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Article Citation - WoS: 29Citation - Scopus: 31Composition-tuned band gap energy and refractive index in GaSxSe1-x layered mixed crystals(Elsevier Science Sa, 2017) Isik, Mehmet; Gasanly, NizamiTransmission and reflection measurements on GaSxSe1-x mixed crystals (0 <= x <= 1) were carried out in the 400-1000 nm spectral range. Band gap energies of the studied crystals were obtained using the derivative spectra of transmittance and reflectance. The compositional dependence of band gap energy revealed that as sulfur (selenium) composition is increased (decreased) in the mixed crystals, band gap energy increases quadratically from 1.99 eV (GaSe) to 2.55 eV (GaS). Spectral dependencies of refractive indices of the mixed crystals were plotted using the reflectance spectra. It was observed that refractive index decreases nearly in a linear behavior with increasing band gap energy for GaSxSe1-x mixed crystals. Moreover, the composition ratio of the mixed crystals was obtained from the energy dispersive spectroscopy measurements. The atomic compositions of the studied crystals are well-matched with composition x increasing from 0 to 1 by intervals of 0.25. (C) 2016 Elsevier B.V. All rights reserved.Article Citation - WoS: 2Determination of Optical Constants and Temperature Dependent Band Gap Energy of Gas0.25se0.75< Single Crystals(Natl inst Optoelectronics, 2017) Isik, M.; Gasanly, N.Optical properties of GaS0.25Se0.75 single crystals were investigated by means of temperature -dependent transmission and room temperature reflection experiments. Derivative spectrophotometry analysis showed that indirect band gap energies of the crystal increase from 2.13 to 2.26 eV as temperature is decreased from 300 to 10 K. Temperature dependence of band gap energy was fitted under the light of theoretical expression. The band gap energy change with temperature and absolute zero value of the band gap energy were found from the analyses. The Wemple-DiDomenico single effective oscillator model and Sellmeier oscillator model were applied to the spectral dependence of room temperature refractive index to find optical parameters of the GaS0.25Se0.75 crystal. Chemical composition of the crystal was determined using the energy dispersive spectral measurements.Article Citation - Scopus: 3Determination of optical constants and temperature dependent band gap energy of GaS0.25Se0.75 single crystals(National Institute of Optoelectronics, 2017) Isik,M.; Gasanly,N.Optical properties of GaS0.25Se0.75 single crystals were investigated by means of temperature-dependent transmission and room temperature reflection experiments. Derivative spectrophotometry analysis showed that indirect band gap energies of the crystal increase from 2.13 to 2.26 eV as temperature is decreased from 300 to 10 K. Temperature dependence of band gap energy was fitted under the light of theoretical expression. The band gap energy change with temperature and absolute zero value of the band gap energy were found from the analyses. The Wemple-DiDomenico single effective oscillator model and Sellmeier oscillator model were applied to the spectral dependence of room temperature refractive index to find optical parameters of the GaS0.25Se0.75 crystal. Chemical composition of the crystal was determined using the energy dispersive spectral measurements.Article Citation - WoS: 5Citation - Scopus: 5Absorption Edge and Optical Constants of Tl2ga2< Crystals From Reflection and Transmission, and Ellipsometric Measurements(Elsevier, 2012) Isik, M.; Gasanly, N. M.The optical properties of Tl2Ga2S3Se layered crystalline semiconductors were investigated from transmission, reflection and ellipsometric measurements. The experimental results of the room temperature transmission and reflection measurements performed in the wavelength range of 400-1100 nm showed the presence of both indirect and direct transitions in the band structure of the crystals with 2.38 and 2.62 eV band gap energies. Spectroscopic ellipsometry measurements on Tl2Ga2S3Se crystals were carried out on the layer-plane (0 0 1) surfaces with light polarization E perpendicular to c* in the 1.20-4.70 eV spectral range at room temperature. The real and imaginary parts of the dielectric function as well as refractive and absorption indices were found as a result of analysis of ellipsometric data. The Wemple-DiDomenico single-effective-oscillator model was used to study the dispersion of the refractive index in the below band gap energy range. The structures of critical points have been characterized from the second derivative spectra of the dielectric function. The analysis revealed four interband transition structures with 3.14, 3.40, 3.86 and 4.50 eV critical point energies. (C) 2012 Elsevier B.V. All rights reserved.Article Citation - WoS: 2Citation - Scopus: 2Optical constants and interband transitions of anisotropic layered structured Tl2GaInS4 crystals by spectroscopic ellipsometry(Elsevier Science Sa, 2013) Isik, M.; Gasanly, N. M.; Turan, R.Spectroscopic ellipsometry measurements were carried out on Tl2GaInS4 layered crystals for orientations of electric field vector, parallel (E//c*) and perpendicular (E perpendicular to c*) to optical axis c*. The measurements were performed in the 1.2-6.2 eV spectral range at room temperature. The real and imaginary components of the pseudodielectric function, pseudorefractive index and pseudoextinction coefficient were calculated from the analysis of ellipsometric data. The energies of interband transitions (critical points) have been found from the least-square fitting of the second derivative spectra of the pseudodielectric function. The results indicated five each interband transition structures for E//c* and E perpendicular to c* configurations. The obtained critical point energies were assigned tentatively to interband transitions using the available electronic energy band structure given in literature. (C) 2012 Elsevier B.V. All rights reserved.
