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Journal of Non-Crystalline Solids 352 (2006) 2335–2338

Sol–gel derived highly transparent and conducting yttrium dopedZnO films

Ravinder Kaur, A.V. Singh, R.M. Mehra *

Department of Electronic Science, University of Delhi South Campus, New Delhi 110 021, India

Available online 24 May 2006

Abstract

This paper reports the structural, electrical and optical properties of Yttrium doped zinc oxide (YZO) thin films deposited on Corning(7059) glass substrates by spin coating technique. A precursor solution of ZnO, 0.2 M in concentration was prepared from zinc acetatedissolved in anhydrous ethanol with diethanolamine as a sol gel stabilizer. Yttrium nitrate hexahydrate (Y2NO3 Æ 6H2O) was used as thedopant (3 wt%) in the present study. The films of different thickness in the range (200–500 nm) were prepared. The films were annealed inair at 450 �C for 1 h. It was observed that the c-axis orientation improves and the grain size increases as is indicated by an increase inintensity of the (002) peak and the decrease in the FWHM with the increase of film thickness. The resistivity decreased sharply from2.8 · 10�2 to 5.8 · 10�3 X-cm as the thickness increased from 200 to 500 nm. However, the average transmittance decreased from87% to 82.6% as the film thickness increased to 500 nm. The lowest sheet resistance of �120 X/h was obtained for the 500 nm thick film.� 2006 Elsevier B.V. All rights reserved.

PACS: 73.50.Pz; 73.61.�r; 78.66.�w; 81.20.Fw

Keywords: Films and coatings; Spin coating

1. Introduction

In recent years there has been a renaissance in the elec-tro-optical study of ZnO films as a serious candidate forTCO owing to their optical and electrical properties,together with their high chemical and mechanical stability[1,2]. Moreover the electrical resistivity of ZnO thin filmsis readily modified either by non-stoichiometry of pureZnO films or by the addition of a suitable impurity [3–5].Many research groups [6–8] have reported the effects ofIn, Al and Ga doping in ZnO. The effect of doping ofZnO with rare earth impurities such as Sc and Y has alsobeen reported as their ionic radius is very close to that ofZinc [9–12].

A variety of thin film deposition techniques areemployed to deposit zinc oxide films such as chemical

0022-3093/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2006.03.011

* Corresponding author. Tel.: +91 11 24115849; fax: +91 11 24110876.E-mail address: rammehra2003@yahoo.com (R.M. Mehra).

vapour deposition (CVD) [13], spray pyrolysis [14], rf sput-tering [15], pulse laser deposition (PLD) [8] and sol–geltechnique [16]. Since the electrical and optical propertiesof the films depend strongly on their microstructure, stoi-chiometry and the nature of impurities present, each depo-sition technique with its associated parameters yields filmsof different properties.

Sol–gel technique offers many advantages for the fabri-cation of coatings, including excellent control of the stoi-chiometry of precursor solutions, ease of compositionalmodifications, customizable microstructure, relatively lowannealing temperatures, the possibility of coating deposi-tion on large-area substrates, and simple and inexpensiveequipment. Sol–gel preparations are therefore ideal forexploratory studies for a large numbers of candidate mate-rials, compositions or preparatory conditions that requirescreening.

The present work focuses on deposition of yttriumdoped ZnO (YZO) films using spin coating technique ofsol–gel process. The YZO films (3 wt%) were deposited

20 25 30 35 40

(002)

400 nm

350 nm

250 nm

Inte

nsity

(a.

u.)

2θ (deg.)

Fig. 1. X-ray diffractograms of 3 wt% YZO films with thickness 250, 350and 400 nm.

250 350 400 450 5000.20

0.25

0.30

0.35

0.40

FWHM Intensity

Thickness (nm)

FW

HM

(de

g.)

Intensity (002) peak (a.u.)

300

Fig. 2. Effect of film thickness on FWHM and intensity of (002) peak of3 wt% YZO film.

2336 R. Kaur et al. / Journal of Non-Crystalline Solids 352 (2006) 2335–2338

on corning glass substrates with metal salt as the precursorelement, ethanol as a solvent and diethanolamine (DEA) asthe stabilizer. The films of different thickness (250–500 nm)were annealed in air at 450 �C for 1 h. The structural, elec-trical and optical properties of these films have been inves-tigated as a function of film thickness.

2. Experiment

The key to obtain good quality films using sol–geltechnique is the preparation of a clear, transparent andhomogenous solution without any chemical species. Itwas observed that 0.2 M solutions of zinc acetate (Zn(CH3CO2)2 Æ 2H2O, purity 99.5%) dissolved in anhydrousethanol with an equimolar amount of DEA addedyielded stable solutions [12]. Yttrium nitrate hexahydrate(Y2NO3 Æ 6H2O, purity 99.9%) was used as the dopant(3 wt%) in the present study.

The glass substrates were cleaned ultrasonically in meth-anol and acetone for 10 min each and then cleaned withdeionised water and dried with dry nitrogen. The spinningspeed and time were optimized to 3200 rpm and 30 s toensure that each spun layer is thin enough to ensure thesimultaneous evaporation of all solvent, thus preventingthe cracking of the films. The drying of the films was doneat 300 �C for 20 min. An approximate thickness of 0.02 lmwas obtained for each spin. To increase the thickness of thefilms, the process of spinning and drying was repeatedaccordingly. The deposited films were annealed in air atan optimized temperature of 450 �C for 1 h [12].

