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Journal of Non-Crystalline Solids 353 (2007) 1474–1477

Electrical properties of a-Se85�xTe15Snx thin films

Vineet Sharma a,*, Anup Thakur b, Jeewan Sharma c, Vivek Kumar c,Sanjeev Gautam c, S.K. Tripathi c

a Jaypee University of Information Technology, Waknaghat, Solan-173 215, Indiab U.C.o.E., Punjabi University, Patiala, India

c Center of Advanced Study in Physics, Panjab University, Chandigarh-160 014, India

Available online 30 March 2007

Abstract

Se–Te alloys are an important system of chalcogenide glasses from application point of view. The incorporation of Sn additive alters theelectrical properties of these alloys. The conductivity measurements have been done on the thin films of a-Se85�xTe15Snx (x = 0, 2, 4, 6 and10 at.%) deposited using vacuum evaporation technique. Both dark (rd) and photoconductivity (rph) show a maximum for x = 6 at.% ofSn, which, decreases on further Sn addition to the binary Se–Te alloy. The dark activation energy (DEd) shows a minimum for x = 2 at.%of Sn, but increases on further Sn addition. There is a sharp decrease in photosensitivity (rph/rd) on Sn addition to Se85Te15 alloy. Thecharge carrier concentration (nr) calculated with the help of dc conductivity measurements also show a maximum at x = 6 at.% of Sn.The results are explained on the basis of increase in the density of localized states present in the mobility gap on Sn incorporation.� 2007 Published by Elsevier B.V.

PACS: 72.15.Cz; 61.72.Ss; 73.61.Jc; 71.23.Cq

Keywords: Conductivity; Chalcogenides

1. Introduction

The photoconductivity measurements are valuable toolsfor the study of transport mechanism and defect states inchalcogenide glassy semiconductors. In chalcogenideglasses, it is assumed that the localized states in the mobil-ity gap are the D+ and D� states and that no other levelsare significant [1,2]. So, these measurements in chalcogen-ide glasses may be helpful in understanding the recombina-tion mechanisms, which in turn give information regardingthe localized states present in the mobility gap of thesematerials.

The Se–Te glassy alloy system based on Se is of consid-erable commercial, scientific and technological importance.They are widely used for various applications as opticalrecording media, because of their excellent laser writing

0022-3093/$ - see front matter � 2007 Published by Elsevier B.V.

doi:10.1016/j.jnoncrysol.2006.09.059

* Corresponding author. Fax: +91 1792 245362.E-mail address: vneetsharma@gmail.com (V. Sharma).

sensitivity, xerography, and electrographic applications[3]. Amorphous Se–Te alloys have greater hardness,higher crystallization temperature, higher photosensitivity(rph/rd) and smaller ageing effects than pure Se. As theseglasses have poor thermo-mechanical properties, in orderto enlarge their domain of applications, it is necessary toincrease their softening temperature and mechanicalstrength [4]. The addition of third element (Sn) expandsthe glass forming region and also creates compositionaland configurational disorder in the system. The presentpaper reports the effect of Sn additive on the dark (rd)and photoconductivity (rph) in vacuum evaporated thinfilm of glassy Se85Te15. At this composition, the densityof defect states is minimum [5–7].

2. Experimental procedure

Glassy alloys of Se85�xTe15Snx (x = 0, 2, 4, 6 and 10) areprepared by melt quenching technique. Materials (99.999%

2.90 2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35-26

-25

-24

-23

-22

-21

-20

-19

-18

-17 a-Se85-x

Te15

Snx

lnσ d

(Ω-1cm

-1)

1000/T [K-1]

x = 0 = 2 = 4 = 6 = 10

Fig. 1. Temperature dependence of rd in the thin films of a-Se85�xTe15Snx

alloys.

2.90 2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35

-19.0

-18.8

-18.6

-18.4

-18.2

-18.0

-17.8

-17.6

-17.4

-17.2 a-Se85-x

Te15

Snx

lnσ ph

( Ω-1cm

-1)

1000/T [K-1]

x = 0 = 2 = 4 = 6 = 10

Fig. 2. Temperature dependence of rph of thin films of a-Se85�xTe15Snx

alloys.

