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Electrical switching and aluminium speciation in Al-As-Te glasses
Journal of Non-Crystalline Solids 452 (2016) 253–258
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Electrical switching and aluminium speciation in Al-As-Te glasses Pumlianmunga 1, K. Ramesh ⁎ Department of Physics, Indian Institute of Science, Bangalore 560012, India
a r t i c l e
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Article history: Received 28 June 2016 Received in revised form 6 September 2016 Accepted 7 September 2016 Available online xxxx Keywords: Chalcogenide glasses Electrical switching Network connectivity Defect states Chemical disorder NMR
a b s t r a c t Bulk AlxAs30Te70-x glasses in the range 5 ≤ x ≤ 20 prepared by melt quenching method has been found to exhibit memory for x b 10 and threshold switching for x ≥ 10. Upon heating in a differential scanning calorimeter (DSC), glasses with x ≤ 10 show crystallization reaction whereas glasses with x N 10 do not show crystallization reaction. Glasses with x ≤ 10 crystallized into Te and As2Te3 crystal structures when annealed at their crystallization temperature. Glasses with x ≥ 15 crystallized to As2Te3 structure only. The glasses cooled from its melting temperature (Tm) in water at room temperature (28 °C) solidified into crystalline structure for x = 5, whereas x ≥ 10 solidified into amorphous structure. Magic angle spinning nuclear magnetic resonance (MAS NMR) results show 27Al resides in [4]Al and [6]Al coordination. Raman spectra of the as-quenched, SET and RESET samples of the memory glass reveal that the SET state is crystalline and RESET state is amorphous. Raman spectra of threshold switching glass reveal no structural change even after 50 switching cycles. The progressive addition of Al promotes the transfer of lone-pair electrons of Te to Al atoms. We propose this lone-pair tailoring by Al may form a band above the valence band that decreases the density of charge carriers. As a result, a decrease in threshold current (Ith) and an increase in threshold voltage (Vth) is observed. The higher coordinated Al cross links the structural network heavily. This makes structural reorganization difficult resulting in the observed threshold switching in AlxAs30Te70-x for glasses with x N 5. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Chalcogenide glasses formed from Group VI elements S, Se and Te are known as phase change materials due to the high contrast in electrical resistivity between their amorphous and crystalline states [1–4]. Under high electric field, they switch to a high conducting state [1–7]. This electrical switching is of two types: (i) threshold and (ii) memory. Threshold material switches between the high and low resistance states without any structural transition and is a reversible process. On the other hand, memory switching is a structural transition from a high resistance amorphous (OFF) to a low resistance crystalline (ON) state and is an irreversible transition. By applying a high current pulse (RESET) the memory material can be brought back to its OFF state. In general, threshold switching is understood by electronic transitions (no structural transition) and the memory switching is understood by the thermally induced transitions (structural transition between amorphous and crystalline). The electrical and optical properties of chalcogenide glasses are – greatly influenced by the high density of defect states C+ 3 and C1 present in the mobility gap [8,9]. Here C denotes the chalcogen and the subscript ⁎ Corresponding author. E-mail address: [email protected] (K. Ramesh). 1 On leave from Department of Physics, Jamia Millia Islamia University, New Delhi, 110025.
http://dx.doi.org/10.1016/j.jnoncrysol.2016.09.004 0022-3093/© 2016 Elsevier B.V. All rights reserved.
and superscript indicate the coordination charge state. These defect states pin the Fermi level at the center of the mobility gap. The pinning of the Fermi level prevents doping in the chalcogenide glasses. They also act as trap states for the charge carriers. In a normal chalcogenide glass – C+ 3 and C1 centers are equal. Electrical switching gets initiated by filling of the trap states by field injected charged carriers. At a threshold voltage (Vth) the trap centers are completely filled [7]. The charge carrier can transit the sample with enhanced mobility that leads to the ON state. Removal of the applied field brings back the high resistive state. Hence, in the threshold switching materials the switching between the OFF to ON state occurs without any structural transition [1,7]. Generally the systems or compositions which exhibit memory switching are difficult glass formers and the compositions which exhibit threshold switching are found to be good glass formers [6]. The rate of formation of crystal nuclei should be fast and hence the phase transition of amorphous to crystalline state can be fast in memory materials [5]. A threshold material with high network connectivity and rigidity have greater steric hindrance for structural reorganization due to which they are reluctant to crystallize. The higher coordinated atoms like Ge, Si and Al are known to increase the network connectivity and rigidity of the chalcogenide glasses [10,11]. Chalcogenide glasses obey ‘8-N’ rule [12] and hence the coordination of Group IV (Si, Ge), Group V (As, Sb), Group VI (S, Se and Te) and Group VII (Cl, I, Fl) atoms are 4, 3, 2 and 1 respectively. When metal atoms are added to chalcogenide glasses this rule does not valid as the metal
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atoms are found to be in higher coordinated state. Metal atoms in chalcogenide glasses are generally found to be in 4-fold coordination [13– 18]. To generalise the ‘8-N’ rule, Liu et al. proposed a model called formal valence shell (FVS) [19,20]. This model allows 4-fold coordination for metal atoms. The 4-fold coordination is possible when the lone-pair electrons of chalcogen atoms are formally transferred to the metal atoms. The transfer of LP electrons gradually increases the number of bonds in the chalcogen atom from 2 to 4. So, by increasing the concentration of metal atoms, the network can transform from lone-pair type semiconductor to a tetrahedral type semiconductor. The absence of photodarkening (PD) in Cu doped As\\S and As\\Se glasses is an evidence for this transition [21]. The increase in metal atoms also increases the network connectivity and rigidity which is reflected as an increase in glass transition (Tg) and crystallization (Tc) temperatures. Among the metal doped chalcogenide glasses, Al\\Te, Al\\As\\Te and Al\\Te\\Sb glasses are exception to the FVS model as Al is found to be in 6-fold coordination along with 4-fold coordination [10,22–24]. Binary AlxTe100-x glasses exhibit memory switching [25]. Addition of As to Al\\Te glasses promotes the glass formation [26]. The increase of Al concentration in As\\Te glasses also alters the switching type from memory to threshold [10]. In amorphous thin films of Al doped Sb\\Te which exhibit memory switching, the crystallization temperature (Tc) and crystallization activation energy are found to increase with the increase of Al [27]. The threshold properties of Si-Te-As-Ge (STAG) is improved by doping with nitrogen that cross-links the network. Also the formation of Si3N4 acts as a barrier for crystallization [28]. This combined effect led to the absence of crystallization reaction making the material suitable for threshold switching applications [29]. By varying the concentration of metal atoms the concentration of lone-pairs can be tuned to a desired level. The present work focus on the influence of the variation of lone-pair on Ith and Vth, and hence on the thermal crystallization. I\\V electrical switching, differential scanning calorimetry (DSC) and thermal crystallization were carried out on the glass samples. MAS-NMR has been employed to study the local coordination of Al. Raman spectroscopic measurements are carried out on memory(x = 5) and threshold (x = 20) switching glasses in the SET and RESET states. NMR results show the presence of [4]Al and [6]Al with [4]Al as the dominant sites. With the increase of Al, the threshold current (Ith) decreases and the threshold voltage (Vth) increases. Glass transition (Tg) and crystallization (Tc) temperatures are also increases with the increase of Al. For higher concentrations of Al crystallization reaction is not observed. These observations are understood on the basis of lone-pair variation, network connectivity, rigidity and the chemical disorder present in these glasses.
