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Reviews in Advanced Sciences and EngineeringVol. 1, pp. 292–301, 2012(www.aspbs.com/rase)

Recent Advances in AmorphousSemiconductors—A Correlative Study onSe-Based Metallic Chalcogenide AlloysAbhay Kumar Singh

Department of Physics, Indian Institute of Science, Bangalore 560012, India

ABSTRACT

Chalcogenide glasses or amorphous semiconductors are applicable materials in modern optoelectronics.Understanding of thermal, optical, electrical and structural properties in these materials is useful to demon-strate their potential uses. Particularly physical properties of metal containing chalcogenide glasses are gettingmuch attention owing to their interesting features and wide range structural modification. This work presentsa chronologic development in metal containing chalcogenide glasses and a correlation between optical, elec-trical, thermal parameters for recent developed Se–Zn–In alloys. Specifically, the variation of optical energyband gap (Eg�, electrical conductivity (�av�, crystallization activation energy (Ec� and Hruby number (GFA-glassforming ability parameter) with indium atomic percentage of Se98−xZn2Inx (0 ≤ x ≤ 10) chalcogenide glassesis described. Subsequently, the variation of refractive index (n), Eg, �av, Ec and Hruby number with averagecoordination number �r� of under examined systems is also discussed. Minimum and maximum variations inabove physical parameters are obtained at threshold composition (6 at. wt% of In) and corresponding thresholdstructural unit �r� value.

KEYWORDS: Chalcogenides, Optical Properties, Electrical Properties, Thermal Properties, Glass FormingAbility, Average Coordination Number.

CONTENTS

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2921.1. Background of Metallic Chalcogenide Glasses . . . . . . . . 2921.2. Optical Energy Band Gap . . . . . . . . . . . . . . . . . . . . . 2931.3. Refractive Index . . . . . . . . . . . . . . . . . . . . . . . . . . . 2941.4. Electrical Conductivity . . . . . . . . . . . . . . . . . . . . . . . 2941.5. Crystallization Activation Energy . . . . . . . . . . . . . . . . 2941.6. Hurby Parameter . . . . . . . . . . . . . . . . . . . . . . . . . . . 2941.7. Average Coordination Number . . . . . . . . . . . . . . . . . . 295

2. Materials Preparations . . . . . . . . . . . . . . . . . . . . . . . . . . 2953. Characterizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295

3.1. Optical Characterization . . . . . . . . . . . . . . . . . . . . . . 2953.2. Electrical Characterization . . . . . . . . . . . . . . . . . . . . . 2963.3. Thermal Characterization . . . . . . . . . . . . . . . . . . . . . 296

4. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2964.1. Physical Parameters Variations with Alloys Compositions . 2964.2. Physical Parameters Variations with Alloys Average

Coordination Number �r� . . . . . . . . . . . . . . . . . . . . . 2975. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2986. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299

Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . 300

Email: [email protected]: 10 January 2012Accepted: 10 April 2012

1. INTRODUCTION

1.1. Background of Metallic Chalcogenide Glasses

Ternary composition of chalcogenide glasses have beenbroadly studied more than three decades. Such chalco-genide compositions can be prepared by introducing asuitable additive element in well known or new binarymatrix. First most extensively studied ternary As–S–Sesystem demonstrated by Flaschen et al.,1 they show a wideglass-forming region for this composition. This outcomealso revealed the solid solutions can be formed along theline As2 S3–As2 Se3 which proved via IR spectra and X-ray analysis by Velinov and his coworkers.2 The Cova-lent Random Network (CRN) and the Chemically OrderedNetwork (CON) models both satisfy the 8-N rule underthe distribution of bond types in a covalent network withmulti elements. As-rich glasses can be formed As–As,As–Se, and As–S bonds; thus Se-rich glasses have As–Se,As–S, and Se–Se bonds and S-rich glasses As–Se, As–S,and S–S bonds. The relative weight of each of the aboveunits is expected to be proportionate to the overall com-position of the glass itself.3 Afterword an intensive effortwas made to deduce thermally stable ternary chalcogencompositions by introducing a suitable alloying element

