glass formation in germanium telluride glasses containing metallic additives

Glass formation in germanium telluride glasses containingmetallic additives

K. Ramesha,* , S. Asokanb, K.S. Sangunnia, E.S.R. Gopala

aDepartment of Physics, Indian Institute of Science, Bangalore 560012, IndiabDepartment of Instrumentation, Indian Institute of Science, Bangalore 560012, India

Received 21 October 1998; accepted 28 May 1999

Abstract

Differential scanning calorimetric (DSC) studies have been carried out on germanium telluride glasses containing Cu and Ag.Both CuxGe15Te852x �2 # x # 10� and AgxGe15Te852x �2:5 # x # 21:5� glasses are found to exhibit single glass transition andsingle crystallization. On the basis of the devitrification behaviour of these glasses one can conclude that the network connec-tivity of the parent Ge–Te matrix is not improved by the addition of Cu whereas Ag improves the connectivity. Over-constraining of the structural network forx . 5 is rapid in Cu added glasses and more gradual in Ag added glasses. Thedifference in the glass formation in the Cu–Ge–Te and Ag–Ge–Te systems is understood in light of the above differences in thestructural network.q 1999 Elsevier Science Ltd. All rights reserved.

Keywords:A. Chalcogenides; A. Glasses; C. Differential scanning calorimetry (DSC); C. X-ray diffraction

1. Introduction

The absence of long-range periodic constraints in amor-phous semiconductors compared to their crystalline counter-parts facilitates the preparation of these glasses in variedcompositions. The variation in composition brings changesin short-range order resulting in variation in physical proper-ties, which help in tailoring the properties of these materialsto a desired requirement [1]. For example, in Ge–Se–Bi andGe–Se–Pb glasses the conduction type changes form p-typeto n-type at certain compositions as a function of Bi and Pb[2–4]. While binary Al–Te and As–Te glasses [5,6] exhibitonly memory switching, a change from memory to thresh-old-type switching can be observed in the ternary Al–As–Te glass system by varying the concentration of Al [7].Hence, a wide glass-forming region is necessary to have alarge number of compositions in a system.

In the Cu–Ge–Te system, the bulk glass formation iscentered around 20 at.% of Ge (13–23) and homogeneousglasses can be obtained by progressively replacing Te withCu up to a maximum of 10 at.% [8]. In contrast, in the Ag–

Ge–Te system, a maximum of 23 at.% of silver can beincorporated [9]. Though Cu and Ag belong to the samegroup (Group I), the way in which they get into the Ge–Te glass matrix and the resultant short-range order differ to alarge extent. In the present work, we have attempted tounderstand the difference in glass formation of Ag–Ge–Te and Cu–Ge–Te samples on the basis of network connec-tivity and network rigidity.

2. Experimental

CuxGe15Te852x �2 # x # 10� and AgxGe15Te852x �2:5 #x # 21:5� glasses were prepared by the conventional melt-quenching method. The amorphous nature of the samples isconfirmed by X-ray diffraction. The glass transition andcrystallization temperatures of the prepared glasses aremeasured using a differential scanning calorimeter (DSC).

To identify the devitrified phases, which appear duringcrystallization, the samples were sealed under a vacuum ofmore than 1024 Torr in a quartz ampoule and annealed attheir respective crystallization temperatures (72 h for Cu–Ge–Te samples and 48 h for Ag–Ge–Te samples). Theseannealed samples were then analysed by X-ray diffraction.

Journal of Physics and Chemistry of Solids 61 (2000) 95–101

0022-3697/00/$ - see front matterq 1999 Elsevier Science Ltd. All rights reserved.PII: S0022-3697(99)00239-5

* Corresponding author.E-mail address:[email protected] (K. Ramesh)

www.elsevier.nl/locate/jpcs

3. Results and discussion

3.1. Network connectivity and glass formation

Figs. 1 and 2 show the DSC traces of CuxGe15Te852x andAgxGe15Te852x glasses. All these glasses are found to exhibita single glass transition and a single crystallization uponheating. Determination of the glass transition temperatureTg corresponds to the point of intersection of the tangentsdrawn to the baseline and the endothermic baseline shift.Determination of the crystallization temperatureTc is takenas the maximum of the exothermic peak. Generally, thedifferenceDT � Tc 2 Tg gives a measure of the glass-form-ing ability [10]; DT of CuxGe15Te852x and AgxGe15Te852x

glasses as a function of composition is shown in Fig. 3(a)and (b). It can be seen thatDT of CuxGe15Te852x glassesinitially increases up tox� 5 and then rapidly decreasesindicating an increase in crystallization tendency for highervalues of Cu. In the case of Agx–Ge15–Te852x glasses,DTdecreases up tox� 5, and then starts increasing, resultingin a wide range of glass formation. The composition depen-dence ofTg and Tc of CuxGe15Te852x and AgxGe15Te852x

glasses are discussed in detail elsewhere [11,12].Fig. 4 shows the representative X-ray patterns of

