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Influence of indium content on the optical, electrical and crystallization kinetics of Se100−xInxthin films deposited by flash evaporation technique
Physica B 407 (2012) 356–360
Contents lists available at SciVerse ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Influence of indium content on the optical, electrical and crystallization kinetics of Se100 xInxthin films deposited by flash evaporation technique A.H. Ammar a, N.M. Abdel-Moniem b, A.A.M. Farag a,n, El-Sayed M. Farag c a
Physics Department, Faculty of Education, Ain shams University, Roxy Square, Cairo, Egypt Physics Department, Faculty of Science, Tanta University, Tanta, Egypt c Physics Department, Faculty of Engineering, Monoufia University, Shebin EI-Koum, Egypt b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 September 2011 Accepted 31 October 2011 Available online 12 November 2011
Thin films of Se 100 xInx (x ¼10, 20 and 30 at%) have been prepared by the flash evaporation technique. The effect of the indium content on optical band gap of the Se100 x Inx films has been investigated by the optical characterization. The optical band gap values of the Se100 x Inx thin films were determined and are found to decrease with increasing indium content. This indium content changes the width of localized states in the optical band gaps of the thin films. It was found that the optical band gap, Eg, of the Se100 x Inx films changes from 1.78 to 1.37 eV with increasing indium content from 10 to 30 at%, while the width of localized states in optical band gap changes from 375 to 342 meV. The temperature dependence of the dark electrical conductivity were studied in the temperature range 303–433 K and revealed two activation energies providing two electrical conduction mechanisms. The activation energy of the Se100 x Inx films in the high temperature region changes from 0.49 to 0.32 eV with increasing indium content from 10 to 30 at%, while the hopping activation energy in the lower temperature region changes from 0.17 to 0.22 meV. The change in the electrical conductivity with time during the amorphous-to-crystalline transformation is recorded for amorphous Se100 xInx films at two points of isothermal temperatures 370 and 400 K. The formal crystallization theory of Avrami has been used to calculate the kinetic parameters of crystallization. & 2011 Elsevier B.V. All rights reserved.
Keywords: Chalcogenide thin films Se–In Electrical conductivity Optical properties Crystallization kinetics
1. Introduction In recent years, the efforts have been devoted to the development of chalcogenides materials suitable for optoelectronics. It is well known that chalcogenide semiconducting alloys have found application not only due to their electrical and thermal properties, but also due to their optical properties [1]. These glasses have recently attracted a great deal of interest because of many applications as solid state devices both in scientific and technological fields [2]. Moreover, it exhibits unique IR transmission and electrical properties that make them useful for several applications such as threshold switching, memory switching, inorganic photoreceptors, IR transmission, detection through lenses and optical waveguide e.g. in welding and surgery [3,4]. Many chalcogenide glasses, in particular selenium, exhibit a unique property of reversible transformation that makes these glasses very useful in optical memory devices [1,2]. However, the shortcomings of pure glassy selenium for its practical application include its short lifetime, low sensitivity and thermal instability.
n
Corresponding author. Tel.: þ2033518705; fax: þ2022581243. E-mail address: [email protected] (A.A.M. Farag).
0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.10.053
To overcome these difficulties certain additives are used and specially the used to Se–Te, Se–Sb and Se–In binary alloys are of interest owing to their various properties like greater hardness, higher sensitivity, higher crystallization temperature, higher conductivity and smaller ageing effect as compared to pure selenium [1,2,5–8]. The electrical properties of chalcogenide glasses have attracted more attention because of their technical importance. It has been found that the amorphous In–Se films with composition around In2Se3 exhibit several anomalous features such as a negative Seebeck coefficient, large Fermi level shift and large photovoltaic effect [9]. Se–In alloys are considered to be a layer semiconductor and the structure is described as a regular stacking of layers with covalent bonding of the Van der Waals type between the layers [9]. Since, network connectivity, rigidity and nature of bonding do play important roles in electronic conduction process and optical characteristics [10–12]; hence in order to understand the conduction phenomena, a great deal of experimental data is required. Many workers have carried out the investigations of electrical and optical properties of these chalcogenide glasses [10–12]. A further progress in the performance of Se–In devices may be achieved if the main physical properties of this alloy are better
A.H. Ammar et al. / Physica B 407 (2012) 356–360
x= 10 20 30
Intensity (a.u.)
understood. The present work is an extension of these studies that aims to characterize the structural, electrical and optical properties of Se 100 x Inx(x¼10, 20 and 30). The specific aims are related to improve the characteristics of this alloy by controlling the stoichiometry of Se 100 xInx films prepared by the flash evaporation technique. Moreover, further investigations for the electrical and optical characteristics are presented. In addition, crystallization kinetics of these alloys are also considered.
