- Email: [email protected]
Study on electrical switching behaviour of Ge-Se-S and Ge-Se0.5-S1.5 thin films
Optik 152 (2018) 1–8
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Study on electrical switching behaviour of Ge-Se-S and Ge-Se0.5 -S1.5 thin films R.T. Ananth Kumar a , Saleh T. Mahmoud a , D. Pathinettam Padiyan b , N. Qamhieh a,∗ a b
Department of Physics, UAE University, P.O. Box 15551, Al-Ain, United Arab Emirates Department of Physics, Manonmaniam Sundaranar University, Tirunelveli 627012, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 26 March 2017 Received in revised form 18 September 2017 Accepted 18 September 2017 Keywords: Thin films Chalcogenide Thickness Electrical switching
a b s t r a c t Ternary glassy chalcogenides of Ge-Se-S and Ge-Se0.5- S1.5 systems were prepared by vacuum evaporation technique. Their electrical switching characteristics were examined in detail, where the resistance of the prepared thin films changed from mega to several tens of ohms. The amorphous-crystalline transition temperature (TC ) for GeSeS and GeSe0.5 S1.5 thin films are around 380 K and 360 K, respectively. Due to the electrothermal mechanism a rapid transition from resistive to conductive nature was observed and the results obtained were discussed by Joule heating effect. In the prepared compositions of Ge-Se-S and GeSe0.5 S1.5 , sulphur atoms get their preferred bonding when connected to the two-fold coordinated selenium atoms, and the Se-S chains are cross linked by the four-fold germanium atoms. Incorporating more sulphur to Ge-Se-S system, gives the sulphur more space to adapt in the network due the smaller size of the sulphur atoms when compared to selenium. Furthermore, this method of preparation and results open-up a new approach towards the phase change memory devices that rely on easy structural transformation. © 2017 Elsevier GmbH. All rights reserved.
1. Introduction Recent advancement on amorphous systems based on Ge-Se has become significant in the field of multimedia, data storage, phase change and switching applications [1–3]. The switching effect has been fashionable in chalcogenide materials than other oxide type materials [4,5]. The chalcogenide materials show phase change behaviour that is attained by Joule heating effect [6,7]. However, a lot of research has been focused on this area to know the factual knowledge of the structural change on these materials. Quite a few sulphur based materials like Ge46 S54, GeS2 , and GeS exhibited memory type electrical switching [8–10]. The inclusion of sulphur into the Se based system with fourfold coordinated Ge forms a stable glassy chalcogenide material [11]. In our previous reported data on GeSeS and GeSe0.5 S1.5 compositions [12,13], thickness dependent structural disorder was studied using Raman spectra, Tauc parameter and Urbach energy. Nevertheless, in this article the research work is continued on thin films of GeSeS and GeSe0.5 S1.5 to investigate the amorphous-crystalline transition temperature (TC ) followed by electrical switching of those alloys. The experimental observations concerning the dependence of electrical switching on film thickness for GeSeS and GeSe0.5 S1.5 chalcogenide alloys have been discussed. This study would explore the feasibility of using these phase change materials in memory devices.
∗ Corresponding author. E-mail address: [email protected] (N. Qamhieh). http://dx.doi.org/10.1016/j.ijleo.2017.09.074 0030-4026/© 2017 Elsevier GmbH. All rights reserved.
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Fig. 1. Resistance as a function of temperature for GeSeS and GeSe0.5 S1.5 thin films of thickness 110 nm and 122 nm.
2. Experimental details Bulk GeSeS and GeSe0.5 S1.5 glasses have been prepared by vacuum-sealed melt quenching technique. Appropriate quantities of high purity (99.99%) constituent elements of Ge, Se and S were weighed according to their stoichiometric ratio and sealed into an evacuated quartz ampoule at a vacuum of 10−5 Torr. Melting was performed at 1050 ◦ C for 36 h inside a horizontal type furnace (Strohlein instruments) that allows the ampoule containing the sample to rotate with a rotational speed of 10 rpm to ensure homogeneity of the melt. Then the bulk glasses of GeSeS and GeSe0.5 S1.5 were obtained by quenching the ampoules in a bath of ice water-NaOH mixture. The amorphous nature of the quenched materials is confirmed by X-ray diffraction (XRD) analysis using PANalytical X’Pert PRO (Cu as target and Ni as filter, = 1.5406 Å). Thin films of GeSeS and GeSe0.5 S1.5 were deposited by thermal evaporation technique (HINDHIVAC 12A4D) on cleaned glass substrates held at room temperature in a vacuum of 2 × 10−5 Torr. Three different thicknesses 110, 200, and 710 nm of GeSeS thin films were deposited, and for GeSe0.5 S1.5 films the thicknesses were 122, 297, 420 and 590 nm. The substrates were located at a distance of 10 cm vertically above the source. During each deposition, the bulk material was completely evaporated at a fast rate so that no residual alloy would remain. This procedure is necessary to ensure homogeneity due to preferential evaporation of the volatile sulphur and selenium. The thickness of the films was measured using the surface roughness tester (SJ-210, MITUTOYO). For the lower thickness film of 110 nm (GeSeS) and 122 nm (Ge-Se0.5- S1.5 ), the resistance measurement were carried out using Keithley 590 at different temperatures using Lakeshore 335 temperature controller in the range of 300–400 K with heating rate of 5 K/min. During the measurement, the samples were heated under vacuum of 10−3 Torr using Janis ST-100H cryostat. X-ray diffraction analysis was made for 