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DC Electrical measurements on evaporated thin films of vanadium pentoxide
Physica B 254 (1998) 273—276
DC Electrical measurements on evaporated thin films of vanadium pentoxide R.M. Abdel-Latif Physics Department, Faculty of Science, Minia University, Minia, Egypt Received 11 November 1997; accepted 24 April 1998
Abstract The current—voltage characteristics of the Au—V O —Au thin film sandwich system at different temperatures were studied. The DC conduction mechanism was explained using Jonscher—Ansari modified Poole—Frenkel type. Both freshly evaporated and annealed vanadium pentoxide films are amorphous. 1998 Elsevier Science B.V. All rights reserved.
1. Introduction A large number of amorphous-appearing insulator materials are known which in thin film form exhibit a current flow which increases roughly exponentially with the square root of the applied voltage [1,2]. This type of voltage—current characteristic is usually ascribed to the Schottky emission mechanism whereby electrons are emitted from the material electrode into interfacial barrier. Vanadium pentoxide (V O ) crystallizes in a layered structure and has been widely used in a variety of scientific and technological applications. It can be used as a catalyst [3—5], as a cathode for solid-state batteries [6—8], as a window for solar cells [9,10] and for electrochromic devices [11] as well as for electronic and optical switches [12,13]. Various properties of V O including structural, electronic, optical and electrical properties have been investigated. A particular interest has been devoted to the study of the electrical properties
[14—18]. This paper reports an investigation of current transport mechanisms in vanadium pentoxide thin films as a function of applied voltage and temperature.
2. Experimental The evaporation processes were carried out in a high-vacuum evaporation unit (Edwards 306 A) at a pressure of 5;10\ bar. Gold electrode was evaporated from a molybdenum boat onto carefully cleaned glass substrates through suitable masks to form the base electrode. Pure V O powder was evaporated through a mask from molybdenum boat to form the film over the gold eleectrode. The substrate temperature was maintained at room temperature. Then another gold counterelectrode was evaporated to form the metal—insulator—metal (MIM) sandwich of Au—V O —Au. The completed devices had an active area of about 7;10\ m.
0921-4526/98/$ — see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 4 1 1 - 6
274
R.M. Abdel-Latif / Physica B 254 (1998) 273—276
X-ray diffraction carried on both freshly deposited films and annealed at 473 K for 24 h shows amorphous structure. The DC current across the Au—V O —Au sand wich structure was measured as a function of the applied voltage at different temperatures using Keithley 610 electrometer and autoranging power supply model AP6050A. Sample temperatures were varied using a heater and a temperature control unit. Temperature was measured by means of a chromal/alumel thermocouple mounted in close proximity of the sample. Capacitance measurements were made at a frequency 100 kHz using a Stanford Research systems model SR720 LCR meter. Fig. 2. Plot of log I versus » for a film thick 180 nm.
3. Results and discussion Fig. 1 shows the dependence of capacitance C on reciprocal film with thickness 1/d. The linearity of the plot may be analysed in terms of the capacitance of a parallel plate capacitor, the slope of the line being ee A where e is the permittivity of V O film, e the permittivity of free space and A is the film area (7;10\ m). The value of ee estimated from Fig. 1 was found to be 2.25;10\ F m\ (e"2.54) and this value has been used in the analysis of the following results.
Various type of conduction mechanism were considered, in particular space-charge-limited conductivity and the Poole—Frenkel (PF) or Richardson—Schottky effects (RS). The I—» characteristics on a log—log scale did not explicitly show any conduction mechanism and hence the data were replotted on a log I versus » scale. Fig. 2 shows such a plot for a typical film 180 nm thick at different temperatures. It is seen that the plots become linear when the applied field is greater than 5.56;10 V m\. This suggests that the conduction process is either of the Schottky emission at the electrodes or of the Poo"e—Frenkel bulk mechanism. In the Poole—Frenkel mechanism, the conduction is limited by the field-enhanced thermal emission of electrons from a discrete trap level into the conduction band. In the Schottky emission process, the electrons are emitted from the metal electrode into the conduction band of the insulating film over the image force interfacial barrier under careful lowering of the applied electric field. Several dielectric and semiconducting thin films [18,19] exhibit a current—voltage of the form [19,20] IJexp
Fig. 1. Dependence of capacitance C on reciprocal thickness 1/d.
