20120140505007

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20120140505007

  1. 1. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 59-65 © IAEME 59 EFFECT OF COMPOSITION AND THICKNESS ON THE ELECTRICAL BEHAVIOR OF Ge – As – Se - Te CHALCOGENIDE GLASSES H. H. Amer* and K. E. Ghareeb** *Solid State Department, National Center for Radiation and Technology (NCRT), Atomic Energy Authority (AEA) - Cairo – Egypt **Metallurgy Department, Nuclear Research Center, AEA - Cairo – Egypt ABSTRACT Thin films of thickness in the range of 500 – 3000 A° of the composition Ge20As30 SeXTe(50- X)(with x = 0 and 40) were prepared by thermal evaporation. D.C. conductivity was reported for the investigated films. It was found that the activation energy decreases by increasing the thickness of the film. It was observed that the increase of Se was followed by increase in the glass transition temperature. Also, it is noticed that increasing Se content affected the average heat of atomization and cohesive energy (C.E.) of the composition and the energy gap decreases by increasing Se content. Keywords: Amorphous, Chalcogenide, Conductivity, Activation Energy, Cohesive Energy. INTRODUCTION In Chalcogenide glasses, there are different mechanisms which can be observed in different temperature regions. The electrical conductivity (σ) in these glasses can be written (1) in three terms arise from three different conduction mechanisms as: σ = σo exp (-∆E/KBT) + σ1 exp (-E1/KBT) + σ2 exp (-E2/KBT) (1) The first term describes the high temperature region where the dominant mechanism is the band conduction through the extended states. In this term the constant σo varies from 10-5 to 10-9 -1 cm-1 and depends on the composition, where ∆E is the activation energy, KB is Boltzman’s constant and T is the absolute temperature. The second region represents the hopping conduction via localized states. Here the conduction arises from tunneling through unoccupied levels of the nearest neighboring centers. The value of σ1 is approximately 102 -104 less than σo, partly because of the smaller density of localized states and their low mobilities. The third region represents the hopping INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN ENGINEERING AND TECHNOLOGY (IJARET) ISSN 0976 - 6480 (Print) ISSN 0976 - 6499 (Online) Volume 5, Issue 5, May (2014), pp. 59-65 © IAEME: www.iaeme.com/ijaret.asp Journal Impact Factor (2014): 7.8273 (Calculated by GISI) www.jifactor.com IJARET © I A E M E
  2. 2. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 59-65 © IAEME 60 conduction near Fermi level. This third contribution to conductivity in an amorphous semiconductor is analogous to impurity conduction in heavily doped semiconductors. In this case the conductivity is given by the third term on the R.H.S. of equation (1)(2) . In this paper the results of D.C. conductivity for thin film samples of amorphous Ge20As30SeXTe(50-X) (with x = 0 and 40) are presented and discussed in a frame of chemical bonds involved. In addition, the average coordination number (NCO), the average heat of atomization (Hs), the constraints (Ns) and the cohesive energy (CE) of the Ge20As30SeXTe(50-X) glasses have been examined theoretically. EXPERIMENTAL WORK 1-Preparation of Bulk Amorphous Ge20As30 SeXTe(50-X) (with x = 0 and 40) Two glasses of bulk amorphous of the system Ge20As30 SeXTe50-X (where x = 0 and 40) were prepared. These glasses were prepared from Ge, As, Se and Te elements with purity 99.999%. These glasses are reactive at high temperature with oxygen. Therefore, synthesis was accomplished in evacuated clean silica tubes. The tubes were washed with distilled water, and then dried in a furnace whose temperature was about 100o C. For each composition, the proper amounts of materials were weighed using an electrical balance type with accuracy ±10-4 gm. The weighted materials were introduced in the cleaned silica tubes and then evacuated to about 10-6 torr and sealed. The sealed tubes were placed inside the furnace and the temperature of the furnace was raised gradually up to 850o C within one hour and kept constant for 8 hours. Moreover, shaking of the constituent materials inside the tube in the furnace was necessary for realizing the homogeneity of the composition. After synthesis, the tubes were quenched in liquid nitrogen. The glassy ingots could be obtain by drastic quenching. Then, the material was removed from the tubes and kept in dry atmosphere. The ingot materials were identified as glass due to their bright features. The proper ingots were confirmed to be completely amorphous using X-ray diffraction and differential thermal analysis. Homogeneity of prepared samples was proved by determination of density of different parts (3) . 