Thin Solid Films 519 (2010) 1078–1081

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Thin Solid Films

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Structural and electrical studies of thermally evaporated nanostructuredCdTe thin films

Sukhvir Singh ⁎, Rajeev Kumar, K.N. SoodNational Physical Laboratory, Dr. K.S. Krshnan Marg, New Delhi, 110012, India

⁎ Corresponding author.E-mail address: sukhvir@mail.nplindia.ernet.in (S. S

0040-6090/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.tsf.2010.08.047

a b s t r a c t

a r t i c l e i n f o

Available online 25 August 2010

Keywords:NanostructuresSemiconductorsMicrostructuresVDS TechniqueThermal evaporation

CdTe thin films were deposited on KCl and glass substrates using thermal evaporation technique under highvacuum conditions. CdTe bulk compound grown by vertical directional solidification (VDS) technique wasused as the source material to deposit thin films. Powder X-ray diffraction technique was employed to identifythe phase of the as grown bulk CdTe compound as well as its thin films. Surface morphology and thestoichiometry of the bulk compound and thin films was carried out by using scanning electron microscope(SEM) with an attachment of energy dispersive spectrometer(EDS). Microstructural features associated withthe as deposited CdTe thin films were studied by using transmission electron microscope (TEM). The filmsdeposited on to glass substrates at different temperatures have been used to study the I–V characteristics of thefilms. These parameters have been studied in detail in order to prepare good quality nanostructured thin filmsof CdTe compound. CdTe bulk compound grown by VDS method and its thin films prepared by thermalevaporation method found to have single phase with cubic structure. Size of the particles in the as depositedfilms vary between 5 and 40 nm In the present study efforts have been made to correlate the electrical andoptical properties of the CdTe thinfilmswith the correspondingmicrostructural features associatedwith them.

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1. Introduction

Numerous materials have been investigated for solar cell applica-tions [1]. This leads to the development of new and innovative classesof semiconductor devices: Crystalline silicon, amorphous silicon andthin film polycrystalline materials like cadmium telluride (CdTe),Cadmium zinc Telluride (CZT), Zinc Telluride (ZnTe), etc. areprominent materials. The development of thin film solar cells is anactive area of research at this time. Among the II–VI compounds, CdTeis one of the suitable candidates for the production of thin films solarcells due to its ideal band gap(1.45 eV), high absorption coefficientand ease of thin film fabrication. Recently nanostructured thin films ofCdTe semiconducting compound prepared through chemical routeare found to have application as glucose biosensors [10].

The choice of the material for solar cells depends upon thepotential conversion efficiency expected from the devices made fromthem. CdTe is found to be promising material [2] for the highefficiency solar cells. CdTe has the flexibility to be deposited by anumber of deposition techniques such as screen printing [3],electrodeposition [4,5] etc. Thermal evaporation technique has beenthe most productive method owing to the very high deposition rate,low material consumption and low cost of operation. Enormousamount of work has already been reported on structure, optical and

electrical properties of CdTe thin films [6–8]. However it still requiresfurther investigation to optimize structural, optical and electricalparameters. In the present study, VDS technique [9] grown CdTe bulkcompound has been used as the source material to deposit thin filmsusing thermal evaporation technique under high vacuum conditions.After optimizing the certain parameters to obtain good qualitynanostructured thin films of CdTe compound, a further study will bedevoted to utilize these films as biosensors.

2. Experimental

Stoichiometric CdTe bulk compound has been grown by using highpurity cadmium and tellurium by vertical directional solidification(VDS) method. In this method high purity Cd (99.999%) and Te(99.999%) were taken in stoichiometric proportions and loaded in thequartz ampoule of 12–15 mm diameter and 150 mm in length withconical shape at one end. The ampoule loaded with the charge wasevacuated under high vacuum and sealed. Later the ampoule washeated in a system consisting of a vertical single zone resistancefurnace which is fully temperature controlled. VDS method was usedto grow good quality, single phase CdTe bulk compound. A small piecetaken from this bulk compound was used as the source material todeposit thin films of CdTe compound .Using this process it is expectedto get good quality, single phase CdTe film having particle size in nanodimensions. Powder X-ray diffraction technique was employed toidentify the phase of the as grown bulk CdTe compound as well as itsthin films. The as grown CdTe compound was used as the source

Fig. 1. XRD pattern of the as grown bulk CdTe compound.

Fig. 3. FTIR spectra of commercial and VDS grown CdTe bulk compound.

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material to deposit nanostructured thin films by thermal evaporationmethod under vacuum conditions.

The films were deposited onto freshly cleaved KCl crystal and glasssubstrates at temperatures of 300, 373 and 473 K. The thickness of thefilms were varied from 100 to 120 nm and was measured by thethickness monitor attached in the thermal evaporation system.

The films deposited onto KCl, crystal were used for the micro-structural investigations under transmission electron microscope(TEM) model JEM 200CX accelerated at 160 kV .The films depositedonto glass substrate were used for studying the electrical properties.Thin films thus prepared were examined for elemental analysis andstoichiometry by energy dispersive spectrometer (EDS) oxford ISIS

Fig. 2. (a): SEM images of CdTe bulk compound. (b) EDS spectra of the bulk CdTecompound.

