Session Mo-A2

Chalcogenides: Electronic Structure, Defects, Metastability and Transport II

Chair: Maria Mitkova, Boise State University

Mo-A2.1 16:00–16:20

Relationship Between Photo-induced Surface Relief Formation and Birefringence in Soft Materials

Janis Teteris

Institute of Solid State Physics, University of Latvia, 8 Kengaraga Str., Riga, LV-1063, Latvia

An interaction between optical field with various modulation of intensity and polarization direction, and soft materials (amorphous chalcogenide and organic polymer films) was studied. An interference pattern of two coherent beams was used for optical field formation with various intensity and polarization configuration. Under intensive illumination the formation of surface relief gratings (SRG) on the surface of amorphous films caused by lateral mass transport regarding the light propagation direction has been observed.

The amorphous films of chalcogenides (As-S, As-S-Se, As-Se and Ge-Se systems), azo-dye containing organic polymers and low molecular organic glasses were used for the studies. The influence of the amorphous film thickness, recording laser wavelength in the spectral range of 248 nm–671 nm, grating period, light intensity and polarization state on the relief formation process in amorphous inorganic and organic films was studied. It was shown that the efficiency of the surface-relief formation strongly depends on the recording light polarization state. The best surface relief grating (SRG) formation was observed with (+45° : –45°) and (RCP : LCP) configuration of recording beams, which involve primarily variation in polarization state across the film. The relief grating profile on amorphous films was analyzed by means of atomic force microscope (AFM).

A strong relationship between SRG and polarization holographic grating formation was observed. It is known that polarization gratings can be recorded in a material which possesses photo-induced birefringence and dichroism. Therefore a detailed study of a photo-induced birefringence and changes of optical properties in studied films was performed. The lasers with wavelengths of 375 nm, 448 nm, 473 nm, 532 nm, and 632.8 nm were used as a pumping beam for sample excitation. The photo-induced birefringence was measured at 532 nm and 632.8 nm wavelengths. The birefringence phenomenon can be explained by trans-cis isomerisation of azo dyes and photochemical reactions of polymer under pump light radiation. The sign and numeral values of photoinduced birefringence and their dependence on the pumping beam wavelength and intensity were evaluated.

The study of the phase relationship between the exciting light field and the resulting surface deformation is crucial in understanding the mechanism of SRG formation. It was observed that the peak of the SRG on chalcogenide films was formed at the position of p-polarization state in the polarization modulation pattern. This fact is crucial to infer the direction of the force to form the SRG. That is, since the SRG is formed by molecular migration, we can confirm that the force drives the migration toward the position of p-polarization state in the polarization modulation. The peak formation of the surface relief on organic azobenzene doped polymers is opposite and was observed at the position of s-polarization state.

The mechanism of the direct recording of surface-relief on amorphous films based on the photo-induced softening of the matrix, formation of the defects with enhanced or decreased polarizability, and their drift under the optical field gradient force has been discussed.

Keywords: chalcogenide films, azo-benzene polymers, surface relief gratings

Mo-A2.2 16:20–16:40

Electrochemical metalization cells: Nanoscale memories in Chalcogenide Glass Films

T. Wágner (1), J. Kolář (1), S. Valkova (1), I. Voleská (1), M. Krbal (1), J. Macák (1), M. Frumar (1), and K. Terabe (2)

1. Department of General and Inorganic Chemistry, "ReAdMat" project team, Faculty of Chemical Technology, and Centre for Materials Science, University of Pardubice, Cs. Legion's Sq. 565, Pardubice 532 10, Czech Republic

2. International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan

A range of material systems exist in which nanoscale ionic transport and redox reactions provide the essential for switching as platform for reconfigurable electronic devices and biological like computing. One class relies on mobile cations, which are easily created by electrochemical oxidation of the corresponding electrode metal, transported in the insulating layer, and reduced at the inert counter electrode. These devices are termed electrochemical metallization memories (EMC) or conductive bridge random access memories [1]. The material candidates for electrolytes in such devides have been recently studied. They are amorphous chalcogenides [2, 3] and also oxides (SiO2, WO3, TiO2 and others [1]) containing metal elements (Ag, Cu) or their compounds (Ag2S, CuS) and gaining some portion of ionic conductivity and becoming mixed ionic-electronic conductors [3–7].

