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Effect of the deposition rate on the phase transition in silver telluride thin films
Materials Science in Semiconductor Processing 47 (2016) 12–15
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Effect of the deposition rate on the phase transition in silver telluride thin films P. Gnanadurai a,n, N. Soundararajan b, C.E. Sooriamoorthy c a
Department of Physics, NMSSVN College, Madurai, Tamilnadu 625019, India School of Physics, Madurai Kamaraj University, Madurai 625021, India c School of energy Sciences, Madurai Kamaraj University, Madurai 625021, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 30 October 2015 Received in revised form 2 February 2016 Accepted 15 February 2016
Silver telluride thin films of thickness 50 nm have been deposited at different deposition rates on glass substrates at room temperature and at a pressure of 2 10 5 mbar. The electrical resistivity was measured in the temperature range 300–430 K. The temperature dependence of the electrical resistance of Ag2Te thin films shows structural phase transition and coexistence of low temperature monoclinic phase and high temperature cubic phase. The effect of deposition rate on the phase transition and the electrical resistivity of silver telluride thin films in relation to carrier concentration and mobility are discussed. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Silver telluride Deposition rate Electrical resistivity Phase transition
1. Introduction Silver telluride is an n-type narrow band gap semiconductor that undergoes a structural phase transition with hysteresis[1–6]. There have been several investigations on the electrical, the magnetic, the optical and the structural properties in thin films and bulk of Ag2Te [1–25]. From the temperature dependence of electrical conductivity and the thermoelectric power, Das and Karunakaran [1] have observed that the temperature of the phase transition varies inversely with film thickness. Gnanadurai et al. [3] have investigated the effect of heating rate on the hysteresis in phase transition in silver telluride thin films. They have inferred that the phase transition temperatures as well as the hysteresis width are influenced by heating rate. They [4] have also studied the temperature dependence of the electrical resistance of Ag2Te thin films of different thicknesses and have found that resistivity of silver telluride thin films decrease with the decrease in film thickness. Li et al. [6] have also observed hysteresis from the electrical conductivity of Ag2Te nano wire. From the studies on the polymorphism of silver telluride and silver sulphate, Frueh [23] has found that the phase transition temperature of Ag2Te varies from 380 K to 420 K from 3% silver deficiency to 3% excess of silver atoms in silver telluride. Dhere and Goswami [10] observed the reversible structural phase transition from monoclinic α phase to n
Corresponding author. E-mail address: [email protected] (P. Gnanadurai).
http://dx.doi.org/10.1016/j.mssp.2016.02.009 1369-8001/& 2016 Elsevier Ltd. All rights reserved.
cubic β phase in Ag2Te films on different kinds of substrates by reflection and transmission methods of electron diffraction. Apart from hysteresis on structural transformation Ag2Te films, Sharma [11] has observed that even after repeated studies there is no compositional change on Ag2Te thin films. The deposition rate is one of the parameters that influence the growth of films. The size and concentration of nuclei during the initial stage of the film growth depends on the deposition rate also, since the surface mobility of adatoms increases with increasing deposition rate. As there is no investigation on the influence of the deposition parameters such as the substrate temperature and the deposition rate, on the electrical conduction in silver telluride thin films, the present work is the study of the effect of the deposition rate on the electrical conduction of silver telluride thin films in the temperature range 300–430 K.
2. Experimental Silver telluride alloy was prepared from a stoichiometric mixture of spectroscopically pure (99.999%) silver and tellurium placed in a vacuum sealed quartz tube. The quartz tube was heated to 1300 K in a furnace for two days and kept at that temperature for a day. Subsequently, it was annealed at 1000 K for a day and then cooled slowly to room temperature. The X-ray diffraction study of Ag2Te carried out in the range of scanning angle 20° to 80° with cuKα line(λ ¼1.5406 Å). The formations of the monoclinic
P. Gnanadurai et al. / Materials Science in Semiconductor Processing 47 (2016) 12–15
phase of the silver telluride have been confirmed by X-ray powder diffraction, at room temperature. Glass slides of dimension 0.075 0.025 0.01 m, well cleaned with warm chromic acid, distilled water, acetone and again with distilled water, were used as substrates. First thick silver films were deposited on the substrates using a suitable mask for good contacts. Using suitable mask, silver telluride thin films of thickness 50 nm and of dimensions 0.03 0.02 m were deposited on glass substrates kept at room temperature at different deposition rates from 0.02 to 0.90 nm/s, at a pressure of 2 10 5 mbar. A digital quartz crystal thickness monitor was used to monitor the thickness of films and their deposition rate. The films were uniformly annealed at 430 K for about an hour and then slowly cooled. The electrical resistance of the annealed silver telluride thin films of different deposition rate was measured in the temperature range 300–430 K using a high impedance digital multimeter and the temperature was measured using copper constantan thermocouple at the rate of 1 K/min.