Fig. 1. XRD of the bulk melt quenched AlxAs30Te70-x samples showing the amorphous nature.
which did not exhibit crystallization peak in the DSC thermogram were annealed at 300 °C well below the melting temperature for 2 h; (ii) heated to its melting temperature (Tm) and quenched in water at 28 °C. Electrical switching studies were carried out using a Keithley Sourcemeter (Model: 2410C) interfaced with a PC controlled by Lab VIEW 8.5 (National Instruments, Austin, TX) [30]. Sample polished to a thickness of 0.30 mm was placed between a point contact top electrode and a flat plate bottom electrode using a spring loading mechanism. Both the electrodes were made of brass. A controlled current was passed through the sample and the corresponding switching voltage developed was measured. Magic angle spinning nuclear magnetic resonance (MAS-NMR) was carried out using Joel 400 MHz. Finely powdered samples were spun at 10 KHz to reduce peak broadening due to dipolar interactions. Raman studies were carried using Horiba Jobin Yvon (LabRAM HR) laser Raman Spectrometer in the backscattering mode. The sample is illuminated by 514.5 nm line of an argon ion laser focused using a 100 X objective. The spectral resolution for the Stokes-side Raman is 0.5 cm−1. All the spectra were recorded for 50 s with laser power of b0.2 mW to avoid undesired crystallization.
2. Methods Bulk AlxAs30Te70-x glasses in the composition range 5 ≤ x ≤ 20 were prepared by melt quenching method. Appropriate amounts of high purity elements (99.999%) of 1.5g were taken in flattened quartz ampoules of 8 mm diameter and sealed under a vacuum 10−6 Torr. These ampoules were loaded into resistive furnaces. The temperature of the furnace was increased to 600 °C at a rate of 100 °C/h and held for 6 h. The temperature was further increased to 900 °C and held at that temperature for 48 h under constant agitation to homogenize the melt. After 48 h the temperature was reduced to 800 °C and then quenched in ice-water + NaOH mixture. X-ray diffraction (XRD) shown in Fig. 1 confirms the amorphous nature of the prepared samples. The glass transition (Tg), crystallization (Tc) and melting (Tm) temperatures were measured using PerkinElmer differential scanning calorimeter (DSC) at a scan rate of 10 °C/min. About 10 mg of the sample taken in an aluminium pan was sealed with a crimper. An empty crimped aluminium pan was used as the reference. Thermal crystallization of these glasses were carried out in quartz ampoules sealed under a vacuum of 10−5 Torr in two ways: (i) glasses with x ≤ 10 were annealed at their respective crystallization temperature (Tc) for 2 h and the glasses x ≥ 15,
Fig. 2. I\ \V characteristics of AlxAs30Te70-x glasses showing memory switching for x b 10 and threshold switching for x ≥ 10.
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3. Results and discussions Fig. 2 shows the I\\V characteristics of AlxAs30Te70-x (5 ≤ x ≤ 20) with 3 mA (ramped at 0.6 mA/s) input current. For x b 10, memory switching is observed whereas for x ≥ 10, threshold switching is observed. Initially I and V are linaer and follows Ohmic behaviour. On increasing the current, I\\V deviates from its Ohmic behaviour and follows a non-linear path exhibiting a negative differential resistance (NDR), leading to a low resistance state [1,31]. On removing the current, the memory glass (x = 5) retains its high conducting state, whereas glasses (x ≥ 10) returned to their amorphous state. Interestingly, the above I\\V characteristic is accompanied by a decrease in Ith and an increase in Vth (with the increase of Al) as shown in Fig. 3. This can be explained on the basis of network connectivity and formal valence shell (FVS) model [19–21]. According to the FVS model, the lone-pair (LP) electrons of chalcogen atoms are formally transferred to the metal atoms. When Al atoms are introduced to As30Te70 glass, Te donates its lone-pair electrons to Al. The valence of Al is 3 and to get a valence of 4, it needs an electron from Te. When the Te donates one of its LP, its coordination increases from 2 to 3. This higher coordination of Al and Te increases the network connectivity and rigidity. In glasses having large density of LP electrons, the repulsive interaction between the lone-pairs will be strong. As a result, the LP orbitals will be positioned energetically and spatially above the valence band [32]. Electrons from energetically higher LP that do not participate in bonding will be easier to access for charge transport under applied field rather than by disrupting the bonded electron just above the valence band. When Al concentration is low few of the LP electrons of Te atoms will be tailored by Al and the density of LP electrons available for charge transport will be high. Consequently, the high density of LP electrons will lead to a larger charge clouding (density) and hence higher Ith. The high Ith indicates higher Joule heating effect which results in lower Vth. This shows that the lone-pair band can be tailored in a controlled way by adjusting the concentration of metallic atoms. So the progressive addition of Al to the base composition (As30Te70) results in the decrease of Ith and correspondingly an increase in Vth is observed (Fig. 3). As stated before, the LP tailoring can increase the number of bonds per atom in Te from 2 to 3. In Fig. 4, the schematic of the energy band structure of chalcogenide glass with the addition of metal atoms is shown. Fig. 4a shows the band structure of – a chalcogenide glass without metal atoms where C+ 3 and C1 centers are equal in number. Addition of certain metallic atoms alters this balance to a larger extent and hence unpins the Fermi level [33]. Tetrahedral coordination of Al atoms in the Al-As-Te glasses is favoured by the formation of coordinated type bond between Al and Te. This is possible if Te transfers one of its lone-pair electrons to Al and becomes 3fold coordinated (C+ 3 ). MAS-NMR spectroscopic measurements on AlxAs30Te70-x glasses show that Al also exist in 6-fold coordination along with 4-fold coordination which we will discuss later. For 6-fold coordinated Al, three of Te atoms need to donate one electron from each. In this process three C+ 3 centers will be created. As a result, the – concentration of C+ 3 centers will increase while the C1 centers remain
Fig. 3. Ith and Vth as a function of Al concentration. The lines are only drawn to guide the eyes.
