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Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing 13 (2010) 298–302

1369-80

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/mssp

Effect of Ar+ ion irradiation on structural and optical propertiesof e-beam evaporated cadmium telluride thin films

S. Shanmugan n, D Mutharasu

Nano Optoelectronics Research Laboratory, School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia

a r t i c l e i n f o

Available online 6 January 2011

Keywords:

Thin films

Ar+ ion irradiation

E-beam

CdTe

Structural properties

Optical properties

01/$ - see front matter & 2010 Elsevier Ltd. A

016/j.mssp.2010.12.007

esponding author. Tel.: +60 04 6533672; fax

ail address: shagan77in@yahoo.co.in (S. Shanm

a b s t r a c t

CdTe thin films were prepared using e-beam evaporation technique. The prepared films

were irradiated by Ar+ ions at different fluencies using multipurpose aluminum (Al)

probe as in-situ. This could also be used in ion bombardment for cleaning the substrate

prior to coating. The as grown and Ar+ ion irradiated films were confirmed to be of

polycrystalline nature with X-ray technique. Ar+ ion irradiation enhances the growth of

(1 1 1) oriented CdTe crystals and the Cd enrichment on the surface of CdTe thin films.

Higher Ar+ ion flux helps to grow (2 2 0) oriented CdTe thin film. A considerable change in

structural parameters like crystallite size, lattice parameter, internal strain, etc. could be

observed as a result of high Ar+ ion flux. The applied in-plan stress in both as grown and

irradiated film was identified to be of tensile nature. The applied stress was observed

between 0.016 and 0.067 GPa for all Ar+ ion irradiated samples. As a result of the Ar+ ion

irradiation, the in-plan stress varies between 1.38�109 and 5.58�109 dyn/cm2. The

observed bad gap was increased for higher Ar+ ion flux. It shows the effect of Ar+ ion

irradiation on the modifications of optical properties. The observed results were

encouraging on the use of simple multipurpose Al probe for Ar+ ion irradiation process

as in-situ.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

In the recent years, thin films are of much interestbecause of their various applications in semiconductingdevices, photovoltaics, optoelectronic devices, solar energyconverters, etc. [1,2]. Ion bombardment plays an importantrole in the film properties such as crystallinity, packingdensity, surface roughness, etc.[3]. Parish et al. [4] studiedthe lattice disorder produced in CdS crystals on implanta-tion with different types of ions. If the semiconductor isimplanted with electrically inactive ions then the resultanteffects can be attributed exclusively to the lattice disorder.The modification on the optical bandgap and the absorp-tion coefficient was observed from the Ar+ and N+ ionimplanted CdS thin films and also the formation of Cd

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ugan).

metallic clusters was reported elsewhere [5]. Normally ionirradiation process had been carried out using separate ionsources in a separate vacuum chamber. It may affect theproperties of thin films as well as the device while transitfor process after deposition. The same authors [6] tried toirradiate the O8 + ion generated by simple multipurpose Alprobe to study the influence on structural and opticalproperties of e-beam evaporated CdTe thin films. In thepresent work, e-beam evaporated CdTe thin films wereirradiated by Ar+ ions as in-situ using multipurpose Alprobe located inside the vacuum chamber and the effect ofAr+ ion flux on the structural and optical properties isreported here.

2. Experimental techniques

CdTe thin films were deposited over the glass substrateby e-beam evaporation technique. To get the uniform film

S. Shanmugan, D Mutharasu / Materials Science in Semiconductor Processing 13 (2010) 298–302 299

thickness, rotary drive assembly was used and all filmswere coated with constant speed of 40 rpm. Prior tocoating, the substrates were cleaned by O8 + ion bombard-ment process in presence of highly pure O2 using multi-purpose Al probe, which is followed by conventionalcleaning. The probe has the surface area of 49.5 cm2. Thesubstrate–source distance was fixed at 14 cm. The CdTethin films with 240 nm thickness were prepared in thevacuum of 7�10�6 mbar. The rate of deposition wasmaintained at 2 A/s.