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Singh Recent Advances in Amorphous Semiconductors—A Correlative Study on Se-Based Metallic Chalcogenide Alloys

into binary alloys like; Ge–Se–Te, Si–As–Te, As–Te–Ge,Ge15Te80As5, As–Ag–Se, As2S3Gex, PdTeI, Pd2SeI3,Ge–S–Sb, Ge4Se5Te, Se–Ge–As, As40−xS60Ix, AgTISe2,GexTe10Se90−x, Pb0�8Sn0�2Te, CuBiS2, GexSyBrz, Li2S-P2S5-B2S3, Ge25Se75−xBix, (FexTi1−x�3Se4, As–Se–Te,Cu–As–Se, AgInTe2, As0�20Se0�40Te0�40, Ge1−xSnxSe2,Ag–As–S, Ga1−xGexTe, Te–Se–I, Se–Ag–Br, Se–Ag–Cl,(Se0�7Te0�3�100−xInx, As–Sb–S, Tl–GeTe4, Cs2Au2Se3,As25S65Ag10, Cu35As20Te45, Se1−x−y TexPy , Cu–Te–Ietc.4–39 for various optoelectronics objectives.In the decade Se-based metal containing chalcogenide

glassy alloys became attractive materials for investiga-tions in optoelectronics and photonics.40–50 These materialsexhibited kind of thermal, electrical and optical propertiesduring the performances.51–56 Specifically their variablestructural property makes them interesting for investigationowing to that, they extensively studied40–50 by the inves-tigators. Predominantly VI–II–III group alloying glasseshave been getting much attention in last a few years causedrastic change in their thermal, optical and electrical prop-erties at threshold composition/- threshold structural unit(or average coordination number) value.To explore the different aspects of such materials sev-

eral investigators have been reported their work addressesthe various properties; Garrido et al.57 have studied the ionselective application of Ag–Ge–Se chalcogenide glasses.Tomova et al.58 have reported the Cu-ion as selectivemembranes based on chalcogenide glasses. Vassilev et al.have reported the Cd(II) and Zn(II) can be used as selec-tive electrodes59�60 with chalcogenide glasses. VeroniqueSousa61 has studied the chalcogenide materials can be usedas non-volatile memories. Madhavan et al.62 have exploredthe spectral properties of Co–Ge–Te amorphous thin film.Kim et al. and Tver et al.63�64 have demonstrated thethin film from laser ablation and formation of ferromag-netic spinel microcrystals in metal containing chalcogenidealloys. Hyuk Choi et al.65 have demonstrated the applica-tion of Ag doped thin film in programmable metallization

Abhay Kumar Singh has born 1976 in India, he received the B.Sc. and M.Sc. degreesfrom Chhatrapati Shahu Ji Maharaj University, in 1998, 2000 and Ph.D. degree in physicsfrom Banaras Hindu University, Varanasi, India, in 2009, respectively. He is presently work-ing as a Dr. D. S. Kothari Post-Doctoral Fellow (funded by university grand commissionNew Delhi) in the Department of Physics, Indian Institute of Science Bangalore, India.His current research interest including, chalcogenide photovoltaic solar cell, chalcogen-nanocomposites, chalcogenide metallic/-non-metallic multicomponent alloys preparations andtheir bulk as well as thin films structural, thermal, optical and electrical characterizations.He was successfully introduced the Se–Zn–In and Se–Zn–Te–In two new series of chalco-genide glasses in 2009, 2010, with Banaras Hindu University, research group. Experimentalfindings on these two series as well as other compositions glasses he has been demonstrated

in more than 20 technical research papers in reputed international journals. From January 2005 to February 2004, healso worked as a research staff in Notational Metallurgical Laboratory, Jamshedpur, (CSIR Lab), In applied chemistryand corrosion division, under the project on hot dip galvanizing and their metallographic analysis. He has extensiveexperiences with the process of hot dip galvanizing and their thin films structural analysis by using various characterizingtools and material analysis by using metal analyzer.