110 nm (GeSeS) and 122 nm (Ge-Se0.5- S1.5 ) thin films at 300 K and 400 K to identify the amorphous-crystal phase change. Other several characterizations such as differential scanning calorimetry, X-ray diffraction, energy dispersive X-ray analysis attached to scanning electron microscopy, photo-acoustics, transmittance and Raman spectra are reported in earlier publications [12,13]. The electrodes were prepared in sandwich geometry using aluminium as top and bottom electrodes for a thickness of 100 nm. The electrodes were evaporated through a suitable mask and they were used to perform the I–V characteristics to study the electrical switching behaviour of GeSeS and GeSe0.5 S1.5 thin films. The I–V curves were recorded using a Keithley source-meter (Model 2410c ) controlled by LabVIEW 6i (National Instruments). For the I–V measurements using the sandwiched electrodes, a constant current of few mA was applied and the corresponding voltages developed were observed. All the measurements were repeated at different portions of a thin film in order to check the reproducibility, and the error was about ±2%. The standard deviation method has been used for error analysis of the switching voltage measurements. 3. Results The resistance as a function of temperature is shown in Fig. 1 for the film of thickness 110 nm (GeSeS) and 122 nm (GeSe0.5 S1.5 ). At room temperature of 300 K the figure displays a high resistance of the film characterizing the amorphous nature. With the raise in temperature from 300 K, the resistance is found to decrease exponentially and an abrupt drop in resistance was noted around 360 K and 380 K for GeSe0.5 S1.5 and GeSeS thin films respectively. This temperature at 360 K and 380 K is known as the amorphous-crystalline transition temperature (TC ). The high electrical contrast of four orders of magnitude between the amorphous and crystalline states of the film and the fast transition in the resistivity could be utilized for high-speed memory devices. Fig. 2 shows the X-ray diffraction pattern for (a) GeSeS thin film of thickness 110 nm at 300 K of temperature, (b) GeSeS thin film of thickness 110 nm at 400 K of temperature, (c) GeSe0.5 S1.5 thin film of thickness 122 nm at 300 K of temperature and (d) GeSe0.5 S1.5 thin film of thickness 122 nm at 400 K of temperature. At 300 K in both the compositions no any Bragg’s peak were found indicating the amorphous nature (Fig. 1a & c). From Fig. 1 it is known that a phase change was observed at 360 K and 380 K for GeSeS and GeSe0.5 S1.5 composition. These results are in well agreement with results obtained from the XRD which indicates the presence of Bragg’s diffraction peak at 400 K (Fig. 2b & d). Fig. 2b
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Fig. 2. shows the XRD pattern for (a) GeSeS thin film of thickness 110 nm at 300 K of temperature, (b) GeSeS thin film of thickness 110 nm at 400 K of temperature, (c) GeSe0.5 S1.5 thin film of thickness 122 nm at 300 K of temperature and (d) GeSe0.5 S1.5 thin film of thickness 122 nm at 400 K of temperature.
◦
◦
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shows two Bragg peaks at 23.59 and 26.07 , whereas Fig. 2d shows two Bragg X-ray peak at 23.07◦ and 28.20 respectively. From XRD it is noted that, the appearance of crystalline peak is found after the amorphous-crystalline transition temperature (TC ) which indicates a phase change. The (I–V) characteristics of GeSeS thin films with thickness 110, 200 and 710 nm are displayed in Fig. 3(a)–(c). Fig. 3(a) shows that an increase in the applied current produces a very small rise in voltage, representing the OFF state with high resistance of the switch (the region “oa” of the curve). Above the threshold voltage (VT ) the samples exhibit switching which leads to ON state (part “ab”). Since the transition to ON state is very fast, no data points are observed in this range (ab). As the current is applied further, it does not make any change in the potential drop across the thin film (part “bc” of the curve), which represents the ON state with very low value of resistance. When the applied current is decreased, the voltage is found to decrease to a value of zero in the path “co” of the curve. The behaviour of the curve has low and high conducting branches (oa and bc) connected by two branches (ab and co) represents a typical I–V characteristic curve for a memory switch. Figs. 3 (b) and 1 (c) of GeSeS thin films are also illustrate the same memory switching behaviour but for different thicknesses with different switching voltages. Fig. 4(a)–(d) shows the identical memory switching nature of GeSe0.5 S1.5 thin films of thickness 122, 297, 420 and 590 nm respectively. When compared to other chalcogenides such as Cux (AsSe1.4 I0.2 )100−x and Sbx Se55−x Te45 , the obtained electrical switching in GeSeS and GeSe0.5 S1.5 thin films does not show any fluctuation in switching [14,15]. At VT , all the samples switch from a highresistance state (OFF) to a low-resistance (ON) state. A direct resistance measurement of the as deposited thin films has been done using Keithley 614 electrometer. Fig. 5(a) represents the resistance values obtained before and after the switching occurred. The resistance of the as prepared film is greater than the mega-ohms’ range and it decays to the range of ohms once VT is reached. The GeSe0.5 S1.5 thin film of thickness 590 nm shows a resistance of 125 M before switching and after it reduced to 26 . Fig. 5(b) pictures the variation of the threshold switching voltage with the film thickness. Perusal of Fig. 5 shows that the threshold voltage and resistance are found to increase as a result of the increase in the film thickness.
