Electrical conductivity and crystallization kinetics of amorphous Ag10Te90 and Ag20Te80 thin films

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Solid State Sciences 10 (2008) 1416e1421 www.elsevier.com/locate/ssscie

Electrical conductivity and crystallization kinetics of amorphous Ag10Te90 and Ag20Te80 thin films A. Abu El-Fadl a,*, M.M. Hafiz a, M.M. Wakkad b, A.S. Aashour a b

a Physics Department, Faculty of Science, Assiut University, Assiut 71516, Egypt Physics Department, Sohag Faculty of Science, South Valley University, Sohag, Egypt

Received 6 August 2007; received in revised form 20 January 2008; accepted 26 January 2008 Available online 20 March 2008

Abstract The d.c. conductivity of thermally evaporated Ag10Te90 and Ag20Te80 thin films has been studied. Measurement of the temperature dependence of the electrical conductivity indicates that the increase of annealing temperature leads to a decrease in the activation energy for conduction Ea. In this case the pre-exponential factor and the activation energy Ea satisfies MeyereNeldel rule. Using the electrical conductivity as a characteristic quantity to follow the growth of crystalline phases in the amorphous matrix, the crystallization kinetics have been studied in the glassy AgeTe films. The results indicated that, the activation energy for crystallization Ec under isothermal conditions of the glassy Ag10Te90 and Ag20Te80 films is 44.723 and 81.228 kJ/mol, respectively. The obtained value of the Avrami index is between 0.867 and 0.952 for all isotherm temperatures, which means that one-dimensional growth mechanism is working in the crystallization process. Ó 2008 Elsevier Masson SAS. All rights reserved. PACS: 61.43.Fs; 64.70.Pf; 05.70.Fh Keywords: Ag10Te90 and Ag20Te80 thin films; Crystallization kinetics; D.C. conductivity

1. Introduction There has been a rapidly growing interest in the physical properties of amorphous solids during the last decades. Chalcogenide glasses have drawn great attention from scientists and engineers because of their extensive use in solid-state devices. Optical memory effects in amorphous semiconducting films have been investigated and utilized for various applications [1e3]. Ag-doped chalcogenide glasses and thin films become attractive materials for fundamental research of their structures, properties, and preparations. The interest for silver telluride semiconducting thin films is motivated by their applications in optical recording and polymorphic phase transition [4]. Thin amorphous films of Ag-containing glasses have mainly been prepared by thermal vacuum evaporation [5], sputtering

* Corresponding author. Tel.: þ20 88 2412244; fax: þ20 88 2333837. E-mail address: [email protected] (A.A. El-Fadl). 1293-2558/$ - see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2008.01.035

[6], and flash-evaporation [7,8], and also the Ag-containing films can be prepared by so-called photo doping of chalcogenide, where a bilayer (Ag or Ag compound)/chalcogenide is illuminated by light or exposed by an e-beam [9]. For the present study, by thermally evaporated technique, Ag10Te90 and Ag20Te80 thin films have been deposited on glass substrates. Our intentions are to check the effects of different annealing temperatures on the structural and electrical properties and to study the crystallization kinetics by continuously following the changes in the electrical conductivities during isothermal annealing. 2. Experimental details Two compositions of the system AgxTe1x, named Ag10Te90 and Ag20Te80 glasses were prepared from Ag and Te elements with purity 99.999%. The weighed materials were introduced to quartz ampoules evacuated to (105 Torr.), then sealed. The sealed ampoules were placed inside a furnace and the

