Effect of Sb Addition in GeTeSb Crystallization

Transcription

1 Scientia Agriculturae E-ISSN: X / P-ISSN: DOI: /PSCP.SA Sci. Agri. 6 (3), 2014: PSCI Publications Effect of Sb Addition in GeTeSb Crystallization Abdelhamid Badaoui 1* and Maamar Belhadji 1,2 1. Laboratoire de Chimie des Polymères, LCP, Es-Senia University, Oran, 31000, Algeria 2. Physics Department, Es-Senia University, Oran, 31000, Algeria Corresponding Author *aeh_badaoui@yahoo.fr Paper Information A B S T R A C T The crystallization process of ternary chalcogenide system GeTeSb is Received: 20 April, 2014 studied using Differential Scanning Calorimetry (DSC) and X-Ray Diffraction (XRD). Antimony Sb effect is discussed. It is found that the Accepted: 29 May, 2014 Ge 15.5-xTe 84.5Sb x system presents a double phase separation. The eutectic composition exhibits the same behavior but when thermally treated. The Published: 20 June, 2014 activation energy of the process is calculated according to Kissinger method and the results show that the activation energy varies with Sb content. By X-ray patterns, the identification of the crystallized compounds show that Tellurium Te crystallizes first with 1.8 ev then (Te+GeTe) with 2.1 ev PSCI Publisher All rights reserved. Key words: Switching Materials, Chalcogenide GeTeSb, DSC, Activation energy, Crystallization. Introduction Chalcogenide glasses exhibit photo-induced properties which allow them to be well-used in storage media (S. M. El-Sayed et al. 2007). These glasses have a relatively high atomic mass and weak bond strength resulting in lower phonon energy than other glasses; in consequence, they are highly transparent for light in mid-infrared region (P. Sharma et al. 2009, V. Pamukchieva et al. 2009). Chalcogenide glasses of GeTeSb system have received much consideration because of their interesting applications in modern technology especially in recording domain (F. H. Wu et al. 2003) and the possibility to prepare electrical and storage memories (D. H. Kang et al. 2003, N. Yamada et al. 1987, K. Uchino et al. 1993). In this work, we study some different compositions of GeTeSb system by Differential Scanning Calorimetry (DSC), the activation energies of crystallization are calculated and we show the thermal effect on different X-Ray Diffraction (XRD) traces. The effect of Antimony (Sb) content in the system is discussed. Experimental Setup The preparation of alloys was in two steps. The three elements ( % purity) were weighted in suitable quantities and introduced in a quartz ampoule and sealed in vacuum of 10-5 Pa. Then, the ampoules were placed in a horizontally rotating oven and annealed at 1000 C for 3 hours. The ampoules were finally quenched into ice-cold water to avoid crystallization. After breaking the quartz ampoules, amorphous nature of these alloys were verified by X-Ray diffraction (XRD) technique. The XRD spectra do not contain any prominent peak, which confirms the amorphous nature of the samples. Results and Discussion Theoretical basis The study of the crystallisation process under non-isothermal conditions is generally based on Johnson-Mehl- Avrami (JMA) (M. Avrami, ) equation: n xt ( ) 1exp Kt 1 where x(t) is the volume fraction of the initial material transformed at time t, n is the Avrami exponent and K is the reaction rate constant which is related to the temperature as: E K K 0 exp a 2 kt where K 0 is the frequency factor, E a denotes the activation energy for the crystallisation process, k is the Boltzmann constant and T is the isothermal temperature. One of the theoretical bases for interpreting the DSC results, especially for determining the activation energy, is Kissinger (H. E. Kissinger, 1957) method using the highest rate of the process at maximum peak:

2 Ea st ln 2 C 3 T p ktp dt Where T p is temperature at maximum peak, is the heating rate and E a is the activation energy. dt Thermal Study Figure 1 shows the typical thermogram obtained for Ge 14 Te 84.5 Sb 1.5 glassy alloy under non-isothermal condition. The presence of two T g and two T c is evident (same remark for all the system Ge 15.5-x Te 84.5 Sb x ) and points to a phase separation in the glassy system, which suggests the formation of more than one crystalline structure during the crystallization process. This phenomenon of double glass transition has been observed in many glass systems (M. A. Abdel-Rahim et al , M. A. Urena et al. 2003, Shamshad et al. 2002). Figure 1. Differential ther mal analysis of Ge 14.5 Te 84.5 Sb 1 at rate of 10 C/min The characteristic temperatures i.e., transition glass temperature T g and crystallization temperatures T c are listed in Table 1 for the different compositions of the system. Table 1. Characteristic temperatures of GeTeSb system: Composition T g1 (K) T g2 (K) T c1 (K) T c2 (K) Ge 15Te 84.5Sb Ge 14.5Te 84.5Sb Ge 14Te 84.5Sb Ge 13Te 84.5Sb When considering the crystallisation zone in the thermograms, as shown in Figure 2, it is seen that the two peaks approach when the content of Sb increases and the values of all characteristic temperatures decrease. Figure2. DSC of Ge 15.5-x Te 84.5 Sb x system at heating rate of 10 C/min The heating rate effect on the sample is seen in Figure 3 for the Ge 14 Te 84.5 Sb 1.5 glass as an example, where the crystallization peaks shift towards high the temperatures when the heating rate increases. 131

