Journal of Non-Crystalline Solids 368 (2013) 34–39

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Journal of Non-Crystalline Solids

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Correlation between crystallization behavior, electrical switching and local atomicstructure of Ge–Te glasses

Manisha Upadhyay, Sevi Murugavel ⁎Department of Physics and Astrophysics, University of Delhi, Delhi – 110007, India

⁎ Corresponding author. Tel.: +91 1127667061.E-mail address: murug@physics.du.ac.in (S. Muruga

0022-3093/$ – see front matter. Crown Copyright © 20http://dx.doi.org/10.1016/j.jnoncrysol.2013.02.028

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 December 2012Received in revised form 20 February 2013Available online 22 March 2013

Keywords:chalcogenide glass;crystallization kinetics;electrical switching

We report the results of the crystallization behavior, electrical switching and structure of the bulk GexTe100-xglasses to ascertain the role of composition on phase change behavior. Obtained quantities like crystallizationtemperature and switching parameters exhibit threshold behavior at x = 17 and 22. Within this compositionalrange, we find that measured properties are insensitive to x content and beyond this composition window itshows strong composition dependence andmay be driven by the ordering among the structural units. The order-ing of the glassmolecular structure becomesmaximized and shows anomalous behavior at x = 20 (xc). An efforthas been made to understand the electrical switching characteristics and variation of threshold switching fieldswith Ge content on the basis of local atomic structure. The thickness dependence of switching fields andcyclability of these glasses identifies themechanism of switching and influence of local atomic structure. The ob-served compositional variations are interpreted on the basis of the local atomic structure of the given glass andthe corresponding crystalline phase.

Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

1. Introduction

Chalcogenide glasses are interesting class ofmaterialswhichpossessunique combination of properties including fast crystallization of theamorphous state accompanied by a drastic change in their electronicproperties such as optical reflectivity and electrical resistance. Suchlarge contrast in the optical and electrical properties of these materialsmakes us to use them as potential candidate for the optical and electricaldata storage applications. Nearly four decades back Ovshinsky reportedthe first switching and memory effects in these classes of materials withcomplex systems like Si12Te48As30Ge10, Ge15Tl81Sb2S2, Ge10Te5In2.5Ga2.5etc. that draw much attention of various researchers [1–3]. Later, it hasbeen found that the pseudobinary compounds of GeTe–Sb2Te3 (GST) al-loys are the most suitable candidate for the commercial phase changememory (PCM) devices. In 1987, these GST alloys were introduced bySONY with 500 Mb in the rewritable optical disc, since then memoryhas increased to 50 Gb in blu-ray disc. The flash memory has beenobeying Moore's law of scaling and it has achieved limiting value of64 Gb NAND flash memory with 30 nm feature size. Hence, the sizelimiting feature can be overcome by the PCM based materials and itcould be the next generation non-volatile memory for the successfulreplacement of currently used flash memory. More recently, it hasbeen shown that nanostructured devices such as GeTe nanowiresof as small as two to three times the lattice constant retain their

vel).

13 Published by Elsevier B.V. All rig

phase change properties and it could remove the scaling limit offlash memory with phase change technology [4].

The increasing demand on such devices largely depends on thethermal parameters such as crystallization temperature (Tc), crys-tallization speed and melting temperature (Tm) of the given glasscomposition. Since, the phase change technology is mainly based onthe reversible switching between the crystalline and amorphousstate induced by an electric field, light or combination of both [5]. Inthis context, the binary Ge–Te alloys, which exhibit superior thermalproperties (higher Tc) than ternary Ge–Sb–Te (GST) alloy composi-tions, appear attractive, if their crystallization speed becomes ade-quately fast [6]. Furthermore, a sufficient knowledge of the thermalcrystallization kinetics is necessary for the development of suitablephase-change materials with optimized parameters. Within the Ge–Te alloys, the stoichiometric Ge50Te50 has attracted considerable in-terest due to the rapid crystallization and relative stability at ambientconditions. Recently, it has been shown that the eutectic, GeTe6 ap-pears to be potential selector device, which exhibits Ovonic thresholdswitching behavior with superior performance in the switching pa-rameters. In this context, significant progress has beenmade in binaryGe–Te glass system with various compositions and found that the Tcand crystallization speed exhibit a strong compositional dependence[7,8]. However, the reason behind such strong dependence of crystal-lization kinetics on chemical composition remains to be established. Itis interesting to note that an early work done by Barton et al. revealedthat the minimum pulse width required for crystallization of Ge–Tefilms has been found to be least for stoichiometric Ge50Te50 composi-tion and it increases while shifting away from this composition [7].

hts reserved.

