Effect of Composition and Thickness on the Electrical Behavior of Ge-Se- Sb-Te Amorphous Films

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1 Effect of Composition and Thickness on the Electrical Behavior of Ge-Se- Sb-Te Amorphous Films H.H.Amer 1,A. Abdel-Mongy 2, and A.A.Abdel-Wahab 1 (1) Solid State Department, National Center for Radiation Research and Technology, Atomic Energy Authority, Cairo, Egypt (2) Physics Department, Faculty of Science, Helwan University, Ain Helwan, Cairo, Egypt Received: 20/4/2016 Accepted: 1/9/2016 ABSTRACT The effect of replacement of Tellurium by Germanium on the electrical properties of Ge 19-xSe 63.8Sb17.2Te x (where x=0, 1, 3 and 5 at. %) was studied. Thin films with thickness in the range A o were prepared by thermal evaporation. It was found that the activation energy decreases with increasing Te content and also with thickness. It was observed that the increase of Te was followed by an increase in the glass transition temperature. In other words, the addition of Te decreases the stability of the formed glass. Increasing Te content was found to affect the average heat of atomization and cohesive energy (C.E) of the composition. Keywords: Amorphous, Chalcogenide, Electrical Properties. 1.INTRODUCTION Chalcogenide glasses containing metal atoms form an interesting class of amorphous semiconductors (a.s.c) because of their potential application as switching and memory devices. The addition of Te to these glasses increases the average coordination number. The family obtained by adding Te to glasses is due to the decrease in electrical conductivity and the decrease in the activation energy of conduction. In chalcogenide glasses there are different conduction mechanisms which can be observed. The conductivity (σ) in the chalcogenide can be written as (1) : σ =σ o +exp (-ΔE/KT) + σ 1 exp(-e 1 /KT)+σ 2 exp(-e 2 /KT) (1) The three terms arise from three different conduction mechanisms and they will be discussed separately as follows: i) The High Temperature Region The dominant mechanism is the band conduction through the extended states. This region is expressed by the first term of the R.H.S of equation(1) (2).The constant σ o for the chalcogenide glasses varies from Ω -1 cm -1 and is found to depend on composition, where ΔE is the activation energy, K is the Boltzman constant, and T is the absolute temperature. ii) Hopping Conduction via localized States This is responsible for the conduction in the second region. Here the conductivity arises from tunneling through unoccupied levels of the nearest neighboring center. The value of σ 1 is approximately times less than σ o partly because of the smaller density of localized states and their low mobility. iii) Hopping Conduction Near the Fermi Level This third contribution to conductivity in an amorphous semiconductor is analogous to impurity conduction in a heavily doped semiconductor. In this case the conductivity is given by the third term on the right-hand side of equation (1). in this paper results of the d.c conductivity of thin film samples of amorphous Ge 19-x Se 63.8 Sb 17.2 Te x semiconductor with( x=0,1,3&5%) are represented and discussed in a frame of chemical bonds involved. 136

2 2. EXPERIMENTAL WORK Bulk amorphous Ge 19-x Se 63.8 Sb 17.2 Te x (where x=0, 1, 3 and 5 at %) glasses were prepared. These glasses were prepared from Ge, Se, Sb and Te elements with high- purity ( % ).These glasses are reactive at high temperature with oxygen. Therefore, synthesis was accomplished in evacuated clean Silica tubes; (10-5 Torr), which were then heated slowly to a temperature of 1000 for 33 h. The ampoules were shaken several times to ensure homogeneity and then quenched in ice-cold water to avoid crystallization (3). Film samples were deposited by the thermal evaporation technique using (Edwared 306E) coating unit. Cleaned glass slides were used as substrates. The thickness of these films was determined using thickness monitor in the range ( )A. X-ray diffractometery proved the nature of the formed ingots. The homogeneity and transition temperature of their compositions were identified by differential thermal analysis (DTA). Thin films employed for D.C conductivity measurements were deposited onto glass substrates previously equipped with coplanar gold electrodes separated by a gap of width about 0.2 cm. The D.C conductivity was measured at different temperatures between 358 K and 77 K (liquid nitrogen temperature). The density of the considered samples was determined using the method of hydrostatic weight using toluene. A single crystal of germanium was used as a reference material for determining the toluene density. The density of the considered samples was determined using the method of hydrostatic weight using the toluene. The latter has been determined from the formula: d toluene = W air W toluene d W Ge (2) air Where, w' is the weight of Ge single crystal. Then, the sample density was calculated from the formula d sample = W air W air W toluene d toluene (3) Where w is the weight of the sample (4). 3- RESULTS AND DISCUSSION 1. The density dependence of (Te) content The density of the prepared glasses of the system Ge 19-x Se 63.8 Sb 17.2 Te x are given in Table (1), where it is noticed that the density decreases by increasing Te from gm/cm 3 for the composition Ge 19-x Se 63.8 Sb 17.2 Te x at x=0% down to gm/cm 3 for composition Ge 19-x Se 63.8 Sb 17.2 Te x at x=5%. The atomic volume of Te is greater than that of Ge which means that the molecular weight of a given volume increases by increasing Te content. Table (1): The composition dependence on density Te at.% Composition d th(gm/cm 3 ) dexp(gm/cm 3 ) Ge Se 63.8 Sb Ge Ge Ge14Se 63.8 Sb Se 63.8 Sb 17.2 Te1 3 16Se 63.8 Sb 17.2 Te3 5 Te5 Fig (1) represents the relation between the density and Te content. It is clear that there is a linear dependence up to a volume of almost 5 at. %. It does not show an appreciable behavior change between d th and d exp. 137