3. Results Fig. 1 shows the X-ray diffractrogram of a silver telluride thin film. It shows that the films are polycrystalline and homogeneous. It is seen from Fig. 1 the diffraction peaks at 22.9°, 46.7°, 73°,correspond to the (200), ( 1̅ 42)and( 5̅ 25) planes of the monoclinic phase of Ag2Te thin films which is consistent with the literature data (JCPDS: 34-0142). Fig. 2 displays the variation of the resistance as a function of temperature for an annealed silver telluride thin film deposited at 0.08 nm/s. The temperature dependence of resistance of Ag2Te thin films during heating and cooling can be divided into three regions, the region before phase transition or low temperature phase region, the region during phase transition and the region after phase transition or high temperature phase region. In each region, the temperature dependence of resistance is distinct during heating and cooling cycles. The low temperature phase with monoclinic structure is semiconducting and the high temperature cubic phase exhibits metallic behavior [14]. The monoclinic phase in the low temperature region shows different temperature dependence of resistance during heating and cooling cycles. In low temperature region during heating the observed temperature dependence of resistance is due to the semiconducting behavior of monoclinic phase. But during cooling, the resistance of the film decreases near phase transition region and increases in near room temperature region. The observed behavior can be explained by the incomplete phase transition, coexistence of high temperature phase with low temperature during cooling. In the low temperature range, the variation in the temperature dependence of resistance in the cooling is
Fig. 1. X-ray diffractrogram of silver telluride thin films.
13
Fig. 2. The variation of resistance as a function of temperature for annealed silver telluride thin film of thickness 50 nm prepared with deposition rate 0.08 nm/s.
independent of the effect of defects [27] because of the reversibility of temperature dependence of resistance. Impurities can influence resistivity of films by changing the Fermi level [28]. But the films are free from impurities as confirmed by the XRD studies on films. Hence, the observed temperature dependence of resistivity in low temperature phase is due to the semiconducting behavior of monoclinic phase and the metallic behavior of high temperature cubic phase. As these variations in resistance as a function of temperature in the cooling cycle is reversible there is no effect of defects on the resistivity of films. The films were annealed for homogenization and for removal of defects at 430 K for an hour. Since the temperature variations of resistances in the low temperature phase are reversible in many cycles, the observed temperature dependence of resistance could be due to the structural phase transition as observed by Das and Karunakaran [2]. As seen in Fig. 2, the electrical resistance of the silver telluride thin film decreases with temperature upto 375 K. Around 375 K, the electrical resistance increases slowly with increasing temperature and from 386 K, it increases with temperature very rapidly for about 11 K increase in temperature. Then the electrical resistance increases almost linearly with temperature upto 430 K. The rapid change in the electrical resistance is due to the structural phase transition in silver telluride thin film as in bulk material, from the monoclinic phase to the cubic phase [5,6]. Silver telluride thin film exhibits semiconducting behavior in the monoclinic phase and metallic behavior in the cubic phase, as observed in the bulk material. The slow increase in the resistance from 375 K is due to the nucleation of the cubic phase in the monoclinic phase. During cooling the resistance monotonically decreases with temperature and at 373 K, the resistance decreases very rapidly for about 6 K temperature interval with decreasing temperatures. The rate of decrease is reduced notably with decreasing temperature down to 320 K and then the resistance increases with decreasing temperature. The rapid change in the resistance is due to the structural phase change from the cubic phase to the monoclinic phase. During cooling, the cubic phase is not completely transformed to the monoclinic phase. It coexists with the monoclinic phase in a small proportion at room temperature. Dhere and Goswami [10] have also observed the coexistence of these two phases in silver telluride thin films, in scanning electron microscopy. Similar temperature dependence of resistance has been observed for the silver telluride thin film of thickness 50 nm, deposited at different rates. The temperature of phase initiation is the temperature at which the resistance starts increasing with increasing temperature in the monoclinic phase. The temperature of the rapid phase change is the average of the temperatures at which the rapid change in resistance starts and ends. Finally, the temperature of completion is the temperature at which the resistance starts increasing as appropriate to the cubic phase. Table 1 displays the temperatures of the phase initiation, rapid transition and the phase completion in
14
P. Gnanadurai et al. / Materials Science in Semiconductor Processing 47 (2016) 12–15
Table 1 Phase transition temperature as a function of deposition rate. Deposition rate (nm/s)
Phase transition temperature (in K) Initiation
0.02 0.06 0.08 0.20 0.40 0.50 0.90
Hysteresis width (in K)
Completion
Rapid
Heating
Cooling
Heating
Cooling
Heating
Cooling
373.5 358.3 375.6 367.2 369.5 394.6 371.7
356.2 342.4 345.1 340.3 351.9 342.0 356.0
413.3 396.6 396.6 386.2 411.0 419.1 419.1
388.3 373.5 373.5 369.5 390.5 398.0 392.7
404.9 387.3 389.5 379.8 403.9 414.2 410.0
382.9 367.3 371.5 363.5 388.1 393.6 388.5
Initiation
Completion
Rapid
17.3 15.9 30.5 26.9 17.6 52.6 15.7
25.0 23.1 23.1 16.7 20.5 21.1 26.4
22.0 20.1 18.0 16.3 15.8 20.6 21.5
Fig. 3. Differential scanning calorimetric curves of bulk silver telluride with the heating rate of 1 °C/min.
