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Materials Chemistry and Physics 104 (2007) 153–157

Electrical conduction mechanism in amorphousthin films of the system PbxGe42−xSe58

Syed Rahman ∗, Shashidhar Bale, K. Siva KumarDepartment of Physics, Osmania University, Hyderabad 500007, India

Received 12 July 2006; received in revised form 17 February 2007; accepted 27 February 2007

bstract

Bulk glasses of composition PbxGe42−xSe58 (x = 7, 8 and 9) were prepared by melt quench technique. The thermal behaviour of the glass systemas investigated by differential scanning calorimetry. The double stage crystallization was observed in these glasses. Amorphous thin films ofbxGe42−xSe58 (x = 7, 8 and 9) were prepared by thermal evaporation of bulk glasses of the same composition on the glass substrates. Electrical

onduction mechanism in PbxGe42−xSe58 thin films in the temperature range of 300–425 K with both symmetric and asymmetric electrodes iseported. The results were interpreted in terms of a Poole–Frenkel type of conduction mechanism over the entire range of temperatures and fieldtrengths. Tunnel characteristics are reported in ultra thin films and the metal semiconductor interface barrier heights are determined. 2007 Elsevier B.V. All rights reserved.

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eywords: Glasses; Thin films; Double stage crystallization; Poole–Frenkel eff

. Introduction

The focus of doping attempts in most chalcogenide glassesas so far been to obtain n-type conduction with the addition ofufficient amount of bismuth or lead [1]. Some results of n-typeonductivity glasses were summarized in articles by Nagels etl. [2], Elliot and Steel [3], Thoge et al. [4] and Asha Bhat et al.5]. Recently, several workers [6–9] have reported the impurityffects in chalcogenide glasses. These amorphous semiconduc-ors with tailored properties have a potential for thermo electricnd photovoltaic applications and can be used as core materialsor optical fiber transmission [10,11].

To explore the possibility of such applications, it is neces-ary to obtain the materials in thin film form and to understandheir physical properties and the transport mechanism. Somettempts have been made to study the conduction mechanism inhalcogenide glasses on the basis of contact controlled mech-nism [12], space-charge limited current [13], Poole–Frenkelffect [14] and Davis–Mott model [15].

In the present paper, the authors studied the thermal behaviourf the bulk PbxGe42−xSe58 (x = 7, 8 and 9) glasses and maden attempt to understand the nature of conduction mech-

∗ Corresponding author. Tel.: +91 98 49 82 30 78; fax: +91 40 27 07 39 07.E-mail address: syedrahman848@yahoo.co.in (S. Rahman).

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254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2007.02.101

C electrical conductivity; Barrier potentials

nism in thin film form by studying the conductivity andurrent–voltage–temperature characteristics. The glass compo-ition PbxGe42−xSe58 is specifically chosen since the p-type to-type transitions has been to occur at x = 9. Also, the authorstudied the tunnel characteristics of ultra thin films (d ≈ 500 A)nd the metal semiconductor interface potential barriers wereetermined.

. Experimental procedure

Quaternary PbGeSe alloys were prepared by melt quenching [16,17]. Forhis purpose high purity (5N) lead, germanium and selenium were accuratelyeighed and sealed in quartz ampoules in a vacuum better than 10−5 torr. The

ealed ampoules were then baked in a high temperature furnace at 1000 ◦Cor about 48 h. The ampoules were frequently rotated to obtain homogeneousixing of the starting materials. The quartz ampoules were then quenched in

n ice-water mixture. For all samples, scooped off from the different ingots, thebsence of any Bragg diffraction peak in the X-ray powder diffraction patternonfirms the amorphous nature and homogeneity of the samples.

The thermal behaviour of the bulk glass samples were investigated using au Pont 1090 differential scanning calorimeter at a heating rate of 10 ◦C min−1.morphous films of the bulk glass composition can be prepared by vacuum

vaporation with good reproducibility over a sufficient thickness range [18].hin amorphous films of the glass system PbxGe42−xSe58 (x = 7, 8 and 9) were

repared by thermal evaporation of the bulk alloys from a tantalum boat in a vac-um better than 10−5 torr on to an ultrasonically cleaned silica glass substratesaintained below room temperature. The ultra thin films (d ≈ 500 A) were also

repared using the same technique. The amorphous nature of the prepared thinlms was determined using XRD technique. Fig. 1 shows the XRD pattern of the

154 S. Rahman et al. / Materials Chemistry and Physics 104 (2007) 153–157

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ig. 1. X-ray diffractograms of PbGeSe thin film (a) amorphous, (b) at firstrystallization, (c) at second crystallization.

repared amorphous thin film, at first crystallization and at second crystalliza-ion. To make electrical contacts aluminum electrodes were deposited on eitheride of the film there by forming a sandwich structure (aluminum–PbGeSe thinlm–aluminum).

