ARTICLE IN PRESS

0022-0248/$ - se

doi:10.1016/j.jc

�CorrespondE-mail addre

(K. Kumar).

Journal of Crystal Growth 289 (2006) 405–407

www.elsevier.com/locate/jcrysgro

A novel in situ synthesis and growth of ZnAl2O4 thin films

K. Kumara,�, K. Ramamoorthya, P.M. Koinkarb, R. Chandramohanc, K. Sankaranarayanana

aCrystal Research Centre, Department of Physics, Alagappa University, Karaikudi 630 003, IndiabThin Film Laboratory, Department of Physics, University of Pune, Pune 410 007, India

cDepartment of Physics, Sree Sevugan Annamalai College, Devakottai 630302, India

Received 10 October 2005; received in revised form 27 October 2005; accepted 3 November 2005

Available online 4 January 2006

Communicated by R. Kern

Abstract

ZnAl2O4 is a spinel oxide, widely used as high-temperature material, catalyst, catalyst support, optical coating for spacecrafts and

emerging as one of the best wide band gap compound semiconductor (Eg ¼ 3:8 eV) for UV optoelectronic applications. In this work,

spinel oxide is synthesized by the simple, in situ, acid-based chemical reaction and the thin film was deposited as a product of reaction by

liquid-phase deposition (LPD). Material to be grown as metal incorporated thin film was taken as precursor and put into a bath

containing acid, catalyst. The acid also serves as solvent with a metal foil as cation scavenger. Uniform and highly adherent ZnAl2O4 thin

films were prepared. Structural, compositional and surface morphological properties of thin films were studied using Philips, Xpert—

MPD: X-ray diffractometer and Philips, ESEM—TMP+EDAX, Hitachi S-450: SEM, Nanoscope—III: AFM.

r 2005 Elsevier B.V. All rights reserved.

Keywords: A1. X-ray diffraction; B1. Oxides; B2. Semiconducting ternary compounds

1. Introduction

Transition metal oxides possess a number of interestingoptical and electrical properties. As thin films, they findapplications in many electrical and optical devices. Zincaluminate(ZnAl2O4) is a widely used ceramic, electronicand catalytic material with spinel structure [1]. ZnAl2O4 isalso transparent to light with wavelengths above 320 nmand is hence suitable for UV optoelectronic applicationand for use in thermal control coatings for spacecraft [2–4].

Conventionally, zinc aluminate powder was prepared athigh temperature by many techniques such as sintering,solid-state reaction [5], sol–gel [2], hydrothermal [6], andmicroemulsions techniques [7]. The thin films weredeposited by dry process such as vacuum evaporation,sputtering and chemical vapor deposition [8], etc. In thiswork, we have developed a single-step acid-based chemicalreaction for synthesis and to grow thin films using the same

e front matter r 2005 Elsevier B.V. All rights reserved.

rysgro.2005.11.007

ing author. Tel.: +914561 261620.

sses: secltkumar@military.com, kumardrf@rediffmail.com

optimized chemical bath at room temperature. Hence, thismethod is found to be a novel, in situ, and low costtechnique for the growth of ternary oxide materials.

2. Experimental processes

ZnO was selected as a suitable material to be grown asmetal (Al) incorporated thin films. Hydrofluoric acid (HF)was used as solvent and reaction catalyst. ZnO was dissoledin 2.5% aqueous solution of HF acid, i.e., nearly 1.2206 gof ZnO powder was mixed with aqueous HF, in a glassbeaker at room temperature. A constant size (5 cm2) ofaluminum foil with thickness 0.5mm was chosen as relatedcation (F� ion) scavenger and introduced into the chemicalbath. The chemical bath was optimized using the variationin HF concentration (5ml to 2.5ml in steps of 0.5ml) of6 trials.Afterwards, hot chromic-acid-treated glass substrates

(cleaned with trichloro ethylene, methanol and acetone)were immediately dipped vertically into the freshlyprepared bath. The resultant solution with immersed glasssubstrate was maintained at room temperature for 15 h. As

ARTICLE IN PRESSK. Kumar et al. / Journal of Crystal Growth 289 (2006) 405–407406

a result of chemical reaction, Zinc–fluro complex initiallyformed immediately reacts with aluminum metal ions(Al+). Finally, single-phase ZnAl2O4 thin films wereformed. The gently harvested thin films on glass substratewere dried at normal atmospheric condition. Thus grownthin films were characterized for their structure, composi-tion and surface morphology using Philips, Xpert—MPD:X-ray diffractometer and Philips, ESEM—TMP+EDAX,Hitachi S-450: SEM, Nanoscope—III: AFM.

Fig. 2. EDAX spectra of ZnAl2O4 thin films deposited HF concentration

of 2.5ml.