Fig. 4. (a) Schematic electronic band structure of chalcogenide glass with equal – concentration of C+ 3 and C1 defects (As30 Te70). (b) The electronic band structure of AlxAs30Te70-x glasses. The increase in C+ 3 defect centers by the increase of Al atoms shifts the Fermi level towards valence band.
unchanged. This results in the shift of Fermi level towards the valence band which is depicted in Fig. 4b. Fig. 5 shows the DSC thermograms of AlxAs30Te70-x glasses recorded at 10 °C/min. It can be clearly seen that both Tg and Tc increases with the increase of Al. Interestingly, the crystallization reaction is absent for x ≥ 15. Similar kind observation has been reported earlier on Al-As-Te glasses [34,35]. Glass with x = 10 has a very weak crystallization peak. The absence of crystallization for x N 10, indicates that these glasses do not undergo structural re-organization upon heating. Structural re-organization and network rigidity depends upon the network connectivity and the bond strength. The transfer of lone-pair electrons from Te increases the 4-fold coordinated Al and 3-fold coordinated Te in the network. This increases the cross-linking and the rigidity of the structural network. The increase in the concentration of Al at the expense of Te reduces the flexible Te chains in the Al-As-Te glass network. When Te concentration is high, the number of Te chains are also high which favours flexibility, crystallization and phase separation. Both Al and As increases the stability of the glasses and also promotes the glass formation. With increasing Al, the 3-fold coordinated Te also increases which increases the cross-linking in the network. For glasses
Fig. 5. DSC thermograms recorded at a scan rate of 10 °C/min of AlxAs30Te70-x glasses showing Tg (first arrow down), Tc (arrow up), Tm (second arrow down). Tg increases with the increase of Al.
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with x ≥ 15, the cross-linking is high enough to constraint the network and hence crystallization becomes difficult. It is also known that the wider glass forming region in As-based systems is related to the broken chemical order introduced by As [32]. This chemical disorder is due to the high bond strength of As\\As homopolar bonds. In the AlxAs30Te70-x glasses, As\\As bond strength (91.3 kcal/mol) is higher than the other possible bonds, like As\\Te (77.5 kcal/mol), Al\\As (69.8 kcal/mol), Al\\Te (64 kcal/mol), Te\\Te (63.2 kcal/mol) and Al\\Al (31.79 kcal/mol). Being higher in energy, the As\\As homopolar bonds have highest possibility for bond formation. The concentration of As is high in AlxAs30Te70-xglasses and hence the concentration of As\\As homopolar bonds is also high. Due to this, the structural network already has the chemical disorder. The role of homopolar As\\As bonds in promoting the glass formation has been established earlier [36]. The general observation is that the threshold switching is observed in ternary chalcogenide glasses which have arsenic as a major constituent. Hence, the threshold switching in the arsenic containing bulk glasses may be due to this broken chemical order and its role in promoting the glass formation by suppressing the crystallization. The concentration of Te is also critical in determining the switching type in Al-As-Te glasses. It is suggested that the Te rich glasses are not homogeneous hence upon heating they phase separate [34,35]. As\\Te based glasses having Te N60% crystallize into As2Te3 and Te. Also the energy barrier for diffusion above the softening point is weak in Te- rich glasses which makes the transition of glass to crystal extremely rapid [37]. The bond length of As\\As (2.49 Ǻ) is very much less than the Te\\Te (2.86 Ǻ) [34]. This makes the rotation of molecules difficult in As-rich compositions. Due to this, in As- rich glasses, “As-pair locks” forms and prevents the free rotation of molecules [37]. This increases the energy barrier for crystallization and correspondingly, the crystallization temperature increases and the crystallization develops very slowly. So for As- rich samples, crystallization becomes difficult. In the present AlxAs30Te70-x glasses, the increase in Al increases the ratio of As to Te and hence the structural network becomes As rich, consequently, the crystallization is difficult for glasses with x N 10. This is also supported by the XRD of the thermally crystallized samples. Fig. 6 displays the XRD diffractrograms of thermally crystallized samples. Glasses in the range 5 ≤ x ≤ 10 crystallized into As2Te3 and Te structures. Glasses with x ≥ 15 crystallized into As2Te3 only. Another
Fig. 6. XRD of the annealed AlxAs30Te70-x samples. Memory switching glasses (x ≤ 10) annealed at their respective crystallization temperatures show As2Te3 and Te phases. Threshold switching glasses (x N 20) annealed at 300 °C show As2Te3 phase only.
interesting observation is that in many of the metal doped chalcogenide glasses the crystalline phases involving metal atoms are absent. For example, in thermally crystallized AlmAsnTe100-m-n, Ge15Te85-sCus, Ge20Te80-tSnt, glasses, there is no Al, Sn or Cu involved crystalline phases are observed [34,38–40]. In the binary AluTe100-u (17 ≤ u ≤ 30), the formation of Φ(Al\\Te) crystalline phase has been observed [41]. This underlines the role of As in the structural reorganization in Al-As-Te glasses. When CuyAs40Se60-y glasses are annealed at their respective crystallization temperatures, Cu3AsSe4 and As2Se3 crystalline phases for y b 15 and Cu3AsSe4 and Cu2As3 phases for y ≥ 25 are observed. The intermediate compositions 15 ≤ y ≤ 20 crystallized into Cu3AsSe4 phase with amorphous background. Prolonged annealing (for 48 h) yields As2Se3 phase for this intermediate range (15 ≤ y ≤ 20) [42,43]. Thermogravimetric analysis (TGA) showed weight loss for glasses with y ≥ 20. The evaporating material deposited on the sides of quartz ampoule was found to be amorphous. This amorphous structure was reluctant to crystallize when annealed for short duration (48 h). When annealed for 30 days at 270 °C, yield As4Se4 and As crystalline phases [44,45]. So in metal doped chalcogenide glasses, some of the crystalline phases involving metal are not favoured at normal conditions. Prolonged annealing may result in the formation of crystalline phases involving the metal atoms. They may need higher energy for crystallization. In the context of the present study, prolonged annealing will not be useful as the switching occurs in a short time. In AlxAs30Te70-x glasses, the steric hindrance to structural re-organization increases with the increase of Al due to the increase in chemical disorder, crosslinking and rigidity. Some of the structural units involving Al may find difficulty in reorganization and hence structural units consisting of Al\\Te are absent. Apart from As\\As bonds the next higher energy bonds are As\\Te and the formation of structural units involving As\\Te is expected. In As\\Te system, As40Te60 (As2Te3) is a difficult glass former [37]. Hence, in As\\Te system, As2Te3 crystallizes out. When AlxAs30Te70-x glasses are annealed As2Te3 is observed for As- rich compositions and As2Te3 + Te for Te- rich compositions. Hence, in AlxAs30Te70-x glasses, the conducting filament formed at the time of switching consists of As2Te3 and Te for x b 10. The excess Al may be redistributed in the surrounding amorphous matrix [40]. When AlxAs30Te70-x glasses in the range 10 ≤ x ≤ 20 are melted and quenched in water at room temperature (28 °C) solidifies into amorphous structure and x = 5 solidified into crystalline structure. This indicates that the compositions in the range 10 ≤ x ≤ 20 are good glass formers and the composition x = 5 is relatively a poor glass former [5]. The solidification of the melted samples into amorphous structure when quenched in water may indicate the high viscosity of the melt. The viscosity of the melt is high due to the increased cross-linking rendered by the higher coordinated Al and Te atoms. This decreases the mobility of the atoms and hence glass formation becomes easy. The absence of crystallization reaction in the thermograms and amorphitization of the melts cooled in water may be used as a measure of the goodness of threshold switching property. It should be noted that, many threshold switching glasses show crystallization (exothermic reaction in DSC), indicating threshold switching glasses also undergo structural transition [43]. These glasses may not be stable for large number of switching cycles. Fig. 7 shows the 27Al MAS-NMR spectra of AlxAs30Te70-x glasses. The observed chemical shifts at 78 and 0 ppm are corresponding to 4- and 6fold coordinated Al respectivity [46]. Among [4]Al and [6]Al sites in AlxAs30Te70-x glasses [4]Al sites are dominant than [6]Al sites in the entire range of composition. This is in contrast to previous reports on Al-As-Te glasses, where [6]Al sites are dominant [10,22,47]. This may be attributed to the sample preparation conditions. Particularly metal added chalcogenide glasses are sensitive to preparation conditions. In covalently bonded chalcogenide glasses each atom is allowed to satisfy its valence requirements by obeying the ‘8-N’ rule where N is the element's column in the Periodic Table. When metal atoms are added to the chalcogenide glasses this rule is violated as metal atoms and the chalcogen atoms found to be in higher coordination states. The lone-pair transfer and
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Fig. 7. NMR spectra showing [4]Al and [6]Al sites of Al residing at an energy corresponding to 78 and 0 ppm.