357
2. Experimental
3. Results and discussion 3.1. X-ray diffraction patterns of Se100 x Inx thin films Fig. 1 shows the X-ray diffraction patterns of the Se100 x Inx films with x¼10, 20 and 30 at%. The patterns reveal no sharp diffraction lines thus confirming the non-crystalline structure of the studied films. 3.2. Optical characteristics of Se100 xInx films The optical absorption spectrum is an important tool to obtain optical band gap of crystalline and amorphous materials. In order to determine optical band gap of the films, T and R spectra of the films
10
20
30
40
50
60
70
2θ° Fig. 1. XRD of Se100 x Inx films with x¼ 10, 20 and 30 at%.
6.0x103
x = 10 x = 20 x = 30
4.0x103 α (cm-1)
All investigated samples of Se100 x Inx (x ¼10, 20 and 30 at%) were prepared by the conventional melting quenching technique. High purity elements (99.999%) of the constituents Se and In were sealed under vacuum (10 5 Torr) in silica ampoules and heated gradually up to 1200 K for 10 h. During the course of heating the ampoules were shaken several times. Finally the molten alloys were quenched in ice water. Thin films of the systems were prepared by flash evaporation technique using vacuum coating unit type Edwards E306A. Precleaned corning and quartz glass substrates were fixed onto horizontal rotating holder (with rotation speed of 4 cycles/min) to obtain homogeneous film thickness and composition. All the films used were deposited at the same evaporation rate. The distance separates the evaporated source material and the substrates were 14 cm. The temperature of the substrate was kept constant at room temperature ( 300 K). A rapid flash evaporation of a compound can be obtained by continuously dropping fine particles of the material onto a hot boat surface so that numerous discrete evaporation occurs. The base pressure of the vacuum system was 6 10 6 Torr. During evaporation the vacuum pressure was stable at 1.5 10 6 Torr and the evapora˚ tion speed was about 100–200 A/s. The film thickness was monitored using a thickness monitor model Edward FTM5. After evaporation the thickness of the virgin films was accurately determined by an optical interference method. The film thickness is about 680 nm. The structure of the obtained films were checked by X-ray diffraction (XRD) patterns with a Cu Ka radiation source. The transmission, T, and reflection, R, spectra of these films for different concentrations of indium in Se–In system have been taken by spectrophotometer (Cary 2360, Varian) at normal incidence of light in a wavelength range 320–2500 nm at room temperature and at normal pressure. The electrical conductivity is recorded for amorphous thin films as a function of temperature in the temperature range 303–433 K. To measure the resistance, silver paste is used to attach the terminals. Electrical conductivity is also recorded as a function of annealing time at 370 K and 400 K. The kinetics of phase-change are calculated from the conductivity data as a function of annealing time.
2.0x103
0.0
400
500
600 λ (nm)
700
Fig. 2. Optical absorption spectra of Se100 x Inx thin films.
(not shown here) can be converted to the absorption coefficient using the following relation [13]: 2 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 1 4ð1R2 Þ ð1RÞ4 ð1Þ þ a ¼ ln þ R2 5 d 2T 4T 2 where a is the absorption coefficient and d is the film thickness. Fig. 2 depicts the variation of the spectral dependence of the absorption coefficient. The optical band gaps of the thin films were determined from the absorption spectra near the absorption edge. In non-crystalline systems the indirect optical transitions are most likely to occur due to the absence of translation symmetry. For indirect transition, the absorption coefficient dependence on photon energy is expressed as [14]
ahn ¼ AðhnEg Þ2
ð2Þ
where A is a constant, hn is the photon energy and Eg is the optical band gap. The plots of (ahn)1/2 vs. hn are shown in Fig. 3. From the linear parts of these curves, the Eg values of the films were calculated and are given in Table 1. The absorption edge shifts to the higher wavelengths with increasing In content. This suggests that the optical band gap decreases with increasing In content. This behavior may be attributed to the formation of In–Se bonds and the decrease in concentrations of other bonds, which exist in the glass. This may result in a perturbation in the system, which will broaden the valence and conduction band edges in the mobility gap [14,15]. In other words, low In concentration leads to the decrease of conductivity, followed by increasing in both the activation energy
358
A.H. Ammar et al. / Physica B 407 (2012) 356–360
120
2x102 80
α (cm-1)
(αhν)1/2,(eV/cm)1/2
3x102
x = 10 x = 20 x = 30
x = 10 x = 20 x = 30
102
40
0
1.6
2.0
2.4
2.8
3.2
1.7
1.8
Fig. 3. Plots of (ahn)1/2 vs. hn of Se100 x Inx thin films.
2.0
Fig. 4. Plots of log a vs. hn of Se100 x Inx thin films.
