Ž .Applied Surface Science 147 1999 19–26

Photoelectrical properties of amorphous Se Ge Te25 20 55

Shelly Jain ), Sanjeev Gautam, D.K. Shukla, Navdeep GoyalCentre of AdÕanced Study in Physics, Panjab UniÕersity, Chandigarh, India

Received 27 March 1998; accepted 17 September 1998

Abstract

This paper reports photoelectrical properties of Se Ge Te through measurements of ‘transient’ and ‘steady state’25 20 55

photocurrents. The material displayed great sensitivity to optical and thermal stresses. A study of photoconductivity ofSe Ge Te at different temperatures and at different levels of light intensity reveals that photoconductivity increases25 20 55

Ž . Ž .exponentially with increase in temperature. Photocurrent I when plotted against light intensity F follows a power law:ph

I AFg. The exponent g has been found nearly 0.5 suggesting bimolecular recombination. The decay portion ofph

photocurrent has two components, i.e., a fast component in the beginning followed by a slow decay. The decay time constantt for the fast decay process increases with increasing temperature indicating the existence of deeper traps in the materiald

whereas for slow decay process, it decreases with increasing temperature indicating the presence of gap states near themobility edge. q 1999 Elsevier Science B.V. All rights reserved.

PACS: 72.40

Keywords: Chalcogenides; Amorphous semiconductors; Photoconductivity; Photoelectrical properties; Photocurrent; Recombination me-chanics

1. Introduction

Photoconductivity in amorphous chalcogenideglasses has drawn a great deal of attention in recentyears due to their basic and applied aspects in vari-ous solid state devices. Amorphous semiconductorsare characterized by the presence of localized statesin the mobility gap and absence of long range orderw x1 . Because photocurrent is controlled by carrier

w xlocalization and delocalization processes 2 , the pho-

) Corresponding author. Fax: q91-172-541741; E-mail:jains%phys@puniv.chd.nic.in

toconductivity has proven to be a valuable tool inunderstanding the recombination kinetics which, inturn, provides information about the localized statesin these materials and throws light on the photosensi-

w xtivity of materials for device applications 3–6 . Thestudy of photoelectrical properties also provides anunderstanding of photo generations and transport offree carriers in the solids. Photoconducting propertiesof several compounds have been investigated by

w xmany workers 7–10 .Although the electrical conductivity and dielectric

Ž .behaviour of Se–Ge–Te glasses have already beenexplained on the basis of ‘dangling bond’ theory toprovide information about the existence of localized

0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-4332 98 00588-1

( )S. Jain et al.rApplied Surface Science 147 1999 19–2620

w xstates. However, these studies 11,12 did not pro-vide any information about the recombination mech-anisms shown by Se–Ge–Te alloys. So an attempthas been made by present authors to accomplish thistask by carrying out photoconductivity measure-ments on the cross linked chalcogenide Se Ge Te25 20 55

Ž .over a wide range of temperature 269 to 313 K andŽlight intensity 0 to 26 A.U. where A.U.sarbitrary

.unit .The study of ‘transient’ and ‘steady state’ pho-

tocurrent concluded that the combined mechanismsof recombination from the gap states near the mobil-ity edge and deeper traps contribute to photoconduc-tivity in Se Ge Te .25 20 55

2. Experimental details

2.1. Preparation of material and samples

Ingots of glassy material Se Ge Te were pre-25 20 55

pared by melt quench method. The desired quantityŽ .of constituent 99.99% pure elements was sealed in

an evacuated quartz ampoule and heated at approx.10008C. The homogeneous melt was finally quenchedin liquid nitrogen. The composition of the materialwas verified by EDAX and amorphous nature wasensured by X-ray diffraction pattern using X-ray

Ž .diffractometer Phillips PW 1130r60 .The samples in the form of thin circular pellets of

diameter 0.677 cm were prepared by grinding theingots finely and then compressing the powder in adie under hydraulic pressure of f107 Pa. All thesamples were then annealed at a temperature of 608Cfor nearly 24 h. The annealing of samples reduce theeffect of grain boundaries. This way the effect ofinhomogeneities of the pressed pellets are reduced toa great extent. Earlier work has reported that mea-surements made on annealed pellets and cut ingots

w xgive same results 13 . Measurements were made onvarious sample configurations. However, we reporthere the measurements for planer samples whichwere prepared by coating silver on one face of thepellet, leaving in between a narrow slit of the mate-rial for exposure to light radiations.