2- Preparation of Thin Film Samples Thin films of the selected compositions were prepared by thermal evaporation technique. Edward 306E coating unit was used for thin film deposition. The vacuum system consists mainly of a rotary pump, diffusion pump, penning bridge for measuring vacuum, high A.C. current source and the bell jar. A specially designed silica boat was used for evaporation instead of using metallic boats to obtain highly homogeneous uniform films. The silica boat was used instead of metallic boat to omit the probability of contamination. The heating of the silica was achieved by spiral tungsten wire. The silica boat had to be cleaned every time before evaporation. This was accomplished by using hydrochloric acid, then washing several times with boiled distilled water and finally it was dried in a furnace, whose temperature was about 100o C. The thin films were checked by x-ray diffraction. The obtained data reveal no sharp peaks. 3- Density Determination 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 latter has been determined from the formula: Ge air tolueneair toluene dx W WW d ' '' − = (2) Where, W` is the weight of single Ge crystal and dGe is the density of Ge. dGe = 5.32 gm/cm3
  3. 3. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 59-65 © IAEME 61 dtoluene = 5.15 gm/cm3 Then, the sample density was calculated from the formula toluene tolueneair air sample dx WW W d − = (3) Where W is the weight of the sample (4) . Devices used in different measurements are as follows 1- X-ray Philips diffractometer was used to investigate and characterize the structure of the samples. 2- A micro-Data apparatus, Shimadzu DT-30 model was used for the measurements of DTA. 3- Double-Beam Jasco V-530 UV/V is spectrophotometer. It was used for optical measurements. 4- Edward 306E coating unit was used for thin film deposition. 5- Thickness monitor MAXTEK Model TM-200. RESULTS AND DISCUSSION Figure (1) shows that the density increases by increasing the Se content. 4.7 4.8 4.9 5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 0 500 1000 1500 2000 2500 3000 3500 Thickness (t Ao ) Density(gm/cm3 ) X = 0 X= 40 Fig.(1): Dependence of density on thickness in the system Ge20As30 SeXTe(50-X) (with x = 0 and 40) -Effect of thickness on electrical conductivity The dependence of ln (σ) on 1/T for the investigated films with different thicknesses with x = 0 and x = 40 are shown in Figures (2) and (3), respectively. All samples follow a common pattern, where two regions of conductivity are observed: the first is for high temperature range which is for
  4. 4. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 59-65 © IAEME 62 high described by the first term of equation (1). In this region, band like conduction through extended states is the dominant mechanism. -33 -31 -29 -27 -25 -23 -21 -19 -17 1 2 3 4 5 6 7 8 9 10 11 12 13 1000/T (K -1 )lnσσσσ(ΩΩΩΩ -1 cm -1 ) X = 0, t = 500A° X = 0, t =1000A° X = 0, t = 2000A° X = 0, t = 3000A° Fig.(2): Variation of (ln σ)versus reciprocal absolute temperature for films of (Ge20As30SeXTe50- X with x = 0) system at different thickness (500A°, 1000A°, 2000A° and 3000A°) -35 -33 -31 -29 -27 -25 -23 -21 -19 -17 -15 2 3 4 5 6 7 8 9 10 11 1000/T (K -1 ) lnσσσσ(ΩΩΩΩ -1 cm-1 ) X = 40, t = 500A° X = 40, t = 1000A° X = 40, t = 2000 A° X = 40, t = 3000A° Fig. (3): Variation of (ln σ) versus reciprocal absolute temperature for films of (Ge20 As30SeXTe50-X with x = 40) system at different thickness (500A°, 1000A°, 2000A° and 3000A°)
  5. 5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 59-65 © IAEME 63 Values of σo and σRT (conductivity of room temperature) for different films were estimated and listed in Table (1). Table (1): Compositional and thickness dependence of the electrical characteristics quantities for the thin film glasses in the system Ge20As30SeXTe50-X) with x = 0 and 40 Comp. Thickness (t Ao ) Tg(o C) Density (gm/cm3 ) ∆E.(eV) σRT ( -1 cm-1 ) Ln σRT σo ( -1 cm-1 ) E1(eV) X = 0 500 1000 2000 3000 245 4.81 5.22 5.31 5.53 0.58 0.35 0.28 0.18 3.86x10-12 1.12x10-11 3.8x10-11 2.4x10-11 -26.28 -25.2 -23.99 -23.9 26.579 25.05 24.12 23.98 1.16 0.69 0.56 0.35 X = 40 500 1000 2000 3000 258 4.9 5.3 5.35 5.6 0.35 0.25 0.16 0.13 4.59x10-13 1.36x10-10 1.7x10-10 3.01x10-9 -28.4 -22.7 -22.5 -21.92 28.2 22.81 22.57 21.97 0.71 0.499 0.322 0.27 To check the validity of compensation law, the pre-exponential factor σo against the activation energy ∆E is shown in fig.(4). The observed dependence indicates that Ge -As -Se –Te obeys the compensation law(5,6) . Such behavior was also observed by Majied(7) . It was observed that the electrical activation energy decreases by increasing both Se content and the thickness of the thin film for the composition Ge20As30SeXTe(50-X) with x = 0 and 40. 23 24 25 26 27 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 ∆∆∆∆E(e.v) σσσσo(ΩΩΩΩ -1 cm -1 ) 0 5 10 15 20 25 30 σσσσοοοο(ΩΩΩΩ -1 cm -1 ) X = 0 X = 40 Fig.(4): Variation of σo as a function of activation energy for the composition (Ge20 As30SeXTe50-X) with X = 0 , 40 at different thickness (500A° , 1000A° , 2000A°, 3000A°)
