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Switching behaviour of plasma-polymerized thin polyfuran films
Thin Solid Films, 164 (1988) 353-356
353 SWITCHING BEHAVIOUR OF PLASMA-POLYMERIZED THIN P O L Y F U R A N FILMS* P. K. ABRAHAM AND K. SATHIANANDAN
Department of Physics, Cochin University of Science and Technology, Cochin 682022 (India)
Thin polyfuran films are prepared by low frequency plasma polymerization. Bistable memory switching is observed in these films even though chemically and electrochemically prepared films are reported to be conducting. Dependence of switching on the temperature in the range 298-373 K is investigated. A careful study of the dependence of the switching on the thickness of the films demonstrates that films of thicknesses between 700 and 1500/~ exhibit switching behaviour. A threshold field of 2.12 x 106 V c m - 1 is required for switching. The activation energy is found to be 1.55 eV. The high activation energy and temperature-dependent threshold voltage do not support a purely electronic mechanism. From the investigations, it is concluded that the Poole-Frenkel effect followed by the formation of conducting channels is responsible for the switching behaviour. 1. INTRODUCTION Bistable memory switching and monostable threshold switching using thin film devices have been subjects of interest in recent years. The motivation for these studies is the use of thin film memory elements in place of sophisticated transistor devices. In fact, an application in the case of computer memory is reported by Tauc 1. According to Ovshinsky 2, memory switching requires two stable states at zero applied bias, one of them a highly conductive O N state and the other a poorly conductive O F F state. Switching is reported in a wide variety of materials such as inorganic chalcogenide glasses 3, organic materials 4, pure elements s, polymeric materials such as polymethyl methacrylate 6, polyacrylonitrile 7 and polystyrene 8, and heterostructure junctions 9. Both amorphous and crystalline materials exhibit switching. The phenomenon is explained by a number of mechanisms such as thermal ~o, electrothermal11, phase change 2 and electronic9'11-14 Plasma polymerization being an easy method for preparing good quality uniform thin films, it was felt that it was important to undertake a detailed study of the switching behaviour of these films. We have succeeded in observing switching in a few plasma-polymerized organic compounds. We report here the detailed observations of plasma-polymerized thin polyfuran films.
* Paper presentedat the 7th International Conferenceon Thin Films, New Delhi, India, December7-11, 1987. 0040-6090/88/$3.50
© ElsevierSequoia/Printedin The Netherlands
354 2.
P.K.
A B R A H A M , K. S A T H I A N A N D A N
E X P E R I M E N T A L DETAILS
A thoroughly cleaned glass substrate is suitably masked and a silver electrode of thickness 1500~ is evaporated onto it to form the b o t t o m electrode. The electrode-coated substrate is now placed in the plasma polymerization chamber. The discharge electrodes are kept at a distance of 3 cm. Polymerization is carried out at a constant m o n o m e r pressure of 0.6 Torr with a current density of 0.01 mA cm-2. The silver counterelectrode is evaporated to form 1 cm 2 of a m e t a l - p o l y m e r - m e t a l sandwich structure. The film thicknesses used for the investigations varied from 700 to 1500~. The sample is kept in a metal chamber with electrical connections, evacuated to 1 0 - 2 T o r r and annealed at 100°C, before any measurements are obtained. The current-voltage characteristic is plotted on an X - Y c h a r t recorder. The voltage is linearly increased at a rate of 0.5 V m i n - 1 by using a sweep voltage generator. A series resistance Rs (100 f~) is connected to the film to limit the current through the film. 3.
