Multimode phonon structure of Be‐containing II ‐ VI mixed crystals determined by IR spectroscopic ellipsometry

Spectroscopic ellipsometry in the infrared spectral range 250‐5000 cm^‐1 is used for analysis of the dielectric response of Zn_1‐x‐yBe_xMg_ySe and Zn_1‐x‐yBe_xMn_ySe crystals grown by a high‐pressure Bridgman method. Ellipsometric spectra display features in the spectral range 390‐500 cm^‐1 associated with BeSe‐type phonon modes. In the optical spectra of Zn_1‐x‐yBe_xMg_ySe crystals both BeSe‐type and MgSe‐type lattice absorption bands are detected. The MgSe‐like modes are located at approximately 300 cm^‐1. The complex dielectric functions can be reproduced using a model with two or three and one or two classical damped oscillators corresponding to the BeSe‐like and the MgSe‐like transverse‐optical phonon modes, respectively. The frequencies of longitudinal‐optical phonons have been derived from the dielectric loss functions. A red‐shift of the BeSe‐like phonons frequencies with a mean rate 0.42 cm^‐1 (0.50 cm^‐1) per mole percent of Mg (Mn) incorporated to the alloy has been found for examined concentration range x, y ≤ 0.25. A noticeable damping the intensities of BeSe‐type modes with increasing fraction of Mg and Mn dopant is observed in comparison to the strengths of BeSe‐type modes in Zn_1‐xBe_xSe crystals. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoelectric and photothermal investigations of Zn1‐x‐yBexMnySe solid solutions

Zn_1‐x‐yBe_xMn_ySe solid solutions were grown from the melt by the high pressure Bridgman method. Optical, luminescence, photoelectric and photothermal properties of this material were investigated. Interpretation of the piezoelectrically detected photoacoustic spectra was performed using Jackson and Amer theory. From the spectral dependence of the amplitude and phase of photoacoustic signal as well as from photoluminescence spectra the variation of energy gap with composition was determined. The values of thermal diffusivity were derived from the dependence of photoacoustic signal on modulation frequency of the laser radiation illuminating the sample. The photoluminescence‐excitation spectra and photoconductivity spectra were measured. The origin of luminescence in Zn_1‐x‐yBe_xMn_ySe was discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure and optical property of BexZn1‐xO nanorod arrays

Be_xZn_1‐xO nanorod arrays with high crystalline quality were fabricated on Si substrate by a simple, low‐cost hydrothermal method. The effect of Be‐corporation on the structure, morphology and optical property of ZnO nanorod arrays was investigated. The diameter of Be_xZn_1‐xO nanorods gradually decreased and the length of them increased with increasing Be concentration. Edge emissions of the Be_xZn_1‐xO nanorods show a obvious blue shift upon the increase of the Be content.     

Growth and photoluminescence properties of CdS solid solution semiconductor

Photoluminescence (PL) emitted from Cd_1‐xZn_xS and CdS_1‐ySe_y solid solution semiconductor was significantly stronger than PL from the pure CdS and CdSe semiconductor. The samples were prepared using an improved Se‐S‐Na₂S flux route. Photoluminescence in Cd_1‐xZn_xS crystal was brightly yellow at the room temperature under VU radiation. The phase and composition of the solid solution was measured by the XRD and was confirmed by UV‐NIS spectrum as x of 0.3 and y of 0.2. The enhanced photoluminescence was presumably due to the introduction of extra defect (vacancies) by solid solution action and consequently the increasing of luminescence center concentration. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermoelectrical properties of In1‐xGaxAs and InAs crystals irradiated with fast electrons

The thermoelectric power in In_1‐xGa_xAs (x = 0,01;0,04) solid solutions and InAs crystals irradiated with fast electrons by the energy of 6 MeV and dose of 10¹⁶‐ 2 x 10¹⁷ el/cm^‐2 on the interval 80‐400 K have been investigated. It is revealed that in the all crystals the value of the thermoelectric power is decreased under irradiation that resulted from the growth of the free electron concentration to form radiation induced defects of the donor type. It has been determined that in the initial InAs after irradiation, the charge carriers scatter on optical phonons and in In_1‐xGa_xAs solid solutions they do on optical phonons and ionized impurities. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Raman scattering study on Ga1–x Mnx As prepared by Mn ions implantation,deposition and post‐annealing