Table 1 The energy gaps and Urbach energies for a-Se100 xInx films.
x=
-4
x¼10 at%
x¼ 20 at%
x¼ 30 at%
1.78 0.375
1.53 0.352
1.37 0.342
and optical energy gap. This is most likely due to the compensation of hole traps. Increasing In content leads to an opposite behavior. This could be accounted by the generation of excess electronic delocalized states. The decrease in the optical band gap can be explained by the increase in the degree of disorder in the films. The exponential density of states is explained by exponential absorption and the exponential density of states is defined as [16] hn N ¼ N0 exp ð3Þ Eu where N0 is a constant and Eu is the Urbach energy i.e., width of localized states in the optical band gap. In amorphous semiconductors, the absorption coefficient at below absorption edge indicates an exponential dependence on photon energy. This dependence is described as [16] hn a ¼ a0 exp ð4Þ Eu where a0 is a constant. The curves of the semigarithmic plot of a vs. hn are shown in Fig. 4. The Eu values were calculated from the slope of these figures and are given in Table 1. It is seen that the indium content changes the width of localized states in the optical band gap of the films. The Eu values are indicators for the various defect levels taking place in optical band gap of the films. 3.3. Temperature dependence of electrical conductivity of Se100 x Inx thin films The measured dc conductivities, sdc of amorphous Se100 x Inx films with x ¼10, 20 and 30 at% are plotted and shown in Fig. 5 as a function of the reciprocal temperature. As seen in the figure, sdc can be expressed in the form [14]: DE1 DE2 þ s02 exp ð5Þ sdc ¼ s01 exp kT kT The two terms on the right-hand side arise from two different conduction processes: the first term is related to band conduction in extended states where s01 and DE1 are the pre-exponential
10 20 30
-5
ln σ
Eg (eV) Eu (eV)
1.9 hν (eV)
hν (eV)
-6 -7 -8 -9 2.2
2.4
2.6
2.8
3.0
3.2
3.4
-1
1000/T (K ) Fig. 5. Plots of ln s vs. 1000/T of Se100 x Inx thin films.
Table 2 The activation and hopping energies for a-Se100 xInx films.
DE1 (eV) DE2 (eV)
x¼ 10 at%
x¼20 at%
x ¼30 at%
0.49 0.17
0.36 0.22
0.32 0.22
factor and the activation energy of conduction, respectively. The second term is due to hopping conduction in localized states with s02 and DE2 that refer to the pre-exponential factor and the activation energy of hopping conduction, respectively. The calculated values of s01, DE1, s02 and DE2 are listed in Table 2. It is observed that s01, DE1, s02 and DE2 vary slightly with the incorporation of In content. The increasing of the electrical conductivity and decreasing activation energy Se100 x Inx films may be attributed to the formation of In–Se bonds and then generation of excess electronic delocalized states [14]. 3.4. Kinetics of phase transformation on the electrical conductivity Se100 x Inx thin films Studies of the crystallization kinetics in chalcogenide glasses thin films are of particular interest because they are connected with such important phenomena as memory type of switching,
A.H. Ammar et al. / Physica B 407 (2012) 356–360
ac ðtÞ ¼ 1exp½ðkn tÞn
400K
160 σx10-4 (Ω-1.cm-1)
reversible optical recording, imaging media and absorption filters [17]. Thermal analysis is a very useful tool for describing the crystallization phenomena as it is rapid and convenient. The kinetics of the first-order phase transformation are important in physics, chemistry, ceramic and material sciences [17]. The isothermal crystallization is usually described by Avrami, sometime called Johnson–Mehl–Avrami (JMA), equation relating the fraction of the crystallized volume, ac(t), grown from amorphous phase during the time, t, of thermal annealing to crystalline phase as follows [18–20]:
359
120
80
n
ð6Þ
x= 10 20 30
n
where k is the rate constant of crystallization and the exponent, nn is a dimensionless parameter, which characterized the morphology of the crystal growth. The crystallization rate, ac(t), can also be calculated using the relative increase in the electric conduction during the crystallization growth. The empirical formula, which relates the amount of material crystallized at time t with the basis of the experimental results is represented by [21]:
sc st sc sa
80 120 t (min)
160
200
Fig. 7. Plots of time dependent of conductivity of Se100 x Inx thin films annealed at 400 K.
370 K
T =370 K 1
16
0
x= 10 20 30
-1
7.5
8.0
8.5 ln t
9.0
9.5
Fig. 8. Plots of ln [ ln(1 ac)] vs. ln t of Se100 x Inx thin films annealed at 370 K.