Cu2Ge15Te83 and Cu8Ge15Te77 samples annealed atTc for72 h. The diffraction patterns of crystallized Cu–Ge–Tesamples are fully indexable with hexagonal Te and rhombo-hedrala -GeTe phases [13,14]. We have not been able toidentify elemental Cu or compounds of Cu with Te or Ge inthe crystallized CuxGe15Te852x glasses. The X-ray diffraction

patterns of crystallized AgxGe15Te852x samples forx� 5,10, 15 and 20 are shown in Fig. 5. Diffraction peaks of thecrystallized samples can be indexed with hexagonal Te;monoclinic AgTe, orthorhombic Ag2Te, cubic GeTe4 andcubic GeTe4 phases [13,15,16]. With the increase of Agcontent the ternary Ag8GeTe6 phase increases resulting in agreater number of lines in the XRD pattern of AgxGe15Te852x

samples for higher values ofx (Fig. 5). The lines corre-sponding to AgTe and Ag2Te starts appearing only forx $ 15, through the intensity of the lines corresponding toAg2Te does match the reported values exactly. It is alsoknown that twinning took place during the formation ofAg2Te, which probably results in the intensity variations[17].

In general a third element, added to a binary glass, can

K. Ramesh et al. / Journal of Physics and Chemistry of Solids 61 (2000) 95–10196

Fig. 1. Differential scanning calorimetric traces of CuxGe15Te852x

glasses recorded at a heating rate of 108C/min.

Fig. 2. Differential scanning calorimetric traces of AgxGe15Te852x

glasses recorded at a heating rate of 108C/min.

form its own structural units in the system. If the newlyformed structural units interact with the parent glass matrixand increase the network connectivity, there will be anenhancement in the glass formation in the system [8].However, if the added impurity does not interact with thehost matrix, it forms a phase-separated network of its own.The phase-separated units can remain in the parent glassmatrix as a micro-inclusion [8]. Another factor controllingthe glass-forming ability is frustration. Crystallization in amulticomponent melt is prevented either by a competitionbetween the formation of several different types of crystal-line structures (with different compositions), or simply dueto the difficulty in rearranging many different types of atomsto form a multicomponent crystal.

In this regard, the glass-forming ability of the binarysystem AsxS(Se)1002x is of interest. The addition of As toS(Se) increases the cross-linking of the S(Se) chains by thethree-fold coordinated As atoms. The cross-linkingincreases the connectivity uptox� 40. Forx . 40, discrete

molecular species (As4S4 and As4Se4) are formed which arespherical in shape and do not improve the connectivity. Inaddition, the constraints provided by these spherical shapedare insufficient to prevent As rich As–S(Se) melts to solidifyinto crystalline structural arrangement [18,19]. This limitsthe glass formation in AsxS1002x �5 # x # 46� andAsxSe1002x �0 # x # 60� glasses. In PxSe1002x system, atx� 57, P4Se3 quasi-spherical molecular structural unitsform which cannot be vitrified [8]. Consequently, theglass-forming region in PxSe1002x system is divided intotwo regions with 0# x # 50 and 63# x # 85.

In the Ge–Se system, the addition of group III elementslike Ga and Tl forms structural units of its own kind with Geand Se. These structural units enter the network withoutaffecting the structural continuity of the parent glass matrixand improve the structural connectivity. This promotes theglass formation in these systems [8]. In the same way, theaddition of Al to As–Te glasses introduces structural unitsinvolving Al, As and Te, which increase the glass

K. Ramesh et al. / Journal of Physics and Chemistry of Solids 61 (2000) 95–101 97

Fig. 3. Thermal stability (DT) of (a) CuxGe15Te852x; and (b) AgxGe15Te852x samples as a function of composition. The lines drawn are a guide forthe eye.

formability of the Al–As–Te system by enhancing theconnectivity of the network [7,8,20]. Interestingly in theAl–As–Te system, Al resides in two different environmentswith four- and six-fold coordination. The ratio of four- andsix-fold coordinated sites decides the glass formation in theAl–As–Te system [7].

