Theoretical Predictions on Physical Properties of Se-Sb-Bi Glass System with Compositional Variations

Transcription

1 International Journal of Engineering Technology, Management and Applied Sciences Theoretical Predictions on Physical Properties of Se-Sb-Bi Glass System with Compositional Variations Department of Physics, M. L. V. Govt. College, Bhilwara, Rajasthan, India ABSTRACT The investigation of composition dependence of various properties of chalcogenide glasses has been increased in recent years. Since reversible switching phenomenon in certain types of chalcogenide glasses was first reported, a lot of attention has been given to characterization and improvement of the properties of chalcogenide glasses in general and the materials exhibiting the switching phenomenon in particular. In the present work, the effect on the physical properties with the variation in bismuth content has been studied theoretically for Se 90-x Sb 10 Bi x (x= 4, 8, 12, 16, 20 at. %) glassy alloys. The impact of variation of bismuth (Bi) on the system has been discussed in terms of average coordination number, heat of atomization, and optical band gap of the system. Almost all the parameters are found to vary linearly with the variation in Bi content. It has been found that almost all the parameters, studied here, were increased with the increase in Bi content, thus making this suitable for phase change optical recording and find applications in rewritable optical recording. Keywords: Chalcogenide Glasses, Coordination Number, Average Heat of Atomization, energy band gap. 1. INTRODUCTION Recently, chalcogenide glasses have attracted much attention amongst researchers due to their vast applications in various fields. These glasses are based on the chalcogen elements S, Se, and Te. These are formed by the addition of other elements such as Ge, As, Sb, Ga,etc. These glasses are optically highly nonlinear and could therefore be useful for all-optical switching (AOS). Chalcogenide glasses are sensitive to the absorption of electromagnetic radiation and show a variety of photo-induced effects as a result of illumination. Chalcogenide glasses have drawn prodigious attention because of their potential use in photo-resist,microelectronic,optoelectronic, holographic applications and especially their ability to transmit light in the mid to far-infrared region [1-5]. Some chalcogenide materials experience thermally has driven amorphous crystalline phase changes, enabling the encoding of binary information on thin films of chalcogenides, forming the basis of rewritable optical discs and non-volatile memory devices. Chalcogenide glasses are the most promising materials for a wider range of wavelengths, near and mid-infrared. In general, there are two main sources of infrared absorption in chalcogenide glasses, affecting the transmittance near 10.6 m wavelength. As selenium exhibits the unique property of reversible phase transformation and also applications like photocells, xerography, memory switching etc., it seems attractive, but pure selenium has disadvantage like short life time and low photo sensitivity. To overcome this problem, some impurity atoms like Ge, In, Bi, Te, Sb, Ag, etc. can be used to make alloys with Se, which may enhance sensitivity, crystallization temperature and reduce ageing effects [6].The addition of third element bismuth (Bi) increases the chemical durability and broadens the IR transparency region.the compositional dependence studies on glassy alloys were reported for the alloys based on the 219

