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Electrical switching and thermal studies on bulk Ge–Te–Bi glasses
Journal of Non-Crystalline Solids 357 (2011) 165–169
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
Electrical switching and thermal studies on bulk Ge–Te–Bi glasses Chandasree Das, 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 21 November 2009 Received in revised form 26 August 2010 Available online 19 October 2010 Keywords: Electrical switching; Chalcogenide glasses; Differential scanning calorimetry
a b s t r a c t Bulk, melt quenched Ge18Te82-xBix glasses (1 ≤ x ≤ 4) have been found to exhibit memory type electrical switching behavior, which is in agreement with the lower thermal diffusivity values of Ge–Te–Bi samples. A linear variation in switching voltages (Vth) has been found in these samples with increase in thickness which is consistent with the memory type electrical switching. Also, the switching voltages have been found to decrease with an increase in temperature which happens due to the decrease in the activation energy for crystallization at higher temperatures. Further, Vth of Ge18Te82-xBix glasses have been found to decrease with the increase in Bi content, indicating that in the Ge–Te–Bi system, the resistivity of the additive has a stronger role to play in the composition dependence of Vth, in comparison with the network connectivity and rigidity factors. In addition, the composition dependence of crystallization activation energy has been found to show a decrease with an increase in Bi content, which is consistent with the observed decrease in the switching voltages. X-ray diffraction studies on thermally crystallized samples reveal the presence of hexagonal Te, GeTe, Bi2Te3 phases, suggesting that bismuth is not taking part in network formation to a greater extent, as reflected in the variation of switching voltages with the addition of Bi. SEM studies on switched and unswitched regions of Ge–Te–Bi samples indicate that there are morphological changes in the switched region, which can be attributed to the formation of the crystalline channel between two electrodes during switching. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Certain crystalline and amorphous materials, including chalcogenide glasses, are found to exhibit a non destructive electrical switching, from a low conducting (OFF) state to a high conducting (ON) state, upon application of a high electric field. In chalcogenide glasses, the electrical switching phenomenon is of two types, namely memory switching and threshold switching [1–3]. If the ON state of the material is retained even after the removal of electric field, it is called memory switching; whereas if the material reverts back to the high resistance OFF state after removing the electric field, it is called threshold switching. Threshold switching materials require a holding current and voltage to sustain the ON state. Once the holding current is removed, they revert back to their original OFF state. Thermal and electronic are the predominant mechanisms proposed for understanding the electrical switching in amorphous chalcogenides. While the response of electrons to the applied high field is used to explain threshold switching, an additional thermally induced transition to the crystalline state in the electrode region explains memory switching in chalcogenide glasses [4–10]. The phenomenon of memory switching in chalcogenide glasses has recently found applications in Phase Change Memories (PCM)
⁎ Corresponding author. Fax: +91 80 23608686. E-mail address: [email protected] (S. Asokan). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.09.046
which are being considered as a possible replacement for conventional Non Volatile Random Access Memories (NVRAMs). The 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, easiness to integrate with logic, etc. [11,12]. Generally, tellurides have a higher crystallizability in comparison with sulfides and selenides. This fact makes the phase change between amorphous and crystal phases easier in Te based glasses and hence, more useful materials for PCM applications. In particular, the GeTe based glasses have been widely investigated for their suitability for application in phase change memories. Chalcogenide glasses prepared by melt quenching technique are normally p-type conductors. However, it has been found that n-type semiconducting glasses can be