4. Discussions
Fig. 4. The variation of resistivity as a function of deposition rate for annealed silver telluride thin films.
films of different deposition rates. It displays the hysteresis width, the temperature of the phase transition initiation, temperature of rapid transition and the temperature at which phase transition completion at different deposition rates also. It is clear from the table that the temperature of rapid transition, the completion of phase transition and the hysteresis width for the rapid transition and the phase transition completion decrease initially and then increase with increasing deposition rates of 0.02 nm/s to 0.9 nm/s. All these parameters show a minimum around 0.2 nm/s while the temperature of phase transition initiation and the hysteresis width show random dependence on the deposition rate. Fig. 3 shows the differential scanning calorimetric analysis of bulk material during heating and cooling cycles, exhibiting the hysteresis on the structural phase change in bulk also. Fig. 4 displays the variation of the resistivity as a function of the deposition rate for the annealed films.
The unique way of film growth influences many properties of films like resistivity and Seeback coefficient. The thickness of the films may influence many properties of films by surface scattering of electrons. Similarly as the grain size depends on film thickness, thickness is the significant factor. Apart from thickness, structural details and size of the grains are influenced by deposition rate, ambient pressure, nature of the substrate, substrate temperature at which films are prepared. Therefore these properties also has influenced on the properties of films. So in general, properties of films are influenced by thickness, deposition rate, substrate temperature, ambient pressure and nature of the substrate. As phase transition is a property of silver telluride, these parameters also influence the phase transition of Ag2Te thin films. The bulk silver telluride undergoes first order reversible transition with hysteresis in the temperature range 380 K to 410 K. Natarajan et al. [24] have observed the effect of grain sizes on the phase transition temperature and the hysteresis in quartz and potassium sulphate. Mamedov et al. [9] have found that the phase transition in silver telluride and shows hysteresis. Aliev and Aliev [8] have studied the influence of the phase transition on the electronic process in silver telluride. They have also observed that the resistivity increases during the phase transition during heating. It is well known that the deposition parameters like the deposition rate, the substrate temperature, and the nature of the substrate influence the growth of thin films, and thereby affect the properties of films. Sharma [11] and Dhere and Goswami [10] have studied the influence the substrate temperature and the nature of
P. Gnanadurai et al. / Materials Science in Semiconductor Processing 47 (2016) 12–15
substrates on the phase transition of silver chalcogenide films by scanning electron microscopic analysis. They have observed the hysteresis in the phase transition. Das and Karunakaran [2] have also observed the hysteresis in the phase transition of silver telluride from the studies on the thickness dependence of the electrical resistance of silver telluride thin films. They have also observed the hysteresis in the phase transition of the bulk sample using differential scanning calorimetric studies. We have also observed the hysteresis in the phase transition of bulk from the differential scanning calorimetric studies in Fig. 2. Dhere and Goswami [10] have observed that during cooling, at room temperature, the cubic phase coexists with the orthorhombic phase. Though the other investigators have not observed the coexistence of the phases, still the complex structural details observed by them indicate that the coexistence of the phase does coexist. It is seen from the Table 1 that the hysteresis width and the phase transition temperature during rapid transition and phase transition completion show a minimum around 0.2 nm/s. At room temperature the coexistence of cubic phase along with monoclinic phase in arbitrary ratio may be the cause for randomness in initiation temperature. But the hysteresis width and the phase transition temperature during rapid phase transition and the completion of phase transition show a minimum around at the deposition rate of 0.2 nm/s as shown in Table1. The capillarity model [26] predicts