2 International Journal of Engineering Technology, Management and Applied Sciences materials like Ge, Sb, Ag, As, Pb, Ga, Sn, Bi, Se, Te etc. [7-12].. Earlier some scholars measured the optical gap and electrical resistivity of a series of Se 90-x Sb 10 Bi x glasses with (x=0, 2, 4, 6, 8, 10 at %) [8]. They analyzed the transmission spectra of films with various Bi compositions to determine the optical band gap. It was reported that the optical band gap decreased sharply from 1.46 ev to 1.24 ev with the increase in the Bi contents from 0 to 10 at. %. The optical and electrical results suggest that the addition of Bi produces localized states near the conduction band edge, so the electrical transport is due to hopping of electrons after being excited into localized states at the conductionband edge. In the present work, we have taken a quaternary alloy comprising of Se-Sb-Bi. Here we have fixed of Sb at 10 at %, and then studied the variation of various important physical parameters by varying the concentration of Bi from4-20 at%. The variation in concentration of Bi element used to create compositional and configurationally disorder in the material with respect to the ternary alloys [9, 10].It has been established that physical properties in this system are highly composition dependent. The Se 90-x Sb 10 Bi x glass system is of special interest as it forms glasses over a wide domain of compositions. The present paper is concerned with the theoretical prediction of some physical parameters related to composition, viz. coordination number, heat of atomization, and optical band gap, etc for Se 90 Sb 10 Bi x (x= 4, 8, 12, 16, 20 at. %) glassy alloys. 2. THEORETICAL STUDIES, RESULTS AND DISCUSSION 2.1. AVERAGE COORDINATION NUMBER & BONDING CONSTRAINTS Two models namely, RCNM (Random Covalent Network Model) and CONM (Chemically Ordered Network Model) have been reported to be consistent with the bonding schemes (the 8-N rule) in chalcogenide network glasses [11,12].Out of these two, the CONM has been adopted to understand the features observed in the property-composition dependence of a very wide range of chalcogenide glasses [13,14]. The CONM is also referred as the change crossing model or as the COCN (Chemically Ordered Covalent Network Model) [15]. The formation of hetropolar bonds is favoured over the formation of homopolar bonds in CONM. According to CONM the glass structure can be composed of cross linked structural units of SbSe 2 and Bi 2 Se 3 with excess of Sb or Se dispersed among Se-Sb-Bi glass system.though the CONM was adopted successfully to understand the features observed in the property composition dependence of several systems, later it was found that aspects such as the ease of glass formation and stability of specific composition were not addressed to in this model. With the advent of efficient preparation techniques, the glass formation regions of some systems could be extended. Out of these models, one of which was developed by Phillips and Thrope [16, 17], the dynamics of network topology has been given importance. The CONM can account for only one of the features observed in V-VI (P, As, Sb, S, Se, Te) and IV-V (Ge, Si, S, Se, Te) systems. The topological models on the other hand can account for features at both the Z values (Z ~2.40 and 2.67). It was, therefore, thought that the topological models are of a more general nature and therefore these models began to get wider acceptance in understanding the features observed in the property-composition dependence of chalcogenide network glasses. The ease of glass formation and the stability of the resulting network are also associated with the value of Z. Special features observed in the Z data in a large number of systems around the threshold value of Z have been attributed to this topological transition which has been probed by studying both macroscopic and microscopic properties [18, 19].The average coordination number (Z) is calculated using standard method [20] for the composition of Se-Sb-Bi system, Z is given by 220

3 Nc Z International Journal of Engineering Technology, Management and Applied Sciences Where a, b and c are the at % of Se, Sb and Bi respectively and NSe(2), NSb(3), NBi(3) are their respective coordination number [15,16]. From the calculated values of average coordination number for Se 90-x Sb 10 Bi x, (x = 4, 8, 12, 16, 20, at. %) systems, it is clear from figure 1 that values of Z increase with increase in concentration of Bi from 4 to 20 in Se Sb-Bi system. 2.5 Z Fig1-: Variation of average coordination number with Bi content The glassy network are influenced by mechanical constraints (Nc) associated with the atomic bonding and anaverage coordination number Z which is also related to Nc. There are two types of near-neighbor bonding forces in covalent solids; bond-stretching (α- forces) and bond-bending (βforces) [17]. The number of Lagrangian bond-stretching constraints per atom is Nα = Z/2 and, of bond-bending constraints is Nβ = 2Z 3. The total number of constraints is given by Nc = Nα + Nβ Fig. 2 depicts the variation of Nc with for Se 90-x Sb 10 Bi x. Here Nc increase from 2.58 to 3.35 with increase in Bi at.% from 4 to 20, which shows in our composition that the number of constraints Nc acting on the network are balanced by the number of degrees of freedom N available from the atoms in the network. This means that network is isostatically rigid, no stress is present i.e. Nc = Nd. 3.3 Nc Fig. 2: Variation of number of constraints with Bi content 221

4 f X International Journal of Engineering Technology, Management and Applied Sciences The cross-linking density (X) is equal to the average coordination number of cross linked chain less the coordination number of initial chain [21]. X = Nc 2 The values of cross linking density (X) are calculated using above mentioned relations. From fig. 3, it is clear that the value X increase with increase in Bi content. 1.2 X Fig. 3: Variation of cross-linking density with Bi content According to Thorpe, the uncoordinated network having finite fraction of zero frequency normal vibrations modes termed as floppy modes in absence of weak long range forces. The fraction of floppy modes available in a network is given by From the fig. 4, the value of f becomes more and more negative with increase in Bi content from 4 to 20 at. %. This shows that the system becomes more and more rigid, which corresponds to a strong tendency for making glass f Fig. 4: Variation of fraction of floppy modes with Bi content 222