1.5
T=400 K
1.0 ln[-ln(1-αc)]
σx10-4 (Ω-1.cm-1)
40
ð7Þ
where sa and sc are the prescriptive conductivity at the beginning and the end of the crystallization process and st is the conductivity at time t between these two limits as shown in Figs. 6 and 7. These figures represent the time dependence of the dc electrical conductivity for our samples at T¼370 and 400 K, respectively. In these figures, it can be shown that, the conductivity increases by increasing the thermo-annealing with three thermo-phase transition processes. At first, in the beginning of the time annealing, there are some fluctuations due to phase transition. These fluctuations can be explained as follows: in the beginning the indium nuclei in the phase transition fluctuated between birth and decay until they reach to a certain critical volume, then, birth and growth of thermal crystallization states. The second period is the thermo-crystallization period, during which the Indium nuclei are born with a certain critical volume and started to increase on the expense of amorphous matrix to reach maximum constant value as the whole sample matrix transformed from amorphous to crystalline phase by increasing the time of annealing. The third period is at the end of the time of annealing, which gives a nearly constant value of conductivity when all sample transformed to crystalline phase. The crystallization rates, ac(t) are calculated from Eq. (6) and plotted for different crystallization temperatures (T¼370 and
20
0
ln[-ln(1-αc)]
ac ðtÞ ¼
40
0.5 0.0 -0.5 x=
-1.5 8
4
10 20 30
-1.0
12
x= 10 20 30
0
40
80
120
160
200
t (min) Fig. 6. Plots of time dependent of conductivity of Se100 x Inx thin films annealed at 370 K.
7.5
8.0
8.5
9.0
9.5
ln t Fig. 9. Plots of ln [ ln(1 ac)] vs. ln t of Se100 x Inx thin films annealed at 400 K.
400 K) as shown in Figs. 8 and 9. The crystallization temperatures are selected to be between the glass temperature, Tg, and the crystallization temperature, Tc, as it is drawn out from DTA traces [22]. In Figs. 8 and 9, the ln[-ln(1 ac(t))] values are plotted
360
A.H. Ammar et al. / Physica B 407 (2012) 356–360
Table 3 Avrami kinetic parameters nn and kn as a function of isothermal annealing temperature for a-Se100 xInx films. Annealing Temperature
nn kn
T ¼370 K
T¼400 K
x ¼10 at%
x¼ 20 at%
x¼ 30 at%
x¼10 at%
x ¼20 at%
x¼ 30 at%
1.37 0.206
1.4 0.242
1.43 0.25
1.47 0.15
1.48 0.18
1.51 0.198
1.37 eV. It can be concluded that the optical band gap of the Se100 x Inx thin films decrease with increasing In content due to the formation of In–Se bonds. The width of localized states of the films changes from 375 to 342 meV due to the increasing of In content from 10 to 30 at%. The incorporation of In atoms in amorphous Se matrix leads to an increase in the electrical conductivity and a decrease in the thermal activation energy of the films in the temperature range 303–433 K from 0.49 to 0.32 eV. References
against ln t. However, the kinetics parameters nn and kn are evaluated for the two temperatures T¼ 370 and 400 K, which are tabulated in Table 3. From this table, the values of nn and kn are increased with increasing the Indium percentage at each crystallization temperature in our films. Also, by increasing the crystallization temperature, the mean values of nn (the nucleation and crystallization growth) are increased while, the mean value of kn (the crystallization rate) are decreased due to the rearrangement of Se–In chains [22].
4. Conclusions The results of crystallization kinetics for amorphous Se100 xInx films have been indicated that the crystallization mechanism of the films occurs in two and three dimension growth according to the Avrami parameters nn and kn. These results are due to the limitation of crystallization process to include only growth of entities by small relaxation time. The incorporation of In atoms in amorphous Se matrix leads to an increase in the electrical conductivity and a decrease in the thermal activation energy of the films in the temperature range 303–433 K from 0.49 to 0.32 eV. The indirect optical band gap was changed from 1.78 to
