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Journal of Non-Crystalline Solids 353 (2007) 2079–2083

Transport properties of Se80�xTe20Bix (0 6 x 6 1) system

Nikhil Suri, K.S. Bindra, R. Thangaraj *

Semiconductors Laboratory, Department of Applied Physics, Guru Nanak Dev University, Amritsar, Punjab 143 005, India

Available online 3 April 2007

Abstract

Thermal studies using differential scanning calorimetry under non-isothermal conditions with the samples heated at different rates (5,10, 15 and 20 K/min) have been reported and discussed. The glass transition activation energy and crystallization activation energy havebeen determined. Thermal stability and glass-forming tendency have also been studied. The activation energy in dark and optical band-gap have been calculated. It has been found that optical bandgap is equal to twice the dark thermal activation energy.� 2007 Elsevier B.V. All rights reserved.

PACS: 61.10.NZ; 64.70.Pf; 67.80.Gb; 71.23.�k

Keywords: Crystallization; Conductivity

1. Introduction

Chalcogenide glasses have attracted much attentionbecause of their potential application in various solid-statedevices. The studies of these amorphous chalcogenideglasses are of great interest also due to their importancein preparing electronic memories [1] and their optical appli-cation as good IR transmitting materials [2]. Moreover,they are interesting as core materials for optical fibers forlight transmission, especially when short lengths and flexi-bility are required [3,4]. In recent years, chalcogenide thinfilms have become a subject of systematic research becauseof the changes in physio-chemical properties that arecaused by thermal annealing [5–7]. Tellurium alloys haveoften been used for the active layer of memory devices [8]because they have low melting points. However, experi-mental results reported by various researchers have shownthat the addition of impurity atoms in binary systems doeschange the electrical properties of chalcogenide glasses sig-nificantly. Therefore, investigations on the influence ofimpurities on chalcogenide glasses are of relevance fromthe basic science and application point of view.

0022-3093/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2007.01.071

* Corresponding author.E-mail address: rthangaraj@rediffmail.com (R. Thangaraj).

The Se–Te–Bi is one of the most characteristic chalco-genide glasses, which is why it has been chosen for thestudy. In the present work, differential scanning calorimet-ric (DSC) studies on the bulk glass have been carried out todetermine glass transition temperature (Tg) and peak crys-tallization temperature (Tp) the apparent activation energyfor glass transition and activation energy for crystallizationhave been investigated. Electrical and optical properties ofthe thin films of these glassy samples have also beenstudied.

2. Experimental

Bulk samples of Se80�xTe20Bix (x = 0,0.2, 0.5,1.0) wereprepared by conventional melt quenching technique. Mate-rials of 5 N purity were sealed in a cleaned and evacuatedquartz ampoule (length �5 cm and internal diameter�0.8 cm) in a vacuum of 10�5 mbar. The ampoule was keptinside a vertical furnace and the temperature was raised upto 1123 K slowly (3–4 K/min) and kept for 48 h. Theampoule was inverted at regular intervals to ensure homo-geneous mixing of the constituents. The ampoule was thenquenched in ice-cold water. The bulk material was sepa-rated from the quartz ampoule by dissolving the ampoulein HF + H2O2 solution for approx. 48 h. The ingots that

5K/min

10K/min

15K/min

20K/min

303 423363 483 543Temperature (K)

ΔQ (

mW

)E

xoE

ndo

Fig. 1. DSC thermograms for Se80�xTe20Bix (x = 1.0) at different heatingrates.

338

342

346

350

0.0 0.4 0.8 1.2 1.6Bi (at%)

T g (K

)

5 K/min 10 K/min 15 K/min 20 K/min

Fig. 2. Variation of Tg with composition of Se80�xTe20Bix at differentheating rates.