bE , k¹
(1)
where I is the current, E"»/d the applied electric field, d is the thickness of the film, » the applied
R.M. Abdel-Latif / Physica B 254 (1998) 273—276
275
Table 1 Values of b at various temperatures Temperature (°C)
b;10\(eV V\ m )
20 50 75 100 130
7.29 7.51 7.82 8.15 8.45
voltage, k the Boltzman constant, ¹ the absolute temperature and b is given by
e b"b "2b " , (2) .$ 1 pee where b and b are the Schottky and Poole—Fren1 .$ kel field lowering coefficients, respectively, and e is the electronic charge. Values of b calculated from the slope of the linear portions of the plots shown in Fig. 2 are tabulated in Table 1. The value of b calculated from Eq. (2) using e"2.54 is 4.76;10\ eV V\ m . It is seen from Table 1 that the value of b increases with increasing temperature. Furthermore, the experimental value of b is approximately 1.5—1.8 times the theoretical value. The conduction mechanism may be determined from whether b is dependent on the electrode work function (RS) or not (PF). The I—» characteristics were therefore measured using electrodes with different work functions, namely indium and aluminium. However, it was found that b did not depend on the electrode material, which led us to rule out the Richardson—Schottky type, and thus we conclude that the conduction mechanism is Poole—Frenkel type for electric field above 5.56;10 V m\. The high value of b (henceforth b ) cannot be .$ explained using the classical PF equation and we therefore examined the data in terms of the modified PF processes. The three-dimensional PF process gives the equation for the current as [21]
IJ
b b .$!1 exp .$ E , k¹ ak¹
(3)
Fig. 3. Plot of log I/(1.88;10\ E !1) versus E for the same film as in Fig. 2 at 20°C.
where a"1 for the normal PF process and a"2 for the modified PF process [22]. The value of a can be determined from a plot of log[I/+(b /k¹)E !1,] versus E . Fig. 3 .$ shows such a plot for the same film with b "4.76;10\ eV V\ m and ¹ " 293 K. .$ The value of a determined from the slope of the plot is 0.73, indicating that neither of the above theories can account for the experimental value. Jonscher and Ansari [23] have suggested that electrons produced by thermal ionization of donorlike centers might hop between the localized sites as a result of thermal activation. The applied field lowers the potential barrier between the hopping sites from the initial value of to " !cE , where the coefficient c has the same theoretical value as b . Thus, decreases with increasing .$ applied field. The activation energy was determined from plots of log I versus 1/¹ at various constant voltages by equating the slope to /k. Fig. 4 shows such a plot for the same film. To determine c, was plotted against » which yielded a straight line as shown in Fig. 5. The slope of the straight line gives c"b "4.45;10\ .$ eV V\ m . It is thus seen that the modified theory of Jonscher and Ansari can give a value for the Poole—Frenkel factor b which agrees approx.$ imately with the theoretical value.
276
R.M. Abdel-Latif / Physica B 254 (1998) 273—276
Schottky or Poole—Frenkel mechanisms. Further studies indicate that at an average electric field less than 5.56;10 V m\, the conduction mechanism is Schottky and at higher field it is Jonscher—Ansari modifed Poole—Frenkel. From the X-ray diffraction studies it is found that both freshly evaporated and annealed films are amorphous.
References
Fig. 4. Plot of I versus 1/¹ for the same film as in Fig. 1 at different voltage.
Fig. 5. Plot of the activation energy versus » .