2.2. Measurements

Photoconductivity measurements were made us-Žing computer aided impedance system Model 5206,

.EG&G, PARC, USA . The impedance system alsoŽ .has a facility to obtain I–V current vs. voltage and

Ž .I–T current vs. time characteristics. These charac-teristics have been used to study the photoconductiv-

Ž .ity. Ealing UK fibre optics illumination set, with aŽlight source of variable intensity tungsten–halogen,

.sylvania lamp, 150 W was used to illuminate thesample with the help of fibre bundle. The intensity ofthe light source was measured with the help of aphotodiode. The current of the photodiode was di-rectly proportional to the intensity of the light. Thecurrent so measured has been used as arbitrary unitsŽ .A.U. in finding out the relative intensity of thelight.

The sample was kept in vacuum in a sampleholder to ensure that temperature gradient between

w xoppose faces of the pellet is f"18C 14 . Thedifferent temperatures were obtained using a closed

Ž .cycle refrigerator Model F-70, Julabo which canmaintain constant temperature within "1 K for allthe measurements.

3. Results and discussion

3.1. Steady state photoconductiÕity

The temperature dependence of dark current andphotocurrent for Se Ge Te is shown in Fig. 1. It25 20 55

is evident from the figure that the plots of ln I vs.ph

1000rT and ln I vs. 1000rT are linear, whichd

indicates that conduction in this glass is through anactivated process having a single activation energy inthe temperature range 269–313 K. In most of the

Ž .chalcogenides, dc conductivity s can be ex-dc

pressed by the relation

s ss exp yD ErkT 1Ž . Ž .dc 0

where D E is the activation energy for dc conductionand k is Boltzmann’s constant. The slopes of thecurves in Fig. 1 are used to calculate the activationenergies for dark and illuminated conditions. It isalso observed that the activation energy gets reducedafter illumination and is almost independent of thelight intensity used. Similar results have also beenreported for some other chalcogenides and chalcopy-

w xrites 15–17 . It may also be noted that no maxima in

( )S. Jain et al.rApplied Surface Science 147 1999 19–26 21

Ž . Ž .Fig. 1. Temperature dependence of dark current 0 A.U. and photocurrent 2 A.U., 12 A.U., 26 A.U. .

Fig. 2. Variation of photocurrent with illumination level at different temperatures.

( )S. Jain et al.rApplied Surface Science 147 1999 19–2622

Fig. 3. Photocurrent vs. time plots at different levels of illumination.

Fig. 4. Photocurrent vs. time plots at different temperatures.

( )S. Jain et al.rApplied Surface Science 147 1999 19–26 23

Fig. 5. ln I vs. time plots at different temperatures for decay processes at 26 A.U.ph

Fig. 6. ln I vs. time plots at different intensities for decay processes at 303 K.ph

( )S. Jain et al.rApplied Surface Science 147 1999 19–2624

Table 1Values of t at different times at different temperaturesd

Ž . Ž . Ž . Ž .Time t 269 K t 293 K t 303 K t 313 Kd d d dŽ . Ž . Ž . Ž . Ž .s s s s s

8 18.18 30.22 39.76 45.6350 87.53 135.0 148.0 136.8

500 1865.0 1727.0 939.4 896.3800 3470.0 2812.0 1530.33 1506.0

w xI has been observed. Mott and Davis 18 haveph

reported that a maxima is observed where I 4 I .ph d

A similar absence of maxima was reported by othersw x19,20 .

In order to investigate the nature of recombinationprocess taking place in Se Ge Te , intensity de-25 20 55

pendence of steady state photocurrent has been stud-ied. The variation of photocurrent I at differentph

illumination levels is shown in Fig. 2. Plots of ln Iph

vs. ln F are again linear at all temperatures whichfurther indicates power law dependence for

w xSe Ge Te 21,22 , i.e.,25 20 55

I AFg 2Ž .ph

The exponent g determines whether the recombina-tion process is monomolecular or bimolecular. In the

w xsingle trap analysis 23 , it is equal to 0.5 for bi-molecular recombination and 1.0 for monomolecularrecombination. However, in case of continuous dis-tribution of traps, the value of g lies between 0.5

w xand 1.0 24 . The values of g have been calculatedfrom the slopes of ln I vs. ln F plots and theyph

have been found close to 0.5 which indicates theexistence of bimolecular recombination in which re-combination rate of electrons is proportional to the