that the deposition rate, one of the deposition parameter, influences the growth of films by affecting the size and the concentration of the nuclei in thin films Chopra and Randlett [29] have made careful observations of the influence of the deposition rate on the agglomeration in silver and gold films, deposited on NaCl substrates. They have observed increase in agglomeration for increasing deposition rates, and thereafter, decrease in agglomeration with the increase in deposition rate. The minimum observed in the phase transition temperature against the deposition rate of 0.2 nm/s indicates that there is increased agglomeration of thin films upto 0.2 nm/s and then the agglomeration decreases with the increase deposition rate. The randomness of the temperature of phase transition with the deposition rate indicates that the internal structure of the film may influence the nucleation of the cubic phase. Boakye and Grassie [30] have found that the resistivity of manganese films decreases with the increase in deposition rate from 0.5 nm/s to 1.5 nm/s. They attributed this to the increase in the mobility due to decrease in defect concentration with the increase in agglomeration. From galvanomagnetic studies, Okuyama et al. [31] have observed that with increasing deposition rate up to 20 nm/s, the mobility of holes remains constant but the carrier concentration increases with increased deposition rates in tellurium films along with the increased agglomeration. Vos and Aerts [32] have found that in tellurium films mobility of carriers increases with the increase in film thickness due to increased agglomeration. We have observed that the resistivity is almost independent of the deposition rate. This may be due to the increase in defect concentration with decreasing agglomeration at high deposition rates. As silver telluride is a defect semiconductor, the increase in Frenkel defects of silver atoms leads to the increase in free electrons. But the increase in the defect concentration
15
decreases the mobility of free carriers. So the increase in the carrier concentration is approximately compensated by the decrease in mobility of carriers, which leads to the fact that the resistivity is essentially independent of the deposition rate.
5. Conclusion We have measured the electrical resistivity in the temperature range 300–430 K in silver telluride thin films of thickness 50 nm prepared at different deposition rates. It is found that the agglomeration increases with the deposition rate and reaches a maximum around 0.2 nm/s. Subsequently, agglomeration decreases with increasing deposition rate. The maximum agglomeration at low deposition rate of 0.2 nm/s may be due to the influence of the phase transition at around 380 K in films as a result of condensation of vapours. The resistivity is almost independent of the deposition rate. However, the phase transition temperature shows a minimum at maximum agglomeration due to particle size effect.
References [1] V.D. Das, D. Karunakaran, J. Phys. Chem. Solids 46 (1985) 551. [2] V.D. Das, D. Karunakaran, J. Appl. Phys. 66 (1989) 1822. [3] P. Gnanadurai, N. Sundararajan, C.E. Sooriamoorthy, Phys. Status Solidi B 236 (2003) 472. [4] P. Gnanadurai, N. Sundararajan, C.E. Sooriamoorthy, Vacuum 81 (2006) 1. [5] A.K. Samal, T. Pradeep, J. Phys. Chem. C 113 (2009) 13539. [6] F. Li, C. Hu, Y. Xiong, B. Wan, W. Yan, M. Zhang, J. Phys. Chem. C 112 (2008) 16130. [7] S.K. Sharma, J. Mater. Sci. 4 (1969) 189. [8] S.A. Aliev, F.F. Aliev, Inorg. Mater. 25 (1989) 204. [9] K.P. Mamedov, M.F. Godzhiev, Z.D. Nurieva, Sov. Phys. Solid State 19 (1977) 1285. [10] N.G. Dhere, A. Goswami, Thin Solid Films 5 (1970) 137. [11] S.K. Sharma, Nature 198 (1963) 280. [12] C. Ching-Liang, R.M. Imamov, Z.G. Pinsker, Sov. Phys. Crystallogr. 6 (1961-62) 618. [13] G.E. Gottlieb, W.M. Kane, J.F. Walsh, C. Wood, J. Phys. Chem. Solids 15 (1959) 183. [14] V.D. Das, D. Karunakaran, J. Appl. Phys. 54 (1983) 5252. [15] Y. Izumi, S. Miyatani, J. Phys. Soc. Jpn. 35 (1973) 312. [16] S.A. Aliev, F.F. Aliev, Inorg. Mater. 24 (1988) 1187. [17] V.V. Gorbachev, I.M. Putilin, Inorg. Mater. 11 (1975) 1329. [18] S.A. Aliev, U. Suyunov, M.I. Aliev, Sov. Phys. Semicond. 7 (1974) 1383. [19] R. Hismam, A. Xu, T.S. Rosenbaum, M.L. Saboungi, I.E. Enderby, P.B. Littlewood, Nature 390 (1997) 57. [20] L.S. Chuprakov, K.H. Dahmen, Appl. Phys. Lett. 7221 (1998) 65. [21] B.Q. Liang, X. Chen, Y.J. Wang, Y.J. Tang, Phys. Status Solidi B 21 (1999) 51145. [22] J. Appel, Z. Naturforsch 10A (1955) 530. [23] A.J. Frueh, Am. Mineral. 