Optical parameters of ternary Te15(Se100−xBix)85 thin films deposited by thermal

evaporation

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Phys. Scr. 84 (2011) 045703 (6pp) doi:10.1088/0031-8949/84/04/045703

Optical parameters of ternaryTe15(Se100−xBix)85 thin films deposited bythermal evaporationKameshwar Kumar1, Pankaj Sharma2, S C Katyal3 and Nagesh Thakur1

1 Department of Physics, HP University, Shimla 171005, India2 Department of Physics, Jaypee University of Information Technology, Waknaghat, Solan, HP 173215,India3 Department of Physics, Jaypee Institute of Information Technology, Noida, UP, India

E-mail: kameshwarkumar01@gmail.com, pankaj.sharma@juit.ac.in and ntb668@yahoo.co.in

Received 4 March 2011Accepted for publication 16 August 2011Published 20 September 2011Online at stacks.iop.org/PhysScr/84/045703

AbstractThin films of Te15(Se100−x Bix )85 (x = 0, 1, 2, 3, 4 and 5 at.%) glassy alloys were deposited bythermal evaporation (at 10−4 Pa) from bulk samples. Optical characterization of the films wasdone by analysing their transmission spectra taken in the spectral range 400–2300 nm.Swanepoel’s method was used to calculate the refractive index (n) and extinction coefficient(k). It was found that the refractive index increases with an increase in Bi content. TheWemple–DiDomenico single-oscillator approach was used to calculate the average band gapenergy (Eo), dispersion energy (Ed) and static refractive index (no). The absorption coefficient(α) and film thickness were calculated from the transmission spectra of the films. The opticalband gap (Eg) was estimated using Tauc’s extrapolation and was found to decrease from 1.37to 1.21 eV with Bi addition from 0 to 5 at.% in glassy alloys. The decrease in optical band gapis explained on the basis of the decrease in cohesive energy of the samples and the differenceof electronegativity of the atoms involved. The real (εr) and imaginary parts (εi) of thedielectric constant for the films were also calculated and reported.

PACS number: 78.20.Ci

1. Introduction

Chalcogenide glasses are formed by alloying chalcogenelements (S, Se and Te) with Ge, Bi, As, Sb, Cd, Ga,etc [1]. These glasses have attracted much attention becauseof their interesting optical [2] and thermal [3] properties,which are highly composition dependent. These materialshave current and potential applications in optical memories,photonic crystals, transistors, reversible phase change opticalrecording materials, infrared (IR) lasers and IR transmittingoptical fibres [4–6]. Efforts are on to develop the chalcogenidematerials for rewritable optical memories [7]. To use thesematerials in optical fibres and reflecting coating requiresaccurate knowledge of their optical constants over a widerange of wavelengths. The optical properties of thesematerials are also related to their atomic structure, electronicband structure and electrical properties [8]. These glasseshave a relatively high atomic mass and a weak bond strength,

resulting in low characteristic phonon energies. Thus, theseglasses are highly transparent in the mid- to far-IR region andare used in the IR window and optics [9]. The transparencywindow of sulfide glasses is 0.5–10 µm, that for selenideglasses is 0.8–15 µm and that for telluride glasses extends tothe 18 µm wavelength range [10, 11]. Chalcogenide glassesare mostly selenium based because of their glass-formingability, the unique property of reversible transformationand applications such as switching, optical memory andxerography [12, 13]. Pure Se has disadvantages such as ashort lifetime and a low photosensitivity. These problems canbe overcome by alloying Se with impurity atoms such as Te,Ge, Ga and As [14, 15]. It has been found that substitution ofTe for Se breaks the Se8 ring structure and slightly increasesthe chain fraction but reduces the chain length [16]. Se–Teglassy alloys have gained much importance because of theirhigh photosensitivity, greater hardness, high crystallizationtemperature and smaller ageing effects [17]. The properties

0031-8949/11/045703+06$33.00 Printed in the UK & the USA 1 © 2011 The Royal Swedish Academy of Sciences

Phys. Scr. 84 (2011) 045703 K Kumar et al

of chalcogenide glasses are usually affected by the additionof a third element. We have chosen Bi as the third elementbecause it creates compositional and configurational disorderin Se–Te glassy alloys. Bi addition also produces remarkablechanges in the optical properties and causes p-to-n transitionin conductivity in chalcogenide glasses [18–21].