RESULTS AND DISCUSSION
M e m o r y switching is observed in these polyfuran films even though chemically and electrochemically prepared samples are reported to be conducting 15. Silver is found to be a good ohmic contact to the film for lower fields (less than 1.5 x 1 0 3 V cm - 1). For fresh films a forming process is found to occur as described by Chopra 16 and Simmons 17. On increasing the sweep voltage at a rate of 0.5 V min -~, the current rose from 10- 9 to 10- 3 A as shown in Fig. 1. The first current rise is always at a higher threshold voltage but, for the subsequent switching events, a lower threshold voltage is found to be sufficient. The switched film showed a non-volatile memory at zero bias. Initially the impedance of the film was high ( O F F state) and after switching the resistance became few tens of ohms (ON state). The two impedance states are stable for m a n y days. A change from the O N state to the O F F state takes place on the application of a pulse voltage of sufficient power. Thus the breakdown is regenerative and O N - O F F commutation is possible several hundred times. The films showed switching in air also. The variation in switching threshold voltage for different thicknesses from 700 to 1500 ~ is illustrated in Fig. 2. A threshold field of2.12 x 1 0 6 V cm - 1 is required for
]
I
1
2
3
4
L
5
VOLTAGE
Fig. l. Current-voltage characteristic of silver/polyfuran/silversandwich structure.
355
SWITCHING BEHAVIOUR OF POLYFURAN FILMS
the switching behaviour. For lower thicknesses (less than 700 A) the films are found to short easily. The dependence of switching on the temperature in the range 298373 K is presented in Fig. 3. The threshold voltage of the film is found to decrease on increasing the temperature. The film is found to switch up to a temperature of 383 K. A positive temperature coefficient of resistance (TCR) of 5 x 10 -4 K - ~ in the O F F state is obtained for the formed film. The T C R of the film in the O N state is found to be negligibly small compared with that of silver, which is the electrode material. Hence the formation of a metallic bridge is ruled out.
o2 r-
I
800
I I 7200 7600 TH ICKNESS ( ~ )
Fig. 2. Threshold voltage Fig. 3. Threshold voltage
vs. vs.
[ 2000
I 29.3
I I ,353 575 TEMPERATURE ( K )
I 4t5
thickness of polyfuran film. temperature of polyfuran films of thickness 1200 ~, (O) and 1536/~. (0).
For a polymer, according to Segui et al. 8, a transition from an a m o r p h o u s state to a more long-range-ordered state would never give the conductivity reported here. Hence an order-disorder mechanism is not a favoured process for the observed switching in this polymer. F r o m the analysis of the I - V characteristic we find that ohmic conduction takes place for lower fields. The In I vs. V 1/2 plot for the fields in the range from 5.74 x 1 0 3 to 1.15 x 105 V cm -1 yields a straight line with a slope of 1.6. This suggests that either Poole-Frenkel or Schottky conduction takes place. Using electrodes of dissimilar work functions, i.e. with the silver-polymer-aluminium configuration, the I - V characteristic is symmetric for both of the polarities. Hence the conduction mechanism is a bulk phenomenon and is Poole-Frenkel. The trap density would be high as a result of the a m o r p h o u s nature of the polymer which leads to such a mechanism. Reviews relating to switching in a m o r p h o u s materials have been given by Drake 18, Tauc 1 and Ray and Hogarth ~9. Thermally assisted detrapping could take place. However, the temperature-dependent threshold voltage and the high activation energy rule out a purely electronic mechanism for switching. The observed temperature dependence of Vth is characteristic of a thermoelectronic mechanism 2° in which conducting channels are formed. This is supported by the fact that a change in electrode area does not affect the threshold voltage. The observed variations in the O N state resistance for different numbers of switching cycles can be attributed to the variations in number and size of the conducting channels taking part. The application of a suitable pulse causes a sufficient temperature rise to rupture the conducting channels, resulting in the device's reverting to the high impedance state.
