plane orientation, which results in a reduced lattice mismatch when the CdO[001] direction is perpendicular to the sapphire c axis, has been proposed. Although the measured lattice parameters indicate that the layers are almost fully relaxed, an anisotropic mosaicity is detected with symmetrical rocking curves attaining minimum values when measured along the CdO[110] direction. The layer morphology consists of a regular ridge-and-valley structure which defines, again, a preferential in-plane direction. The grooves run parallel to the CdO[001] axis and exhibit lateral surfaces sloped at 28° with respect to the (110) surface. The influence of growth temperature and VI/II molar ratio on the anisotropic mosaicity and morphology has been analyzed. [ABSTRACT FROM AUTHOR]

X-ray-absorption fine-structure experiments at different temperatures in ZnSexTe1-x (x=0, 0.1, 0.2, 0.55, 0.81, 0.93, 0.99, and 1.0) have been performed in order to obtain information about the structural relaxation and disorder effects occurring in the alloys. First and second neighbor distance distributions have been characterized at the Se and Zn K edges, using multiple-edge and multiple-scattering data analysis. The first neighbor distance distribution was found to be bimodal. The static disorder associated with the Zn–Te distance variance did not depend appreciably on composition. On the other hand, the static disorder associated with the Zn–Se distance increased as the Se content diminished. Using the bonding angle information provided by our experiments the point of view of the anion has been related to that of the cation. The resulting structural model indicates that Zn tetrahedra surrounding the anions remain essentially undistorted, but forced to tilt from their ideal zincblende orientation to accommodate the minority element. The main origin of structural disorder is suggested. [ABSTRACT FROM AUTHOR]

Emission spectra of GaTe single crystals in the range of 1.90-1.38 eV have been analyzed at different temperatures and excitation intensities by photoluminescence, photoluminescence excitation, and selective photoluminescence. A decrease in band gap energy with an increase in temperature was obtained from the redshift of the free exciton recombination peak. The energy of longitudinal optical phonons was found to be 14 ± 1 meV. A value of 1.796 ± 0.001 eV for the band gap at 10 K was determined, and the bound exciton energy was found to be 18 ± 0.3 meV. The activation energy of the thermal quenching of the main recombination peaks and of the ones relating to the ionization energy of impurities and defects was analyzed. The results obtained show the existence of two acceptor levels with ionization energies of 110 ± 5 and 150 ± 5 meV, respectively, and one donor level with an ionization energy of 75 ± 5 meV. The study of chemical composition by inductively coupled plasma-optical emission spectroscopy and x-ray energy dispersion spectroscopy shows the existence of Na, Li, and Si. Sodium and lithium impurities could be associated with acceptor levels at gallium substitutional sites, and silicon ones with a donor level at Ga sites, whose vacancies can also be involved in these electronic levels. [ABSTRACT FROM AUTHOR]