the increase in coordination number of metal and chalcogen atoms may be affected by the temperature of the melt and the quenching rates. The configurational changes may differ at different temperatures as the melt viscosity largely depends on the temperature. So samples quenched from high temperatures may relatively differ from samples quenched from slightly above the melt temperatures for metal doped chalcogenide glasses. In oxide glasses, Al is known to reside in [4,5,6]Al sites with [4] Al as the dominant site [48–50]. The melt of Al2O3 contains mainly 4-fold coordinated Al [51]. Samrat et al. reported [4]Al site only in CaAl2O4 glasses [46]. Amorphous chalcogenide film of Al-Sb-Te has [4] Al and [6]Al sites of which [4]Al is more dominant [47]. In crystalline Al23Te77 compound Al is found to be in 4 and 6- fold coordination states with [6]Al being dominant [22]. In stoichiometric Al2Te3 and Al2O3 compounds only 6- fold coordinated Al is found [22,52,53]. Probably in the stable equilibrium state (low energy state), Al based oxide and nonoxide glasses have Al in 6- fold coordination. In the AlxAs30Te70-x glasses, the 6- fold coordinated Al increases with the increase of Al
Fig. 8. Variation of [6]Al with respect to Al percentage. The lines are only drawn to guide the eyes.
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concentration as shown in Fig. 8. The increase in 6- fold coordinated Al further increases the cross-linking, connectivity and rigidity. So the structural reorganization is difficult and the glasses with x N 5 are difficult to crystallize. To get a better insight into the local structural changes in the ON and RESET (OFF) states of a memory switching sample (x = 5), Raman spectra were recorded on fresh (as quenched) as well as SET and RESET states. Also, the stability of the threshold (x = 20) glasses can be ascertained by examining the switching area. Fig. 9 shows the Raman spectra of memory (x = 5) glass. The structural differences between the OFF (amorphous) and the ON (crystalline) states can be clearly seen. The RESET and OFF states are similar indicating they are amorphous. The Raman band at 70 cm−1 (A) in the OFF/RESET samples is due to the bending of disordered Te\\Te bonds [54]. Band at 157 cm−1 (B) is attributed to the vibration of short Te\\Te amorphous chain, and peak at ~ 190 cm− 1 (C) is due to vibration of disordered AsTe3/2 pyramidal units [55,56]. The ON (SET) state sample has Raman bands at 119 (D), 138 (E) and a hump-like-peak at 178 cm−1 (F). The bands at 119 (D) and 138 cm−1 (E) can be attributed to crystalline Te (Te3 pyramid) vibrations [54–56]. Raman peaks at 178 cm−1 (F) is attributed to crystalline AsTe3/2 units [55]. The red-shift in the Raman band from 190 to 178 cm−1 is due to the long range interaction effect in crystalline structure. Fig. 10 shows the Raman spectra of threshold switching glass (x = 20) taken from the as-quenched glass and the sample subjected to 50 switching cycles. The spectra are similar indicating there is no structural variation. Raman bands are observed at 70 (A), 157 (B) and 190 cm−1 (C) respectively [54–57]. The similarity in the bands show that there is no significant diffusion of Te atoms that could lead to crystal nucleation in the threshold glass after 50 switching cycles. This structural and thermal stability in the threshold sample is consistent with the LP tailoring and higher crosslinking of Te engendered by transfer of lone-pair electrons to Al. The Raman bands at 70 cm−1 (A) is due to the bending modes of amorphous Te\\Te vibrations [54]. Peak at 157 cm−1 (B) is due to the vibration of short Te\\Te chains [54]. Peak at 190 cm−1 (C) is from disordered units of AsTe3/2 vibrations [56]. This similarity in the Raman bands of quenched and the sample subjected to 50 switching cycles indicates the stable nature of Al-As-Te glasses. So the stable and the non-crystallizing glasses (x N 10) should be suitable for threshold switching devices and the glasses which undergo crystallization easily (x b 10) should be suitable for memory switching devices.
Fig. 9. Raman spectra of as-quenched, SET (ON) and RESET (OFF) states of memory switching composition (x = 5).
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Fig. 10. Raman spectra taken from as-prepared and after 50 switching cycles in threshold glass (x = 20) showing no structural variation.
