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Electrical switching and thermal behavior of ternary Si15Te85-xBix (0 ≤ x ≤ 2) chalcogenide glasses
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 21292–21298
www.materialstoday.com/proceedings
ICSEM 2016
Electrical switching studies of ternary Si15Te85-xBix (0 ≤ x ≤ 2) chalcogenide glasses Brian Jeevan Fernandes a* , Pumlian Munga b, K. Ramesh b, N. K. Udayashankar a b
a Department of Physics, National Institute of Technology Karnataka, Surathkal 575025, India. Department of Physics, Indian Institute of Science, C.V.Raman Avenue, Bengaluru 560012,India.
Abstract Bulk semiconducting Si15Te85-xBix(0 ≤ x ≤ 2) chalcogenide glasses have been prepared using a well established melt-quenching technique. Electrical switching studies have been undertaken on Si 15Te85-xBix(0 ≤ x ≤ 2) chalcogenide glasses. The results indicate that these samples exhibit memory type electrical switching behavior. It has been observed that the switching voltage of the glasses decreases with the addition of Bi. In addition, OFF state resistivity of the samples have been found to decrease with the increase in Bi concentration and are related to the observed decrease in switching voltages. The switching voltage ( has been found to increase with the thickness of the sample and decrease with increase in temperature confirming the thermal origin of the memory switching process. Further, scanning electron microscopy (SEM) studies reveal the formation of a crystalline channel indicating the conducting path between the two electrodes in the switched region. © 2018 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of INTERNATIONAL CONFERENCE ON SMART ENGINEERING MATERIALS (ICSEM 2016). Keywords: chalcogenide glasses; electrical switching; OFF state resistivity; scanning electron microscopy(SEM)
1. Introduction Ovshinsky [1] discovered one of the scientifically interesting and technologically important phenomena called electrical switching which is exhibited by amorphous/glassy chalcogenides (semiconductors!). By applying a
* Corresponding author. Tel.: = +91- 9945362005. E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of INTERNATIONAL CONFERENCE ON SMART ENGINEERING MATERIALS (ICSEM 2016).
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suitable electric field, the chalcogenide glasses can be switched from a high resistance amorphous (OFF/RESET) state to a low resistance (ON/SET) crystalline state. The voltage at which switching occurs is known as a threshold or switching voltage (VT). There are two types of switching, namely the threshold and memory switching. The threshold type is reversible while memory switching is an irreversible process. The memory material undergoes a phase change (amorphous to crystalline) that can be brought back to the amorphous state by applying an appropriate RESET current pulse. Hence memory material is known as Phase Change Memory (PCM). Owing to the unique properties of amorphous chalcogenides such as scalability, adequate switching speed and the simplicity of the storage concept, they have attracted the interest of industry for being a potential candidate to replace the present day data storage [2-5]. Several Te and Se-based chalcogenide glasses have been found to exhibit electrical switching [6-18]. Further, the addition of metallic impurities to the chalcogenide glasses is found to bring significant changes in the electrical switching behavior of these glasses, such as variation in VT values, a cross-over in switching type, etc. Defects play a major role in the switching process. Switching occurs when the charged defect states present in the chalcogenide glasses are filled by the field injected charge carriers[3, 19-24]. Thermal effect plays a key role in memory switching which guides to the creation of a conducting crystallization channel in the electrode region[25 ]. Generally, prepared chalcogenide glasses are p-type of semiconductor in nature, though it has been found that we obtain n-type of semiconducting glasses by doping Bismuth (Bi) to certain chalcogenide systems [26] . Bismuth is mainly metallic and least abundant of the elements in the nitrogen group. It is hard, brittle, shiny, and roughly crystalline material. Attractive material properties and p-n transition behavior of certain chalcogenide systems containing Bi have motivated the present investigations on electrical switching of Si15Te85-xBix ( 0 ≤ x ≤ 2) glasses with a focus on the distinction of threshold voltages with composition, temperature and thickness. SEM morphological studies have been carried out on the switched and un-switched surface of the sample. 2. Experimental Details Bulk Si15Te85-xBix(0 ≤ x ≤ 2) glasses were prepared by melt quenching method. Suitable quantities of constituent elements (99.99% pure) were weighed and transferred into a flattened quartz ampoule of 6mm inner diameter and 8mm outer diameter, which was evacuated to a pressure of ~ 10 -5 Torr (Avac Vacuum, India). The ampoules were maintained at this pressure for 30 min, after which they were sealed under vacuum. The sealed ampoule was loaded into a custom built electric furnace (Indfurr, India) slowly heated to 1273K at the rate of 373K/h. The ampoule was maintained at this temperature for 24h under continuous rotation at 10 rpm to ensure homogeneity of the melt. The ampoules were subsequently quenched in an ice water bath mixed with NaCl to obtain bulk glasses. The amorphous nature of the quenched glasses was confirmed by X-Ray diffraction. The electrical switching studies were undertaken using a programmable dc source-meter (Keithley 2410c, Canada). Samples polished to about 0.30mm thickness were mounted in a two probe sample holder made of brass between a flat bottom electrode and a pointed electrode using a spring loaded mechanism. A current of 0 – 2 mA was passed through the sample and the voltage developed across the sample was measured. Temperature dependence of switching voltage was studied in the range of 303-383K using a custom built heater cell (Indfurr, India) provided with a thermocouple. The micro-structural analysis of as-prepared samples was performed with a high-resolution scanning electron microscope (JEOL JSM 6380AL, Japan) in the magnification range of 5000 X. To avoid charging effect, the specimen was coated with gold. 3. Results and Discussions 3.1. XRD Studies Figure 1.shows the XRD pattern of the as-prepared representative Si15Te84Bi1 glass samples. The amorphous nature of as-prepared samples is proved by the absence of sharp diffraction peaks in the XRD patterns.