4. Discussion From Fig. 1 it is noted that the TC in GeSe0.5 S1.5 (360 K) is lower than GeSeS (380 K) composition thin film. As the sample stoichiometry is varied as sulphur rich (GeSe0.5 S1.5 ), the TC is found to be lower. It is reported that the crystallization temperature in Ge-based chalcogenides is very sensitive to the stoichiometry of the film and a small change in the variation of chalcogenides will lead to a large variation in the TC value [16]. Generally, a higher TC will improve the thermal stability of the amorphous films to achieve a better data retention [17]. The annealed GeSeS and GeSe0.5 S1.5 thin films of thickness 110 nm and 122 nm at 400 K showed the presence of crystalline peaks which confirms the occurrence of structural transition. The electrical switching observed in GeSeS and GeSe0.5 S1.5 thin films of Figs. 3 and 4 implies three different features. First is the OFF state with a high resistance in the order of mega ohms. The second is a switching region in which no data points are observed beyond the threshold voltage. Finally, the third feature is the ON state which has a very low resistance. In both GeSeS and GeSe0.5 S1.5 thin films, the memory switching holds the high conductive ON state even for very small currents. Using the electrothermal model [18] the pre-switching region obtained in the I–V characteristics is discussed further as follow: when a low current is applied to the sandwiched sample, the path in the GeSeS and GeSe0.5 S1.5 thin films where the current is flowing gets heated and as a result the conduction in the sample increases due to Joule heating effect. As the voltage is increased further till VT (Figs. 3 and 4), more current will flow through the heated region of the sample that causes more joule heating. At the threshold voltage region, a thermal breakdown occurs due to the rise in temperature by the applied current. This breakdown region leads to electrical switching in GeSeS and GeSe0.5 S1.5 thin films.
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Fig. 3. (a)–(c) The (I–V) characteristics of GeSeS thin films with thickness 110, 200 and 710 nm.
It is worth-noting that some fluctuation in the electrical switching was observed in our previous report on GeSe1.5 S0.5 composition [11]. In Figs. 3 and 4, the region “oa” before VT shows a linear region for each sample with a slope that raises with increasing in the sample’s thickness. In practical for memory devices, the switching speed depends on the phasechange nature from amorphous to crystalline. During the switching, the higher is the value of VT the longer time is required for reaching the crystallization temperature, which means that the switching speed may be delayed due to the higher VT . In the present work, the switching speed is not measured. However, the switching speed can be revealed by the slope of the resistance change around the region “oa” before VT . The linear relation between the voltage and the current has a slope which is the reciprocal of the resistance, and is found to increase with the increase in the thickness of GeSeS and GeSe0.5 S1.5 thin films. As a result a time delay happens before switching based on the resistance of each sample and a different nature of data points with linearity is observed in the region “oa” (Figs. 3 and 4) [2]. In this work incorporating more sulphur such as in GeSeS and GeSe0.5 S1.5 alloys, a more stable switching has been observed. This phenomenon could be related to the fact that changing in the composition of chalcogenide materials alters the disorder in the structure which may allow reorientation more easily and results in modifying the density of states [19]. However, the addition of sulphur into the Se system strengthens the twofold coordination bonding, which when coordinated with Ge improves the fourfold coordination, thus crosslinking the chains of Ge-Se-S system to form a stable tetragonal glass forming structure [11]. The outermost electronic configuration of Ge is (4s)2 (4p)2 and that of Se is (4s)2 (4p)4 . In Se atom, two 4 s electrons are low in energy and they do not participate in bonding. Two electrons among four p electrons are used to form the covalent bonds with other two atoms, hence Se atom makes two-fold coordination. The remaining two p electrons do not participate in bonding and form the ‘lone-pair electrons’. In Ge-Se-S system when the amount of sulphur is large enough in the compositions, a tetrahedral structure is formed centering a Ge atom [13]. Comparing sulphur and Se, the structure of sulphur has more rings
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Fig. 4. (a)–(d) I–V characteristics of GeSe0.5 S1.5 thin films of thickness 122, 297, 420 and 590 nm.
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Fig. 5. (a) The resistance values obtained before and after the switching in GeSeS and GeSe0.5 S1.5 thin films. (b) The variation of the threshold switching voltage with the film thickness.
than Se. Normally sulphur consists of very long chains of sulphur atoms rather than S8 rings. Incorporating more sulphur to Ge-Se-S and during rearrangement, one of the S–S bonds in some of the rings breaks and the rings open forming chains of eight sulphur atoms. The sulphur atoms at each end of the chain have only seven valence electrons. Therefore, they have a tendency to attach an additional electron and are very reactive. This process continues leading to the formation of very long chains of sulphur. This increase in sulphur chains causes the structure of the chalcogenide glasses, GeSeS and GeSe0.5 S1.5 , endure atomic rearrangements more easily and exhibit an ease memory switching [20]. It is reported that increasing the film thickness of GeSeS and GeSe0.5 S1.5 leads to a more ordered atomic network with less defects in the structure [12,13]. In the prepared thin films, sulphur atoms get their preferred bonding when connected to the two-fold coordinated selenium atoms, and the Se-S chains are cross linked by the four-fold germanium atoms. Incorporating more sulphur to Ge-Se-S gives sulphur more space to adapt in the network due the smaller size of sulphur atoms when compared to selenium. Less defects and a more ordered structure of the atomic network, resulted from increase in film thickness, suggests a more rigid atomic structure of the prepared composition of GeSeS and GeSe0.5 S1.5 . For similar thicknesses of samples, the higher resistance for the sulphur rich samples could be attributed to its low conductivity. The lower atomic number of sulphur makes the valence electrons closer to the nucleus. This means that the selenium valence electrons are at higher energy levels than those of sulphur and, hence, require a smaller additional amount of energy to escape from the atom. This property makes the selenium more conductive than sulphur, and consequently the sulphur rich samples are more resistive than the selenium rich ones (Fig. 5a). The increase in the resistance with increasing the film thickness (Fig. 5a) could be attributed to the increase in the length of the resistive element between the electrodes in the sandwich sample (thickness of the film), in