Raman scattering measurements have been performed in Ga_1–x Mn_x As crystals prepared by Mn ions implantation, deposition, and post‐annealing. The Raman spectrum measured from the implanted surface of the sample shows some weak phonon modes in addition to GaAs‐like phonon modes, where the GaAs‐like LO and TO phonons are found to be shifted by approximately 4 and 2 cm^‐1, respectively, in the lower frequency direction compared to those observed from the unimplanted surface of the sample. The weak vibrational modes observed are assigned to hausmannite Mn₃O₄ like. The coupled LO‐phonon plasmon mode (CLOPM), and defects and As related vibrational modes caused by Mn ions implantation, deposition, and post‐annealing are also observed. The compositional dependence of GaAs‐like LO phonon frequency is developed for strained and unstrained conditions and then using the observed LO_GaAs peak, the Mn composition is evaluated to be 0.034. Furthermore, by analyzing the intensity of CLOPM and unscreened LO_GaAs phonon mode, the hole density is evaluated to be 1.84×10¹⁸ cm^‐3. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Soft‐Mode Spectroscopy in Fe:KTa1‐xNbxO3 Crystals

Fe‐doped potassium tantalite niobate, Fe:KTa_1‐xNb_xO₃ (Fe:KTN), crystallizes with x = 0.48 and a perovskite‐type structure in tetragonal system with point group 4mm, conforming space group P4mm. The paraelectric‐ferroelectric structural transition of the Fe:KTN is studied by Raman scattering investigations. A condensed soft lattice vibrational mode at the phase transition has been analyzed. It originates from the symmetric O2/O3‐Nb/Ta‐O3/O2 in‐plane bending of the Nb/TaO₆ group. The soft optical phonon mode concerns the extraordinary transverse optical phonons propagating along the [110] direction. The Raman spectra measured reflect the crystal disorder. Curie temperature measured by two methods is within 353 and 356.5 K.     

Optical phonons and Raman scattering in ternary II–VI spheroidal nanocrystals embedded in a glass matrix

Theoretical and experimental studies of the spatial phonon confinement in ternary CdS_xSe_1−x nanocrystals embedded in a glass matrix formed by the composites (40)SiO₂₋(30)Na₂CO₃–(29)B₂O₃–(1)Al₂O₃ (mol%) + [(2)CdO + (2)S + (2)Se] (wt%) were carried out. From the analysis of the surface phonon modes, the theoretical procedure has allowed the determination of the geometrical characteristics of the nanocrystals. The calculated frequencies were compared with the experimental values obtained from the Raman spectra of CdS_xSe_1−x nanocrystals grown under different thermal treatments. A good correlation between the experimental and calculated CdS-like and CdSe-like surface optical modes was observed. The Raman selection rules and their connection with the nature of the surface optical phonons is discussed in order to use Raman spectroscopy as a probe to determine the composition x and the geometrical shape of the semiconductor nanocrystals.     

Optical Properties of Bi2Se3‐xAsx Single Crystals

Bi₂Se_3‐xAs_x single crystals with the As content of c_As = 0 to 2.0x10¹⁹ atoms/cm³ prepared from the elements of 5N purity by means of a modified Bridgman method were characterized by measurements of infrared reflectance and transmittance. Values of the plasma resonance frequency omega_p, optical relaxation time tau, and high‐frequency permittivity were determined by fitting the Drude‐Zener formulas to the reflectance spectra. It was found that the substitution of As atoms for Se atoms in the Bi₂Se₃ crystal lattice leads to a decrease in the omega_p values. This effect is accounted for by a model of point defects in the crystal lattice of Bi₂Se_3‐xAs_x. The dependences of the absorption coefficient K on the energy of incident photons were determined from the transmittance spectra. The optical width of the energy gap is found to decrease with increasing As content. The values of the exponent b from the relation of K ∼ lamda^b for the long‐wavelength absorption edge range within the interval 2.0 to 2.3, i.e. the dominant scattering mechanism of free current carriers in Bi₂Se_3‐xAs_x crystals is the scattering by acoustic phonons.     

Growth and dielectric properties of KTiOPO<Subscript>4</Subscript> and K<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Rb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>TiOPO<Subscript>4</Subscript> crystals

Methods of growth of KTiOPO₄ and K_{1 ? x} Rb _x TiOPO₄ crystals of high optical quality have been optimized. The dielectric properties (permittivity and conductivity) of the crystals grown have been investigated at frequencies from 10² to 10⁶ Hz in the temperature range from 100 to 350 K, along the [001] crystallographic direction. It is established that partial substitution of K⁺ ions with Rb⁺ ions leads to a decrease in the permittivity and conductivity.     

