Applied Surface Science 255 (2008) 2791–2795

Effect of Bi addition on the optical behavior of a-Ge–Se–In–Bi thin films

Ishu Sharma a,*, S.K. Tripathi b, P.B. Barman a

a Department of Physics, Jaypee University of Information Technology, Waknaghat, Solan, HP 173215, Indiab Department of Physics, Panjab University, Chandigarh, India

A R T I C L E I N F O

Article history:

Received 5 May 2008

Received in revised form 6 August 2008

Accepted 9 August 2008

Available online 19 August 2008

Keywords:

Chalcogenide glasses

Thin films

Refractive index

Optical band gap

A B S T R A C T

An optical study of vacuum evaporated (10�4 Pa) Ge20Se70–xIn10Bix (x = 2, 4, 6, 8, 10) thin films is reported

in the present work. The optical constants viz. refractive index (n) and extinction coefficient (k) have been

accurately determined by envelope method using transmission spectra in the range of 400–1800 nm. The

dispersion of the refractive index of a-Ge–Se–In–Bi thin films is analyzed in terms of the Wemple–

DiDomenico single-effective-oscillator model. Refractive index increases with increase in Bi content,

whereas optical band gap (calculated using Tauc plot) decreases significantly from 1.63 eV to 0.87 eV. The

variation in the optical behavior for different thin films is explained on the basis of defect states and the

decrease in average bond energy of the system. The dielectric constants (er and ei) and optical

conductivity (s) of the thin films are also reported.

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1. Introduction

Being a glass, chalcogenides are versatile platform. The freedomallowed in the preparation of glasses in varied compositions bringsabout changes in their short range order and thus results invariation of their physical properties. Therefore it is possible totailor their various properties, as desired for technologicalapplications. They have well defined niches in fiber optics, opticalrecording, switching, regeneration, wavelength conversion, ampli-fication etc. Besides the wide commercial/device applications likeswitching memory and xerography etc. of Se, it also exhibits aunique property of reversible transformation [1–5]. This veryproperty makes it very useful in optical memory devices.

Ge–Se system is a widely studied system. On addition of Ge toSe, Ge atoms act as bond modifiers; they strengthen the averagebond by cross linking the Se chain structure, thereby increasing theglass transition temperature and resistivity [6] and also overcomesome difficulties of pure Se, like short life time and low sensitivity.The Ge20Se80 glassy alloy lies at the threshold of mode change i.e.floppy to intermediate region [7]. The addition of Indium (In) toGe20Se80 system in an effective way, control it’s electrical andoptical properties which leads the system towards intermediateregion. Moreover, Ge–Se–In system is of special interest because ofthe fact that it forms glasses over a wide domain of compositions

* Corresponding author.

E-mail addresses: ishuphy@gmail.com (I. Sharma), surya@pu.ac.in

(S.K. Tripathi), parthabarman@yahoo.com (P.B. Barman).

0169-4332/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2008.08.011

[8]. The incorporation of In to Ge–Se alloy expands the glassforming area and also creates compositional and configurationaldisorder in the system. Optical and electrical study of Ge20Se80–xInx

(x = 0, 5, 10, 15, 20) thin films has already been reported by Ishuet al. [9,10]. The non-linear and photosensitive properties togetheroffer numerous applications to a number of integrated opticalfunctions. Since Ge20Se70In10 system shows maximum value ofoptical band gap and photosensitivity, so in continuation authorshave decided to study the effect of Bi addition to a-Ge–Se–Insystem for their optical behavior. Since Bi addition to Ge–Se systemincreases the chemical durability, the replacement of Se with Bi inGe–Se host matrix leads to the decrease in optical band gap [11,12]and significantly broadened the transparency range towards NIRregion. Maximum of Bi content which can be incorporated to Ge–Se matrix (when Ge content is 20 at. %) is only 13 at. % [13]. It is welldocumented that the incorporation of about 6–9 at. % Bi into theglass matrix of Ge–Se leads to the change of the conductivity fromp to n type [14]. Earlier experiments have shown that the additionof Bi in Ge–Se system brings down the optical band gap upto 1.2 eVfor n-type conducting samples, resulting in an enhancement ofphotoconduction for n-type samples [15,16].

Knowledge of the optical properties of these amorphousmaterials is obviously necessary for exploiting their interestingtechnological potential. The present work deals with determina-tion of optical band gap ðEopt

g Þ, absorption coefficient (a), refractiveindex (n) and extinction coefficient (k) for Ge20Se70–xIn10Bix (x = 2,4, 6, 8, 10) thin films by analyzing transmission spectra in thewavelength range from 400–1800 nm. The dispersion of refractiveindex was analyzed with Wemple–DiDomenico single oscillator

Fig. 1. Plot of Transmission versus wavelength (nm) for Ge20Se70–xIn10Bix (x = 2, 4, 6,

8, 10) thin films.

