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Direct observation of switching filaments in chalcogenide glasses
Journal of Non-Crystalline Solids 22 (1976) 223-227 © North-Holland Publishing Company
DIRECT OBSERVATION OF SWITCHING FILAMENTS IN CHALCOGENIDE GLASSES M. SAJI * and K.C. KAO Materials Research Laboratory, Department of Electrical Engineering, University o f Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
Received 15 December 1975 Revised manuscript received 20 April 1976
It is well known that the switching phenomena in chalcogenide glasses involve the formation of a switching filament [1-3], though the mechanisms responsible for these phenomena are still in dispute [4]. Sieet al. [5] have observed the filament formed in a thin film memory device of film thickness 1.5/am under a scanning electron microscope, and Uttecht et al. [6] have observed the surface filament formed on the surface of a piece of As55Te35Gelo glass. However, little has been reported about the shape and the size of the filaments, particularly those inside the bulk of the material. The information on the shape and the size of the filaments, and the factors controlling them, is extremely important to the understanding of the switching phenomena. In this note we report some observed results along this line. The Si12Ge10As30Te48 chalcogenide glass was supplied by the Royal Radar Estab. lishment, UK, and the specimens were prepared using the techniques reported earlier [7], the thickness of all specimens being about 90/am. All devices were fabricated in a sandwich configuration with vacuum-deposited molybdenum electrodes, the diameter of the upper electrode being about 60/am and that of the bottom electrode about 1 cm. The device holder is similar to that used by Saji and Kao [7]. We also used a molybdenum probe with a radius of curvature of 30/am at the end to contact the top surface of the specimen as upper electrode instead of the vacuum-deposited one, and obtained the same results. To observe the filament formed in such devices, we used a high-power Vickers microscope with a magnification ×800. We first fitted the geometric shape of the device to the rectangular coordinates with the anode surface at z = 0 and the cathode surface at z = d, where d is the thickness of the specimen, so that any position inside the specimen can be defined by x , y and z. We then removed the upper electrode by polishing with Al203 (2.5 /am) powder, then photographed the cross-section of the filament, and then removed a portion of about 10
* Present address: Nagoya Institute of Technology, Nagoya, Japan. 223
224
M. Sail and K.C. Kao / Switching filaments in chalcogenide glasses
/am thick by polishing and again photographed the cross-section of the filament, and so on. Using this step-by-step technique with the removal of a portion of about I0 /am thick at each step, we can picture the shape of the filament by assembling all the photographs of the cross-sections, and measure the cross-section area at any position along the filament. All experiments were performed at the ambient temperature of 20°C, and the results are summarized as follows: (1) At low fields the current voltage ( I - V ) relation is linear (I cc V), and at higher fields this relation follows a square law ( / = V2), indicating that the current becomes space-charge limited due mainly to the carrier injection from electrodes. At fields close to the threshold for the onset of switching I increases vary rapidly with V (I cc V n with n larger than 6). At the threshold field the device is switched to the ON state. The threshold voltage Vth is about 380 V and the current corresponding to V~ at the switching point (Ith) is about 10 -5 A. (2) Figures 1 and 2 show the general shape of a typical filament and that of a typical filament cross-section. The filaments were formed after the devices had been switched to the ON state at a peak current level of 10 mA and then to the OFF state for about 50 cycles to allow the filaments to reach their thermal equilibrium condition, each switching cycle taking about 1 min. We did not measure the variation of the filament size with the accumulated time for which the current of 10 mA was left, but we noticed that the peak current and hence the filament size were not stable at the first few cycles but gradually increased and finally reached steady values after about 25 cycles. The filament sizes given in figs. 1 - 4 were obtained after such a thermal equilibrium condition had been reached. The filament generally follows a very irregular path, and the filament cross-section is generally not circular but, rather, oval in shape. In fig. 2 the cross-section area at the distance of 29/am from the anode surface is about 70/am 2, while the cross-section areas of this filament at the anode
Z
NEGATIVE
Z
IELECTROOZ' ~/~ '
] I
. -,80
.60
40,
40
POSITIVE ~ 1
t
-40
-20
't~ cT~°*tt 20 -20
0
~ ' x 20 30
Fig. 1. The general shape of a typical switching filament across the device of sandwich configuration with the plane at z = 0 as the anode and that at z = 88 pm as the cathode. The specimen thickness is 88 pm and the current passing through the device is 10 mA. The valuesshown are in P.m.