addition to the local atomic rearrangements in chalcogenide materials. A similar increase in resistance with film thickness is reported by Domtau et al. [21]. The reduction in the resistance after switching could be attributed to the phase-change of the films which is evidenced by the sudden rise in current which leads to large adiabatic Joule heating and as a result the temperature at the electrode region gets increased suddenly which results in a phase change in the material [22,23]. Yeonwoong et al. [24] have observed crystalline phases in Ge2 Sb2 Te5 films after switching which is evidenced from the electron microscopy measurements. The high voltage (electric field) applied between the two electrodes allows more current to flow in the film and consequently causes a local increase in the temperature of the film. At the threshold voltage, the temperature is sufficient and phase change has occurred which is owing to the Joule heat effect as a result of the collisions among the free electrons and between the free electrons and the lattice. The Joule heating in the current carrying path and a consequent increased mobility of atoms lead to local structural rearrangements which results in the ON state. The memory switching occurs if the Joule heating is sufficient
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to induce a phase transition in the material (in the electrodes’ region). Whether a chalcogenide glass would exhibit memory switching or not depends on many material parameters such as crystallization tendency of the glass, network connectivity, and thermal diffusivity of the material [25]. Post crystallization, the variation in the band structure causes a reduction in the energy gap and excess in the free carrier’s concentration. Consequently, a drastic drop is observed in the resistance from mega ohm to the range of ohms as observed in Fig. 5(a). Due to this drastic drop in the resistance of the device, after switching it remains in the vastly conducting ON state [4,26]. In Fig. 5(b) a sublinear rise in the threshold voltage is observed when the thickness increases for GeSeS thin films which means a slower rise than a linear one. Whereas for sulphur rich composition of GeSe0.5 S1.5 a superlinear rise in threshold voltage is noticed which means a faster rise than a linear one. In the prepared thin films, the order gets increased by increasing the thickness [12,13]. The increase in the order with thickness suggests that more applied voltage is needed to attain the ON state of sulphur rich GeSe0.5 S1.5 thin films that shows superlinear behaviour of switching voltage to 123 V for the film of thickness of 590 nm. The individual elements’ resistance plays an important role on switching which leads to a higher threshold voltage with thickness [1]. This result is supported by the high resistance values observed in Fig. 5(a) for the samples of both GeSeS and GeSe0.5 S1.5 thin films [27–29]. Generally, it is known that in most of the amorphous materials dipoles are being present randomly due to their high resistance nature. When high electric field is applied the dipoles are tend to orient in the direction of the applied field. Normally this orientation depends on the viscosity of the amorphous matrix and on the resistance of the present GeSeS and GeSe0.5 S1.5 materials [30]. Due to the applied electric field the temperature at the conduction path get increased. As a result an increase in the orientation process till the threshold switching point is obtained. At this stage, the resistance of the viscous amorphous medium gets diminished. Thus, as the surrounding temperature increases, the viscosity of the conduction path decreases and the field required to cause maximum dipole orientation would decrease and the switching takes place [31]. 5. Conclusion Thin films of GeSeS and GeSe0.5 S1.5 deposited using thermal evaporation technique exhibit memory type electrical switching nature as a thermal breakdown occurs at the threshold voltage region due to the increase in temperature by the applied current. The increase in sulphur composition strengthens the twofold- and fourfold- coordination and ease the atomic rearrangement to occur, thus exhibiting easiness in memory switching. As the thickness of GeSeS and GeSe0.5 S1.5 thin films increases, the threshold voltage for switching is found to increase and the mechanism is understood by the electrothermal model. More ordered structure and less defects in the atomic network are resulted from the increase in the film’s thickness, demonstrating a more rigid atomic structure of the prepared films of GeSeS and GeSe0.5 S1.5 . The results highlight on the mechanism of electrical switching and the dependence of the threshold potential on the geometry of the films, which is an important parameter to be counted for materials utilized in memory devices. Acknowledgments This work is supported by the United Arab Emirates University Program for Advanced Research (UPAR) Grants Nos. 31S112 and 31S207. One of the authors R.T. Ananth Kumar is gratefully acknowledges the University Grant Commission (UGC), New Delhi, Govt. of India, for the financial support under the UGC project No. 37-270/2009(SR). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
A.M. Shakra, J. Alloys Compd. 699 (2017) 722. E.B. Lee, B.K. Ju, Y.T. Kim, Microelectron. Eng. 86 (2009) 1950. F.A. Al Agel, Mater. Sci. Semicond. Process. 18 (2014) 36. Hellmut Fritzsche, J. Phys. Chem. Solids 68 (2007) 878. S. Moller, C. Perlov, W. Jackson, C. Taussig, S.R. Forrest, Nature 426 (2003) 166. S.T. Mahmoud, H. Ghamlouche, N. Qamhieh, Sadiqa Ahmed, J. Non Cryst. Solids 354 (2008) 1976. N. Qamhieh, S.T. Mahmoud, A.I. Ayesh, H. Ghamlouche, Phys. Status Solidi A 209 (2012) 2476. N. Sulitanu, Mater. Sci. Eng. B 83 (2001) 84. R.T. Ananth Kumar, Chandasree Das, S. Asokan, C. Sanjeeviraja, D. Pathinettam Padiyan, Appl. Phys. A 115 (2014) 1151. H.K.C. Chan, G.D. Pitt, M.K.R. Vyas, J. Non Cryst. Solids 17 (1975) 401. R.T. Ananth Kumar, Chandasree Das, P. Chithra Lekha, S. Asokan, C. Sanjeeviraja, D. Pathinettam Padiyan, J. Alloys Compd. 615 (2014) 629. R.T. Ananth Kumar, N.N. Qamhieh, S.T. Mahmoud, P. ChithraLekha, K. Jeyadheepan, C. Sanjeeviraja, D. PathinettamPadiyan, J. Non Cryst. Solids 450 (2016) 135. R.T. Ananth Kumar, P. ChithraLekha, C. Sanjeeviraja, D. Pathinettam Padiyan, J. Non Cryst. Solids 405 (2014) 21. M.P. Slankamenac, R. Svetlana, Lukic-Petrovic, M.B. Zivanov, Kristina Cajko, Solid State Commun. 152 (2012) 1160. S.B. Bhanu Prashanth, S. Asokan, J. Non Cryst. Solids 355 (2009) 164. S.T. Mahmoud, N. Qamhieh, A.I. Ayesh, Phys. Status Solidi A 211 (2014) 645. Y. Lu, Y. Hu, L. Yuan, X. Zhu, H. Zou, L. Shen, Z. Song, J. Non Cryst. Solids 432 (2016) 505. R.M. Mehra, Radhey Shyam, P.C. Mathur, J. Non Cryst. Solids 31 (1979) 435. I. Sharma, S.K. Tripathi, A. Monga, P.B. Barman, J. Non Cryst. Solids 354 (2008) 3215. Y. Sakayuchi, D.A. Tenne, M. Mitkova, J. Non Cryst. Solids 355 (2009) 1792. D.L. Domtau, J. Simiyu, E.O. Ayieta, B. Muthoka, J.M. Mwabora, J. Mater. Phys. Chem. 4 (2016) 1. H. Lee, Y.K. Kim, D. Kim, D.H. Kang, IEEE Trans. Magn. 41 (2005) 1034.