46 (1961) 654. [24] M. Natarajan, A.R. Das, C.N.R. Rao, Trans. Faraday Soc. 65 (1969) 3081. [25] P. Gnanadurai, N. Sundararajan, C.E. Sooriamoorthy, Vacuum 67 (2002) 273. [26] L. Maissel, R. Glang, Hand Book of Thin Film Technology, McGraw Hill, NY, 1970 8–8. [27] N.E. Gorji, J. Semicond. 35 (2014) 122001. [28] N.E. Gorji, J. Semicond. 36 (2015) 054001. [29] K.L. Chopra, M.R. Randlett, J. Appl. Phys. 24 (1967) 1874. [30] F. Boakye, A.D.C. Grassie, Thin Solid films 201 (1992) 224. [31] K. Okuyama, H. Yamamoto, Y. Kumagai, J. Appl. Phys. 46 (1975) 105. [32] A.D. Vos, J. Aerts, Thin Solid Films 40 (1977) 223.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Effect of the deposition rate on the phase transition in silver telluride thin films P. Gnanadurai a,n, N. Soundararajan b, C.E. Sooriamoorthy c a
Department of Physics, NMSSVN College, Madurai, Tamilnadu 625019, India School of Physics, Madurai Kamaraj University, Madurai 625021, India c School of energy Sciences, Madurai Kamaraj University, Madurai 625021, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 30 October 2015 Received in revised form 2 February 2016 Accepted 15 February 2016
Silver telluride thin films of thickness 50 nm have been deposited at different deposition rates on glass substrates at room temperature and at a pressure of 2 10 5 mbar. The electrical resistivity was measured in the temperature range 300–430 K. The temperature dependence of the electrical resistance of Ag2Te thin films shows structural phase transition and coexistence of low temperature monoclinic phase and high temperature cubic phase. The effect of deposition rate on the phase transition and the electrical resistivity of silver telluride thin films in relation to carrier concentration and mobility are discussed. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Silver telluride Deposition rate Electrical resistivity Phase transition
1. Introduction Silver telluride is an n-type narrow band gap semiconductor that undergoes a structural phase transition with hysteresis[1–6]. There have been several investigations on the electrical, the magnetic, the optical and the structural properties in thin films and bulk of Ag2Te [1–25]. From the temperature dependence of electrical conductivity and the thermoelectric power, Das and Karunakaran [1] have observed that the temperature of the phase transition varies inversely with film thickness. Gnanadurai et al. [3] have investigated the effect of heating rate on the hysteresis in phase transition in silver telluride thin films. They have inferred that the phase transition temperatures as well as the hysteresis width are influenced by heating rate. They [4] have also studied the temperature dependence of the electrical resistance of Ag2Te thin films of different thicknesses and have found that resistivity of silver telluride thin films decrease with the decrease in film thickness. Li et al. [6] have also observed hysteresis from the electrical conductivity of Ag2Te nano wire. From the studies on the polymorphism of silver telluride and silver sulphate, Frueh [23] has found that the phase transition temperature of Ag2Te varies from 380 K to 420 K from 3% silver deficiency to 3% excess of silver atoms in silver telluride. Dhere and Goswami [10] observed the reversible structural phase transition from monoclinic α phase to n
Corresponding author. E-mail address: [email protected] (P. Gnanadurai).
http://dx.doi.org/10.1016/j.mssp.2016.02.009 1369-8001/& 2016 Elsevier Ltd. All rights reserved.
cubic β phase in Ag2Te films on different kinds of substrates by reflection and transmission methods of electron diffraction. Apart from hysteresis on structural transformation Ag2Te films, Sharma [11] has observed that even after repeated studies there is no compositional change on Ag2Te thin films. The deposition rate is one of the parameters that influence the growth of films. The size and concentration of nuclei during the initial stage of the film growth depends on the deposition rate also, since the surface mobility of adatoms increases with increasing deposition rate. As there is no investigation on the influence of the deposition parameters such as the substrate temperature and the deposition rate, on the electrical conduction in silver telluride thin films, the present work is the study of the effect of the deposition rate on the electrical conduction of silver telluride thin films in the temperature range 300–430 K.