3. Results and discussion

ZnAl2O4 thin films were deposited by a principle called‘liquid phase deposition’ [9,10]. This ‘electroless fluid basedeposition’ is also defined as auto-catalytic deposition of ametal-incorporated material from a chemical bath of itsown ions by interaction with a reaction accelerationreagent (catalyst). This reaction acceleration reagentprovides sticking surface for the ions by self-catalyticactivity. The choice of solvent may also provide somecontrol over thin film growth, i.e., deposition of a newpolycrystalline material on the etched surface. The slowetching on the substrate surface provides the sites forfurther growth. The XRD studies showed that the as-deposited films have high crystallinity and highly adherentwith glass substrates. Fig. 1 shows the XRD spectrumof a typical thin film grown for the HF concentration of2.5ml. From the spectrum fine texturing of deposited thinfilm was observed. For example the repeated order of(2 2 0), (4 4 0) and (6 2 0) peaks enumerate the layer by layerof lattice formation in particular direction (h k 0). Similarly(3 1 1), (5 1 1) peaks exhibit the layer by layer of latticeformation towards the direction (h k l) with k ¼ l ¼ 1.Likewise (2 2 2), (4 2 2) and (3 3 1), (5 3 1) emphasized thegrowth of lattices in (h k l) with k ¼ l ¼ 2 and (h k l) withk ¼ 3, l ¼ 1 respectively. The calculated lattice parameter(a ¼ 7:9512 A) has good agreement with standard data ofICCD card file No. 5-669 (a ¼ 8:0848 A). Polycrystallinethin films were obtained from initial trails of high HFconcentration and the degree of crystallinity increased withthe decrease of HF. XRD also revealed the significantchanges which occurred in the crystalline arrangement and

Fig. 1. X-ray diffractograms (y–2y scan) of ZnAl2O4 thin films deposited

at HF concentration of 2.5ml.

orientation with reduction in HF concentration. TheEDAX spectrum presented in Fig. 2 for ZnAl2O4 thinfilms on glass substrate confirms the presence of elementsZn, Al and O. The Si is due to glass substrate. The presenceof elements Ca and Na may be attributed as impuritieseither from the source materials or from the glass substrate.Fig. 3 shows SEM micrograph obtained for a typical

ZnAl2O4 thin film at room temperature. The surfacefeature reveals the uniformity of the thin film. The filmswere of adherent nature. Higher magnification(250KV� 1000) of the surface revealed the presence ofcrystallites. The spherical crystallites are clustered aroundthe etched pits. The grains were having an average sizearound a micron.

Fig. 3. SEM photograph illustrates wave like pattern of ground spread

dry leaves.

ARTICLE IN PRESS

Fig. 4. AFM photograph reveals uniformity of thin film surface without

any voids.

K. Kumar et al. / Journal of Crystal Growth 289 (2006) 405–407 407

Fig. 4 shows the AFM of ZnAl2O4 thin films depositedat room temperature. The AFM studies revealed uniformsurface without any valleys. The thickness of the film is0.9 mm as calculated using optical interference method. Theabsence of grains or grain boundaries in the AFM picturereveals that the films are continuous and void free. This isattributed to nearly amphorous film formation at roomtemperature. Further annealing studies are under progressto study the orientation of crystallites.

4. Conclusion

The paper reports the synthesis and deposition of aspinel oxide for the first time using a single step in situprocess at room temperature. Spontaneous room tempera-ture growth of ZnAl2O4 thin films with crystalline qualitywas achieved. This process is found to be new, low cost andsuitable for the growth of thin films of high-temperatureoxide material. The process is found to be highlyreproducible.

References

[1] S.K. Sampath, D.G. Kandive, Ravindra Pandey, J. Phys.: Condens.

Matt. 11 (1999) 3635.

[2] S. Mathur, M. Veith, M. Haas, Haoshen, N. Lecerj, V. Huch, J. Am.

Ceram. Soc. 84 (2001) 1921.

[3] J.F. Cordoro, US Patent No. 6,099,637 (8 August 2000).

[4] J.F. Cordoro, et al., US Patent No. 5,807,909 (15 September 1998).

[5] C.O. Arean, B.S. Sintes, G.T. Palomino, C.M. Carbonell, E. Scalona

Platero, J.B. Parra Soto, Microporous Mater. 8 (1997) 187.

[6] Z. Chen, E. Shi, Y. Zheng, W. Li, N. Wu, W. Zhong, Mater. Lett. 56

(2002) 601.

[7] A.E. Giannakas, T.C. Vaimakis, A.K. Ladavos, P.N. Trikalitis,

P.J. Pomonis, J. Colloid Interface Sci. 259 (2003) 244.

[8] G. Fang, D. Li, B.-L. Yao, J. Crystal growth 247 (2003) 393.

[9] H. Nagayama, H. Honda, H. Kawahara, J. Electrochem. Soc. 135

(1998) 2013.

[10] A. Hishinuma, I. Goda, M. Kitaoka, S. Hayashi, H. Kawahara,

Appl. Surf. Sci. 48/49 (1991) 405.