4. Conclusions In summary, electrical switching behaviour has been studied in bulk AlxAs30Te70-x (5 ≤ x ≤ 20) glasses. The glasses exhibits memory for x ≤ 5 and threshold for x ≥ 10. Thermally crystallized samples show the presence of Te and As2Te3 crystallites for x ≤ 10, and only As2Te3 for x ≥ 15. The 27Al MAS NMR results show the presence of [4,6]Al with the dominance of [4]Al sites. The 4- and 6- fold coordination of Al necessitated by the transfer of lone-pair electrons of Te to Al. This increases the network connectivity and rigidity. The lone-pair transfer also affects the – balance between the C+ 3 and C1 defect states present in the mobility gap thus influences the electrical properties like electrical switching and electrical conductivity to a larger extent. The chemical disorder due to the As\\As homopolar bonds also promotes the glass formation and decreases the crystallizing ability. With Raman spectra the difference in the structure of SET and RESET states have been clearly demonstrated. On the other hand, Rama spectra of the threshold glass having maximum Al (x = 20) show no signature of structural changes even after 50 switching cycles. This stable nature indicates Al-As-Te glasses are promising candidates for threshold switching devices. References [1] [2] [3] [4] [5] [6] [7]
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Electrical switching and aluminium speciation in Al-As-Te glasses Pumlianmunga 1, K. Ramesh ⁎ Department of Physics, Indian Institute of Science, Bangalore 560012, India
a r t i c l e
i n f o
Article history: Received 28 June 2016 Received in revised form 6 September 2016 Accepted 7 September 2016 Available online xxxx Keywords: Chalcogenide glasses Electrical switching Network connectivity Defect states Chemical disorder NMR
a b s t r a c t Bulk AlxAs30Te70-x glasses in the range 5 ≤ x ≤ 20 prepared by melt quenching method has been found to exhibit memory for x b 10 and threshold switching for x ≥ 10. Upon heating in a differential scanning calorimeter (DSC), glasses with x ≤ 10 show crystallization reaction whereas glasses with x N 10 do not show crystallization reaction. Glasses with x ≤ 10 crystallized into Te and As2Te3 crystal structures when annealed at their crystallization temperature. Glasses with x ≥ 15 crystallized to As2Te3 structure only. The glasses cooled from its melting temperature (Tm) in water at room temperature (28 °C) solidified into crystalline structure for x = 5, whereas x ≥ 10 solidified into amorphous structure. Magic angle spinning nuclear magnetic resonance (MAS NMR) results show 27Al resides in [4]Al and [6]Al coordination. Raman spectra of the as-quenched, SET and RESET samples of the memory glass reveal that the SET state is crystalline and RESET state is amorphous. Raman spectra of threshold switching glass reveal no structural change even after 50 switching cycles. The progressive addition of Al promotes the transfer of lone-pair electrons of Te to Al atoms. We propose this lone-pair tailoring by Al may form a band above the valence band that decreases the density of charge carriers. As a result, a decrease in threshold current (Ith) and an increase in threshold voltage (Vth) is observed. The higher coordinated Al cross links the structural network heavily. This makes structural reorganization difficult resulting in the observed threshold switching in AlxAs30Te70-x for glasses with x N 5. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Chalcogenide glasses formed from Group VI elements S, Se and Te are known as phase change materials due to the high contrast in electrical resistivity between their amorphous and crystalline states [1–4]. Under high electric field, they switch to a high conducting state [1–7]. This electrical switching is of two types: (i) threshold and (ii) memory. Threshold material switches between the high and low resistance states without any structural transition and is a reversible process. On the other hand, memory switching is a structural transition from a high resistance amorphous (OFF) to a low resistance crystalline (ON) state and is an irreversible transition. By applying a high current pulse (RESET) the memory material can be brought back to its OFF state. In general, threshold switching is understood by electronic transitions (no structural transition) and the memory switching is understood by the thermally induced transitions (structural transition between amorphous and crystalline). The electrical and optical properties of chalcogenide glasses are – greatly influenced by the high density of defect states C+ 3 and C1 present in the mobility gap [8,9]. Here C denotes the chalcogen and the subscript ⁎ Corresponding author. E-mail address: [email protected] (K. Ramesh). 1 On leave from Department of Physics, Jamia Millia Islamia University, New Delhi, 110025.
http://dx.doi.org/10.1016/j.jnoncrysol.2016.09.004 0022-3093/© 2016 Elsevier B.V. All rights reserved.
and superscript indicate the coordination charge state. These defect states pin the Fermi level at the center of the mobility gap. The pinning of the Fermi level prevents doping in the chalcogenide glasses. They also act as trap states for the charge carriers. In a normal chalcogenide glass – C+ 3 and C1 centers are equal. Electrical switching gets initiated by filling of the trap states by field injected charged carriers. At a threshold voltage (Vth) the trap centers are completely filled [7]. The charge carrier can transit the sample with enhanced mobility that leads to the ON state. Removal of the applied field brings back the high resistive state. Hence, in the threshold switching materials the switching between the OFF to ON state occurs without any structural transition [1,7]. Generally the systems or compositions which exhibit memory switching are difficult glass formers and the compositions which exhibit threshold switching are found to be good glass formers [6]. The rate of formation of crystal nuclei should be fast and hence the phase transition of amorphous to crystalline state can be fast in memory materials [5]. A threshold material with high network connectivity and rigidity have greater steric hindrance for structural reorganization due to which they are reluctant to crystallize. The higher coordinated atoms like Ge, Si and Al are known to increase the network connectivity and rigidity of the chalcogenide glasses [10,11]. Chalcogenide glasses obey ‘8-N’ rule [12] and hence the coordination of Group IV (Si, Ge), Group V (As, Sb), Group VI (S, Se and Te) and Group VII (Cl, I, Fl) atoms are 4, 3, 2 and 1 respectively. When metal atoms are added to chalcogenide glasses this rule does not valid as the metal
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atoms are found to be in higher coordinated state. Metal atoms in chalcogenide glasses are generally found to be in 4-fold coordination [13– 18]. To generalise the ‘8-N’ rule, Liu et al. proposed a model called formal valence shell (FVS) [19,20]. This model allows 4-fold coordination for metal atoms. The 4-fold coordination is possible when the lone-pair electrons of chalcogen atoms are formally transferred to the metal atoms. The transfer of LP electrons gradually increases the number of bonds in the chalcogen atom from 2 to 4. So, by increasing the concentration of metal atoms, the network can transform from lone-pair type semiconductor to a tetrahedral type semiconductor. The absence of photodarkening (PD) in Cu doped As\\S and As\\Se glasses is an evidence for this transition [21]. The increase in metal atoms also increases the network connectivity and rigidity which is reflected as an increase in glass transition (Tg) and crystallization (Tc) temperatures. Among the metal doped chalcogenide glasses, Al\\Te, Al\\As\\Te and Al\\Te\\Sb glasses are exception to the FVS model as Al is found to be in 6-fold coordination along with 4-fold coordination [10,22–24]. Binary AlxTe100-x glasses exhibit memory switching [25]. Addition of As to Al\\Te glasses promotes the glass formation [26]. The increase of Al concentration in As\\Te glasses also alters the switching type from memory to threshold [10]. In amorphous thin films of Al doped Sb\\Te which exhibit memory switching, the crystallization temperature (Tc) and crystallization activation energy are found to increase with the increase of Al [27]. The threshold properties of Si-Te-As-Ge (STAG) is improved by doping with nitrogen that cross-links the network. Also the formation of Si3N4 acts as a barrier for crystallization [28]. This combined effect led to the absence of crystallization reaction making the material suitable for threshold switching applications [29]. By varying the concentration of metal atoms the concentration of lone-pairs can be tuned to a desired level. The present work focus on the influence of the variation of lone-pair on Ith and Vth, and hence on the thermal crystallization. I\\V electrical switching, differential scanning calorimetry (DSC) and thermal crystallization were carried out on the glass samples. MAS-NMR has been employed to study the local coordination of Al. Raman spectroscopic measurements are carried out on memory(x = 5) and threshold (x = 20) switching glasses in the SET and RESET states. NMR results show the presence of [4]Al and [6]Al with [4]Al as the dominant sites. With the increase of Al, the threshold current (Ith) decreases and the threshold voltage (Vth) increases. Glass transition (Tg) and crystallization (Tc) temperatures are also increases with the increase of Al. For higher concentrations of Al crystallization reaction is not observed. These observations are understood on the basis of lone-pair variation, network connectivity, rigidity and the chemical disorder present in these glasses.
