YongGu Shim

Refractive indices of layered semiconductor ferroelectrics TlInS2, TlGaS2, and TlGaSe2 from ellipsometric measurements limited to only layer-plane surfaces

The ternary layered Tl-contained semiconductor ferroelectrics TlInS2, TlGaS2, and TlGaSe2, exhibiting an incommensurate phase and a relaxor state with temperature, were approached down to 0.76eV at room temperature by spectroscopic phase modulated ellipsometry in a region below the energy gap of each material. By using both coherent and incoherent reflection modes on only the layer-plane surfaces, the refractive indices in the parallel- and perpendicular-to-the-layers directions of the light propagation were determined. Sellmeier single-oscillator form was used to fit the model data to the experimental intensities. The discrepancy between the model and experimental data was shown to be negligible. Besides, a self-consistent picture of the obtained refraction indices was emerging after the discussion based on the available data for the studied materials. The applied experimental approach was thus found to be quite effective for layered materials, and the obtained refractive indices might be used for databa...

Polarized Transmission Intensity Studies of Off-Zone-Center Incommensurate Semiconductors-Ferroelectrics TlMeX2

Temperature dependent polarized transmission intensity studies in strongly converging and parallel light beams had been accomplished for off-zone-center incommensurate semiconductors-ferroelectrics TlInS2, TlGaSe2 and TlGaS2 having a layered structure and reportedly a C2h6 space group of symmetry at room temperature. All above materials were biaxial. TlInS2, as compared to TlGaS2 and TlGaSe2, displayed a stronger wavelength dispersion of the normal-to-layer-plane optical anisotropy due to the strong polarization of the excitonic transitions near the absorption edge. Temperature dispersion of the dielectric axes of TlInS2 and TlGaSe2 was shown to include forbidden rotation that might be associated with a lower than monoclinic symmetry or spatially dispersed microscopic domains. Macroscopic, spatially ordered domain structure was found in TlGaS2, for which, however, above-like rotation was not observed. Unusual effect with temperature behavior encountered in soliton-like incommensurate phases was observed for TlInS2. The optical anisotropy of the incommensurate phase was estimated at ~ 10-3 with the background value of ~ 10-4 for basic lattice.

Linearized Augmented Plane Wave Band Structure Calculations and Dielectric Function of Layered TlGaSe2

The electronic energy-band structure of the room temperature phase of the ternary layered TlGaSe2 has been approached using linearized augmented plane wave (LAPW) method. Band structure and dielectric function have been calculated. This function has also been obtained in a wide range of photon energies from 0.76 to 6.5 eV by using spectroscopic phase modulated ellipsometry and a good agreement between the calculated and measured values has been observed. As derived from the used LAPW wave function set, the valence band top, occupying the center of the Brillouin zone originates from Tl 6s and Se 4p anti-bonding states, while the conduction band bottom, occupying a shifted-from-the center position on Γ–Y symmetry line is due to Tl 6p and Se 4p anti-bonding states. Higher than this bottom by 50 meV, there is one more conduction band minimum in the center of the Brillouin zone. It has been shown that the calculated dielectric function accounts for all polarization peculiarities of the direct band-gap transitions in TlGaSe2 at room temperature.

Spectroscopic Ellipsometry Studies of CdS:O Layers for Solar Cells

This work reports the results of spectroscopic ellipsometry study of CdS thin films, deposited on glass substrates in the presence of oxygen at room temperature by rf sputtering. The data acquisition, together with the subsequent linear regression analysis to restore dielectric function has been performed over the photon energy range 1.5–5.0 eV. The obtained oxygen-free CdS films and reference bulk CdS have been found to be very much the same regarding their dielectric function spectra. On the other hand, blue-shift of the main structures in dielectric function has been observed for CdS:O films deposited at 5% value of O/Ar ratio. In spite of the broadening taking place on a par with the shift, the last turns out to be so large that provides transparency of CdS:O thin films in a spectral range above the energy gap of CdS. The obtained results allow to considering CdS:O as an improved window-layer material for solar cell application.

Phonon Spectra of Chain TlSe and TlInSe2: Density Functional Theory Based Study

The density functional theory (DFT) based ab initio calculations of the phonon spectra of the chain compounds, TlSe and TlInSe2, with analogous crystal structures have been carried out. For both materials, the derived frequencies and symmetry of the phonon modes in the center of Brillouin zone agree well with the experimental data on Raman scattering and infrared absorption. For TlSe, the available phonon dispersion and density-of-states data obtained by neutron scattering are also reproduced well by the results of calculations. In both compounds, the acoustic and low-frequency optic branches greatly overlap, which is a characteristic feature of structurally low-dimensional materials. For both compounds, the calculated and measured heat capacities are in a good agreement up to the temperatures limiting the validity of the used harmonic approximation. Strong anharmonisity of TlInSe2 as compared to TlSe, as well as the probable connection of this anharmonicity with incommensurate phase of TlInSe2 at temperatures above 100 K is figured out.