I. Sharma et al. / Applied Surface Science 255 (2008) 2791–27952792

model. The dielectric constants and optical conductivity are alsodetermined using n, k and a.

2. Experimental details

Bulk samples of the Ge20Se70–xIn10Bix (x = 2, 4, 6, 8, 10)chalcogenide system are prepared using melt quench technique.The materials (Ge, Se, In and Bi) (5 N pure) are crushed to powderform and then sealed in evacuated (�10�4 Pa) quartz ampoules,each of 4 g batch, weighed according to their atomic weightpercentage. The sealed ampoules are kept inside a furnace wherethe temperature is increased up to 1000 8C gradually at a heatingrate of 2–3 8C/min. The ampoules are frequently rocked for 24 h atthe highest temperature to make the melt homogeneous. Thequenching is done in ice cold water. Thin films of bulk samples areprepared on cleaned glass substrates (microscopic glass slides).The substrates are cleaned with soap solution, ultrasonicallycleaned by trichloroethylene, acetone followed by methyl alcohol.In last the substrates are washed by DI water and dried in oven atapproximately 110 8C. Thin films of the alloys are prepared bythermal evaporation technique [vacuum coating unit HINDHIVAC12A4D Model] at room temperature and base pressure of�10�4 Pausing a molybdenum boat. The thickness of the deposited thinfilms has been measured by thickness monitor (DTM-101). Thecompositions of evaporated samples have been measured by anelectron microprobe analyzer (JEOL 8600 MX) on different spots(size �2 mm). For the composition analysis, the constitutionalelements (Ge, Se, In and Bi) and the bulk original alloys i.e.Ge20Se70–xIn10Bix are taken as reference samples. The compositionof 2 � 2 cm2 sample is uniform within the measurement accuracyof about � 2-3%. Amorphous nature of the bulk samples and thinfilms are checked by XRD technique. No prominent peak is observedin the bulk as well as in thin films. The normal incidence transmissionspectra of Ge20Se70–xIn10Bix thin films are obtained by a double beamUV–vis-near infrared spectrophotometer [Perkin Elmer Lambda 750],in the transmission range 400–1800 nm. All optical measurementsare performed at room temperature (300 K).

3. Results and discussion

Optical transmission spectra of Ge20Se70–xIn10Bix thin films aregiven in Fig. 1. A noteworthy red shift is observed in theinterference free region of spectra with Bi addition. The valuesof the thickness (d), refractive index (n), extinction coefficient (k)and absorption coefficient (a) for all the films subjected to studyhave been determined only from their normal-incidence transmis-sion spectra, using the envelope method given by Swanepoel [17],which is based on Manificier et al. [18].

3.1. Determination of refractive index and extinction coefficient

The values of the thickness (d), refractive index (n), extinctioncoefficient (k) of the thin films are obtained by the envelope

Table 1Values of thickness (d*) and (d), refractive index (n), extinction coefficient (k), absorpt

constant (ei), and optical conductivity (s) are given at 800 nm, oscillator strength (E0), dis

Ge20Se70In10 [9] and Ge20Se70–xIn10Bix (x = 2, 4, 6, 8, 10) thin films

x d* � 25(nm) d(nm) n k Ed (eV) E0 (eV)

0 700 777 2.76 0.0098 54.22 7.15

2 700 679 3.18 0.0242 35.78 4.44

4 700 723 3.26 0.0283 32.76 3.96

6 700 692 3.35 0.0396 34.21 3.92

8 700 707 3.44 0.0537 33.67 3.72

10 700 695 3.50 0.0889 32.69 3.54

method proposed by Swanepoel [17] and is described in detailedform elsewhere [9].

The thickness of the thin films calculated from the transmissionspectrum is reported in Table 1. It is in consistent with thethickness (d*) measured directly from the thickness monitor whiledepositing the films. The differences between the mechanicallymeasured and the optically-calculated values varied within 2–3%which shows the precision of reported results. The variation ofrefractive index (n) and extinction coefficient (k) with wavelengthis shown in Figs. 2 and 3, respectively. From the Figs. 2 and 3, it isclear that both refractive index and extinction coefficient decreasewith the increase of wavelength for the films. This decrease in thevalue of the refractive index with wavelength attributes to thesignificant normal dispersion behavior of the material. The Fig. 2also indicates that with the increase of Bi content in Ge20Se70–

xIn10Bix thin films, refractive index increases for the entirewavelength range under investigation. This high refractive indexvalue is advantageous for strong optical field confinement whichallows small waveguide bend radii (leading to compact circuit

ion coefficient (a), real part of dielectric constant (er), imaginary part of dielectric

persion energy (Ed), static refractive index (n(0)), optical band gap ðEoptg Þ are given for