M. Saji and K.C. Kao / Switching filaments in chaleogenide glasses
225
i~
%
10.
Fig. 2. The photograph showing the shape of the cross-section of a typical switching filament on the x - y plane and at the position 29 ~m from the anode surface (z = 29 t~m). The specimen thickness is 90 #m and the current passing through the device is 10 mA.
and the cathode surfaces are, respectively, about 1300 and 280/am 2. The switching threshold voltage for a virgin device is about 3 7 0 - 4 0 0 V, but it decreases to about 8 0 - 1 0 0 V after a few switching cycles at the peak current level of 10 mA. (3) The cross-section area varies from position to position along the filament, with the largest area at the anode surface and the smallest area in the bulk at a distance of about two-thirds of the specimen thickness from the anode surface (close to the cathode surface) as shown in fig. 3. The smallest cross-section area is about 40/am 2 with the narrowest width about 4 - 5 / a m . As the shape of the cross-section is very irregular, particularly at the anode and at the cathode surfaces, the areas were determined by measuring directly the areas enclosed by the filament; boundary. The fact that the cross-section area at the anode surface is much larger than that at the cathode surface may indicate that a higher Joule heating occurs near the anode, possibly due to a higher density of injected hole carriers near there, and that for this case the hole injection from the anode is predominant. Mthough the cross-section area is dependent on the current passing through the device in the ON state to establish the filament, the general distribution of the cross-section areas along the filament remains unaltered. Furthermore, the tendency shown in fig. 3 is not affected
•2 2 6
M. Sail and 1(,.C. Kao / Switching filaments in chalcogenide glasses 2000~
--"-
~E 1 6 0 0 ~
•z
~
10 m A 3mA
120()
i( ~.
800
•-,
400
=
Ol
20 40 60 80 88 DISTANCE from SURFACE (pn~
F i g . 3. Filament cross-section area as a function of the distance from the anode surface for the currents o f 1 0 a n d 3 m A passing through the device.
by reversing the polarity of the applied voltage for creating the filament (with the upper electrode either as an anode or as a cathode) or by changing the electrode surface areas. (4) Figure 4 shows that the input power and the cross-section areas at both anode and cathode surfaces increase with increasing current passing through the device in the ON state. At low current levels the change of these quantities with current is small and the input remains practically constant at about 10 mW. But at current levels higher than 10 -4 A these quantities increase very rapidly as the current is increased. It is interesting to note that the variation of the cross-section at the anode surface with current follows the same pattern for input power, indicating that the
50
E !000~
/ INPUT POWER
/
~40
-x- POSITIVE POLARITY
:o
~
/
NEGATIVE POLARITY
/
/
16002 /
20 •
~-
800
~ 010-5
,oo
2
! i 5
i
:,~ ,
10 -4 2
! i
I
i
4oo~
i
5 10-3 2 CURRENT (A)
i
S
0
10-2
Filament cross-section areas at the anode surface (positive polarity) and at the cathode surface (negative polarity), and input power as functions of current passing through the device.
Fig. 4.