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Figure 1: XRD patterns of the representative (Si15Te84Bi1) sample showing absence of sharp diffraction peaks
3.2. I-V characteristics of the Ge20Te80-xSnx glasses and thermal mechanism Figure 2(a)-(c). show the I-V characteristics of Si15Te85-xBix(0 ≤ x ≤ 2) glasses. It is clear from the graph that at lower applied current, the samples exhibit an ohmic behavior. At a critical voltage called threshold voltage V T, the samples exhibit a rapid switching from a high resistance OFF state to a low resistance ON state. It is found that samples do not revert back to their original OFF state resistance even after removing the applied electric field. This observation indicates that in Si15Te85-xBix(0 ≤ x ≤ 2) glasses, memory switching behavior occurs at a comparatively lower applied current (2 mA).
Figure 2: I-V characteristics of (a) Si15Te85, (b) Si15Te84Bi1, (c) Si15Te83Bi2 and (d) composition dependence of threshold voltage (VT) and OFF sate resistivity of Si15Te85-xBix(0 ≤ x ≤ 2) chalcogenide glasses as a function of atomic percentage of Bi
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There are many factors which determine memory switching, yet thermal effect plays a major role. Memory switching in chalcogenide involves the formation of a crystalline channel in the material between the electrodes. Memory switching is usually observed in poorly connected Te-rich glasses because of their greater conductance that results in greater power dissipation and ease of formation of conducting channel. The formation of conducting paths is more drastic in glass composition which devitrifies easily [27]. Glasses with lower thermal diffusivity are likely to exhibit memory switching [28]. These discussions suggest that thermal mechanism is involved in memory switching . It has been observed that tellurium-rich glasses exhibit a clean electrical switching and low voltage without any fluctuations in the I-V characteristics during the transition to the ON state[6]. 3.3. Composition dependence of threshold voltage (V T) Figure 2(d) shows the compositional dependence of switching voltages and starting electrical resistivity values of Si15Te85-xBix(0 ≤ x ≤ 2) glasses. The graph indicates that V T of these glasses decreases with increase in Bi content. We can see a considerable decrease in switching voltage with a small increase in the atomic percentage of Bi. Previous studies have revealed that the composition dependent switching voltage is determined mainly by the metallic factor of the additives, the connectivity or the rigidity of the network. According to the network constraint theory[29], the addition of higher coordinated atoms increases the network connectivity and hence the switching voltages. This is because of the increase in complexity of the structural reorganization required for memory switching. However, it is believed that the addition of metallic impurities changes the network connectivity and rigidity [30]. It is important to note in the present study that the results show a decrease in V T with an increase in Bi content. In Si15Te85-xBix(0 ≤ x ≤ 2) glasses, the addition of Bi-metal at the cost of Te leads to lower switching voltages due to the fact that resistivity of Bi is very less compared to that of Te (Bi = 1.29 x Ωm and Te=10 x10-5Ωm). These samples exhibit memory switching for the reason that the low resistivity allows higher currents to be carried in the sample and as higher currents flow in the sample, the temperature rises due to Joule’s heating which aids the structural transformation. This phenomenon results in low values of switching voltages [31]. In glasses which contain substantial amounts of Te, the filament consists mainly of degenerate Te with the metallic type of conductivity[3]. The addition of Bi on Si 15Te85 base glass makes it more metallic in nature. In general, the addition of more metallic impurities to the base glass system lowers the resistivity and activation energy required for electrical conductivity which in turn decreases the switching voltage of the sample. The addition of metallic additives such as Sb and Ag, [6,32] to the Si-Te system shows a similar decrease in the switching voltage, thus signifying that the present results are consistent with earlier observations. This seems to rule out the role of network connectivity and rigidity in switching voltage. The decrease in switching voltages of Si15Te85-xBix (0 ≤ x ≤ 2) glasses observed with the addition of Bi indicates that the metallicity of the dopant plays a dominant role in this system over network connectivity and rigidity. In addition, a direct relationship has been established in the composition dependence of switching voltages and starting electrical resistivity or OFF state resistivity. The starting electrical resistance decreases with the increase in Bi content. The same is observed in Figure 2(d). Literature report supports a direct correlation between VT and starting electrical resistivity [6,17]. 