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Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Study on electrical switching behaviour of Ge-Se-S and Ge-Se0.5 -S1.5 thin films R.T. Ananth Kumar a , Saleh T. Mahmoud a , D. Pathinettam Padiyan b , N. Qamhieh a,∗ a b
Department of Physics, UAE University, P.O. Box 15551, Al-Ain, United Arab Emirates Department of Physics, Manonmaniam Sundaranar University, Tirunelveli 627012, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 26 March 2017 Received in revised form 18 September 2017 Accepted 18 September 2017 Keywords: Thin films Chalcogenide Thickness Electrical switching
a b s t r a c t Ternary glassy chalcogenides of Ge-Se-S and Ge-Se0.5- S1.5 systems were prepared by vacuum evaporation technique. Their electrical switching characteristics were examined in detail, where the resistance of the prepared thin films changed from mega to several tens of ohms. The amorphous-crystalline transition temperature (TC ) for GeSeS and GeSe0.5 S1.5 thin films are around 380 K and 360 K, respectively. Due to the electrothermal mechanism a rapid transition from resistive to conductive nature was observed and the results obtained were discussed by Joule heating effect. In the prepared compositions of Ge-Se-S and GeSe0.5 S1.5 , sulphur atoms get their preferred bonding when connected to the two-fold coordinated selenium atoms, and the Se-S chains are cross linked by the four-fold germanium atoms. Incorporating more sulphur to Ge-Se-S system, gives the sulphur more space to adapt in the network due the smaller size of the sulphur atoms when compared to selenium. Furthermore, this method of preparation and results open-up a new approach towards the phase change memory devices that rely on easy structural transformation. © 2017 Elsevier GmbH. All rights reserved.
1. Introduction Recent advancement on amorphous systems based on Ge-Se has become significant in the field of multimedia, data storage, phase change and switching applications [1–3]. The switching effect has been fashionable in chalcogenide materials than other oxide type materials [4,5]. The chalcogenide materials show phase change behaviour that is attained by Joule heating effect [6,7]. However, a lot of research has been focused on this area to know the factual knowledge of the structural change on these materials. Quite a few sulphur based materials like Ge46 S54, GeS2 , and GeS exhibited memory type electrical switching [8–10]. The inclusion of sulphur into the Se based system with fourfold coordinated Ge forms a stable glassy chalcogenide material [11]. In our previous reported data on GeSeS and GeSe0.5 S1.5 compositions [12,13], thickness dependent structural disorder was studied using Raman spectra, Tauc parameter and Urbach energy. Nevertheless, in this article the research work is continued on thin films of GeSeS and GeSe0.5 S1.5 to investigate the amorphous-crystalline transition temperature (TC ) followed by electrical switching of those alloys. The experimental observations concerning the dependence of electrical switching on film thickness for GeSeS and GeSe0.5 S1.5 chalcogenide alloys have been discussed. This study would explore the feasibility of using these phase change materials in memory devices.
∗ Corresponding author. E-mail address: [email protected] (N. Qamhieh). http://dx.doi.org/10.1016/j.ijleo.2017.09.074 0030-4026/© 2017 Elsevier GmbH. All rights reserved.
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Fig. 1. Resistance as a function of temperature for GeSeS and GeSe0.5 S1.5 thin films of thickness 110 nm and 122 nm.