2080 N. Suri et al. / Journal of Non-Crystalline Solids 353 (2007) 2079–2083

we obtained were approx. 2 cm in length and 0.8 cm diam-eter. The amorphous nature of the bulk samples was con-firmed by the absence of peaks in the X-raydiffractogram. The thermal behavior of the samples wasrecorded using Mettler Toledo Stare DSC System. In eachstudy approx. 25 mg of the bulk material was used. DSCruns were taken for four different heating rates i.e., 5, 10,15 and 20 K/min for each of the composition so as to getglass transition, peak crystallization and melting tempera-ture. The glass transition temperature is defined as the tem-perature corresponding to the intersection of two linearportions adjoining the transition elbow of DSC trace offirst endothermic peak. The onset crystallization tempera-ture Tc, has been defined as the temperature correspondingto the intersection of the two linear portions adjoining thetransition elbow of the DSC trace in the exothermic direc-tion. The fraction X crystallized at any temperature T wascalculated using the relation X ¼ AT

A , where A is the totalarea of the exotherm between Tc and the temperature atwhich crystallization is completed, AT is the area of exo-therm between Tc and T [9]. The bulk alloy obtained wasused to prepare thin films on well-cleaned corning glasssubstrates by thermal evaporation technique in a vacuumbetter than 10�5 mbar using Hind High Vacuum coatingunit (model 12A4D). Talysurf, Taylor Hobson make(model no. 2015) was used to measure thickness of thefilms. The thickness was found to be �2300 A for all thesamples. Dark conductivity of all the samples was mea-sured in vacuum. Electrical contacts were made on thinfilms using silver paint electrodes in a co-planar geometry(length 0.9 cm and gap between electrodes 0.3 cm) for allthe samples. The sample was mounted inside a metalliccryostat with a transparent window. All the measurementswere made under a vacuum of 10�5 mbar. The opticaltransmission spectrum was recorded at room temperatureusing UV–visible spectrophotometer (Shimadzu, Japan)in the wavelength interval of 300–1100 nm. The absorptionco-efficient, a was calculated using the relation

a ¼ ð1=tÞ � lnð100=T Þ;

where t is the thickness of the films and T is the percentagetransmission. The optical band gap Eg was obtained from aplot of (ahm)1/2 versus (hm) and taking intercept on energyaxis.

3. Results

3.1. Thermal studies

DSC thermograms for Se80�xTe20Bix (x = 1.0) areshown in Fig. 1. It can be seen from the figure that Tg

and Tp shift towards higher temperature with increase inthe heating rate. Samples with other x values also showsimilar behavior. The variation of Tg in Se80�xTe20Bix sys-tem with Bi concentration at different heating rates isshown in Fig. 2. In all samples Tg obeys the empiricalrelation

T g ¼ Aþ B logðaÞ; ð1Þ

where A and B are constants. The value of B is calculatedfrom the slope of Tg versus log (a) plots and reported in Ta-ble 1. The change in B with Bi concentration indicates thatthe Se–Te–Bi alloy undergoes structural changes with Biaddition [10]. The apparent activation energy for glasstransition Et is calculated using the Kissinger formula [11]

lnT 2

g

a

!þ const: ¼ Et

RT g

; ð2Þ

where R is the gas constant.

The lnT 2

g

a

� �versus 1000/Tg (Fig. 3) is found to be linear

for all the samples and the values of apparent activationenergy Et for glass transition, obtained from their slopes,

Table 1Parameters determined from heating rate dependence of Se80�xTe20Bix glassy system

X B Et (KJ/mol) mEc/n (KJ/mol) n m Ec (KJ/mol) mEc (KJ/mol) Ec (KJ/mol)

0.0 16.61 125.16 97.35 4.05 4 97.35 380.18 95.040.2 12.21 185.70 86.06 2.45 2 86.06 267.00 133.500.5 11.47 197.00 69.20 2.01 2 69.20 237.00 118.501.0 9.74 236.00 63.30 2.30 2 63.30 299.00 149.50

8.5

9.0

9.5

10.0

10.5

2.85 2.88 2.91 2.941000/ Tg (K-1)

ln(T

2 g/α

)

Fig. 3. lnT 2

g

a

� �versus 1000/Tg for Se80�xTe20Bix.

-6.00

-4.00

-2.00

0.00

2.00

2.40 2.50 2.60 2.70 2.80

1000/T(K-1)ln

(-ln(

1-X)

)

5deg/min10deg/min15deg/min20deg/min

Fig. 4. ln (�ln (1 � X)) versus 1000/T for Se80�xTe20Bix (x = 1.0).

