- Email: [email protected]
Electrical switching behavior of amorphous Al23Te77 thin film sample
LETTER TO THE EDITOR Journal of Non-Crystalline Solids 356 (2010) 2203–2206
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Letter to the Editor
Electrical switching behavior of amorphous Al23Te77 thin film sample Chandasree Das, R. Lokesh, G. Mohan Rao, S. Asokan ⁎ Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore 560012, India
a r t i c l e
i n f o
Article history: Received 8 April 2010 Received in revised form 28 July 2010 Available online 26 August 2010 Keywords: Electrical switching; Semiconductors; Thin films
a b s t r a c t The electrical switching behavior of amorphous Al23Te77 thin film devices, deposited by flash evaporation, has been studied in co-planar geometry. It is found that these samples exhibit memory type electrical switching. Scanning Electron Microscopic studies show the formation of a crystalline filament in the electrode region which is responsible for switching of the device from high resistance OFF state to low resistance ON state. It is also found that the switching behavior of thin film Al–Te samples is similar to that of bulk samples, with the threshold fields of bulk samples being higher. This has been understood on the basis of higher thermal conductance in bulk, which reduces the Joule heating and temperature rise in the electrode region. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the group of amorphous solids, semiconducting chalcogenides are interesting due to their applications in optoelectronics [1], infrared optical fibers [2], solar cells [3], optical recording systems [4], Phase Change Memories (PCM), etc. The absence of long-range order in these materials provides the convenience of changing the elemental ratio and hence the properties over a wide range. One of the most interesting as well as remarkable properties of chalcogenides which makes them suitable materials for phase change memories (PCM) is electrical switching, first observed by Ovshinsky nearly four decades ago [5]. Chalcogenide glass based phase change memories are recently being considered as a possible replacement for conventional Non Volatile Random Access memories (NVRAMs). Phase change memories make use of chalcogenide glasses of memory switching type. The main advantages of PCM are their direct write/ over write capability, lower volume operation, write /erase cycle, and easiness to integrate with logic [6,7]. The electrical switching in chalcogenide glasses occurs when an appropriate voltage, known as the threshold voltage (VT), is applied and the glass switches to a high conducting ON state from a low conducting OFF state. This phenomenon can be divided into two general categories [8]: i) Threshold switching, in which continuous electrical power is required to maintain the high conducting ON state and ii) Memory switching, in which the ON state can be maintained without electrical power. Threshold switching is known to be electronic in nature; however, additional thermal effects come into play in memory materials, which ⁎ Corresponding author. E-mail address: [email protected] (S. Asokan). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.07.057
result in the formation of a conducting crystalline channel in the electrode region [9]. Several tellurium based binary amorphous semiconductors such as As–Te, Ge–Te, Si–Te, In–Te and Ga–Te have been found to exhibit memory type of switching in their bulk form, at electrical fields of the order of 105 V/cm [10–17]. Ge–Te–Cu, Ge–Te–Ag, and Ge–As–Te are few examples of ternary tellurium based glasses which exhibit memory switching [18,19]. Apart from that, studies also have been done on tellurium based glasses like As–Te [20], Ge–Te [21] to know the composition dependence of crystallization and the effect of metallic doping on these glasses. Earlier investigations indicate that bulk, glassy AlxTe100 − x samples [22], exhibit memory type electrical switching, though no such work has been found in literature on AlxTe100 − x samples in amorphous thin film form. In this work, amorphous thin films of Al23Te77 have been deposited by flash evaporation method and the I–V characteristics and electrical switching behavior of the films have been studied along with surface morphology and composition. The reasons behind choosing this particular stoichiometry are: a) It can be prepared easily in bulk glass which can be used as a base material for thermal evaporation. b) It has shown a clean memory switching in the bulk glassy form. c) The earlier investigations have indicated that the x = 23 composition in the AlxTe100 − x system corresponds to the rigidity percolation threshold [23] and interesting properties have been observed in chalcogenide glasses at compositions corresponding to the rigidity percolation threshold [24,25]. 2. Experimental techniques The base–glassy semiconductor material with composition Al23Te77 has been prepared by melt quenching technique. Constituent elements (99.999% pure) are weighed to± 0.1 mg accuracy and sealed in a flat