2. Experimental details Bulk GeSeS and GeSe0.5 S1.5 glasses have been prepared by vacuum-sealed melt quenching technique. Appropriate quantities of high purity (99.99%) constituent elements of Ge, Se and S were weighed according to their stoichiometric ratio and sealed into an evacuated quartz ampoule at a vacuum of 10−5 Torr. Melting was performed at 1050 ◦ C for 36 h inside a horizontal type furnace (Strohlein instruments) that allows the ampoule containing the sample to rotate with a rotational speed of 10 rpm to ensure homogeneity of the melt. Then the bulk glasses of GeSeS and GeSe0.5 S1.5 were obtained by quenching the ampoules in a bath of ice water-NaOH mixture. The amorphous nature of the quenched materials is confirmed by X-ray diffraction (XRD) analysis using PANalytical X’Pert PRO (Cu as target and Ni as filter, = 1.5406 Å). Thin films of GeSeS and GeSe0.5 S1.5 were deposited by thermal evaporation technique (HINDHIVAC 12A4D) on cleaned glass substrates held at room temperature in a vacuum of 2 × 10−5 Torr. Three different thicknesses 110, 200, and 710 nm of GeSeS thin films were deposited, and for GeSe0.5 S1.5 films the thicknesses were 122, 297, 420 and 590 nm. The substrates were located at a distance of 10 cm vertically above the source. During each deposition, the bulk material was completely evaporated at a fast rate so that no residual alloy would remain. This procedure is necessary to ensure homogeneity due to preferential evaporation of the volatile sulphur and selenium. The thickness of the films was measured using the surface roughness tester (SJ-210, MITUTOYO). For the lower thickness film of 110 nm (GeSeS) and 122 nm (Ge-Se0.5- S1.5 ), the resistance measurement were carried out using Keithley 590 at different temperatures using Lakeshore 335 temperature controller in the range of 300–400 K with heating rate of 5 K/min. During the measurement, the samples were heated under vacuum of 10−3 Torr using Janis ST-100H cryostat. X-ray diffraction analysis was made for 110 nm (GeSeS) and 122 nm (Ge-Se0.5- S1.5 ) thin films at 300 K and 400 K to identify the amorphous-crystal phase change. Other several characterizations such as differential scanning calorimetry, X-ray diffraction, energy dispersive X-ray analysis attached to scanning electron microscopy, photo-acoustics, transmittance and Raman spectra are reported in earlier publications [12,13]. The electrodes were prepared in sandwich geometry using aluminium as top and bottom electrodes for a thickness of 100 nm. The electrodes were evaporated through a suitable mask and they were used to perform the I–V characteristics to study the electrical switching behaviour of GeSeS and GeSe0.5 S1.5 thin films. The I–V curves were recorded using a Keithley source-meter (Model 2410c ) controlled by LabVIEW 6i (National Instruments). For the I–V measurements using the sandwiched electrodes, a constant current of few mA was applied and the corresponding voltages developed were observed. All the measurements were repeated at different portions of a thin film in order to check the reproducibility, and the error was about ±2%. The standard deviation method has been used for error analysis of the switching voltage measurements. 3. Results The resistance as a function of temperature is shown in Fig. 1 for the film of thickness 110 nm (GeSeS) and 122 nm (GeSe0.5 S1.5 ). At room temperature of 300 K the figure displays a high resistance of the film characterizing the amorphous nature. With the raise in temperature from 300 K, the resistance is found to decrease exponentially and an abrupt drop in resistance was noted around 360 K and 380 K for GeSe0.5 S1.5 and GeSeS thin films respectively. This temperature at 360 K and 380 K is known as the amorphous-crystalline transition temperature (TC ). The high electrical contrast of four orders of magnitude between the amorphous and crystalline states of the film and the fast transition in the resistivity could be utilized for high-speed memory devices. Fig. 2 shows the X-ray diffraction pattern for (a) GeSeS thin film of thickness 110 nm at 300 K of temperature, (b) GeSeS thin film of thickness 110 nm at 400 K of temperature, (c) GeSe0.5 S1.5 thin film of thickness 122 nm at 300 K of temperature and (d) GeSe0.5 S1.5 thin film of thickness 122 nm at 400 K of temperature. At 300 K in both the compositions no any Bragg’s peak were found indicating the amorphous nature (Fig. 1a & c). From Fig. 1 it is known that a phase change was observed at 360 K and 380 K for GeSeS and GeSe0.5 S1.5 composition. These results are in well agreement with results obtained from the XRD which indicates the presence of Bragg’s diffraction peak at 400 K (Fig. 2b & d). Fig. 2b
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Fig. 2. shows the XRD pattern for (a) GeSeS thin film of thickness 110 nm at 300 K of temperature, (b) GeSeS thin film of thickness 110 nm at 400 K of temperature, (c) GeSe0.5 S1.5 thin film of thickness 122 nm at 300 K of temperature and (d) GeSe0.5 S1.5 thin film of thickness 122 nm at 400 K of temperature.
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shows two Bragg peaks at 23.59 and 26.07 , whereas Fig. 2d shows two Bragg X-ray peak at 23.07◦ and 28.20 respectively. From XRD it is noted that, the appearance of crystalline peak is found after the amorphous-crystalline transition temperature (TC ) which indicates a phase change. The (I–V) characteristics of GeSeS thin films with thickness 110, 200 and 710 nm are displayed in Fig. 3(a)–(c). Fig. 3(a) shows that an increase in the applied current produces a very small rise in voltage, representing the OFF state with high resistance of the switch (the region “oa” of the curve). Above the threshold voltage (VT ) the samples exhibit switching which leads to ON state (part “ab”). Since the transition to ON state is very fast, no data points are observed in this range (ab). As the current is applied further, it does not make any change in the potential drop across the thin film (part “bc” of the curve), which represents the ON state with very low value of resistance. When the applied current is decreased, the voltage is found to decrease to a value of zero in the path “co” of the curve. The behaviour of the curve has low and high conducting branches (oa and bc) connected by two branches (ab and co) represents a typical I–V characteristic curve for a memory switch. Figs. 3 (b) and 1 (c) of GeSeS thin films are also illustrate the same memory switching behaviour but for different thicknesses with different switching voltages. Fig. 4(a)–(d) shows the identical memory switching nature of GeSe0.5 S1.5 thin films of thickness 122, 297, 420 and 590 nm respectively. When compared to other chalcogenides such as Cux (AsSe1.4 I0.2 )100−x and Sbx Se55−x Te45 , the obtained electrical switching in GeSeS and GeSe0.5 S1.5 thin films does not show any fluctuation in switching [14,15]. At VT , all the samples switch from a highresistance state (OFF) to a low-resistance (ON) state. A direct resistance measurement of the as deposited thin films has been done using Keithley 614 electrometer. Fig. 5(a) represents the resistance values obtained before and after the switching occurred. The resistance of the as prepared film is greater than the mega-ohms’ range and it decays to the range of ohms once VT is reached. The GeSe0.5 S1.5 thin film of thickness 590 nm shows a resistance of 125 M before switching and after it reduced to 26 . Fig. 5(b) pictures the variation of the threshold switching voltage with the film thickness. Perusal of Fig. 5 shows that the threshold voltage and resistance are found to increase as a result of the increase in the film thickness.