350 400 450 500 550-0.5

0.0

0.5

1.0

Temperature (K)

Hea

t-fl

ow

(m

W) Tc2

Tc1

Tg

Non-reversible HF

Total HF Reversible HF

Fig. 1. MDSC scan of Ge17Te83 glass showing deconvolution of total heat flow (HF) intoreversible and non-reversible components.

35M. Upadhyay, S. Murugavel / Journal of Non-Crystalline Solids 368 (2013) 34–39

Although various studies have been carried out on Ge–Te glasssystem for both bulk and thin films, but the compositional dependenceof structural and thermal parameters is not yet completely understood.However, based on various studies it has been found that the fast crys-tallization speed between the two states is related with structural sim-ilarities of amorphous and crystalline phase, which provides lowerenergy barrier for the reversible local structural order changes in thecase of stoichiometric Ge50Te50 [9].

In recent years, significant progress has been made in this class ofmaterials by using theoretical simulation methods in addition to thevarious experimental techniques [10–12]. Several of these studies in-dicate the presence of considerable amount of vacancies in their crys-talline state of PCM materials. These vacancies could accelerate themovement of Ge atoms from octahedral sites in the crystalline stateto tetrahedral site in the amorphous state. Moreover, it has been sug-gested that the large concentration of pre-existing Ge vacancies couldact as nucleation sites for the phase change process from crystalline toamorphous state [12]. Recently, we have used various experimentaltechniques to characterize the Ge–Te alloy compositions towards un-derstanding the nature of atomic defects with different sizes in glassand the corresponding crystalline phase [13]. Remarkably, these ex-perimental studies indicate that the crystalline state of germaniumtelluride alloys possess the large defect concentration than that ofthe corresponding glassy state.

In thiswork, we have extended our studies on Ge–Te system towardstheir composition dependence of thermal properties and electricalswitching characteristics to understand the phase change mechanismwith the aid of the local atomic structure of these alloys. Based onthese studies, we find the strong correlation between the switchingproperty, thermal parameters and local structure of these alloys. In ad-dition, we report for the first time, the presence of a new type of“intermediate composition” in the binary telluride glasses and found itto exhibit anomalous behavior with Ge content. The obtained electricalswitching parameters suggest that the intrinsic local atomic order is re-sponsible for the cyclability of these materials in non-volatile memoryapplications. Furthermore, we emphasize the significance of composi-tion dependent thermal parameters on the cyclability of PCM material.

2. Experimental

Bulk semiconducting glass samples of GexTe100-x (15 ≤ x ≤ 23)were prepared by conventional melt quench method. An appropriateamount of Ge and Te granules (Sigma Aldrich 99.999%) were trans-ferred in a cleaned quartz ampoule and evacuated (10−6 mB) for suffi-ciently long hours. Following this, the quartz ampoules were sealedwith the same vacuum conditions and kept in a rotating furnace andthen the temperature has been increased continuously with the rateof 5 °C/min to 1000 °C. Alumina tube containing ampoule was rotatedwith 10 rpm to ensure homogeneity of themelt followed by quenchingin ice + NaCl mixture. We were able to obtain bulk and homogenousglass specimens only between 15 ≤ x ≤ 23 compositions. An obtainedbulk glass samples were confirmed to be amorphous by x-ray diffrac-tion (XRD) and the glass composition was examined by energy disper-sive x-ray (EDX) analysis.

The bulk glass mass densities were measured using the Archimedesprinciple (digital balanceMettler model B154). Glass samples of typical-ly 100 mg or larger in size were weighed in air and in pure ethylene gly-col. Thermal analysis has been carried out with Modulated DifferentialScanning Calorimeter (MDSC) (DSC Q200 V24.7 Build 119 from TA in-struments). The glass transition temperature (Tg) was obtained from re-versible component of heat flowmeasurements. On the other hand, thecrystallization temperature (Tc) was obtained from non-reversible heatflow part. Additionally, the bulk glass specimens were recrystallized byheating the glass sample at respective crystallization temperaturefor about 6 h and then cooled to room temperature at the rate of1 K/min. Crystalline phases were identified by the x-ray diffraction

(XRD) technique. The electrical switching studies were performed onthe polished glass samples using Keithley source meter (model 2410)by mounting it in holder made of brass, in between a flat bottomelectrode and a point contact top electrode using a spring loadingmechanism. Additionally, we have carried out the cycling perfor-mance of glass samples with x ≤ 20 and with SET and RESET cur-rents of 0.5–5 mA and 10 mA respectively. An input constant currentis passed through the sample in the form of triangular pulse and voltagedeveloped across the sample ismeasured using twowire sensingmode.