3 Figure 3. DSC of Ge 14.5 Te 84.5 Sb 1 at various heating rates In the other hand, from Figure 4, we see that the eutectic composition Ge 15 Te 82 Sb 3 exhibits only one glass transition (T g = 400 K) and one crystallization (T c = 458 K) peaks, indicating a single glassy phase at heating rate of 3K/min. Figure 4. Differential thermal analysis of Ge 15 Te 82 Sb 3 at rate of 10 C/min The calculation of the activation energy of the crystallization process for various compositions using Kissinger method (Eq. 3) according to linear fits in Figure 5, leads to the results listed in Table 2. The activation energy values are significantly affected by Sb content but also by Te composition. Figure5. Ln(α/T 0 2 ) plot versus 1000/T 0 for different compositions of GeTeSb glassy system 132

4 Table 2. Activation energies for GeTeSb system with different compositions System Ge 20-xTe 80Sb x Ge 18-xTe 82Sb x Ge 15.5-xTe 84.5Sb x x Peak 1 Peak 2 Peak 1 Peak 2 Peak 1 Peak 2 E a (ev) For the systems containing 80% at Te, the activation energy increases with increasing Sb content while for 82% at Te it decreases due to high Te content. For the glasses with 84.5% at Te, the average values are 1.8 ev for the first peak and 2.8 ev for the second one. X-Ray Diffraction In order to clarify the structural changes which occur during the crystallization process, we used X-Ray diffraction of the partial then the complete crystallized samples as shown in Figure 6. Figure 6. DRX spectra of Ge 15 Te 14.5 Sb 0.5 : (a) first crystallization peak, (b) second crystallization peak The system is annealed at a temperature near T c1 leading to the precipitation of tellurium Te in the hexagonal form, which is the same behaviour for the other Te-based amorphous alloys. Then, up to second crystallisation peak (at T c2 ), the system presents the crystallization of the remain vitreous phase (Te + GeTe). While for the eutectic glass, Figure 7 shows the thermal effect on Ge 15 Te 82 Sb 3 X-Ray traces. Figure 7. DRX pattern of Ge 15 Te 82 Sb 3 at different heating rates. 133

5 It is seen a Te precipitation in hexagonal form accompanied by GeTe precipitation rhombohedric structure with no ray corresponding to Sb-based compounds, concluding, according to Moss, that Sb atoms are concentred into GeTe crystals and are not visible in the X-Ray diagram due to their low concentration. Conclusion From the thermal study of the Ge 15-x Te 84.5 Sb x glassy system, we found a double glass transition with two transition and two crystallization temperatures, which decrease when Sb content increases except for second transition temperature. The eutectic composition exhibits the same behavior when thermally treated. During the first crystallization peak, and according to the X-ray diffraction patterns, Te crystallization in hexagonal structure occurs with activity energy of 1.8 ev, while during the second one it occurs crystallization of (Te + GeTe) with 2.8 ev. While there are no rays corresponding to any Sb-based compounds. References Abbdel-Rahim MA, Abdel-Lateif AY, Soltan AS, Abu-el-Oyoun M Crystallization Kinetics of Overlapping Phases in Cu 6Ge 14Te 80 Chalcogenide Glass. Physica B Abdel-Rahhim MA, Hafiz MM, Shamekh AM A Study of Crystallization Kinetics of Some GeSeIn Glasses. Physica B Avrami M. Kinetics of Phase Change. I. General Theory. J. Chem. Phys Avrami M.. Granulation, Phase Change and Microstructure Kinetics of Phase Change. III. J. Chem. Phys Avrami M.. Kinetics of Phase Change. II. Transformation-Time Relations for Random Distribution of Nuclei. J. Chem. Phys El-Sayed SM, Saad HM, Amin GA, Hafez FM, Abdel-Rahman M Physical Evolution in Network Glasses of the Ag As Te System. J. Phys. Chem. Solids Kang DH, Ahn DH, Kim KB One-dimensional heat conduction model for an electrical phase change random access memory device with an 8F2 memory cell (F=0.15 μm). J. Appl. Phys Kissinger HE Reaction Kinetics in Differential Thermal Analysis. Anal. Chem. 29. Pamukchieva V, Szekeres A, Todorova K, Fabian M, Svab E, Revay Zs, Szentmiklosi L Evaluation of Basic Physical Parameters of Quaternary Ge Sb-(S,Te) Chalcogenide Glasses. J. Non-Cryst. Solids Shamshad A, Khan M, Zulfequar M Husain. On the crystallization Kinetics of Amorphous Se 80In 20-xPb x Solid State Commun Sharma P, Katyal SC Effect of Substrate Temperature on the Optical Parameters of Thermally Evaporated Ge Se Te Thin Films. Thin Solid Films Uchino K, Takada K, Ohno T, Yoshida H, Kobayashi Y High-Density Pulse Width Modulation Recording and Rewritable Capability in GeSbTe Phase-Change System Using Visible Laser Beam at Low Linear Velocity. Japan J. Appl. Phys Urena MA, Fontana M, Arcondo B, Clavaguera-Mora MT J. Non-Cryst. Solids Wu FH, Shieh HPD Thermochromism of Silver Oxide for Optical Switching Layers in Volumetric Optical Disks. Japan J. Appl. Phys Yamada N, Ohno E, Akahira N, Nishiuchi K, Nagata K, Takao M Proceedings of the International Symposium on Optical Memory. High Speed Overwritable Phase Change Optical Disk Material. Japan J. Appl. Phys. 26 (Suppl.4)