M. Saji and K. C Kao / Switching filaments in chalcogenide glasses
227
filament formation is energy-controlled and that the carriers to produce Joule heating for this formation process may be originated from the anode due to its hole injection. (5) At low current levels (10 -5 - 1 0 -4 A) it is difficult to observe any significant change at the electrode surface, although the great change in electric current indicates that the device has been switched on and that the filament has been established in the bulk of the device. For such cases we have to remove the upper electrode before we can locate the origin of the filament. However, at high current levels (5 × 1 0 - 4 - 1 0 - 2 A) we could observe a change on the electrode surface, and this change is usually an indication of the origin of the filament. At very high current levels (~10 - 2 A) the glass in molten state beneath the electrode has also been observed. The filament cross-section will expand or grow as the current in the ON state is increased as shown in figs. 3 and 4. The increase in size of the filament due to the increase of the current allowed to flow in the ON state will decrease the resistance of the device in the OFF state and the threshold voltage for the onset of switching. At current levels higher than 10 - 2 A the device is changed from its switching-ON state to a " m e m o r y " state. A current pulse of much higher than 10 - 2 A can erase the memory state as has been observed by other investigators [7,8]. On the basis of the above results we can conclude that the switching process is governed by the injection of charge carriers of a dominant type (holes or electrons, and in our devices holes are the dominant type), and is energy-controlled. The authors wish to thank Mr. D. Mardis for his technical assistance, and the Defence Research Board of Canada (Grant No. 5566-39) and the National Research Council of Canada (Grant No. A-3339) for supporting this research.
References [1 ] L.A. Coward, J. Non-Crystalline Solids 6 (1971) 107. [2] M.H. Cohen, R.G. Neake and A. Paskin, J. Non-Crystalline Solids 8-10 (1972) 885. [3] R.F. Ormondroyd, J. Allison and M.J. Thompson, J. Non-Crystalline Solids 15 (1974) 310. [4] J. Tauc (ed.), Amorphous and liquid semiconductors (Plenum Press, New York, 1974). [5] C.H. Sie, M.P. Dugan and S.C. Moss, J. Non-Crystalline Solids 8-10 (1972) 877. [6] R. Utrecht, H. Stevenson, C.H. Sie, J.D. Griener and K.S. Raghaven, J. Non-Crystalline Solids 2 (1970) 358. [7] M. Saji and K.C. Kao, J. Non-Crystalline Solids 18 (1975) 275. [8] G.A. Petrillo and K.C. Kao, J. Non-Crystalline Solids 16 (1974) 247.
DIRECT OBSERVATION OF SWITCHING FILAMENTS IN CHALCOGENIDE GLASSES M. SAJI * and K.C. KAO Materials Research Laboratory, Department of Electrical Engineering, University o f Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
Received 15 December 1975 Revised manuscript received 20 April 1976
It is well known that the switching phenomena in chalcogenide glasses involve the formation of a switching filament [1-3], though the mechanisms responsible for these phenomena are still in dispute [4]. Sieet al. [5] have observed the filament formed in a thin film memory device of film thickness 1.5/am under a scanning electron microscope, and Uttecht et al. [6] have observed the surface filament formed on the surface of a piece of As55Te35Gelo glass. However, little has been reported about the shape and the size of the filaments, particularly those inside the bulk of the material. The information on the shape and the size of the filaments, and the factors controlling them, is extremely important to the understanding of the switching phenomena. In this note we report some observed results along this line. The Si12Ge10As30Te48 chalcogenide glass was supplied by the Royal Radar Estab. lishment, UK, and the specimens were prepared using the techniques reported earlier [7], the thickness of all specimens being about 90/am. All devices were fabricated in a sandwich configuration with vacuum-deposited molybdenum electrodes, the diameter of the upper electrode being about 60/am and that of the bottom electrode about 1 cm. The device holder is similar to that used by Saji and Kao [7]. We also used a molybdenum probe with a radius of curvature of 30/am at the end to contact the top surface of the specimen as upper electrode instead of the vacuum-deposited one, and obtained the same results. To observe the filament formed in such devices, we used a high-power Vickers microscope with a magnification ×800. We first fitted the geometric shape of the device to the rectangular coordinates with the anode surface at z = 0 and the cathode surface at z = d, where d is the thickness of the specimen, so that any position inside the specimen can be defined by x , y and z. We then removed the upper electrode by polishing with Al203 (2.5 /am) powder, then photographed the cross-section of the filament, and then removed a portion of about 10