Fig. 1. XRD of the bulk melt quenched AlxAs30Te70-x samples showing the amorphous nature.
which did not exhibit crystallization peak in the DSC thermogram were annealed at 300 °C well below the melting temperature for 2 h; (ii) heated to its melting temperature (Tm) and quenched in water at 28 °C. Electrical switching studies were carried out using a Keithley Sourcemeter (Model: 2410C) interfaced with a PC controlled by Lab VIEW 8.5 (National Instruments, Austin, TX) [30]. Sample polished to a thickness of 0.30 mm was placed between a point contact top electrode and a flat plate bottom electrode using a spring loading mechanism. Both the electrodes were made of brass. A controlled current was passed through the sample and the corresponding switching voltage developed was measured. Magic angle spinning nuclear magnetic resonance (MAS-NMR) was carried out using Joel 400 MHz. Finely powdered samples were spun at 10 KHz to reduce peak broadening due to dipolar interactions. Raman studies were carried using Horiba Jobin Yvon (LabRAM HR) laser Raman Spectrometer in the backscattering mode. The sample is illuminated by 514.5 nm line of an argon ion laser focused using a 100 X objective. The spectral resolution for the Stokes-side Raman is 0.5 cm−1. All the spectra were recorded for 50 s with laser power of b0.2 mW to avoid undesired crystallization.
2. Methods Bulk AlxAs30Te70-x glasses in the composition range 5 ≤ x ≤ 20 were prepared by melt quenching method. Appropriate amounts of high purity elements (99.999%) of 1.5g were taken in flattened quartz ampoules of 8 mm diameter and sealed under a vacuum 10−6 Torr. These ampoules were loaded into resistive furnaces. The temperature of the furnace was increased to 600 °C at a rate of 100 °C/h and held for 6 h. The temperature was further increased to 900 °C and held at that temperature for 48 h under constant agitation to homogenize the melt. After 48 h the temperature was reduced to 800 °C and then quenched in ice-water + NaOH mixture. X-ray diffraction (XRD) shown in Fig. 1 confirms the amorphous nature of the prepared samples. The glass transition (Tg), crystallization (Tc) and melting (Tm) temperatures were measured using PerkinElmer differential scanning calorimeter (DSC) at a scan rate of 10 °C/min. About 10 mg of the sample taken in an aluminium pan was sealed with a crimper. An empty crimped aluminium pan was used as the reference. Thermal crystallization of these glasses were carried out in quartz ampoules sealed under a vacuum of 10−5 Torr in two ways: (i) glasses with x ≤ 10 were annealed at their respective crystallization temperature (Tc) for 2 h and the glasses x ≥ 15,
Fig. 2. I\ \V characteristics of AlxAs30Te70-x glasses showing memory switching for x b 10 and threshold switching for x ≥ 10.
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3. Results and discussions Fig. 2 shows the I\\V characteristics of AlxAs30Te70-x (5 ≤ x ≤ 20) with 3 mA (ramped at 0.6 mA/s) input current. For x b 10, memory switching is observed whereas for x ≥ 10, threshold switching is observed. Initially I and V are linaer and follows Ohmic behaviour. On increasing the current, I\\V deviates from its Ohmic behaviour and follows a non-linear path exhibiting a negative differential resistance (NDR), leading to a low resistance state [1,31]. On removing the current, the memory glass (x = 5) retains its high conducting state, whereas glasses (x ≥ 10) returned to their amorphous state. Interestingly, the above I\\V characteristic is accompanied by a decrease in Ith and an increase in Vth (with the increase of Al) as shown in Fig. 3. This can be explained on the basis of network connectivity and formal valence shell (FVS) model [19–21]. According to the FVS model, the lone-pair (LP) electrons of chalcogen atoms are formally transferred to the metal atoms. When Al atoms are introduced to As30Te70 glass, Te donates its lone-pair electrons to Al. The valence of Al is 3 and to get a valence of 4, it needs an electron from Te. When the Te donates one of its LP, its coordination increases from 2 to 3. This higher coordination of Al and Te increases the network connectivity and rigidity. In glasses having large density of LP electrons, the repulsive interaction between the lone-pairs will be strong. As a result, the LP orbitals will be positioned energetically and spatially above the valence band [32]. Electrons from energetically higher LP that do not participate in bonding will be easier to access for charge transport under applied field rather than by disrupting the bonded electron just above the valence band. When Al concentration is low few of the LP electrons of Te atoms will be tailored by Al and the density of LP electrons available for charge transport will be high. Consequently, the high density of LP electrons will lead to a larger charge clouding (density) and hence higher Ith. The high Ith indicates higher Joule heating effect which results in lower Vth. This shows that the lone-pair band can be tailored in a controlled way by adjusting the concentration of metallic atoms. So the progressive addition of Al to the base composition (As30Te70) results in the decrease of Ith and correspondingly an increase in Vth is observed (Fig. 3). As stated before, the LP tailoring can increase the number of bonds per atom in Te from 2 to 3. In Fig. 4, the schematic of the energy band structure of chalcogenide glass with the addition of metal atoms is shown. Fig. 4a shows the band structure of – a chalcogenide glass without metal atoms where C+ 3 and C1 centers are equal in number. Addition of certain metallic atoms alters this balance to a larger extent and hence unpins the Fermi level [33]. Tetrahedral coordination of Al atoms in the Al-As-Te glasses is favoured by the formation of coordinated type bond between Al and Te. This is possible if Te transfers one of its lone-pair electrons to Al and becomes 3fold coordinated (C+ 3 ). MAS-NMR spectroscopic measurements on AlxAs30Te70-x glasses show that Al also exist in 6-fold coordination along with 4-fold coordination which we will discuss later. For 6-fold coordinated Al, three of Te atoms need to donate one electron from each. In this process three C+ 3 centers will be created. As a result, the – concentration of C+ 3 centers will increase while the C1 centers remain
Fig. 3. Ith and Vth as a function of Al concentration. The lines are only drawn to guide the eyes.