3. Experimental results

3.1. Thermal and XRD studies on Ge–Te glasses

MDSC scans were performed in all the glass samples, which providevarious thermal parameters such as Tg, Tc and non-reversing enthalpyand Fig. 1 shows the typical evaluation of MDSC scans for a representa-tive Ge17Te83 glass sample. In Fig. 2a, we show the composition depen-dent variation of glass transition temperature (Tg) of GexTe100-x glassesobtained from the reversing heat flow measurements. All the glasscompositions under the present study exhibit single Tg and it increaseslinearly with increasing the Ge content. In contrast to the earlier report[14], we do not observe double Tg in the studied glass compositions andthese results are in agreement with those reported in the literature[15–19]. Crystallization of these glasses is found to show both singleand double stage crystallization depending upon on the glass constitu-ents shown in Fig. 2b. The double stage crystallization has been ob-served in various other chalcogenide systems like Si–Te, Ga–Te andAl–Te glasses and discussed in various aspects [20–22]. More common-ly, the first crystallization temperature (Tc1) has been attributed to thecrystallization of a-Te into the hexagonal form and it increases linearlywith Ge content. On the other hand, the second stage crystallizationtemperature (Tc2) is ascribed to the crystallization of the glass networkstructure, i.e. crystallization of Ge–Te in α-GeTe and/or GeTe4 structuredepending on the Ge content. It can be seen that glasses with x b 18.5exhibit the double stage crystallization and it increases with Ge contentup to 21.5 mol%with aminima at x = 20. Remarkably, we find that thecomposition dependence of Tc2 shows the anomalous variations withGe content, i.e. it shows the maxima between x = 18.5 and 21.5along with small dip at x = 20. Beyond the compositions, it decreasescontinuously with Ge content. These results on the crystallization tem-peratures of GexTe100-x glasses are in agreementwith those found in theliterature [17–19,23,24] but differ from the ones reported [14]. The dif-ference between the present results and those of particularly Asokan etal. is probably related to the glass synthesis conditions and homogene-ity of the obtained samples. However, we confirm our glass sample

14 16 18 20 22 24

420

440

Tc2

460

480

500

520

Tc

(K)

Tc1

b

a

Tg (

K)

x (mole %)

Fig. 2. Composition dependence of thermal parameters (a) glass transition tempera-ture (Tg) and (b) crystallization temperature (Tc) of GexTe100-x with 15 ≤ x ≤ 23glasses.

36 M. Upadhyay, S. Murugavel / Journal of Non-Crystalline Solids 368 (2013) 34–39

homogeneity by the observation of single Tg and its linear variationwithGe content as well as by the compositional analysis.

Additionally, we have carried out detailed analysis to identify thetypes of crystalline phases present by using the powder XRD patternon bulk crystalline GexTe100-x samples with (15 ≤ x ≤ 23) and areshown in Fig. 3. At the eutectic composition (x = 15), the observeddiffraction peaks (dhkl) correspond to the Te chains and distortedα-GeTe phase. However, with an increase of Ge content, we observethe appearance of new phase like GeTe4 at x ≥ 18.5 along withα-GeTe and Te. However, in the case of Ge20Te80 composition, wefind only tetragonal GeTe4 phase without α-GeTe phase, which indi-cates the presence of another stoichiometric cubic GeTe4 phase [25].Within the compositions studied in the present work, the structure

20 25 30 35 40 45 50

x = 23

x = 22

x = 20

x = 18.5

x = 15

/

Inte

nsi

ty

2θ (degrees)

TeGeTe4

GeTe

/

Fig. 3. The experimental powder XRD pattern of bulk crystalline GexTe100-x (15 ≤ x ≤ 23)samples recrystallized at their respective crystallization temperatures.

of c-Ge20Te80 is distinctly different from the other compositions dueto the presence of cubic GeTe4 phase. On the other hand, when thecomposition moves away from xc, i.e. 18.5 ≤ x ≤ 22, we identifythat the crystalline phases consist of mixture of GeTe4 and α-GeTealong with Te. The intensity ratio of GeTe4 and α-GeTe directly re-flects the presence of corresponding phases and their variation withGe content. Therefore, we identify that the evolvement of GeTe4phase starts at x = 18.5 and maximizes at x = 20 and then it be-comes minimal at x = 22. In parallel with thermal characterizations,we have determined the mass density of GexTe100-x glasses to a highaccuracy and the typical error in the measurement is within ±3%. Theresulting density values and molar volumes as a function of glasscomposition are shown in Fig. 4 and found that Vm(x) decreaseswith increasing x. However, the variation of mass density shows thestrikingly similar results between 17 ≤ x ≤ 22, where it becomes in-dependent of Ge content. Beyond this compositional window, itshows the strong dependency with composition.