3.4. Thickness and Temperature dependence of switching voltages. Figure 3 (a). shows a variation of switching voltage with thickness (in the range 0.10 mm to 0.60 mm) for the Si15Te83Bi2 glass sample. The overall features of I-V characteristics of Si15Te83Bi2 glass sample are not altered with the sample thickness. In the case of samples exhibiting memory switching, threshold voltage V T varies linearly or has a square root dependence (d1/2) on thickness. In the case of threshold switching, square dependence(d 2) on thickness is observed[33]. However, the switching voltage is found to be linearly increasing with an increase in sample thickness. It is apparent that a linear relationship between the switching voltage and the thickness of the device is relevant. This result appears to exclude a purely thermal theory [34,35] which predicts that the switching voltage should change with the square root of the device thickness.
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Figure 3 : (a) Variation of switching voltage (VT ) with respect to thickness of a representative Si15Te83Bi2 chalcogenide glass sample. (b) Variation of switching voltage (VT ) with respect to temperature of a representative Si15Te83Bi2 chalcogenide glass sample
Figure 3 (b). shows decreasing trend of a Si15Te83Bi2 glass at different temperatures. The switching voltage is reduced with increasing temperature staying to the fact that activation energy required for the phase change reduces. The energy barriers for crystallization are reduced at elevated temperatures resulting in the decrease of switching voltage of memory type materials [36]. It is observed that this decrease in the switching voltage with an increase in temperature is moderate. This suggests that memory samples have moderate thermal stability. The electrical conductivity increases with increase in temperature which confirms the semiconducting nature of the sample. The temperature dependence on the threshold switching voltage can be expressed as [37],
(1) Where VT is the threshold switching voltage, is the threshold voltage - activation energy, is the Boltzmann constant (8.617 x eV ) and T is the temperature in Kelvin. The threshold voltage-activation energy at elevated temperatures calculated from the above expression is tabulated in Table 1. It can be noted that the value of the activation energy decreases with an increase in the temperature which can be directly related to the decrease in VT observed at elevated temperatures. Table 1: The values of activation energy (
at different temperature ranges.
Temperature range (K) 303-323 323-343 343-363 363-383
Activation energy (eV) 0.16 0.11 0.02 0.01
These observations made on the thickness and temperature dependence of switching voltages provide a definite experimental evidence of thermal effect playing a significant role in the switching process. 3.5. Microscopic study of switched region. Figure 4. shows the scanning electron micrograph of the surface morphology of the unswitched and switched samples. The conducting filament is formed by glass- melt- crystal transition. This transition can be observed after the switching is complete. It is known that once the filaments are formed, the crystalline phases are retained even after the removal of the voltage which causes the memory switching [38]. In SEM images we can see crystallized
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melt representing the conducting filament formed during switching on the surface of the sample which is observed after the switching process has been completed. The occurrence of this channel is an additional experimental evidence for the filament formation, hence the thermal mechanism plays a major role in the memory switching process.
Figure 4: SEM micrographs of (a) unswitched (b) switched Si15Te84Bi2 chalcogenide glass sample. SEM image in (b) clearly shows a crystallized melt representing the conducting filament formed during switching.