LETTER TO THE EDITOR C. Das et al. / Journal of Non-Crystalline Solids 356 (2010) 2203–2206
bottom quartz ampoule under a vacuum of 10−5 mbar. The sealed ampoule is heated in a horizontal rotary furnace at about 1000 °C at the rate of 100 °C/h. The ampoule containing the melt is continuously rotated for nearly 36 h at 10 RPM to ensure homogeneity of the melt. The molten sample is subsequently quenched in ice water and sodium hydroxide (NaOH) mixture to get bulk glassy sample. The ingots of the sample are taken out by breaking the ampoules. These ingots are then ground into a fine powder for use as evaporation source material. Detergent solution, acetone and ultrasonic cleaning clean the 25 mm× 75 mm glass slides used as substrates. Flash evaporation technique has been used to prepare thin films. A mechanical mask has been used to obtain the required structure of the device. One extra slide is also coated for structural and other characterization. The thickness of the films measured by stylus profilometer (Form Taly-surf plus) has been found to be in the range of about 150 nm, for about 5 min deposition time. Aluminum electrodes have been deposited on the film in a co-planar geometry, using an appropriate mask. First, the chalcogenide film is coated in a rectangular shape on a glass slide. Secondly, aluminum is coated as the electrode, with a thin mask wrapped on the film. Subsequently, the electrode mask is removed. The active area of the device is the film area between two aluminum electrodes. In this work, the co-planar geometry has been deliberately chosen in order to compare the electrical switching characteristics of bulk glassy and amorphous thin film samples. Nevertheless, the sandwich geometry can be used to deposit the electrodes which would bring down the switching voltages of the devices. During all the depositions, the pressure inside the chamber is maintained at 10−5 mbar. The electrical contacts from the aluminum electrodes have been made using silver paste and copper wire. The switching behavior of the sample is studied by recording the current–voltage (I–V) characteristics using a Keithley Source-Meter (Model 2410) with a basic accuracy of ± 0.012%, controlled by Lab View 6i (National Instruments). The sourcing capacity of the source meter is in the range of 0–20 mA at a compliance voltage of 1100 V (maximum). A constant current is passed through the sample and the voltage across the sample is recorded. Scanning electron microscopy, optical microscopy and energy dispersive X-ray analysis have also been done on the sample for measuring the distance between the electrodes, surface morphology and chemical composition of the film respectively.
3. Results and discussion Fig. 1 shows the X-ray diffraction pattern of the as-deposited Al23Te77 film from which it is evident that it is amorphous in nature. Table 1 gives the composition of the different constituent elements in the as-deposited films obtained by Energy Dispersive X-ray Analysis (EDAX). It can be seen that the composition of the film is close to the composition of starting material, Al23Te77. The resistance of the asdeposited devices has been found to be in Mega Ohm range. Fig. 2 (a) and (b) shows the typical I–V characteristics of two representatives Al23Te77 thin film devices, which have been deposited in co-planar geometry with aluminum as electrode. It can be seen from these figures that the voltage across the devices initially increases linearly with current, indicating an Ohmic behavior. After a particular threshold voltage, the voltage across the sample starts decreasing with current, which indicates a negative resistance behavior. The negative resistance region leads to a low resistance state, which latches the device in that state. The device remains in low resistance state even if the current reduces to zero, which indicates and shows that amorphous Al23Te77 films exhibit memory type electrical switching. The insets in figures in Fig. 2(a) and (b) show the I–V characteristics around the threshold voltage, which reveal in a detailed manner, the negative resistance and the switching regions.
70 60 50
Intensity
2204
40 30 20 10 0 0
10
20
30
40
50
60
70
80
90
2θ Fig. 1. X-ray diffraction pattern of the as-deposited Al23Te77 film.