A.A. El-Fadl et al. / Solid State Sciences 10 (2008) 1416e1421

temperature of the furnace was raised from room temperature to 800 K and kept at this temperature for 2 h. Then the temperature was raised to 1200 K for 24 h. Constituent materials inside the ampoule were manually stirred for realizing the homogeneity of the composition. After synthesis the ampoules were quenched into ice-cooled water. The fast cooling of the melt enables us to obtain the glassy materials. After quenching the melt, the ingots were removed from the ampoules and kept in dry atmosphere. The amorphous structure and phases of the films were confirmed by using X-ray diffractometer (Philips type PW ˚ ). 1710 with Cu as a target and Ni as a filter, l ¼ 1.5418 A Amorphous Ag10Te90 and Ag20Te80 thin films were prepared by thermal evaporation (Edward 306E) of the bulk glass (at normal incidence) on to ultrasonically cleaned glass substrates. The film thickness was fixed at 100 nm, which accurately measured using a quartz crystal monitor (Edward model FTM5). Annealing of the evaporated films was carried out in quartz tube furnace in nitrogen flow at 373, 423, 473 and 523 K. The annealing time was 1 h. Phase and surface morphology was analyzed using scanning electron microscopy (SEM) technique, Jeol (JSM)-T200 type. Thick aluminum electrodes were thermally deposited as ohmic contact. A planar geometry of the amorphous films (electrode gap approximately equals 2 mm) was used for the electrical measurement. To prevent oxidation of the films, the measurement was taken under vacuum z105 Torr. Keithly 610 C electrometer was used as ohmmeter. The temperature was measured using a calibrated copper constantan thermocouple. 3. Results and discussion 3.1. X-ray diffraction Analysis of the X-ray diffraction data reveals the amorphous state of the as-prepared specimen as shown in Fig. 1(a e I). As-prepared Ag10Te90 does not show any diffraction peaks and the amorphous structure of the as-deposited film is expected because the evaporating molecules precipitate randomly on the surface of the substrate and all the following condensed molecules also adhere randomly leading to disordered films. The XRD of the as-prepared Ag20Te80 is presented in Fig. 1(b e I). From this curve, it can be seen that, the film was semipolycrystalline (between polycrystalline and amorphous). It also reveals some phases, which may be related to Ag2Te and Ag5Te3 phases. This behavior may be attributed to the probability of the increase in AgeTe coalescence by increasing Ag content, therefore these phases early begin to appear. This behavior agrees with the results obtained by Marinkovic and Simic [10] regarding the reaction kinetics in thin silveremetal couples at room temperature; the Ag2Te and Ag5Te3 phases are formed during sputtering or thermal evaporation of tellurium on the silver layer. Fig. 1(a e II, III) and (b e II, III) shows the X-ray diffraction patterns for 1 h annealing at two different temperatures (373 and 473 K) for the two compositions. Fig. 1 shows well-defined peaks suggesting the formation of polycrystalline films due to annealing. A comparison of the observed pattern

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Fig. 1. X-ray diffraction patterns for the as-prepared and annealed (a) Ag10Te90 (b) Ag20Te80 films, I e as-prepared, II e annealed at 373 K and III e annealed at 473 K.

with the standard JCPDS cards shows that Te (JCPDS 850563), Ag2Te (JCPDS 34-0142) and Ag5Te3 (JCPDS 861168) phases are identified in the above-mentioned figure. It is also noticed that, the intensities of the parasite phase, which enhanced with an increase in post-heating temperature, may be due to the sufficient increase in the thermal energy for re-crystallization and the grain growth with temperature. 3.2. Scanning electron microscopy Fig. 2 depicts the scanning electron micrographs (SEM) of the two AgeTe compositions. The figure indicates that, the grain sizes of studied films increased with increase in the annealing temperature, from 373 K to 473 K and the amount of polycrystallinity increases. However, the crystallization degree was improved, as the annealing temperature increases, and the crystalline morphology is not distinct due to the fact that the crystallization is only in its initial stages. SEM observations indicate an amorphous-tocrystalline transformation of AgeTe films after annealing. The amount of transformed crystalline phases depends on the original composition as well as on the annealing temperature. 3.3. D.C. conductivity Fig. 3 shows the temperature dependence of dark conductivity (s) for Ag10Te90 and Ag20Te80 films annealed at

A.A. El-Fadl et al. / Solid State Sciences 10 (2008) 1416e1421

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Fig. 2. SEM micrographs of: (a) Ag10Te90 annealed at 373 K (b) Ag10Te90 annealed at 473 K; (c) Ag20Te80 annealed at 373 K (d) Ag20Te80 annealed at 473 K.

different temperatures in the range 373e523 K. ln s versus 1000/T curves are straight lines, which indicate that, the conduction in these glasses is through an activated process having a single activation energy in the temperature range 303e450 K. The d.c. conductivity (sd.c.) can, therefore, be expressed by the usual relationship [11]: 1.5

ln σ [

-1.cm-1]

1.0 0.5 0.0 -0.5 -1.0

-1.cm-1]