Conclusion Electrical studies indicate that Si15Te85-xBix ( 0 ≤ x ≤ 2) chalcogenide glasses exhibit memory type of switching. The switching voltages are found to decrease with Bi content. In addition, OFF state resistance of Si 15Te85-xBix ( 0 ≤ x ≤ 2) glasses have been found to decrease with Bi concentration which is in agreement with the observed decrease in switching voltages with composition. Thickness dependence of switching voltages of the prepared glasses is found to be linear and in agreement with the memory type of switching behavior shown by these glasses. Temperature dependence of suggests that thermal effects play a major role during the switching process. Formation of conduction channel on the surface of the sample gives additional evidence on the role of thermal effect in memory switching process. The prepared glasses may find their applications in display or RF devices. Acknowledgements The authors thank the Department of Materials and metallurgical Engineering, NITK Surathkal for the SEM facility. Authors thank the Department of Physics, NITK Surathkal for extending the XRD facility. References [1] S.R. Ovshinsky, Reversible electrical switching phenomena in disordered structures, Phys. Rev. Lett. 21 (1968) 1450–1453. [2] S.R. Ovshinsky, H. Fritzsche, Amorphous semiconductors for switching, memory, and imaging applications, IEEE Trans. Electron Devices. 20 (1973) 91–105. [3] D. Adler, H.K. Henisch, S.N. Mott, The mechanism of threshold switching in amorphous alloys, Rev. Mod. Phys. 50 (1978) 209–220. [4] M. Wuttig, Phase-change materials: Towards a universal memory?, Nat. Mater. 4 (2005) 265–266. [5] A.E. Owen, J.M. Robertson, Electronic conduction and switching in chalcogenide glasses, IEEE Trans. Electron Devices. 20 (1973) 105– 122. [6] R. Lokesh, N.K. Udayashankar, S. Asokan, Electrical switching behavior of bulk Si15Te85−xSbx chalcogenide glasses – A study of compositional dependence, J. Non-Cryst. Solids. 356 (2010) 321–325. [7] V.C. Selvaraju, S. Asokan, V. Srinivasan, Electrical switching studies on As 40Te60-xSex and As35Te65-xSex glasses, Appl. Phys. A. 77 (2003) 149–153. [8] C. Das, M.G. Mahesha, G.M. Rao, S. Asokan, Electrical switching and optical studies on amorphous GexSe35−xTe65 thin films, Thin Solid Films. 520 (2012) 2278–2282.
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[9] B.H. Sharmila, S. Asokan, Studies on electrical switching behavior of As-Te-Tl glasses – effect of local structure on switching type and composition dependence of switching voltages, Appl. Phys. A. 82 (2005) 345–348. [10] M. Dominguez, E. Marquez, P. Villares, R. Jimenez-Garay, On the electrical conduction and current-controlled negative differential resistance with memory in bulk samples of glassy semiconductor Cu0.05As0.50Te0.45: NTC thermistor-type behavior, Semicond. Sci. Technol. 3 (1988) 1106. [11] C.N. Murthy, V. Ganesan, S. Asokan, Electrical switching and topological thresholds in Ge-Te and Si-Te glasses, Appl. Phys. A. 81 (2005) 939–942. [12] R. Chatterjee, S. Asokan, S.S.K. Titus, Current-controlled negative-resistance behavior and memory switching in bulk As-Te-Se glasses, J. Phys. Appl. Phys. 27 (1994) 2624. [13] M. Anbarasu, S. Asokan, Electrical switching behavior of bulk As–Te–Si glasses: composition dependence and topological effects, Appl. Phys. A. 80 (2004) 249–252. [14] S. Prakash, S. Asokan, D.B. Ghare, Easily reversible memory switching in Ge - As - Te glasses, J. Phys. Appl. Phys. 29 (1996) 2004. [15] K. Ramesh, S. Asokan, K.S. Sangunni, E.S.R. Gopal, Electrical switching in germanium telluride glasses doped with Cu and Ag, Appl. Phys. A. 69 (1999) 421–425. [16] P. Pattanayak, S. Asokan, Anomalous electrical switching behavior in phase-separated bulk Ge–Se–Ag chalcogenide glasses, Europhys. Lett. EPL. 75 (2006) 778–783. [17] B.J. Fernandes, K. Sridharan, P. Munga, K. Ramesh, N.K. Udayashankar, Memory type switching behavior of ternary Ge20Te80−xSnx (0 ⩽ x ⩽ 4) chalcogenide compounds, J. Phys. Appl. Phys. 49 (2016) 295104. [18] Pumlianmunga, R. Venkatesh, E.S.R. Gopal, K. Ramesh, The mechanism of memory and threshold switching in (GeTe4)100−x(As2Se3)x glasses, J. Non-Cryst. Solids. 452 (2016) 210–219. [19] D. Adler, M.S. Shur, M. Silver, S.R. Ovshinsky, Threshold switching in chalcogenide‐glass thin films, J. Appl. Phys. 51 (1980) 3289–3309. [20] D. Adler, Defects in amorphous semiconductors, J. Non-Cryst. Solids. 