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Switching mechanism in ZnTe films
Thin Solid Films, 113 (1984) 185-188 ELECTRONICS AND OPTICS
185
S W I T C H I N G M E C H A N I S M IN ZnTe F I L M S S. M. PATEL AND N. G. PATEL Department of Physics, Sardar Patel University, Vallabh Vidyanagar 388120. Gujarat (India)
(ReceivedOctober 11, 1983; accepted January 11, 1984)
The memory switching observed in ZnTe films has been analysed using an A1-ZnTe-A1 coplanar device prepared on a glass substrate. The transition from the O F F to the O N state is associated with the formation of filaments with an excess tellurium content and which show metallic behaviour. The transition from the O N to the O F F state has been observed to be due to thermal rupture of the filament resulting in discrete tellurium islands. 1. INTRODUCTION Several electronic and thermal theories have been proposed to explain the switching processes observed in chalcogenide semiconductors 1'2. The validity of these theories for I I - V I compounds such as ZnTe, however, should be checked carefully since ZnTe differs very much in structure from the amorphous chalcogenides and has quite different material constants. The electrical properties of sandwiched thin ZnTe films have been investigated by several researchers. Shirakawa et al. 3 have reported a space-charge-limited current in their A1/ZnTe/A1 sandwich devices. Mufti and Holt 4 have interpreted the electrical conduction in ZnTe films in terms of the Poole-Frenkel or the Schottky mechanism. Burgelman 5 has reported that the most probable mode of conduction in the O F F state of ZnTe thin films is by a modified Poole-Frenkel mechanism. Ota and Takahashi 6 have reported that non-polarized devices show two stable states: the O N state conduction is due to the formation of filaments which have a metallic nature, the transition from the O N to the O F F state is due to thermal rupture of the filaments and the O F F state shows non-ohmic behaviour. All published reports are on metal/ZnTe/metal sandwich devices. A direct visual confirmation of the existence of such filaments in ZnTe thin films during switching has not been reported. Only a coplanar device can be of use in the confirmation of the formation of filaments. Further, the possibility of shorting of the sandwich structure due to pinholes in the test film can also be avoided. Hence a coplanar A1-ZnTe-A1 structure was used in the present investigation. 2. EXPERIMENTALPROCEDURE ZnTe films were prepared by thermal evaporation of pure ZnTe powder (Koch0040-6090/84/$3.00
© ElsevierSequoia/Printed in The Netherlands
186
s.M. PATEL, N. G. PATEL
Light) onto ultrasonically cleaned glass slides at a substrate temperature of 550 K in a vacuum of the order of 10 4 Pa. The rate of evaporation was about 5 6 n m s 1 and the thickness of the film was 120 nm. An aluminium film was deposited onto the ZnTe films to act as electrodes with a separation ranging from 0.1 to 0.5 m m and a constant width of 1.5 mm. The electrode separation was obtained by means of a wire mask. A d.c. bias voltage from a power supply was applied across the electrodes with a suitable current-limiting resistor in series. The c u r r e n t - v o l t a g e [l- V) characteristics were measured with a high impedance electrometer; simultaneously the sample surface was observed with an optical microscope. The films for transmission electron microscope studies could be stripped from the glass substrate by carefully dipping them in distilled water, after the application of a thin backing layer of carbon. The structure before and after switching cycles was also studied with an electron microscope (Philips EM 400 plus an S(t) EM system P W 6585), and energydispersive analysis of X-rays (EDAX) (with an E D A X 9100/60) was used for elemental analysis. 3. RESULTSAND I)IS('USSION Figure 1 shows an electron micrograph and the corresponding electron diffraction pattern for an as-deposited ZnTe thin film before any switching cycle. E D A X revealed that the composition was stoichiometric.