4. Conclusion The vanadium pentoxide thin films show a log IJ» dependence which is indicative of
[1] D. Meyerbofer, S.A. Ochs, J. Appl. Phys. 34 (1963) 2535. [2] J.E. Hartman, J.C. Blair, R. Bauter, J. Appl. Phys. 37 (1966) 2468. [3] M.N. Colpaert, Z. Phys. Chem. Neue Folge 84 (1973) 150. [4] R. Ramirez, B. Casal, L. Utrera, E. Ruiz-Hitzky, J. Phys. Chem. 94 (1990) 8965. [5] A. Legrouri, T. Baird, J.R. Fryer, J. Catal. 140 (1993) 173. [6] I. Kosacki, M. Massot, M. Balkanski, H.L. Tuller, Mater. Sci. Eng. B 12 (1992) 345. [7] Y.S.T. Nomura, H. Tanaka, K. Kobayakawa, J. Electrochem. Soc. 138 (1991) L37. [8] J.R. Akkkride, M. Balkanski, Nato Adv. Stud. Inst. Ser. B (1990) 217. [9] C.R. Aita, Y.-L. Liu, M.L. Kao, S.D. Hansen, J. Appl. Phys. 60 (2) (1986) 749. [10] S.D. Hansen, C.R. Aita, J. Vac. Sci. Technol. A 3 (3) (1985) 660. [11] S.F. Cogan, N.M. Nguyen, S.T. Perrotti, R.D. Rauph, Proc. Soc. Photo-Opt. Instrum. Eng. 57—62 (1988) 1016. [12] G.S. Nadkarni, V.S. Shirodkar, Thin Solid Films 105 (1983) 115. [13] H. Hirashima, M. Ide, T. Yoshida, J. Non-Cryst. Solids 86 (1986) 327. [14] C. Sanchez, R. Morineau, J. Livage, Phys. Stat. Sol. A 76 (1983). [15] C. Sanchez, J. Livage, J. Non-Crystal Solids 65 (1984) 285. [16] V.S. Pankajakshan, K. Neelakandan, C.S. Menon, Thin Solid Films 215 (1992) 196. [17] M. Benmoussa, E. Ibnouelghazi, A. Bennouna, E.L. Ameziane, Thin Solid Films 265 (1995) 22. [18] P.A. Walley, Thin Solid Films 2 (1968) 213. [19] D.K. Jain, J.C. Garg, Indian J. Pure Appl. Phys. 17 (1979) 281. [20] R.D. Gould, J.C. Bowler, Thin Solid Films 164 (1988) 281. [21] J.L. Hartke, J. Appl. Phys. 39 (1968) 4871. [22] J.G. Simmons, Phys. Rev. 155 (1967) 657. [23] A.K. Jonscher, A.A. Ansari, Phil. Mag. 23 (1971) 205.
DC Electrical measurements on evaporated thin films of vanadium pentoxide R.M. Abdel-Latif Physics Department, Faculty of Science, Minia University, Minia, Egypt Received 11 November 1997; accepted 24 April 1998
Abstract The current—voltage characteristics of the Au—V O —Au thin film sandwich system at different temperatures were studied. The DC conduction mechanism was explained using Jonscher—Ansari modified Poole—Frenkel type. Both freshly evaporated and annealed vanadium pentoxide films are amorphous. 1998 Elsevier Science B.V. All rights reserved.
1. Introduction A large number of amorphous-appearing insulator materials are known which in thin film form exhibit a current flow which increases roughly exponentially with the square root of the applied voltage [1,2]. This type of voltage—current characteristic is usually ascribed to the Schottky emission mechanism whereby electrons are emitted from the material electrode into interfacial barrier. Vanadium pentoxide (V O ) crystallizes in a layered structure and has been widely used in a variety of scientific and technological applications. It can be used as a catalyst [3—5], as a cathode for solid-state batteries [6—8], as a window for solar cells [9,10] and for electrochromic devices [11] as well as for electronic and optical switches [12,13]. Various properties of V O including structural, electronic, optical and electrical properties have been investigated. A particular interest has been devoted to the study of the electrical properties
[14—18]. This paper reports an investigation of current transport mechanisms in vanadium pentoxide thin films as a function of applied voltage and temperature.
2. Experimental The evaporation processes were carried out in a high-vacuum evaporation unit (Edwards 306 A) at a pressure of 5;10\ bar. Gold electrode was evaporated from a molybdenum boat onto carefully cleaned glass substrates through suitable masks to form the base electrode. Pure V O powder was evaporated through a mask from molybdenum boat to form the film over the gold eleectrode. The substrate temperature was maintained at room temperature. Then another gold counterelectrode was evaporated to form the metal—insulator—metal (MIM) sandwich of Au—V O —Au. The completed devices had an active area of about 7;10\ m.