V. Sharma et al. / Journal of Non-Crystalline Solids 353 (2007) 1474–1477 1475

pure) are weighed according to their atomic percentage andsealed in quartz ampoules in a vacuum �2 · 10�5 mbar.The sealed ampoules are heated and rocked to make themelt homogeneous inside a furnace heated upto 920 K ata rate of 2–3 K/min. The quenching is done in liquid N2.Thin films of the alloys are prepared by vacuum evapora-tion technique keeping substrates at room temperatureand base pressure of �2 · 10�5 mbar using a Mo boat.The amorphous nature of the films is confirmed by X-raydiffraction (XRD). The composition of evaporated samplesis measured by an electron microprobe analyzer (JEOL8600 MX) on different spots (size �2 lm) on the samples.The composition of a 2 · 2 cm2 sample is uniform, withinthe measurement accuracy of about ±0.1%. Pre-depositedthick indium electrodes on well-degassed corning 7059glass substrates are used for the electrical contacts. A pla-nar geometry of the film (length �1.78 cm; electrode gap�8 · 10�2 cm) is used for electrical measurements. Thethickness of the film is �5000 �A. The films are kept inthe deposition chamber in dark for 24 h before mountingthem in the metallic sample holder to attain thermody-namic equilibrium as stressed by Abkowitz et al. [8].

The rph of the amorphous films is studied by mountingthem in a specially designed metallic sample holder whereheat filtered white light (200 W tungsten lamp) can beshone through a transparent glass window. A vacuum ofabout 10�3 mbar is maintained throughout these measure-ments. Light intensity is measured by a digital Luxmeter(Testron, model TES-1332). The photocurrent (Iph) isobtained after subtracting dark current (Id) from the cur-rent measured in the presence of light. The accuracy inIph measurements is typically 1 pA. The films are annealedin the range 312–331 K for 2 h in a vacuum of about10�3 mbar for different samples and the dark and photo-conductivity measurements are carried out.

3. Results

Fig. 1 shows the temperature dependence of rd in thinfilms of a-Se85�xTe15Snx (x = 0, 2, 4, 6 and 10). The lnrd

versus 1000/T curves are straight lines in the measured tem-perature range (301–340 K). The narrow temperaturerange of the present measurements is due to the low glasstransition temperature (e.g. Tg = 336 K for a-Se85Te15) ofthe studied samples. The measurements at temperaturemuch lower than Tg could have helped for better analysis,but, it has not been possible to go below 301 K in the pres-ent experiment because of the high resistance of the films.This indicates that the conduction in these glasses isthrough an activated process. The rd can, therefore, beexpressed by the usual relation:

rd ¼ ro expð�DEd=kBT Þ

where ro is the pre-exponential factor and DEd is the acti-vation energy for dark conduction.

The values of DEd are calculated from Fig. 1. The valueof rd at 301 K increases from (5.22 ± 1.00) · 10�12

1476 V. Sharma et al. / Journal of Non-Crystalline Solids 353 (2007) 1474–1477

X�1 cm�1 to (1.72 ± 0.12) · 10�9 X�1 cm�1 for x = 0to x = 6 at.% of Sn addition followed by a decrease to(2.73 ± 0.67) · 10�11 X�1 cm�1 for x = 10 at.%. The valueof DEd decreases from (0.93 ± 0.01) eV to (0.61 ±0.01) eV for x = 2 at.% and then increases to (0.94 ±0.01) eV as the Sn concentration increases to x = 10 at.%.Fig. 2 shows the temperature dependence of rph at anintensity (1035 Lux). The value of rph increases exponen-tially in the temperature range 301–340 K. A maximumin photoconductivity is observed for samples added withx = 6 and x = 10 at.% of Sn concentration. At lower Snadditive concentration (x = 2 and x = 4), rph reaches amaximum with increase in temperature and saturates withfurther increase in temperature in the measured tempera-ture range. The maximum in rph is absent in binarySe85Te15, i.e. at x = 0. The rph simply increases slowlyand monotonically with increasing temperature. Thisoccurs due to the thermally activated process, which isexplained on the basis of bimolecular recombination. Theactivation energy of photoconduction (DEph) is smallerthan the dark conduction.

