ARTICLE IN PRESS

Physica B 404 (2009) 1591–1594

Contents lists available at ScienceDirect

Physica B

0921-45

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/physb

Effect of Bi incorporation on defect state density in a-Se85�xTe15Bix thin films

Ambika Sharma �, P.B. Barman

Department of Physics, Jaypee University of Information Technology, Waknaghat, Solan, H.P. 173215, India

a r t i c l e i n f o

Article history:

Received 14 August 2008

Received in revised form

13 January 2009

Accepted 20 January 2009

PACS:

61.43 Dq

81.15 Ef

71.55 Jv

Keywords:

Chalcogenide glasses

Space charge limited conduction

Density of states

26/$ - see front matter & 2009 Elsevier B.V. A

016/j.physb.2009.01.026

esponding author. Tel.: +919817 164795; fax:

ail address: ambikasharma2004@yahoo.co.in

a b s t r a c t

Current voltage (I–V) characteristics have been studied at various temperatures in vacuum evaporated

thin films of a-Se85�xTe15Bix (x ¼ 0, 1, 2, 3, 4, 5) glassy alloys. Ohmic behavior is observed at low electric

field while at high electric field (E�104 V/cm) current becomes superohmic. An analysis of the

experimental data reveals that in case of samples with 0–2 at% Bi, the experimental data fit well with

the theory of space charge limited conduction (SCLC) for uniform distribution of localized states in the

mobility gap. Such type of behavior is not observed at higher concentration of Bi in the present glassy

system. Due to high conductivity in these samples, joule heating due to large currents may prohibit the

SCLC mechanism. Density of localized defect states near Fermi level is determined for these samples by

fitting the data in the theory of SCLC. The results indicate that density of defect states increases with Bi

addition to binary Se85Te15 alloy. This is explained in terms of electronegativity difference of Bi as

compared to host elements.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

In contrast to amorphous silicon and other group IV tetra-hedrally bonded semiconductors, the chalcogenide glasses haveattracted much attention due to their interesting optical, electricaland physical properties which can be controlled by changing theirchemical composition [1,2]. This property of chalcogenide glassesmakes these materials useful for technological applicationsincluding phase change memories [3]. Amorphous selenium (Se)rich semiconducting alloys are of particular interest due to theircurrent uses as photoconductors in high definition TV pick uptubes and particularly in digital X-ray imaging. When Te is addedin a-Se, it enters the structure by an isoelectronic substitution sothat its coordination remains two. Addition of Te brings aboutchanges in Vander Waals bonds or interchain secondary bonds,because the Te atom is larger than the Se atom and has moreelectrons in its orbital. One can therefore argue that increase insecondary bonding between chains is due to Te addition. Teaddition will also increase the valence alteration pair (VAP) typedefects [4], connecting neighboring chains and limiting molecularmobility. The composition Se85Te15 has been studied by severalauthors [5–7], because of its ease of fabrication in the form of bulkand thin films. The glassy alloys of the Se–Te system are widelyused for various applications in many fields as optical recording

ll rights reserved.

+911792 245362.

(A. Sharma).

media because of their excellent laser writer sensitivity, xero-graphy and electrographic applications such as photoreceptors inphotocopying and laser printing, infrared spectroscopy and laserfiber techniques [8–10]. Moreover the technological application ofSe–Te glassy alloy enhances due to their greater hardness, highercrystallization temperature and higher photosensitivity [11]. Theaddition of third element expands the glass forming area and alsocreates compositional and configurational disorder in the system.Several authors [12–14] have reported the effect of impurities invarious chalcogenide glasses. The addition of impurities like Bihas produced a remarkable change in the optical, electrical andthermal properties of chalcogenide glasses especially in Se85Te15

glassy alloy. While the optical band gap of Se85Te15 binary alloydecreases with Bi incorporation, the glass transition temperatureis found to increase [15]. Density of the system also found toincrease with increase in Bi concentration [16], indicating therigidity and hence density of defect states (DOS) also increase.The conductivity of chalcogenide glasses changes from p to n typedue to Bi addition [17–20]. The effect of this impurity on DOS hasalways been a subject of curiosity, as the knowledge of thisparameter is a key factor in chalcogenide glasses for determiningthe semiconducting properties of these materials.

One of the most useful method for the determination of DOSinvolves the measurement of space charge limited conduction(SCLC). In chalcogenide glasses, the DOS reported by differentauthors [21–25] ranges from 1013 to 1017 eV�1 cm�3. The presentpaper reports the SCLC measurements in Se85�xTe15Bix (x ¼ 0, 1, 2,3, 4, 5) thin films.