The insets also show the errors involved in the voltages which are not otherwise clearly visible in the I–V characteristics. As mentioned earlier, the application of a high electric field leads to the formation of a conducting crystalline filament during memory switching, because of which the sample remains in the low resistance ON state even after the removal of the field. The memory switching behavior of telluride glasses arises because of two general factors: The local structure of these samples is characterized by long Te chains in which atomic rearrangements and devitrification can occur easily [26]. Glassy tellurides also carry a higher electrical conductance in comparison with other chalcogenides like sulphides and selenides. The larger Joule heating due to higher conductance and easy crystallizability, lead to memory switching in telluride glasses such as Al23Te77. Fig. 3 (a) and (b) shows the scanning electron micrographs of unswitched and switched devices respectively. It is clearly seen that the un-switched film is featureless with a smooth topography. On the other hand, the phase change in the electrode region is revealed by an image contrast in the SEM picture; the image contrast in SEM after switching can be due to any of the following aspects: a) The phenomenon of electrical switching in chalcogenides, especially memory, is associated with local melting and re-solidification into the crystalline state of the active material. This process can lead to a surface relief which can give rise to an image contrast. b) The glass to crystal phase change is also associated with local structural changes and densification in the electrode region as the crystalline phase is denser than the corresponding amorphous phase. The density changes in the switched region can contribute for the image contrast. c) There is a large change in the electrical resistance of the active material in the electrode region (about three orders of magnitude). The conductivity changes can also lead to an image contrast in SEM. Detailed investigations are necessary to comprehend which of the above mentioned aspects is responsible for the observed changes in the SEM images of the switched sample. However, it can be noted here that a similar SEM image have been obtained earlier during the SET process of the device which has been attributed to the change of state of the material from amorphous to crystalline [11]. Table 1 Atomic percentage of Al23Te77 film obtained by EDAX analysis. Element
Atomic % in film
Al Te
22.54 77.46
LETTER TO THE EDITOR C. Das et al. / Journal of Non-Crystalline Solids 356 (2010) 2203–2206
Earlier studies on bulk Al–Te glasses of different compositions reveal that these samples exhibit memory type switching [22]. Fig. 4 shows the I–V characteristic of bulk Al23Te77 sample of thickness of 0.2 mm [22] and in ambient conditions, which indicates that the switching behavior of bulk and thin film Al–Te samples is similar. Table 2 shows the comparison of switching fields of the two representative thin film devices and the bulk Al23Te77 glass [22]. It can be seen that the switching fields of the two devices are comparable to each other. The difference in the switching fields of two films is 0.4 kV/cm, which is due to the difference in separation between two electrodes. However, the switching field of the bulk glass is much higher than that of the thin film samples. In a bulk sample, the heat is likely to be dissipated away faster from the electrode region to the larger thermal mass surrounding it. Consequently the Joule heating and temperature rise in the electrode region for a given applied voltage will be lower for the bulk, requiring a larger voltage for switching. Hence the threshold field for the bulk sample is higher than that of the thin film samples. 4. Conclusions Amorphous thin films of Al23Te77 have been deposited by flash evaporation method. The amorphous nature and the composition of films are confirmed by X-ray diffraction and EDAX respectively. The I– V characteristics and electrical switching behavior of the films have been studied using a Keithley source-meter. It is found that thin film
Fig. 3. Scanning electron micrograph of (a) un-switched and (b) switched Al23Te77 thin film samples respectively.
amorphous Al23Te77 samples exhibit memory type electrical switching. The image contrast in the switched region of the Scanning Electron Micrograph shows the phase change (formation of a crystalline filament), which is responsible for the switching of the device from high resistance OFF state to low resistance ON state.
2.5
2.0
Current (mA)
Fig. 2. (a) and (b). I–V characteristics of two representative Al23Te77 thin film devices, exhibiting memory switching. The insets in figures in Fig. 2(a) and (b) show the I–V characteristics around the threshold voltage, which reveal in a detailed manner, the negative resistance and the switching regions.
2205
Al23Te77 1.5
1.0
0.5
0.0 0
20
40
60
80
100
120
140
voltage (V) Fig. 4. I–V characteristics of bulk Al23Te77 glass showing memory switching [22].