N. Suri et al. / Journal of Non-Crystalline Solids 353 (2007) 2079–2083 2081

are listed in Table 1. Activation energy for crystallization iscalculated from the modified Kissinger equation [12,13]

lnan

T 2p

!¼ �mEc

RT p

þ ln K 0; ð3Þ

where K 0 is a constant, containing the factors depending onthe thermal history of samples, n and m are constantsdepending upon the morphology of the growth. The valueof mEc

n calculated from the slope of loga versus 1000/Tp andreported in Table 1.

According to Matusita relation [14]

ln½� lnð1� X Þ� ¼ �n lnðaÞ � 1:052mEc

RTþ const: ð4Þ

where X is the volume fraction of crystals precipitated inthe glass heated at a uniform rate, Ec is the activation en-ergy for crystallization and R is the gas constant. The slopeof ln [�ln (1 � X)] versus ln (a) gives the value of n reportedin Table 1. The values of both m and n are reported in Ta-ble 1. The slope of ln [�ln (1 � X)] versus 1000/T gives thevalue of mEc shown in Fig. 4.

The ease in glass formation is determined by calculatingthe reduced glass transition temperature T rg ¼ T g

T m[15]. The

value obtained obeys the two-thirds rule which states

T g

T m

¼ 2

3: ð5Þ

Two-third rule holds well for this composition. The glassforming parameter is calculated using Hruby’s parameter[16]

Kgl ¼T c � T g

T m � T c

: ð6Þ

The values of Kgl are found to decrease with increasing Bicontent. The values of Tc � Tg, Kgl and Trg are listed inTable 2.

3.2. Electrical studies

The dark conductivity of the Se80�xTe20Bix (0 6 x 6 1)samples, measured as a function of temperature, is shownin Fig. 5. The conductivity of the samples increases withincrease in temperature as observed from the figure. Therelation can express the dc conductivity

r ¼ r0 exp½DEd=kT �; ð7Þ

where r0 is the pre-exponential factor, DEd is the dc activa-tion energy and is calculated from the slope of r and 1000/T plots, k is the Boltzmann constant and T is the absolutetemperature. Table 3 shows the variation of DEd with bis-muth concentration. It is seen from the table that the acti-vation energy decreases with increase in bismuth content.Fig. 6 shows a plot (ahm)1/2 versus (hm) for different bismuthconcentrations. The values of bandgap calculated from theabove plot for different bismuth concentrations are given in

Table 2Values of (Tc � Tg), Kgl and Trg calculated from DSC thermograms taken at 10 K/min heating rate for Se80�xTe20Bix glassy system

x Tg (K) Tc (K) Tm (K) (Tc � Tg) (Tm � Tc) Kgl Trg

0.0 337 402 521 65 119 0.54 0.640.2 343 389 529 46 140 0.32 0.640.5 344 384 530 40 146 0.27 0.641.0 345 380 534 35 154 0.22 0.64

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

3.0 3.5 4.0

1000/T (K-1)

cond

uctiv

ity (O

hm-1

m-1

)

x = 0.0

x = 0.2

x = 0.5

x = 1.0

Fig. 5. Variation of dark conductivity with temperature in Se80�xTe20Bix(x = 0,0.2,0.5,1.0) thin films.

Table 3Showing the variation of optical band gap and electrical activation energyvalues for Se80�xTe20Bix thin films

x Optical bandgap (eV) Electrical activation energy (eV)

0.0 1.46 0.730.2 1.41 0.600.5 1.38 0.561.0 1.32 0.50

0

300

600

900

1.2 1.7 2.2hν (eV)

( αh ν

)1/2 (c

m)-1

/2(e

V)1/

2

x = 1.0x = 0.5x = 0.2x = 0.0

Fig. 6. Plot showing the variation of (ahm)1/2 with (hm) for Se80�xTe20Bix(x = 0,0.2,0.5,1.0) thin films.

2082 N. Suri et al. / Journal of Non-Crystalline Solids 353 (2007) 2079–2083

Table 3. From this table it can be seen that optical bandgapdecreases with bismuth concentration.