3 d th 5.8 d exp 5.6 density (gm/cm 3 ) Te content at.% Fig. (1): Dependence of density on Te content in the system Ge 19-x Se 63.8 Sb 17.2 Te x (with x=0, 1, 3, 5at. %) 2. The DC Conductivity Dependence of (Te) Content The dependence of lnσ on the reciprocal of temperature ( 1 ) for films with different thicknesses T of the composition Ge 19-x Se 63.8 Sb 17.2 Te x (where x=0, 1, 3&5at %) are shown in figures (2, 3, 4, &5), respectively. All samples follow a common pattern, where two regions of conductivity are observed. The activation energy pre-exponential factors of the two regions were estimated and listed in table (2). The values obtained for the first region suggest that the conduction mechanism, in the high temperature regions, is band like conduction (through extended states). It is also observed that σ o is composition dependent where it decreases linearly by increasing Te content. The values of ΔE decrease linearly by increasing Te content (5). This effect is most likely due to the reduction of the average binding energy by Te addition. To check this argument the cohesive energy (C.E) was calculated for the investigated four compositions Table (3). It is clear that C.E decreases with increasing Te content which is in good agreement with the above assumptions.on the other hand, in the low temperature range it seems that the dominant mechanism is the hopping around Fermi level T=500 T=1000 T=2000 T= LN /T Fig. (2): Variation of (lnσ) versus reciprocal absolute temperature for films of Ge 19-x Se 63.8 Sb 17.2 Te x (where x=0 at. %) system at different thicknesses (500, 1000, 2000, 3000A ) 138

4 T=500 T=1000 T=2000 T= LN /T Fig. (3): Variation of (lnσ) versus reciprocal absolute temperature for films of Ge 19-x Se 63.8 Sb 17.2 Te x (where x=1 at. %) system at different thicknesses (500, 1000, 2000, 3000A ) T=500 T=1000 T=2000 T= LN /T Fig. (4): Variation of (lnσ) versus reciprocal absolute temperature for films of Ge 19-x Se 63.8 Sb 17.2 Te x (where x=3at. %) system at different thicknesses (500, 1000, 2000, 3000A ) T=500 T=1000 T=2000 T= LN /T Fig. (5): Variation of (lnσ) versus reciprocal absolute temperature for films of Ge 19-x Se 63.8 Sb 17.2 Te x (where x=5at. %) system at different thicknesses (500, 1000, 2000, 3000A ) 139

5 X= 0.0% X=0.1% X=0.3% X=0.5% LN /T Fig. (6): Variation of (lnσ) versus reciprocal absolute temperature for films of Ge 19-x Se 63.8 Sb 17.2 Te x (where x=0, 1,3 &5 at.%) system at constant thickness T=500 A Table (2): Compositional and thickness dependence of the electrical conductivity for the thin film glasses in the Ge 19-x Se 63.8 Sb 17.2 Te x (where x=0, 1, 3&5at. % ) with different thicknesses (500, 1000,2000&3000) Å Ratio of Te % Thickness (ta ) Tg ( C) ΔE (ev) Density (gm/cm 3 ) E1 (ev) Σ (Ω -1 Cm -1 ) σo (Ω -1 Cm -1 ) X=0% x x x x X=1% x x x x X=3% x x x x X=5% x x x x