LETTER TO THE EDITOR 2206
C. Das et al. / Journal of Non-Crystalline Solids 356 (2010) 2203–2206
Table 2 Comparison of switching fields of two representative devices and the bulk. Device
Switching voltage (V) in volt
Distance (d) between two electrodes in micron
Electric field kV/cm
1 2 Bulk
276 ± 2 195 ± 2 135 ± 2
976.8 ± 5 789 ± 5 200 ± 5
2.8 2.4 6.75
Further, the switching behavior of bulk and thin film Al23Te77 samples are found to be similar, with both types of samples exhibiting memory switching. The threshold fields of bulk samples are much higher compared to those of the thin film samples, which can be understood based on the higher thermal conductance in bulk samples which reduces the Joule heating and temperature rise in the electrode region. Acknowledgements Authors are thankful to Kesab Barai of Nanocentre for Scanning Electron Microscopy and also to Mr. Shamasundar H.S for helping to measure thickness by Profilometer. References [1] P. Sharma, M. Vashistha, I.P. Jain, Chalc.Lett. 2 (2005) 113. [2] H.A. Abd El Ghani, M.M. Abd El raham, M.M. Wakkad, A. Abosehli, N. Assraan, Physica B 381 (2006) 156. [3] J. Fusong, M. Okuda, Jpn. J. Appl. Phys. 30 (1991) 07.
[4] M.A. Majeed Khan, M. Zulfequar, S. Kumar, M. Husssain, J. Mod. Opt. 50 (2003) 251. [5] S.R. Ovshinsky, Phys.Rev.lett. 21 (1968) 1450. [6] Martin H.R. Lankhorst, Bas W.M.M. Ketelaars, R.A.M. Wolters, Nat. Mater. 4 (2005) 347. [7] S. Lai, T. Lowrey, Tech.Dig.-Int.Electron devices Meet. (2001) 803. [8] A.E. Owen, P.G. Le Comber, J. Hajto, M.J. Rose, A.J. Snell, Int.J.electronics 73 (1992) 897. [9] R. Narayan, S. Asokan, A. Kumar, Phys.Rev.B 63 (2001) 92203. [10] S.R. Ovshinsky, H. Fritzche, IEEE Trans. Electron Devices 20 (1973) 91. [11] H. Fritzsche, Amorphous and Liquid Semiconductors, Pletinum Press, London, 1974. [12] N.F. Mott, E.A. Davis, Electronic Processes in Non-Crystalline Materials, Clarendon Press, London, 1979. [13] P. Dargan, P. Burton, S.V. Phillips, A.S. Bloor, P. Nesvadba, J. Mater. Sci. 9 (1974) 1595. [14] E. Bebenskas, S. Balyavichyus, A. Deksins, A. Poshkus, N. Shiktorov, J. Non. Cryst. Solids 90 (1987) 601. [15] D.P. Gosain, M. Nakamura, T. Shimizu, M. Suzuki, S. Okano, Jap.J.Appl.Phys. 28 (1989) 1013. [16] S. Balyavichyus, A. Deksins, A. Poshkus, N. Shiktorov, Sov. Phys. Semocond. 18 (1984) 947. [17] S.S.K. Titus, R. Chatterjee, S. Asokan, A. Kumar, Phys.Rev.B 48 (1993) 1460. [18] K. Ramesh, S. Asokan, K.S. Sangunni, et al., Appl. Phys. A: Mater. Sci. Process. 69 (1999) 421. [19] S. Prakash, S. Asokan, D.B. Ghare, J. Phys. D Appl. Phys. 29 (1996) 2004. [20] S.S.K. Titus, S. Asokan, E.S.R. Gopal, Solid State Commun. 83 (1992) 745. [21] K. Ramesh, S. Asokan, K.S. Sangunni, et al., J. Phys. Chem. Solids 61 (2000) 95. [22] S. Prakash, S. Asokan, D.B. Ghare, Semicond. Sci. Technol. 9 (1994) 1484. [23] R. Ramakrishna, S. Asokan, G. Parthasarathy, S.S.K. Titus, E.S.R. Gopal, J. Non-Cryst. Solids 139 (1992) 120. [24] S. Asokan, G. Parthasarathy, E.S.R. Gopal, Phill Mag. B 57 (1988) 49. [25] S. Asokan, M.V.N. Prasad, G. Parthasarathy, E.S.R. Gopal, Phys. Rev. Lett. 62 (1989) 108. [26] S.R. Ovshinsky, K. Sapree, Proceedings of the Seventh International Conference on Amorphous and Liquid Semiconductors, Institute of Physics, Bristol, 1977.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Letter to the Editor
Electrical switching behavior of amorphous Al23Te77 thin film sample Chandasree Das, R. Lokesh, G. Mohan Rao, S. Asokan ⁎ Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore 560012, India
a r t i c l e
i n f o
Article history: Received 8 April 2010 Received in revised form 28 July 2010 Available online 26 August 2010 Keywords: Electrical switching; Semiconductors; Thin films
