Thin Solid Films 520 (2012) 1757–1761

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Optical and electrical properties of Te-substituted Sn–Sb–Se semiconductingthin films

Ravi Chander ⁎, R. ThangarajSemiconductor Laboratory, Department of Physics, Guru Nanak Dev University, Amritsar, India

⁎ Corresponding author.E-mail address: rcohri@yahoo.com (R. Chander).

0040-6090/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.tsf.2011.08.054

a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 July 2010Received in revised form 16 August 2011Accepted 17 August 2011Available online 23 August 2011

Keywords:Amorphous semiconductorsOptical band gapActivation energydc-conductivity

Thin films of Sn10Sb20Se70-XTeX (0≤X≤14) composition were deposited using thermal evaporationtechnique. As-prepared films were amorphous as studied by X-ray diffraction. Surface morphology studiesrevealed that films have surface roughness ~2 nm and av. grain size ~30 nm. Optical band gap Eg showed asharp decrease for initial substitution of Se with Te upto 2 at.%. A broad hump in the optical band gap isobserved for further substitution of Se with Te. The trend of optical band gap variation with tellurium contenthas been qualitatively explained using band model given by Kastner. The dc-conductivity measurementsshowed thermally activated conduction with single activation energy for the measured temperature regimeand followedMeyer–Neldel rule. The dc-activation energy has nearly half the value as that of optical band gapthat revealed the intrinsic nature of semiconductor. The annealing below glass transition Tg led to decrease inoptical band gap as well as dc-activation energy that might be related to increase of disorder in material withannealing.

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1. Introduction

Amorphous chalcogenide semiconductors are promising materialsfrom their technological andbasic physics point of viewandare used invarious optical and electronic devices [1–4]. The applications includexerography, chemical sensing, optical fibers and photonics circuits,optical amplifiers, grating elements, holography and optical andrandom access memories. Binary and ternary alloys of chalcogenidematerials have advantage over their elemental counterpart in respectof greater photosensitivity [2], hardness, ability to inhibit crystalliza-tion and smaller ageing effects and thus aremore promising for deviceapplications. These materials have high value of third-order non-linearity (two orders of magnitude larger than Silica based glasses)along with large refractive index range that makes them potentialcandidate for all-optical based signal processing devices with powerand device length requirement order of magnitude smaller than theirSilica based counterpart [5]. Moreover chalcogenide materials havelow to moderate two photon absorption, negligible free carrierabsorption at communication wavelengths [6] and its remarkablephoto-induced effects that are useful for fabricating low loss photonicscircuits and thus open up immense opportunity for various ultrafastall-optical signal processing devices such as switching, regeneration,wavelength conversion, pulse compression [3] and on-chip analog to

digital conversion [7] etc. On the other hand, Te based chalcogenidecompounds such as Ge2Sb2Te5 are particularly used for optical andelectronic recordingmedia applications [1,8] since thesematerials canexist in both amorphous and crystalline phase with remarkablecontrast in optical and electrical properties of these two phases andfast transition from one phase to another utilizing these materials fordigital data storage.

From basic physics point of view chalcogenide materials showanomalous behavior in their physical properties around two types ofthresholds namely rigidity percolation threshold (RPT) [9,10] andchemical threshold [11] at mean coordination no. brN=2.4 and 2.67respectively. This abrupt change in properties is related to increase inmean coordination no. brN from 2 (for elemental chalcogenide chainlike structure) to higher value by alloyingwith non-chalcogen elementhaving coordination higher than two (having rigid three dimensionalstructure) thus exhibiting a floppy to rigid transition at brN=2.4. Atchemical threshold, the systemhasmaximumno. of heteropolar bondsand this is normally observed for stochiometeric composition and thussystem behaves as least constrained. The change in physicalproperties, with brN around 2.4 and 2.67 has been studied by manyresearchers [12,13]. But the effect of substituting one chalcogen withanother has not been studied much [14]. It is interesting to study theeffect of substituting one chalcogen with another on physicalproperties of the materials and in the present study it has been donefor a systemhavingmean coordinationno.brN=2.4 i.e. at (RPT). In thepresent study, Sn10Sb20Se70 was used as base system and Se wassubstituted with Te and its effect was studied on electrical and opticalproperties of as-prepared and annealed thin films of the system.

Fig. 1. XRD pattern of as-prepared Sn10Sb20Se70-XTeX (X=0, 2, 4, 6, 8, 10, 12) thin films.