0921-4526/98/$ — see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 4 1 1 - 6
274
R.M. Abdel-Latif / Physica B 254 (1998) 273—276
X-ray diffraction carried on both freshly deposited films and annealed at 473 K for 24 h shows amorphous structure. The DC current across the Au—V O —Au sand wich structure was measured as a function of the applied voltage at different temperatures using Keithley 610 electrometer and autoranging power supply model AP6050A. Sample temperatures were varied using a heater and a temperature control unit. Temperature was measured by means of a chromal/alumel thermocouple mounted in close proximity of the sample. Capacitance measurements were made at a frequency 100 kHz using a Stanford Research systems model SR720 LCR meter. Fig. 2. Plot of log I versus » for a film thick 180 nm.
3. Results and discussion Fig. 1 shows the dependence of capacitance C on reciprocal film with thickness 1/d. The linearity of the plot may be analysed in terms of the capacitance of a parallel plate capacitor, the slope of the line being ee A where e is the permittivity of V O film, e the permittivity of free space and A is the film area (7;10\ m). The value of ee estimated from Fig. 1 was found to be 2.25;10\ F m\ (e"2.54) and this value has been used in the analysis of the following results.
Various type of conduction mechanism were considered, in particular space-charge-limited conductivity and the Poole—Frenkel (PF) or Richardson—Schottky effects (RS). The I—» characteristics on a log—log scale did not explicitly show any conduction mechanism and hence the data were replotted on a log I versus » scale. Fig. 2 shows such a plot for a typical film 180 nm thick at different temperatures. It is seen that the plots become linear when the applied field is greater than 5.56;10 V m\. This suggests that the conduction process is either of the Schottky emission at the electrodes or of the Poo"e—Frenkel bulk mechanism. In the Poole—Frenkel mechanism, the conduction is limited by the field-enhanced thermal emission of electrons from a discrete trap level into the conduction band. In the Schottky emission process, the electrons are emitted from the metal electrode into the conduction band of the insulating film over the image force interfacial barrier under careful lowering of the applied electric field. Several dielectric and semiconducting thin films [18,19] exhibit a current—voltage of the form [19,20] IJexp
Fig. 1. Dependence of capacitance C on reciprocal thickness 1/d.
bE , k¹
(1)
where I is the current, E"»/d the applied electric field, d is the thickness of the film, » the applied
R.M. Abdel-Latif / Physica B 254 (1998) 273—276
275
Table 1 Values of b at various temperatures Temperature (°C)
b;10\(eV V\ m )
20 50 75 100 130
7.29 7.51 7.82 8.15 8.45
voltage, k the Boltzman constant, ¹ the absolute temperature and b is given by
e b"b "2b " , (2) .$ 1 pee where b and b are the Schottky and Poole—Fren1 .$ kel field lowering coefficients, respectively, and e is the electronic charge. Values of b calculated from the slope of the linear portions of the plots shown in Fig. 2 are tabulated in Table 1. The value of b calculated from Eq. (2) using e"2.54 is 4.76;10\ eV V\ m . It is seen from Table 1 that the value of b increases with increasing temperature. Furthermore, the experimental value of b is approximately 1.5—1.8 times the theoretical value. The conduction mechanism may be determined from whether b is dependent on the electrode work function (RS) or not (PF). The I—» characteristics were therefore measured using electrodes with different work functions, namely indium and aluminium. However, it was found that b did not depend on the electrode material, which led us to rule out the Richardson—Schottky type, and thus we conclude that the conduction mechanism is Poole—Frenkel type for electric field above 5.56;10 V m\. The high value of b (henceforth b ) cannot be .$ explained using the classical PF equation and we therefore examined the data in terms of the modified PF processes. The three-dimensional PF process gives the equation for the current as [21]
IJ
b b .$!1 exp .$ E , k¹ ak¹
(3)
Fig. 3. Plot of log I/(1.88;10\ E !1) versus E for the same film as in Fig. 2 at 20°C.