a b s t r a c t The electrical switching behavior of amorphous Al23Te77 thin film devices, deposited by flash evaporation, has been studied in co-planar geometry. It is found that these samples exhibit memory type electrical switching. Scanning Electron Microscopic studies show the formation of a crystalline filament in the electrode region which is responsible for switching of the device from high resistance OFF state to low resistance ON state. It is also found that the switching behavior of thin film Al–Te samples is similar to that of bulk samples, with the threshold fields of bulk samples being higher. This has been understood on the basis of higher thermal conductance in bulk, which reduces the Joule heating and temperature rise in the electrode region. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the group of amorphous solids, semiconducting chalcogenides are interesting due to their applications in optoelectronics [1], infrared optical fibers [2], solar cells [3], optical recording systems [4], Phase Change Memories (PCM), etc. The absence of long-range order in these materials provides the convenience of changing the elemental ratio and hence the properties over a wide range. One of the most interesting as well as remarkable properties of chalcogenides which makes them suitable materials for phase change memories (PCM) is electrical switching, first observed by Ovshinsky nearly four decades ago [5]. Chalcogenide glass based phase change memories are recently being considered as a possible replacement for conventional Non Volatile Random Access memories (NVRAMs). Phase change memories make use of chalcogenide glasses of memory switching type. The main advantages of PCM are their direct write/ over write capability, lower volume operation, write /erase cycle, and easiness to integrate with logic [6,7]. The electrical switching in chalcogenide glasses occurs when an appropriate voltage, known as the threshold voltage (VT), is applied and the glass switches to a high conducting ON state from a low conducting OFF state. This phenomenon can be divided into two general categories [8]: i) Threshold switching, in which continuous electrical power is required to maintain the high conducting ON state and ii) Memory switching, in which the ON state can be maintained without electrical power. Threshold switching is known to be electronic in nature; however, additional thermal effects come into play in memory materials, which ⁎ Corresponding author. E-mail address: [email protected] (S. Asokan). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.07.057
result in the formation of a conducting crystalline channel in the electrode region [9]. Several tellurium based binary amorphous semiconductors such as As–Te, Ge–Te, Si–Te, In–Te and Ga–Te have been found to exhibit memory type of switching in their bulk form, at electrical fields of the order of 105 V/cm [10–17]. Ge–Te–Cu, Ge–Te–Ag, and Ge–As–Te are few examples of ternary tellurium based glasses which exhibit memory switching [18,19]. Apart from that, studies also have been done on tellurium based glasses like As–Te [20], Ge–Te [21] to know the composition dependence of crystallization and the effect of metallic doping on these glasses. Earlier investigations indicate that bulk, glassy AlxTe100 − x samples [22], exhibit memory type electrical switching, though no such work has been found in literature on AlxTe100 − x samples in amorphous thin film form. In this work, amorphous thin films of Al23Te77 have been deposited by flash evaporation method and the I–V characteristics and electrical switching behavior of the films have been studied along with surface morphology and composition. The reasons behind choosing this particular stoichiometry are: a) It can be prepared easily in bulk glass which can be used as a base material for thermal evaporation. b) It has shown a clean memory switching in the bulk glassy form. c) The earlier investigations have indicated that the x = 23 composition in the AlxTe100 − x system corresponds to the rigidity percolation threshold [23] and interesting properties have been observed in chalcogenide glasses at compositions corresponding to the rigidity percolation threshold [24,25]. 2. Experimental techniques The base–glassy semiconductor material with composition Al23Te77 has been prepared by melt quenching technique. Constituent elements (99.999% pure) are weighed to± 0.1 mg accuracy and sealed in a flat
LETTER TO THE EDITOR C. Das et al. / Journal of Non-Crystalline Solids 356 (2010) 2203–2206