1758 R. Chander, R. Thangaraj / Thin Solid Films 520 (2012) 1757–1761

2. Experimental details

Bulk samples of Sn10Sb20Se70-XTeX (0≤×≤14) were preparedusing a conventional melt quenching technique [14]. The appropriateamounts of constituent elements of 5 N purity were weighed andsealed in the cleaned ampoules under vacuum sealing at ~10—3 Pa.The sealed ampoules were heated in a furnace at 5 K/min heating rateup to 1123 K andwere frequently rocked inside the furnace in order toensure homogenous mixing. After 48 h of heating, ampoules werequenched in ice-cold water to obtain amorphous material. The bulksamples were extracted by dissolving the ampoules in HF+H2O2

solution. The as-prepared bulk samples of different compositionswere used for depositing thin films on cleaned glass slides by thermalevaporation technique using HINDHIVAC coating unit (Model No.12A4D). The deposition was carried out at pressure ~10−3 Pa andthickness of the films was kept~250 nm. The deposited thin filmswere kept inside vacuum chamber for 24 h for attaining metastableequilibrium as suggested by Abkowitz [15]. Thin films were annealedat 130 °C (well below the glass transition temperature Tg for system[14]) for one hour under the running vacuum of 10−3 Pa.

The thickness of films was measured using surface profiler (KLATencor P15). The as-deposited and annealed thin films were charac-terized with X-ray diffraction (XRD) using Phillips PAN analyticalmachine in θ–θ geometrywith X-ray of Cu Kα line. Surfacemorphologyof thin films were studied with Atomic force microscope (AFM) incontact mode using diCaliber of make Vecco instruments Inc. andcantilever of spring constant k ~0.16 N/m. Composition and micro-structure were studied using Scanning electron microscope (SEM) ofmake Phillips XL30 ESEM operated at 20 kV with energy dispersiveX-ray analysis (EDAX) attachment operated at 20 kV with detectorSUTW- Sapphire and tilt ~0.3–0.7°. The transmittance (T) and specularreflectance (R) studies were performed using UV–VIS-NIR Spectro-photometer (VARIAN Cary 500) in the wavelength range 200 nm–

3000 nm with slit width of 1 nm. Al electrodes were deposited withgap ~0.5 mm onto as-deposited and annealed thin film in coplanargeometry for carrying out electrical conductivity measurements.Electrical conductivity measurements were performed in cryostatunder 10−2 Pa vacuum in the temperature range 253 K–403 K.Current wasmeasured using digital picoammeter (DPM-111 ScientificInstruments, Roorkee). The linear behavior of V-I characteristicsproved the ohmic nature of the Al electrode contacts.

3. Results and discussions

Fig. 1 shows the XRD pattern of as-deposited films of compositionSn10Sb20Se70-XTeX (0≤×≤12). The absence of any sharp peak in XRDpattern revealed the amorphous nature of thin films. Fig. 2 shows the2 μm X 2 μm AFM image for X=2 sample. Structural analysis showsthat all as-deposited thin films are highly smooth with r.m.s. surfaceroughness~2 nm and an average grain size ~30 nm. Elementalcomposition of thin films was checked with EDAX attached to SEMmachine. The EDAX studies revealed that thin films of all compositionscontain higher Sb content and lower Se and Te contents than as-prepared bulk source material used for thermal evaporation. Theaverage observed atomic% of each element measured at various spotsonfilms is given in Table 1. The observed compositional variancemightbe due to incongruent sublimation of different elements present in thesource material. This kind of compositional variance is inevitablypresent in multi-component thin films deposited with thermalevaporation technique. The observed variance in elemental composi-tion of thin filmsmight also because of compositional in-homogeneityin the as-prepared bulk samples. Optical band gap Eg, calculated fromtransmittance and reflectance spectra of as- prepared samples usingTauc relation [16] exhibited a sharp decrease for initial substitution ofSe with Te upto 2 at.% and thereafter a broad hump is observed forfurther substitution of Se with Te [14]. The sharp decrease in optical

band gap was similar to the observed decrease in glass transitiontemperature Tg for tellurium content in the sample [17]. The observedtrend in compositional variance of Eg can be qualitatively explainedusing bond formation energy [17] and band model given by Kastner[18]. The band gap narrowing of Te containing samplesmight be due tointeraction of lone pair and anti-bonding states of Te, which broadensinto a band just above the lone pair band and just below the anti-bonding band of Se respectively, as reflected from ionization energylevels of elemental Te and Se [18]. The observed steepdecrease in Eg forX=2 composition might be due to the fact that this compositioncontains Te–Te weakest bonds [17] that further shifts down the anti-bonding band of the material as emphasized by Kastner [18].

3.1. Electrical properties

Electrical conductivity measurement in dark for as-prepared thinfilms of different compositions showed an exponential increase withtemperature [19] and followed the Arrhenius relation.