The DC electrical conductivity of PbGeSe film was measured as a functionf temperature ranging from 300 to 425 K. The current–voltage temperatureharacteristics were studied using a Keithley 616 digital electrometer and aeithley 181 nanovoltmeter.

. Results and discussion

Fig. 2 shows the DSC thermograms of bulk PbxGe42−xSe58x = 7, 8 and 9) glasses at a heating rate of 10 ◦C min−1.he samples were scanned through their liquidus temperature.

hermodynamical and the glass stability parameters such aslass transition temperature (Tg), crystallization temperatureTc), melting temperature (Tm), liquidus temperature (Tl), heatbsorbed during melting (�Hm) were evaluated using standard

a

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able 1hermodynamical and glass stability parameters of the PbGeSe glasses

omposition Tg (◦C) Tc1 (◦C) Tc2 (◦C) Tp (◦C) Tm

b7Ge35Se58 247 308 378 330 470b8Ge34Se58 255 352 395 374 472b9Ge33Se58 260 285 400 320 474

g: glass transition temperature; Tm: melting temperature; Tc1 and Tc2: crystallizationHm: melting enthalpy; S: glass stability; Trg: reduced glass transition temperature; K

ig. 2. DSC thermograms of bulk PbxGe42−xSe58 (x = 7, 8 and 9) glasses.

rocedures. The DSC thermogram shows a single endothermiclass transition peak (Tg) and two exothermic crystallizationeaks (Tc1 and Tc2). There is an increase of glass transition tem-erature by about 13 ◦C as the lead content is increased from 7o 9 at%. The thermodynamical parameters of the present glassystem are given in Table 1.

The empirical parameters used to parameterize the sta-ility of the glass are the glass stability S defined as theifference between the crystallization temperature and thelass transition temperature, the reduced glass transition tem-erature Trg = Tg/Tm and the Hruby parameter defined asgl = (Tc − Tg)/(Tm − Tc). The Hruby parameter characterizes

he tendency of the melts to form glasses. The glass formingbility parameters for PbGeSe glasses are presented in Table 1.

The phenomenon of double stage crystallization is a directonsequence of the phase separation occurring in the glasses ands found to occur in a variety of glass systems [19–22]. The dou-le stage crystallization observed in the present PbGeSe glassystem may be due to the fact that in selenium rich glass system,ermanium atoms tetrahedrally bonded with selenium atomsorms a loosely packed random structure and the excess seleniumtoms forms a complete disordered segregated matrix. There-ore the first crystallization might arise due to the crystallizationf excess selenium matrix and the second crystallization peakccurs when the crystallization of Pb-Se tetrahedrally bonded

toms takes place [23,24].

Fig. 3 shows the temperature dependence of electrical con-uctivity of thin films of PbxGe42−xSe58 (x = 7, 8 and 9). Theoom temperature conductivity was found to be of the order

(◦C) Tl (◦C) �Hm (J g−1) S (◦C) Trg Kgl

480 9.14 61 0.69 0.376485 17.8 97 0.70 0.810500 40.4 25 0.71 0.132

temperatures; Tl: liquidus temperature; Tp: peak temperature of crystallization;

gl: Hruby parameter.

S. Rahman et al. / Materials Chemistry and Physics 104 (2007) 153–157 155

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woHSto the theoretically calculated βPF and for the Poole–Frenkeleffect the experimental value of β should be larger than the the-oretical value. From Table 2 it is clear that the experimental valueof β is greater than that of the theoretical value, suggesting that

Table 2Experimental and theoretical values of β

Composition Temperature(K)

Experimental105 β (V1/2)

Theoretical105 β (V1/2)

Pb7Ge35Se58 300 3.93 2.77325 3.86

ig. 3. Conductivity plots of PbxGe42−xSe58 (x = 7, 8 and 9) films (thicknessndicated).

f 10−12 (� cm)−l. Activation energies were obtained from thelopes of conductivity plots which varied from 0.4 to 0.76 eV inhe entire composition range.

In order to investigate the conduction mechanism in theselasses the current–voltage temperature characteristics weretudied and are presented below.

The variation of log I as a function of field (F) at different tem-eratures 300, 325 and 350 K showed an exponential increaseith the F1/2 and obeys the following relation

= Io exp

{βF1/2

kT

}

here I is the current, k the Boltzmann’s constant, F the fieldnd T is the Kelvin temperature. The current–voltage character-stics of amorphous Pb Ge Se (x = 7) film (d = 3420 A) at

x 42−x 58ifferent temperatures are shown in Fig. 4. In the temperatureange of present study, log I showed a linear variation with field1/2.