In the present work, we report the effect of Bi additionon the optical properties of Te15(Se100−x Bix )85 (x = 0, 1, 2,3, 4 and 5 at.%) thin films. The optical properties, namelythe refractive index (n), extinction coefficient (k), absorptioncoefficient (α) and optical band gap (Eg), of thin filmsare calculated by analysing their transmission spectra inthe spectral range 400–2300 nm using Swanepoel’s method[22, 23]. The dispersion of the refractive index is studied interms of the Wemple–DiDomenico (WDD) single oscillatormethod.

2. Experimental details

Glassy alloys of Te15(Se100−x Bix )85 (x = 0, 1, 2, 3, 4 and5 at.%) were prepared by the melt quenching technique. Se, Teand Bi of high purity (99.999%) were weighed according totheir atomic percentages and were sealed in quartz ampoulesevacuated at 10−3 Pa. The sealed ampoules were kept insidea vertical furnace, where the temperature was raised up to1073 K at a heating rate of 3–4 K min−1. The ampoules wereheated at the highest temperature for 12 h. During this heating,the ampoules were frequently rocked to make the melthomogeneous. Quenching was done in ice-cold water. Thequenched samples were obtained by breaking the ampoules.The amorphous nature of the glasses was confirmed, as therewas no prominent peak in their x-ray diffraction spectra.

Thin films of Te15(Se100−x Bix )85 glassy alloys weredeposited on the cleaned microscopic glass substrates bythe thermal evaporation process at 10−4 Pa base pressure.The evaporation process was carried out in the coatingsystem (HINDIHIVAC, model 12A4D, India). The filmswere kept in the deposition chamber for 24 h to achievemetastable equilibrium. The amorphous nature of the thinfilms was verified, as there was no prominent peak in theirx-ray diffraction spectra. The normal incidence transmissionspectra of the thin films were obtained in the spectralrange of 400–2300 nm using a double-beam UV–vis–NIRspectrophotometer (PerkinElmer Lambda 750). All thereported measurements were taken at room temperature(300 K).

3. Results and discussion

3.1. Refractive index and film thickness

The optical transmission spectra of thin films are shown infigure 1. Optical transmission is a very complex functionand is strongly dependent on the wavelength and absorptioncoefficient. It is observed that the transmission shifts to higherwavelength with an increase in Bi content. The refractiveindex (n), absorption coefficient (α) and thickness (d) arefound using Swanepoel’s method, which is based on the ideaof Manifacier. Continuous envelopes are generated throughthe maximum (TM) and minimum (Tm) of transmission

0 500 1000 1500 2000 25000

20

40

60

80

100

Tra

nsm

issi

on %

Wavelength (nm)

x=0 x=1 x=2 x=3 x=4 x=5

Figure 1. Transmission spectra for Te15(Se100−x Bix )85 (x = 0, 1, 2,3, 4 and 5 at.%) thin films.

data. The advantage of using envelopes of the transmissionspectrum compared to using only the transmission spectrumis that the envelopes are slowly changing functions ofwavelength, whereas the spectrum varies rapidly withwavelength. In the transparent region (α ∼ 0), the refractiveindex, according to Swanepoel [23], is given by

n = [N + (N 2− s2)1/2]1/2, (1)

where

N =2s

Tm−

(s2 + 1)

2, (2)

Tm is the envelope function of minimum transmittance ands is the refractive index of the substrate having the value1.51. In the weak absorption region (α 6= 0), the transmittancedecreases due to absorption and the value of N is given by