obtained by adding Bi to certain chalcogenide glassy systems such as Ge–Se and Ge–Se–Te, etc. [13,14]. The p–n transition has also been found in Ge–Te–Bi glasses [15], which has prompted the present investigations on the electrical switching behavior on Ge18Te82-xBix (1 ≤ x ≤ 4) glasses, with a focus on the variation of threshold voltages with composition, temperature and thickness. In addition, the crystallization process in Ge18Te82-xBix (x = 1, 2, 3) alloys at different heating rates has also been studied by Differential Scanning Calorimetry (DSC), as it is important for phase change applications; the composition dependence of crystallization temperatures and the activation energies for crystallization (obtained
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using the Kissinger's method) have been investigated. Further, Xray diffraction investigations have been undertaken on crystallized Ge–Te–Bi samples to identify the phases formed during devitrification. Finally, the scanning electron microscopic studies have been carried out on the switched and un-switched regions of a representative Ge18Te81Bi1 sample. 2. Experimental procedures Chalcogenide glasses are reactive at high temperature, particularly with O2, and also the constituent elements frequently differ widely in volatility. This necessitates the preparation of these glasses in sealed containers. Further, impurities can alter the electrical switching and thermal behavior of these materials, though chalcogenide glasses are generally not as sensitive to impurities as intrinsic semiconductors. Therefore, the elementary materials used for preparing bulk samples are always of a high purity (99.999%). Bulk Ge18Te82-xBix (1 ≤ x ≤ 4) glasses have been prepared by the conventional melt quenching method. Appropriate quantity of high purity components (Ge, Te and Bi) have been mixed in a quartz ampoule of 6 mm inner diameter and then evacuated to a pressure of 10−5 mbar. The ampoules are maintained at this pressure for about 30 min and sealed under vacuum. The sealed ampoules are heated to a temperature of 1000 °C in a horizontal rotary furnace and held at this temperature for 36 h to homogenize the melt. Subsequently, the ampoules are quenched in ice water + NaOH mixture. The amorphous nature of the quenched samples is confirmed by X-ray diffraction (XRD). The electrical switching characteristics of these glasses have been studied using a Keithley source-measure unit (Model 2410c). Samples polished to a thickness of about 0.25 mm are mounted between a flat bottom electrode and a point contact top electrode. A constant current is passed through the sample and the voltage developed across the sample is measured. The standard deviation method is used for the analysis of errors in the measurement of switching voltages. The thermal parameters such as crystallization temperatures (Tc) of Ge18Te82-xBix glasses have been obtained using a Mettler Toledo Differential Scanning Calorimeter. Samples have been heated at four different heating rates and the activation energy for crystallization has been calculated using Kissinger's equation, with the calorimetric scans made in the range room temperature to 350 °C. The typical error in the measurement is found to be ±1 °C for Tc at different scanning rates. Crystallization studies of two representative Ge18Te81Bi1 and Ge18Te79Bi3 samples have been undertaken by annealing the samples at the respective crystallization temperatures for about 2 h and X-ray diffraction studies have been carried out on the annealed samples in a Philips powder diffractometer with CuKα radiation. The Scanning Electron Microscopic (SEM) images of the switched and un-switched regions of a representative Ge18Te81Bi1 sample has been taken by ESEM Quanta machine. 3. Results Fig. 1 shows the I–V characteristics of Ge18Te82-xBix series of glasses, from which it is clear that these samples exhibit memory type electrical switching. Fig. 2 shows the variation of threshold voltage of Ge18Te82-xBix glasses with Bi content, which indicates that switching voltage decreases with increase in bismuth content. The threshold voltage (Vth) of Ge18Te81Bi1 glass is found to decrease with increase in temperature above room temperature as shown in Fig. 3, which is a common feature exhibited by many memory switching chalcogenides glasses [16–18]. Fig. 4 shows thickness dependence of threshold voltage, indicating an almost linear variation.