* Present address: Nagoya Institute of Technology, Nagoya, Japan. 223
224
M. Sail and K.C. Kao / Switching filaments in chalcogenide glasses
/am thick by polishing and again photographed the cross-section of the filament, and so on. Using this step-by-step technique with the removal of a portion of about I0 /am thick at each step, we can picture the shape of the filament by assembling all the photographs of the cross-sections, and measure the cross-section area at any position along the filament. All experiments were performed at the ambient temperature of 20°C, and the results are summarized as follows: (1) At low fields the current voltage ( I - V ) relation is linear (I cc V), and at higher fields this relation follows a square law ( / = V2), indicating that the current becomes space-charge limited due mainly to the carrier injection from electrodes. At fields close to the threshold for the onset of switching I increases vary rapidly with V (I cc V n with n larger than 6). At the threshold field the device is switched to the ON state. The threshold voltage Vth is about 380 V and the current corresponding to V~ at the switching point (Ith) is about 10 -5 A. (2) Figures 1 and 2 show the general shape of a typical filament and that of a typical filament cross-section. The filaments were formed after the devices had been switched to the ON state at a peak current level of 10 mA and then to the OFF state for about 50 cycles to allow the filaments to reach their thermal equilibrium condition, each switching cycle taking about 1 min. We did not measure the variation of the filament size with the accumulated time for which the current of 10 mA was left, but we noticed that the peak current and hence the filament size were not stable at the first few cycles but gradually increased and finally reached steady values after about 25 cycles. The filament sizes given in figs. 1 - 4 were obtained after such a thermal equilibrium condition had been reached. The filament generally follows a very irregular path, and the filament cross-section is generally not circular but, rather, oval in shape. In fig. 2 the cross-section area at the distance of 29/am from the anode surface is about 70/am 2, while the cross-section areas of this filament at the anode
Z
NEGATIVE
Z
IELECTROOZ' ~/~ '
] I
. -,80
.60
40,
40
POSITIVE ~ 1
t
-40
-20
't~ cT~°*tt 20 -20
0
~ ' x 20 30
Fig. 1. The general shape of a typical switching filament across the device of sandwich configuration with the plane at z = 0 as the anode and that at z = 88 pm as the cathode. The specimen thickness is 88 pm and the current passing through the device is 10 mA. The valuesshown are in P.m.
M. Saji and K.C. Kao / Switching filaments in chaleogenide glasses
225
i~
%
10.
Fig. 2. The photograph showing the shape of the cross-section of a typical switching filament on the x - y plane and at the position 29 ~m from the anode surface (z = 29 t~m). The specimen thickness is 90 #m and the current passing through the device is 10 mA.
and the cathode surfaces are, respectively, about 1300 and 280/am 2. The switching threshold voltage for a virgin device is about 3 7 0 - 4 0 0 V, but it decreases to about 8 0 - 1 0 0 V after a few switching cycles at the peak current level of 10 mA. (3) The cross-section area varies from position to position along the filament, with the largest area at the anode surface and the smallest area in the bulk at a distance of about two-thirds of the specimen thickness from the anode surface (close to the cathode surface) as shown in fig. 3. The smallest cross-section area is about 40/am 2 with the narrowest width about 4 - 5 / a m . As the shape of the cross-section is very irregular, particularly at the anode and at the cathode surfaces, the areas were determined by measuring directly the areas enclosed by the filament; boundary. The fact that the cross-section area at the anode surface is much larger than that at the cathode surface may indicate that a higher Joule heating occurs near the anode, possibly due to a higher density of injected hole carriers near there, and that for this case the hole injection from the anode is predominant. Mthough the cross-section area is dependent on the current passing through the device in the ON state to establish the filament, the general distribution of the cross-section areas along the filament remains unaltered. Furthermore, the tendency shown in fig. 3 is not affected
•2 2 6
M. Sail and 1(,.C. Kao / Switching filaments in chalcogenide glasses 2000~
--"-
~E 1 6 0 0 ~
•z
~
10 m A 3mA
120()
i( ~.