cell. Samson et al.66 have demonstrated the Metamate-rial electro-optic switching at nanoscale thickness. Skouget al.67 have studied the effect of structure on thermal con-ductivity of metallic chalcogenide glasses. Silva et al.68

have studied the temperature dependence thermo-opticalproperties of metallic chalcogenide glasses. Hu et al.69

have reported the structural, electrical and optical proper-ties of Cu doped chalcogenide glasses. Sharma and Kumarhave demonstrated70 the role of Cu additive on the dielec-tric relaxation of Se75Te25 and Se85Te15 glassy alloys.Gresty et al.71 have demonstrated the structural phase tran-sitions and superconductivity in Fe1+�Se0�57Te0�43 glass.Selby et al.72 have suggested the new phases in K/Cu/Th/Sglassy systems. Matsushita et al.73 have demonstratedthe new magnetophenomena in Fe0�47Pb8�04In17�37Se34 sys-tem. Poudeu et al.74 have introduced the new class ofmetallic semiconductors. In order to this Zhou et al.75

have introduce the promising thermoelectric properties inAg–Mo–Se compounds.Investigations on metallic chalcogenides not limited

to above described studies. It is still ongoing towardto forward direction to deduce new future prospectiveadvanced amorphous chalcogenide/semiconducting alloys.Amorphous semiconductors which full fill the essentialrequirements of modern optoelectronics. Thus, it can beoutlined the potential field of optoelectronics and advancedmaterials is rapidly growing due to their adequate uses.Therefore, it is worth to have an understanding on variousparameters (like, optical, electrical, thermal and structuralunits) of chalcogenide glasses to describe the recent devel-oped ternary materials.

1.2. Optical Energy Band Gap

Optical energy band gap describes the minimum energyrequired for optical excitation of a material. To explain

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Recent Advances in Amorphous Semiconductors—A Correlative Study on Se-Based Metallic Chalcogenide Alloys Singh

the optical energy band gaps in chalcogenide glasses sev-eral model have been proposed by the investigators. Streetet al.76 have demonstrated the existence of dangling bondsin density of localized states and subsequently defines the-ses defects or dangling bonds responsible for the physi-cal variation in chalcogenide glasses. Adler et al.77 haveproposed the existence of localized electronic states inamorphous semiconductors. Biegelsen et al.78 have stud-ied the light induced defects for chalcogenide glasses anddemonstrate the optical energy band in these materialswidely depend on light exposure time. Arkhipov et al.79

have demonstrated an electronic model of photoinducedoptical anisotropy for chalcogenide glasses. Subsequently,they have also explained the occurrence of electron–holepairs sufficiently close and sufficiently deep in localizedstates (or traps), in literal meaning they interpreted as;such pairs occur in glassy materials due to random spatialdistribution of both electrons and holes traps and occa-sionally some traps close to each other. The addition ofspecific foreign defects or structural units can provide thecorrelation between position in deep localized states forthe electrons and holes. The foreign specific defects alsocreate the photoexcited geminate pairs of carriers whichoccupying correlated traps. These traps can be consid-ered as a change in electronic structure owing to defects,which arises due to structural rearrangements in vicinity ofdefects. Afterward, Phillips80 was extended this study anddemonstrated that the occurrence of vibrational thresholdnear to the average coordination number for the networkglasses.

1.3. Refractive Index

Refractive index is the fundamental optical property ofa substance which defines the working performance ofa system. Fundamentally, refractive index of a materialdescribes the relative permittivity of the incident light.Therefore, this physical quantity extensively depends onthe wave length of incident light. Because the incidentlight slow down in the material microscopic complexstructure cause disturbance in relative charge distributionof atoms. In the complex alloys or materials (more thantwo elements) charges oscillate slightly out of phase withrespect to driving electric field and radiate their own elec-tromagnetic wave with same frequency but having a phasedelay. The macroscopic sum of this kind contribution inthe material is a wave with same frequency but shorterwavelength than the original incident light. Owing to this,the most of the radiation from oscillating material chargesmodify with incoming wave as well as concentration ofalloy. For chalcogenide glasses it is well describe therefractive index widely depend on density of localizedstates of the materials and it can be influenced from theaddition of foreign element.

1.4. Electrical Conductivity

In chalcogenide glasses electrical conduction can be takeplace by means two parallel processes namely band con-duction and hopping conduction. Predominantly the bandconduction occurs when the carriers are excited beyond themobility edges into non-localized states. The excitationsof the carriers into localized states at the band edges causethe hopping conduction.81 Therefore, the total electricalconductivity of a glassy system can be described with helpthese two conduction mechanisms. In order to describe theelectrical conductivity in such glasses, we can not ignorethe thermal influences because it affects the electrical con-ductivity of glassy materials. To overcome such thermalinfluences Jonscher and Hill82 have been defined an empir-ical relation for chalcogenide glasses, and Pool-Frenkel83

have been established a conduction mechanism for non-linear materials. This conduction mechanism predomi-nantly deals with conduction in these materials in whichtraps are involved and defects impurity generates electrons.The structural defects in the amorphous semiconductors orchalcogenide glasses cause additional energy close to theband edges called traps. The existed traps restricted thecurrent flow in this kind materials owing to a capture andemission process; therefore they play a dominant role incurrent mechanism.

