Ž .Thin Solid Films 373 2000 235]238

Ž . Ž .Structural and optical properties of ZnO CdO thin filmsx 1yxobtained by spray pyrolysis

Guillermo Santanac,1, Arturo Morales-Acevedoc,U, Osvaldo Vigila,1, Lıdice Vaillant b,´Francisco Cruza, Gerardo Contreras-Puentea

a ´Escuela Superior de Fısica y Matematica del IPN, Edificio 9 U.P.Z., Mexico D.F., C.P. 07738´ ´bFacultad de Fısica-IMRE, Uni ersidad de la Ha¨ana, 43100 La Ha¨ana, Cuba´

c ( )Seccion de Electronica del Estado Solido SEES , Departamento de Ingenierıa Electrica, CINVESTAV del IPN, Apartado Postal 14-740,´ ´ ´ ´ ´Mexico D.F.

Abstract

Ž . Ž .Thin films of ZnO CdO oxides were deposited on glass substrates by spray pyrolysis and annealed in air at 4508C. Thex 1yxstructural and optical properties of the as-grown and thermally annealed thin films are presented. The crystalline structure was

Ž .studied by X-ray diffraction XRD having found the presence of the CdO cubic phase pattern for low Zn concentrations and amixing of cubic-CdO and hexagonal-ZnO phases for low Cd concentrations. The crystallinity of all samples improves with thethermal annealing. The optical band-gap was also studied from the optical transmittance for the as-grown and annealed samples.As expected, the band-gap changes between that for pure CdO and that for ZnO. Q 2000 Elsevier Science S.A. All rightsreserved.

Keywords: Spray pyrolysis; X-Ray diffraction; Thermal annealing

1. Introduction

Ž .Transparent conducting oxide TCO thin films havebeen widely used in solar cell applications. CdO andZnO have high transparency in the visible region of theelectromagnetic spectrum and show n-type conductiv-ity, mainly due to oxygen vacancies. With a ranging

w xband gap of 2.2]2.7 eV 1 , CdO presents the advan-tage of a low resistivity with respect to the high values

U Corresponding author. CINVESTAV-IPN, Electrical Engineer-ing Department, Avenida IPN No. 2508, 07360 Mexico D.F., Mexico.´ ´Fax: q52-5747-7114.

ŽE-mail address: amorales@gasparin.solar.cinvestav.mx A..Morales-Acevedo .

1Permanent address: Facultad de Fısica-IMRE, Universidad de la´Habana, 43100, La Habana, Cuba.

obtained for ZnO, but this exhibits a higher trans-parency, having a band gap of approximately 3.2 eV. Itis known that it is difficult to obtain simultaneously ahigh transmission coefficient in the visible region and

w xgood conductivity qualities 2 , however, a ternary com-pound which combines these properties in a controlledway may allow the optimization of the window layer.

On the other hand, it is usual that for solar cellfabrication several deposition and thermal treatmentsteps are necessary in order to improve the photovol-taic characteristics. Many of these steps are made inthe temperature range of 350]5008C, after the deposi-

w xtion of the window materials 3]5 . From this point ofview, it is very important to know the post-thermalannealing effect on the properties of the window thinfilms, if we want to use them as heterojunction partnerson different solar cells. In this paper, we report theeffect that a post-annealing at 4508C in air has on the

0040-6090r00r$ - see front matter Q 2000 Elsevier Science S.A. All rights reserved.Ž .PII: S 0 0 4 0 - 6 0 9 0 0 0 0 1 1 4 2 - 1

( )G. Santanaı et al. r Thin Solid Films 373 2000 235]238´236

structural and optical properties of cadmium]zinc ox-ide with variable composition x, grown by spray pyroly-sis.

2. Experimental

The films were grown onto corning glass substrates,using a typical spray pyrolysis system. The experimental

w xsetup used is similar to that described in Vigil et al. 6 .The spray solution was prepared by mixing the ap-

Ž .propriate volumes of cadmium acetate 0.1 M and zincŽ .acetate 0.1 M dissolved in a mixture of methanol and

Ž .deionized water 1:1 . The substrate temperature wasfixed at 2508C and was controlled within "58C through

Ž .a thermocouple chromel-alumel as a sensor for thetemperature controller. The solution flow rate and gaspressure was kept constant at 5 mlrmin and 5=104

kgrm2, respectively. N was used as the carrier gas.2The distance between the nozzle and the substrate was0.25 m. Taking into account the growth rate for each

w xcomposition x 6 , the samples were grown for differentdeposition times in order to obtain similar thickness forall films of approximately 0.35 mm. The X-ray diffrac-tion spectra were obtained by means of a D-500 SiemensX-ray system using the Cu-K line. Optical transmis-a

sion data were obtained with an UV-Visible Shimadzu3101 PC double beam spectrophotometer. The band-gap was obtained from the transmission spectra, andthe layer thickness was measured with a Sloan DektalII profilometer. The sample post-deposition thermaltreatments were performed in an open quartz tubefurnace in an air atmosphere at 4508C for 2 h. For each

Žsample with a different composition xs0; 0.25; 0.50;.0.75 and 1 , the structural and optical properties were

measured, before and after the annealing.