4. Discussion From Fig. 1 it is noted that the TC in GeSe0.5 S1.5 (360 K) is lower than GeSeS (380 K) composition thin film. As the sample stoichiometry is varied as sulphur rich (GeSe0.5 S1.5 ), the TC is found to be lower. It is reported that the crystallization temperature in Ge-based chalcogenides is very sensitive to the stoichiometry of the film and a small change in the variation of chalcogenides will lead to a large variation in the TC value [16]. Generally, a higher TC will improve the thermal stability of the amorphous films to achieve a better data retention [17]. The annealed GeSeS and GeSe0.5 S1.5 thin films of thickness 110 nm and 122 nm at 400 K showed the presence of crystalline peaks which confirms the occurrence of structural transition. The electrical switching observed in GeSeS and GeSe0.5 S1.5 thin films of Figs. 3 and 4 implies three different features. First is the OFF state with a high resistance in the order of mega ohms. The second is a switching region in which no data points are observed beyond the threshold voltage. Finally, the third feature is the ON state which has a very low resistance. In both GeSeS and GeSe0.5 S1.5 thin films, the memory switching holds the high conductive ON state even for very small currents. Using the electrothermal model [18] the pre-switching region obtained in the I–V characteristics is discussed further as follow: when a low current is applied to the sandwiched sample, the path in the GeSeS and GeSe0.5 S1.5 thin films where the current is flowing gets heated and as a result the conduction in the sample increases due to Joule heating effect. As the voltage is increased further till VT (Figs. 3 and 4), more current will flow through the heated region of the sample that causes more joule heating. At the threshold voltage region, a thermal breakdown occurs due to the rise in temperature by the applied current. This breakdown region leads to electrical switching in GeSeS and GeSe0.5 S1.5 thin films.
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Fig. 3. (a)–(c) The (I–V) characteristics of GeSeS thin films with thickness 110, 200 and 710 nm.
It is worth-noting that some fluctuation in the electrical switching was observed in our previous report on GeSe1.5 S0.5 composition [11]. In Figs. 3 and 4, the region “oa” before VT shows a linear region for each sample with a slope that raises with increasing in the sample’s thickness. In practical for memory devices, the switching speed depends on the phasechange nature from amorphous to crystalline. During the switching, the higher is the value of VT the longer time is required for reaching the crystallization temperature, which means that the switching speed may be delayed due to the higher VT . In the present work, the switching speed is not measured. However, the switching speed can be revealed by the slope of the resistance change around the region “oa” before VT . The linear relation between the voltage and the current has a slope which is the reciprocal of the resistance, and is found to increase with the increase in the thickness of GeSeS and GeSe0.5 S1.5 thin films. As a result a time delay happens before switching based on the resistance of each sample and a different nature of data points with linearity is observed in the region “oa” (Figs. 3 and 4) [2]. In this work incorporating more sulphur such as in GeSeS and GeSe0.5 S1.5 alloys, a more stable switching has been observed. This phenomenon could be related to the fact that changing in the composition of chalcogenide materials alters the disorder in the structure which may allow reorientation more easily and results in modifying the density of states [19]. However, the addition of sulphur into the Se system strengthens the twofold coordination bonding, which when coordinated with Ge improves the fourfold coordination, thus crosslinking the chains of Ge-Se-S system to form a stable tetragonal glass forming structure [11]. The outermost electronic configuration of Ge is (4s)2 (4p)2 and that of Se is (4s)2 (4p)4 . In Se atom, two 4 s electrons are low in energy and they do not participate in bonding. Two electrons among four p electrons are used to form the covalent bonds with other two atoms, hence Se atom makes two-fold coordination. The remaining two p electrons do not participate in bonding and form the ‘lone-pair electrons’. In Ge-Se-S system when the amount of sulphur is large enough in the compositions, a tetrahedral structure is formed centering a Ge atom [13]. Comparing sulphur and Se, the structure of sulphur has more rings
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Fig. 4. (a)–(d) I–V characteristics of GeSe0.5 S1.5 thin films of thickness 122, 297, 420 and 590 nm.