The Ar+ ion irradiation processes were performed forabout 10 min with three different ion flux densities(3.14�1016, 6.294�1016 and 9.44�1016 ions/cm2 s) in pre-sence of ultrahigh pure Ar gas. The sample names areidentified for different process conditions throughout thistext as follows: ArI=3.14�1016 ions/cm2 s, ArII=6.294�1016

ions/cm2 s and ArIII=9.44�1016 ions/cm2 s. The chamberpressure of about 2–5�10�3 mbar was maintained for allion irradiation processes. The flux densities were fixed byadjusting both Al probe current and gas pressure inside thechamber. The distance between the probe and substrate wasfixed as 6 cm. The structural properties were analyzed by XRDtechnique using Cu-Ka radiation (l=1.5406 A) in Bruker D8Advance diffractometer. The transmittance spectra wererecorded using a double beam Shimadzu UV 160 A spectro-photometer in the wavelength range of 400–1100 nm.

Fig. 1. XRD pattern of as grown and Ar+ ion irradiated CdTe thin film.

Table 1Structural parameters of (1 1 1) peak of as grown and Ar+ ion irradiated CdTe t

Processing parameter

(ion irradiation)2y FWHM

(12Th.)

Internal stress

s (GPa)

d space

(A)

Lattice

consta

As grown 23.8165 0.1968 �0.0166 3.736 6.471

ArI 23.8650 0.2969 �0.0467 3.726 6.453

ArII 23.9116 0.3000 �0.0671 3.718 6.441

ArIII 23.8462 0.3000 �0.0385 3.728 6.458

3. Results and discussion

3.1. Structural properties

Ar+ ion irradiated CdTe thin films were analyzed withXRD technique and the observed spectra are shown inFig. 1. The sharp diffraction lines indicate a good crystallinequality with the (1 1 1) preferred crystalline orientation ofall Ar+ ion irradiated CdTe thin films. The calculatedstructural parameters are given in Table 1. From the table,it is found that the full-width half-maximum (FWHM) of(1 1 1) orientation increases with the increase in Ar+ ionflux from condition I to III. It is the evidence of improvedcrystallinity of the Ar+ irradiated film. It also shows that theintensity of preferred (1 1 1) oriented peak increases withthe increase in Ar+ ion flux. The change in intensity of(1 1 1) oriented peak is clearly indicated in Fig. 2. It can alsobe observed that the Ar+ ion flux helps to increase thecrystallinity of the films, which may be attributed to thereduction of Cd vacancy [7]. From Table 1, the position of(1 1 1) crystalline line shifts slightly to higher angularposition as Ar+ ion flux increases. It is attributed to theformation of Cd rich CdTe thin film and hence extra Cdreduces the lattice constant (see Table 1) due to reductionin the bond length produced by the interstitial Cd atoms inCdTe lattice [8]. It is also confirmed by observing thedecrease in d space value with the increase in Ar+ ion flux.

Another reason for the decrease in d spacing with theincrease in Ar+ ion flux is described as follows: the

hin films.

nt (a)

Micro-strain, e(�10�2 lin�2 m�4)

Dislocation density, d(�1020 lin/m2)

Crystallite

size, D (nm)

4.81 18.57 43.09

7.26 8.16 28.56

7.34 7.99 28.27

7.34 7.99 28.27

Fig. 2. Change in intensity of (1 1 1) orientated peak of CdTe thin film with

Ar+ ion irradiation.

S. Shanmugan, D Mutharasu / Materials Science in Semiconductor Processing 13 (2010) 298–302300

interplanar spacing decreases gradually (from 3.736 to3.726 A) as the Ar+ ion flux increases. Since the contam-inations (e.g. O2, N2, etc.) are in general of smaller atomicsize compared with both Cd and Te, the Ar+ ion irradiationhelps to remove/reduce these contaminants in the originalCdTe lattice. Thus, the decrease in their concentrationresults in shorter interplanar spacing. The two types ofbonding should also be considered, as the ionic radii ofTe�2 and Cd+2 are 0.221 and 0.097 nm and their tetra-hedral covalent radii are 0.132 and 0.148 nm, respectively.Therefore, the decrease in d spacing as the Te/Cd ratioincreases (decreases in CdTe antisites) may indicate theincreasing contribution of the covalent-type [9] as thedensity of lattice defect decreases.