1.5. Crystallization Activation Energy

Crystallizations activations energies of the glasses reflectthe contributions of molecular motions and rearrangementsof the atoms around the critical transitions temperatures.84

Crystallizations activations energies of such materialscan be evaluated from the interpretation of isothermalor non-isothermal modes of DSC measurements results.In isothermal and non-isothermal DSC measurementsatoms undergo infrequent transitions between the local (ormetastable state) potential minima states which separatedfrom the different energy barriers in configuration spacewhere each local minima represent a different state struc-ture. For a stable glassy system, configuration must havelocal minima structure state. By mean, atoms possessingminimum crystallization activation energy have a higherprobability to jump in metastable state of lower internalenergy.85 Hence the activations energies of the glasses areamount of energies which absorbed by a group of atomsfor a jump from one metastable state to another state.85�86

Generally activations energies of the glasses are describedat the critical transitions temperatures like, glass transitiontemperature, onset crystallization temperature and peakcrystallization temperature.

1.6. Hurby Parameter

Hubry parameter describes the glass forming ability (GFA)and thermal stability of the glassy materials.87�49 GFA

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and thermal stability of vitreous materials can be estab-lished from the DSC measurements analysis. Generallyunstable glass is exhibited crystallization peak near to theglass transition temperature and stable glass peak closeto melting temperature. GFA and thermal stability canbe evaluated by meen of difference between the criticaltransitions temperatures like, the crystallization tempera-ture (peak temperature Tp and/or onset temperature Tc�and the glass transition temperature (Tg�. This differencevaries with alloys concentration and higher and lower fora certain composition. To obtain the GFA and thermalstability of such materials, several quantitative methodshave been introduced by the investigators.88�89 Dietzel88

has introduced the first GFA criterion DT = Tc − Tg andHruby90 developed the important HR GFA criterion [HR =Tc−Tg/Tm −Tc]. Hruby developed HR GFA criterion alsodescribes the thermal stability of amorphous materials.

1.7. Average Coordination Number

It is well established the physical properties of chalco-genide glasses closely related to average coordinationnumber �r�. First Phillips in 1979,91 Thorpe in 198392

and Tanaka in 198993 established the relationship between�r� and properties of covalent network glasses. Phillips-Thorpe fundamental studies were also demonstrated theexistence of two topological threshold magic numbers�r� = 2�4 and �r� = 2�6 for network solids.Phillips-Thorpe91�92 were demonstrated the threshold

corresponds to a network arrangement when ideal mechan-ical stability of a glassy system reached at �r� = 2�4. Theyalso demonstrated the number of interatomic force-fieldconstraints per atom equal to number of vector degrees offreedom in per atom. At the critical threshold �r� = 2�4the network becomes rigid and rigid clusters percolate thennetwork goes through under a phase transition. Respectiveabove and below the critical thresholds 2�4< �r�< 2�4 thenetworks belongs to underconstrained (floppy) and over-constrained (rigid) structural modes.Afterward Tanaka93 extended this original idea of

Phillips for two-dimensional glassy structures. Essentiallyhe assumed a planar lattice-like entity of the glassy struc-ture as a basic molecular entity lies in a three-dimensionalspace and demonstrated that the number of angular con-straints is reduced and consequent rigidity threshold appearat �r� = 2�67. At the critical threshold �r� = 2�67 a two-dimensional glass appears to be stable in three-dimensionalspace. Thus Tanaka’s average coordination number thresh-olds (�r�< 2�67 and �r�> 2�67) divides the molecular-likeconfiguration which continuous in three-dimensional net-work structure.After, these reports several investigators extensively94–97

inspect the dependence properties on �r� and theextremal points addressed which attributed to topologicalthresholds. Subsequently, Mahadevan and Giridhar97 and