Glass formation in Cu–As–Se system, depends on theinteraction of Cu with the host As–Se matrix. In the caseof glasses with high Se content, the added Cu does notinteract with the As–Se glass matrix and enters the networkas a micro-inclusion. These micro-inclusions are not boundto the As–Se glass matrix and obstruct the connectivity ofthe parent glass structure [8]. However, in glasses with lowSe content, Cu interacts with the As–Se glass matrix andforms structural species involving Cu, As and Se. They enteras a whole unit into the structural network without disturb-ing the continuity of the parent (As–Se) matrix. Thisimproves the network connectivity resulting in a largeglass-forming region in the Cu–As–Se system (a maximumof 28 at.% of Cu can be added).

The above discussions reveal that glass formation in anysystem depends mainly on the way in which the addedimpurities enter the parent glass matrix. In this context thepresent devitrification studies on the Cu–Ge–Te and Ag–Ge–Te samples give a clue to understand the glass forma-tion in these systems. Glass formation in the Cu–Ge–Tesystem is limited to a maximum of 10 at.% of Cu [8]. Inthese glasses the interaction of Cu with the Ge–Te matrix is

not significant as the X-ray diffraction patterns do not revealany Cu involved structural species (Fig. 4). The atomic radiiof Ge (1.25 A) and Cu (1.35 A˚ ) are comparable [21]. Theelectronegativities of Ge (2.01) and Cu (1.9) are also of thesame order. Earlier structural investigations suggest that Cuis four-fold coordinated in the Cu–Ge–Te glasses [8,22,23].It is possible that the similar atomic radii, coordination andelectronegativity of Ge and Cu in CuxGe15Te852x samplesmay lead to the random replacement of Ge by Cu [11].Thus, there is no net increase in the overall network connec-tivity with the addition of Cu, which limits the glass-forma-tion in the Cu–Ge–Te system. In other words the Ge–Te–Cu system acts like a pseudo-binary (Ge/Cu)151xTe852x

system.In the Ag alloyed Ge–Te system, the atomic radii and

electronegativity of Ag atoms differ considerably fromthose of Ge atoms. In this system, the silver atoms interactwith the host Ge–Te matrix and form a ternary phaseAg8GeTe6 in addition to AgTe, Ag2Te, GeTe4 and Te (Fig.5). The formation of a greater number of structural units(which is absent in Cu–Ge–Te system) also provides suffi-cient constraints to prevent the rearrangement of thenetwork into a regular structure (crystalline form). Conse-quently, the glass-forming region in Ag–Ge–Te system ismuch larger compared to that of the Cu–Ge–Te system.

3.2. Phillips constraint theory and glass formation

The differences in the glass-forming ability of Ge–Teglasses doped with Cu and Ag can also be understood onthe basis of the Phillips constraint theory proposed for cova-lent network glasses [24,25]. The constraint theory suggeststhat covalent network glasses (chalcogenide glasses) consistof under-cross-linked floppy and over-constrained rigidnetworks. It also predicts a critical composition correspond-ing to an average coordination numberZav � 2:40, at whichthe number of constraints (bond-stretching and bond-bend-ing forces) acting on the network are balanced by thenumber of degrees of freedom available for the atoms innetwork. So glasses withZav , 2:40 are under-cross-linkedor loosely connected, and glasses withZav . 2:40 are overconstrained or rigidly connected. A transformation from afloppy to a rigid network structure called the rigidity perco-lation threshold occurs atZav � 2:40. Glass-forming abilityof many alloys is generally found to be easy at the composi-tion (corresponding toZav � 2:40) at which the rigiditypercolation threshold occurs.

In addition to the rigidity percolation threshold, a chemi-cal ordering also occurs in chalcogenide glasses (usually athigher coordination numbers), where the structural networkis maximally ordered [26]. The chemically ordered networkhas a maximum molar volume and a minimum density.Maximum ordering in the network indicates that they areclosest to its crystalline state. The glass-forming ability ofchalcogenide glasses is found to be difficult at the composi-tion corresponding to the chemically ordered network as


Fig. 4. The X-ray diffraction pattern of (a) Cu2Ge15Te83; and (b)Cu8Ge15Te77; glasses annealed at their crystallization temperaturesfor 72 h.

they attain their crystalline state easily. Unusual changesin various properties are expected at these criticalcompositions. For example, glass transition temperature,semiconductor-to-metal transition pressure and theconductivity activation energy of GexTe1002x glassesexhibit unusual variations atx� 20, corresponding toZav � 2:40 [27,28]. Experimentally the glass-formingability of GexSe1002x glasses is found to be easy atx�20 (Zav � 2:40, rigidity percolation threshold) and rela-tively difficult at x� 33:33 (Zav � 2:67, chemical order-ing threshold) [29]. The unusual variations inDT, Tg

and Tc of both CuxGe15Te852x and AgxGe15Te852x glassesat x� 5 �Zav � 2:40� are associated with the rigiditypercolation threshold. In AgxGe15Te852x glasses, a maxi-mum in Tg and a minimum inTc, and DT observed at

x� 20 �Zav � 2:70� are an indication of the chemicalordering in the network (Figs. 3 and 6).