The photosensitivity (rph/rd) is an important and usefulparameter in the photoconductivity measurements, whichdecides the use of that material in photoconductive devices.From the measured values of rd and rph, we have thereforecalculated the rph/rd at various temperatures for all the

0 2 4 6 8 10

0

200

400

600

800

1000

1200

301 K1035 lux

a-Se85-x

Te15

Snx

σσ

ph/

d

Sn Concentration (x)

Fig. 3. Composition dependence of photosensitivity (rph/rd) at roomtemperature for different a-Se85�xTe15Snx thin films.

samples at a particular intensity (1035 Lux) using theresults shown in Figs. 1 and 2.

The results of these calculations are shown in Fig. 3 atroom temperature (301 K) as a function of x in thea-Se85�xTe15Snx. The value of rph/rd decreases quite appre-ciably after the incorporation of Sn and having a minimumat x = 4. The value of rph/rd increases as the concentrationof Sn in the binary Se–Te alloy is further increased. Similarresults are also obtained at other temperatures.

4. Discussion

The increase in rd with a corresponding decrease in DEd

is generally found to be associated with the shift of Fermilevel in impurity doped chalcogenide glasses [9]. Theincrease in rd after impurity doping is also explained interms of the increased hopping conduction in impuritydefect induced states [10]. In the first interpretation, i.e.the shift of the Fermi level with Sn addition, the densityof states should not increase. Hence, it can not explainthe rd increase with Sn addition, which also lowers therph/rd at a particular temperature and intensity. However,in the second interpretation, the density of localised statesshould increase resulting in lowering of rph/rd. Since, therph/rd decreases quite sharply after Sn incorporation(Fig. 3), the first interpretation seems unlikely in the pres-ent case. Therefore, this justifies the latter explanation forincrease of rd and decrease of rph/rd. The incorporationof Sn increases the density of localized states. The chargecarrier concentrations (nr) for different samples are calcu-lated using the equation [11]:

nr ¼ 22pmkBT

h2

� �3=2

exp�DEr

kBT

� �" #;

where ‘m’ is the mass of the charge carrier. The value of nr

increases with the increase of Sn concentration as shown inFig. 4. The value of rph/rd depends on the lifetime of theexcess charge carriers which in turn depends on the den-sity of localized states in a particular material. Therefore,rph/rd should decrease with the increase in the density ofdefect states. The higher the density of states, the lower willbe lifetime, as these defect states may act as recombinationcenters in presence of light. This is found to be due to theformation of intimate valence alternation pairs (IVAPs)under illumination. These defects are converted by bondswitching reactions to random pairs of charged defects,i.e. light induce metastable defects. Therefore, rph/rd

should decrease with the increase in the density of defectstates.

The increase in the density of defect states after incorpo-ration of Sn additive may be understood interms of theelectron affinity (EA) of Sn as suggested by Onozukaet al. [12]. Se and Te belong to the group VI of the periodictable. When Te is added to Se, some of the Te atoms maynot be incorporated in chains. So, some of the Te atomswould act as ionized impurities (EA of Te is lower than

0 2 4 6 8 1022

23

24

25

26

27

28

29301 K

a-Se85-x

Te15

Snx

lnn

σ (cm

-3)

Sn concentra tion (x)

Fig. 4. Variation of nr with change of Sn concentration in a-Se85�x-Te15Snx thin films.

V. Sharma et al. / Journal of Non-Crystalline Solids 353 (2007) 1474–1477 1477

that of Se). The Te atoms are therefore, expected to formthe positively charged localized states. Similarly, Sn addi-tion will induce more and more positively charged localizedstates, since EA of Sn (1.11 eV) is lower than that of Se(2.02 eV) and Te (1.97 eV). So, an increase in defect states

after adding Sn in binary Se85Te15 alloy may be due tolower EA of Sn as compared to Se or Te. These increasedcharged localised states act as defect states, which subse-quently increase rd and hence decrease rph/rd.

5. Conclusion

The increase in rd is interpreted in terms of increasedhopping conduction due to the increase in localized states.However, a decrease in rd, rph and nr and increase in DEd,rph/rd can be understood in the structural changes whichare taking place in the material at such a high Sn additiveconcentration (i.e. x = 10 at.%).

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