4. Discussion

In the system under study, the glass transition tempera-ture was found to increase with the increase in heating rateas well as with the addition of bismuth content. Thisincrease in the value of Tg with bismuth content can beexplained with the help of chemically ordered network(CON) model according to which the heteropolar bond isfavoured over the homopolar bond. The various types ofbonds involved in the present system are Se–Se, Te–Te,Se–Te, Bi–Se, Bi–Te, etc. When the percentage of Bi isincreased, Bi is expected to form bond with Se rather thanwith Te, as the bond energy of Bi–Se (170.4 KJ/mol) is lar-ger than the bond energy of Bi–Te (125.6 KJ/mol) [17].This explains the increase in Tg with increase in Bi content.The glass transition activation energy is the amount ofenergy that is absorbed by a group of atoms in the glassyregion so a jump from one metastable state to another[18]. In other words, the activation energy is involved inthe molecular motions and rearrangements of the atomsaround the glass transition temperature [19]. When thesample is reheated in the DSC furnace, the atoms undergoinfrequent transitions between the local potential minimaseparated by different energy barriers in the configurationspace where each local minimum represents a differentstructure. The most stable local minimum in the glassyregion has lower internal energy. Accordingly, the atomsin a glass having minimum activation energy have a higherprobability to jump to the metastable state (or local) oflower internal energy and hence is the most stable [18].Therefore, one can argue that the addition of Bi reducesthe stability of the glasses under investigation. The valueof n calculated from the slope of ln [�ln (1 � X )] versuslna gives the value of n reported in Table 1. The value ofm is taken as m = n � 1 if there is no previous heat treat-ment and m = n if the sample undergone a heat treatment[20]. In the present case no heat treatment was given; how-ever, the presence of an endothermic peak indicates that athermal relaxation has taken place. Also, due to the lowervalue of the Tg, it can be reasonably assumed that the sam-ple might have undergone a heat treatment at room tem-perature. Hence, the value of m is taken as equal to n inthe present case for further analysis. The values of bothm and n are reported in Table 1. The calculated values ofEc shown in columns 7 and 9 of Table 1 show similar

N. Suri et al. / Journal of Non-Crystalline Solids 353 (2007) 2079–2083 2083

behavior but a discrepancy in the values of Ec evaluatedmay be attributed to the different approximations that havebeen adopted while arriving at the final equation. In addi-tion, the temperature and pressure gradients in the sample(resulting from heat effect e.g. heating rate) vary randomlyand such random variations have predominant effects onkinetic parameters. Hence, the variation in the value ofEc is expected to occur in the light of the reasons mentionedabove. Similar variations in the activation energy have alsobeen observed in various other chalcogenide glasses [21,22].The difference between Tc and Tg, which is an indication ofthe thermal stability of glasses against crystallizationdecreases with increase in bismuth content. This indicatesa decrease in the thermal stability of glass with increasein bismuth concentration in Se–Te–Bi glass system.

The activation energy decreases with increase in bismuthcontent. Increase in dark conductivity and decrease in acti-vation energy as seen from the Fig. 6 and Table 3 respec-tively are found to be associated with the shift in Fermilevel in impurity-doped chalcogenides glasses [23–26].From Table 3 it can be seen that optical bandgap decreaseswith bismuth concentration. Optical absorption dependsupon both, the short-range order in amorphous state andon the defects associated with it. Decrease in optical band-gap in our system may be due to reduction in amount ofdisorder in the system and increase in the density of defectstates [27]. It may be noted from the results that the bandgap is approximately twice that of the dc activation energy.The variation of both DEd and Eopt

g follow the same trendas reported for antimony addition to the Se–Te system[28,29].

5. Conclusion

The high value of the crystallization temperature wellabove the room temperature in these materials can preventself-transition between the two phases making them attrac-tive for recording applications. The decrease of Tc � Tg athigher bismuth concentration leads to the conclusion thatstable glasses can be obtained only with low bismuth con-centration. Thin films of Se80�xTe20Bix were grown bythermal evaporation process. The activation energy and

optical energy gap decrease with increasing bismuthcontent.

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