It seems that the Ar+ ion influences the crystal growthespecially in (1 1 1) orientation with high crystallinenature. From Fig. 1, it is also found that a (3 1 1) orientedpeak with very small intensity is observed in as grown film.However it disappears when the as grown sample isexposed to Ar+ ion irradiation with flux of 3.14�1016

ions/cm2 s. Meanwhile, a peak related to (2 2 0) phase isobserved with high Ar+ ion flux (9.44�1016 ions/cm2 s).It seems that the Ar+ ion irradiation with higher fluxsupports the growth of the crystalline CdTe thin films intheir preferred (1 1 1) and (2 2 0) orientations. In addition,it is also observed that the (1 1 1) peak position shiftsslightly toward lower angular position at high Ar+ ionflux (ArIII). This may be due to the presence of (2 2 0)oriented CdTe phase at this flux. To understand theinfluence of Ar+ ion on structure parameters of CdTe thinfilm in detail, the structural parameters were calculatedfrom the XRD spectra (see Table 1) and discussed in thefollowing section.

The crystallite size (D) was calculated using the DebyeScherer formula [10] from the full-width at half-maximum(w) measurements:

D¼ 0:94l=wcosy ð1Þ

The strain (e) was calculated from

e¼wcosy=4 ð2Þ

The dislocation density (d), defined as the length ofdislocation lines per unit volume of the crystal, wasevaluated from [11]

d¼ 1=D2 ð3Þ

The lattice parameter ‘a’ was evaluated from

a2 ¼ d2ðh2þk2þ l2Þ ð4Þ

where h, k and l are the Miller indices. The internal stress(s) in the deposited film is calculated using

s¼�Eðda�doÞ=ð2doYÞ ð5Þ

where do and da are the d spacing of CdTe bulk and thin filmforms, respectively. E and Y are Young’s modulus andPoisson’s ratio of CdTe, respectively [12].

From Table 1, it is observed that the crystallite sizedecreases from 43.09 (as grown) to 28.27 nm (irradiated)as Ar+ ion flux increases. The decrease in crystallite sizeseems to be due to the presence of low dislocation densityin the irradiated films compared to the as grown film.

In addition to this, the calculated lattice parameter isdecreased considerably as Ar+ ion flux increases. Thisbehavior is also observed for CdCl2 treated CdTe thin films[13] and Cd rich CdTe thin films [8]

It is due to the formation of dense crystallites as a result ofcell volume contraction. The observed lattice constant a of Ar+

ion irradiated CdTe thin films is less compared to that of thepowder sample (6.481 A) [14] suggesting that these films aresubjected to a tensile stress in the plane of the substratesurface as a result of internal stress on the crystallites anddue to the lattice mismatch and differences in thermalexpansion coefficients [15]. The smaller value of a is due tothe recrystallized lattice. A further decrease in a is an evidencefor the stress applied as a result of Ar+ ion flux. A decrease in d

space value also demonstrates the effect of Ar+ ion irradiationon an increase in the applied tensile stress parallel to thesubstrate surface clearly. Due to ion irradiation on the surfaceof the prepared films, there may be a chance of atomicvibration as a result of ion collisions near the surface. Thiseffect may develop the stress, which is applied perpendicularto the thin film surface. Stress affects the mechanical proper-ties of the films such as the stability of microstructure, theadhesion between film and substrate and the optoelectronicproperties of the deposited films.

Using Eq. (5), the applied stress was evaluated and thesign of the internal stress observed from the calculationreveals the nature of stress applied on the film surface astensile nature. We can see from Table 1 that the tensilestress increases from 0.016 to 0.067 GPa for Ar+ ion fluxincreases up to ArII. It is also observed that the appliedtensile stress of Ar+ ion irradiated thin films is compara-tively more than that of as grown samples. However anoticeable decrease could also be observed with high Ar+

ion flux (ArIII). All observed value except dislocationdensity shows the inverse effect of heat treated CdTe thinfilms prepared by chemical bath deposition [16].

Stress in films can be intrinsic, caused by the conditionsprevailing during deposition (temperature, deposition rate,impurities, etc.). On the other hand stress can be extrinsicto the film, but intrinsic to the composite film-substratesystem, caused by the difference in the thermal expansioncoefficients [17].