300 system attached to scanning electron microscope (SEM) Leo 440.Transmittance of the as grown and the commercially available CdTecompound was measured by the FTIR system, make Perkin Elmer.

3. Results and discussion

The bulk CdTe compound grown by VDS method was found to beof good quality having no voids, pits on the surface of the lump andhave smooth surface and grayish finish. Powder X-ray diffractionstudies of the powdered as grown lump show very sharp and wellresolved peaks of single phase cadmium telluride having cubic

a

b

Fig. 4. (a) & (b) TEM Bright field image of CdTe thin film deposited on KCl substrate at300 K and 473 K.

a

b

Fig. 5. (a) & (b) XRD pattern of CdTe thin film deposited on KCl substrate at 300 K and473 K.

Fig. 6. I–V characteristics of CdTe thin films at deposited at 300 K, 373 K and 473 K.

Fig. 7. Variation of resistivity of CdTe thin films at deposited substrate temperature.

1080 S. Singh et al. / Thin Solid Films 519 (2010) 1078–1081

structure (Fig. 1). The powder data are in good agreement with theJCPDS file of CdTe (cubic) JCPDS (75-2086.

A small broken piece taken from the compound was thoroughlyscanned under SEM to examine the morphological details. From themicrographs in Fig. 2(a) show the presence of set of uniform cleavageplanes revealing the formation of ordered structure of CdTecompound has taken place during the synthesis. No voids andinclusions have been noticed in the bulk compound. EDS analysis ofthe as grown bulk compound revealed the formation of stoichiometricCdTe bulk compound as depicted in EDS pattern shown in Fig. 2(b).The FTIR spectra as shown in Fig. 3 revealed the transmittancefeatures of the as grown bulk CdTe compound as well as thecommercially available CdTe compound. From the FTIR data it isobserved that both the compounds show the absorbance peaks at thesame wave numbers revealing growth of good quality compoundusing VDS method. However the variation in the transmittance asdepicted in Fig. 3 may be due to the presence of defects introducedduring the synthesis of the compound. The noise present in thespectra is due the fact that the CdTe compounds under investigation isnot polished since we have taken only the broken piece from thecompound in both the cases.

Microstructural features associated with CdTe thin films depositedon to KCl substrate at 300 K and 473 K prepared by using thermalevaporation technique have been investigated by using TEM. From theTEM image as shown in Fig. 4a, it is observed that film is uniform,continuous and consists of randomly oriented fine crystallites of sizevarying between 5 and 20 nm. The shapes of the particles are found tobe round and polyhedral distributed uniformly throughout the film.Thin film when deposited at 473 K (Fig. 4b) shows that the particlesize was found to be increased and vary between 15 and 40 nm.

X-ray diffraction patterns of CdTe thin films prepared at 300 K and473 K are shown in Fig. 5(a) and (b) respectively. From the XRDmeasurements of the CdTe thin film deposited at 300 K it is observed

that the polycrystalline growth has taken place. The sharp peakrepresents (111) plane of CdTe compound having cubic structure.Broadening of diffraction peak along (111), (220) and (311) planeindicates the smaller particle size in the film deposited at 300 K.Presence of weak and broad peaks are may be due to the stress,vacancies and defects introduced during the synthesis of thin films.However the peaks become sharp as the deposition temperatureincreased to 473 K as revealed in Fig. 5(b).

This indicated the formation of a more ordered structure, havingless number of defects and only thermodynamically stable phases arepresent in the film deposited at 473 K temperature. All the peaksreflected in the XRD patterns are in good agreement with the JCPDSfile of single phase CdTe (cubic) JCPDS (75-2086).

A four probe technique was used to determine the I–V character-istics of the thin films deposited at 300, 373 and 473 K temperaturesshown in Fig. 6. It was observed that the current increases withincreasing substrate temperature for the same voltages. This was dueto the fact that an increase in grain size thereby reduces the number ofgrain boundaries and resulting in formation of more ordered structureof the film at a higher deposition substrate temperature.

It is well known that the electrical transport properties ofpolycrystalline thin film is markedly different from those of singlecrystals due to the presence of grain boundaries. The structure of grainboundaries is complex and process dependent. These grain bound-aries generally contain a high density of trapping centers which areresponsible for grain boundary space-charge potential barriers. Thesepotential barriers oppose the passage of carriers from a grain to theneighboring ones. Thus the study of the electrical properties of grainboundaries has become extremely important. A plot betweenresistivity and temperature as shown in Fig. 7 revealed that theresistivity of CdTe thin films decrease as the deposition temperature

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increases suggesting the semi conducting nature of the thin filmswhich was expected in our study.

4. Conclusion

A good quality CdTe bulk compound having no voids and pits onthe surface and have smooth surface has been grown using the VDSmethod. The compound was found to be stoichiometric and consistsof a single phase cubic structure FTIR data of as grown compoundmatches with commercially available CdTe compound. Thin filmsprepared by thermal evaporation technique are found to be uniformand homogeneous having randomly oriented fine grains withpolycrystalline nature. The I–V measurement study revealed thesemi conducting behavior of thin films. We have succeeded inpreparing good quality nanostructured thin films of CdTe compoundhaving a single phase and in stoichiometric proportion. A furtherstudy in view to utilize these films for biosensors is underway.

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