Structural and optical properties of Zn(Mg,Cd)O alloy films grown by remote-plasma-enhanced MOCVD

We report the structural and optical properties of wurtzite-structure Zn(Mg,Cd)O ternary alloys. Wurtzite (0 0 0 1) Zn_1−xCd_xO and Mg_yZn_1−yO films were grown on (11–20) sapphire substrates using remote-plasma-enhanced metalorganic chemical vapor deposition. The large bowing parameters of Zn_1−xCd_xO and Mg_yZn_1−yO ternary alloys are 3.0 and 3.5, respectively, which reflects the large difference of each binary’s electronegativity. We have analyzed the broadening of photoluminescence (PL) in Zn(Mg,Cd)O alloys on alloy content by taking into account the statistical alloy fluctuation and the localization of the exciton, and have clarified that the localization of the exciton strongly affects to PL full-width at half-maximum (FWHM) in Zn(Mg,Cd)O alloys. The alloy broadenings in steady-state PL of Zn(Mg,Cd)O alloys are in good agreement with the calculated tendency by the theoretical model based on the statistical alloy fluctuation, while PL FWHM of Zn_1−xCd_xO is three times larger than the calculated results. Moreover, as another way to confirm alloy broadening, we also have done time-resolved PL measurements and derived the localized depth of the exciton in ZnO-based system, indicating a good agreement with the tendency of PL FWHM broadening.     

Optical Phonons and Chemical Bonding in TlSbS2, TlSbSe2, and Tl3SbS3 Crystals

The optical phonons at k = 0 of TlSbS₂, TlSbSe₂, and Tl₃SbS₃ have been investigated by infrared reflectivity measurements from 50 to 4000 cm⁻¹ at 300 K. The factor group analysis of vibrational modes of TlSbS₂ and Tl₃SbS₃ crystal lattices has been made. The dielectric constant dispersion for E ‖ a and E ‖ b has been determined by classical dispersion relations. The Szigeti effective charges, the Born dynamic effective charge, and the Tl, Sb, S (Se) relative ion charges were calculated in dependence on the polarization of the incident light.     

Temperature dependences of the optical characteristics of ZnMgSe single crystals

The temperature dependences of the refractive index of wide-gap Zn_{1 − x} Mg _x Se (x ∼ 0.5) single crystals are measured. It is established that, in the temperature range 300–530 K, the value of dn/dT for such crystals grown by the vertical Bridgman method is 9.31 × 10⁻⁵ at λ = 0.63 μm and 5.29 × 10⁻⁵ at λ = 10.6 μm. The obtained values of the coefficients dn/dT for hexagonal Zn_{1 − x} Mg _x Se single crystals are close to the corresponding values for cubic ZnSe single crystals and are much lower than those for hexagonal CdS. Examples of practical application of single crystals of the ZnMgSe substitutional solid solution as a thermostable material for polarization optical elements in the IR range are given.     

Grain boundary defects induced room temperature ferromagnetism in V doped ZnO thin films

Vanadium (V) doped ZnO thin films (Zn_1‐xV_x O, where x = 0, 0.05, 0.10) have been grown on sapphire substrates by RF magnetron sputtering to realize room temperature ferromagnetism (RTFM). The grown films have been subjected to X‐ray diffraction (XRD), resonant Raman scattering, photoluminescence (PL) and vibrating sample magnetometer (VSM) measurements to investigate their structural, optical and magnetic properties, respectively. The full width at half maximum of XRD and Raman scattering peaks increases with V ion concentration indicates that the V ions have been substituted on Zn²⁺ ions in the ZnO matrix. The increase in oxygen vacancies with V concentration is evidenced by PL measurements. Rutherford backscattering spectrometry analysis confirms the presence of the V ions in the films. The room temperature VSM measurements reveal the signature of ferromagnetism in V doped ZnO thin films. It has been observed that the grain boundary defects, i.e., oxygen vacancies play a crucial role in inducing RTFM in V doped ZnO films. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature dependence of Raman‐active mode frequencies and linewidths in TlGaSe2 layered crystals

Raman spectra of TlGaSe₂ crystal at different temperatures are discussed. The temperature dependence of frequency shifts and linewidths of the Raman peaks in the frequency region of 10‐320 cm^‐1 have been measured in the range from 50 to 320 K. The analysis of the experimental data showed that the temperature dependencies of phonon frequencies and linewidths are well described by considering the contributions from thermal expansion and lattice anharmonicity. The anharmonic contribution (phonon‐phonon coupling) is found to be due to three‐phonon processes. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical constants of DC magnetron sputtered titanium dioxide thin films measured by spectroscopic ellipsometry

