Materials Letters 63 (2009) 1189–1191

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Materials Letters

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Synthesis and characterization of 10% Sb doped CdTe thin films by stacked elementallayer (SEL) method

S. Shanmugan a,⁎, S. Balaji a, D. Mutharasu b

a Materials Laboratory, Thiagarajar Advanced Research Centre, Thiagarajar College of Engineering, Madurai – 625 015, Tamil Nadu, Indiab School of Physics, Universiti Sains Malaysia (USM), 11800, Minden, Pulau Penang, Malaysia

⁎ Corresponding author. Tel.: +91 452 2482240; fax:E-mail address: shanmugan@tce.edu (S. Shanmugan

0167-577X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.02.031

a b s t r a c t

a r t i c l e i n f o

Article history:

Antimony doped CdTe thin Received 25 November 2008Accepted 17 February 2009Available online 23 February 2009

Keywords:Thin filmsDepositionStacked elemental layerStructural propertiesSolar energy materials

films have been prepared by stacked elemental layer (SEL) method. The X-raydiffraction spectra have demonstrated that the structure of the annealed films are polycrystalline in natureand mixed CdTe and Sb2Te3 phases have been observed at high annealing temperature (500 °C). Theincreased texture coefficient has been observed for planes (311) and (220) rather than (111) plane of CdTe inannealed stack (Te/Cd/Sb). Transmission spectra have been recorded and the calculated band gap liesbetween 1.45 and 1.68 eV. A pronounced PL spectrum has been noticed at 532 nm and depicted the presenceof nano size particles in annealed thin films.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Cadmium telluride (CdTe) is one of the most promising poly-crystalline materials for producing solar cells because of its highabsorption coefficient (N104 cm−1) and optimum band gap (1.5 eV).To date, about ten methods of fabricating CdTe thin films have beendeveloped [1–3]. One of the critical stages in device fabrication is thedeposition of CdTe films with controllable composition, since a thinTe-rich layer on the film surface is necessary for the formation of highquality ohmic contacts. To overcome this problem, a buffer layer ofhigh carrier concentration is often applied. Romeo et al. [4] havedemonstrated that rf-sputtered Sb2Te3 is a suitable buffer material forhigh efficiency cells.

SEL technique was originally developed to produce CuInSe2 thinfilms [5]. But it has also been used to produce CdTe films [6–8]. It isparticularly suitable for deposition of compound semiconductor films,as it provides good control of composition; also, it seems to be apromising method for producing highly efficient CdTe/CdS solar cells[7]. This work briefly reports the structural and optical properties ofthe Te/Cd/Sb stacked layers produced by SEL method.

2. Experimental work

Sb doped CdTe thin films were prepared at room temperature bySEL method using PVD unit supplied by Hindhivac Company,Bangalore (model BC 300). The 5N purity Te, Sb and Cd powderswere used. Sequential layer of Te/Cd/Te/Sb/Cd were coated on soda

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lime glass. The deposition rate for Cd & Te and Sb films was main-tained as 3 Å/s and 1.5 Å/s respectively. To achieve the desired sto-chiometry, Te, Sb and Cd elemental layer thickness was adjusted. Theratio of the thickness of elemental layers was maintained as tTe/ tCd=1.53 (Te & Cd) and tTe/ tSb=1.57 (Sb) and the thickness of Te, Sb andCd elemental layers were maintained at 400 nm, 24 nm and 230 nmrespectively for 10% of Sb doping. The distance between the substrateand source was fixed at 10 cm. To enhance the film uniformity, rotarydrive assembly has been used.

The stacked layers of Te/Cd/Te/Sb/Cd were allowed to isochronalannealing from 200 °C–500 °C for about 1 h in Ar gas atmosphere in aseparate vacuum furnace. The structural studies were analyzed byXRD technique using CuKα radiation [λ=1.5406 (Å)] in a Bruker D8Advance diffractometer. The luminescence spectra of stacked layerswere recorded and the filmswere excited by Ar ion laser (λ=390 nm)at room temperature. The transmittance spectra were recorded byusing a Double Beam Shimatzu UV 160A spectrophotometer in awavelength range from 200 to 1100 nm.

3. Experimental results

3.1. Structural studies

The XRD spectra of Sb doped CdTe thin films shows that thestructure of the films is in polycrystalline nature and presented inFig.1. The XRD pattern of an as-deposited glass/Te/Cd/Sb film showedno CdTe and other peaks. The patterns display that the compoundSb2Te3 with (1010) orientation started to form at above 175 °C. As thetemperature increases, peaks related to free elements, Sb and Cddisappear as a result of the reaction between the elements by thermal

Fig. 1. XRD spectra of 10% Sb doped CdTe thin film for different annealing temperatures.

Table 1Structural parameters of Sb doped CdTe thin film by SEL method.