6 Table (3): Some physical Parameters as functions of Te Content for Ge 19-x Se 63.8 Sb 17.2 Te x (where x=0, 1, 3&5at. % ) specimens Composition Nco Ns R Hs Kcal/mol H/Nco Eg,exp Eg,th (ev) dth gm/cm 3 dexp gm/cm 3 CE ev/atom Ge 19Se63.8Sb Te 1Ge 18Se63.8Sb Te 3Ge 16Se63.8Sb Te 5Ge 14Se63.8Sb Ioffe and Regel (6) have suggested that the bonding character in the nearest neighbor region, which is the coordination number, characterizes the electronic properties of the semiconducting materials. The coordination number obeys the so- called 8-N rule, where N is the valence of an atom; the number of the nearest-neighbour atoms for Ge, Sb,Se and Te are calculated and listed in table(4). The average coordination number in the quaternary compounds A α B β C γ D λ is as (7): N co =(αnco(a) + βnco(b) + γnco(c) + λnco(d))/(α + β + γ + λ (4) Where: α, β,γ,λ are the valence of the elements of compound A α B β C γ D λ Table (4): Values of energy gap, density, coordination number, heat of atomization (H s ), bond energy and electro negativities of Ge, Se, Sb and Te used for calculations Physical characteristics Ge Se Sb Te Energygap(ev) Density(g/cm3) coordination no Bondenergy(kcal/mol) Hs(kcal/mol) Electronegativity Radius(pm) C.E(ev/atom) The determination of N CO allows the estimation of the number of constraints (N s ). This parameter is closely related to the glass-transition temperature and associated properties. For a material with coordination number N co, N s can be expressed as the sum of the radial and angular valence force constraint. (8) N S = N CO 2 + (2N CO 3) (5) The calculated values of N CO and N S for the Ge (19-x) Se 63.8 Sb 17.2 Te x system are given in table(3). The parameter R, which determines the deviation of stoichiometry and is expressed by ratio of 141

7 the covalent bonding possibility of chalogen atoms to that of non-chalcogen atoms, was calculated using the following relation (9,10): R = (a Nse + cnte)/(bnge + dnsb) (6) The calculated values of R for the Ge (19-x) Se 63.8 Sb 17.2 Te x system are given in Table (3) showing that the composition` Ge (19-x) Se 63.8 Sb 17.2 Te x system represents the so-called stoichiometric composition (R=1). Using this composition as a reference, glasses with Se content of more than 60 at% can be called Se-rich glasses(r>1)and those with Se content of less than 60 at% can be called Se-poor glasses(r<1) (11,12). According to Pauling (13) the heat of atomization H s (A-B) at standard temperature and pressure of a binary semiconductor formed atoms A and B is the sum of the heats of formation ΔH and the average of the heats of atomization H S A and H S B corresponding to the average non-polar bond energy of the two atoms (14,15) is proportional to the square of the difference between the electro negativities χ A and χ B of the two atoms. H S (A-B)=ΔH+1/2(H A S+H A S) (7) ΔH α(χ A - χ B ) 2 (8) The heat of formation (ΔH) which is strongly correlated with the difference in the ionicities of different atoms is small compared to the cohesive energy because the electro negativities of the constituent elements such as Ge, Sb, Se are very similar. In most cases the heat of formation of chalcogenide glasses is unknown, the heat of formation ΔH is about 10% of the heat of atomization and, therefore, can be neglected. To extend the idea to ternary and higher order semiconductor compounds, the average heat of atomization is defined for a compound A α B β C γ D λ as (16,17) Hs = αhs(a)+βhs(b)+γhs(c)+λhs(d) α +β + γ +λ Values of H S of Ge, Sb, Se and Te are given in Table(3).The values of H S increase with increasing Te content. To correlate H S with E g in non-crystalline solids, it is reasonable to use the average coordination number instead of the iso- structure of crystalline semiconductors. It was found that the variation in the theoretical values of the energy gap (E g,th ) with composition in quaternary alloys can be described by the following simple relation (18) : E g,ab (Y)=YE ga +(1-Y)E Gb (10) Where Y is the volume fractions of element. For quaternary alloys: E g,th(abcd)=aeg(a)+beg(b)+ceg(c)+deg(d) (11) Where a, b, c and d are the volume fractions of the elements A, B, C and D respectively. E g (A), E g (B), E g (C) &E g (D) are the corresponding optical gaps. The conversion from a volume fraction to atomic percentage is carried out using the atomic weights and densities tabulated in Table (4). The calculations of (E g, th ) based on the above equation for the alloys, in Ge 19-x Se 63.8 Sb 17.2 Te x are given in Table(3),which reveal that the addition of Te lead to change in the considered properties. The increase in Te % leads to decrease in E g,th,e g,exp and N co shown in Figs. (7,8,9,10). The various bond strengths of the compound decrease and hence E g will decrease shown in fig (11). Knowing the bond energies shown in Table (5), we can estimate the cohesive energy (CE), i.e. the stabilization energy of an infinitely large cluster of the material per atom, by summing the bond energies over all the bonds expected in the system under test. The CE of the prepared samples is evaluated from the following equation (11) (19) : C. E = Ci Di/100 (12) (9) 142