3. Results

Fig. 1 shows the X-ray diffraction patterns for sam-ples grown with different compositions at 2508C. For

Ž .xs0 CdO , two diffraction peaks were identified asŽ . Ž .the 111 and 200 reflections of the cubic structure

˚ Ž .with a lattice parameter of 4.71 A. For xs1 ZnOŽ . Ž . Ž .three peaks related to the 100 , 002 and 101 planes

of the hexagonal structure are observed with lattice˚ ˚parameters as3.24 A and cs5.20 A. Notice that the

XRD patterns for xs0.25 resemble the pattern forpure CdO. This fact could be associated to the poorcrystallinity of ZnO resulting from the low temperatureused. When the samples were annealed in an air atmo-

Ž .sphere during 2 h Fig. 2 , the peaks corresponding tothe CdO and ZnO structures were present for samples

Ž .with intermediate compositions xs0.50 and xs0.75 ,telling us about the re-crystallization and the formationof a mixed oxide. For these compositions a definedmixing of CdO and ZnO phases is observed. A relative

Ž .Fig. 1. X-Ray diffraction patterns for different compositions x ofŽ . Ž .as-grown ZnO CdO thin films.x 1yx

lower intensity of the peaks is obtained when theconcentration of Zn is increased. This fact could beassociated to a poor crystallinity of ZnO respect toCdO. For the ZnO sample, the film as deposited ex-

Ž .hibits a 002 preferential orientation, but it changes toŽ .a 100 orientation after the thermal annealing. This

result is in agreement with those obtained forw xZn Cd O compounds grown by sol-gel 5 .x 1yx

Optical transmission spectra for as-grown films areshown in the Fig. 3. When the samples are annealed, ashift to lower energy in the transmission threshold anda reduction of the average transmission are observed,as can be seen in the Fig. 4. This behavior varies with

Žthe value of the composition x. The band-gap calcu-2 .lated by using a vs. photon energy plot , for the

as-grown and annealed samples as a function of thecomposition x is shown in Fig. 5. Note that the band-

Ž .Fig. 2. X-Ray diffraction patterns for different compositions x ofŽ . Ž .ZnO CdO films annealed for 2 h in air.x 1yx

( )G. Santanaı et al. r Thin Solid Films 373 2000 235]238´ 237

Ž . Ž .Fig. 3. Optical transmission spectra of as grown ZnO CdOx 1yxfilms.

gap decreases with thermal annealing, except for xs0.75, for which the band-gap energy is above that forthe as grown sample. This behavior may be attributedto a transition from a poor crystalline phase to thecubic-hexagonal mixing phase. A similar behavior hasbeen found for CdS thin films grown by chemical bathand annealed at different temperatures and atmo-spheres, where a transition from a cubic to a hexagonal

w xphase has been reported 7,8 . The band-gap energycorresponding to xs0 decreases to 2.29 eV with an-nealing for 2 h. The values of the band-gap energy andthe lattice parameter corresponding to xs0 for thisannealing time are in agreement with the reported

w xvalues reported for bulk CdO 9 . This observationshows that the changes in the band-gap values in theannealed samples are associated to a size growth of thecrystallites. This mechanism is related to the post-de-position thermal treatment and will be studied in detailin future works.

4. Conclusions

Structural and optical analysis show that the spraypyrolysis technique is a useful method for the deposi-

Ž . Ž .tion of CdO ZnO thin films. For xG0.5 a mixing1yx xphase corresponding to cubic-CdO and hexagonal-ZnOis formed. The hexagonal-ZnO phase has a poorercrystallinity than the cubic-CdO phase, and the preva-lent presence of a cubic phase throughout the range0 F x - 0.5 agrees with the trend observed forŽ . Ž .ZnO . CuO thin films grown by sol-gel. The latticex 1-xparameters variation at approximately xs0 can beunderstood due to the lower size of the Zn atom ascompared to the Cd atom resulting in a lattice reduc-tion and a band-gap energy increase. Changes in thetransmission spectra and on the corresponding values

Ž . Ž .Fig. 4. Optical transmission spectra of ZnO CdO films an-x 1yxnealed for 2 h in air.

of the band-gap energy are correlated with the in-termixing of transparent ZnO crystallites in-betweenCdO crystallites.

Post-thermal annealing improves the crystallinity ofŽall samples the FWHM decreases as a consequence of

.the post-thermal annealing processes . The band-gapenergy and lattice parameter of pure CdO and pureZnO reach the reported bulk values at room tempera-ture. Usually a wide variety of band-gap values arereported in the literature for these semiconductor com-pounds. The band-gap change from 2.29 to 3.28 eVwhen the nominal composition x is changed fromxs0 to xs1, respectively, for the annealed samples.The band-gap for each composition is also changed bythe post-thermal annealing.

Fig. 5. Band-gap variation for the as-grown and annealed samples asa function of the composition x.

( )G. Santanaı et al. r Thin Solid Films 373 2000 235]238´238

Acknowledgements

This work was partially supported by the MexicanAgency CONACyT.

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