[1] K. Singh, N.S. Saxena, O.N. Srivastava, D. Patidar, T.P. Sharma, Chalcogenide Lett. 3 (2006) 33. [2] K. Singh, D. Patidar, N.S. Saxena, J. Phys. Chem. Solids 66 (2005) 946. [3] P. Nemec, M. Frumar, J. Non-Cryst. Solids 299– 302 (2002) 1018. [4] Y.G. Ghai, K.H. KimB, J. Park, J. Heo, Appl. Phys. Lett. 78 (2001) 1249. [5] N. Mehta, V.S. Kushwaha, A. Kumar, Vacuum 83 (2009) 1169. [6] M.M.A. Imran, N.S. Saxena, Y.K. Vijay, R. Vijayvergiya, N.B. Maharjan, M. Husain, J. Non-Cryst. Solids 298 (2002) 53. [7] N. Mehta, A. Kumar, J. Mater. Sci. 42 (2007) 490. [8] V.K. Saraswat, V. Kishore, K. Singh, N.S. Saxena, T.P. Sharma, Chalcogenide Lett. 3 (2006) 61. [9] S. Abou El-Hassan, Physica B 307 (2001) 86. [10] V.K. Saraswat, V. Kishore, N.S. Saxena, T.P. Sharma, Indian J. Pure Appl. Phys 44 (2006) 196. [11] A.S. Soltan, Appl. Phys. A 80 (2005) 117. [12] R. Rajesh, J. Philip, Semicond. Sci. Technol 18 (2003) 133. [13] A.A.M. Farag, M. Cavas, F. Yakuphanoglu, F.M. Amanullah, J. Alloys Compd. 509 (2011) 7900. [14] S.A. Fayek, A.F. Maged, M.R. Balboul, Vacuum 53 (1999) 447. [15] M.M. El-Ocker, M.H. El-Fouly, S.A. Fayek, H. Talaat, G.A.M. Amin, Appl. Phys. A 60 (1995) 233. [16] F. Yakuphanoglua, G. Barımb, I. Erol, Physica B 391 (2007) 136. [17] K.A. Aly, M.A. Osman, A.M. Abosehly, A.A. Othman, J. Phys. Chem. Solids 69 (2008) 2514. [18] El-Sayed M. Farag, Indian J. Phys. 80 (4) (2006) 367. [19] A.A. Al-Ghamdi, M.A. Alvi, S.A. Khan, J. Alloys Compd. 509 (2011) 2087. [20] M.S. Abo Ghazala, E. Aboelhasan, A.H. Amar, W. Gamel, Physica Status Solidi C 8 (2011) 3095. [21] S. Kirkpatrick, Phys. Rev. Lett. 27 (1971) 17. [22] M.K. El-Mously, M.M. El-Zaidia, J. Non-Cryst. Solids 27 (1978) 265.
Contents lists available at SciVerse ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Influence of indium content on the optical, electrical and crystallization kinetics of Se100 xInxthin films deposited by flash evaporation technique A.H. Ammar a, N.M. Abdel-Moniem b, A.A.M. Farag a,n, El-Sayed M. Farag c a
Physics Department, Faculty of Education, Ain shams University, Roxy Square, Cairo, Egypt Physics Department, Faculty of Science, Tanta University, Tanta, Egypt c Physics Department, Faculty of Engineering, Monoufia University, Shebin EI-Koum, Egypt b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 September 2011 Accepted 31 October 2011 Available online 12 November 2011
Thin films of Se 100 xInx (x ¼10, 20 and 30 at%) have been prepared by the flash evaporation technique. The effect of the indium content on optical band gap of the Se100 x Inx films has been investigated by the optical characterization. The optical band gap values of the Se100 x Inx thin films were determined and are found to decrease with increasing indium content. This indium content changes the width of localized states in the optical band gaps of the thin films. It was found that the optical band gap, Eg, of the Se100 x Inx films changes from 1.78 to 1.37 eV with increasing indium content from 10 to 30 at%, while the width of localized states in optical band gap changes from 375 to 342 meV. The temperature dependence of the dark electrical conductivity were studied in the temperature range 303–433 K and revealed two activation energies providing two electrical conduction mechanisms. The activation energy of the Se100 x Inx films in the high temperature region changes from 0.49 to 0.32 eV with increasing indium content from 10 to 30 at%, while the hopping activation energy in the lower temperature region changes from 0.17 to 0.22 meV. The change in the electrical conductivity with time during the amorphous-to-crystalline transformation is recorded for amorphous Se100 xInx films at two points of isothermal temperatures 370 and 400 K. The formal crystallization theory of Avrami has been used to calculate the kinetic parameters of crystallization. & 2011 Elsevier B.V. All rights reserved.
Keywords: Chalcogenide thin films Se–In Electrical conductivity Optical properties Crystallization kinetics
1. Introduction In recent years, the efforts have been devoted to the development of chalcogenides materials suitable for optoelectronics. It is well known that chalcogenide semiconducting alloys have found application not only due to their electrical and thermal properties, but also due to their optical properties [1]. These glasses have recently attracted a great deal of interest because of many applications as solid state devices both in scientific and technological fields [2]. Moreover, it exhibits unique IR transmission and electrical properties that make them useful for several applications such as threshold switching, memory switching, inorganic photoreceptors, IR transmission, detection through lenses and optical waveguide e.g. in welding and surgery [3,4]. Many chalcogenide glasses, in particular selenium, exhibit a unique property of reversible transformation that makes these glasses very useful in optical memory devices [1,2]. However, the shortcomings of pure glassy selenium for its practical application include its short lifetime, low sensitivity and thermal instability.
n
Corresponding author. Tel.: þ2033518705; fax: þ2022581243. E-mail address: [email protected] (A.A.M. Farag).