8 Where Ci and Di are the numbers of the expected chemical bonds and the energy of each corresponding bond, respectively. The calculated values of CE for all compositions are presented in Table (3). With the exception of Ge (19-x) Se 63.8 Sb 17.2 Te x glass,ce increases with increasing Te content. Increasing the Te content leads to an increase in the average molecular weight, which increase the rigidity (strength) of the system. Table (5): Bond energy probabilities and relative probabilities of formation of various bonds in glassesge 19-x Se 63.8 Sb 17.2 Te x (where x=0, 1, 3&5at %)system at taking the probability of (Ge-Se) bond as unity Bond Bond energy Relative probability(at Probability Kcal/mol T= (Ge-Se) * (Te-Se) * *10-6 (Se-Se) * *10-8 (Sb-Se) * *10-8 (Ge-Ge) * *10-9 (Ge-Te) * *10-17 (Ge-Sb) * *10-17 (Te-Te) * *10-18 (Sb-Te) * *10-20 (Sb-Sb) * *10-23 To emphasize the relationship between E g and the average bond strength more clearly, E g is compared with H S /N co which is the average single bond energy in the alloy. One observes that E g, as well as H S /N co, increase with increasing In content, suggests that one of the main factors determining E g is the average single bond in the alloy. Fig (12) shows the X-ray diffraction patterns (20) for thin films Ge 19-x Se 63.8 Sb 17.2 Te x (where x=0, 1, 3 and 5 at %) system. The absence of diffraction lines in the X-ray patterns indicates that the films have amorphous structures Nco Te at % Fig. (7): Variation of coordination number with Te content 143

9 R Te % Fig. (8): Variation of Stoichiometry parameter (R) with Te content C.E Te % Fig (9): Variation of cohesive energy with Te content Hs Te % Fig. (10): Variation of heat of atomization (H s ) with Te content 144

10 Eg Te % Fig. (11): Optical band gap with Te content Fig. (12): X-Ray diffraction Patterns of the amorphous Ge (19-x) Te x Se 63.8 Sb 17.2 (where x=0,1,3&5at.%) 145

11 CONCLUSION The physical interpretations and their result make the composition Ge 19-x Se 63.8 Sb 17.2 Te x the most cross linked and a suitable one. The coordination number and heat of atomization increase with Te content. It is concluded that an increase in Te content decreases the Stoichiometry (R), cohesive energy and dc conductivity. It was found that the activation energy decreases with increasing the thickness of the film. REFERENCES (1) N.F.Mott and E.A.Davis,"Electronic process in Non-Crystalline Materials, (2) H.H.Amer, A.E.H.Zekry, S.M.S.EL.Arabi,k.E.Ghareeb and A.A.ELShazly; J.Rad,Res.Appl.Sci.Vol.6 No 1.pp ,2013. (3) J.A.Savoge;''Infrared optical Materials and their Antireflection Coatings",Adam Higher Bristal(1983). (4) T.Qzawa,Bull.Chem.Soc.Jpn.38(1965)1881. (5) Pankaj Sharma, and Vineet SharmaInt. J. New. Hor. Phys. 2, No. 1, (2015). (6) A.F. Ioffe and A.R. Regel, Prog. Semicond (1960). (7) A.F. Ioffe and A.R. Regel; Journal of prog.semicond., (1994). (8) 8.J.C.Phillps and M.F.Thorpe ; Journal of Solid State Commun Vol.53, ,(1985). (9) L. Tichy and H. Ticha, Mater. Lett (1994). (10) 10.L. Tichy and H. Ticha, J. Non-Cryst. Solids (1995). (11) 11. Satya Prakash Saxena1, Sunita Chawla, Shilpa Gupta, Nikhil Rastogi, Manish Saxena, I J I R SET, Vol. 2, Issue 9, September (12) 12.Neha Sharma, Sunda Sharda, Vineet Sharma and Pankaj Sharda, (13) Chalcogenide Letters Vol. 9, No. 8, August 2012, p (14) 13. L. Pauling, J. Phys. Chem (1954). (15) 14. S.A. Fayek and S.S. Fouad, Vacuum (1998). (16) 15. L. Brewer, in Electronic Structure and Alloy Chemistry of the Transition Elements, edited by P.A. Beck (Interscience, New York, 1963), p (17) 16. V. Sadagopan and H.C. Gotos, Solid State Electron (1965). (18) 17. S.S. Fouad, Vacuum (1999). (19) 18. S. Mahadevan, A. Giridhar and A.K. Singh, J. Non-Cryst. Solids (1994). (20) 19.S.A.Fayek Journal of Phys. Chem.Solids Vol.62, (2001). (21) 20. S.S. Fouad, A.H. Ammar and M. Abo-Ghazala, Vacuum (1997). 146