bottom quartz ampoule under a vacuum of 10−5 mbar. The sealed ampoule is heated in a horizontal rotary furnace at about 1000 °C at the rate of 100 °C/h. The ampoule containing the melt is continuously rotated for nearly 36 h at 10 RPM to ensure homogeneity of the melt. The molten sample is subsequently quenched in ice water and sodium hydroxide (NaOH) mixture to get bulk glassy sample. The ingots of the sample are taken out by breaking the ampoules. These ingots are then ground into a fine powder for use as evaporation source material. Detergent solution, acetone and ultrasonic cleaning clean the 25 mm× 75 mm glass slides used as substrates. Flash evaporation technique has been used to prepare thin films. A mechanical mask has been used to obtain the required structure of the device. One extra slide is also coated for structural and other characterization. The thickness of the films measured by stylus profilometer (Form Taly-surf plus) has been found to be in the range of about 150 nm, for about 5 min deposition time. Aluminum electrodes have been deposited on the film in a co-planar geometry, using an appropriate mask. First, the chalcogenide film is coated in a rectangular shape on a glass slide. Secondly, aluminum is coated as the electrode, with a thin mask wrapped on the film. Subsequently, the electrode mask is removed. The active area of the device is the film area between two aluminum electrodes. In this work, the co-planar geometry has been deliberately chosen in order to compare the electrical switching characteristics of bulk glassy and amorphous thin film samples. Nevertheless, the sandwich geometry can be used to deposit the electrodes which would bring down the switching voltages of the devices. During all the depositions, the pressure inside the chamber is maintained at 10−5 mbar. The electrical contacts from the aluminum electrodes have been made using silver paste and copper wire. The switching behavior of the sample is studied by recording the current–voltage (I–V) characteristics using a Keithley Source-Meter (Model 2410) with a basic accuracy of ± 0.012%, controlled by Lab View 6i (National Instruments). The sourcing capacity of the source meter is in the range of 0–20 mA at a compliance voltage of 1100 V (maximum). A constant current is passed through the sample and the voltage across the sample is recorded. Scanning electron microscopy, optical microscopy and energy dispersive X-ray analysis have also been done on the sample for measuring the distance between the electrodes, surface morphology and chemical composition of the film respectively.
3. Results and discussion Fig. 1 shows the X-ray diffraction pattern of the as-deposited Al23Te77 film from which it is evident that it is amorphous in nature. Table 1 gives the composition of the different constituent elements in the as-deposited films obtained by Energy Dispersive X-ray Analysis (EDAX). It can be seen that the composition of the film is close to the composition of starting material, Al23Te77. The resistance of the asdeposited devices has been found to be in Mega Ohm range. Fig. 2 (a) and (b) shows the typical I–V characteristics of two representatives Al23Te77 thin film devices, which have been deposited in co-planar geometry with aluminum as electrode. It can be seen from these figures that the voltage across the devices initially increases linearly with current, indicating an Ohmic behavior. After a particular threshold voltage, the voltage across the sample starts decreasing with current, which indicates a negative resistance behavior. The negative resistance region leads to a low resistance state, which latches the device in that state. The device remains in low resistance state even if the current reduces to zero, which indicates and shows that amorphous Al23Te77 films exhibit memory type electrical switching. The insets in figures in Fig. 2(a) and (b) show the I–V characteristics around the threshold voltage, which reveal in a detailed manner, the negative resistance and the switching regions.
70 60 50
Intensity
2204
40 30 20 10 0 0
10
20
30
40
50
60
70
80
90
2θ Fig. 1. X-ray diffraction pattern of the as-deposited Al23Te77 film.