5 R International Journal of Engineering Technology, Management and Applied Sciences 2.2. DEVIATION FROM THE STOICHIOMETRY OF COMPOSITION The parameter R also plays an important role in the analysis of the results. Depending on R values, the chalcogenide systems can be organized into three different categories: For R = 1, the system reaches the stoichiometric composition since only hetero polar bonds are present. For R > 1, the system is chalcogen-rich. There are hetero-polar bonds and chalcogen chalcogen bonds present. For R < 1, the system is chalcogen-poor. There are only hetero-polar bonds and metal metal bonds present. The parameter R which determines the deviation of stoichiometry is defined as the ratio of covalent bonding possibilities of chalcogen atom to that of nonchalcogen atom.the quantity R is given by [22,23] R = Where a, b and c are the atomic fractions of Se, Sb and Bi respectively R Fig. 5: Variation of parameter R with Bi content 2.3 AVERAGE HEAT OF ATOMIZATION As proposed by Pauling [24,25], in case of ternary and higher order semiconductor materials, the average heat of atomization Hs is defined for a compound Se, Sb, Bi is considered as a direct measure of the cohesive energy and thus average bond strength, as Hs = Where a, b, c and d are the atomic % of Se, Sb, and Bi respectively. The values of average heat of atomization for all the compositions of Se-Sb-Bi ternary system is calculated by using this relation and the values of heat of atomization in the units of KJ/mol, i.e., 237, 265, 210 for,se, Sb 223

6 Eg Hs/Z International Journal of Engineering Technology, Management and Applied Sciences and Bi respectively. Average single bond energy Hs/Z which is a measure of cohesive energy, decreases with increase in Bi content from 4 to 20 at % for the compositions. A graphical representation of average heat of atomization per single bond Hs/Z with the variation in Bi content is given in figure Hs/Z Figure 6: Variation of Hs/Z with Bi content 2.4 ENERGY BAND GAP The band gap values may be correlated to the bond strength of the system. Average single bond energy (Hs/z) specifies the bond strength and it decreases with an increase in the Bi at. %. This decreases the bond strength of the system. The band gap values have been calculated using Shimakawa s relation as [26], E g (ABC) = A Eg(Se) + B Eg(Sb) + C Eg(Bi) Where A, B, C, D are the volume fractions and Eg (Se) = 1.95eV, Eg (Sb) = ev, Eg (Bi) = 0.06 ev, are the energy gaps of Se, Sb and Bi respectively. The variation of Eg decrease with increase in content of Bi follows from the above discussion. Fig. 7 shows that values of Eg decrease from 1.25eV to 0.83eV with increase in concentration of Bi from 4 to 20 at % Eg Figure 7: Variation of Eg with Bi content