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ig. 4. Variation of current with field at different temperatures in Pb7Ge35Se58

lm.

The observed variation between current and electric fieldtrength suggest a conduction mechanism in which charge carri-rs are released by thermal activation over a coulombic potentialarrier that is decreased by the applied electric field. The physi-al nature of such potential can be interpreted in two basic ways:1) it can be a transition of electrons over the cathode and the filmSchottky emission effect), (2) charged carriers can be releasedy the ionization of impurity centers in the film (Poole–Frenkelffect).

To determine the actual conduction mechanism the values ofhe Poole–Frenkel constant β at different temperatures deducedrom the slopes of the plots of log I versus F1/2 are comparedith the theoretical values in Table 2. The theoretical value of β

ay be deduced from the following relation.

PF = 2βRS = 2

{e

4πε εo

}1/2

here ε is the dielectric constant of the glass. Dielectric constantf the glass was measured by using impedance analyzer (ModelP 4192) at 1 kHz frequency and was found to be 7.5. For achottky type of barrier the experimental slope β should be equal

350 4.44

b9Ge33Se58 300 3.003.60 4.02

156 S. Rahman et al. / Materials Chemistry and Physics 104 (2007) 153–157

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fisented in Table 3. It is observed that the barrier heights remainconstant.

ig. 5. Current–voltage characteristics of PbGeSe films with different electrodeaterials.

he Poole–Frenkel type of conduction mechanism is dominantn this glass system.

However, the mere coincidence of β values may not be suffi-ient evidence for a firm conclusion on the nature of conductionechanism [25,26]. This may be better determined by studying

he effect of different electrode materials on the current–voltageharacteristics, since the Richardson–Schottky mechanism islectrode dependent where as Poole–Frenkel is not [27].

The current–voltage–temperature characteristics of amor-hous films using different electrode materials of different workunctions have been studied and are presented in Fig. 5. Fromhe above figure, it is clear that the effect of electrode materialsn the I–V characteristics is found to be negligible within thexperimental error confirming that the Poole–Frenkel conduc-ion mechanism is the dominant one.

Fig. 6 shows the plots of log σ versus V1/2. It is seen that log σ

aries linearly with V1/2 and this linear variation once againupports the Poole–Frenkel type of conduction mechanism inbGeSe films.

Fig. 7 illustrates the tunnel resistance ρ = (V/J), as a functionf voltage for ultra thin films of various composition, whereis the current density. The plot of ρ versus V is chosen for

llustration, because it is a more efficacious way of illustratinghe effect of junction parameters on the electrical characteris-ics. It was observed that at very low voltages, the curves areorizontal which means that the junction resistance is ohmic

nd the junction resistance falls off rapidly with the increase inoltage.

In order to determine the electrode–semiconductor interfacearrier height Φ, a graph between the percentage change in the

ig. 6. Conductivity as a function of (voltage)1/2 in PbGeSe films at differentemperatures.

urrent density versus voltage is drawn. The importance of thislot is now apparent; the J maxima occur at voltage equal to thelectrode semiconductor interface barrier height and becausef the prominence of the peak, it provides a good method ofxperimentally determining the barrier height.

Fig. 8 shows the plots of %�J versus V for ultra thinlms. The barrier height and the breakdown voltages are pre-

Fig. 7. Tunnel characteristics (ρ/V) of PbGeSe thin films.

S. Rahman et al. / Materials Chemistr

Fig. 8. Plots of %�J vs. V for ultra thin films.

Table 3Breakdown voltage and barrier potentials of ultra thin films

Film composition Thickness of thefilm (A)

Breakdownvoltage (V)

Barrierpotentials (V)

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b7Ge35Se58 495 4.0 0.4b8Ge34Se58 540 4.6 0.4b9Ge33Se58 510 4.4 0.4

. Conclusions

The thermal behaviour of bulk PbxGe42−xSe58 (x = 7, 8 and) glasses was investigated using DSC technique at a heatingate of 10 ◦C min−1. It was observed that there is an increasef glass transition temperature by about 13 ◦C as the lead con-ent is increased from 7 to 9 at%. The double stage crystallizationhenomenon is the unique observation in lead contained chalco-enide system.

DC electrical conductivity and current–voltage behaviour ofmorphous thin films of PbGeSe were studied as a functionf temperature. The current–voltage characteristics were inter-reted in terms of Poole–Frenkel type of conduction mechanism.

[[[[[

y and Physics 104 (2007) 153–157 157

Tunneling characteristics were studied in ultra thin films.etal semiconductor interface barrier potentials were deter-ined. The junction resistance was observed to be ohmic in

ature at low voltages.

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