Varshneya et al.98 demonstrated the most probable chem-istry and described the property-�r� of dependence forthese materials. Tichly et al.99 were reported the extremain property dependence on �r� around the chemical thresh-old rather than Phillips-Thorpe or Tanaka, thresholds. Theyalso demonstrated the optical properties of chalcogenideclosely related to structural parameters such as averagecoordination number. In order to this several authors100

contributed their work on extensive study of �r� depen-dence for network glasses. But controversy remains aliveand it is believe for a real chalcogenide glass �r� thresholdcan be lay between 2 to 3.Goal of this work to demonstrate the recent advanced

in metallic (ternary) amorphous semiconductors or chalco-genide glasses and show the composition and averagecoordination �r� dependence correlative thermal, optical,electrical properties of recent developed Se–Zn–In metallicchalcogenide glasses. Here additive element Zn has beentaken due to their metallic character and In due to semi-metallic character.

2. MATERIALS PREPARATIONS

Bulk glassy materials were prepared by conventional melt-quenched technique. The high purity elements Selenium,Zinc and Indium were used. The appropriate amountsof elements were weighed by electronic balance and putinto clean quartz ampoules (length of ampoules 8 cm anddiameter 14 mm). All the ampoules were evacuated andsealed under at a vacuum of 1�33× 10−3 Pa to avoid thereaction of glasses with oxygen at high temperature. Abunch of sealed ampoules was heated in electric furnaceup to 1173 K at a rate of 5–6 K/min and kept at thattemperature for 10–11 h. During the hole melting processampoules were frequently rocked to ensure the homogene-ity of molten materials. After achieving the desired meltingtime, the ampoules with molten materials were quenchedinto ice cooled water. Finally ingots of the glassy materialswere obtained by breaking the ampoules.

3. CHARACTERIZATIONS

3.1. Optical Characterization

For optical characterization powder of bulk glassy materi-als were dissolves into nuzoul to prepare a homogeneoussolution. The prepared solutions of bulk glassy materialswere deposited on transparent object to form a thin film onobject. During the transparent object preparations almostcare was taken to ensure the homogeneous thickness ofthin film of each system. UV/VIS absorption spectra ofprepared thin films on transparent object were recordedfrom SHIMADZU, UV-1700 model spectrometer in rangeof 200 nm to 1100 nm.

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3.2. Electrical Characterization

For the electrical characterization the prepared glassymaterials were cursed into fine powder and make its pel-let (12 mm dia and ∼2 mm thickness) under a 5 tonsload. I–V measurement were performed across the planesurface of the pellet samples with help of Keithely 2400source measure unit under fixed bias (V), in voltage range10 to 200 V at normal room temperature. To ensure thehomogeneity of experimental data each observation wereperformed several times.

3.3. Thermal Characterization

Thermal characterizations were performed from the DSCmeasurement (METTLER DSC-25) with an accuracy of±1 K. For each heating rate measurement instrumentwas calibrated with materials of well-known meltingtemperatures and the melting enthalpy of indium, whichwere supplied with the instrument. The prepared glassysamples (5–8 mg) were loaded into aluminum pans foreach continuous heating rates (� = 5, 10, 15 and 20 K(min)−1� and scanned from 303 to 473 K.

4. RESULTS

4.1. Physical Parameters Variations with AlloysCompositions

Optical energy band gaps of the under test materialsare illustrated from the band tail analysis of UV/Visibleabsorption spectra in spectral range 400 nm to 600 nm,the complete optical study was demonstrated in our paststudy.42 Width of broad absorbance peaks were measuredfor interval 20 nm in spectral tail region. Using the valuesof spectrum width the optical energy gaps were obtainedby employing the well known Tauc’s relation101�102 ofamorphous semiconductors. While, electrical conductivi-ties of present glasses are described between the abovesaid voltage range (10 to 200 V), at room temperature.Here average values of electrical conductivity have to betaken to ensure the minimum deviation and good under-standing of the overall electrical conduction behaviour ofthe materials.Variations of optical energy band gaps and electrical

conductivities with indium atomic percentage is given inFigure 1. It has been observed both optical energy bandgap and electrical conductivity in decreasing and increas-ing order upto threshold (6 at wt% of In) indium con-centration afterward both physical quantities in vice-versa.Obtained outcomes reveal the maximum variation occursfor threshold composition glass.41�42 This change in opti-cal energy bad gap and electrical conductivity arise dueto existance of large number impurity levels and gemi-nate traps of electron–hole pairs in forbidden gap of glassymaterial.103�104 Existance of large number close impurity