356
P . K . ABRAHAM, K. SATHIANANDAN
4. CONCLUSION Plasma-polymerized polyfuran films showed memory switching. The effects of temperature and thickness on these films were investigated. From the observations, a purely electronic mechanism is ruled out. A Poole-Frenkel mechanism followed by the formation of conducting channels at a threshold field of 2.12 x 106 V c m - 1 is the probable mechanism of the switching behaviour. REFERENCES 1 2 3 4
J. Tauc, Phys. Today, 29 (1976) 23. S.R. Ovshinsky, Phys. Rev. Lett., 21 (1968) 1450. R.G. Neale, D. L. Nelson and G. E. Moore, Electronics, 43 (1970) 56. J. Swiatek, Thin Solid Films, 61 (1979) 321.
5 G. JonesandR. A. Collins, ThinSolidFilms, 40(1977) L15. 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
A. Sa Neto, M. Octavio, R. C. Callarotti, P. E. Schmidt and P. Esqueda, J. Appl. Phys., 51 (1980) 3827. J.A. Mayers, J. Non-Cryst. Solids, 79 (1986) 57. Y. Segui, Bui Ai and H. Carchano, J. Appl. Phys., 47 (1976) 140. H.J. Hovel, Appl. Phys. Lett., 17 (1970) 141. G.C. Vezzoli, P. J. Walsh and L. W. Doremus, J. Non-Cryst. Solids, 18 (1975) 333. J. Feinleib, J. de Neufville, S. C. Moss and S. R. Ovshinsky, Appl. Phys. Left., 18 (1971) 254. J.G. Simmons, Phys. Rev., 166 (1968) 1912. K.G. Peterson and D. Adler, J. Appl. Phys., 47 (1976) 256. H.J. Hovel and J. J. Urgell, J. Appl. Phys., 42 (1971) 5076. K. Yoshino, S. Hayashi and R. Sugimoto, Jpn. J. Appl. Phys., 23 (1984) L899. K.L. Chopra, J. Appl. Phys., 36 (1965) 184.
J.G. Simmons, J. Phys. D, 4(1971)613. C.F. Drake, Thin Solid Films, 50(1978) 125. A . K . RayandC. A. Hogarth, lnt. J. Electron.,57(1984) l. J. Swiatek, Thin Solid Films, 41 (1977) 5.
353 SWITCHING BEHAVIOUR OF PLASMA-POLYMERIZED THIN P O L Y F U R A N FILMS* P. K. ABRAHAM AND K. SATHIANANDAN
Department of Physics, Cochin University of Science and Technology, Cochin 682022 (India)
Thin polyfuran films are prepared by low frequency plasma polymerization. Bistable memory switching is observed in these films even though chemically and electrochemically prepared films are reported to be conducting. Dependence of switching on the temperature in the range 298-373 K is investigated. A careful study of the dependence of the switching on the thickness of the films demonstrates that films of thicknesses between 700 and 1500/~ exhibit switching behaviour. A threshold field of 2.12 x 106 V c m - 1 is required for switching. The activation energy is found to be 1.55 eV. The high activation energy and temperature-dependent threshold voltage do not support a purely electronic mechanism. From the investigations, it is concluded that the Poole-Frenkel effect followed by the formation of conducting channels is responsible for the switching behaviour. 1. INTRODUCTION Bistable memory switching and monostable threshold switching using thin film devices have been subjects of interest in recent years. The motivation for these studies is the use of thin film memory elements in place of sophisticated transistor devices. In fact, an application in the case of computer memory is reported by Tauc 1. According to Ovshinsky 2, memory switching requires two stable states at zero applied bias, one of them a highly conductive O N state and the other a poorly conductive O F F state. Switching is reported in a wide variety of materials such as inorganic chalcogenide glasses 3, organic materials 4, pure elements s, polymeric materials such as polymethyl methacrylate 6, polyacrylonitrile 7 and polystyrene 8, and heterostructure junctions 9. Both amorphous and crystalline materials exhibit switching. The phenomenon is explained by a number of mechanisms such as thermal ~o, electrothermal11, phase change 2 and electronic9'11-14 Plasma polymerization being an easy method for preparing good quality uniform thin films, it was felt that it was important to undertake a detailed study of the switching behaviour of these films. We have succeeded in observing switching in a few plasma-polymerized organic compounds. We report here the detailed observations of plasma-polymerized thin polyfuran films.