3.2. Electrical switching characteristics of Ge–Te glasses.

The current–voltage characteristics of all the glass samples havebeen found to exhibit memory type of switching. The lock on timeis an important feature of the device to set the material in the mem-ory state which depends on the characteristics of the sample as wellas on the applied over voltage. In the present work, we have foundthat minimum set current of 5 mA to achieve the complete ON stateand voltage developed across the sample has been measured. InFig. 5, we show the composition dependent switching field (Eth) onGexTe100-x glasses, where it shows the distinct behavior with x con-tent. It is interesting to note that threshold switching field increaseswith Ge content linearly up to x = 17 then it shows less dependencytill x = 21.5 and then it increases super linearly.

Again, it was found that the glass composition at x = 20 behavesdifferently from the other composition upon application of externalelectric field. It was observed that all these glasses switch from high re-sistance OFF state to a low resistance ON state at threshold voltage.Upon reducing the current in the ON state, the sample remains in thelow resistance state due to the larger Joule heating that leads to crystal-lization of the active region between the two electrodes. The resultingactive crystalline region could be recycled only by applying shortRESET pulse, which would locally melt and form amorphous phase.However, we have found that the c-Ge20Te80 composition shows theswitching process without application of RESET pulse where the Vth

nearly identical to corresponding Ge20Te80 glass sample.

3.3. Thickness dependence of threshold field

In Fig. 6, we show the variation of threshold voltage (Vth) withthickness (d) for Ge15Te85, Ge17Te83 and Ge20Te80 glass samples. Thedependence of Vth on the sample thickness provides further insight

14 16 18 20 22 24

5.52

5.56

5.60

5.64

5.68

20.2

20.6

21.0

21.4

21.8

x (mole %)

Mo

lar volu

me (cm

3/mo

l)

Den

sity

(g

/cm

3)

Fig. 4. Composition dependence of density and molar volume for GexTe100-x with(15 ≤ x ≤ 23) glasses.

14 16 18 20 22 24

5

6

7

8

9

Eth

(kV

/cm

)

x (mole %)

Fig. 5. Composition dependence of threshold field (Eth) for GexTe100-x with 15 ≤ x ≤ 23glasses.

37M. Upadhyay, S. Murugavel / Journal of Non-Crystalline Solids 368 (2013) 34–39

into the electrical switching mechanism on these glasses. Clearly, weobserve distinct behavior of Vth with thickness on the studied glasscompositions. For x = 15, and 17 the dependence of threshold volt-age shows the d1/2 behavior and a similar behavior has been observedin which the chalcogenide glasses have been found to exhibit memorytype switching [26,27]. On the other hand, the case of x = 20 doesnot exhibit square-root dependence instead we observe the d2 depen-dence although it is found to exhibit memory type switching. It is im-portant to note that such d2 dependence has been observed in thechalcogenide glasses which show the threshold type switching behav-ior [26]. This contrasting behavior of thickness dependence of Vth withcomposition identifies the influence of local atomic structure on thephase change behavior in Ge–Te glasses.

3.4. SET–RESET process in Ge–Te glasses

We have carried out the SET and RESET operations in differentglass compositions with x ≤ 20 by applying 5 mA triangular currentpulse for the SET event and 10 mA rectangle pulse of ≈ 10 ms widthfor RESET operations respectively. In Fig. 7, we show the SET–RESETprocess in representative of x = 15, 17 and 20 glass compositions. Itwas noticeable to observe a “forming” effect and was more pronouncedin Ge15Te85 glass composition than the other glass compositions [28]. Itis interesting to note that the Ge20Te80 glass composition withstandsrelatively large number of SET–RESET cycles without any degradation.

0.010 0.015 0.020 0.025 0.030 0.035

60

80

100

120

140

160

180

Vth

(V)

Thickness (cm)

Ge15

Te85

Ge17

Te83

Ge20

Te80

Fig. 6. Thickness (d) dependence of switching voltages (Vth) of Ge15Te85, Ge17Te83 andGe20Te80 glasses. The symbol represents the obtained experimental data and the lineshows the fit to d1/2 and d2.

The cyclability of Ge20Te80 glass composition has been performedabout 1 × 103 SET–RESET cycles without any damage in the device.