The insets also show the errors involved in the voltages which are not otherwise clearly visible in the I–V characteristics. As mentioned earlier, the application of a high electric field leads to the formation of a conducting crystalline filament during memory switching, because of which the sample remains in the low resistance ON state even after the removal of the field. The memory switching behavior of telluride glasses arises because of two general factors: The local structure of these samples is characterized by long Te chains in which atomic rearrangements and devitrification can occur easily [26]. Glassy tellurides also carry a higher electrical conductance in comparison with other chalcogenides like sulphides and selenides. The larger Joule heating due to higher conductance and easy crystallizability, lead to memory switching in telluride glasses such as Al23Te77. Fig. 3 (a) and (b) shows the scanning electron micrographs of unswitched and switched devices respectively. It is clearly seen that the un-switched film is featureless with a smooth topography. On the other hand, the phase change in the electrode region is revealed by an image contrast in the SEM picture; the image contrast in SEM after switching can be due to any of the following aspects: a) The phenomenon of electrical switching in chalcogenides, especially memory, is associated with local melting and re-solidification into the crystalline state of the active material. This process can lead to a surface relief which can give rise to an image contrast. b) The glass to crystal phase change is also associated with local structural changes and densification in the electrode region as the crystalline phase is denser than the corresponding amorphous phase. The density changes in the switched region can contribute for the image contrast. c) There is a large change in the electrical resistance of the active material in the electrode region (about three orders of magnitude). The conductivity changes can also lead to an image contrast in SEM. Detailed investigations are necessary to comprehend which of the above mentioned aspects is responsible for the observed changes in the SEM images of the switched sample. However, it can be noted here that a similar SEM image have been obtained earlier during the SET process of the device which has been attributed to the change of state of the material from amorphous to crystalline [11]. Table 1 Atomic percentage of Al23Te77 film obtained by EDAX analysis. Element
Atomic % in film
Al Te
22.54 77.46
LETTER TO THE EDITOR C. Das et al. / Journal of Non-Crystalline Solids 356 (2010) 2203–2206
Earlier studies on bulk Al–Te glasses of different compositions reveal that these samples exhibit memory type switching [22]. Fig. 4 shows the I–V characteristic of bulk Al23Te77 sample of thickness of 0.2 mm [22] and in ambient conditions, which indicates that the switching behavior of bulk and thin film Al–Te samples is similar. Table 2 shows the comparison of switching fields of the two representative thin film devices and the bulk Al23Te77 glass [22]. It can be seen that the switching fields of the two devices are comparable to each other. The difference in the switching fields of two films is 0.4 kV/cm, which is due to the difference in separation between two electrodes. However, the switching field of the bulk glass is much higher than that of the thin film samples. In a bulk sample, the heat is likely to be dissipated away faster from the electrode region to the larger thermal mass surrounding it. Consequently the Joule heating and temperature rise in the electrode region for a given applied voltage will be lower for the bulk, requiring a larger voltage for switching. Hence the threshold field for the bulk sample is higher than that of the thin film samples. 4. Conclusions Amorphous thin films of Al23Te77 have been deposited by flash evaporation method. The amorphous nature and the composition of films are confirmed by X-ray diffraction and EDAX respectively. The I– V characteristics and electrical switching behavior of the films have been studied using a Keithley source-meter. It is found that thin film
Fig. 3. Scanning electron micrograph of (a) un-switched and (b) switched Al23Te77 thin film samples respectively.
amorphous Al23Te77 samples exhibit memory type electrical switching. The image contrast in the switched region of the Scanning Electron Micrograph shows the phase change (formation of a crystalline filament), which is responsible for the switching of the device from high resistance OFF state to low resistance ON state.
2.5
2.0
Current (mA)
Fig. 2. (a) and (b). I–V characteristics of two representative Al23Te77 thin film devices, exhibiting memory switching. The insets in figures in Fig. 2(a) and (b) show the I–V characteristics around the threshold voltage, which reveal in a detailed manner, the negative resistance and the switching regions.
2205
Al23Te77 1.5
1.0
0.5
0.0 0
20
40
60
80
100
120
140
voltage (V) Fig. 4. I–V characteristics of bulk Al23Te77 glass showing memory switching [22].