Table 1 shows the crystallization temperature (Tc) of three different compositions of Ge–Te–Bi glassy system, at four different heating rates (β). The Kissinger's plot [19] of the variation of log (β/T2c ) versus (1000/ Tc) for a representative Ge18Te81Bi1 has been shown in Fig. 5. The activation energy for crystallization (Ec) has been calculated using the Kissinger's equation [20], ln (β/T2c ) = −Ec/RTc + D. Fig. 6 shows the X-ray diffraction patterns of two representative Ge18Te81Bi1 and Ge18Te79Bi3 samples respectively annealed at their respective crystallization temperature for about 2 h. It can be seen that the diffraction patterns of crystallized sample are indexable with hexagonal Te, GeTe, BiTe and Bi2Te3. Fig. 7 shows the scanning electron micrograph of switched and un-switched regions of Ge18Te81Bi1 sample. 4. Discussion 4.1. Electrical switching behavior of Ge–Te–Bi glasses As mentioned earlier, the memory switching in chalcogenide glasses has a thermal origin involving the formation of a conducting crystalline channel between the electrodes, with the energy required for crystallization provided by the electric field. The Te-rich chalcogenide glasses are normally found to exhibit memory switching because of their comparatively larger electrical conductance which results in a higher power dissipation and therefore the conducting channel could be formed easily. It is also known that weaker bond, poor structural cross-linking and more lone-pair interactions [21] favor memory switching in telluride glasses. The present studies reveal that Ge18Te82-xBix glasses also exhibit memory type electrical switching like many other binary and ternary telluride glasses such as Ge–Te, Si–Te, As–Te, Al–Te, As–Se–Te, Ge–As–Te, Ge–Te–Cu [22–29]. In this context, it is interesting to note that the thermal diffusivity (α) of representative Ge–Te–Bi samples, namely Ge18Te81Bi1, Ge18Te80Bi2 and Ge18Te79Bi3, have been found to be 0.015, 0.0108 and 0.00926 respectively, which is comparatively lower than other chalcogenide glasses [30,31]. Usually, chalcogenide glasses which have higher thermal diffusivity exhibit threshold type electrical switching and those with a lower α show memory behavior [32]. The memory behavior seen in Ge–Te–Bi glasses is therefore consistent with thermal diffusivity values of these samples. The composition dependence of threshold/switching voltage in chalcogenide glasses is determined by three main factors, namely the resistivity of the additive elements [33], network connectivity, rigidity [34] and chemical ordering [35]. The addition of more metallic elements usually brings down the switching voltages of chalcogenide glasses; this is also the case in the present Ge18Te82-xBix system in which Bi is more metallic compared to Te (ρBi = 1.29 × 10−6 Ω m and ρTe = 1 × 10−4 Ω m) and the replacement of Te with Bi leads to the reduction in the threshold voltages (Fig. 2). The rigidity percolation deals with dimensionality and rigidity of a glassy network. For a covalent network glass, the network connectivity & rigidity increases with the increase in average coordination number and at a mean coordination number brcN = 2.4 (known as the Stiffness Threshold or Rigidity Percolation Threshold) [36–38], the glassy system exhibits a transition from a floppy polymeric glass to a rigid amorphous solid. In general, the memory switching voltages of chalcogenide glasses are found to increase with an increase in network connectivity and rigidity as the structural rearrangements become more difficult with increasing network rigidity [34]. In the Ge18Te82-xBix system, the addition of Bi with a higher coordination number of 3 [39], at the expense of Te with the coordination number of 2 [40], results in a progressive increase in network connectivity and rigidity. Hence, based on the network connectivity rigidity, one could expect an increase in switching voltages with the addition of Bi. However, the present results show that the switching voltages of Ge18Te82-xBix
C. Das et al. / Journal of Non-Crystalline Solids 357 (2011) 165–169
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Fig. 1. I–V characteristics of (a) Ge18Te81Bi1 (b) Ge18Te80Bi2 (c) Ge18Te79Bi3 and (d) Ge18Te78Bi4 representing the Ge18Te82-xBix series.
glasses show a decrease with the increase in Bi concentration, which indicates that in this system the effect of resistivity of the dopant is more pronounced than the network connectivity and rigidity. Further, in many chalcogenide systems, a sharp signature of the rigidity percolation is seen in the variation of switching voltages at brcN. However, in the present Ge18Te82-xBix system, it has not been possible to probe the effect of stiffness transition on switching voltages as the composition range of bulk glass formation is limited only to lower brN values (brN ≤2.4). 4.2. Temperature dependence of threshold voltages
chalcogenide glasses [16–18]. The variation of threshold voltage of Ge18Te81Bi1 glass with temperature (Fig. 3) shows that the Vth of Ge18Te82-xBix glasses decrease as the temperature is increased above room temperature. In case of memory switching glasses the decrease in switching voltages with temperature has been explained on the basis of a configurational free-energy diagram [25], according to which the decrease in Vth is due to the decrease in the energy barrier required for crystallization of a sample with the increase in temperature. In addition, at higher temperature, the charged defect centers are filled up by thermally excited charge carriers, which are in addition to field-injected carriers, resulting in decrease in switching voltage [41].