In order to discuss in detail, the change in in-plan stresswith Ar+ ion irradiation can be explained by knowing themagnitude of strain (e), it is possible to calculate the in-plane stress (s) measured normal to the sample surfacefrom [18]

e¼ dfilm�dpowder=dpowder ¼sð2S11þ4S12�S44=3þS44=2sin2cÞ

ð6Þ

where dfilm and dpowder are the interplanar distances forCdTe film deposited on glass substrates and powdersample, respectively, c=0 and S11, S12 and S44 are thecomponents of compliance. For CdTe, S11=4.27�10�12

dyn/cm2, S12=1.73�10�12 dyn/cm2 and S44=5�10�12

dyn/cm2 [19].Fig. 3 shows the variation in stress in CdTe thin films

with the Ar+ ion flux. The minus sign indicates compres-sive stress and plus sign indicates tensile stress. As we cansee from the figure, the irradiated film is under tensile

Fig. 3. Change in in-plan, extrinsic and intrinsic stress applied during the

process of Ar+ ion irradiation.Fig. 4. Transmittance spectra of Ar+ ion irradiated CdTe thin films

prepared by e-beam technique.

Fig. 5. Bandgap of Ar+ ion irradiated CdTe thin films prepared by e-beam

technique.

Fig. 6. Absorption coefficient of Ar+ ion irradiated CdTe thin films

prepared by e-beam technique.

S. Shanmugan, D Mutharasu / Materials Science in Semiconductor Processing 13 (2010) 298–302 301

stress and the stress increases with Ar ion irradiation andreaches a maximum value and then starts to decrease. Evenon further irradiation, the change in stress is observedwithin the tensile region. Due to irradiation, the in-planstress varies between 1.38�109 and 5.58�109 dyn/cm2.These values are very much larger than the critical value offormation of structural defects for CdTe, which is approxi-mately equal to 109 dyn/cm2 [20]. We have also estimatedthe intrinsic and extrinsic components of the stress and theresults are presented in the same Fig. 3. The extrinsiccomponent of the strain (eext) originating from the differ-ence in thermal expansion coefficients between CdTe andthe substrate can be estimated using [21]

sext ¼

Z To

T1

ðaf�asÞdT ¼ ðaf�asÞðTo�T1Þ ð7Þ

where af and as are the thermal expansion coefficients of thefilm and the substrate, respectively, To is the room temperatureand T1 the annealing temperature. Here we have consideredthe term To – T1 as To since all our experiments were carried outat room temperature. Assuming that the change in thermalexpansion coefficient of the CdTe film is negligible in theroom temperature, we have taken 5.9�10�6 K�1 [22]. as is9�10�6 K�1 at room temperature [23]. The extrinsic stress(sext) was calculated using Eq. (6), and the difference betweenthe in-plan stress and the extrinsic component is the intrinsicstress (sint). From Fig. 3, a change in intrinsic stress fromcompressive (as grown) to tensile (irradiated) as a result of Ar+

ion irradiation is also observed.

3.2. Optical properties

In order to study the effect of Ar+ ion irradiations on theoptical behavior, transmission spectra were recorded for allsamples and shown in Fig. 4. It shows that the Ar+ ion fluxdoes not affect the transmission curve considerably.However the presence of small interference curve revealsthe uniform film thickness even after ion irradiationprocess. Variation in bandgap with respect to the photonenergy is represented in Fig. 5. It shows that the bandgapvalue increases as Ar+ ion flux increases.

S. Shanmugan, D Mutharasu / Materials Science in Semiconductor Processing 13 (2010) 298–302302

The absorption coefficient (a) was calculated from thetransmittance spectra using

a¼ 4pkf=l ð8Þ

The optical bandgap of all films was analyzed using

ahu¼ Aðhu-EgÞ1=2

ð9Þ

where A is a constant.The observed bandgap value lies between 1.55 (as

grown) and 1.62 eV (for higher Ar+ ion flux). The calculatedabsorption coefficient is plotted against the photon energyin Fig. 6. It shows that no considerable influence on theabsorption coefficient could be observed for various Ar+ ionfluxes.

4. Conclusion

E-beam evaporated CdTe thin films were irradiated withAr+ ions produced by simple multipurpose Al probe. Thestructural analysis states the possibility of the formationof preferred (1 1 1) oriented CdTe thin films by Ar+ ionirradiation as in in-situ process. The observed resultsevidenced the growth (2 2 0) oriented CdTe thin film athigh Ar+ ion flux. From the observed results, Ar+ ion fluxsupports the growth of Cd rich CdTe thin film surface. Theapplied in-plan stress was evaluated to be of tensile natureand increased with the increase in Ar+ ion flux. Theirradiation process also helps to modify the structuralparameters for various ion fluxes. From the observedoptical properties, it is concluded that Ar+ ion irradiationhelps to improve the bandgap of the prepared CdTe thinfilms as well as the structural parameters.

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