35–36, Part 2 (1980) 819–824. [21] D. Adler, Amorphous-Semiconductor Devices, Sci. Am. 236 (1977) 36–48. [22] S.R. Ovshinsky, H. Fritzsche, Reversible structural transformations in amorphous semiconductors for memory and logic, Metall. Trans. 2 (n.d.) 641–645. [23] A.E. Owen, J.M. Robertson, C. Main, The threshold characteristics of chalcogenide-glass memory switches, J. Non-Cryst. Solids. 32 (1979) 29–52. [24] K.E. Petersen, D. Adler, On state of amorphous threshold switches, J. Appl. Phys. 47 (1976) 256–263. [25] B.G. Bagley, H.E. Bair, Thermally induced transformations in glassy chalcogenides, J. Non-Cryst. Solids. 2 (1970) 155–160. [26] N. Tohge, T. Minami, Y. Yamamoto, M. Tanaka, Electrical and optical properties of n-type semiconducting chalcogenide glasses in the system Ge‐Bi‐Se, J. Appl. Phys. 51 (1980) 1048– 1053. [27] H.J. Stocker, Phenomenology of switching and memory effects in semiconducting chalcogenide glasses, J. Non-Cryst. Solids. 2 (1970) 371– 381 [28] F.M. Collins, Switching by thermal avalanche in semiconducting glass films, J. Non-Cryst. Solids. 2 (1970) 496–503. [29] J.C. Phillips, Topology of covalent non-crystalline solids I: Short-range order in chalcogenide alloys, J. Non-Cryst. Solids. 34 (1979) 153– 181. [30] J.T. Devaraju, B.H. Sharmila, S. Asokan, K.V. Acharya, Threshold electrical switching in As45Te55-xInx and As50Te50-xInx glasses, Appl. Phys. A. 75 (2002) 515–518. [31] S.R. Gunti, S. Asokan, Thermodynamic, Raman and electrical switching studies on Si15Te85-xAgx (4 ≤ x ≤ 20) glasses, J. Appl. Phys. 111 (2012) 33518. [32] G. Jones, R.A. Collins, Threshold and memory switching in amorphous selenium thin films, Phys. Status Solidi A. 53 (1979) 339–350. [33] J.J. O’Dwyer, Theory of Dielectric Breakdown in Solids, J. Electrochem. Soc. 116 (1969) 239–242. [34] K. Nakashima, K.C. Kao, Conducting filaments and switching phenomena in chalcogenide semiconductors, J. Non-Cryst. Solids. 33 (1979) 189–204. [35] S. Murugavel, S. Asokan, Composition tunable memory and threshold switching in Al20AsxTe80−x semiconducting glasses, J. Mater. Res. 13 (1998) 2982–2987. [36] K. Shimakawa, Y. Inagaki, T. Arizumi, Thermal switching in chalcogenide glass semiconductors, Jpn. J. Appl. Phys. 12 (1973) 1043–1046. [37] R. Karuppannan, V. Ganesan, S. Asokan, Electrical switching in Cu–As–Se glasses, Int. J. Appl. Glass Sci. 2 (2011) 52–62. [38] V. Modgil, V.S. Rangra, Effect of Sn addition on thermal and optical properties of glass, J. Mater. 2014 (2014) e318262.
ScienceDirect Materials Today: Proceedings 5 (2018) 21292–21298
www.materialstoday.com/proceedings
ICSEM 2016
Electrical switching studies of ternary Si15Te85-xBix (0 ≤ x ≤ 2) chalcogenide glasses Brian Jeevan Fernandes a* , Pumlian Munga b, K. Ramesh b, N. K. Udayashankar a b
a Department of Physics, National Institute of Technology Karnataka, Surathkal 575025, India. Department of Physics, Indian Institute of Science, C.V.Raman Avenue, Bengaluru 560012,India.
Abstract Bulk semiconducting Si15Te85-xBix(0 ≤ x ≤ 2) chalcogenide glasses have been prepared using a well established melt-quenching technique. Electrical switching studies have been undertaken on Si 15Te85-xBix(0 ≤ x ≤ 2) chalcogenide glasses. The results indicate that these samples exhibit memory type electrical switching behavior. It has been observed that the switching voltage of the glasses decreases with the addition of Bi. In addition, OFF state resistivity of the samples have been found to decrease with the increase in Bi concentration and are related to the observed decrease in switching voltages. The switching voltage ( has been found to increase with the thickness of the sample and decrease with increase in temperature confirming the thermal origin of the memory switching process. Further, scanning electron microscopy (SEM) studies reveal the formation of a crystalline channel indicating the conducting path between the two electrodes in the switched region. © 2018 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of INTERNATIONAL CONFERENCE ON SMART ENGINEERING MATERIALS (ICSEM 2016). Keywords: chalcogenide glasses; electrical switching; OFF state resistivity; scanning electron microscopy(SEM)
1. Introduction Ovshinsky [1] discovered one of the scientifically interesting and technologically important phenomena called electrical switching which is exhibited by amorphous/glassy chalcogenides (semiconductors!). By applying a
* Corresponding author. Tel.: = +91- 9945362005. E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of INTERNATIONAL CONFERENCE ON SMART ENGINEERING MATERIALS (ICSEM 2016).