The decreasing trend in switching voltages with increase in temperature is a common feature in many memory switching
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Fig. 4. Thickness dependence of threshold voltages of a representative Ge18Te81Bi1 glass.
4.3. Thickness dependence of switching voltages
Fig. 5. The Kissinger's plot showing the variation of ln (β/T2c ) Vs. (1000/Tc) for Ge18Te81Bi1 glass.
reflected in the variation of switching voltages which shows no increase with the addition of Bi.
It is known that in memory switching samples, the threshold voltage (Vth) is found to be proportional to “t” or “t1/2”, where “t” is the thickness of the sample. A linear variation of Vth with t, has been seen in samples like Ge–Te–Al [42], As–Se [43], Ge–Se–Tl [44]. However, a few samples like Ge–As–Te [45], Ge–Te [22] have been found to exhibit a t1/2 variation with Vth. In the present sample, the variation of threshold voltage Vth with thickness has been observed to be linear (Fig. 4).
4.6. Electron microscopic analysis of Ge–Te–Bi It is clear from the SEM images of switched and un-switched regions of Ge18Te81Bi1 sample which indicate that there are morphological changes in the switched region. The morphological
4.5. Crystallization behavior of Ge18Te82-xBix glasses X-ray diffraction studies on Ge18Te81Bi1 and Ge18Te79Bi3 annealed for 2 h under vacuum at their respective crystallization temperatures reveal the presence of hexagonal Te and hexagonal GeTe. In addition, hexagonal BiTe and hexagonal Bi2Te3 have also been found in the crystallized Ge–Te–Bi samples. In this context, it is interesting to compare the earlier studies on the crystallization behavior of Ge–Te–Ag [46] glasses; X-ray investigations on crystallized Ge–Te–Ag samples show that silver forms a ternary phase along with possible binary phases in this system [46]. However, in the case of Ge–Te–Bi samples, no ternary phase has been found and in fact, more number of binary phases is found. The possible reason for not finding any ternary phases of bismuth or more number of binary phases in Ge18Te79Bi3 could be that bismuth is not taking part in network formation to a greater extent. This aspect is also
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The activation energy for devitrification of the Ge–Te–Bi samples, estimated from the Kissinger's plots, has been found to decrease with the addition of Bi (Table 1). The observed decrease in Ec with Bi content is consistent with the decrease in memory switching voltages with Bi content.
Intensity (arb. units)
4.4. Activation energy for crystallization
Table 1 The crystallization temperatures at different heating rates and the estimated crystallization activation energies of Ge18Te82-xBix glasses of different compositions. Composition x (at.%)
Tc at 4 K/min
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2θ Fig. 6. The X-ray diffraction patterns of Ge18Te81Bi1 and Ge18Te79Bi3 glasses annealed at their respective crystallization temperatures.
C. Das et al. / Journal of Non-Crystalline Solids 357 (2011) 165–169
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play in comparison with the network connectivity and rigidity factors. The composition dependence of activation energy for crystallization of Ge18Te82-xBix glasses, exhibits a decrease with increase in Bi content, which is consistent with the decrease in switching voltages with Bi content. Based on the X-ray diffraction studies of thermally crystallized samples, it is proposed that bismuth is not taking part in network formation in a greater extent, which is reflected in the variation of switching voltages with the addition of Bi. SEM studies on switched and un-switched regions of Ge–Te–Bi sample indicate that there are morphological changes in the switched region, which can be attributed to the formation of the crystalline channel formation between two electrodes during switching. References
b
Fig. 7. Scanning electron micrographs of (a) un-switched and (b) switched regions of Ge18Te81Bi1 glass.