Fig. 1. Variation of Ec and �av with indium atomic percentage ofSe98−xZn2Inx (0 ≤ X ≥ 10) chalcogenide glasses.

levels in forbidden gap makes easy to photons jump onemetastable energy level to other105–108 and trap electron–hole pairs produced more electronic conduction centers.Therefore, collectively the impurity levels photons requiresless amount of energy for optical excitations and trapselectron–hole pairs electronic centers allow to easier pathfor conduction. This could be accounted for minimumand maximum Eg and �av values at threshold compositionglass.Crystallization activation energy (Ec� is the result of

phase transformation which arise due to thermal vibra-tions or motions of the atoms. Heat conduction in randomnetwork materials widely affected from the compositionalstructures. The phonons wavelengths and phonons den-sity of states109 play an important role to increase anddecrease in crystallizations activations energies of glassymaterials. Variations of Ec and Eg with indium atomic per-centage of alloys is given in Figure 2. Here Ec valueshave been taken from our pervious study110 which wasdescribed from Augis and Bennet111 method by using the

Fig. 2. Variation of Ec and Eg with indium atomic percentage ofSe98−xZn2Inx (0 ≤ X ≥ 10) chalcogenide glasses.

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Differential Scanning Calorimetry (DSC) measurements atheating rates 5, 10, 15 and 20 K/min. Both Ec and Eg val-ues vary with alloying additive element concentrations andfound to be minimum at threshold composition.The crystallizations activations energies variations in

under examined materials could be expressed in thesewords, the powdered or bulk solid consist of a lattice ofmicro-crystallites with missing of certain lattice points.These missing points creates the voids in the systemsowing to incorporation of foreign/or addition element con-centration in matrixes. The scattering of phonons from themicro-crystallites or solid particles and presence of voidsinfluences the process of heat transfer through the sys-tems. As consequence the phonons wave lengths in suchmaterials is reduced largely. Reduction in phonon-phononinteractions in the test materials ultimately tends towardto decrease in crystallization activation energy beyond thethreshold concentration.83�84�112

Thermal stability and glass forming ability (GFA) playan important role in determination of the utility of alloys asrecording materials. Owing to truth, the phase change opti-cal (PCO) recording and erasing techniques based on laser-induced thermal amorphization and crystallization.113�56

GFA of glasses are related to simply, by which melt canbe cooled to avoid the crystal formation. GFA of the sub-jected materials can be obtained from the analysis of DSCmeasurements at heating rates 5, 10, 15 and 20 K/min byemploying the Hruby Hr parameter relation114

HR =(Tc −Tg

Tm−Tc

)

Variation of Ec and GFA with indium atomic percentage isgiven in Figure 3. Obtained results demonstrate the GFAof studied glasses are in increases order with indium alloy-ing concentration and utmost at threshold compositionthen reduce for 10 % indium composition glass. In con-trast to GFA values Ec is vice-versa. Therefore, these two

Fig. 3. Variation of Ec and Hruby number with indium atomic percent-age of Se98−xZn2Inx (0 ≤ X ≥ 10) chalcogenide glasses.

outcomes correlatively can be interpreted as; the thermalstability and GFA of a glassy material becomes maximumwhen activation energy of crystallization is minimum.

4.2. Physical Parameters Variations with AlloysAverage Coordination Number �r�

Structure of chalcogenide glasses are closely related tostructural unit or average coordination number whichextensively varied with alloying networks.100 Severalapproaches have been proposed by the investigators, toexplain the composition dependence of physical propertiesof these glasses.115–121 Most common accepted approachesare called chemically ordered network models,115–118 inwhich the formation of heteropolar bonds is predomi-nant over the formation of homopolar bonds. These mod-els demonstrate the glass structure is assumed to becomposed of cross-linked structural unit of the stablechemical compound (heteropolar bonds) of the systemand excess of the elements (homopolar bonds). Owingto occurrence of chemical ordering features the physi-cal properties significantly changes at the so-called tieline or stoichiometric composition at which the glassstructure is made up cross-linked structural unit con-sisting of heteropolar bonds only. The tie line compo-sition, at which the features have been seen chemicalorigin are also referred as the chemical threshold of thealloy.122�123 Other existing approaches are so-called topo-logical models which essentially described with help of theconstraint theory91�93�119�120 and structural dimensionalityconsiderations121 In topological models changes in proper-ties discussed in term of the average coordination number�r� which indiscriminate of the species or valence bond.In the constraints models91�93�119�120 number of operatingconstraints equating with number of degrees of freedomand most stable glass �r� is demonstrated around ∼2.4.At the critical value of �r�, the glass network transformsinto an elastically floppy (polymeric glass) type to a rigid(amorphous solid) type configuration. In subsequent of thisseveral investigators are also demonstrated the existence ofmedium-range structures and their physical significance115

at the critical threshold �r�∼2.67 in extensive study of thetopological models.Figure 4 shows the variations of refractive (n) and