N = 2sTM − Tm

TMTm+

s2 + 1

2, (3)

where TM is the envelope function of transmittance maxima.The refractive index is found to decrease with an increase inwavelength for the thin films. The increase in transmittancewith wavelength (λ) is responsible for the decrease inrefractive index (n). The increase in refractive index withBi addition is probably due to the replacement of lighter Se(density 4.79 g cm−3) by heavier Bi (density 9.78 g cm−3).This increases the density of bulk Te15(Se100−x Bix )85 glassesand hence the refractive index. The increase in refractiveindex is further explained on the basis of polarizability. Therefractive index and polarizability are related through theLorentz–Lorentz equation [24]

n2− 1

n2 + 2=

1

3ε0

∑i

Niαpi , (4)

where ε0 is the permittivity of free space and Ni is thenumber of polarizable units of type i per unit volume withpolarizabilityαpi . The atomic radius of Bi (160 pm) is morethan that of Se (116 pm) and Te (137 pm). This increase inradius with Bi addition increases the polarizability and henceincreases the refractive index of the films. Figure 2 shows the

2

Phys. Scr. 84 (2011) 045703 K Kumar et al

600 800 1000 1200 1400 1600 1800 2000 2200

2.8

3.0

3.2

3.4

3.6

3.8

Ref

ract

ive

inde

x

Wavelength (nm)

x=0 x=1 x=2 x=3 x=4 x=5

Figure 2. The variation of the refractive index (n) with wavelength(λ) for Te15(Se100−x Bix )85 (x = 0, 1, 2, 3, 4 and 5 at.%) thin films.

Table 1. Thickness (d), refractive index (n) and extinctioncoefficient (k) at 800 nm, static refractive index (no) andhigh-frequency dielectric constant (ε∞) for Te15(Se100−x Bix )85

(x = 0, 1, 2, 3, 4 and 5 at.%) thin films.

Composition d (nm) n k no ε∞

x = 0 334 2.95 0.059 2.730 7.454x = 1 443 3.03 0.058 2.737 7.493x = 2 405 3.09 0.072 2.744 7.528x = 3 444 3.27 0.067 2.752 7.577x = 4 393 3.39 0.064 2.753 7.584x = 5 335 3.54 0.072 2.784 7.755

variation of refractive index with wavelength for the thin filmsunder study.

The thickness of the films was determined using therelation [23]

d =λ1λ2

λ1n2 − λ2n1, (5)

where n1 and n2 are refractive indices of the film for twoadjacent maxima or minima at wavelengths λ1 and λ2.Calculated values of the refractive index and thickness for thefilms are listed in table 1.

3.2. Average band gap energy, dispersion energy and staticrefractive index

The energy dependence of the refractive index hasbeen studied by using the single-effective-oscillator modelproposed by WDD [25, 26]. According to this model the dataon dispersion of the refractive index can be described to an

excellent approximation by the WDD dispersion equation

n2= 1 +

Ed Eo

E2o − (hν)2

, (6)

where hv is the photon energy, n is the refractive index, Eo isthe average band gap energy and Ed is the dispersion energy.Eo and Ed values were calculated from the slope (Eo Ed)

−1

and intercept (Eo/Ed) on the vertical axis of the (n2− 1)−1

versus (hν)2 plot given in figure 3. The calculated values ofEd and Eo are given in table 2. The average band gap energy,Eo, corresponds to the distance between the centres of gravityof valence and conduction bands. The dispersion parametersEo and Ed decreases with an increase in Bi content from 0 to5 at.%. Eo is related to the optical band gap Eg (calculated insection 3.3) through Tanaka’s relation [27]

Eo ≈ 2Eg. (7)

It is clear from table 2 that Tanaka’s relation is partlyapplicable to the films under consideration. The dispersionenergy follows a simple empirical relationship [26],

Ed = βNc Za Ne, (8)

where β is a constant having the value 0.37 ± 0.04 eV forcovalent amorphous materials. Nc is the coordination numberof the cation that is nearest neighbour to the anion, Za isthe formal chemical valency of the anion and Ne is the totalnumber of valence electrons per anion. Nc is calculated usingequation (8) and is found to decrease from 4.66 to 2.95 forTe15(Se100−x Bix )85 (x = 0, 1, 2, 3, 4 and 5 at.%) thin films.The calculated values of Ne and Nc are listed in table 2.

The value of the static refractive index (no) has beencalculated by extrapolating the WDD dispersion equation ashν approaches zero, and is given by

n20 = 1 +

Ed

Eo. (9)

The high-frequency dielectric constant ε∞ has been calculatedfrom the relation ε∞ = (no)

2.The values of no and ε∞ for the films are listed in table 1.