changes are due to the formation of the crystalline channel between two electrodes during switching. 5. Conclusions Melt quenched Ge18Te82-xBix glasses have been found to exhibit memory switching. The lower thermal diffusivity values of these samples are in agreement with the observed memory switching behavior. A linear increase has been found in switching voltages with increase in sample thickness which is also consistent with the memory switching exhibited by Ge–Te–Bi samples. Also, the switching voltages have been found to decrease with increase in temperature due to decrease in the activation energy for crystallization at higher temperatures. Further, the switching voltages of the Ge–Te–Bi samples have been found to decrease with increase in Bi content, which indicates that resistivity of the additive has a stronger role to
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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
Electrical switching and thermal studies on bulk Ge–Te–Bi glasses Chandasree Das, 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 21 November 2009 Received in revised form 26 August 2010 Available online 19 October 2010 Keywords: Electrical switching; Chalcogenide glasses; Differential scanning calorimetry
a b s t r a c t Bulk, melt quenched Ge18Te82-xBix glasses (1 ≤ x ≤ 4) have been found to exhibit memory type electrical switching behavior, which is in agreement with the lower thermal diffusivity values of Ge–Te–Bi samples. A linear variation in switching voltages (Vth) has been found in these samples with increase in thickness which is consistent with the memory type electrical switching. Also, the switching voltages have been found to decrease with an increase in temperature which happens due to the decrease in the activation energy for crystallization at higher temperatures. Further, Vth of Ge18Te82-xBix glasses have been found to decrease with the increase in Bi content, indicating that in the Ge–Te–Bi system, the resistivity of the additive has a stronger role to play in the composition dependence of Vth, in comparison with the network connectivity and rigidity factors. In addition, the composition dependence of crystallization activation energy has been found to show a decrease with an increase in Bi content, which is consistent with the observed decrease in the switching voltages. X-ray diffraction studies on thermally crystallized samples reveal the presence of hexagonal Te, GeTe, Bi2Te3 phases, suggesting that bismuth is not taking part in network formation to a greater extent, as reflected in the variation of switching voltages with the addition of Bi. SEM studies on switched and unswitched regions of Ge–Te–Bi samples indicate that there are morphological changes in the switched region, which can be attributed to the formation of the crystalline channel between two electrodes during switching. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Certain crystalline and amorphous materials, including chalcogenide glasses, are found to exhibit a non destructive electrical switching, from a low conducting (OFF) state to a high conducting (ON) state, upon application of a high electric field. In chalcogenide glasses, the electrical switching phenomenon is of two types, namely memory switching and threshold switching [1–3]. If the ON state of the material is retained even after the removal of electric field, it is called memory switching; whereas if the material reverts back to the high resistance OFF state after removing the electric field, it is called threshold switching. Threshold switching materials require a holding current and voltage to sustain the ON state. Once the holding current is removed, they revert back to their original OFF state. Thermal and electronic are the predominant mechanisms proposed for understanding the electrical switching in amorphous chalcogenides. While the response of electrons to the applied high field is used to explain threshold switching, an additional thermally induced transition to the crystalline state in the electrode region explains memory switching in chalcogenide glasses [4–10]. The phenomenon of memory switching in chalcogenide glasses has recently found applications in Phase Change Memories (PCM)
⁎ Corresponding author. Fax: +91 80 23608686. E-mail address: [email protected] (S. Asokan). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.09.046