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suitable electric field, the chalcogenide glasses can be switched from a high resistance amorphous (OFF/RESET) state to a low resistance (ON/SET) crystalline state. The voltage at which switching occurs is known as a threshold or switching voltage (VT). There are two types of switching, namely the threshold and memory switching. The threshold type is reversible while memory switching is an irreversible process. The memory material undergoes a phase change (amorphous to crystalline) that can be brought back to the amorphous state by applying an appropriate RESET current pulse. Hence memory material is known as Phase Change Memory (PCM). Owing to the unique properties of amorphous chalcogenides such as scalability, adequate switching speed and the simplicity of the storage concept, they have attracted the interest of industry for being a potential candidate to replace the present day data storage [2-5]. Several Te and Se-based chalcogenide glasses have been found to exhibit electrical switching [6-18]. Further, the addition of metallic impurities to the chalcogenide glasses is found to bring significant changes in the electrical switching behavior of these glasses, such as variation in VT values, a cross-over in switching type, etc. Defects play a major role in the switching process. Switching occurs when the charged defect states present in the chalcogenide glasses are filled by the field injected charge carriers[3, 19-24]. Thermal effect plays a key role in memory switching which guides to the creation of a conducting crystallization channel in the electrode region[25 ]. Generally, prepared chalcogenide glasses are p-type of semiconductor in nature, though it has been found that we obtain n-type of semiconducting glasses by doping Bismuth (Bi) to certain chalcogenide systems [26] . Bismuth is mainly metallic and least abundant of the elements in the nitrogen group. It is hard, brittle, shiny, and roughly crystalline material. Attractive material properties and p-n transition behavior of certain chalcogenide systems containing Bi have motivated the present investigations on electrical switching of Si15Te85-xBix ( 0 ≤ x ≤ 2) glasses with a focus on the distinction of threshold voltages with composition, temperature and thickness. SEM morphological studies have been carried out on the switched and un-switched surface of the sample. 2. Experimental Details Bulk Si15Te85-xBix(0 ≤ x ≤ 2) glasses were prepared by melt quenching method. Suitable quantities of constituent elements (99.99% pure) were weighed and transferred into a flattened quartz ampoule of 6mm inner diameter and 8mm outer diameter, which was evacuated to a pressure of ~ 10 -5 Torr (Avac Vacuum, India). The ampoules were maintained at this pressure for 30 min, after which they were sealed under vacuum. The sealed ampoule was loaded into a custom built electric furnace (Indfurr, India) slowly heated to 1273K at the rate of 373K/h. The ampoule was maintained at this temperature for 24h under continuous rotation at 10 rpm to ensure homogeneity of the melt. The ampoules were subsequently quenched in an ice water bath mixed with NaCl to obtain bulk glasses. The amorphous nature of the quenched glasses was confirmed by X-Ray diffraction. The electrical switching studies were undertaken using a programmable dc source-meter (Keithley 2410c, Canada). Samples polished to about 0.30mm thickness were mounted in a two probe sample holder made of brass between a flat bottom electrode and a pointed electrode using a spring loaded mechanism. A current of 0 – 2 mA was passed through the sample and the voltage developed across the sample was measured. Temperature dependence of switching voltage was studied in the range of 303-383K using a custom built heater cell (Indfurr, India) provided with a thermocouple. The micro-structural analysis of as-prepared samples was performed with a high-resolution scanning electron microscope (JEOL JSM 6380AL, Japan) in the magnification range of 5000 X. To avoid charging effect, the specimen was coated with gold. 3. Results and Discussions 3.1. XRD Studies Figure 1.shows the XRD pattern of the as-prepared representative Si15Te84Bi1 glass samples. The amorphous nature of as-prepared samples is proved by the absence of sharp diffraction peaks in the XRD patterns.
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Figure 1: XRD patterns of the representative (Si15Te84Bi1) sample showing absence of sharp diffraction peaks
3.2. I-V characteristics of the Ge20Te80-xSnx glasses and thermal mechanism Figure 2(a)-(c). show the I-V characteristics of Si15Te85-xBix(0 ≤ x ≤ 2) glasses. It is clear from the graph that at lower applied current, the samples exhibit an ohmic behavior. At a critical voltage called threshold voltage V T, the samples exhibit a rapid switching from a high resistance OFF state to a low resistance ON state. It is found that samples do not revert back to their original OFF state resistance even after removing the applied electric field. This observation indicates that in Si15Te85-xBix(0 ≤ x ≤ 2) glasses, memory switching behavior occurs at a comparatively lower applied current (2 mA).