0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.10.053
To overcome these difficulties certain additives are used and specially the used to Se–Te, Se–Sb and Se–In binary alloys are of interest owing to their various properties like greater hardness, higher sensitivity, higher crystallization temperature, higher conductivity and smaller ageing effect as compared to pure selenium [1,2,5–8]. The electrical properties of chalcogenide glasses have attracted more attention because of their technical importance. It has been found that the amorphous In–Se films with composition around In2Se3 exhibit several anomalous features such as a negative Seebeck coefficient, large Fermi level shift and large photovoltaic effect [9]. Se–In alloys are considered to be a layer semiconductor and the structure is described as a regular stacking of layers with covalent bonding of the Van der Waals type between the layers [9]. Since, network connectivity, rigidity and nature of bonding do play important roles in electronic conduction process and optical characteristics [10–12]; hence in order to understand the conduction phenomena, a great deal of experimental data is required. Many workers have carried out the investigations of electrical and optical properties of these chalcogenide glasses [10–12]. A further progress in the performance of Se–In devices may be achieved if the main physical properties of this alloy are better
A.H. Ammar et al. / Physica B 407 (2012) 356–360
x= 10 20 30
Intensity (a.u.)
understood. The present work is an extension of these studies that aims to characterize the structural, electrical and optical properties of Se 100 x Inx(x¼10, 20 and 30). The specific aims are related to improve the characteristics of this alloy by controlling the stoichiometry of Se 100 xInx films prepared by the flash evaporation technique. Moreover, further investigations for the electrical and optical characteristics are presented. In addition, crystallization kinetics of these alloys are also considered.
357
2. Experimental
3. Results and discussion 3.1. X-ray diffraction patterns of Se100 x Inx thin films Fig. 1 shows the X-ray diffraction patterns of the Se100 x Inx films with x¼10, 20 and 30 at%. The patterns reveal no sharp diffraction lines thus confirming the non-crystalline structure of the studied films. 3.2. Optical characteristics of Se100 xInx films The optical absorption spectrum is an important tool to obtain optical band gap of crystalline and amorphous materials. In order to determine optical band gap of the films, T and R spectra of the films
10
20
30
40
50
60
70
2θ° Fig. 1. XRD of Se100 x Inx films with x¼ 10, 20 and 30 at%.
6.0x103
x = 10 x = 20 x = 30
4.0x103 α (cm-1)
All investigated samples of Se100 x Inx (x ¼10, 20 and 30 at%) were prepared by the conventional melting quenching technique. High purity elements (99.999%) of the constituents Se and In were sealed under vacuum (10 5 Torr) in silica ampoules and heated gradually up to 1200 K for 10 h. During the course of heating the ampoules were shaken several times. Finally the molten alloys were quenched in ice water. Thin films of the systems were prepared by flash evaporation technique using vacuum coating unit type Edwards E306A. Precleaned corning and quartz glass substrates were fixed onto horizontal rotating holder (with rotation speed of 4 cycles/min) to obtain homogeneous film thickness and composition. All the films used were deposited at the same evaporation rate. The distance separates the evaporated source material and the substrates were 14 cm. The temperature of the substrate was kept constant at room temperature ( 300 K). A rapid flash evaporation of a compound can be obtained by continuously dropping fine particles of the material onto a hot boat surface so that numerous discrete evaporation occurs. The base pressure of the vacuum system was 6 10 6 Torr. During evaporation the vacuum pressure was stable at 1.5 10 6 Torr and the evapora˚ tion speed was about 100–200 A/s. The film thickness was monitored using a thickness monitor model Edward FTM5. After evaporation the thickness of the virgin films was accurately determined by an optical interference method. The film thickness is about 680 nm. The structure of the obtained films were checked by X-ray diffraction (XRD) patterns with a Cu Ka radiation source. The transmission, T, and reflection, R, spectra of these films for different concentrations of indium in Se–In system have been taken by spectrophotometer (Cary 2360, Varian) at normal incidence of light in a wavelength range 320–2500 nm at room temperature and at normal pressure. The electrical conductivity is recorded for amorphous thin films as a function of temperature in the temperature range 303–433 K. To measure the resistance, silver paste is used to attach the terminals. Electrical conductivity is also recorded as a function of annealing time at 370 K and 400 K. The kinetics of phase-change are calculated from the conductivity data as a function of annealing time.
2.0x103
0.0
400
500
600 λ (nm)
700
Fig. 2. Optical absorption spectra of Se100 x Inx thin films.