which are being considered as a possible replacement for conventional Non Volatile Random Access Memories (NVRAMs). The 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, easiness to integrate with logic, etc. [11,12]. Generally, tellurides have a higher crystallizability in comparison with sulfides and selenides. This fact makes the phase change between amorphous and crystal phases easier in Te based glasses and hence, more useful materials for PCM applications. In particular, the GeTe based glasses have been widely investigated for their suitability for application in phase change memories. Chalcogenide glasses prepared by melt quenching technique are normally p-type conductors. However, it has been found that n-type semiconducting glasses can be obtained by adding Bi to certain chalcogenide glassy systems such as Ge–Se and Ge–Se–Te, etc. [13,14]. The p–n transition has also been found in Ge–Te–Bi glasses [15], which has prompted the present investigations on the electrical switching behavior on Ge18Te82-xBix (1 ≤ x ≤ 4) glasses, with a focus on the variation of threshold voltages with composition, temperature and thickness. In addition, the crystallization process in Ge18Te82-xBix (x = 1, 2, 3) alloys at different heating rates has also been studied by Differential Scanning Calorimetry (DSC), as it is important for phase change applications; the composition dependence of crystallization temperatures and the activation energies for crystallization (obtained
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using the Kissinger's method) have been investigated. Further, Xray diffraction investigations have been undertaken on crystallized Ge–Te–Bi samples to identify the phases formed during devitrification. Finally, the scanning electron microscopic studies have been carried out on the switched and un-switched regions of a representative Ge18Te81Bi1 sample. 2. Experimental procedures Chalcogenide glasses are reactive at high temperature, particularly with O2, and also the constituent elements frequently differ widely in volatility. This necessitates the preparation of these glasses in sealed containers. Further, impurities can alter the electrical switching and thermal behavior of these materials, though chalcogenide glasses are generally not as sensitive to impurities as intrinsic semiconductors. Therefore, the elementary materials used for preparing bulk samples are always of a high purity (99.999%). Bulk Ge18Te82-xBix (1 ≤ x ≤ 4) glasses have been prepared by the conventional melt quenching method. Appropriate quantity of high purity components (Ge, Te and Bi) have been mixed in a quartz ampoule of 6 mm inner diameter and then evacuated to a pressure of 10−5 mbar. The ampoules are maintained at this pressure for about 30 min and sealed under vacuum. The sealed ampoules are heated to a temperature of 1000 °C in a horizontal rotary furnace and held at this temperature for 36 h to homogenize the melt. Subsequently, the ampoules are quenched in ice water + NaOH mixture. The amorphous nature of the quenched samples is confirmed by X-ray diffraction (XRD). The electrical switching characteristics of these glasses have been studied using a Keithley source-measure unit (Model 2410c). Samples polished to a thickness of about 0.25 mm are mounted between a flat bottom electrode and a point contact top electrode. A constant current is passed through the sample and the voltage developed across the sample is measured. The standard deviation method is used for the analysis of errors in the measurement of switching voltages. The thermal parameters such as crystallization temperatures (Tc) of Ge18Te82-xBix glasses have been obtained using a Mettler Toledo Differential Scanning Calorimeter. Samples have been heated at four different heating rates and the activation energy for crystallization has been calculated using Kissinger's equation, with the calorimetric scans made in the range room temperature to 350 °C. The typical error in the measurement is found to be ±1 °C for Tc at different scanning rates. Crystallization studies of two representative Ge18Te81Bi1 and Ge18Te79Bi3 samples have been undertaken by annealing the samples at the respective crystallization temperatures for about 2 h and X-ray diffraction studies have been carried out on the annealed samples in a Philips powder diffractometer with CuKα radiation. The Scanning Electron Microscopic (SEM) images of the switched and un-switched regions of a representative Ge18Te81Bi1 sample has been taken by ESEM Quanta machine. 3. Results Fig. 1 shows the I–V characteristics of Ge18Te82-xBix series of glasses, from which it is clear that these samples exhibit memory type electrical switching. Fig. 2 shows the variation of threshold voltage of Ge18Te82-xBix glasses with Bi content, which indicates that switching voltage decreases with increase in bismuth content. The threshold voltage (Vth) of Ge18Te81Bi1 glass is found to decrease with increase in temperature above room temperature as shown in Fig. 3, which is a common feature exhibited by many memory switching chalcogenides glasses [16–18]. Fig. 4 shows thickness dependence of threshold voltage, indicating an almost linear variation.