The aim of this work is to present our current results on syntesis and resistive switching of chalcogenide based nanowire array cells.

The authors thanks to project CZ.1.07/2.3.00/20.00254 "Research Team for Advanced Non-crystalline Materials" realized by European Social Fund and Ministry of Education, Youth and Sports of The Czech Republic within The Education for Competitiveness Operational Programme for financial support.

[1] W. Lu, D. S. Jeong, M. Kozicki, R. Waser, MRS Bulletin, 37 (2012) 124

[2] M. Frumar and T. Wágner, Curr. Opinion Solid St. Mat. Sci. 7 (2003) 117

[3] M. Frumar, B. Frumarova, T. Wágner, Amorphous and Glassy Semiconducting Chalcogenides. In: Bhattacharya P, Fornari R, and Kamimura H, (eds.), Comprehensive Semiconductor Science and Technology, volume 4, pp. 206–261 (2011) Amsterdam: Elsevier

[4] Š. Stehlík, J. Kolář, M. Bartoš, Mil. Vlček, M. Frumar, V. Zima, T. Wágner, Sol. State Ionics 181 (2010) 1625

[5] J. Kolář, T. Wágner, V. Zima, Š. Stehlík, B. Frumarová, L. Beneš, Mil. Vlček, M. Frumar, J. Non-Cryst. Solids, 357 (2011) 2223

[6] I. Kaban, P. Jóvári, T. Wágner, M. Bartoš, M. Frumar, B. Beuneu, W. Hoyer, N. Mattern, J., Eckert, J. Non-Cryst. Solids, 357 (2011) 3430

[7] Š. Stehlik, K. Shimakawa, T. Wágner, M. Frumar, J. Phys. D: Appl. Phys. 45 (2012) 205304

Keywords: amorphous chalcogenides, photoinduced dissolution of silver, electronic properties, memory

Mo-A2.3 16:40–17:00

Temperature Dependence of Photodarkening Kinetics in a-Se

Anastasiia Mishchenko (1) and Alla Reznik (1,2)

1. Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada

2. Thunder Bay Regional Research Institute, 290 Munro St., Thunder Bay, ON, P7A 7T1, Canada

The kinetics of the photodarkening (PD) effect for wide temperature range from –100°C to 35°C has been studied experimentally in amorphous selenium (a-Se) stabilized with arsenic (0.2%). By switching an intense pumping light on and off with a period of 200 s, the kinetics of both the buildup of photodarkening and its relaxation (recovery) was investigated[1]. Both reversible and irreversible photodarkening component were found for all temperatures except of T-region close to glass transition (Tg) where only reversible process was found, which is in a good agreement with previously published data [1]. The observed temperature behavior defines three distinct regimes. In the temperature regions: from 35°C to 20°C (Regime I) and from –5°C to −100°C (Regime III) irreversible component was always present and increased with decrease in temperature, moreover darkening changes with T were much more rapid for Regime I. In contrast, an intermediate range between 20°C and –5°C (Regime II) was observed, where irreversible component was temperature independent. These results are consistent with previously reported Raman studies of photo-crystallization (PC) in a-Se [2], where similar temperature regimes where defined and in particular, for temperatures close to Regime II PC rates were temperature independent. This suggests that both PD and PC are of the same nature and are governed by competing temperature dependent processes which seem to cancel themselves in the Regime II.

[1] Alla Reznik, et al., J. of Applied Physics 100, 113506 (2006)

[2] Robert E.Tallmen, et al., J. of Non-Crystalline Solids 354, 4577–4581 (2008)

Keywords: amorphous selenium, photostructural changes, metastability, amorphous semiconductors.