Treated temp.(°C)

(h k l) Lattice parameter(a) Å

Cell volume(Å)3

Texture coefficient(TC)

200 (111) c 6.519 277.16 –

275 (111) c 6.464 270.16 1.350645(311) c 6.491 273.52 0.652151

350 (111) c 6.515 276.59 0.927886(311) c 6.523 277.65 1.072121

425 (111) c 6.514 276.41 0.696524(220) c 6.503 274.95 1.081121(311) c 6.499 274.62 1.222474

500 (111) c 6.525 277.85 0.969274(220) c 6.505 275.31 1.190753(311) c 6.504 275.12 1.320797(400) c 6.497 274.27 1.080741(331) c 6.496 274.13 0.461374(422) c 6.495 273.94 0.977028

1190 S. Shanmugan et al. / Materials Letters 63 (2009) 1189–1191

diffusion. Meanwhile, the intensity of (111), (220) and (311) CdTepeaks increased. At higher annealing temperature, mixed cubic andhexagonal structure and phases of CdTe, Sb2Te3, Sb7Te, Sb2Te2 etc.could be observed. The patterns display sharp and strong reflectionsindicating that annealing temperatures above 300 °C improve filmcrystallinity, probably by promoting grain growth and reducinginhomogeneous stress [8].

The doublet peaks near the 2θ value between 36–40° reflect theamphoteric behavior of Sbmaterials, in that Sb replaces bothCd andTe inthe CdTe lattice with equal probability as indicated by the presence ofmixed phases (CdTe and Sb2Te3) in higher temperature [9].

The lattice parameter ‘a’ can be evaluated from the relation

a2 = d2 h2 + k2 + l2� �

ð1Þ

where h, k, l are Miller indices.In addition, the lattice parameter, cell volume and the texture

coefficient of the Sb doped CdTe thin films are presented in Table 1. Itreveals that the Sb incorporation increases both the lattice parameter[aCdTe=6.48 Å] and also the cell volume [a3CdTe=272.1] as thetemperature increases. It is a consequence of the substitutional atomicsites of Sb and the temperature effects; and finally the structure of thematerial acquires a metastable amorphous phase [9].

In order to investigate the possibility of preferred orientation, theHarris analysis [10] was performed using the following relationshipfor the texture coefficient:

Pi TCð Þ = N Ii = I0ð ÞPNi = 1 Ii = I0ð Þ ð2Þ

where Pi is the texture coefficient of the plane I, Ii is the measuredintegral intensity, I0 is the integral intensity of the JCPDS powderdiffraction pattern of the corresponding peak and N is the number of

reflections considered for the analysis.Pi is the unity for each reflection inthe case of a randomly oriented sample and values of Pi greater thanunity indicate the preferred orientation of the crystallites in thatparticular direction. Table 1 summarizes the calculated values of thetexture coefficients. Note that the texture coefficient value of plane (111)decreased drastically and the value increases significantly for (311) and(220) planes [11] due to the formation of a compound such as Sb2Te3 asthe annealing temperature increases. Therefore, our results suggest thatthe addition of Sb by SEL method assists in promoting the orientation of(311) and (220) rather than the (111) orientation.

3.2. Optical studies

Optical transmittance spectra were performed and presented inFig. 2 (a). The figure reveals that the Sb incorporation reduces thetransmittance of pure CdTe thin film prepared by SEL method.

Fig. 3. PL spectra of Sb doped CdTe thin film annealed in 425 °C for 1 h.

1191S. Shanmugan et al. / Materials Letters 63 (2009) 1189–1191

As a consequence of the Sb introduction into the CdTe lattice, itcould observed near the IR region that the valley disappears and abroad band centered approximately at 1030 nm is created. This bandcan be associated with deep level centers originated by the Sbintroduced into the CdTe layer, and a similar effect originated in CdTe:Ni, CdTe:Co in the presence of the transitionmetal impurities [12]. Theinfluence of the increasing free carrier concentration on the spectraldependence of transmittance in the near IR region manifests in adecrease of the transmittance due to the free carrier absorption [13].

The optical band gap of all films was analyzed by using the relation

αhυ = A hυ−Eg� �1=2 ð5Þ

where A is a constant. The variation of (αhυ)2 with photon energy hυfor the prepared Sb doped CdTe thin films is shown in Fig. 2 (b). It hasbeen observed that the plots of (αhυ)2 versus hυ are mostly linearover a wide range of photon energies indicating the direct type oftransitions. The intercepts (extrapolations) of these plots (straightlines) on the energy axis [(αhυ)2=0] give the energy band gap. Fig. 2(b) reveals that the band gap varied from 1.6–1.68 eV and theobserved value is slightly higher than that of pure CdTe (1.5 eV). Thephotoluminescence spectra of Sb doped CdTe thin film were recordedat room temperature and presented in Fig. 3. The spectra reveal thatthe sharp high-energy peak of excitonic luminescence exists at532 nm with a wide low-energy band of emission from trap states.This band has been inhomogeneously broadened due to the sizedistribution of CdTe crystallites and fluctuations in their microenvir-onment [14]. Also we could observe one more peak at 485 nmexpressing the same effect on the size of the particles. This emission

Fig. 2. Optical studies (a) transmittance spectra and (b) band gap variation of 10% of Sbdoped CdTe thin film with different annealing temperatures.

line explains that the prepared films have nanocrystals and similarresults have been reported elsewhere [15].

4. Conclusion

In this paper, antimony doped CdTe films were prepared using theSEL method. The XRD results show that the films are polycrystallineand exhibit the presence of mixed structure and phases (CdTe andSb2Te3 compounds) at a higher annealing temperature. In opticalstudies, a significant improvement in both transmittance andabsorption could be observed. The calculated band gap values liebetween 1.45–1.68 eV. PL spectra reveal that the prepared thin filmshave nano size particles. The observed results facilitate the formationof Sb2Te3 over the CdTe thin film by stacking the elemental layer of Sbover the Te/Cd stack by the SEL method.

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