New Developments in Experimental and Analytical Aspects of Light Figure Spectroscopy

Recent developments in light figure spectroscopy are reviewed with a purpose to promote their large-scale application to anisotropic materials and thin films. Both absolute and relative analytical approaches and the results of the experimental approval on the well-known uniaxial calcite, CaCO3, are given. Further generalization and extension onto optically active materials are described with a stress on a shrinkage effect that was predicted and observed for the first time. The numbering of the interference curves in the presence of gyration is shown to be different in comparison with gyration-free case. A universal approach to the determination of the correct interference order is proposed and found to be in a good agreement with experimental data on CaCO3, TeO2 and AgGaS2. The effectiveness of the light figure method with regard to the nanomodulated structures is demonstrated by the recent application to the novel thallium-contained incommensurate materials, TlInS2 and TlGaSe2. Thin film applications are discussed in terms of the merits of homogeneously and non-homogeneously polarized testing beams and isotropic and anisotropic substrates. A preference is given to anisotropic substrates which can provide creation of the optical vortexes with ellipsometric or higher sensitivity to film parameters and open a gateway to the light-figure-based characterization of the anisotropic films with vanishing thickness and optical anisotropy.

Extended Application of Light Figures to Optically Active Materials with Transversally Isotropic Dielectric Function

An analytical approach to optic-axis light figures of optically active transparent materials with transversally isotropic linear dielectric function is developed. The ratio of the gyration constant to the parameter of anisotropy of this function is shown to be essential for the scale of the gyration effect on light figures. An algorithm for determination of all material-related optical parameters is proposed in the form covering both symmetries possible for optical activity in uniaxial materials, such as AgGaS2 and TeO2. For AgGaS2, a deviation of the shape of the interference curves from circular is expected at enough large angles of interference. In full agreement with the performed theoretical analysis, the shrinking of interference curves into zero-size isochromates is observed at multiples of π for rotation angle of a thick 1420 µm-plate of TeO2. Transversal component of gyration pseudo-tensor of TeO2 is determined for the first time. The recently proposed relative light-figure method is re-formulated and briefly discussed with allowance for optical activity.

Light figure spectroscopy of optically active anisotropic materials

Abstract Low order interference curves in optic-axis light figures of the optically active AgGaS 2 and TeO 2 are investigated in the visible spectral range at room temperature. Light figure patterns of AgGaS 2 are found not to be affected by optical activity within the incidence angles of 10° for which, according to our symmetry considerations, the normal component of gyration tensor is small and evolution of light figures with wavelength is overwhelmingly determined by optical anisotropy. The absolute value of this anisotropy is falling down to null when passing through the isotropic point, with the result of a remarkable change of the parameters of light figure patterns. Unlike the case of AgGaS 2 and consistently with our symmetry considerations, light figures of TeO 2 are influenced by optical activity especially strongly in low orders of interference. The obtained results are compared with similar results for optically inactive CaCO 3 and discussed in the frameworks of our recent analyses.

Phase transitions and Raman scattering spectra of TlGaSe2

In this work we examined the phase transitions in the ternary thallium chalcogenide TlGaSe2 by studying the temperature dependence of the Raman spectra with the aid of confocal microscopy. The unpolarized Raman scattering spectra of TlGaSe2 single crystals were measured over the temperature range 78- 300 K (which includes the range of the successive phase transitions) in the frequency region of 50 – 300 cm-1. The Raman spectra exhibited 12 lines at 300 K, but the number of lines rose to 17 at 78 K. In the temperature interval between 107 K and 120 K, where the phase transitions take place, the temperature dependence of the phonon frequencies showed discontinuities for several of the Raman lines.

Numerical and experimental approbation of extended application of light figures

The light figure based method proposed recently for determination of the relative dispersion of the optical constants has been applied to the uniaxial material, CaCO3, with accurately known refraction indices in a wide spectral range. The dispersion term determined directly from the spectral data for the ordinary refraction index and the term calculated from the approximation given by the light figure based approach coincide with high accuracy. Distribution of light intensity in light figures of 1.225×103 μm thick CaCO3 has also been measured and analyzed to ascertain the obtained value of the dispersion term. Agreement is very good. The use of the plate with known optical constants is shown to enable calibration of the deployed optical system.