Eg with �r�, where values of �r� vary between 2.04 to2.14. The increasing and decreasing reversal trends in opti-cal parameters are observed at �r� = 2�1, which is smallerthan well known first threshold value �r� = 2�4 of chalco-genide glasses. This may be due to fact the applicabilityof topological models has been generally verified for idealnon-metallic/semi-metallic binary and ternary glassy sys-tems. In regard to real metallic alloys, the mechanical sta-bilized structure can be, therefore, observed at lower valueof average coordination number. Increasing or decreasingreversal trends in binary and ternary chalcogenide alloys

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Fig. 4. Variation of n and Eg with average coordination number ofSe98−xZn2Inx (0 ≤ X ≥ 10� chalcogenide glasses.

at lower value of average coordination number has beenalso verified by the several authors.124–128

Figure 5 shows the variations of Eg and electrical con-ductivity (�av� with �r�. An increasing and decreasingphase reversal is observed in the both physical parameters.This result revealed optical energy band gap and electri-cal conductivity are minimum and maximum respectivelyat the critical �r� or threshold composition. Figures 6and 7 exhibit the variations of Eg, �av and Hruby num-ber with �r� of under test materials. These two outcomesdemonstrate at the threshold value �r� the thermal stabil-ity, GFA and electrical conductivity of the system maxi-mum when it optical energy band gap is minimum. FurtherFigure 8 shows a correlation between crystallization acti-vation energy and glass forming ability with �r� of the testmaterials. These two temperature dependence parametershave shown the decreasing and increasing phase reversalwith �r�. Ec and GFA values have been achieved minimumand maximum at the threshold value �r�. Thus outcomesof the described correlative physical properties of under

Fig. 5. Variation of Eg and �av with average coordination number ofSe98−xZn2Inx (0 ≤ X ≥ 10) chalcogenide glasses.

Fig. 6. Variation of Eg and Hruby number with average coordinationnumber of Se98−xZn2Inx (0 ≤ X ≥ 10) chalcogenide glasses.

test materials demonstrates a system should have least Eg,Ec and high n, �av and GFA values at the threshold �r�for the most stable glass configuration.

5. DISCUSSION

Composition and average coordination number depen-dence variations of thermal, electrical, optical parametersof under examined materials could be explained on thebasis of the chemical-bond theory of solids. If bond ener-gies are assumed to be additive, the cohesive energiescan be calculated by summation over the bonds presentwithin the glassy structure.129 In the glasses studied here,it is expected to metallic Zn bonds are dissolved in Sechains to make Zn–Zn, Se–Se and Se–Zn homonuclear andheteronuclear bonds having bond energies 204, 104 and161 kJ (mole)−1. Essentially the binary Se–Zn glassy alloyhas a Se2Zn4 cross-linked heteronuclear metastable struc-ture and rigidity of the structural unit belong in the floppymode.56�110 Cause the thermal agitation or fluctuations130

Fig. 7. Variation of Hruby number and �av with average coordinationnumber of Se98−xZn2Inx (0 ≤ X ≥ 10) chalcogenide glasses.

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Fig. 8. Variation of Ec and Hruby number with average coordinationnumber of Se98−xZn2Inx (0 ≤ X ≥ 10) chalcogenide glasses.