-1.5

ln σ [

a

1.0

Ann.Temp. [K] As 373 423 473 523

b

0.5

-1.0

Ea sd:c: ¼ s0 exp  kB T

Ann.Temp. [K] As 373 423 473 523 2.4

ð1Þ

Ea is the activation energy for d.c. conduction, s0 is the pre-exponential factor, and kB is the Boltzmann constant. The values of Ea calculated by least squares fitting are listed in Table 1. The values of resistance at room temperature RRT (303 K) are also given. It is clear from Table 1 that, Ea decreases with an increase in annealing temperature. This decrease in the values of Ea with the increase of annealing temperatures up to about 523 K in the present study reveals the decrease of structural disorder in the investigated films, which leads to an improvement of the electronic properties. These changes may be interpreted in terms of amorphous-tocrystalline transformation. Such transformations can be emphasized from the XRD diffractograms on Fig. 1. During this transformation, the thermally induced crystalline phase grows and causes a rapid increase of the electrical conductivity

Annealing temperature [K]

-1.5 2.2



Table 1 The room temperature resistance RRT, activation energy Ea, and pre-exponential factors ln s0 as a function of annealing temperatures for Ag10Te90 and Ag20Te80 films

0.0 -0.5



2.6

2.8

3.0

3.2

3.4

103/T [K-1] Fig. 3. Variation of d.c. conductivity with temperature for the as-prepared and annealed (a) Ag10Te90 (b) Ag20Te80 thin films at different annealing temperatures.

As prepared 373 423 473 523

Ag10Te90

Ag20Te80

RRT  103 U

Ea [meV]

ln s0

RRT  103 U

Ea [meV]

ln s0

31.800 23.437 21.579 16.596 13.668

149.89 134.92 127.35 120.51 111.21

4.713 4.443 4.318 4.232 4.094

39.389 29.144 27.705 19.603 15.945

142.62 130.78 126.52 117.81 103.86

4.180 4.029 4.011 3.923 3.691

A.A. El-Fadl et al. / Solid State Sciences 10 (2008) 1416e1421

and consequently a decrease of the activation for conduction [12]. It is also seen that, Ea decreases with increasing Ag content and this change may be explained in terms of the change in chemical bonding. To explain this behavior, let us calculate the average coordination number Navc, which suggests the bonding character in the nearest-neighbor region in a binary compounds AxB100x according to the following equation [13,14]: Navc ¼ ½xNcA þ ð100  xÞNcB =100

ð2Þ

The calculated coordination number Navc, for the AgxTe100x (x ¼ 10, 20) system, using the values of Nc for Ag ¼ 7 and Te ¼ 2 are found to be 2.5 and 2.8, respectively. It can be seen that Navc increases with increasing Ag content since AgeTe bonds are formed when Ag is added into the Te-based glasses. The values of Navc give us an impression that the bonds of the investigated compositions are a mixture of homopolar and heteropolar bonds. The density of homopolar bonds is decreased with increasing Ag content. Besides, the decrease of Ea with increasing Ag content could be correlated to the increase of the concentration of AgeTe structural units. From the compensation law, in many organic and amorphous semiconductors, the magnitudes of the pre-exponential factor ln s0 are found to increase exponentially with the activation energies Ea [15e21]. This is called the MeyereNeldel rule [19], which is given by the following expression: d½ln s0  ¼ ln s00 þ

d½Ea  EMN

at different annealing temperatures. The values of EMN and s00, evaluated by using the slope of the curve and its intercept on Ea ¼ 0 axis, are found to be 62.5 meV, 11.51 U1 cm1 and 83.33 meV, 10.11 U1 cm1 for Ag10Te90 and Ag20Te80, respectively. The value of EMN increases as Ag content increases which reflects the decrease of the degree of disorder in thin films samples. 3.4. Kinetics of phase transformation Fig. 5 represents the dependence of the electrical conductivity (s) on the annealing time (t) at different isothermal annealing temperatures during the amorphous-to-crystalline transformation of Ag10Te90 and Ag20Te80 glassy films. This figure indicates that, as the annealing temperature increases from 453 to 493 K, the maximum conductivity attained increases. Also, it was observed that, increasing the temperature of annealing leads to decrease in the total time necessary for completing the crystallization process. This is due to the increase in the degree of crystallization with the annealing temperature as checked using XRD patterns. The point sa represents the amorphous state and sc corresponds to the complete crystalline state. At any intermediate point s(t), the conductivity corresponds to a mixture of amorphous and crystalline states. In accord to Kotkata et al. [22], the experimental values of ln s were used to determine the value of crystallization fraction x(t) at any intermediate time by the relation:

ð3Þ xðtÞ ¼

where s00 and EMN are constants related to the characteristics of the material. Fig. 4. shows the typical variation of the logarithmic value of s0 plotted against the corresponding Ea obtained from the variation of s versus inverse of temperature

ln st  ln sa ln sc  ln sa

-0.7

lnσ [ -1.cm-1]

5.0

1419

a

ð4Þ

lnσc

-0.8

-0.9 T[K] 453 463 473 483 493

-1.0 lnσa -1.1

4.5

0

50

100

150

200

250

lnσ0

time t [min] -0.8

b

lnσc

-1.cm-1]

4.0

-1.0

lnσ [

-0.9

-1.1

T[K] 453 463 473 483 493

-1.2 lnσa 3.5 0.10

0.11

0.12

0.13

0.14

0.15

0.16

Ea [eV] Fig. 4. Variation of ln s0 versus Ea obtained at different isotherm temperatures for (-) Ag10Te90 (B) Ag20Te80 films.

-1.3 0

20

40

60

80

100

120

140

time t [min] Fig. 5. Time dependence of d.c. conductivity for (a) Ag10Te90 (b) Ag20Te80 films crystallized at different isotherm temperatures.

A.A. El-Fadl et al. / Solid State Sciences 10 (2008) 1416e1421

1  xðtÞ ¼ exp½ktn 

ð5Þ

where n is the Avrami exponent and can take only one of the three integer values, 1, 2 or 3 depending on whether it represents one-, two- or three-dimensional growth, respectively, and reflects the characteristics of nucleation and growth processes and k is the rate constant, which reflects the rates of both nucleation and growth, which is usually assumed to have an Arrhenian temperature dependence:   Ec ð6Þ k ¼ k0 exp  RT where k0 is the frequency factor, R is the universal gas constant, and Ec is the effective activation energy for the whole crystallization process. The parameter k0 takes into account both the nucleation frequency and the crystal growth rate; these assumptions are appropriate when a broad range of temperature is considered [22]. Taking logarithms and rearranging Eq. (5) gives:   1 ¼ ln k þ n ln t ð7Þ ln ln 1x

1.0

0.8 0.6 0.4

T [K] 453 463 473 483 493

0.2 0.0 0

60

120

180

time (t) [min]

b

1.5

1.0

1.0

0.5 0.0 -0.5

T [K] 453 463 473 483 493

-1.0 -1.5 1

2

3

4

5

0.5 0.0 T [K] 453 463 473 483 493

-0.5 -1.0 6

1

2

3

4

5

ln t [min]

ln t [min]

Fig. 7. Avrami plots for (a) Ag10Te90 (b) Ag20Te80 films crystallized at different isotherm temperatures.

According to this equation, a plot of ln[ln(1x)] versus ln t yields a straight line of slope n. This has been verified for Ag10Te90 and Ag20Te80 in Fig. 7. This figure shows that the slope is almost constant with the increase in the annealing temperatures. The obtained value of the Avrami index is in between 0.867 and 0.952 for all isotherm temperatures, which indicates that, the process of crystallization takes place in one-dimension. The energy of the whole process of crystallization has been calculated by taking the logarithm of the two sides of Eq. (6). Fig. 8 illustrates the dependence of ln k on 1/T for the two compositions where the activation energies of crystallization Ec are found to be 44.723 and 81.228 kJ/mol for Ag10Te90 and Ag20Te80, respectively. The increase in the activation of crystallization with the addition of Ag may be due to the difference in binding nature between TeeTe and Age Te bonds which can probably affect the rigidity of the glassy network. 4. Conclusions  X-ray diffraction analyses showed that, the as-prepared Ag10Te90 films have amorphous structure, while it is semipolycrystalline for Ag20Te80. On annealing at temperature 373 K, the films have a polycrystalline structure.

b

-0.5 -1.0

0.8

-1.5

0.6 0.4

T [K] 453 463 473 483 493

0.2 0.0

240

2.0

a

1.5

lnk

a crystallization fraction (x)

crystallization fraction (x)