Fig. 1. Electron micrograph and the corresponding electron diffraction pattern of a ZnTe region before any switching cycle. Initially all the A I - Z n T e - A I devices were in the O F F state and showed nonlinear I - V characteristics (Fig. 2, branch ODA). W h e n the voltage exceeds the threshold voltage V~h, the device switches to the O N state (branch OBC) which is ohmic. The device could be switched back to the O F F state by applying a current higher than the threshold current I,a. Since both states are maintained after removal of the bias voltage, these devices behave as a m e m o r y switch. The ratio of the O F F state resistance Roy v to the O N state resistance RaN is about 104. Figure 3 shows the threshold voltage V~h for the O F F - , O N switch as a function of the electrode separation S for the coplanar devices. V,a varies linearly with S and shows no dependence on the electrode area, thus indicating that a constant field strength of the order of 104 V cm 1 is necessary for the transition.
SWITCHING MECHANISMIN ZnTe FILMS
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Fig. 2. I - V characteristics of A1-ZnTe-AIcoplanardevices. Fig. 3. Variationin the threshold voltage Vtthwith the electrode separation S. Observation of the sample surface with an optical microscope revealed that the O F F ~ O N transition is accompanied by the formation of a filament which bridges the electrodes. Figure 4 shows a scanning electron micrograph of a filament after the first O F F ~ O N switching cycle. The elemental composition of the filament was analysed with the help of an electron beam 4 nm in diameter and EDAX, which showed the filament region to contain an excess of tellurium. Similar tellurium bridges have also been observed in As16Tea 3Ge thin film coplanar switching devices by Per~in e t al. 7 The filaments are about 1-2 Ixm in diameter and carry currents of up to about 100 mA. This corresponds to a current density of about 107 A cm -2. Such current densities might normally be expected to give rise to a heating effect. In the filament the temperature rise due to self-heating may be sufficiently high to start the decomposition of the compound. Figure 5 shows that switching back to the O F F state was associated with
Fig. 4. Backscatteredelectron imageof a filament after the first switchingcycle. Fig. 5. Backscatteredelectron imageof ruptured filaments after a few switchingcycles.
188
S.M. PATEI+, N. G. PATEL
rupture of the filament and the formation of tellurium islands resulting from melting followed by rapid cooling and solidification of the ruptured filaments. Figure 6 is an electron micrograph and electron diffraction pattern of the ruptured filament region and shows the presence of single-crystal tellurium together with the polycrystalline ZnTe phase. The formation of tellurium islands in the ruptured filament breaks the continuity between the electrodes and hence the O F F state is regained.
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+,. +o +. ++. +. Fig. 6. Electron micrograph and the corresponding electron diffraction pattern of a ruptured filamen! region.
Repeated switching cycles performed by gradually increasing the electric field show that the O F F - ~ O N transitions are accompanied by the formation of filaments, while the O N ~ O F F transitions are associated with the zupture of these filaments. These phenomena take place only along the filaments without destroying the sample provided that the current is not too high. In addition to the filaments responsible for the O F F ~ O N transition, it is possible that unterminated filaments may also contribute to the conduction in the O N state by a tunnelling process as they are also ruptured as the total current through the device increases. Thus the present investigation provides conclusive evidence for the formation of filamentary paths during the O F F --* O N transition and the rupture of these filaments when the device is switched back to the O F F state. ACKNOWLEDGMENT
The authors would like to thank Dr. G. K. Shivakumar for helpful discussions. REFERENCES I 2 3 4 5 6 7
S.R. Ovshinsky, Phys. Rev. Lett., 21 (1968) 1450. H. Fritzsche, in J. Tauc (ed.), Amorphous and Liquid Semiconductors, Plenum, New York, 1974. T. Shirakawa, A. Hayashi and J. Nakai, Jpn. J. Appl. Phys., 9 (1970) 420. A.R. Mufti and D. B. Holt, Solid-State Electron., 16 (1973) 1214. M. Burgelman, Solid-State Electron., 20 (1977) 523. T. Ota and K. Takahashi, Solid-State Electron., 16 (1973) 1089. M. Per~in, D. Kunstelj, A. Per~in and H. Zorc, Thin Solid Films, 36 ( 1976} 475.