3.3. Absorption coefficient, optical band gap and dielectricconstant

The absorption coefficient (α) has been calculated using therelation [28]

α =1

dln

(1

T

), (10)

Table 2. Optical band gap (Eg), average energy gap (Eo), dispersion energy (Ed), cohesive energy (CE), Nc and Ne parameters forTe15(Se100−x Bix )85 (x = 0, 1, 2, 3, 4 and 5 at.%) thin films.

Composition Eg (eV) Eo (eV) Ed (eV) CE (kcal mol−1) Ne Nc

x = 0 1.37 3.772 24.345 43.15 7.06 4.66x = 1 1.29 3.377 21.930 43.09 7.12 4.16x = 2 1.27 3.163 20.650 43.03 7.18 3.89x = 3 1.26 2.792 18.360 42.97 7.25 3.42x = 4 1.25 2.510 16.529 42.91 7.31 3.05x = 5 1.21 2.386 16.122 42.84 7.38 2.95

3

Phys. Scr. 84 (2011) 045703 K Kumar et al

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

0.115

0.120

0.125

0.130

0.135

0.140

0.145

0.150

0.155

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.115

0.120

0.125

0.130

0.135

0.140

0.145

0.150

0.155

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50.10

0.11

0.12

0.13

0.14

0.15

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.10

0.11

0.12

0.13

0.14

0.15

0.0 0.5 1.0 1.5 2.0 2.5

0.09

0.10

0.11

0.12

0.13

0.14

0.15

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.08

0.09

0.10

0.11

0.12

0.13

0.14

A=0.1550B=-0.0108

x=0

(n2 -1

)-1

A=0.1540B=-0.0135

x=1

A=0.1532B=-0.0153

x=2

(n2 -1

)-1

A=0.1521B=-0.0195

x=3

A=0.1512B=-0.0241

(n2 -1

)-1

(hν)2 (eV)2

x=4

A InterceptB Slope

A=0.1481B=-0.0259

(hν)2 (eV)2

x=5

Figure 3. Plot of the refractive index factor (n2− 1)−1 versus (hv)2 for Te15(Se100−x Bix )85 (x = 0, 1, 2, 3, 4 and 5 at.%) thin films.

where d is the thickness of the film and T is the transmittance.The extinction coefficient (k), which is a measure of fractionof light lost due to scattering and absorption per unit distanceof the participating medium, was calculated using the relation

k =αλ

4π. (11)

Figure 4 shows the variation of the extinction coefficient withthe wavelength of incident light for the thin films under study.In amorphous semiconductors the transmission spectra havethree distinct regions, namely a high-absorption region, anexponential edge region and a weak absorption tail. The highabsorption part is caused by the band-to-band transition and

is followed by an Urbach’s tail. The weak absorption tailoriginates from defects and impurities. The optical band gaphas been calculated from the absorption coefficient data as afunction of photon energy using Tauc’s relation [29]

αhν = B(hν − Eg)m, (12)

where B is a constant and Eg is the optical band gap ofthe films. In equation (11), m = 1/2 for a direct allowedtransition, m = 3/2 for a direct forbidden transition, m = 2for an indirect allowed transition and m = 3 for an indirectforbidden transition. After fitting all the values of m inTauc’s relation, the value m = 2 was found to hold good,leading to indirect transitions. Figure 5 shows the plots of

4

Phys. Scr. 84 (2011) 045703 K Kumar et al

600 800 1000 1200 1400 1600 1800 2000 2200 2400

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0.11

0.12

0.13

0.14

Ext

inct

ion

coef

fici

ent

Wavelength (nm)

x=0 x=1 x=2 x=3 x=4 x=5

Figure 4. The variation of the extinction coefficient (k) withwavelength (λ) for Te15(Se100−x Bix )85 (x = 0, 1, 2, 3, 4 and 5 at.%)thin films.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

0

20

40

60

80

100

120

140

160

180

200

220

(αhν

)0.5 (

eV c

m-1)0.

5

hν (eV)

x=0 x=1 x=2 x=3 x=4 x=5

Figure 5. Plot of (αhv) 1/2 versus hv for Te15(Se100−x Bix )85 (x = 0,1, 2, 3, 4 and 5 at.%) thin films.