800
•-,
400
=
Ol
20 40 60 80 88 DISTANCE from SURFACE (pn~
F i g . 3. Filament cross-section area as a function of the distance from the anode surface for the currents o f 1 0 a n d 3 m A passing through the device.
by reversing the polarity of the applied voltage for creating the filament (with the upper electrode either as an anode or as a cathode) or by changing the electrode surface areas. (4) Figure 4 shows that the input power and the cross-section areas at both anode and cathode surfaces increase with increasing current passing through the device in the ON state. At low current levels the change of these quantities with current is small and the input remains practically constant at about 10 mW. But at current levels higher than 10 -4 A these quantities increase very rapidly as the current is increased. It is interesting to note that the variation of the cross-section at the anode surface with current follows the same pattern for input power, indicating that the
50
E !000~
/ INPUT POWER
/
~40
-x- POSITIVE POLARITY
:o
~
/
NEGATIVE POLARITY
/
/
16002 /
20 •
~-
800
~ 010-5
,oo
2
! i 5
i
:,~ ,
10 -4 2
! i
I
i
4oo~
i
5 10-3 2 CURRENT (A)
i
S
0
10-2
Filament cross-section areas at the anode surface (positive polarity) and at the cathode surface (negative polarity), and input power as functions of current passing through the device.
Fig. 4.
M. Saji and K. C Kao / Switching filaments in chalcogenide glasses
227
filament formation is energy-controlled and that the carriers to produce Joule heating for this formation process may be originated from the anode due to its hole injection. (5) At low current levels (10 -5 - 1 0 -4 A) it is difficult to observe any significant change at the electrode surface, although the great change in electric current indicates that the device has been switched on and that the filament has been established in the bulk of the device. For such cases we have to remove the upper electrode before we can locate the origin of the filament. However, at high current levels (5 × 1 0 - 4 - 1 0 - 2 A) we could observe a change on the electrode surface, and this change is usually an indication of the origin of the filament. At very high current levels (~10 - 2 A) the glass in molten state beneath the electrode has also been observed. The filament cross-section will expand or grow as the current in the ON state is increased as shown in figs. 3 and 4. The increase in size of the filament due to the increase of the current allowed to flow in the ON state will decrease the resistance of the device in the OFF state and the threshold voltage for the onset of switching. At current levels higher than 10 - 2 A the device is changed from its switching-ON state to a " m e m o r y " state. A current pulse of much higher than 10 - 2 A can erase the memory state as has been observed by other investigators [7,8]. On the basis of the above results we can conclude that the switching process is governed by the injection of charge carriers of a dominant type (holes or electrons, and in our devices holes are the dominant type), and is energy-controlled. The authors wish to thank Mr. D. Mardis for his technical assistance, and the Defence Research Board of Canada (Grant No. 5566-39) and the National Research Council of Canada (Grant No. A-3339) for supporting this research.
References [1 ] L.A. Coward, J. Non-Crystalline Solids 6 (1971) 107. [2] M.H. Cohen, R.G. Neake and A. Paskin, J. Non-Crystalline Solids 8-10 (1972) 885. [3] R.F. Ormondroyd, J. Allison and M.J. Thompson, J. Non-Crystalline Solids 15 (1974) 310. [4] J. Tauc (ed.), Amorphous and liquid semiconductors (Plenum Press, New York, 1974). [5] C.H. Sie, M.P. Dugan and S.C. Moss, J. Non-Crystalline Solids 8-10 (1972) 877. [6] R. Utrecht, H. Stevenson, C.H. Sie, J.D. Griener and K.S. Raghaven, J. Non-Crystalline Solids 2 (1970) 358. [7] M. Saji and K.C. Kao, J. Non-Crystalline Solids 18 (1975) 275. [8] G.A. Petrillo and K.C. Kao, J. Non-Crystalline Solids 16 (1974) 247.