Though an unusual variation in a property is generallyexpected at the percolation threshold, the exact nature ofthe variation (slope change/maxima/minima) depends onthe property studied and it also varies from one system toanother. For example, binary telluride glasses such as Ge–Te and Si–Te show a minimum inTg at the rigidity percola-tion threshold [27,30], whereas the glassy selenides such asGe–Se [31], Ge–Ga–Se, Ge–In–Se [32,33], show a slopechange. TheTg of CuxGe15Te852x and AgxGe15Te852x glassesshows a minimum (Fig. 6) at the rigidity percolation thresh-old and the behaviour is consistent with telluride glasses inthis respect.

The Tc of Cu–Ge–Te and Ag–Ge–Te glasses exhibits


Fig. 5. The X-ray diffraction pattern of (a) Ag5Ge15Te80; (b) Ag10Ge15Te75; (c) Ag15Ge15Te70; and (d) Ag20Ge15Te65 samples annealed at theircrystallization temperatures for 48 h.

interesting variations with compositions. A minimum isobserved inTc at x� 5 in AgxGe15Te852x glasses, whereasCuxGe15Te852x glasses exhibit a local maximum. Moreover,Tc rapidly decreases forx . 5 in CuxGe15Te852x glasses andshow an increasing trend in AgxGe15Te852x glasses. Thisindicates that the tendency of crystallization of the Cu–Ge–Te glasses is more when the Cu content increases. Inthe case of Ag–Ge–Te glasses the crystallization tendencyis not increased. TheDT of both Cu–Ge–Te and Ag–Ge–Teglasses mimics that ofTc indicating thatDT is a consequencemore ofTc.

The glass transition temperature of CuxGe15Te852x andAgxGe15Te852x glasses as a function of composition (Cu/Ag) is shown in Fig. 6. The addition of Ag to Ge–Teglasses introduces structural units involving Ag, Ge andTe. These units can enter into the Ge–Te matrix and forma part of the structural network. As discussed earlier, Cuin the Cu–Ge–Te glasses does not interact with Ge–Teglass matrix. It randomly replaces Ge sites or enters as aphase-separated unit or micro-inclusion. In these situa-tions, the added Cu can obstruct the continuity of the

Ge–Te structural network and over constrains the networkmore rapidly afterx� 5. The rapid increase ofTg in Cu–Ge–Te glasses afterx� 5 �Zav . 2:40� is an indication ofrapid over-constraining. In Ag–Ge–Te glasses the addi-tion of Ag forms its own structural units and enter theGe–Te matrix without affecting the continuity andenhances the network connectivity forx . 5�Zav . 2:40�. In this case the over-constraining of thenetwork is more gradual. The increase inTg is not asfast as in the Cu–Ge–Te glasses, probably, indicating aslower-constraining of the network. In this way the glass-forming range in CuxGe15Te852x glasses is restricted and inAgxGe15Te852x glasses is extended.

It is worth mentioning that the glass formation in Cu–Ge–Se system is also limited to 10 at.% of Cu though Ge–Se is an excellent glass former [34]. However, in the case ofAg–As–Se and Ag–Ge–Se systems [8,35], bulk glasses canbe obtained even up to 30% of Ag. The arguments discussedabove in the context of Cu–Ge–Te and Ag–Ge–Te glassesare probably also valid for Cu–Ge–Se, Ag–Ge–Se and Ag–As–Se systems.


Fig. 6. Glass transition and crystallization temperatures of CuxGe15Te852x and AgxGe15Te852x glasses as a function of composition. The linesdrawn are a guide for the eye.

4. Conclusions

The glass formation in Ag–Ge–Te and Cu–Ge–Tesystems have been understood based on how the impuritiesenter the parent glass matrix. In Cu–Ge–Te glasses Cu doesnot form its own structural units with the germanium tell-uride glass matrix. It randomly replaces the Ge sites orenters the Ge–Te network as a micro-inclusion. Therefore,the network is rapidly over constrained forx . 5 �Zav .2:40� which restricts the glass formability of Cu–Ge–Tesystem. On the other hand, Ag in Ge–Te glasses, forms itsown connected structures and constrains the structuralnetwork more gradually, which enhances the glass-formingability of Ag–Ge–Te system.

Acknowledgements

The authors thank the Department of Science and Tech-nology (DST) and the Council of Scientific and IndustrialResearch (CSIR) for financial help. They also thank thereferee for various suggestions.

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glass formation in germanium telluride glasses containing metallic additives

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