Mo-A2.4 17:00–17:20

Raman and AFM Mapping Studies of Photo-induced Crystallization in a-Se Films: Substrate-strain and Thermal Effects

G. P. Lindberg (1), B. A. Weinstein (1), A. Reznik (2), S. Abbaszadeh (3), K. S. Karim (3), T. O'Loughlin (4), G. Belev (5), M. J. Yaffe (6), and D. M. Hunter (6)

(1) SUNY at Buffalo, Department of Physics, Buffalo, NY 14260-1500 USA

(2) Thunder Bay Regional Research Institute, Thunder Bay, Ontario P7B 5E1 Canada

(3) University of Waterloo, Waterloo, Ontario, N2L 3G1 Canada

(4) SUNY at Buffalo, Department of Chemistry, Buffalo, NY 14260 USA

(5) University of Saskatchewan, Saskatoon, Saskatchewan S7N 5A9 Canada

(6) Sunnybrook Research Institute, Toronto, Ontario M4N 3M5 Canada

Photo-induced crystallization (PC) is studied in several amorphous selenium (a-Se) films deposited on either glass or Si substrates in geometries with and without intermediate layers of conducting (ITO), charge blocking, or polymer material. The spatial profile of photo-crystallized domains is investigated by co-localized scanning atomic force microscopy (AFM) and Raman mapping. These profiles show that the crystalline (trigonal-Se) domains grow from the substrate to the surface in pyramidal structures, and that the boundary region where the Raman spectrum has mixed crystalline-amorphous character is ≤500 nm wide. The thermal behavior of the onset and growth of PC domains in the different a-Se films is explored by Raman spectroscopy measurements for a range of temperatures spanning the a-Se glass transition (Tg ~ 313K). In many films the onset time for PC shows a surprising discontinuous increase with increasing temperature near Tg, and there even can be a "dead zone" of no crystallization in this range [1]. Other films only show a minimum in the onset time at similar temperatures, but no discontinuity. We find that inserting a thin polyimide layer between the a-Se film and the substrate tends to inhibit the PC effect. Our results point to the importance of the substrate for PC in a-Se films. We find that the observed discontinuities and minima in the PC onset times, the shape of the domain mapping profiles, and the effects of having a soft polymer interface layer can be well understood by models that take into account the substrate shear-strain and its relaxation near Tg. Within this context, several similarities between the thermal and substrate effects observed in the present PC experiments, and effects recently found in related photo-induced darkening studies on a-Se films are noted and discussed.

[1] R. E. Tallman, et al., J. Non-Cryst. Solids 354, 4577 (2008)

Mo-A2.5 17:20–17:40

Thermo Analytical, Thermal Transport, Dielectric, and Mechanical Properties of Chalcogenide Se98−xAg2Inx (x=0, 2, 4, 6) System

C. Dohare and N. Mehta

Department of Physics, Solid State Lab, Banaras Hindu University, Varanasi 221005, India

Thermo analytical analysis, i.e. glass transition and crystallization kinetics of chalcogenide Se98–xAg2Inx (x=0, 2, 4, 6) system is studied under non-isothermal condition using differential scanning calorimetry (DSC). Thermal transport observations, i.e. thermal conductivity (λe), thermal diffusivity (χe) and specific heat per unit volume (ρCv) have been carried out by transient plane source (TPS) technique. Temperature and frequency dependence of dielectric constant (ε1) and dielectric loss (ε2) for the same system measured in the frequency (20 Hz–500 kHz) and temperature (297K–400K) range. The experimental results illustrated that the values of dielectric constant (ε1) and dielectric loss (ε2) decreases with frequency and increases with temperature. The maximum barrier height Wm was calculated using the dielectric measurements according to the Guintini equation. The results indicate that the dielectric relaxation exist in the above mentioned frequency and temperature range. Mechanical load dependent properties such as micro hardness (Hv) and modulus of elasticity (E) are calculated by well known empirical relations. The minimal energy of micro-void (Ev) in micro void volume (Vv), Density, and covalence character of glassy system are discussed in term of micro hardness (Hv). The morphology and micro structural analysis of as prepared alloy was confirmed by XRD, SEM, EDX, and TEM.

Keywords: chalcogenide, glass transition, crystallization, thermal transport, dielectric constant, hardness