ARTICLE IN PRESS

-18.2

-18.1

-18.0

-17.9

-17.8

-17.7

-17.6

-17.5

ln (

I/V

)Ω-1

303 K308 K313 K318 K323 K328 K333 K

A. Sharma, P.B. Barman / Physica B 404 (2009) 1591–15941592

2. Experimental details

Glasses of Se85�xTe15Bix, where x ¼ 0, 1, 2, 3, 4, 5 were preparedby the melt quenching technique. The materials (5 N pure) wereweighed according to their atomic percentages and sealed inevacuated (at �10–4 Pa) quartz ampoules. The sealed ampouleswere kept inside a furnace where the temperature was increasedup to 1000 1C at a heating rate of 3–4 1C min–1. The ampoules werefrequently rocked for 15 h at the highest temperature to make themelt homogeneous. The quenching was done in ice-cold water.Glasses were obtained by breaking the ampoules.

Thin films of the alloys are prepared by thermal evaporationtechnique at �10�4 Pa base pressure on properly cleaned glasssubstrates in a vacuum coating system (HINDHIVAC 12A 4Dmodel). The compositions of evaporated films were analyzed byan electron microprobe analyzer (JEOL 8600 MX) on the differentspots (size �2mm) on the films. For the compositional analysis,the constitutional elements (Se, Te and Bi) and the bulk originalalloys, i.e. Se85�xTe15Bix, were taken as reference samples. Thecompositions of a 2�2 cm2 samples were found to be uniformwithin the measurement accuracy of about 72%. Well degassedcorning glass plates, having predeposited indium electrodes wereused as a substrate for depositing amorphous films in the planergeometry (length �1.2 cm and electrode gap �0.1 cm). Thethickness of the films was kept constant (�5000 A). The filmswere annealed below their glass transition temperature in orderto attain a metastable thermodynamic equilibrium and then keptin dark for about 24 h before mounting them in the sample holder.

For the measurement of SCLC the samples were mounted in aspecially designed metallic sample holder, in which a vacuum�10�3 Torr can be maintained throughout the measurement. A DCvoltage (0–300 V) was applied across the sample and the resultingcurrent was measured by a digital picometer (Keithley, model6487). Current–voltage (I–V) characteristics were recorded atvarious fixed temperature (303–333 K) for all the glassy alloysunder study. The temperature was measured by mounting acopper-constant thermocouple near to the sample.

-50-18.5

-18.4

-18.3

Se85Te15

V (volt)

350300250200150100500

3. Results and discussion

Fig. 1 shows the typical I–V plots for Se85�xTe15Bix glassy alloyat 303 K. While an ohmic behavior is observed at low electricfields (Eo103 V/cm), a superohmic behavior is observed at highfields (E–10�4 V/cm) in all the samples. ln (I/V) vs. V curves are

0

0.000000

0.000007

0.000014

0.000021

0.000028

0.000035T = 303 K

I (A

)

V (volt)

x = 0

x = 1

x = 2

x = 3

x = 4

x = 5

30025020015010050

Fig. 1. Plot of I–V characteristics for Se85�xTe15Bix (x ¼ 0, 1, 2, 3, 4, 5) at 303 K

temperature.

found to be straight lines at all measuring temperatures(303–333 K) as shown in Fig. 2a, b for x ¼ 0 and 3. Similar resultshave been also been obtained in other glassy alloys (result notshown here). It is clear from figures that the slope S of ln (I/V) vs. V

curves varied at all measuring temperatures. At lower bias theinjected charge carrier density is lower than the thermallygenerated carrier density and that leads to the ohmic behavior.At higher bias it may be suggested that the conduction in thisregion is dominated by a trap limited SCLC mechanism. Accordingto the theory of SCLC; in the case of uniform distribution oflocalized states the current I is related to the voltage V by theequation [26]

I ¼ eAmn0V=d� �

expðSVÞ (1)

where d is the electrode spacing, n0 is the density of thermallygenerated charge carriers, m is the mobility, e is the electroniccharge, A is the area of cross section of films and S is given by

S ¼ 2�0�r=eg0kTd2 (2)

where g0 is the density of localized states near Fermi level, er is therelative dielectric constant, e0 is the permittivity of free space andk is the Boltzman constant.

As is evident from Eqs. (1) and (2), for SCLC, the ln (I/V) vs. V

curves should be a straight line and slope S, of these curves should

-50

-18.5

-18.0

-17.5

-17.0

-16.5

-16.0

-15.5

Se82Te15Bi3.0

ln (

I/V

)Ω−1

V (volts)

303 K

308 K

313 K

318 K

323 K

328 K

333 K

350300250200150100500

Fig. 2. (a, b) Plot of ln(I/V) vs. V for Se85�xTe15Bix (x ¼ 0, 3) alloy at different

temperatures.