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Fig. 5. (a) The resistance values obtained before and after the switching in GeSeS and GeSe0.5 S1.5 thin films. (b) The variation of the threshold switching voltage with the film thickness.
than Se. Normally sulphur consists of very long chains of sulphur atoms rather than S8 rings. Incorporating more sulphur to Ge-Se-S and during rearrangement, one of the S–S bonds in some of the rings breaks and the rings open forming chains of eight sulphur atoms. The sulphur atoms at each end of the chain have only seven valence electrons. Therefore, they have a tendency to attach an additional electron and are very reactive. This process continues leading to the formation of very long chains of sulphur. This increase in sulphur chains causes the structure of the chalcogenide glasses, GeSeS and GeSe0.5 S1.5 , endure atomic rearrangements more easily and exhibit an ease memory switching [20]. It is reported that increasing the film thickness of GeSeS and GeSe0.5 S1.5 leads to a more ordered atomic network with less defects in the structure [12,13]. In the prepared thin films, sulphur atoms get their preferred bonding when connected to the two-fold coordinated selenium atoms, and the Se-S chains are cross linked by the four-fold germanium atoms. Incorporating more sulphur to Ge-Se-S gives sulphur more space to adapt in the network due the smaller size of sulphur atoms when compared to selenium. Less defects and a more ordered structure of the atomic network, resulted from increase in film thickness, suggests a more rigid atomic structure of the prepared composition of GeSeS and GeSe0.5 S1.5 . For similar thicknesses of samples, the higher resistance for the sulphur rich samples could be attributed to its low conductivity. The lower atomic number of sulphur makes the valence electrons closer to the nucleus. This means that the selenium valence electrons are at higher energy levels than those of sulphur and, hence, require a smaller additional amount of energy to escape from the atom. This property makes the selenium more conductive than sulphur, and consequently the sulphur rich samples are more resistive than the selenium rich ones (Fig. 5a). The increase in the resistance with increasing the film thickness (Fig. 5a) could be attributed to the increase in the length of the resistive element between the electrodes in the sandwich sample (thickness of the film), in addition to the local atomic rearrangements in chalcogenide materials. A similar increase in resistance with film thickness is reported by Domtau et al. [21]. The reduction in the resistance after switching could be attributed to the phase-change of the films which is evidenced by the sudden rise in current which leads to large adiabatic Joule heating and as a result the temperature at the electrode region gets increased suddenly which results in a phase change in the material [22,23]. Yeonwoong et al. [24] have observed crystalline phases in Ge2 Sb2 Te5 films after switching which is evidenced from the electron microscopy measurements. The high voltage (electric field) applied between the two electrodes allows more current to flow in the film and consequently causes a local increase in the temperature of the film. At the threshold voltage, the temperature is sufficient and phase change has occurred which is owing to the Joule heat effect as a result of the collisions among the free electrons and between the free electrons and the lattice. The Joule heating in the current carrying path and a consequent increased mobility of atoms lead to local structural rearrangements which results in the ON state. The memory switching occurs if the Joule heating is sufficient
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to induce a phase transition in the material (in the electrodes’ region). Whether a chalcogenide glass would exhibit memory switching or not depends on many material parameters such as crystallization tendency of the glass, network connectivity, and thermal diffusivity of the material [25]. Post crystallization, the variation in the band structure causes a reduction in the energy gap and excess in the free carrier’s concentration. Consequently, a drastic drop is observed in the resistance from mega ohm to the range of ohms as observed in Fig. 5(a). Due to this drastic drop in the resistance of the device, after switching it remains in the vastly conducting ON state [4,26]. In Fig. 5(b) a sublinear rise in the threshold voltage is observed when the thickness increases for GeSeS thin films which means a slower rise than a linear one. Whereas for sulphur rich composition of GeSe0.5 S1.5 a superlinear rise in threshold voltage is noticed which means a faster rise than a linear one. In the prepared thin films, the order gets increased by increasing the thickness [12,13]. The increase in the order with thickness suggests that more applied voltage is needed to attain the ON state of sulphur rich GeSe0.5 S1.5 thin films that shows superlinear behaviour of switching voltage to 123 V for the film of thickness of 590 nm. The individual elements’ resistance plays an important role on switching which leads to a higher threshold voltage with thickness [1]. This result is supported by the high resistance values observed in Fig. 5(a) for the samples of both GeSeS and GeSe0.5 S1.5 thin films [27–29]. Generally, it is known that in most of the amorphous materials dipoles are being present randomly due to their high resistance nature. When high electric field is applied the dipoles are tend to orient in the direction of the applied field. Normally this orientation depends on the viscosity of the amorphous matrix and on the resistance of the present GeSeS and GeSe0.5 S1.5 materials [30]. Due to the applied electric field the temperature at the conduction path get increased. As a result an increase in the orientation process till the threshold switching point is obtained. At this stage, the resistance of the viscous amorphous medium gets diminished. Thus, as the surrounding temperature increases, the viscosity of the conduction path decreases and the field required to cause maximum dipole orientation would decrease and the switching takes place [31]. 5. Conclusion Thin films of GeSeS and GeSe0.5 S1.5 deposited using thermal evaporation technique exhibit memory type electrical switching nature as a thermal breakdown occurs at the threshold voltage region due to the increase in temperature by the applied current. The increase in sulphur composition strengthens the twofold- and fourfold- coordination and ease the atomic rearrangement to occur, thus exhibiting easiness in memory switching. As the thickness of GeSeS and GeSe0.5 S1.5 thin films increases, the threshold voltage for switching is found to increase and the mechanism is understood by the electrothermal model. More ordered structure and less defects in the atomic network are resulted from the increase in the film’s thickness, demonstrating a more rigid atomic structure of the prepared films of GeSeS and GeSe0.5 S1.5 . The results highlight on the mechanism of electrical switching and the dependence of the threshold potential on the geometry of the films, which is an important parameter to be counted for materials utilized in memory devices. Acknowledgments This work is supported by the United Arab Emirates University Program for Advanced Research (UPAR) Grants Nos. 31S112 and 31S207. One of the authors R.T. Ananth Kumar is gratefully acknowledges the University Grant Commission (UGC), New Delhi, Govt. of India, for the financial support under the UGC project No. 37-270/2009(SR). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
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