1.0

2.0

ln[-ln(1-x)]

where sc and sa are the conductivities of the crystalline and amorphous phases having volume fractions x and (1x), respectively, and st is the conductivity of a mixture during amorphous to crystalline transformation. Fig. 6 shows the crystallized fraction (x) versus time (t) plots at five isothermal temperatures. Increasing the annealing temperature increases the steepness of the resulting curve which means that the crystallization process proceeds faster at higher temperatures than that at lower ones. Such crystallization plotting has been found to shift toward lower time with increasing temperature consistent with the formal theory of crystallization. The isothermal crystallization is usually described by JohnsoneMehleAvrami (JMA) equation relating the fraction of the crystallized volume (x) grown from an amorphous phase, to the time (t) of thermal annealing as follows [23]:

ln[-ln(1-x)]

1420

0

30

60

90

-2.0 -2.5 -3.0

120

time (t) [min]

Fig. 6. Crystallized fractions (x), calculated from the conductivity data, versus t for (a) Ag10Te90 (b) Ag20Te80 films at different isotherm temperatures.

-3.5 2.00

2.05

2.10

2.15

2.20

1000/T [K-1] Fig. 8. ln k versus1/T for (-) Ag10Te90 and (B) Ag20Te80 films.

2.25

A.A. El-Fadl et al. / Solid State Sciences 10 (2008) 1416e1421

 From the present results it is evident that the conductivity in the studied glassy system is thermally activated and exhibits a temperature dependence and the activation energy Ea in this case, depends on the annealing temperature.  The pre-exponential factor and the activation energy Ea satisfy the same MeyereNeldel rule.  The activation energy Ec for the isothermal crystallization for the two compositions analyzed in terms of (JMA) method are 44.723 and 81.228 kJ/mol for Ag10Te90 and Ag20Te80 films, respectively.  The obtained value of the Avrami index (n) approximately equals 1, indicating that, the crystallization of Ag10Te90 and Ag20Te80 amorphous matrixes is a one-dimensional diffusion controlled growth with a decrease in nucleation rate.

References [1] K.A. Rubin, M. Chen, Thin Solid Films 181 (1989) 129. [2] P.E. Chrcia, F.D. Kalk, P.E. Bierstedt, A. Ferretti, G.A. Jones, D.G. Swarizfager, J. Appl. Phys. 64 (1988) 1715. [3] S. Zembutsu, Y. Toyoshima, T. Igo, H. Nagi, Appl. Opt. 14 (1975) 3073.

1421

[4] S. Mahadevan, A. Giridhar, J. Non-Cryst. Solids 197 (1996) 219. [5] R. Kumur, A. Singh, A. Mahajan, Indian J. Eng. Mater. Sci. 7 (2000) 411. [6] M.D. Mikhailov, I.I. Kryshanowski, I.M. Petcherizin, J. Non-Cryst. Solids 1 (2000) 256. [7] G.H. Chandra, O.M. Hussain, S. Uthanna, Vacuum 62 (2001) 39. [8] G.V. Rao, G.H. Chandra, O.M. Hussain, J. Alloys Comp. 325 (2001) 12. [9] M. Frumar, T. Wagner, Curr. Opin. Solid State Mater. Sci. 7 (2003) 117. [10] Z. Marinkovic, V. Simic, Thin Solid Films 195 (1991) 127. [11] P.G. Shewmon, Diffusion in Solids, McGraweHill, New York, 1963. [12] M.M. Hafiz, A.H. Moharram, A.A. Abu-Shely, Appl. Surf. Sci. 352 (1998) 207. [13] K. Tanaka, Phys. Rev. B 39 (1989) 1270. [14] J.Z. Liu, P.C. Taylor, Solid State Commun. 70 (1989) 81. [15] T. Dosdale, R.J. Brook, J. Mater. Sci. 13 (1978) 167. [16] D.P. Almond, A.R. West, Solid State Ionics 18 (1986) 1105. [17] R. Arora, A. Kumar, Phys. Status Solidi A 125 (1991) 273. [18] S.K. Dwivedi, M. Dixit, A. Kumar, J. Mater. Sci. Lett. 17 (1998) 233. [19] W. Meyer, H. Neldel, Z. Tech. Phys. 18 (1937) 588. [20] A. Many, E. Harnik, D. Gerlick, J. Chem. Phys. 23 (1955) 1733. [21] D.D. Eley, J. Polym. Sci. C17 (1967) 73. [22] M.F. Kotkata, M.F. El-Mously, F.M. Ayad, Acta Phys. Hung. 46 (1979) 19. [23] M. Avrami, J. Chem. Phys. 8 (1940) 212.