4. Discussions

In the present investigations, we have reported on the behavior ofthe thermal parameters and the electrical switching characteristics ofGexTe100-x glasses as a function of composition. In general, we findthat the Ge20Te80 glass composition behaves uniquely from the otherglass compositions. However, the variation of Tg with compositionshows the monotonous increase with Ge content, which has been ob-servedmore commonly in different chalcogenide glasseswith the addi-tion of either IV or V group elements. The central result of the presentwork is the observed maxima in the Tc (at x = 18.5 and 21.5) alongwith a dip at x = 20 followed by a decreasewith the addition of Ge con-tent. First, it is interesting to note that the crystallization phenomenonobserved in these glasses is distinctly different from the other IV–VIgroup glasses [29,30]. In most of the studied glass compositions Tcwould vary in parallel with the corresponding Tg, i.e. if the Tg is highthen Tc would tend to be higher. However, in GexTe100-x glasses Tcdoes not vary in parallel with Tg and shows the anomalous variationswith composition around the critical composition xc.

The experimental powder XRD pattern of bulk crystalline GexTe100-xsamples provides valuable information about the crystallization behav-ior of these glasses. In the following, we explain the composition depen-dent Tc of Ge–Te glasses closely with the obtained XRD pattern of bulkcrystalline sample. The random covalent network (RCN) model hasbeen used to describe the local atomic structure of amorphousmaterialsand it assumes that every atom combines with every other atom by co-valent bonds. The spectroscopic studies onM-X (M = Ge, Si and X = S,Se, Te) binary glasses had shown the presence of GeX4 corner shared(CS) tetrahedral unit followed by the emergence of edge shared (ES)tetrahedra as M content increases. Based on the RCN model, theGe–Te glasses can be described in a tetrahedral configuration onein which the Ge atoms have only Te atoms as the nearest neighbors.Thus, every Ge and Te in GexTe100-x glasses is fourfold and twofoldcoordinated, respectively, and that the number of bond pairs doesnot depend on the bond energy or strength. However, its total num-ber strongly depends only on the chemical constituents.

Most of the experimental works have focused on the compositionaround the stoichiometric Ge50Te50, where the Ge atom prefers to bein distorted octahedral environment when the amorphous films crys-tallized upon application of electric/light pulse or by thermal. The crys-tallization of the eutectic composition leads to the Ge atoms in distortedoctahedral environment in similar to the stoichiometric Ge50Te50. How-ever, the compositions away from the eutectic point, the crystallizationbehavior appears to be different, wherewe find anomalous variations inthe measured physical properties [25,31–33]. On the basis of RCN

0 250 500 750 1000

60

80

100

Ge15Te85

Ge17Te83

Ge20Te80

Number of cycles

Th

resh

old

Vo

ltag

e (V

)

Fig. 7. Threshold voltage (V) vs number of set–reset cycles for Ge15Te85 (black),Ge17Te83 (red) and Ge20Te80 (green) glassy samples.

38 M. Upadhyay, S. Murugavel / Journal of Non-Crystalline Solids 368 (2013) 34–39

model and the present XRD results, we can discuss the local environ-ment of Ge atom changes when the crystallization takes place fromthe glassy state.

It is known that the chemical surrounding of Ge in the amorphousstate is characterized by the sp3 hybrid orbital's,while the bonds aroundTe consists of p orbital's. One would expect the similar bonding consid-erations in the case of GeTe4 phase of the crystalline state. On the otherhand, in the case of α-GeTe the situation becomes completely differentwhere the Ge atoms possess the resonance bonding character withrhombohedral distortion in their cubic crystalline phase [34]. In thiscase, the symmetry is broken by the six identical bonds of octahedralcoordination into three short and three long bonds. Therefore, theα-GeTe structure is stabilized by the resonance bonding, i.e., a superpo-sition of configurationswith saturated covalent bonds. The recent in situEXAFS studies on Ge2Sb2Te5 (GST) films reveal that anharmonic tem-perature dependence of bond lengths in the crystalline state comparedto the more harmonic character in the amorphous phase [35,36]. Astemperature rises, the longer Ge\Te bond begins to fluctuate consider-ably and increases monotonously with temperature. Therefore, we be-lieve that the large fluctuation of the Ge\Te bond that consists of s2p3

orbitals contributes to the crystallization behavior, i.e. decrease orlower in the Tc values. On the other hand, the intermediate composi-tions where we find higher Tc as well as independence of compositionare attributed to the retainment of GeTe4 phase with saturated covalentbonds.