2. Experimental Silver telluride alloy was prepared from a stoichiometric mixture of spectroscopically pure (99.999%) silver and tellurium placed in a vacuum sealed quartz tube. The quartz tube was heated to 1300 K in a furnace for two days and kept at that temperature for a day. Subsequently, it was annealed at 1000 K for a day and then cooled slowly to room temperature. The X-ray diffraction study of Ag2Te carried out in the range of scanning angle 20° to 80° with cuKα line(λ ¼1.5406 Å). The formations of the monoclinic
P. Gnanadurai et al. / Materials Science in Semiconductor Processing 47 (2016) 12–15
phase of the silver telluride have been confirmed by X-ray powder diffraction, at room temperature. Glass slides of dimension 0.075 0.025 0.01 m, well cleaned with warm chromic acid, distilled water, acetone and again with distilled water, were used as substrates. First thick silver films were deposited on the substrates using a suitable mask for good contacts. Using suitable mask, silver telluride thin films of thickness 50 nm and of dimensions 0.03 0.02 m were deposited on glass substrates kept at room temperature at different deposition rates from 0.02 to 0.90 nm/s, at a pressure of 2 10 5 mbar. A digital quartz crystal thickness monitor was used to monitor the thickness of films and their deposition rate. The films were uniformly annealed at 430 K for about an hour and then slowly cooled. The electrical resistance of the annealed silver telluride thin films of different deposition rate was measured in the temperature range 300–430 K using a high impedance digital multimeter and the temperature was measured using copper constantan thermocouple at the rate of 1 K/min.
3. Results Fig. 1 shows the X-ray diffractrogram of a silver telluride thin film. It shows that the films are polycrystalline and homogeneous. It is seen from Fig. 1 the diffraction peaks at 22.9°, 46.7°, 73°,correspond to the (200), ( 1̅ 42)and( 5̅ 25) planes of the monoclinic phase of Ag2Te thin films which is consistent with the literature data (JCPDS: 34-0142). Fig. 2 displays the variation of the resistance as a function of temperature for an annealed silver telluride thin film deposited at 0.08 nm/s. The temperature dependence of resistance of Ag2Te thin films during heating and cooling can be divided into three regions, the region before phase transition or low temperature phase region, the region during phase transition and the region after phase transition or high temperature phase region. In each region, the temperature dependence of resistance is distinct during heating and cooling cycles. The low temperature phase with monoclinic structure is semiconducting and the high temperature cubic phase exhibits metallic behavior [14]. The monoclinic phase in the low temperature region shows different temperature dependence of resistance during heating and cooling cycles. In low temperature region during heating the observed temperature dependence of resistance is due to the semiconducting behavior of monoclinic phase. But during cooling, the resistance of the film decreases near phase transition region and increases in near room temperature region. The observed behavior can be explained by the incomplete phase transition, coexistence of high temperature phase with low temperature during cooling. In the low temperature range, the variation in the temperature dependence of resistance in the cooling is
Fig. 1. X-ray diffractrogram of silver telluride thin films.
13
Fig. 2. The variation of resistance as a function of temperature for annealed silver telluride thin film of thickness 50 nm prepared with deposition rate 0.08 nm/s.