(αhν)1/2 versus hv. The optical band gap has been calculatedfrom these plots by taking the intercepts of extrapolationto zero absorption with the photon energy axis as (αhν)1/2

approaches zero. From table 2 it is clear that the optical bandgap decreases from 1.37 to 1.21 eV as Bi increases from0 to 5 at.% in Te15(Se100−x Bix )85 glasses. Similar trends inoptical band gap have also been observed by earlier workers[28, 32]. Various optical parameters reported in tables 1 and 2for x = 0 composition are in good agreement with earlierreported work [30–33]. The optical band gap decreases withan increase in Bi content for the considered thin films. Thisdecrease in the optical band gap is related to a decrease incohesive energy (CE) [34] as less CE means lower bondingstrength and hence a decrease in optical band gap. Further,the decrease in optical band gap with an increase in Bicontent is probably due to the larger electronegativity of Bithan that of Se. The lone pair p-orbitals form the valenceband [35]. The energy of lone pair states is raised due tothe addition of an electropositive atom, which broadens thevalence band inside the forbidden gap. Electronegativities ofSe, Te and Bi are 2.55, 2.1 and 2.0, respectively. Since Bi ismore electropositive than Se, replacing Se by Bi may raise the

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

8

9

10

11

12

13

14

ε r

hν (eV)

x=0 x=1 x=2 x=3 x=4 x=5

Figure 6. The variation of the real part of the dielectric constant(εr) with photon energy for Te15(Se100−x Bix )85 (x = 0, 1, 2, 3, 4 and5 at.%) thin films.

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.00.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

ε ι

hν (eV)

x=0 x=1 x=2 x=3 x=4 x=5

Figure 7. The variation of the imaginary part of the dielectricconstant (εi) with photon energy for Te15(Se100−x Bix )85 (x = 0, 1, 2,3, 4 and 5 at.%) thin films.

energy of some lone pair states and broaden the valence band,which leads to a decrease in optical band gap.

The complex dielectric constant is a fundamental intrinsicproperty of the materials. The real and imaginary parts ofthe dielectric constant of the thin films have been calculatedusing the refractive index and extinction coefficient. The realpart of the dielectric constant is calculated using the relationεr= n2

− k2, whereas the imaginary part of the refractiveindex is calculated using the relation εi = 2nk. The variationof εr and εi with photon energy for the films is shown infigures 6 and 7, respectively.

The dielectric constant of a material affects the movementof electromagnetic signals through the material. A high valueof dielectric constant makes the distance inside the materiallook greater, thereby slowing the velocity of light. The realpart of the dielectric constant describes how the material willslow down the speed of light, whereas the imaginary partdescribes how the dielectric absorbs energy from the electricfield due to dipole motion. From figures 6 and 7, it is clearthat both the real and imaginary parts of dielectric constant

5

Phys. Scr. 84 (2011) 045703 K Kumar et al

increase with an increase in photon energy. This increase inreal and imaginary parts of the dielectric constant with photonenergy may be due to an increase in the absorption coefficient.

4. Conclusions

The refractive index of thermally evaporated thin films ofTe15(Se100−x Bix )85 (x = 0, 1, 2, 3, 4 and 5 at.%) glasses hasbeen found to increase with an increase in Bi content, which isrelated to the increased polarizability of larger Bi atoms. Theaverage band gap energy (Eo) decreases with an increase inBi content and the static refractive index (no) increases withan increase in Bi content. The optical band gap calculatedusing Tauc’s extrapolation method has been found to decreasewith Bi content from 1.37 to 1.21 eV. The decrease in averageband gap energy and optical band gap is interpreted in termsof the CE of the glasses and the electronegativity differenceof the atoms involved. The real and imaginary parts of thedielectric constant are found to follow the trends followed bythe refractive index and extinction coefficient, respectively.

Acknowledgments

KK thanks the UGC, Delhi, India for providing a teacherfellowship under FIP. NT thanks the UGC and DSTfor providing necessary instrumentation facilities in theDepartment of Physics at HP University, Shimla in theframework of the SAP and FIST programmes.

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