* Paper presentedat the 7th International Conferenceon Thin Films, New Delhi, India, December7-11, 1987. 0040-6090/88/$3.50
© ElsevierSequoia/Printedin The Netherlands
354 2.
P.K.
A B R A H A M , K. S A T H I A N A N D A N
E X P E R I M E N T A L DETAILS
A thoroughly cleaned glass substrate is suitably masked and a silver electrode of thickness 1500~ is evaporated onto it to form the b o t t o m electrode. The electrode-coated substrate is now placed in the plasma polymerization chamber. The discharge electrodes are kept at a distance of 3 cm. Polymerization is carried out at a constant m o n o m e r pressure of 0.6 Torr with a current density of 0.01 mA cm-2. The silver counterelectrode is evaporated to form 1 cm 2 of a m e t a l - p o l y m e r - m e t a l sandwich structure. The film thicknesses used for the investigations varied from 700 to 1500~. The sample is kept in a metal chamber with electrical connections, evacuated to 1 0 - 2 T o r r and annealed at 100°C, before any measurements are obtained. The current-voltage characteristic is plotted on an X - Y c h a r t recorder. The voltage is linearly increased at a rate of 0.5 V m i n - 1 by using a sweep voltage generator. A series resistance Rs (100 f~) is connected to the film to limit the current through the film. 3.
RESULTS AND DISCUSSION
M e m o r y switching is observed in these polyfuran films even though chemically and electrochemically prepared samples are reported to be conducting 15. Silver is found to be a good ohmic contact to the film for lower fields (less than 1.5 x 1 0 3 V cm - 1). For fresh films a forming process is found to occur as described by Chopra 16 and Simmons 17. On increasing the sweep voltage at a rate of 0.5 V min -~, the current rose from 10- 9 to 10- 3 A as shown in Fig. 1. The first current rise is always at a higher threshold voltage but, for the subsequent switching events, a lower threshold voltage is found to be sufficient. The switched film showed a non-volatile memory at zero bias. Initially the impedance of the film was high ( O F F state) and after switching the resistance became few tens of ohms (ON state). The two impedance states are stable for m a n y days. A change from the O N state to the O F F state takes place on the application of a pulse voltage of sufficient power. Thus the breakdown is regenerative and O N - O F F commutation is possible several hundred times. The films showed switching in air also. The variation in switching threshold voltage for different thicknesses from 700 to 1500 ~ is illustrated in Fig. 2. A threshold field of2.12 x 1 0 6 V cm - 1 is required for
]
I
1
2
3
4
L
5
VOLTAGE
Fig. l. Current-voltage characteristic of silver/polyfuran/silversandwich structure.
355
SWITCHING BEHAVIOUR OF POLYFURAN FILMS
the switching behaviour. For lower thicknesses (less than 700 A) the films are found to short easily. The dependence of switching on the temperature in the range 298373 K is presented in Fig. 3. The threshold voltage of the film is found to decrease on increasing the temperature. The film is found to switch up to a temperature of 383 K. A positive temperature coefficient of resistance (TCR) of 5 x 10 -4 K - ~ in the O F F state is obtained for the formed film. The T C R of the film in the O N state is found to be negligibly small compared with that of silver, which is the electrode material. Hence the formation of a metallic bridge is ruled out.
o2 r-
I
800
I I 7200 7600 TH ICKNESS ( ~ )
Fig. 2. Threshold voltage Fig. 3. Threshold voltage
vs. vs.
[ 2000
I 29.3
I I ,353 575 TEMPERATURE ( K )
I 4t5
thickness of polyfuran film. temperature of polyfuran films of thickness 1200 ~, (O) and 1536/~. (0).