independent of the effect of defects [27] because of the reversibility of temperature dependence of resistance. Impurities can influence resistivity of films by changing the Fermi level [28]. But the films are free from impurities as confirmed by the XRD studies on films. Hence, the observed temperature dependence of resistivity in low temperature phase is due to the semiconducting behavior of monoclinic phase and the metallic behavior of high temperature cubic phase. As these variations in resistance as a function of temperature in the cooling cycle is reversible there is no effect of defects on the resistivity of films. The films were annealed for homogenization and for removal of defects at 430 K for an hour. Since the temperature variations of resistances in the low temperature phase are reversible in many cycles, the observed temperature dependence of resistance could be due to the structural phase transition as observed by Das and Karunakaran [2]. As seen in Fig. 2, the electrical resistance of the silver telluride thin film decreases with temperature upto 375 K. Around 375 K, the electrical resistance increases slowly with increasing temperature and from 386 K, it increases with temperature very rapidly for about 11 K increase in temperature. Then the electrical resistance increases almost linearly with temperature upto 430 K. The rapid change in the electrical resistance is due to the structural phase transition in silver telluride thin film as in bulk material, from the monoclinic phase to the cubic phase [5,6]. Silver telluride thin film exhibits semiconducting behavior in the monoclinic phase and metallic behavior in the cubic phase, as observed in the bulk material. The slow increase in the resistance from 375 K is due to the nucleation of the cubic phase in the monoclinic phase. During cooling the resistance monotonically decreases with temperature and at 373 K, the resistance decreases very rapidly for about 6 K temperature interval with decreasing temperatures. The rate of decrease is reduced notably with decreasing temperature down to 320 K and then the resistance increases with decreasing temperature. The rapid change in the resistance is due to the structural phase change from the cubic phase to the monoclinic phase. During cooling, the cubic phase is not completely transformed to the monoclinic phase. It coexists with the monoclinic phase in a small proportion at room temperature. Dhere and Goswami [10] have also observed the coexistence of these two phases in silver telluride thin films, in scanning electron microscopy. Similar temperature dependence of resistance has been observed for the silver telluride thin film of thickness 50 nm, deposited at different rates. The temperature of phase initiation is the temperature at which the resistance starts increasing with increasing temperature in the monoclinic phase. The temperature of the rapid phase change is the average of the temperatures at which the rapid change in resistance starts and ends. Finally, the temperature of completion is the temperature at which the resistance starts increasing as appropriate to the cubic phase. Table 1 displays the temperatures of the phase initiation, rapid transition and the phase completion in
14
P. Gnanadurai et al. / Materials Science in Semiconductor Processing 47 (2016) 12–15
Table 1 Phase transition temperature as a function of deposition rate. Deposition rate (nm/s)
Phase transition temperature (in K) Initiation
0.02 0.06 0.08 0.20 0.40 0.50 0.90
Hysteresis width (in K)
Completion
Rapid
Heating
Cooling
Heating
Cooling
Heating
Cooling
373.5 358.3 375.6 367.2 369.5 394.6 371.7
356.2 342.4 345.1 340.3 351.9 342.0 356.0
413.3 396.6 396.6 386.2 411.0 419.1 419.1
388.3 373.5 373.5 369.5 390.5 398.0 392.7
404.9 387.3 389.5 379.8 403.9 414.2 410.0
382.9 367.3 371.5 363.5 388.1 393.6 388.5
Initiation
Completion
Rapid
17.3 15.9 30.5 26.9 17.6 52.6 15.7
25.0 23.1 23.1 16.7 20.5 21.1 26.4
22.0 20.1 18.0 16.3 15.8 20.6 21.5
Fig. 3. Differential scanning calorimetric curves of bulk silver telluride with the heating rate of 1 °C/min.
4. Discussions
Fig. 4. The variation of resistivity as a function of deposition rate for annealed silver telluride thin films.
films of different deposition rates. It displays the hysteresis width, the temperature of the phase transition initiation, temperature of rapid transition and the temperature at which phase transition completion at different deposition rates also. It is clear from the table that the temperature of rapid transition, the completion of phase transition and the hysteresis width for the rapid transition and the phase transition completion decrease initially and then increase with increasing deposition rates of 0.02 nm/s to 0.9 nm/s. All these parameters show a minimum around 0.2 nm/s while the temperature of phase transition initiation and the hysteresis width show random dependence on the deposition rate. Fig. 3 shows the differential scanning calorimetric analysis of bulk material during heating and cooling cycles, exhibiting the hysteresis on the structural phase change in bulk also. Fig. 4 displays the variation of the resistivity as a function of the deposition rate for the annealed films.