Table 1 shows the crystallization temperature (Tc) of three different compositions of Ge–Te–Bi glassy system, at four different heating rates (β). The Kissinger's plot [19] of the variation of log (β/T2c ) versus (1000/ Tc) for a representative Ge18Te81Bi1 has been shown in Fig. 5. The activation energy for crystallization (Ec) has been calculated using the Kissinger's equation [20], ln (β/T2c ) = −Ec/RTc + D. Fig. 6 shows the X-ray diffraction patterns of two representative Ge18Te81Bi1 and Ge18Te79Bi3 samples respectively annealed at their respective crystallization temperature for about 2 h. It can be seen that the diffraction patterns of crystallized sample are indexable with hexagonal Te, GeTe, BiTe and Bi2Te3. Fig. 7 shows the scanning electron micrograph of switched and un-switched regions of Ge18Te81Bi1 sample. 4. Discussion 4.1. Electrical switching behavior of Ge–Te–Bi glasses As mentioned earlier, the memory switching in chalcogenide glasses has a thermal origin involving the formation of a conducting crystalline channel between the electrodes, with the energy required for crystallization provided by the electric field. The Te-rich chalcogenide glasses are normally found to exhibit memory switching because of their comparatively larger electrical conductance which results in a higher power dissipation and therefore the conducting channel could be formed easily. It is also known that weaker bond, poor structural cross-linking and more lone-pair interactions [21] favor memory switching in telluride glasses. The present studies reveal that Ge18Te82-xBix glasses also exhibit memory type electrical switching like many other binary and ternary telluride glasses such as Ge–Te, Si–Te, As–Te, Al–Te, As–Se–Te, Ge–As–Te, Ge–Te–Cu [22–29]. In this context, it is interesting to note that the thermal diffusivity (α) of representative Ge–Te–Bi samples, namely Ge18Te81Bi1, Ge18Te80Bi2 and Ge18Te79Bi3, have been found to be 0.015, 0.0108 and 0.00926 respectively, which is comparatively lower than other chalcogenide glasses [30,31]. Usually, chalcogenide glasses which have higher thermal diffusivity exhibit threshold type electrical switching and those with a lower α show memory behavior [32]. The memory behavior seen in Ge–Te–Bi glasses is therefore consistent with thermal diffusivity values of these samples. The composition dependence of threshold/switching voltage in chalcogenide glasses is determined by three main factors, namely the resistivity of the additive elements [33], network connectivity, rigidity [34] and chemical ordering [35]. The addition of more metallic elements usually brings down the switching voltages of chalcogenide glasses; this is also the case in the present Ge18Te82-xBix system in which Bi is more metallic compared to Te (ρBi = 1.29 × 10−6 Ω m and ρTe = 1 × 10−4 Ω m) and the replacement of Te with Bi leads to the reduction in the threshold voltages (Fig. 2). The rigidity percolation deals with dimensionality and rigidity of a glassy network. For a covalent network glass, the network connectivity & rigidity increases with the increase in average coordination number and at a mean coordination number brcN = 2.4 (known as the Stiffness Threshold or Rigidity Percolation Threshold) [36–38], the glassy system exhibits a transition from a floppy polymeric glass to a rigid amorphous solid. In general, the memory switching voltages of chalcogenide glasses are found to increase with an increase in network connectivity and rigidity as the structural rearrangements become more difficult with increasing network rigidity [34]. In the Ge18Te82-xBix system, the addition of Bi with a higher coordination number of 3 [39], at the expense of Te with the coordination number of 2 [40], results in a progressive increase in network connectivity and rigidity. Hence, based on the network connectivity rigidity, one could expect an increase in switching voltages with the addition of Bi. However, the present results show that the switching voltages of Ge18Te82-xBix
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Fig. 1. I–V characteristics of (a) Ge18Te81Bi1 (b) Ge18Te80Bi2 (c) Ge18Te79Bi3 and (d) Ge18Te78Bi4 representing the Ge18Te82-xBix series.
glasses show a decrease with the increase in Bi concentration, which indicates that in this system the effect of resistivity of the dopant is more pronounced than the network connectivity and rigidity. Further, in many chalcogenide systems, a sharp signature of the rigidity percolation is seen in the variation of switching voltages at brcN. However, in the present Ge18Te82-xBix system, it has not been possible to probe the effect of stiffness transition on switching voltages as the composition range of bulk glass formation is limited only to lower brN values (brN ≤2.4). 4.2. Temperature dependence of threshold voltages
chalcogenide glasses [16–18]. The variation of threshold voltage of Ge18Te81Bi1 glass with temperature (Fig. 3) shows that the Vth of Ge18Te82-xBix glasses decrease as the temperature is increased above room temperature. In case of memory switching glasses the decrease in switching voltages with temperature has been explained on the basis of a configurational free-energy diagram [25], according to which the decrease in Vth is due to the decrease in the energy barrier required for crystallization of a sample with the increase in temperature. In addition, at higher temperature, the charged defect centers are filled up by thermally excited charge carriers, which are in addition to field-injected carriers, resulting in decrease in switching voltage [41].