The structural properties of the films were investigatedby Phillips–Holland X-ray diffractometer (Model PW1830/00). The surface morphology of the films was ana-lyzed by Scanning Electron Microscopy (JEOL JSM-6300). The thickness of the films was measured by theDEKTECK3-STsurface profilometer. The electrical resistiv-ity and Hall coefficient of the films at room temperaturewas measured by van-der Pauw technique. The opticaltransmittance measurements were carried out in the wave-length range of 200–800 nm using a double beam SHIMA-DZU-330 spectrophotometer.

3. Results

3.1. Structural properties

The structural properties of the films were investigatedby X-ray diffraction studies and SEM micrographs.

3.1.1. X-ray diffraction studies

Fig. 1 shows the X-ray diffractograms of 3 wt% YZOfilms annealed at 450 �C as a function of film thickness(250–400 nm). It is seen from the figure that as the filmthickness increases from 250 to 400 nm, the intensity of(002) peak (c-axis orientation) increases, implying animprovement in the crystalline quality of the film. Fig. 2shows the variation of intensity and FWHM of (002)-

reflection peak of the YZO films. As the film thickness isincreased, FWHM decreases and obtains its minimumvalue of 0.22� for a film thickness of 400 nm. With furtherincrease in thickness of the films up to 500 nm, the intensityof (002) peak did not show a significant change whereasthe FWHM showed a slight increase. It has been reportedin our earlier work [12] that the YZO films with yttriumcontent up to 3 wt% do not exhibit a significant degrada-tion in the crystalline quality of the film.

3.1.2. Surface morphology

The SEM micrographs of YZO films with varying thick-ness showed uniform tightly packed grains whose sizeincreased with an increase in thickness. The planar andcross-sectional SEM micrographs of the film with a thick-ness of 500 nm are shown in Fig. 3(a) and (b) respectively.The cross-sectional image clearly indicates a columnargrowth perpendicular to the surface of the substrate.

Fig. 3. (a) Planar SEM micrograph of 500 nm thick 3 wt% YZO film and (b) cross-sectional image.

R. Kaur et al. / Journal of Non-Crystalline Solids 352 (2006) 2335–2338 2337

3.2. Electrical properties

The sign of Hall coefficient confirmed that YZO filmsexhibit n-type conductivity. Resistivity (q), carrier concen-tration (n) and Hall Mobility (lH) of YZO films were mea-sured as a function of film thickness. It was observed thatthe resistivity decreased from �2.2 · 10�2 X-cm to5.8 · 10�3 X-cm as the film thickness increases from 250to 400 nm.

The variation of sheet resistance with film thicknessof the above YZO films is shown in Fig. 4. It is seen fromthe figure that sheet resistance is strongly dependent on thethickness and the minimum sheet resistance of �120 X/h isobtained for the 500 nm thick film.

3.3. Optical properties

Transmittance measurements were carried out in 200–800 nm range of spectrum to study the effect of film thick-ness on the optical properties such as transmittance, bandgap and absorption edge shift of YZO films. All the sam-

250 300 350 400 50060

70

80

90

T Rs

Film thickness (nm)

Ave

rage

tran

smitt

ance

(%

)

0

200

400

600

800

Rs

Fig. 4. Average transmittance and sheet resistance of 3 wt% YZO films asa function of film thickness.

ples showed high and constant transparency >80% in thevisible range (400–800 nm).

To judge the suitability of these films as TCOs, the aver-age transmittance as a function of thickness is also plottedin Fig. 4. It can be seen from the figure that average trans-mittance is >80% for the film having lowest value of sheetresistance. Shift of the optical absorption edge has beenobserved with increase in the film thickness. The bandgap was observed to increase from 3.35 to 3.42 eV as thefilm thickness increase from 250 to 400 nm. Beyond thisthickness (400 nm), no appreciable change in the bandgap is observed. The obtained optical band gap for allthe films is enhanced as compared to that of pure ZnO�3.3 eV.

4. Discussion

An improvement of the structural and electrical proper-ties of the sol–gel derived YZO films is observed with anincrease in film thickness. The grain size as determinedfrom the value of FWHM using Scherer’s relation is foundto increase from 25 to 42 nm with an increase in film thick-ness. The thick film (�500 nm) having improved crystallin-ity exhibit a columnar growth. The band gap widening withthickness is also observed in the films. This is normallyexplained in terms of B.M. shift [17] due to the increasein carrier concentration. An increase in carrier concentra-tion is also observed in the present work.

5. Conclusion

High quality transparent and conducting yttrium dopedZnO films were obtained by spin coating using sol–gel tech-nique. The quality and the growth pattern of the film iscontrolled by its thickness. Highly c-axis oriented polycrys-talline films with an average transmittance of 84% and asheet resistance of �120 X/h were obtained for a film

2338 R. Kaur et al. / Journal of Non-Crystalline Solids 352 (2006) 2335–2338

thickness of 500 nm. Such films would find application assolar cell window material.

Acknowledgements

The authors wish to acknowledge the financial supportof DRDO, Government of India, India.

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