As pointed out by several others [37–40], the composition depen-dence of switching fields in chalcogenide glasses is determined by dif-ferent factors such as network connectivity, additive element, localatomic structure and resistivity, etc. In general, many binary and ter-nary Te based glasses show that the switching voltages increase withan increase in network connectivity and a sharp change in slope(lower to higher) is seen in the composition dependence of switchingfields at the xc [41,42]. The observed trend could be directly associat-ed with the increased network connectivity of the Te matrix with theaddition of Ge content. More commonly, in chalcogenide glasses it isexpected that the addition of Ge to X (X = Te, Se and S) crosslinksthe X chain and provides an increased stability to the amorphousnetwork.

The composition-dependent threshold switching fields (Eth) onGexTe100-x glasses provide the microscopic energy barrier between theamorphous and crystalline state. This energy barrier is determined bythe different factors including the local atomic order in an amorphousstate and is reflected in the change of the slope in Eth vs x content(Fig. 4). First, with an increase of x content the heteropolar bonds ofhigher strength are formed and the network connectivity and rigidityincrease. This leads to the greater Eth to achieve the higher conductingON state and hence, the slope is positive. Secondly, an ordering amongthe structural units in particularly the presence of medium rangeorder in the amorphous network structure would tend to decrease theEth to some extent. Now, let usmore closely look in detail how these fac-tors determine the Eth upon x content in the present system. Before pro-ceeding, wemention that the sharp changes in the slope seen in the Ethvs x content are below 17 and above 22 mol%.Within this intermediatecomposition, we find that Eth becomes least sensitive to the x contentdue to the influence of ordering among the structural units. It hasbeen shown by the spectroscopic and microscopic studies on Ge20Te80glass that presence of intermediate range order in the range of15–20 nm [43–46]. As seen in the composition dependent Tg plot,where the continuous increase is simply due to the increased net-work connectivity, whereas the composition-dependent Eth showsthe microscopic energy barriers between the two states. Therefore,we suggest that the Eth is solely determined by the chemical bondenergy and local ordering among the structural units in the glassystate.

The dependence of threshold switching voltage on the sample thick-ness (d) provides further insight into the microscopic mechanism of

phase change phenomenon. For instance, the Al–As–Te glass systemhas been found to exhibit composition dependent memory andthreshold type switching behavior. The corresponding d depen-dence of threshold voltage shows d1/2 dependence for memoryswitching and d2 dependence for the threshold switching samples[26]. Although all the glass compositions studied under the presentinvestigations are found to exhibit memory type switching, the ob-served dependence is distinctly different at x = 20 than the othercompositions. Remarkably, the observed d2 dependence on thememorytype chalcogenide glass becomes subject of much interest and it is indis-pensable to know the origin of this behavior. The irreversible memorytype switching is understood on the basis of thermal model [47],according to which electrical switching process occurs due to theformation of a conducting crystalline channel with a help of externalenergy supplied by Eth. In order to understand the thickness depen-dent Vth in Ge–Te glasses, we consider the thermal diffusivity (αs) ofthese samples which provides the atomistic mechanism of the ob-served behavior.

With an application of high electric field, the current carried be-tween the electrodes in the glass specimen with low resistivity willexperience a higher Joule heating (I2R) on account of the conductingcrystalline state. This implies that the corresponding glass would ex-hibit low αs since the rate at which the heat is dissipated within theglass, away from the region carrying the current is low, i.e. higher im-pedance to the flow of the diffusional thermal waves. On the otherhand, the case of samples having higher thermal diffusivity may pre-clude a high enough temperature rise in the conducting region to risein temperature and the contribution of Joule heating becomes negligi-ble, which leads to observed d2 dependence on Ge20Te80 glass sample.Remarkably, the composition-dependent thermal diffusivity and elec-trical resistivity (ρ) results which have been reported by Lima et al.support the observed thickness dependent Vth in Ge–Te glasses [31].They reported that the composition dependent αs and ρ on Ge–Teglasses shows the maxima at x = 20 than from the other composi-tions. In addition, we suggest that the higher αs implies that theheat dissipation is easier or minimum resistance to the thermalwave propagation offered by the highly ordered amorphous networkstructure [43–46]. It seems that formation of crystalline conductingchannel is more confined in other compositions than the Ge20Te80composition. An earlier SEM and TEM studies on Ge–Te systems re-veal the conducting filament size of 1.9 μm corresponding to crystal-line tellurium [48,49].