σ = σoe−ΔE =kTð Þ ð1Þ

where, σo and ΔE are pre-exponential factor and dc-activation energyrespectively for thermally activated conduction process. Upper part ofFig. 3 represents the plot of lnσ vs 1000/T for as-prepared thin films ofvarious studied compositions. The value ofΔE andσ0 can be calculatedfrom the slope and intercept of linear plot of lnσ vs 1000/Trespectively. For X=10, 12, 14 compositions, there has been steepincrease in conductivity (data for that has not been shown in Fig. 3)that might be related to amorphous to crystalline transformation inthese compositions. The value ofΔE andσ0 for these compositionswascalculated from linear fit of data before transformation. The variationof dc-activation energy with tellurium content exhibit a similar steepdecrease upto X=2 and small increase for X=4 composition asobserved for optical band gap[14] and glass transition temperature[17] respectively as given in Table 2. There is an observation of smallhump in dc-activation energywith further substitution of Sewith Te inthe sample. The observed increase in dc-activation energy for samplewith higher tellurium content might be related to statistical shift ofFermi level with increased tellurium content in the material. Theobserved values of activation energy, optical band gap and pre-exponential factor for all studied compositions are given in Table 2. Theobserved higher values of pre-exponential factor σoN104 Ω−1 cm−1

Fig. 2. AFM images of as-prepared Sn10Sb20Se68Te2 thin film.

Fig. 3. ln(σ) vs 1000/T (K−1) for as-prepared (upper part) and annealed (lower part)Sn10Sb20Se70-XTeX (X=0, 2, 6, 8, 10, 12, 14) films.

Table 2

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for as-prepared thin films represents the conduction of charge carriersthrough extended states constituting the valence and conductionbands [19]. Pre-exponential values also showed a sharp decrease oftwo orders of magnitude for initial substitution of Se with Te. Furthertellurium substitution showed insignificant changes in the pre-exponential factor. Furthermore the variation of pre-exponential

Table 1Observed elemental composition of thin films.

Initial composition Observed atomic% with EDAX

%Sn %Sb %Se %Te

Sn10Sb20Se70 10.2 21.3 68.5 0Sn10Sb20Se68Te2 10.3 21.2 67.2 1.3Sn10Sb20Se66Te4 10.3 21.5 65.1 3.1Sn10Sb20Se64Te6 10.2 22.5 61.8 5.5Sn10Sb20Se62Te8 10.4 23.1 59.3 7.2Sn10Sb20Se60Te10 10.2 23.6 57.8 8.4Sn10Sb20Se58Te12 10.3 24.3 55.6 9.8

factor with activation energy ΔE can be fitted with relation (Meyer–Neldel (MN rule)) [20] except for pointΔE=0.52 eV (X=2 sample) asshown in Fig. 4.

σo = σooeΔE =EMNð Þ ð2Þ

where, σoo is constant and EMN is Meyer–Neldel characteristic energyrespectively. The value of σoo and EMN can be obtained from interceptand slope of the plot of ln σo vs ΔE. In the present case the value of σoo

and EMN is found to be 10.7×10−3 Ω−1 cm−1 and 39.6±3.2 meVrespectively. The obtained values are in good agreement with thoseobtained for different chalcogenide systems [20,21]. The observed shiftof point ΔE=0.52 eV corresponding to X=2 sample might be relatedto tellurium phase separation as observed in this particular compo-sition in bulk form annealed at 250 °C [17]. Note however that XRDpatterns of the thermally evaporated film prepared from a sourcematerial of sameX=2composition (Fig. 1) and of the as-prepared bulksample [17] do not have the same characteristics. MN rule have beenobserved in many materials viz. crystalline, amorphous, liquid andorganic semiconductors that invoke thermally activated physical

Various optical and electrical parameters for as-prepared and annealed Sn10Sb20Se70-XTeX(X=0, 2, 4, 6, 8, 10, 12, 14) thin films.

X (As-prepared) (Annealed)

ΔE(eV)

σ0

(×104 Ω−1 cm−1)Εg (eV)(Ref. [14])

ΔE(eV)

σ0

(×103 Ω−1 cm−1)Εg(eV)

0 0.73 155.8 1.34 0.60 9.64 1.222 0.52 4.93 1.16 0.43 3.97 1.164 0.60 3.49 1.18 0.46 6.90 1.096 0.60 5.24 1.18 0.38 5.25 1.078 0.62 5.76 1.17 0.55 6.97 1.0810 0.72 51.48 1.16 0.33 0.07 1.0412 0.63 9.5 1.13 0.31 0.13 1.0414 0.64 10.6 0.3 3.65

Fig. 4. ln(σo) vs ΔE for as-prepared Sn10Sb20Se70-XTeX (X=0, 2, 4, 6, 8, 10, 14) films.

