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Vacuum 75 (2004) 313–320

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Delhi.

*Correspondin

E-mail addre

0042-207X/$ - see

doi:10.1016/j.vac

Effect of some metallic impurities on the density oflocalized states in a-Se80Te20 thin films$

S.P. Singha, S. Kumara,*, A. Kumarb

aDepartment of Physics, Christ Church College, Kanpur 208001, IndiabDepartment of Physics, H.B. Technological Institute, Kanpur 208002, India

Abstract

The present paper reports the d.c. conductivity measurements at high electric fields in vacuum-evaporated thin films

of amorphous Se80Te20, Se75Te20Ge5 and Se75Te20Sb5 systems. Current–voltage (I–V) characteristics have been

measured at various fixed temperatures. In all the samples, at low electric fields ohmic behavior is observed. However,

at high electric fields (EB104V/cm), non-ohmic behavior is observed. An analysis of the experimental data confirms the

presence of space charge limited conduction (SCLC) in all the glassy materials studied in the present case. From the

fitting of the data to the theory of SCLC, the density of defect states (DOS) near Fermi level is calculated. The role of

the aforesaid impurities in a-Se80Te20 is found to be entirely different. In case of Sb, an increase in DOS is observed.

However, a decrease is observed in case of Ge. The change in DOS on impurity incorporation is explained in terms of

the change in structure of these glasses.

r 2004 Elsevier Ltd. All rights reserved.

Keywords: Thin films; Chalcogenide glasses; SCLC; DOS

1. Introduction

Se–Te alloys have gained much importanceamong chalcogenide glasses because of their higherphotosensitivity, greater hardness, higher crystal-lization temperature and smaller aging effects ascompared to pure Se glass. Because of theseadvantages, these alloys are now-a-days preferredin xerography [1]. Attempts have been made to

rted by University Grants Commission, New

g author. Tel.: +91-512-2573069.

ss: satya12m@hotmail.com (S. Kumar).

front matter r 2004 Elsevier Ltd. All rights reserv

uum.2004.04.002

produce stable glasses, which have good photo-sensitive properties and can be doped n- or p-type.The effect of incorporation of third element inbinary chalcogenide glassy alloys has alwaysbeen an interesting problem in getting relativelystable glassy alloys as well as to change theconduction from p to n as most of these glassesshow p-type conduction only. In Ge–Se and Se–Insystems, some metallic additives have been found[2–7] to change conduction from p- to n-typeand hence these binary systems are of greatimportance.The effect of incorporation of Ge and Sb as an

impurity in Se–Te binary glassy system has been

ed.

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S.P. Singh et al. / Vacuum 75 (2004) 313–320314

studied by various workers working in this field[8–10] and their results have been interpreted interms of change in the density of localized states(DOS) near Fermi level. Direct measurements ofthe DOS in the aforesaid glassy system have notbeen reported so far. The present work contributesan attempt in this direction. We have used spacecharge limited conduction (SCLC) as a tool formeasuring the DOS in some of the glassy system.As high-field effects are also most readily observedin these materials because of their low conductivity(Joule heating is negligibly small at moderatetemperatures) and have been studied by variousgroups working in this field [11–18]. The result ofthese workers have been interpreted in terms ofheating effect, SCLC and high-field conductiondue to the Poole–Frenkel effect. This indicates thatthe interpretation of the high-field data is highlyintriguing in these materials and much has to bedone in this field.It is interesting to note that SCLC for a uniform

distribution of traps as well as the high-fieldconduction theories mentioned above lead to asimilar kind of field dependence of the conductiv-ity at different temperatures. To distinguishbetween these two processes, the measurementson samples having different electrode gaps aretherefore necessary.In view of the above, we have measured the

high-field conduction at different temperatures onvacuum-evaporated thin films of glassy alloys ofSe80Te20, Se75Te20Ge5 and Se75Te20Sb5 systemhaving different electrode separation. The depen-dence of d.c. conductivity on the electrodeseparation confirms the presence of SCLC in thepresent samples. Using the theory of SCLC forthe case of uniform distribution of traps, thedensity of localized states near the Fermi level iscalculated for various samples used in the presentstudy.