The unique way of film growth influences many properties of films like resistivity and Seeback coefficient. The thickness of the films may influence many properties of films by surface scattering of electrons. Similarly as the grain size depends on film thickness, thickness is the significant factor. Apart from thickness, structural details and size of the grains are influenced by deposition rate, ambient pressure, nature of the substrate, substrate temperature at which films are prepared. Therefore these properties also has influenced on the properties of films. So in general, properties of films are influenced by thickness, deposition rate, substrate temperature, ambient pressure and nature of the substrate. As phase transition is a property of silver telluride, these parameters also influence the phase transition of Ag2Te thin films. The bulk silver telluride undergoes first order reversible transition with hysteresis in the temperature range 380 K to 410 K. Natarajan et al. [24] have observed the effect of grain sizes on the phase transition temperature and the hysteresis in quartz and potassium sulphate. Mamedov et al. [9] have found that the phase transition in silver telluride and shows hysteresis. Aliev and Aliev [8] have studied the influence of the phase transition on the electronic process in silver telluride. They have also observed that the resistivity increases during the phase transition during heating. It is well known that the deposition parameters like the deposition rate, the substrate temperature, and the nature of the substrate influence the growth of thin films, and thereby affect the properties of films. Sharma [11] and Dhere and Goswami [10] have studied the influence the substrate temperature and the nature of
P. Gnanadurai et al. / Materials Science in Semiconductor Processing 47 (2016) 12–15
substrates on the phase transition of silver chalcogenide films by scanning electron microscopic analysis. They have observed the hysteresis in the phase transition. Das and Karunakaran [2] have also observed the hysteresis in the phase transition of silver telluride from the studies on the thickness dependence of the electrical resistance of silver telluride thin films. They have also observed the hysteresis in the phase transition of the bulk sample using differential scanning calorimetric studies. We have also observed the hysteresis in the phase transition of bulk from the differential scanning calorimetric studies in Fig. 2. Dhere and Goswami [10] have observed that during cooling, at room temperature, the cubic phase coexists with the orthorhombic phase. Though the other investigators have not observed the coexistence of the phases, still the complex structural details observed by them indicate that the coexistence of the phase does coexist. It is seen from the Table 1 that the hysteresis width and the phase transition temperature during rapid transition and phase transition completion show a minimum around 0.2 nm/s. At room temperature the coexistence of cubic phase along with monoclinic phase in arbitrary ratio may be the cause for randomness in initiation temperature. But the hysteresis width and the phase transition temperature during rapid phase transition and the completion of phase transition show a minimum around at the deposition rate of 0.2 nm/s as shown in Table1. The capillarity model [26] predicts that the deposition rate, one of the deposition parameter, influences the growth of films by affecting the size and the concentration of the nuclei in thin films Chopra and Randlett [29] have made careful observations of the influence of the deposition rate on the agglomeration in silver and gold films, deposited on NaCl substrates. They have observed increase in agglomeration for increasing deposition rates, and thereafter, decrease in agglomeration with the increase in deposition rate. The minimum observed in the phase transition temperature against the deposition rate of 0.2 nm/s indicates that there is increased agglomeration of thin films upto 0.2 nm/s and then the agglomeration decreases with the increase deposition rate. The randomness of the temperature of phase transition with the deposition rate indicates that the internal structure of the film may influence the nucleation of the cubic phase. Boakye and Grassie [30] have found that the resistivity of manganese films decreases with the increase in deposition rate from 0.5 nm/s to 1.5 nm/s. They attributed this to the increase in the mobility due to decrease in defect concentration with the increase in agglomeration. From galvanomagnetic studies, Okuyama et al. [31] have observed that with increasing deposition rate up to 20 nm/s, the mobility of holes remains constant but the carrier concentration increases with increased deposition rates in tellurium films along with the increased agglomeration. Vos and Aerts [32] have found that in tellurium films mobility of carriers increases with the increase in film thickness due to increased agglomeration. We have observed that the resistivity is almost independent of the deposition rate. This may be due to the increase in defect concentration with decreasing agglomeration at high deposition rates. As silver telluride is a defect semiconductor, the increase in Frenkel defects of silver atoms leads to the increase in free electrons. But the increase in the defect concentration
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decreases the mobility of free carriers. So the increase in the carrier concentration is approximately compensated by the decrease in mobility of carriers, which leads to the fact that the resistivity is essentially independent of the deposition rate.
5. Conclusion We have measured the electrical resistivity in the temperature range 300–430 K in silver telluride thin films of thickness 50 nm prepared at different deposition rates. It is found that the agglomeration increases with the deposition rate and reaches a maximum around 0.2 nm/s. Subsequently, agglomeration decreases with increasing deposition rate. The maximum agglomeration at low deposition rate of 0.2 nm/s may be due to the influence of the phase transition at around 380 K in films as a result of condensation of vapours. The resistivity is almost independent of the deposition rate. However, the phase transition temperature shows a minimum at maximum agglomeration due to particle size effect.
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