  6. 6. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 59-65 © IAEME 64 - Loffe and Regel (8) 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 nearest-neighbor atoms for Ge, As, Se and Te are listed in table (1).The average coordination number in the quaternary compounds Aα Bβ Cγ Dλ is as: NCO = [α NCO(A)+β NCO(B)+ γ NCO(C)+λ NCO(D)] / [α + β+ γ+ λ] (4) Where: α, β, γ and λ are the valences of the elements of compound. The determination of NCO allows the estimation of the number of constraints (Ns). This parameter is closely related to the glass-transition temperature and associated properties. For a material with coordination number NCO and Ns can be expressed as the sum of the radial and angular valance force constraints(9) . Ns = NCO/2 + (2 NCO-3) (5) To extend the idea to ternary and higher order semiconductor compounds, the average heat of atomization is defined for a compound Aα BβCγ Dλ as(10,11) : Hs = [α Hs(A)+β Hs(B)+ γ Hs(C)+λ Hs(D)] / [α + β+ γ+ λ] (6) Table (2) shows values of the coordination number NCO, the heat of atomization (HS) and the bond energy of Ge, As, Se and Te, used for calculations (12) . Table (2): Values of the coordination number NCO, the heat of atomization (HS) and the bond energy of Ge, As, Se and Te, used for calculations Physical characteristics Ge As Se Te Coordination no. 4 3 2 2 Hs(Kcal/mol) 90 69 54.17 46 Bond energy(Kcal/mol) 37.6 32.1 78.87 33 Knowing the bond energies, we can estimate the cohesive energy (CE), i.e. the stabilization energies of an infinitely large cluster of the materials 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 (13) : CE = Σ (CiDi/100) (7) Where Ci and Di are the number of the expected chemical bonds and the energy of each corresponding bond. The calculated values of NCO, Ns, HS, Hs/ NCO and CE for (Ge20 As30SeXTe50-X) system are given in table(3).
  7. 7. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 59-65 © IAEME 65 Table (3): The calculated values of NCO, Ns, HS, Hs/ NCO and CE for (Ge20 As30SeXTe50-X with x = 0, 40) system Composition NCO Ns HS(Kcal/mol) Hs/ NCO CE(eV/atom) Ge20As30Te50 2.7 3.75 61.7 22.85 3.99 Ge20As30Se40Te10 2.7 3.75 64.97 24.06 5.584 CONCLUSION 1 - The electrical conductivity (σo) decreases with increasing both Se content and the thickness of the thin film for the (Ge20 As30SeXTe50-X with x = 0, 40) composition. 2 - The electrical activation energy (∆E) decreases with increasing in both Se content and the thickness of the thin film for the (Ge20 As30SeXTe50-X with x = 0, 40) composition. 3 - The values of energy gap (E1) decrease with increasing the thickness of the thin film for the (Ge20 As30SeXTe50-X with x = 0, 40) composition. 4 - The values of coordination number (NCO), number of constraints (Ns), heat of atomization (Hs) and cohesive energy (CE) of the (Ge20As30SeXTe50-X with x = 0, 40 system is dependent on the glass composition. The increase in Se content leads to increase in E1, NCO, Hs, Hs/ NCO and CE. REFERENCES (1) N.F.Mott and E.A.Davis.; second edition (oxford: Clarendon Press) (1979). (2) H. H. Amer, A.E.H. Zekry, S.M.S.El.Arabi, K.E.Ghareeb and A.A.EL. Elshazly; J. Rad. Res. Appl. SCi. Vol. 6. No. 1. PP. 105-120 (2013). (3) J. A. Savoge; Adam Higher, Bristal (1983). (4) H. H. Amer, Ph.D. Thesis, Electronic and communication Dept., Faculty of Engineering, El Azhar University, Cairo (1998). (5) H. Meir, Verlag Chemi Gavbh, weinheim (1974). (6) K. Miyair, Ohta Y and Leda M, J. Phys. D. Appl. Phys. Vol. 21, 1519 (1988). (7) C. A. Majid, philosophical Magazine B, Vol.49 (1984). (8) A. F. Loffe and A. R. Regel; Journal of Prog. Semicond., 133-142 (1994). (9) J. C. Phillps and M.F. Thorpe; Journal of solid state Commun Vol. 53, 699-702, (1985). (10) H. H. Amer, A. Abdel-Mongy, A. A. Abdel-wahab; J. Rad. Res. Appl. Sci., Vol. 5, No. 3, pp. 447-461 (2012). (11) M. F. Thorpe; Journal of Non. Cryst. Solids, 182-135, (1995). (12) H. H. Amer, Mohammed Elkordy, Mohammed Zien, A. Dahshan, Randa A. Elshamy; International Journal of Computer Science and Network Security, VoL.11, No. 3 (2011). (13) S. A. Fayek; Journal of Phys. chem. solids VoL. 62, 653-659 (2001). (14) K. Singh, Brajraj Singh and R. Chaudhary, “A New Form of Extended Ziman-Faber Theory for Liquid and Amorphous Binary Alloys”, International Journal of Advanced Research in Engineering & Technology (IJARET), Volume 5, Issue 1, 2014, pp. 36 - 44, ISSN Print: 0976-6480, ISSN Online: 0976-6499.

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