For a polymer, according to Segui et al. 8, a transition from an a m o r p h o u s state to a more long-range-ordered state would never give the conductivity reported here. Hence an order-disorder mechanism is not a favoured process for the observed switching in this polymer. F r o m the analysis of the I - V characteristic we find that ohmic conduction takes place for lower fields. The In I vs. V 1/2 plot for the fields in the range from 5.74 x 1 0 3 to 1.15 x 105 V cm -1 yields a straight line with a slope of 1.6. This suggests that either Poole-Frenkel or Schottky conduction takes place. Using electrodes of dissimilar work functions, i.e. with the silver-polymer-aluminium configuration, the I - V characteristic is symmetric for both of the polarities. Hence the conduction mechanism is a bulk phenomenon and is Poole-Frenkel. The trap density would be high as a result of the a m o r p h o u s nature of the polymer which leads to such a mechanism. Reviews relating to switching in a m o r p h o u s materials have been given by Drake 18, Tauc 1 and Ray and Hogarth ~9. Thermally assisted detrapping could take place. However, the temperature-dependent threshold voltage and the high activation energy rule out a purely electronic mechanism for switching. The observed temperature dependence of Vth is characteristic of a thermoelectronic mechanism 2° in which conducting channels are formed. This is supported by the fact that a change in electrode area does not affect the threshold voltage. The observed variations in the O N state resistance for different numbers of switching cycles can be attributed to the variations in number and size of the conducting channels taking part. The application of a suitable pulse causes a sufficient temperature rise to rupture the conducting channels, resulting in the device's reverting to the high impedance state.
356
P . K . ABRAHAM, K. SATHIANANDAN
4. CONCLUSION Plasma-polymerized polyfuran films showed memory switching. The effects of temperature and thickness on these films were investigated. From the observations, a purely electronic mechanism is ruled out. A Poole-Frenkel mechanism followed by the formation of conducting channels at a threshold field of 2.12 x 106 V c m - 1 is the probable mechanism of the switching behaviour. REFERENCES 1 2 3 4
J. Tauc, Phys. Today, 29 (1976) 23. S.R. Ovshinsky, Phys. Rev. Lett., 21 (1968) 1450. R.G. Neale, D. L. Nelson and G. E. Moore, Electronics, 43 (1970) 56. J. Swiatek, Thin Solid Films, 61 (1979) 321.
5 G. JonesandR. A. Collins, ThinSolidFilms, 40(1977) L15. 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
A. Sa Neto, M. Octavio, R. C. Callarotti, P. E. Schmidt and P. Esqueda, J. Appl. Phys., 51 (1980) 3827. J.A. Mayers, J. Non-Cryst. Solids, 79 (1986) 57. Y. Segui, Bui Ai and H. Carchano, J. Appl. Phys., 47 (1976) 140. H.J. Hovel, Appl. Phys. Lett., 17 (1970) 141. G.C. Vezzoli, P. J. Walsh and L. W. Doremus, J. Non-Cryst. Solids, 18 (1975) 333. J. Feinleib, J. de Neufville, S. C. Moss and S. R. Ovshinsky, Appl. Phys. Left., 18 (1971) 254. J.G. Simmons, Phys. Rev., 166 (1968) 1912. K.G. Peterson and D. Adler, J. Appl. Phys., 47 (1976) 256. H.J. Hovel and J. J. Urgell, J. Appl. Phys., 42 (1971) 5076. K. Yoshino, S. Hayashi and R. Sugimoto, Jpn. J. Appl. Phys., 23 (1984) L899. K.L. Chopra, J. Appl. Phys., 36 (1965) 184.
J.G. Simmons, J. Phys. D, 4(1971)613. C.F. Drake, Thin Solid Films, 50(1978) 125. A . K . RayandC. A. Hogarth, lnt. J. Electron.,57(1984) l. J. Swiatek, Thin Solid Films, 41 (1977) 5.