Fig. 4. (a) Schematic electronic band structure of chalcogenide glass with equal – concentration of C+ 3 and C1 defects (As30 Te70). (b) The electronic band structure of AlxAs30Te70-x glasses. The increase in C+ 3 defect centers by the increase of Al atoms shifts the Fermi level towards valence band.
unchanged. This results in the shift of Fermi level towards the valence band which is depicted in Fig. 4b. Fig. 5 shows the DSC thermograms of AlxAs30Te70-x glasses recorded at 10 °C/min. It can be clearly seen that both Tg and Tc increases with the increase of Al. Interestingly, the crystallization reaction is absent for x ≥ 15. Similar kind observation has been reported earlier on Al-As-Te glasses [34,35]. Glass with x = 10 has a very weak crystallization peak. The absence of crystallization for x N 10, indicates that these glasses do not undergo structural re-organization upon heating. Structural re-organization and network rigidity depends upon the network connectivity and the bond strength. The transfer of lone-pair electrons from Te increases the 4-fold coordinated Al and 3-fold coordinated Te in the network. This increases the cross-linking and the rigidity of the structural network. The increase in the concentration of Al at the expense of Te reduces the flexible Te chains in the Al-As-Te glass network. When Te concentration is high, the number of Te chains are also high which favours flexibility, crystallization and phase separation. Both Al and As increases the stability of the glasses and also promotes the glass formation. With increasing Al, the 3-fold coordinated Te also increases which increases the cross-linking in the network. For glasses
Fig. 5. DSC thermograms recorded at a scan rate of 10 °C/min of AlxAs30Te70-x glasses showing Tg (first arrow down), Tc (arrow up), Tm (second arrow down). Tg increases with the increase of Al.
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with x ≥ 15, the cross-linking is high enough to constraint the network and hence crystallization becomes difficult. It is also known that the wider glass forming region in As-based systems is related to the broken chemical order introduced by As [32]. This chemical disorder is due to the high bond strength of As\\As homopolar bonds. In the AlxAs30Te70-x glasses, As\\As bond strength (91.3 kcal/mol) is higher than the other possible bonds, like As\\Te (77.5 kcal/mol), Al\\As (69.8 kcal/mol), Al\\Te (64 kcal/mol), Te\\Te (63.2 kcal/mol) and Al\\Al (31.79 kcal/mol). Being higher in energy, the As\\As homopolar bonds have highest possibility for bond formation. The concentration of As is high in AlxAs30Te70-xglasses and hence the concentration of As\\As homopolar bonds is also high. Due to this, the structural network already has the chemical disorder. The role of homopolar As\\As bonds in promoting the glass formation has been established earlier [36]. The general observation is that the threshold switching is observed in ternary chalcogenide glasses which have arsenic as a major constituent. Hence, the threshold switching in the arsenic containing bulk glasses may be due to this broken chemical order and its role in promoting the glass formation by suppressing the crystallization. The concentration of Te is also critical in determining the switching type in Al-As-Te glasses. It is suggested that the Te rich glasses are not homogeneous hence upon heating they phase separate [34,35]. As\\Te based glasses having Te N60% crystallize into As2Te3 and Te. Also the energy barrier for diffusion above the softening point is weak in Te- rich glasses which makes the transition of glass to crystal extremely rapid [37]. The bond length of As\\As (2.49 Ǻ) is very much less than the Te\\Te (2.86 Ǻ) [34]. This makes the rotation of molecules difficult in As-rich compositions. Due to this, in As- rich glasses, “As-pair locks” forms and prevents the free rotation of molecules [37]. This increases the energy barrier for crystallization and correspondingly, the crystallization temperature increases and the crystallization develops very slowly. So for As- rich samples, crystallization becomes difficult. In the present AlxAs30Te70-x glasses, the increase in Al increases the ratio of As to Te and hence the structural network becomes As rich, consequently, the crystallization is difficult for glasses with x N 10. This is also supported by the XRD of the thermally crystallized samples. Fig. 6 displays the XRD diffractrograms of thermally crystallized samples. Glasses in the range 5 ≤ x ≤ 10 crystallized into As2Te3 and Te structures. Glasses with x ≥ 15 crystallized into As2Te3 only. Another
Fig. 6. XRD of the annealed AlxAs30Te70-x samples. Memory switching glasses (x ≤ 10) annealed at their respective crystallization temperatures show As2Te3 and Te phases. Threshold switching glasses (x N 20) annealed at 300 °C show As2Te3 phase only.
interesting observation is that in many of the metal doped chalcogenide glasses the crystalline phases involving metal atoms are absent. For example, in thermally crystallized AlmAsnTe100-m-n, Ge15Te85-sCus, Ge20Te80-tSnt, glasses, there is no Al, Sn or Cu involved crystalline phases are observed [34,38–40]. In the binary AluTe100-u (17 ≤ u ≤ 30), the formation of Φ(Al\\Te) crystalline phase has been observed [41]. This underlines the role of As in the structural reorganization in Al-As-Te glasses. When CuyAs40Se60-y glasses are annealed at their respective crystallization temperatures, Cu3AsSe4 and As2Se3 crystalline phases for y b 15 and Cu3AsSe4 and Cu2As3 phases for y ≥ 25 are observed. The intermediate compositions 15 ≤ y ≤ 20 crystallized into Cu3AsSe4 phase with amorphous background. Prolonged annealing (for 48 h) yields As2Se3 phase for this intermediate range (15 ≤ y ≤ 20) [42,43]. Thermogravimetric analysis (TGA) showed weight loss for glasses with y ≥ 20. The evaporating material deposited on the sides of quartz ampoule was found to be amorphous. This amorphous structure was reluctant to crystallize when annealed for short duration (48 h). When annealed for 30 days at 270 °C, yield As4Se4 and As crystalline phases [44,45]. So in metal doped chalcogenide glasses, some of the crystalline phases involving metal are not favoured at normal conditions. Prolonged annealing may result in the formation of crystalline phases involving the metal atoms. They may need higher energy for crystallization. In the context of the present study, prolonged annealing will not be useful as the switching occurs in a short time. In AlxAs30Te70-x glasses, the steric hindrance to structural re-organization increases with the increase of Al due to the increase in chemical disorder, crosslinking and rigidity. Some of the structural units involving Al may find difficulty in reorganization and hence structural units consisting of Al\\Te are absent. Apart from As\\As bonds the next higher energy bonds are As\\Te and the formation of structural units involving As\\Te is expected. In As\\Te system, As40Te60 (As2Te3) is a difficult glass former [37]. Hence, in As\\Te system, As2Te3 crystallizes out. When AlxAs30Te70-x glasses are annealed As2Te3 is observed for As- rich compositions and As2Te3 + Te for Te- rich compositions. Hence, in AlxAs30Te70-x glasses, the conducting filament formed at the time of switching consists of As2Te3 and Te for x b 10. The excess Al may be redistributed in the surrounding amorphous matrix [40]. When AlxAs30Te70-x glasses in the range 10 ≤ x ≤ 20 are melted and quenched in water at room temperature (28 °C) solidifies into amorphous structure and x = 5 solidified into crystalline structure. This indicates that the compositions in the range 10 ≤ x ≤ 20 are good glass formers and the composition x = 5 is relatively a poor glass former [5]. The solidification of the melted samples into amorphous structure when quenched in water may indicate the high viscosity of the melt. The viscosity of the melt is high due to the increased cross-linking rendered by the higher coordinated Al and Te atoms. This decreases the mobility of the atoms and hence glass formation becomes easy. The absence of crystallization reaction in the thermograms and amorphitization of the melts cooled in water may be used as a measure of the goodness of threshold switching property. It should be noted that, many threshold switching glasses show crystallization (exothermic reaction in DSC), indicating threshold switching glasses also undergo structural transition [43]. These glasses may not be stable for large number of switching cycles. Fig. 7 shows the 27Al MAS-NMR spectra of AlxAs30Te70-x glasses. The observed chemical shifts at 78 and 0 ppm are corresponding to 4- and 6fold coordinated Al respectivity [46]. Among [4]Al and [6]Al sites in AlxAs30Te70-x glasses [4]Al sites are dominant than [6]Al sites in the entire range of composition. This is in contrast to previous reports on Al-As-Te glasses, where [6]Al sites are dominant [10,22,47]. This may be attributed to the sample preparation conditions. Particularly metal added chalcogenide glasses are sensitive to preparation conditions. In covalently bonded chalcogenide glasses each atom is allowed to satisfy its valence requirements by obeying the ‘8-N’ rule where N is the element's column in the Periodic Table. When metal atoms are added to the chalcogenide glasses this rule is violated as metal atoms and the chalcogen atoms found to be in higher coordination states. The lone-pair transfer and
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Fig. 7. NMR spectra showing [4]Al and [6]Al sites of Al residing at an energy corresponding to 78 and 0 ppm.