(not shown here) can be converted to the absorption coefficient using the following relation [13]: 2 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 1 4ð1R2 Þ ð1RÞ4 ð1Þ þ a ¼ ln þ R2 5 d 2T 4T 2 where a is the absorption coefficient and d is the film thickness. Fig. 2 depicts the variation of the spectral dependence of the absorption coefficient. The optical band gaps of the thin films were determined from the absorption spectra near the absorption edge. In non-crystalline systems the indirect optical transitions are most likely to occur due to the absence of translation symmetry. For indirect transition, the absorption coefficient dependence on photon energy is expressed as [14]
ahn ¼ AðhnEg Þ2
ð2Þ
where A is a constant, hn is the photon energy and Eg is the optical band gap. The plots of (ahn)1/2 vs. hn are shown in Fig. 3. From the linear parts of these curves, the Eg values of the films were calculated and are given in Table 1. The absorption edge shifts to the higher wavelengths with increasing In content. This suggests that the optical band gap decreases with increasing In content. This behavior may be attributed to the formation of In–Se bonds and the decrease in concentrations of other bonds, which exist in the glass. This may result in a perturbation in the system, which will broaden the valence and conduction band edges in the mobility gap [14,15]. In other words, low In concentration leads to the decrease of conductivity, followed by increasing in both the activation energy
358
A.H. Ammar et al. / Physica B 407 (2012) 356–360
120
2x102 80
α (cm-1)
(αhν)1/2,(eV/cm)1/2
3x102
x = 10 x = 20 x = 30
x = 10 x = 20 x = 30
102
40
0
1.6
2.0
2.4
2.8
3.2
1.7
1.8
Fig. 3. Plots of (ahn)1/2 vs. hn of Se100 x Inx thin films.
2.0
Fig. 4. Plots of log a vs. hn of Se100 x Inx thin films.
Table 1 The energy gaps and Urbach energies for a-Se100 xInx films.
x=
-4
x¼10 at%
x¼ 20 at%
x¼ 30 at%
1.78 0.375
1.53 0.352
1.37 0.342
and optical energy gap. This is most likely due to the compensation of hole traps. Increasing In content leads to an opposite behavior. This could be accounted by the generation of excess electronic delocalized states. The decrease in the optical band gap can be explained by the increase in the degree of disorder in the films. The exponential density of states is explained by exponential absorption and the exponential density of states is defined as [16] hn N ¼ N0 exp ð3Þ Eu where N0 is a constant and Eu is the Urbach energy i.e., width of localized states in the optical band gap. In amorphous semiconductors, the absorption coefficient at below absorption edge indicates an exponential dependence on photon energy. This dependence is described as [16] hn a ¼ a0 exp ð4Þ Eu where a0 is a constant. The curves of the semigarithmic plot of a vs. hn are shown in Fig. 4. The Eu values were calculated from the slope of these figures and are given in Table 1. It is seen that the indium content changes the width of localized states in the optical band gap of the films. The Eu values are indicators for the various defect levels taking place in optical band gap of the films. 3.3. Temperature dependence of electrical conductivity of Se100 x Inx thin films The measured dc conductivities, sdc of amorphous Se100 x Inx films with x ¼10, 20 and 30 at% are plotted and shown in Fig. 5 as a function of the reciprocal temperature. As seen in the figure, sdc can be expressed in the form [14]: DE1 DE2 þ s02 exp ð5Þ sdc ¼ s01 exp kT kT The two terms on the right-hand side arise from two different conduction processes: the first term is related to band conduction in extended states where s01 and DE1 are the pre-exponential
10 20 30
-5
ln σ
Eg (eV) Eu (eV)
1.9 hν (eV)
hν (eV)
-6 -7 -8 -9 2.2
2.4
2.6
2.8
3.0
3.2
3.4
-1
1000/T (K ) Fig. 5. Plots of ln s vs. 1000/T of Se100 x Inx thin films.
Table 2 The activation and hopping energies for a-Se100 xInx films.
DE1 (eV) DE2 (eV)
x¼ 10 at%
x¼20 at%
x ¼30 at%
0.49 0.17
0.36 0.22
0.32 0.22
factor and the activation energy of conduction, respectively. The second term is due to hopping conduction in localized states with s02 and DE2 that refer to the pre-exponential factor and the activation energy of hopping conduction, respectively. The calculated values of s01, DE1, s02 and DE2 are listed in Table 2. It is observed that s01, DE1, s02 and DE2 vary slightly with the incorporation of In content. The increasing of the electrical conductivity and decreasing activation energy Se100 x Inx films may be attributed to the formation of In–Se bonds and then generation of excess electronic delocalized states [14]. 3.4. Kinetics of phase transformation on the electrical conductivity Se100 x Inx thin films Studies of the crystallization kinetics in chalcogenide glasses thin films are of particular interest because they are connected with such important phenomena as memory type of switching,
A.H. Ammar et al. / Physica B 407 (2012) 356–360
ac ðtÞ ¼ 1exp½ðkn tÞn
400K
160 σx10-4 (Ω-1.cm-1)
reversible optical recording, imaging media and absorption filters [17]. Thermal analysis is a very useful tool for describing the crystallization phenomena as it is rapid and convenient. The kinetics of the first-order phase transformation are important in physics, chemistry, ceramic and material sciences [17]. The isothermal crystallization is usually described by Avrami, sometime called Johnson–Mehl–Avrami (JMA), equation relating the fraction of the crystallized volume, ac(t), grown from amorphous phase during the time, t, of thermal annealing to crystalline phase as follows [18–20]:
359
120
80
n
ð6Þ
x= 10 20 30
n
where k is the rate constant of crystallization and the exponent, nn is a dimensionless parameter, which characterized the morphology of the crystal growth. The crystallization rate, ac(t), can also be calculated using the relative increase in the electric conduction during the crystallization growth. The empirical formula, which relates the amount of material crystallized at time t with the basis of the experimental results is represented by [21]:
sc st sc sa
80 120 t (min)
160
200
Fig. 7. Plots of time dependent of conductivity of Se100 x Inx thin films annealed at 400 K.