where a"1 for the normal PF process and a"2 for the modified PF process [22]. The value of a can be determined from a plot of log[I/+(b /k¹)E !1,] versus E . Fig. 3 .$ shows such a plot for the same film with b "4.76;10\ eV V\ m and ¹ " 293 K. .$ The value of a determined from the slope of the plot is 0.73, indicating that neither of the above theories can account for the experimental value. Jonscher and Ansari [23] have suggested that electrons produced by thermal ionization of donorlike centers might hop between the localized sites as a result of thermal activation. The applied field lowers the potential barrier between the hopping sites from the initial value of to " !cE , where the coefficient c has the same theoretical value as b . Thus, decreases with increasing .$ applied field. The activation energy was determined from plots of log I versus 1/¹ at various constant voltages by equating the slope to /k. Fig. 4 shows such a plot for the same film. To determine c, was plotted against » which yielded a straight line as shown in Fig. 5. The slope of the straight line gives c"b "4.45;10\ .$ eV V\ m . It is thus seen that the modified theory of Jonscher and Ansari can give a value for the Poole—Frenkel factor b which agrees approx.$ imately with the theoretical value.
276
R.M. Abdel-Latif / Physica B 254 (1998) 273—276
Schottky or Poole—Frenkel mechanisms. Further studies indicate that at an average electric field less than 5.56;10 V m\, the conduction mechanism is Schottky and at higher field it is Jonscher—Ansari modifed Poole—Frenkel. From the X-ray diffraction studies it is found that both freshly evaporated and annealed films are amorphous.
References
Fig. 4. Plot of I versus 1/¹ for the same film as in Fig. 1 at different voltage.
Fig. 5. Plot of the activation energy versus » .
4. Conclusion The vanadium pentoxide thin films show a log IJ» dependence which is indicative of
[1] D. Meyerbofer, S.A. Ochs, J. Appl. Phys. 34 (1963) 2535. [2] J.E. Hartman, J.C. Blair, R. Bauter, J. Appl. Phys. 37 (1966) 2468. [3] M.N. Colpaert, Z. Phys. Chem. Neue Folge 84 (1973) 150. [4] R. Ramirez, B. Casal, L. Utrera, E. Ruiz-Hitzky, J. Phys. Chem. 94 (1990) 8965. [5] A. Legrouri, T. Baird, J.R. Fryer, J. Catal. 140 (1993) 173. [6] I. Kosacki, M. Massot, M. Balkanski, H.L. Tuller, Mater. Sci. Eng. B 12 (1992) 345. [7] Y.S.T. Nomura, H. Tanaka, K. Kobayakawa, J. Electrochem. Soc. 138 (1991) L37. [8] J.R. Akkkride, M. Balkanski, Nato Adv. Stud. Inst. Ser. B (1990) 217. [9] C.R. Aita, Y.-L. Liu, M.L. Kao, S.D. Hansen, J. Appl. Phys. 60 (2) (1986) 749. [10] S.D. Hansen, C.R. Aita, J. Vac. Sci. Technol. A 3 (3) (1985) 660. [11] S.F. Cogan, N.M. Nguyen, S.T. Perrotti, R.D. Rauph, Proc. Soc. Photo-Opt. Instrum. Eng. 57—62 (1988) 1016. [12] G.S. Nadkarni, V.S. Shirodkar, Thin Solid Films 105 (1983) 115. [13] H. Hirashima, M. Ide, T. Yoshida, J. Non-Cryst. Solids 86 (1986) 327. [14] C. Sanchez, R. Morineau, J. Livage, Phys. Stat. Sol. A 76 (1983). [15] C. Sanchez, J. Livage, J. Non-Crystal Solids 65 (1984) 285. [16] V.S. Pankajakshan, K. Neelakandan, C.S. Menon, Thin Solid Films 215 (1992) 196. [17] M. Benmoussa, E. Ibnouelghazi, A. Bennouna, E.L. Ameziane, Thin Solid Films 265 (1995) 22. [18] P.A. Walley, Thin Solid Films 2 (1968) 213. [19] D.K. Jain, J.C. Garg, Indian J. Pure Appl. Phys. 17 (1979) 281. [20] R.D. Gould, J.C. Bowler, Thin Solid Films 164 (1988) 281. [21] J.L. Hartke, J. Appl. Phys. 39 (1968) 4871. [22] J.G. Simmons, Phys. Rev. 155 (1967) 657. [23] A.K. Jonscher, A.A. Ansari, Phil. Mag. 23 (1971) 205.