the increase in coordination number of metal and chalcogen atoms may be affected by the temperature of the melt and the quenching rates. The configurational changes may differ at different temperatures as the melt viscosity largely depends on the temperature. So samples quenched from high temperatures may relatively differ from samples quenched from slightly above the melt temperatures for metal doped chalcogenide glasses. In oxide glasses, Al is known to reside in [4,5,6]Al sites with [4] Al as the dominant site [48–50]. The melt of Al2O3 contains mainly 4-fold coordinated Al [51]. Samrat et al. reported [4]Al site only in CaAl2O4 glasses [46]. Amorphous chalcogenide film of Al-Sb-Te has [4] Al and [6]Al sites of which [4]Al is more dominant [47]. In crystalline Al23Te77 compound Al is found to be in 4 and 6- fold coordination states with [6]Al being dominant [22]. In stoichiometric Al2Te3 and Al2O3 compounds only 6- fold coordinated Al is found [22,52,53]. Probably in the stable equilibrium state (low energy state), Al based oxide and nonoxide glasses have Al in 6- fold coordination. In the AlxAs30Te70-x glasses, the 6- fold coordinated Al increases with the increase of Al
Fig. 8. Variation of [6]Al with respect to Al percentage. The lines are only drawn to guide the eyes.
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concentration as shown in Fig. 8. The increase in 6- fold coordinated Al further increases the cross-linking, connectivity and rigidity. So the structural reorganization is difficult and the glasses with x N 5 are difficult to crystallize. To get a better insight into the local structural changes in the ON and RESET (OFF) states of a memory switching sample (x = 5), Raman spectra were recorded on fresh (as quenched) as well as SET and RESET states. Also, the stability of the threshold (x = 20) glasses can be ascertained by examining the switching area. Fig. 9 shows the Raman spectra of memory (x = 5) glass. The structural differences between the OFF (amorphous) and the ON (crystalline) states can be clearly seen. The RESET and OFF states are similar indicating they are amorphous. The Raman band at 70 cm−1 (A) in the OFF/RESET samples is due to the bending of disordered Te\\Te bonds [54]. Band at 157 cm−1 (B) is attributed to the vibration of short Te\\Te amorphous chain, and peak at ~ 190 cm− 1 (C) is due to vibration of disordered AsTe3/2 pyramidal units [55,56]. The ON (SET) state sample has Raman bands at 119 (D), 138 (E) and a hump-like-peak at 178 cm−1 (F). The bands at 119 (D) and 138 cm−1 (E) can be attributed to crystalline Te (Te3 pyramid) vibrations [54–56]. Raman peaks at 178 cm−1 (F) is attributed to crystalline AsTe3/2 units [55]. The red-shift in the Raman band from 190 to 178 cm−1 is due to the long range interaction effect in crystalline structure. Fig. 10 shows the Raman spectra of threshold switching glass (x = 20) taken from the as-quenched glass and the sample subjected to 50 switching cycles. The spectra are similar indicating there is no structural variation. Raman bands are observed at 70 (A), 157 (B) and 190 cm−1 (C) respectively [54–57]. The similarity in the bands show that there is no significant diffusion of Te atoms that could lead to crystal nucleation in the threshold glass after 50 switching cycles. This structural and thermal stability in the threshold sample is consistent with the LP tailoring and higher crosslinking of Te engendered by transfer of lone-pair electrons to Al. The Raman bands at 70 cm−1 (A) is due to the bending modes of amorphous Te\\Te vibrations [54]. Peak at 157 cm−1 (B) is due to the vibration of short Te\\Te chains [54]. Peak at 190 cm−1 (C) is from disordered units of AsTe3/2 vibrations [56]. This similarity in the Raman bands of quenched and the sample subjected to 50 switching cycles indicates the stable nature of Al-As-Te glasses. So the stable and the non-crystallizing glasses (x N 10) should be suitable for threshold switching devices and the glasses which undergo crystallization easily (x b 10) should be suitable for memory switching devices.
Fig. 9. Raman spectra of as-quenched, SET (ON) and RESET (OFF) states of memory switching composition (x = 5).
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Pumlianmunga, K. Ramesh / Journal of Non-Crystalline Solids 452 (2016) 253–258 [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
Fig. 10. Raman spectra taken from as-prepared and after 50 switching cycles in threshold glass (x = 20) showing no structural variation.
4. Conclusions In summary, electrical switching behaviour has been studied in bulk AlxAs30Te70-x (5 ≤ x ≤ 20) glasses. The glasses exhibits memory for x ≤ 5 and threshold for x ≥ 10. Thermally crystallized samples show the presence of Te and As2Te3 crystallites for x ≤ 10, and only As2Te3 for x ≥ 15. The 27Al MAS NMR results show the presence of [4,6]Al with the dominance of [4]Al sites. The 4- and 6- fold coordination of Al necessitated by the transfer of lone-pair electrons of Te to Al. This increases the network connectivity and rigidity. The lone-pair transfer also affects the – balance between the C+ 3 and C1 defect states present in the mobility gap thus influences the electrical properties like electrical switching and electrical conductivity to a larger extent. The chemical disorder due to the As\\As homopolar bonds also promotes the glass formation and decreases the crystallizing ability. With Raman spectra the difference in the structure of SET and RESET states have been clearly demonstrated. On the other hand, Rama spectra of the threshold glass having maximum Al (x = 20) show no signature of structural changes even after 50 switching cycles. This stable nature indicates Al-As-Te glasses are promising candidates for threshold switching devices. References [1] [2] [3] [4] [5] [6] [7]
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