7 International Journal of Engineering Technology, Management and Applied Sciences CONCLUSIONS In the present work, various physical parameters properties viz. average coordination number, average heat of atomization(hs), Parameter (R), bandgap (Eg) etc., have been calculated theoretically for combination Se 90-x Sb 10 Bi x (x = 4, 8, 12, 16, 20, at. %) of Se-Sb-Bi glassy alloys. It has been concluded from various figures given above those values of almost all the parameters vary linearly with increase in concentration of Bi from 4 to 20 at. %. Almost all the parameters, except f and R, were found to increase with the variation in Bi content.the stoichiometry R decreases due to decrease in contents of chalcogen Bi. It is concluded that an increase in antimonycontent decreases the stoichiometry (R), and optical band gap and the heat of atomization thus making this combination suitable for phase change optical recording. 4. REFERENCES [1] N. Suri, K.S. Bindra, R. Thangaraj, Electrical conduction and photoconduction in Se 80-x Te 20 Bi x thin films, J. Phys.: Condensed Matter, 18, 9129, [2] A.K. Singh, Recent advancement in metal containing multi component chalcogenide glasses, Opto-Electronics Review, 20, 3, , [3] M. Saxena, A.K. Kukreti, S. Gupta, M.K. Agarwal, N. Rastogi, Effect of Compositional Dependence on Physical Properties of Ge22Se78-xInx Glass System, Archives of Appl. Sci. Research, 4,2, 994, 2012,. [4] S.A. Fayek, Physical evolution and topology of (As 2 Se 3 ) 1-x T lx alloys, Journal of Physics and Chemistry of Solids, 62, , [5] S. Gupta,A.Agarwal,M. Saxena, Study of Crystallization Kinetics of Some Te x (Bi 2 Se 3 ) 1-x Glassy Alloys, Adv. in Appl. Sci. Research, 2, 4, 49-57, [6] K. Tanaka, Structural phase transitions in chalcogenide glass,phys. Rev. B, 39, 1270, [7] A. Giridhar, S. Mahadevan, A.K. Singh, Studies on some Ge -Se-Te Glasses, J. Non-Crystalline Solids, 103, 73, [8] N.Jain, S.L.Kakani, K.C.Pancholi. Effect of Bi Content on Optical Properties of Se-Sb-Bi Chalcogenide Amorphous Thin Films, International Journal for Scientific Research & Development, 2, 9, [9] M. Fadel, The effect of the Sb content on the physical properties of amorphous Se 0.75 Ge 0.25 y thin films Vacuum, 52, 277, [10] A.A.Othman, K.A.Aly, A.M. Abousehly, Effect of Te additions on the optical thin films, Thin Solid Films, 515, 507, [11] G. Lucovsky, R. H. Geils, R. C. Keezer, The Structure of Non-Crystalline Materials, Edited by P. H. Taylor and Francis, London, 127,1977. [12] S.Gupta, A.Agarwal, M.Saxena, Study of Crystallization Kinetics of Some Tex (Bi 2 Se 3 ) 1-x Glassy Alloys, Adv. in Appl. Sci. Research, 2, 4, 49-57, [13] S.Mahadevan, A.Giridhar, Thermal expansion and network constraints in Ge-In-Se glasses, J.Non Cryst. Solids, 162, 294, [14] A.Zakery, S.R.Elliot, Optical properties and applications of chalcogenide glasses: a review, J Non-crystalline Solids, 330, 1, [15] P. Tronc, M. Bensoussan,, A. Brenac,, C. Sebenne,, Optical-Absorption Edge and Raman Scattering in Ge x Se 1-x Glasses, Phys. Rev. B, 8, 5947, 1973.

8 International Journal of Engineering Technology, Management and Applied Sciences [16]J.C.Phillips, Topology of covalent non-crystallinesolids,short-range order in chalcogenide alloys and a-si (Ge), J. Non Cryst. Solids, 34,153, [17] J. C.Phillips, M. F.Thorpe, Constraint Theory, Vector Percolation and Glass Formation, Solid State Comm., 53, 699, [18] R. Kumar, A.Kumar, V.S. Rangra, A study of physical properties of Ge-Se-In glassy semiconductors,optoelec. & Adv. Mater, 4, 10, 1554, [19] A.A. Othman, K.A. Aly, A.M.Abousehly, Effect of Te additions on the optical properties of (As Sb Se) thin films, Thin Solid Films, 515, 507, [20] El. Sayed, M.Farag,, M.M.Sallam,, Composition dependence of the grain size, activation energy and coordination number in Ge 40-x In x Se 60 (10 x 40 at. %) thin films, Egypt J.Solids, 30, 1, 1, [21] D. Singh, S. Kumar, R. Thanraj, Study of the physical properties with compositional dependence in (Se 70 Ge 30 ) 100-x Bi x (0<x< 8) glassy semiconductors, Advances in Applied Science Research, 2, 3, 20, [22] L. Tichy, H. Ticha, On the chemical threshold in chalcogenide glasses, Mater Lett., 21,313, [23] L.Tichy, H. Ticha, Covalent bond approach to the glass-transition temperature of chalcogenide glasses J. Non-Cryst. Solids 189, 141, [24] L.Pauling, The Nature of Chemical Bonds, Cornell University Press, New York, [25] L.Zhenhua, Chemical bond approach to the chalcogenide glass forming tendency, J Non- Crystalline Solids, 127, 298, [26] S.S.Fouad, A theoretical investigation of the correlation between the arbitrarily defined optical gap energy and the chemical bonding in Te 46-x As 32+x Ge 10 Si 12 substances, Vacuum, 52, 505,