the cross-linked heteronuclear metastable structure hasrelaxed into a Se2Zn stable structure and affects the rigid-ity of structural unit.Incorporation of In into the binary Se–Zn glass matrix

creates a ternary Se–Zn–In alloy with Se–In heteronu-clear bonds having a bond energy of 122.4 kJ (mole)−1, inwhich there are strong metallic Zn–In bonds (bond energy165.17 kJ mole−1� present. Addition of the foreign elementincreases the number of inter-atomic force-field constraintsand degree of freedom per atom in three dimensional (3D)space. Further incorporation of In (for X = 4, 6) con-centration in ternary glassy configuration leads to struc-ture being heavily cross-linked and increase the sterichindrance. As consequence corresponding systems struc-tural units tend to transform toward floppy to rigid config-uration. The replacement of weak Se–Se bonds by Se–Inbonds (USe−In − USe−Se� = 24�42 kJ (mole)−1 begin thestructural transformation towards 3D → 2D space, thisleads to an increase in �av, GFA, n, �r� with a decreasein associative Eg, Ec. Thus the cohesive energy as well asaverage coordination number of the glasses increases withincreasing In concentration.In chalcogenide glasses a chemical threshold often

occurs at a specific composition. This event has alsoobserved in Se–Zn–In glasses; at the threshold concentra-tion the glass becomes chemically ordered and containsonly strong Se–In bonds and chemical threshold networkhave most tight bonding, shortest bond lengths with largestcompactness. Thus, the maximum observed deviation at�r� = 2�1 is attributed to complete 2D layered structuraltransformations in floppy to rigid transition.131

Beyond the threshold value, addition of In contentfavours the formation of In–In bonds (bond energy125.58 kJ mole−1� and reduce the density of Se–In bonds.As consequence higher value �r� composition glass net-work transformed into 3D network in 2D → 3D con-figurations transitions.93�132–134 The difference in cohesiveenergy (USe−In−UIn−In�=−124 kJ (mole)−1 and structural

network transitions 2D→ 3D provided the reduce valuesof the average cohesive energy on both sides of the chem-ical threshold cause an increase in Eg, Ec and a decreasein �av, GFA, n. As result one has noticed higher and lowervalues of the thermal, optical and electrical parameterson both sides of the compositional/- structural thresholdnetworks.100�135�136�

Here a small inconsistency appeared in critical �r� =2�1 to well known established structural threshold valuefor chalcogenide glasses. This inconsistency in thresholdvalue of �r� could be attributed to the addition of thesemi metal In in presence of heavy metal Zn in ternaryglassy network.137�138 Several authors also predicted137–142

that, the structural threshold �r� can be differ from wellknown Phillips and Thorpe established critical �r� = 2�4and Tanaka demonstrated the critical �r� = 2�67 in chalco-genide glasses.

6. CONCLUSIONS

In summary, author has critically reviewed the majorachievements in metal containing ternary chalcogenidealloys and discussed the composition/average coordinationnumber �r� dependence correlative physical properties ofSe–Zn–In ternary alloys. It has obtained that the opticalenergy band gap (Eg�, electrical conductivity (�av�, crystal-lization activation energy (Ec�, thermal stability and glassforming ability (GFA) of the materials extensively varywith alloys concentrations. Physical parameters Eg��av, Ec

and GFA values have shown a phase reversal at the chem-ical threshold owing to large number of bonds fluctuationsin glassy networks.Addition of foreign element also influenced the mechan-

ical strengths of the networks of the glasses. As conse-quence, structural rigidity vary with concentrations of analloying elements and configurational networks modifica-tions represents in from of average coordination number�r�. Therefore, here one has demonstrated the correla-tion between n, Eg��av, Ec and GFA on basis of �r�variations. Outcomes demonstrate a mechanical thresholdalso occurred in Se–Zn–In network glasses. At the criticalmechanical threshold (�r� = 2�1) value the thermal, elec-trical and optical parameters n, Eg��av, Ec and GFA havebeen clearly exhibited a phase reversal. This arises due tolarge change in structural constraint unit of the optimumnetwork glass. Below and above the critical threshold con-straints of the structural transitions belong to under andover cross-linked regime. Due to this optical, electrical,thermal properties of the under test glasses have shown asignificant decrease and increase at below and above thecritical threshold value of the average coordination number(�r�).Hence in significance of past different reports and this

work is evident that the intensive investigations in fieldof metallic chalcogenides still on-going to deduce the

Rev. Adv. Sci. Eng., 1, 292–301, 2012 299


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prospective glassy materials. Subsequently it is also con-cluded that the structural unit �r� threshold in chalco-genide glasses has not a well defined value (2.4 or 2.67). Itcan be vary with the selection of glass alloying elements.

Acknowledgment: Abhay Kumar Singh thankful toUniversity Grand Commission (UGC) for support underDr. D. S. Kothari program.

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