185
S W I T C H I N G M E C H A N I S M IN ZnTe F I L M S S. M. PATEL AND N. G. PATEL Department of Physics, Sardar Patel University, Vallabh Vidyanagar 388120. Gujarat (India)
(ReceivedOctober 11, 1983; accepted January 11, 1984)
The memory switching observed in ZnTe films has been analysed using an A1-ZnTe-A1 coplanar device prepared on a glass substrate. The transition from the O F F to the O N state is associated with the formation of filaments with an excess tellurium content and which show metallic behaviour. The transition from the O N to the O F F state has been observed to be due to thermal rupture of the filament resulting in discrete tellurium islands. 1. INTRODUCTION Several electronic and thermal theories have been proposed to explain the switching processes observed in chalcogenide semiconductors 1'2. The validity of these theories for I I - V I compounds such as ZnTe, however, should be checked carefully since ZnTe differs very much in structure from the amorphous chalcogenides and has quite different material constants. The electrical properties of sandwiched thin ZnTe films have been investigated by several researchers. Shirakawa et al. 3 have reported a space-charge-limited current in their A1/ZnTe/A1 sandwich devices. Mufti and Holt 4 have interpreted the electrical conduction in ZnTe films in terms of the Poole-Frenkel or the Schottky mechanism. Burgelman 5 has reported that the most probable mode of conduction in the O F F state of ZnTe thin films is by a modified Poole-Frenkel mechanism. Ota and Takahashi 6 have reported that non-polarized devices show two stable states: the O N state conduction is due to the formation of filaments which have a metallic nature, the transition from the O N to the O F F state is due to thermal rupture of the filaments and the O F F state shows non-ohmic behaviour. All published reports are on metal/ZnTe/metal sandwich devices. A direct visual confirmation of the existence of such filaments in ZnTe thin films during switching has not been reported. Only a coplanar device can be of use in the confirmation of the formation of filaments. Further, the possibility of shorting of the sandwich structure due to pinholes in the test film can also be avoided. Hence a coplanar A1-ZnTe-A1 structure was used in the present investigation. 2. EXPERIMENTALPROCEDURE ZnTe films were prepared by thermal evaporation of pure ZnTe powder (Koch0040-6090/84/$3.00
© ElsevierSequoia/Printed in The Netherlands
186
s.M. PATEL, N. G. PATEL
Light) onto ultrasonically cleaned glass slides at a substrate temperature of 550 K in a vacuum of the order of 10 4 Pa. The rate of evaporation was about 5 6 n m s 1 and the thickness of the film was 120 nm. An aluminium film was deposited onto the ZnTe films to act as electrodes with a separation ranging from 0.1 to 0.5 m m and a constant width of 1.5 mm. The electrode separation was obtained by means of a wire mask. A d.c. bias voltage from a power supply was applied across the electrodes with a suitable current-limiting resistor in series. The c u r r e n t - v o l t a g e [l- V) characteristics were measured with a high impedance electrometer; simultaneously the sample surface was observed with an optical microscope. The films for transmission electron microscope studies could be stripped from the glass substrate by carefully dipping them in distilled water, after the application of a thin backing layer of carbon. The structure before and after switching cycles was also studied with an electron microscope (Philips EM 400 plus an S(t) EM system P W 6585), and energydispersive analysis of X-rays (EDAX) (with an E D A X 9100/60) was used for elemental analysis. 3. RESULTSAND I)IS('USSION Figure 1 shows an electron micrograph and the corresponding electron diffraction pattern for an as-deposited ZnTe thin film before any switching cycle. E D A X revealed that the composition was stoichiometric.