It is interesting to note that the recent theoretical studies on GSTreveal the presence of significant amount of vacancies in the crystal-line state. It has been conjectured that the cation vacancies play animportant role in controlling the stability and rapid reversible phasechange property in this class of materials. In this context, we haveconducted detailed structural and defect analysis by combined useof Raman and positron annihilation spectroscopic studies on Ge–Teglasses and the corresponding crystalline compositions [13]. Basedon these studies, we have found a minimal structural reorganizationtaking place at x = 20 and consequently minimal disorder/defects.Furthermore, these results shed light on observed SET–RESET opera-tions on these samples indicating that Ge20Te80 sample shows thebetter performance in terms of constant threshold voltage for morenumber of cycles due to the minimal structural change between thetwo states.

The cyclability of the PCM strongly depends on several other fac-tors such as electromigration, void formation, relaxation, and elemen-tal segregation but it can be improved further by appropriate dopingor by selecting elements which have lower melting temperatures. Thecase of rapid crystallization time of PCM is related to structural prop-erties, i.e. minimal energy barrier determined by the bonding charac-ter. It is interesting to note that the fast switching materials oftenshow a simple cubic or rocksalt structure that requires little atomicmovement to switch between amorphous and crystalline states [50].

39M. Upadhyay, S. Murugavel / Journal of Non-Crystalline Solids 368 (2013) 34–39

5. Conclusions

In summary, we have established a close correlation between thecrystallization behavior, threshold switching field, electrical switchingmechanism and local atomic structure of Ge–Te glasses. The crystalliza-tion temperature of these glasses is found to strongly depend on thechemical composition and local environment of Ge atom.We have iden-tified the critical composition at which ordering of the glass molecularstructure becomes maximized and shows anomalous behavior in vari-ous physical properties. The thickness dependence of switching voltagesin Ge–Te glasses provides themechanism of electrical switching processinvolved in these glasses.

Acknowledgments

We gratefully acknowledge the Department of Science and Technol-ogy (DST), India for the financial support of this work. M. U. acknowl-edges support from the CSIR, India for the senior research fellowship(SRF). We also acknowledge USIC, University of Delhi for providingthe various characterization facilities.

References

[1] S.R. Ovshinsky, Phys. Rev. Lett. 21 (1968) 1450–1453.[2] S.C. Moss, J.P. De Neufville, Mater. Res. Bull. 7 (1972) 423–442.[3] H.S. Reehal, C.B. Thomas, J. Phys. D: Appl. Phys. 10 (1977) 737–752.[4] H.P. Wong, S. Roux, S. Kim, J. Liang, J.P. Reifenberg, B. Rajendran, M. Asheghi,

K.E. Goodson, Proc. IEEE 98 (12) (2010) 2201–2227.[5] T. Ohta, J. Optoelectron. Adv. Mater. 3 (2001) 609–626;

M. Wuttig, N. Yamada, Nat. Mater. 6 (2007) 824–832.[6] M. Anbarasu, M. Wimmer, G. Bruns, M. Salinga, M. Wuttig, Appl. Phys. Lett. 100

(2012) 143505.[7] M. Chen, K.A. Rubin, R.W. Barton, Appl. Phys. Lett. 49 (1986) 502–504.[8] S. Raoux, H.-Y. Cheng, M.A. Caldwell, H.-S. P. Wong, Appl. Phys. Lett. 95 (2009)

071910.[9] M. Upadhyay, S. Murugavel, M. Anbarasu, T.R. Ravindran, J. Appl. Phys. 110 (2011)

083711.[10] J. Akola, R.O. Jones, Phys. Rev. Lett. 100 (2008) 205502.[11] M. Wuttig, D. Lüsebrink, D. Wamwangi, W. Welnic, M. Gilleben, R. Dronskowski,

Nat. Mater. 6 (2007) 122–128.[12] A.H. Edwards, A.C. Pineda, P.A. Schultz,M.G.Martin, A.P. Thompson, H.P. Hjalmarson,

C.J. Umrigar, Phys. Rev. B 73 (2006) 045210.[13] M. Upadhyay, S. Abhaya, S. Murugavel and G. Amarendra, J. Solid. State Chem.

(submitted for publication).