LETTER TO THE EDITOR 2206
C. Das et al. / Journal of Non-Crystalline Solids 356 (2010) 2203–2206
Table 2 Comparison of switching fields of two representative devices and the bulk. Device
Switching voltage (V) in volt
Distance (d) between two electrodes in micron
Electric field kV/cm
1 2 Bulk
276 ± 2 195 ± 2 135 ± 2
976.8 ± 5 789 ± 5 200 ± 5
2.8 2.4 6.75
Further, the switching behavior of bulk and thin film Al23Te77 samples are found to be similar, with both types of samples exhibiting memory switching. The threshold fields of bulk samples are much higher compared to those of the thin film samples, which can be understood based on the higher thermal conductance in bulk samples which reduces the Joule heating and temperature rise in the electrode region. Acknowledgements Authors are thankful to Kesab Barai of Nanocentre for Scanning Electron Microscopy and also to Mr. Shamasundar H.S for helping to measure thickness by Profilometer. References [1] P. Sharma, M. Vashistha, I.P. Jain, Chalc.Lett. 2 (2005) 113. [2] H.A. Abd El Ghani, M.M. Abd El raham, M.M. Wakkad, A. Abosehli, N. Assraan, Physica B 381 (2006) 156. [3] J. Fusong, M. Okuda, Jpn. J. Appl. Phys. 30 (1991) 07.
[4] M.A. Majeed Khan, M. Zulfequar, S. Kumar, M. Husssain, J. Mod. Opt. 50 (2003) 251. [5] S.R. Ovshinsky, Phys.Rev.lett. 21 (1968) 1450. [6] Martin H.R. Lankhorst, Bas W.M.M. Ketelaars, R.A.M. Wolters, Nat. Mater. 4 (2005) 347. [7] S. Lai, T. Lowrey, Tech.Dig.-Int.Electron devices Meet. (2001) 803. [8] A.E. Owen, P.G. Le Comber, J. Hajto, M.J. Rose, A.J. Snell, Int.J.electronics 73 (1992) 897. [9] R. Narayan, S. Asokan, A. Kumar, Phys.Rev.B 63 (2001) 92203. [10] S.R. Ovshinsky, H. Fritzche, IEEE Trans. Electron Devices 20 (1973) 91. [11] H. Fritzsche, Amorphous and Liquid Semiconductors, Pletinum Press, London, 1974. [12] N.F. Mott, E.A. Davis, Electronic Processes in Non-Crystalline Materials, Clarendon Press, London, 1979. [13] P. Dargan, P. Burton, S.V. Phillips, A.S. Bloor, P. Nesvadba, J. Mater. Sci. 9 (1974) 1595. [14] E. Bebenskas, S. Balyavichyus, A. Deksins, A. Poshkus, N. Shiktorov, J. Non. Cryst. Solids 90 (1987) 601. [15] D.P. Gosain, M. Nakamura, T. Shimizu, M. Suzuki, S. Okano, Jap.J.Appl.Phys. 28 (1989) 1013. [16] S. Balyavichyus, A. Deksins, A. Poshkus, N. Shiktorov, Sov. Phys. Semocond. 18 (1984) 947. [17] S.S.K. Titus, R. Chatterjee, S. Asokan, A. Kumar, Phys.Rev.B 48 (1993) 1460. [18] K. Ramesh, S. Asokan, K.S. Sangunni, et al., Appl. Phys. A: Mater. Sci. Process. 69 (1999) 421. [19] S. Prakash, S. Asokan, D.B. Ghare, J. Phys. D Appl. Phys. 29 (1996) 2004. [20] S.S.K. Titus, S. Asokan, E.S.R. Gopal, Solid State Commun. 83 (1992) 745. [21] K. Ramesh, S. Asokan, K.S. Sangunni, et al., J. Phys. Chem. Solids 61 (2000) 95. [22] S. Prakash, S. Asokan, D.B. Ghare, Semicond. Sci. Technol. 9 (1994) 1484. [23] R. Ramakrishna, S. Asokan, G. Parthasarathy, S.S.K. Titus, E.S.R. Gopal, J. Non-Cryst. Solids 139 (1992) 120. [24] S. Asokan, G. Parthasarathy, E.S.R. Gopal, Phill Mag. B 57 (1988) 49. [25] S. Asokan, M.V.N. Prasad, G. Parthasarathy, E.S.R. Gopal, Phys. Rev. Lett. 62 (1989) 108. [26] S.R. Ovshinsky, K. Sapree, Proceedings of the Seventh International Conference on Amorphous and Liquid Semiconductors, Institute of Physics, Bristol, 1977.