370 K
T =370 K 1
16
0
x= 10 20 30
-1
7.5
8.0
8.5 ln t
9.0
9.5
Fig. 8. Plots of ln [ ln(1 ac)] vs. ln t of Se100 x Inx thin films annealed at 370 K.
1.5
T=400 K
1.0 ln[-ln(1-αc)]
σx10-4 (Ω-1.cm-1)
40
ð7Þ
where sa and sc are the prescriptive conductivity at the beginning and the end of the crystallization process and st is the conductivity at time t between these two limits as shown in Figs. 6 and 7. These figures represent the time dependence of the dc electrical conductivity for our samples at T¼370 and 400 K, respectively. In these figures, it can be shown that, the conductivity increases by increasing the thermo-annealing with three thermo-phase transition processes. At first, in the beginning of the time annealing, there are some fluctuations due to phase transition. These fluctuations can be explained as follows: in the beginning the indium nuclei in the phase transition fluctuated between birth and decay until they reach to a certain critical volume, then, birth and growth of thermal crystallization states. The second period is the thermo-crystallization period, during which the Indium nuclei are born with a certain critical volume and started to increase on the expense of amorphous matrix to reach maximum constant value as the whole sample matrix transformed from amorphous to crystalline phase by increasing the time of annealing. The third period is at the end of the time of annealing, which gives a nearly constant value of conductivity when all sample transformed to crystalline phase. The crystallization rates, ac(t) are calculated from Eq. (6) and plotted for different crystallization temperatures (T¼370 and
20
0
ln[-ln(1-αc)]
ac ðtÞ ¼
40
0.5 0.0 -0.5 x=
-1.5 8
4
10 20 30
-1.0
12
x= 10 20 30
0
40
80
120
160
200
t (min) Fig. 6. Plots of time dependent of conductivity of Se100 x Inx thin films annealed at 370 K.
7.5
8.0
8.5
9.0
9.5
ln t Fig. 9. Plots of ln [ ln(1 ac)] vs. ln t of Se100 x Inx thin films annealed at 400 K.
400 K) as shown in Figs. 8 and 9. The crystallization temperatures are selected to be between the glass temperature, Tg, and the crystallization temperature, Tc, as it is drawn out from DTA traces [22]. In Figs. 8 and 9, the ln[-ln(1 ac(t))] values are plotted
360
A.H. Ammar et al. / Physica B 407 (2012) 356–360
Table 3 Avrami kinetic parameters nn and kn as a function of isothermal annealing temperature for a-Se100 xInx films. Annealing Temperature
nn kn
T ¼370 K
T¼400 K
x ¼10 at%
x¼ 20 at%
x¼ 30 at%
x¼10 at%
x ¼20 at%
x¼ 30 at%
1.37 0.206
1.4 0.242
1.43 0.25
1.47 0.15
1.48 0.18
1.51 0.198
1.37 eV. It can be concluded that the optical band gap of the Se100 x Inx thin films decrease with increasing In content due to the formation of In–Se bonds. The width of localized states of the films changes from 375 to 342 meV due to the increasing of In content from 10 to 30 at%. The incorporation of In atoms in amorphous Se matrix leads to an increase in the electrical conductivity and a decrease in the thermal activation energy of the films in the temperature range 303–433 K from 0.49 to 0.32 eV. References
against ln t. However, the kinetics parameters nn and kn are evaluated for the two temperatures T¼ 370 and 400 K, which are tabulated in Table 3. From this table, the values of nn and kn are increased with increasing the Indium percentage at each crystallization temperature in our films. Also, by increasing the crystallization temperature, the mean values of nn (the nucleation and crystallization growth) are increased while, the mean value of kn (the crystallization rate) are decreased due to the rearrangement of Se–In chains [22].
4. Conclusions The results of crystallization kinetics for amorphous Se100 xInx films have been indicated that the crystallization mechanism of the films occurs in two and three dimension growth according to the Avrami parameters nn and kn. These results are due to the limitation of crystallization process to include only growth of entities by small relaxation time. The incorporation of In atoms in amorphous Se matrix leads to an increase in the electrical conductivity and a decrease in the thermal activation energy of the films in the temperature range 303–433 K from 0.49 to 0.32 eV. The indirect optical band gap was changed from 1.78 to
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