2. Experimental

Glassy alloys of Se80Te20, Se75Te20Ge5 andSe75Te20Sb5 systems are prepared by quenchingtechnique. High purity (99.999%) materials areweighed according to their atomic percentages and

are sealed in quartz ampoules (length B5 cm andinternal dia B8mm) with a vacuum B1.3�10�3 Pa. The ampoules containing the materialsare heated to 1000�C and held at that temperaturefor 10–12 h. The temperature of the furnace israised slowly at a rate of 3–4�C/min. Duringheating, all the ampoules are constantly rocked, byrotating a ceramic rod to which the ampoules aretucked away in the furnace. This is done to obtainhomogenous glassy alloys.After rocking for about 10 h, the obtained

melts are cooled rapidly by removing the ampoulesfrom the furnace and dropping to ice-cooled water. The quenched samples of the glassyalloys are taken out by breaking the quartzampoules.Thin films of these glasses are prepared by

vacuum evaporation technique keeping glass sub-strates at room temperature. Vacuum-evaporatedindium electrodes at bottom are used for theelectrical contact. The thickness of the films isB500 nm. The coplanar structure (length B1.2 cmand electrode separationB0.12–0.37mm) are usedfor the present measurements. A vacuum B1.3 Pais maintained in the entire temperature range (296–337K).The films are kept in the deposition chamber in

the dark for 24 h before mounting them in thesample holder. This is done to allow sufficientannealing at room temperature so that a meta-stable thermodynamic equilibrium may be at-tained in the samples as suggested by Abkowitz[19]. Before measuring the d.c. conductivity, thefilms are first annealed at 340K for 1 h in avacuum B1.3 Pa. I–V characteristics are found tobe linear and symmetric up to 100V. The presentmeasurements are, however, made by applying avoltage from 10 to 300V across the films. Theresulting current is measured by a digital Pico-Ammeter. The heating rate is kept quite small(0.5K/min) for these measurements. Thin filmssamples are mounted in a specially designedsample holder. A vacuum B1.3 Pa is maintainedthroughout the measurements. The temperature ofthe films is controlled by mounting a heater insidethe sample holder, and measured by a calibratedcopper–constantan thermocouple mounted verynear to the films.

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Fig. 1. Plots of ln I/V vs. V curves for (a) a-Se80Te20, (b) a-Se75Te20Ge5, and (c) a-Se75Te20Sb5 at different temperatures.

S.P. Singh et al. / Vacuum 75 (2004) 313–320 315

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Fig. 2. Plots of S vs. 1000/T for Se80Te20, Se75Te20Ge5 and Se75Te20Sb5 glassy systems.

S.P. Singh et al. / Vacuum 75 (2004) 313–320316

3. Results and discussion

Results of I–V characteristics at differenttemperatures show that in all the glassy samplesstudied here, ohmic behavior is observed at lowvoltages, i.e., up to 100V. However, at highervoltages (E B104V/cm), a super ohmic behavior isobserved in all the samples. Here, ln I/V vs. V

curves are found to be straight lines in all thesamples. Fig. 1(a)–(c) show such curves in case ofa-Se80Te20, a-Se75Te20Ge5 and Se75Te20Sb5, re-spectively. According to the theory of SCLC, inthe case of an uniform distribution of localizedstates having density g0, the current (I) at a

particular voltage (V) is given by [20]

I ¼ ð2eAmn0V=dÞðexpðSV ÞÞ: ð1Þ

Here, e is the electronic charge, A is the cross-sectional area of the film, n0 is the density of freecharge carriers, d is the electrode spacing and S isgiven by

S ¼ 2ere0=eg0kTd2; ð2Þ

where er is the static value of the dielectricconstant, e0 is the permittivity of free space, g0 isthe density of traps near the Fermi level and k isBoltzmann’s constant.

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Fig. 3. Plots of ln I/V vs. V curves at room temperature having different electrode gaps for a-Se80Te20.

S.P. Singh et al. / Vacuum 75 (2004) 313–320 317

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Table 1

Values of slopes of ln I/V vs. V curves for different electrode

gaps

Electrode gap (d)

(mm)

1/d2 (mm�2) Slopes (s) of ln I/V

vs. V at Troom

(296K)

0.12 69.44 5.00� 10�3

0.19 27.70 1.39� 10�3

0.30 11.11 9.30� 10�4

0.37 7.31 5.45� 10�4

S.P. Singh et al. / Vacuum 75 (2004) 313–320318

It should be noted that Eq. (1) is not an exactsolution of SCLC equation, but is a very goodapproximation of the one carrier space chargelimited current under the condition of a uniformdistribution of traps. In the present case, the onecarrier assumption is justified as these glassesare known to behave as p-type material. Aspresent measurements scan a very limited rangeof energy near the Fermi level, the assumptionof uniform distribution of traps is also notunjustified.According to Eq. (1), ln I/V vs. V curves should

be a straight lines whose slope should decreasewith increase in temperature as evident fromEq. (2). It is clear from Fig. 1(a)–(c) that the slope(s) of ln I/V vs. V curves is not the same at all themeasuring temperatures. The value of these slopesis plotted as a function of temperature in Fig. 2 forvarious glassy systems used in the present study. Itis clear from this figure that the slope decreaseslinearly with the increase in temperature. Theseresults indicate the presence of SCLC in thepresent samples.It may be mentioned here that nearly linear