n(0) a (cm�1) Eoptg ðevÞ er ei s(s�1) � 1013

2.93 1718.1 1.75 7.62 0.054 0.53

3.01 3801.5 1.63 10.12 0.153 2.88

3.04 4447.4 1.39 10.68 0.185 3.52

3.12 6222.4 1.27 11.24 0.265 4.97

3.17 8443.3 1.16 11.84 0.369 6.93

3.19 13983.1 0.87 12.24 0.623 11.6

Fig. 2. Plot of refractive index versus l (nm) for Ge20Se70–xIn10Bix (x = 2, 4, 6, 8, 10)

thin films. Fig. 3. Plot of extinction coefficient versus l (nm) for Ge20Se70–xIn10Bix (x = 2, 4, 6, 8,

10) thin films.

I. Sharma et al. / Applied Surface Science 255 (2008) 2791–2795 2793

designs) and enhanced optical intensities (for efficient non-linearinteractions). El Samanoudy [12] has also reported that refractiveindex increases with increase of Bi content. The increase in therefractive index with the increase in Bi content at 800 nm isreported in Table 1 and can be attributed to the increase in valenceof current carriers and also presumable due to the increasedpolarizability [19] associated with the larger Bi atom. Larger theatomic radius of the atom, larger will be its polarizability andconsequently, according to Lorentz-Lorenz relation [20] (equation1), larger will be the refractive index.

n2 � 1

n2 þ 2¼ 1

3e0

Xi

Niapi (1)

where e0 is the vacuum permittivity, Ni is the number ofpolarizable units of type i per unit volume with polarizabilityapi. The atomic radius of Se is 1.16 A and Bi is 1.46 A. On the otherhand, a clear red-shift of the optical absorption edge is observed inFig. 1, with increasing Bi content to the host matrix. It is stressedthat, following the fundamental Kramers–Kronig relationships, thered-shift in the absorption spectrum must necessarily give anincreased refractive index value. It is worth to quote here thefundamental relationship, nð0Þ � 1 ¼ ð1=2p2Þ

R10 a dl, as experi-

mentally obtained [19] which allows checking the consistency ofthe values of refractive index. Lezal [21] has also correlated therefractive index with transparency range. As the transmissionrange increases, corresponding to it refractive index also increases.This increase in the refractive index with increase of Bi contentmay be ascribed to the addition of heavier Bi (density = 9.8 g/cc at20 8C) atoms to Se (density = 4.79 g/cc at 20 8C) matrix leading tomake the system more compact.The spectral dependence (dis-persion) of refractive index values for each sample have been fitted

to the single-oscillator model of Wemple and DiDomenico [22,23].

n2 � 1 ¼ EdE0

E20 � ðhnÞ2

(2)

where hn is the photon energy, n is refractive index, E0 is averageenergy gap, also called ‘WDD gap’ and Ed is the dispersion energy.The latter quantity (Ed) measures the average strength of theinterband optical transitions. Plotting refractive index factors (n2–1)�1 against (hn)2, for all the thin films under study, given in Fig. 4,allows us to determine the oscillator parameters by good linear fitsto the larger wavelength data. The value of E0 and Ed can be directlydetermined from the slope (E0Ed)�1 and the intercept (E0/Ed) on thevertical axis. The calculated values of E0 and Ed for all thecompositions are given in Table 1. E0 is related to the bond energyof the different chemical bonds present in the material. Thus, thedecrease observed in the values of E0 with addition of Bi content isdue to the lower bond energies of Bi–Se (40.7 kcal/mol) and Bi–Bi(25.0 kcal/mol) as compared to Ge–Se (49.1 kcal/mol), In–Se(48.2 kcal/mol) and Se–Se (44 kcal/mol) bonds. According toTanaka [24], the single-oscillator energy E0 is approximately equalto twice the optical band-gap Eopt

g . This is quite reasonablysupported by our results also.The value of static refractive indicesn(0) for Ge20Se70–xIn10Bix thin films have been calculated fromWDD dispersion parameters E0 and Ed by using the formula

nð0Þ ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ Ed

E0

s(3)

The values of n(0) are calculated by extrapolating the WDDdispersion equation to hn! 0 (Eq. (2) and Fig. 4). The values of n(0)and high frequency dielectric constant ev = n(0)2 are reported inTable 1. It is clear from the table that with the increase of Bi content

Fig. 4. Plot of (n2–1)�1 versus (hn)2 for Ge20Se70–xIn10Bix (x = 2, 4, 6, 8, 10) thin films.

Fig. 5. Plot of (ahn)1/2 versus hn for Ge20Se70–xIn10Bix (x = 2, 4, 6, 8, 10) thin films.

I. Sharma et al. / Applied Surface Science 255 (2008) 2791–27952794

n(0) increases as expected because of relatively high covalentradius of Bi.