The decreasing trend in switching voltages with increase in temperature is a common feature in many memory switching
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4.3. Thickness dependence of switching voltages
Fig. 5. The Kissinger's plot showing the variation of ln (β/T2c ) Vs. (1000/Tc) for Ge18Te81Bi1 glass.
reflected in the variation of switching voltages which shows no increase with the addition of Bi.
It is known that in memory switching samples, the threshold voltage (Vth) is found to be proportional to “t” or “t1/2”, where “t” is the thickness of the sample. A linear variation of Vth with t, has been seen in samples like Ge–Te–Al [42], As–Se [43], Ge–Se–Tl [44]. However, a few samples like Ge–As–Te [45], Ge–Te [22] have been found to exhibit a t1/2 variation with Vth. In the present sample, the variation of threshold voltage Vth with thickness has been observed to be linear (Fig. 4).
4.6. Electron microscopic analysis of Ge–Te–Bi It is clear from the SEM images of switched and un-switched regions of Ge18Te81Bi1 sample which indicate that there are morphological changes in the switched region. The morphological
4.5. Crystallization behavior of Ge18Te82-xBix glasses X-ray diffraction studies on Ge18Te81Bi1 and Ge18Te79Bi3 annealed for 2 h under vacuum at their respective crystallization temperatures reveal the presence of hexagonal Te and hexagonal GeTe. In addition, hexagonal BiTe and hexagonal Bi2Te3 have also been found in the crystallized Ge–Te–Bi samples. In this context, it is interesting to compare the earlier studies on the crystallization behavior of Ge–Te–Ag [46] glasses; X-ray investigations on crystallized Ge–Te–Ag samples show that silver forms a ternary phase along with possible binary phases in this system [46]. However, in the case of Ge–Te–Bi samples, no ternary phase has been found and in fact, more number of binary phases is found. The possible reason for not finding any ternary phases of bismuth or more number of binary phases in Ge18Te79Bi3 could be that bismuth is not taking part in network formation to a greater extent. This aspect is also
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Intensity (arb. units)
4.4. Activation energy for crystallization
Table 1 The crystallization temperatures at different heating rates and the estimated crystallization activation energies of Ge18Te82-xBix glasses of different compositions. Composition x (at.%)
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2θ Fig. 6. The X-ray diffraction patterns of Ge18Te81Bi1 and Ge18Te79Bi3 glasses annealed at their respective crystallization temperatures.
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play in comparison with the network connectivity and rigidity factors. The composition dependence of activation energy for crystallization of Ge18Te82-xBix glasses, exhibits a decrease with increase in Bi content, which is consistent with the decrease in switching voltages with Bi content. Based on the X-ray diffraction studies of thermally crystallized samples, it is proposed that bismuth is not taking part in network formation in a greater extent, which is reflected in the variation of switching voltages with the addition of Bi. SEM studies on switched and un-switched regions of Ge–Te–Bi sample indicate that there are morphological changes in the switched region, which can be attributed to the formation of the crystalline channel formation between two electrodes during switching. References
b
Fig. 7. Scanning electron micrographs of (a) un-switched and (b) switched regions of Ge18Te81Bi1 glass.
changes are due to the formation of the crystalline channel between two electrodes during switching. 5. Conclusions Melt quenched Ge18Te82-xBix glasses have been found to exhibit memory switching. The lower thermal diffusivity values of these samples are in agreement with the observed memory switching behavior. A linear increase has been found in switching voltages with increase in sample thickness which is also consistent with the memory switching exhibited by Ge–Te–Bi samples. Also, the switching voltages have been found to decrease with increase in temperature due to decrease in the activation energy for crystallization at higher temperatures. Further, the switching voltages of the Ge–Te–Bi samples have been found to decrease with increase in Bi content, which indicates that resistivity of the additive has a stronger role to
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