Optical constants of DC magnetron sputtered TiO₂ thin film have been determined by Spectroscopic Ellipsometry in the photon energy range 1.2 to 5.5 eV at room temperature. The measured dielectric‐function spectra reveal distinct structures at energies of the E1, E1 + Δ1 and E2 critical points are due to interband transitions. The root mean square roughness of the magnetron sputtered TiO₂ thin films evaluated by ex‐situ atomic force microscopy is 5.8 nm. The Dielectric constant values were found to be substantially lower than those for the bulk TiO₂. The dielectric related Optical constants, such as the refractive index, extinction coefficient, absorption coefficient and normal incidence of reflectivity determined from the spectroscopic ellipsometry data are presented and analyzed. The optical constants of the films were also determined using the optical transmittance measurements and the results were discussed. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure and electrical properties of Mg‐doped ZnO nanoparticles

Mg_xZn_1‐xO (x=0.01‐0.3) nanoparticles were synthesized by the sol‐gel technique using solutions of Mg and Zn based organometalic compounds. The electrical properties of Mg doped zinc oxide (ZnO) were studied within wide temperature range from 300 to 500 K under the N₂ gas flow (flow rate: 20 sccm) and in the frequency range from 40 Hz to 1 MHz for ac electrical measurements. The dc conductivities and the activation energies were found to be in the range of 10^‐9‐10^‐6 S/cm at the room temperature and 0.26‐0.86 eV respectively depending on doping rate of these samples. The ac conductivity was well represented by the power law Aω^s. The conduction mechanism for all doped ZnO could be related to correlated barrier hopping (CBH) model. The complex impedance plots (Nyquist plot) showed the data points lying on a single semicircle, implying the response originated from a single capacitive element corresponding to the nanoparticle grains. The crystal structures of the Mg_xZn_1‐xO nanoparticles were characterized using X‐ray diffraction. The calculated average particle sizes values of Zn_1‐xMg_xO samples are found between 29.72 and 22.43 nm using the Sherrer equation. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Experimental and first‐principle investigation of Cu‐doped ZnO ferromagnetic powders

Zn_1‐xCu_x O powders were synthesized by using sol‐gel method. Electronic band structure and ferromagnetic properties of Zn_1‐xCu_x O powders were studied experimentally and theoretically. The simulations are based upon the Perdew‐Burke‐Ernzerhof form of generalized gradient approximation within the density functional theory. Zn_1‐xCu_x O shows dilute ferromagnetism, as a saturated magnetization of 0.9×10^‐3emu/g was observed for Zn_0.95Cu_0.05O powders. The strong p ‐d hybridization between Cu and its four neighbouring O atoms is responsible for the ferromagnetism. Comparing with ZnO whose Fermi level locates at the valence band maximum, the Fermi level of the Zn_1‐xCu_x O shifts upward into the valence band and hence the Zn_1‐xCu_x O system exhibits theoretically a p ‐type metallic semiconducting property. The Zn_1‐xCu_x O system may be a potential candidate in spintronics. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Observation of Meyer–Neldel rule in amorphous thin films of Se100−xSbx in presence of light

In the present paper, we report the thermally activated conductivity in thin films of Se_100−xSb_x in presence of light of different intensities. We found that the conductivity obeys the Meyer–Neldel rule as the pre-exponential factor depends on activation energy. We also found a strong correlation between prefactor σ₀₀ and Meyer–Neldel energy. The origin of the correlation and the MN rule can be described by multiple excitations stimulated by optical phonon energy as described by Yelon and Movaghar. The value of optical phonon energy calculated by experimental data is in good agreement with the YM model which suggests that the optical phonons are the source of the excitation energy in such processes.     

Crystal Data,Photoconductivity and Carrier Scattering Mechanisms in CuIn5S8 Single Crystals

The X‐ray diffraction has revealed that CuIn₅S₈ is a single phase crystal of cubic spinel structure. The value of the unit cell parameter for this crystal is 1.06736 nm. The crystal is assigned to the conventional space group Fd3m. The photocurrent is found to have the characteristic of monomolecular and bimolecular recombination at low and high illumination intensities, respectively. The electrical resistivity and Hall effect of CuIn₅S₈ crystals are measured in the temperature range of 50‐400 K. The crystals are found to be intrinsic and extrinsic above and below 300 K, respectively. An energy band gap of ∼1.35 eV at 0 K, a carrier effective mass of 0.2 m₀ , an acceptor to donor concentration ratio of 0.9, an acoustic phonon deformation potential of 10 eV and a nonpolar optical phonon deformation potential of 15 eV are identified from the resistivity and Hall measurements. The Hall mobility data are analyzed assuming the carrier scattering by polar optical phonons, acoustic combined with nonpolar optical phonons, and ionized impurities.