plots of ln I/V vs. V as well as a linear decrease of S

with temperature can also be explained in terms ofhigh-field conduction due to the Poole–Frenkeleffect of screened charge intrinsic defects and field-induced lowering of energy barriers for the charge-carrier hopping within localized states at the bandedges. However, in the case of field-dependentconductivity, the plot of ln I/V vs. V must beindependent of the electrode spacing ‘d’. On theother hand for any SCLC mechanism, the sameplot gives different curves for different values of‘d’. We have therefore measured I–V character-istics at room temperature (296K) for varioussamples having different electrode spacing. Theresults for one particular sample Se80Te20 areplotted in Fig. 3. It is clear from this figure thatdifferent slopes are obtained at different electrodespacings. The values of these slopes are given inTable 1 and are plotted against 1/d2 in Fig. 4. Thisconfirms the validity of Eq. (2) in the present caseand excludes the possibility of any other high-fieldconduction processes mentioned above. Hence thepresent measurements confirm the presence ofSCLC in the present samples.

Using Eq. (2), we have calculated the density oflocalized states from the slope of Fig. 2. The valueof the relative dielectric constant er is measured byusing capacitance measuring assembly model GR1620 AP, employing the three terminal techniques.The results of these calculations are given in Table2. It is clear from this table that er increasesdrastically on addition of Sb to binary Se80Te20alloy. Such an increase of er has already beenreported by Goel et al. [21].According to Kastner and Fritzsche [22,23],

chalcogenide glasses have co-ordination defectswhich are paired (C3

+ and C1�) as they do not show

electron spin resonance [24]. Such paired defectsare more likely in Se-rich chalcogenide glasses asthey have flexible structure due to twofold co-ordinated structure, i.e., long polymeric chains ofSe type. On the other hand, in case of Ge-richglasses, structure flexibility is reduced due toformation of tetrahedral structure.When Ge is added to Se–Te, Ge–Se tetrahedral

units may be formed on the expense of Se–Temixed rings. Since tetrahedral structure has lessflexibility of structural change as compared to Se–Te binary chains, less defects are expected to beintroduced in Ge-containing alloys as compared toSe–Te glassy system. Our results on photoconduc-tivity measurements also indicate that the densityof defects decreases on introducing Ge in the sameglassy system.On the other hand, introduction of Sb in the

same glassy system does not produce tetrahedralstructure but cross-links with Se–Te chains. Thedefect density will increase further due to differ-ence of electronegativity of Sb with Se and Te.

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Fig. 4. S vs. 1/d2 curve for different electrode gaps for a-Se80Te20.

Table 2

Density of localized states in various glassy alloys

Glassy alloys Slope of S vs.

103/T curve

er (120Hz,303K)

g0(eV�1 cm�3)

Se80Te20 2.52� 10�3 10.16 3.59� 1014

Se75Te20Ge5 1.00� 10�2 9.78 8.70� 1013

Se75Te20Sb5 6.82� 10�3 168.07 2.19� 1015

S.P. Singh et al. / Vacuum 75 (2004) 313–320 319

During the measurements of photoconductivity inSe–Te–Sb system, Mehra et al. [8] have reportedthat Sb increases DOS near Fermi level byintroducing Sb(0,+) (D0,+) states.Goel et al. [10,21] have measured composition

dependence of dielectric constant (e0) and loss (e00)in Se80�xTe20Gex and Se80�xTe20Sbx and foundthat dielectric parameters (e0, e00) increases with Sb

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concentration and decreases with Ge concentra-tion. These results have been correlated in terms ofincrease in DOS in case of Sb while a decrease incase of Ge.It is clear from our results also that the addition

of Ge in Se–Te decreases the DOS; however, theaddition of Sb increases the same which supportsthe results reported by the other workers in thesame glassy systems.

4. Conclusion

I–V characteristics have been studied in amor-phous thin films of Se80Te20, Se75Te20Ge5 andSe75Te20Sb5. At low fields, ohmic behavior isobserved. However, at higher fields (B104V/cm)super ohmic behavior is observed.Analysis of the observed data shows the

existence of SCLC in all the glassy samples usedin the present study. From the fitting of the data inthe theory of SCLC, the density of localized statesnear Fermi-level is calculated. It is clear from theresults obtained that the addition of Ge in Se–Tedecreases the DOS; however, the addition of Sbincreases the same which supports the resultsreported by the other workers in the same glassysystems.

Acknowledgements

S. Kumar is grateful to UGC, New Delhi forproviding a major research project during thecourse of this work.

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