[14] S. Asokan, G. Parthasarathy, E.S.R. Gopal, Int. J. Rapid Solidif. 2 (1987) 257–271.[15] T. Takamori, R. Roy, G.J. McCarthy, Mater. Res. Bull. 5 (1970) 529–540.[16] D.J. Sarrach, J.P. deNeufville, J. Non-Cryst. Solids 22 (1976) 245–267.[17] I. Kaban, E. Dost, W. Hoyer, J. Alloy Comp. 379 (2004) 166–170.[18] D. Oleszak, B. Dobrowski, H. Matyja, J. Mater. Sci. Lett. 8 (1989) 1131–1134.[19] E. Barthélémy, S. Albert, C. Vigreux, A. Pradel, J. Non-Cryst. Solids 356 (2010)

2175–2180.[20] S. Asokan, G. Parthasarathy, E.S.R. Gopal, J. Non-Cryst. Solids 86 (1986) 48–64.[21] G. Parthasarathy, S. Asokan, M.V.N. Prasad, E.S.R. Gopal, J. Mater. Sci. Lett. 6 (1987)

75–77.[22] R. Ramakrishna, S. Asokan, G. Parthasarathy, S.S.K. Titus, E.S.R. Gopal, J. Non-Cryst.

Solids 139 (1992) 129–136.[23] J.A. Savage, J. Non-Cryst. Solids 11 (1972) 121–130.[24] F. Yan, T. Zhu, X. Zhao, S. Dong, J. Univ. Sci. Technol. Beijing 14 (2007) 64–67.[25] A.G. Moore, C. Maghrabi, J.M. Parker, J. Mater. Sci. 13 (1978) 1127–1130.[26] S. Murugavel, S. Asokan, J. Mater. Res. 13 (1998) 2982–2987.[27] H.J. Stocker, C.A. Barlow Jr., D.F. Weirauch, J. Non-Cryst. Solids 4 (1970) 523–535.[28] A.E. Owen, P.G. Le Comber, J. Hajto, M.J. Rose, A.J. Snell, Int. J. Electron. 73 (1992)

897–906.[29] A. Hrubý, Czech. J. Phys. B 23 (1973) 1263–1272.[30] R.W. Johnson, D.L. Price, S. Susman, M. Arai, T.I. Morrison, G.K. Shenoy, J. Non-Cryst.

Solids 83 (1986) 251–271.[31] J.C. de Lima, N. Cella, L.C.M. Miranda, C. Chying An, A.H. Franzan, N.F. Leite, Phys.

Rev. B 46 (1992) 14186–14189.[32] A. Deneuville, J.P. Keradec, P. Gerard, A. Mini, Solid State Commun. 14 (1974)

341–346.[33] R. Azoulay, H. Thibierge, A. Brenac, J. Non-Cryst. Solids 18 (1975) 33–53.[34] G. Lucovsky, R.M. White, Phys. Rev. B 8 (1973) 660–667.[35] A.V. Kolobov, M. Krbal, P. Fons, J. Tominaga, T. Uruga, Nat. Chem. 3 (2011) 311–316.[36] T. Matsunaga, N. Yamada, R. Kojima, S. Shamoto, M. Sato, H. Tanida, T. Uruga,

S. Kohara, M. Takata, P. Zalden, G. Bruns, I. Sergueev, H.C. Wille, R.P. Hermann,M. Wuttig, Adv. Funct. Mater. 21 (2011) 2232–2239.

[37] J.C. Phillips, J. Non-Cryst. Solids 34 (1979) 153–181.[38] M. Anbarasu, S. Asokan, Appl. Phys. A 80 (2005) 249–252.[39] S. Manohar, S. Murugavel, S. Asokan, Solid State Commun. 135 (2005) 323–326.[40] C. Das, G.M. Rao, S. Asokan, J. Non-Cryst. Solids 357 (2011) 165–169.[41] S.S.K. Titus, R. Chatterjee, S. Asokan, Phys. Rev. B 48 (1993) 650–652.[42] M. Anbarasu, S. Asokan, J. Phys. D: Appl. Phys. 40 (2007) 7515–7518.[43] S.G. Bishop, N.J. Shevchik, Solid State Commun. 15 (1974) 629–633.[44] S.R. Elliott, J. Phys. Condens. Matter 4 (1992) 7661–7678.[45] K. Do, D. Lee, D.-H. Ko, H. Sohn, M.-H. Cho, Electrochem. Solid-State Lett. 13 (2010)

H284–H286.[46] K. Ichikawa, Y. Kameda, Q. Xu, M.J. Misawa, J. Non-Cryst. Solids 95 & 96 (1987)

185–192.[47] A.C. Warren, IEEE Trans. Electron Devices 20 (1973) 123–131.[48] C.H. Sie, M.P. Dugan, S.C. Moss, J. Non-Cryst. Solids 8 (1972) 877–884.[49] S.C. Moss, J. de Neufville, J. Non-Cryst. Solids 8 (1972) 45–49.[50] K. Kim, Proc. Symp. Very Large Scale Integr. Tech. Syst. Appl, 2008, pp. 5–9.