Fig. 6. XRD pattern of annealed Sn10Sb20Se56Te14 thin film.

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phenomena [20,22,23]. The wide range of values of σoo for differentsystems has been explained using different models [23,24]. For a-Si:Hsystem the value of σoo ~1 is related with standard band transport,while for chalcogenide systems, the obtained value of σoo is quite lowand have been related with interlayer tunneling of charge carriers forthese systems [20]. Roberts [24] proposed a model based on basicfeatures of band structure to explain the observed MN rule inchalcogenide system. According to this model, MN rule emerges forsystem having exponential nature of the localized states in the bandgap and Fermi level is controlled by dominant holes originating fromdeep states in the gap. Thus the observation of MN rule along withappreciable value of Urbach energy in the present chalcogenidesystem justifies the presence of exponentially varying localized statesin the band gap region for this system.

3.2. Annealing effects

The optical band gap of annealed thin films was calculated byextrapolating the (αhν)½ vs hν plot to zero absorption region asshown in Fig. 5. There has been a decrease of ~0.07–0.12 eV in theoptical band gap for annealed thin films for all compositions exceptX=2 composition. The values of optical band gap for as-prepared and

Fig. 5. (αhν)1/2 vs hν for annealed Sn10Sb20Se70-XTeX (X=0, 2, 6, 8, 12) films inset isvariation of optical band gap with tellurium content in annealed Sn10Sb20Se70-XTeX(X=0, 2, 6, 8, 12) films.

annealed thin films with different tellurium content have beentabulated in Table 2. For X=2 sample there might be rearrangementof weakest Te–Te bonds which compensate for the annealing effect asobserved for other compositions. Dark conductivity measurements ofannealed films showed thermally activated conduction process withsingle activation energy for all compositions as shown in lower part ofFig. 3. There has been a similar decrease of ~0.07–0.2 eV in the dc-activation energy for annealed thin films with X≤8 compositionswhile a decrease of ~0.35 eV has been observed for annealed thin filmsof X>8 compositions as shown in Table 2. The observed lower valuesof dc-activation energy in the high tellurium content samplesmight bedue to crystallization of the amorphous films as shown in XRD patternof X=14 annealed thin film in Fig. 6. An approximate two orderdecrease in pre-exponential factor σ0 values for annealed thin filmsreveals the contribution of conduction of charge carriers throughlocalized states. Many researchers have reported different trends forvariation of optical band gap and dc-activation energy with annealingand different explanations have been given for the observed variation[25,26]. The difference in the observed trendsmight be due to differentgrowth/annealing conditions for different systems. Some researchershave reported opposite variation of optical band gap and dc-activationenergy with annealing [27]. In the present case the observed decreasein both optical band gap and dc-activation energy with annealingrepresents structural changes happening in the material associatedwith annealing process. According to Mott and Davis [19], the densityof localized states within mobility gap is directly related to disorderdue to unsaturateddangling bonds present in the systemandobserveddecrease in optical band gap anddc-activation energy due to annealingmight be related to increase in density of unsaturated dangling bondswhich can be corroborated with the two order of magnitude decreasein pre-exponential factor σ0 associated with conduction for annealedfilms indicating the contribution of conduction of charge carriersthrough localized states.

4. Conclusions

Optical and electrical properties of the as-prepared and annealedfilms of Sn10Sb20Se70-XTeX (0≤X≤14) have been studied. The as-prepared thin filmswere found amorphous by X-ray diffraction studies.The trend of optical band gap variation has been explained with help ofbond formation energy and model given by Kastner. Electricalconductivitymeasurements for as-prepared thin films showed a similarsteep decrease in dc-activation energy for low tellurium content in thesample upto 2 at.% and then small hump is observed for furthersubstitution of Se with Te. The studied system is found to obey MNrule with EMN ~39.6 meV and σoo~10.7×10−3 Ω−1 cm−1. The pre-exponential factor σ0 values suggested the dominant conduction of

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charge carriers through extended states for as-prepared films. Anneal-ing of thin films below glass transition temperature Tg lead to decreasein optical band gap and dc-activation energy. This might be due toincreased density of unsaturated dangling bonds with annealing whichcan be corroborated with decreased values of pre-exponential factor σ0

for annealed films that suggested the increased contribution ofconduction through localized states.

Acknowledgment

Author wants to thank Dr S. Sathiaraj of Botswana University foroptical, AFM and EDAX measurements, RSIC, Punjab UniversityChandigarh for XRD experiments.

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