Ž .Fig. 7. Temperature dependence of differential life time t . Inset: variation of n sd ln t rdln t with temperature.d d

( )S. Jain et al.rApplied Surface Science 147 1999 19–26 25

number of holes. The majority of excess electronsand holes generated by the photoexcitation are lo-cated at D0 centres under nonequilibrium steadystate condition. After removing illumination, D0

centres decrease by recombination process; 2 D0 ™q y w xD qD 25 . Similar behaviour of g has been

w xreported for some other semiconductors 21–25which are known to have a wide distribution oflocalized states including Dq and Dy. One wouldtherefore expect such a distribution to exist inSe Ge Te as well.25 20 55

3.2. Transient photoconductiÕity

Transient photoconductivity has been studied byexposing the sample to light radiations and simulta-neously recording the dc current for a given period.Thereafter the light was turned off and the decay of

Ž .current was recorded. The photocurrent I wasph

obtained by subtracting the dark current from totalcurrent. The results of transient photoconductivitymeasurements, i.e., photocurrent vs. time plots forSe Ge Te at a given temperature for different25 20 55

Ž .illumination levels F are shown in Fig. 3. It isevident from the figure that the inial rise and decayof currents are rapid and thereafter the process be-comes slow. Similar behaviour was observed at othertemperatures also. The photocurrent vs. time plots

Žfor different temperatures at a given intensity Fs26.A.U. are shown in Fig. 4. The decay of photocurrent

as shown in Figs. 5 and 6 can be explained by usingŽthe concept of differential life time t also writtend

. w xas decay time constant 25,26 .Quantitatively, the decay time constant is definedw xas 27

y1Xt sy 1rI r d I rd t 3Ž .Ž .d ph ph

where I X is the maximum photocurrent at ts0 forph

a given temperature. In order to study the timedependence of t , the values of t were calculatedd d

Ž .using Eq. 3 at different intervals of time for a givenŽ .temperature Table 1 .

It is clear from Fig. 7 that t increases linearlyd

with increasing time indicating thereby that the de-Ž yn .cay in Se Ge Te obeys power law t with25 20 55

Ž . Ž .Fig. 8. Temperature dependence of differential lifetime at ts500 s . Inset: temperature dependence of differential lifetime at ts8 s .

( )S. Jain et al.rApplied Surface Science 147 1999 19–2626

Ž . Ž .nsd ln t rd ln t . The parameter n has a valued

0.63 at 313 K and has been found to vary inverselywith temperature as shown in inset of Fig. 7.

The temperature dependence of t was analyzedd

by calculating t at a fixed interval of time ford

different temperatures. The results shown in inset ofŽFig. 8 indicate that for the inial part of decay at

.ts8 s , decay time constant t increases with in-d

crease in temperature which suggests that deeperw xstates exist in the material 27 . For the decay at a

Ž .later time ts500 s it is observed that the decaytime constant decreases with increasing temperatureŽ .Fig. 8 . Similar behaviour has been observed fortimes greater than 500 s. The decrease in value ofdecay time constant with increasing temperature isprobably due to an increase in the number of ther-

Ž .mally generated trap-centres n . Such a behaviour0

of decay time constant has also been reported byw xMoustakas and Weiser 28 for As Te and Main and2 3

w xOwen 29 for As Se . This type of temperature2 3

dependence of decay time constant has been at-tributed to the presence of gap states near the mobil-ity edge. If recombination takes place via thesestates, the calculated decay time will be longer thanthe decay time of mobile carriers.

4. Conclusions

Photoconductivity of Se Ge Te has been stud-25 20 55

ied as a function of temperature and light intensity.Intensity dependence of photocurrent indicates apower law dependence for Se Ge Te . The study25 20 55

of photoconductivity as a function of temperaturereveals that conduction is through an activated pro-cess with a single activation energy in the tempera-ture range studied. It has been shown that the com-bined mechanisms of recombination from the gapstates near the mobility edge and deeper traps con-tribute to photoconductivity in Se Ge Te .25 20 55

Acknowledgements

The useful discussion with Prof. Satya Prakash isgratefully acknowledged. The authors are grateful to

University Grant Commission for providing fundsunder COSIST and CAS programmes for the pur-chase of measurement system which was essentialfor investigations reported in this paper.

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