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A. Sharma, P.B. Barman / Physica B 404 (2009) 1591–1594 1593

be inversely proportional to the temperature. In the presentsamples under study the slope of these curves is inverselyproportional to the temperature for x ¼ 0, 1 and 2 (Fig. 3a–c).However, at higher concentration of Bi (42), S increases withincrease in temperature (Fig. 3d–f) which is against the phenom-enon of SCLC. In these samples (x42) Joule heating due to highconductivity may be predominant which probably prohibits themeasurement of SCLC. We have therefore chosen Se85Te15,Se84Te15Bi1.0 and Se83Te15Bi2.0 samples for further analysis. Similarresults have also been observed by Kushwaha et al. [6] whilestudying the effect of Pb incorporation on the DOS in Se85Te15

glassy alloy. The SCLC mechanism in this composition is alsoobserved upto 2 at% of Pb. The effect of incorporation of somemetallic impurity in Se–Te glassy system have been studied byvarious authors [27–29] and their results have been interpreted interms of change in density of localized states near Fermi level.Khan et al. [24] found that DOS increases with Bi incorporation inSe78Te22 glassy alloy. DOS is also found to increases with Snincorporation in Se85Te15 glassy alloy [5]. The increase in DOSwith these two additives is explained in terms of electronegativitydifference of atoms involved. On the other hand Kumar et al. [30]found that DOS decreases with Ge incorporation in Se85Te15 glassy

0.00056

0.00063

0.00070

0.00

0.00

0.00

0.00

3.0

0.00008

0.00012

0.00016

0.00020

0.

0.0

0.0

0.0

3.0

0.0028

0.0032

0.0036

0.0

0.0

0.0

0.0

x = 2

x = 0

x = 4

S (v

olt-1

)

1

3.1 3.2 3.3

3.1 3.2 3.3

3.0 3.1 3.2 3.3

Fig. 3. (a–f) Plot of slope S vs. 1000/T for S

Table 1Value of slope of S vs. 1000/T, relative dielectric constant er, and density of defect state

Samples Slope of S vs. (1000/T) curve

Se85Te15 6.02�10�4

Se84Te15Bi1.0 4.41�10�4

Se83Te15Bi2.0 3.41�10�4

alloy. They analyzed that the excess carriers experience longerlifetime with higher Ge concentration. Longer lifetime furtherindicates lower recombination rate for the excess carriers andhence gives rise to lesser number of DOS.

Eq. (2) enables us to calculate the value of density of localizedstates by using the slope of Fig. 3. The value of relative dielectricconstant er are measured by using capacitance measuringassembly model GR 1620 AP, employing the three terminaltechnique. The value of er and the calculated value of g0 aretabulated in Table 1. It is clear from table that density of statesincreases with Bi addition.

Chalcogenide materials have a large number of defects due todangling bonds that give rise to a large number of localized defectstates. These localized states act as carrier trapping centers andafter trapping the injected charge from electrodes, they becomecharged and thus are expected to build-up a space charge. Thisbuild-up of space charge by the inherent native defects plays thekey role in the determination of SCLC process. When isoelectronicatom Te is added to the amorphous Se, the DOS is increased. Theresults are well explained on the basis of electronegativitydifference of the atoms involved [31]. Te is assumed to formpositively charged impurities due to its lower electronegativity

3.0

054

060

066

072

3.0

0040

048

056

064

3.0

009

018

027

036

x = 1

x = 3

x = 5

000/T (K-1)

3.1 3.2 3.3

3.1 3.2 3.3

3.1 3.2 3.3

e85�xTe15Bix (x ¼ 0, 1, 2, 3, 4, 5) alloy.

s g0, for Se85�xTe15Bix glassy system.

er (120 Hz, 303 K) Density of states g0 (eV�1 cm�3)

9.63 2.05�1012

11.13 3.2�1012

13.87 5.21�1012

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(2.1) as compared to Se (2.4). When the third element (Bi in ourcase) is introduced to the Se–Te system, the atoms of thirdelement induce structural changes in the host network. This leadsto readjustment in the local environment and results in the shiftof Fermi level. Also on the basis of electronegativity since Bi is lesselectronegative (2.0) than Se, it further increases the concentra-tion of positively charged defects and thus increasing the DOS.

4. Conclusion

I–V characteristics have been studied at various temperaturesin vacuum evaporated thin films of a-Se85�xTe15Bix (x ¼ 0, 1, 2, 3,4, 5) glassy alloys. An ohmic behavior is observed at low electricfield (o103 V/cm) whereas at high fields (104 V/cm) currentbecomes superohmic. The SCLC mechanism holds well in thesamples with 0–2 at % Bi. The density of defect states near Fermilevel is calculated by fitting the data to the theory of SCLC in caseof uniform distribution of localized states. The addition of Biimpurity in a-Se85Te15 increases the density of defect states. Thisis explained according to structural defect model based on theelectronegativity difference. At high concentration of Bi, SCLC isnot observed probably due to higher conductivity. Due to largecurrents at high voltage in these samples (x42), Joule heatingmay be a predominant factor which prohibits the measurement ofSCLC at higher concentration of Bi.

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