3.2. Determination of absorption coefficient and optical band gap

The absorption coefficient (a) of Ge20Se70–xIn10Bix thin filmscan be calculated using the well-known relation

a ¼ 1

d

� �ln

1

X

� �(4)

where X is the absorbance and d is the thickness. The optical bandgap Eopt

g , has been estimated, according to generally accepted ‘non-direct transition’ model for amorphous semiconductor, fromabsorption coefficient data as a function of wavelength by usingTauc relation [25]

ðahnÞ1=2 ¼ B1=2ðhn� Eoptg Þ (5)

where hn is the photon energy,a is the absorption coefficient, Eoptg

the optical band gap, B1/2 is the Tauc Slope. This relationshipassumes that the densities of the electron states in the valence andconduction bands, near the band gap, have a parabolic distributionand the matrix elements for the interband transitions associatedwith the photon absorption are equal for all the transitions. Fig. 5shows the variation of ðahnÞ1=2 with hn. Optical band gap Eopt

g canbe determined by the extrapolation of best fit line betweenðahnÞ1=2 and hn to intercept the hn axis (a = 0) for Ge20Se70–xIn10Bix

system. Variation of optical band gap with different Bi concentra-tion is given in Table 1. It is clear from the table that with theincorporation of Bi content, optical band decreases from 1.63 eV to0.87 eV. Bi impurities induce structural modifications in the parentmatrix which are clearly reflected in the optical band gap values. Itis also suggested by Kitao et al. [26], that the decrease in the band-gap is caused by an alloying effect, namely a compositional change

in the host material itself. This effect is due to the change in bondangles and/or bond lengths. Bi addition to Ge–Se system gives riseto the reduction in the width of two broader transitions. This isnatural if we recall that the broader transitions are assigned torandom VAPs (D+ and D�) which are more likely to get influencedby the external dopants unlike IVAPs (non-random D+ and D�). Thelowering of energy of transitions associated with random VAPs ismight be due to higher atomic radii of Bi atoms added which canpush the corresponding energy levels away from the band edges,towards the Fermi level. The lowering of band gap with the Biaddition to Ge–Se–In matrix can also be explained in accordancewith Mott and Davis model [27] which states that the width of thelocalized states near the mobility edges depends on the degrees ofdisorder and the defects present in the amorphous structure. The Biincorporation to Ge–Se–In matrix thus tends to increase mono-tonically the degree of disorder in the glassy structure. Theaddition of Bi leads to Bi–Se bonding which introduces the largenumber of defects in the system [28]. Moreover this reduction inoptical band gap of the system is also reflected in the reductionin average bond energy with the increase in Bi content (seeSection 3.1).

3.3. Determination of dielectric constants and optical conductivity

The dielectric constant of a-Ge20Se70–xIn10Bix thin films can becalculated with the help of refractive index (n) and extinctioncoefficient (k) [29]. While real dielectric constant (er) can becalculated from the relation er = n2 � k2and the imaginary dielec-tric constant (ei) can be calculated from the following relationer = 2nk. For Ge20Se70–xIn10Bix thin films the variation of both realand imaginary dielectric constants (er and ei) with energy (results

I. Sharma et al. / Applied Surface Science 255 (2008) 2791–2795 2795

not shown) follows the same trend as that of refractive indexand extinction coefficient. Optical response is most convenientlystudied in terms of optical conductivity. Frequency (energy)dependent optical conductivity ‘s’ obtained from absorptioncoefficient (a), velocity of light (c) and refractive index (n) andis given by the relation [30] i.e. s = anc/4p. It has the dimensions offrequency which are valid only in Gaussian system of units.s isalmost proportional to square of frequency/energy which indicatesthat the system may be ascribed by a single relaxation time[31]. The optical conductivity directly depends on absorptioncoefficient and refractive index and is found to increase sharply forhigher energy values due to large absorption coefficient andrefractive index for these values. The values of er, ei and s at 800 nmwavelength are given in Table 1.

4. Conclusions

Optical properties of Ge20Se70�xIn10Bix (x = 2, 4, 6, 8, 10)vacuum evaporated thin films are studied in the wavelength range400–1800 nm, from their normal incidence transmission spectra.Absorption mechanism is due to indirect transition. It is foundthat optical energy gap decreases monotonically whereas therefractive index increases with the increase in Bi content. Thisincrease in refractive index is quite favourable for fabricatingoptical waveguides leading to compact circuit design with highintensities. These results also indicate an increase in chemicaldisorder and a decrease in average bond energy of the systemwith increasing Bi content. The refractive index values for eachsample have been fitted to the single-oscillator model. The valuesobtained for the single-oscillator energy E0 are consistent with ouroptical gap results. The dielectric constants and optical conduc-tivity are found to decrease with the increase in wavelength forentire wavelength range under investigation.

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