Figure 2: I-V characteristics of (a) Si15Te85, (b) Si15Te84Bi1, (c) Si15Te83Bi2 and (d) composition dependence of threshold voltage (VT) and OFF sate resistivity of Si15Te85-xBix(0 ≤ x ≤ 2) chalcogenide glasses as a function of atomic percentage of Bi
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There are many factors which determine memory switching, yet thermal effect plays a major role. Memory switching in chalcogenide involves the formation of a crystalline channel in the material between the electrodes. Memory switching is usually observed in poorly connected Te-rich glasses because of their greater conductance that results in greater power dissipation and ease of formation of conducting channel. The formation of conducting paths is more drastic in glass composition which devitrifies easily [27]. Glasses with lower thermal diffusivity are likely to exhibit memory switching [28]. These discussions suggest that thermal mechanism is involved in memory switching . It has been observed that tellurium-rich glasses exhibit a clean electrical switching and low voltage without any fluctuations in the I-V characteristics during the transition to the ON state[6]. 3.3. Composition dependence of threshold voltage (V T) Figure 2(d) shows the compositional dependence of switching voltages and starting electrical resistivity values of Si15Te85-xBix(0 ≤ x ≤ 2) glasses. The graph indicates that V T of these glasses decreases with increase in Bi content. We can see a considerable decrease in switching voltage with a small increase in the atomic percentage of Bi. Previous studies have revealed that the composition dependent switching voltage is determined mainly by the metallic factor of the additives, the connectivity or the rigidity of the network. According to the network constraint theory[29], the addition of higher coordinated atoms increases the network connectivity and hence the switching voltages. This is because of the increase in complexity of the structural reorganization required for memory switching. However, it is believed that the addition of metallic impurities changes the network connectivity and rigidity [30]. It is important to note in the present study that the results show a decrease in V T with an increase in Bi content. In Si15Te85-xBix(0 ≤ x ≤ 2) glasses, the addition of Bi-metal at the cost of Te leads to lower switching voltages due to the fact that resistivity of Bi is very less compared to that of Te (Bi = 1.29 x Ωm and Te=10 x10-5Ωm). These samples exhibit memory switching for the reason that the low resistivity allows higher currents to be carried in the sample and as higher currents flow in the sample, the temperature rises due to Joule’s heating which aids the structural transformation. This phenomenon results in low values of switching voltages [31]. In glasses which contain substantial amounts of Te, the filament consists mainly of degenerate Te with the metallic type of conductivity[3]. The addition of Bi on Si 15Te85 base glass makes it more metallic in nature. In general, the addition of more metallic impurities to the base glass system lowers the resistivity and activation energy required for electrical conductivity which in turn decreases the switching voltage of the sample. The addition of metallic additives such as Sb and Ag, [6,32] to the Si-Te system shows a similar decrease in the switching voltage, thus signifying that the present results are consistent with earlier observations. This seems to rule out the role of network connectivity and rigidity in switching voltage. The decrease in switching voltages of Si15Te85-xBix (0 ≤ x ≤ 2) glasses observed with the addition of Bi indicates that the metallicity of the dopant plays a dominant role in this system over network connectivity and rigidity. In addition, a direct relationship has been established in the composition dependence of switching voltages and starting electrical resistivity or OFF state resistivity. The starting electrical resistance decreases with the increase in Bi content. The same is observed in Figure 2(d). Literature report supports a direct correlation between VT and starting electrical resistivity [6,17]. 3.4. Thickness and Temperature dependence of switching voltages. Figure 3 (a). shows a variation of switching voltage with thickness (in the range 0.10 mm to 0.60 mm) for the Si15Te83Bi2 glass sample. The overall features of I-V characteristics of Si15Te83Bi2 glass sample are not altered with the sample thickness. In the case of samples exhibiting memory switching, threshold voltage V T varies linearly or has a square root dependence (d1/2) on thickness. In the case of threshold switching, square dependence(d 2) on thickness is observed[33]. However, the switching voltage is found to be linearly increasing with an increase in sample thickness. It is apparent that a linear relationship between the switching voltage and the thickness of the device is relevant. This result appears to exclude a purely thermal theory [34,35] which predicts that the switching voltage should change with the square root of the device thickness.