Fig. 1. Electron micrograph and the corresponding electron diffraction pattern of a ZnTe region before any switching cycle. Initially all the A I - Z n T e - A I devices were in the O F F state and showed nonlinear I - V characteristics (Fig. 2, branch ODA). W h e n the voltage exceeds the threshold voltage V~h, the device switches to the O N state (branch OBC) which is ohmic. The device could be switched back to the O F F state by applying a current higher than the threshold current I,a. Since both states are maintained after removal of the bias voltage, these devices behave as a m e m o r y switch. The ratio of the O F F state resistance Roy v to the O N state resistance RaN is about 104. Figure 3 shows the threshold voltage V~h for the O F F - , O N switch as a function of the electrode separation S for the coplanar devices. V,a varies linearly with S and shows no dependence on the electrode area, thus indicating that a constant field strength of the order of 104 V cm 1 is necessary for the transition.
SWITCHING MECHANISMIN ZnTe FILMS
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Fig. 2. I - V characteristics of A1-ZnTe-AIcoplanardevices. Fig. 3. Variationin the threshold voltage Vtthwith the electrode separation S. Observation of the sample surface with an optical microscope revealed that the O F F ~ O N transition is accompanied by the formation of a filament which bridges the electrodes. Figure 4 shows a scanning electron micrograph of a filament after the first O F F ~ O N switching cycle. The elemental composition of the filament was analysed with the help of an electron beam 4 nm in diameter and EDAX, which showed the filament region to contain an excess of tellurium. Similar tellurium bridges have also been observed in As16Tea 3Ge thin film coplanar switching devices by Per~in e t al. 7 The filaments are about 1-2 Ixm in diameter and carry currents of up to about 100 mA. This corresponds to a current density of about 107 A cm -2. Such current densities might normally be expected to give rise to a heating effect. In the filament the temperature rise due to self-heating may be sufficiently high to start the decomposition of the compound. Figure 5 shows that switching back to the O F F state was associated with
Fig. 4. Backscatteredelectron imageof a filament after the first switchingcycle. Fig. 5. Backscatteredelectron imageof ruptured filaments after a few switchingcycles.
188
S.M. PATEI+, N. G. PATEL
rupture of the filament and the formation of tellurium islands resulting from melting followed by rapid cooling and solidification of the ruptured filaments. Figure 6 is an electron micrograph and electron diffraction pattern of the ruptured filament region and shows the presence of single-crystal tellurium together with the polycrystalline ZnTe phase. The formation of tellurium islands in the ruptured filament breaks the continuity between the electrodes and hence the O F F state is regained.
•
•
+,. +o +. ++. +. Fig. 6. Electron micrograph and the corresponding electron diffraction pattern of a ruptured filamen! region.
Repeated switching cycles performed by gradually increasing the electric field show that the O F F - ~ O N transitions are accompanied by the formation of filaments, while the O N ~ O F F transitions are associated with the zupture of these filaments. These phenomena take place only along the filaments without destroying the sample provided that the current is not too high. In addition to the filaments responsible for the O F F ~ O N transition, it is possible that unterminated filaments may also contribute to the conduction in the O N state by a tunnelling process as they are also ruptured as the total current through the device increases. Thus the present investigation provides conclusive evidence for the formation of filamentary paths during the O F F --* O N transition and the rupture of these filaments when the device is switched back to the O F F state. ACKNOWLEDGMENT
The authors would like to thank Dr. G. K. Shivakumar for helpful discussions. REFERENCES I 2 3 4 5 6 7
S.R. Ovshinsky, Phys. Rev. Lett., 21 (1968) 1450. H. Fritzsche, in J. Tauc (ed.), Amorphous and Liquid Semiconductors, Plenum, New York, 1974. T. Shirakawa, A. Hayashi and J. Nakai, Jpn. J. Appl. Phys., 9 (1970) 420. A.R. Mufti and D. B. Holt, Solid-State Electron., 16 (1973) 1214. M. Burgelman, Solid-State Electron., 20 (1977) 523. T. Ota and K. Takahashi, Solid-State Electron., 16 (1973) 1089. M. Per~in, D. Kunstelj, A. Per~in and H. Zorc, Thin Solid Films, 36 ( 1976} 475.