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Figure 3 : (a) Variation of switching voltage (VT ) with respect to thickness of a representative Si15Te83Bi2 chalcogenide glass sample. (b) Variation of switching voltage (VT ) with respect to temperature of a representative Si15Te83Bi2 chalcogenide glass sample
Figure 3 (b). shows decreasing trend of a Si15Te83Bi2 glass at different temperatures. The switching voltage is reduced with increasing temperature staying to the fact that activation energy required for the phase change reduces. The energy barriers for crystallization are reduced at elevated temperatures resulting in the decrease of switching voltage of memory type materials [36]. It is observed that this decrease in the switching voltage with an increase in temperature is moderate. This suggests that memory samples have moderate thermal stability. The electrical conductivity increases with increase in temperature which confirms the semiconducting nature of the sample. The temperature dependence on the threshold switching voltage can be expressed as [37],
(1) Where VT is the threshold switching voltage, is the threshold voltage - activation energy, is the Boltzmann constant (8.617 x eV ) and T is the temperature in Kelvin. The threshold voltage-activation energy at elevated temperatures calculated from the above expression is tabulated in Table 1. It can be noted that the value of the activation energy decreases with an increase in the temperature which can be directly related to the decrease in VT observed at elevated temperatures. Table 1: The values of activation energy (
at different temperature ranges.
Temperature range (K) 303-323 323-343 343-363 363-383
Activation energy (eV) 0.16 0.11 0.02 0.01
These observations made on the thickness and temperature dependence of switching voltages provide a definite experimental evidence of thermal effect playing a significant role in the switching process. 3.5. Microscopic study of switched region. Figure 4. shows the scanning electron micrograph of the surface morphology of the unswitched and switched samples. The conducting filament is formed by glass- melt- crystal transition. This transition can be observed after the switching is complete. It is known that once the filaments are formed, the crystalline phases are retained even after the removal of the voltage which causes the memory switching [38]. In SEM images we can see crystallized
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melt representing the conducting filament formed during switching on the surface of the sample which is observed after the switching process has been completed. The occurrence of this channel is an additional experimental evidence for the filament formation, hence the thermal mechanism plays a major role in the memory switching process.
Figure 4: SEM micrographs of (a) unswitched (b) switched Si15Te84Bi2 chalcogenide glass sample. SEM image in (b) clearly shows a crystallized melt representing the conducting filament formed during switching.
Conclusion Electrical studies indicate that Si15Te85-xBix ( 0 ≤ x ≤ 2) chalcogenide glasses exhibit memory type of switching. The switching voltages are found to decrease with Bi content. In addition, OFF state resistance of Si 15Te85-xBix ( 0 ≤ x ≤ 2) glasses have been found to decrease with Bi concentration which is in agreement with the observed decrease in switching voltages with composition. Thickness dependence of switching voltages of the prepared glasses is found to be linear and in agreement with the memory type of switching behavior shown by these glasses. Temperature dependence of suggests that thermal effects play a major role during the switching process. Formation of conduction channel on the surface of the sample gives additional evidence on the role of thermal effect in memory switching process. The prepared glasses may find their applications in display or RF devices. Acknowledgements The authors thank the Department of Materials and metallurgical Engineering, NITK Surathkal for the SEM facility. Authors thank the Department of Physics, NITK Surathkal for extending the XRD facility. References [1] S.R. Ovshinsky, Reversible electrical switching phenomena in disordered structures, Phys. Rev. Lett. 21 (1968) 1450–1453. [2] S.R. Ovshinsky, H. Fritzsche, Amorphous semiconductors for switching, memory, and imaging applications, IEEE Trans. Electron Devices. 20 (1973) 91–105. [3] D. Adler, H.K. Henisch, S.N. Mott, The mechanism of threshold switching in amorphous alloys, Rev. Mod. Phys. 50 (1978) 209–220. [4] M. Wuttig, Phase-change materials: Towards a universal memory?, Nat. Mater. 4 (2005) 265–266. [5] A.E. Owen, J.M. Robertson, Electronic conduction and switching in chalcogenide glasses, IEEE Trans. Electron Devices. 20 (1973) 105– 122. [6] R. Lokesh, N.K. Udayashankar, S. Asokan, Electrical switching behavior of bulk Si15Te85−xSbx chalcogenide glasses – A study of compositional dependence, J. Non-Cryst. Solids. 356 (2010) 321–325. [7] V.C. Selvaraju, S. Asokan, V. Srinivasan, Electrical switching studies on As 40Te60-xSex and As35Te65-xSex glasses, Appl. Phys. A. 77 (2003) 149–153. [8] C. Das, M.G. Mahesha, G.M. Rao, S. Asokan, Electrical switching and optical studies on amorphous GexSe35−xTe65 thin films, Thin Solid Films. 520 (2012) 2278–2282.
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