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Al+ZnO

Thin Solid Films 449(2004)86–93

Aluminium-doped zinc oxide films prepared by an inorganic sol–gel

route

Rodrigo F .Silva*,Maria E.D.Zaniquelli

Departamento de Qu?mica,Faculdade de Filosofia,Ciencias e Letras de Ribeirao

Preto,Universidade de Sao Paulo ′???Avenida dos Bandeirantes,3900,Ribeirao

Preto,SP 14040-901,Brazil ?Received 20March 2003;received in revised form 5September 2003;accepted 19October 2003

Abstract

Thin films of aluminium-doped zinc oxide have been formed on glass using an inorganic sol–gel route and the dip-coating

process.The films were formed by the thermal decomposition of a stable precursor colloidal sol prepared by an ethanolic reflux of Zn (CH COO )?2H O and Al (NO )?9H https://www.wendangku.net/doc/b22956956.html,ctic acid was used as hydrolysis catalyst and acetylacetone and diethanolamine 322332were added to improve film homogeneity.Thermal analysis was used to investigate the chemical processes during firing of the sols.Atomic force microscopy data revealed morphological changes in the temperature range 200–5008C.The importance of sample firing after each transfer step was evidenced by the quartz crystal microbalance technique.A red shift of the absorption edge was observed for thicker films and the transmittance of the samples decreased with increasing film thickness.?2003Elsevier B.V.All rights reserved.

Keywords:Sol–gel;Zinc oxide;AFM;UV–Vis

1.Introduction

It is known that the most used transparent conductive oxide (TCO )film for different optoelectronic applica-tions is the tin-doped indium oxide (ITO )film w 1x .In recent years,however,zinc oxide (ZnO )films have also become technologically important due to their range of electrical and optical properties,together with their high chemical and mechanical stabilities,which make them suitable for a variety of applications such as flat panel display electrodes w 2x ,gas sensors w 3x and varistors w 4x .Moreover,these films can be used as surface acoustic wave devices w 5x ,because of their large piezoelectric constant,and also as solar cells w 6x ,since their optical bandgap (3.3eV )is wide enough to transmit most of the useful solar radiation.

ZnO is an n-type semiconductor and its conductivity can be controlled by thermal treatment or by adequate doping.The doping of ZnO films with the group III elements can increase the conductivity of the films.In comparison with other elements,Al and Ga are the best

*Corresponding author.Fax:q 55-16-623-0228.

E-mail address:rofersil2001@https://www.wendangku.net/doc/b22956956.html,.br (R.F .Silva ).dopants because their ionic radii are similar to that of Zn .The broad emission spectrum approximately 5502q nm is known to result from the green emission due to self-activated centers w 7x and its quenching was observed in ZnO doped with Co or Ni .The red emission 2q 2q from Eu ions with complete quenching of the broad 3q ZnO emission has been also reported for mixed powders of ZnO and EuCl w 8x .Many techniques have been used 3for fabricating ZnO films,such as chemical vapor deposition w 9x ,pulsed laser deposition w 10x ,sputtering w 11x ,r.f.magnetron sputtering w 12x ,spray pyrolysis w 13x and the sol–gel process w 14x .

The sol–gel process is based on hydrolysis and polycondensation reactions w 15,16x and has advantages over other processes due to its simplicity and low equipment cost.In general,metal alkoxides are used as raw materials,but the preparation of the sols can be difficult because of their reactivity.Furthermore,the alkoxides are very expensive w 17x and are insoluble in most alcohols w 18x .For this reason,the interest in inorganic sol–gel routes has significantly increased in the last years w 19–24x ,since the raw materials used have lower cost.However,it is verified that few are the

87 R.F.Silva,M.E.D.Zaniquelli/Thin Solid Films449(2004)86–93

papers w22,23x bringing some information on the initial steps of ZnO or doped ZnO film formation via inorganic routes.

This work aimed at investigating the steps that lead to the formation of homogenous aluminium-doped zinc oxide(AZO)films.Some of the results presented here are an extension of our previous works w25,26x.Never-theless,additional information on the relation between the layered structure of the films and their optical properties is given by the quartz crystal microbalance (QCM)technique and the ultraviolet–visible(UV–vis) spectroscopy.

2.Experimental

2.1.Preparation of the sols

The sol preparation and film deposition were described previously w25,26x:aluminium nitrate nona-hydrate(Strem Chemicals)was added to0.22and0.44 mol l ethanolic zinc acetate dihydrate(Merck)solu-y1

tions to render an atomic ratio of Al y Zn s5%

3q2q (mol).The solutions were placed in a100ml round-bottom distillation flask fitted with a condenser and refluxed with stirring at https://www.wendangku.net/doc/b22956956.html,ctic acid(Sigma)was used as hydrolysis catalyst and added dropwise.Stable and translucent precursor colloidal solutions were obtained after the reflux was interrupted and evidence for sol formation was acquired by photon correlation spectroscopy(PCS)measurements(Brook-haven Instruments PCS-100).The proton acceptors ace-tylacetone(aca)(Merck)and diethanolamine(dea) (Synth)were separately added to the sols in the1:1 molar ratio at room temperature.The sols were first transferred to10MHz quartz crystals(ICM Crystals–USA)at room temperature in order to investigate both the kinetics of solvent evaporation and the layered structure of the films.The crystal vibration was moni-tored by a digital frequencimeter(Minipa MF-6120).

2.2.Transfer and drying of the sols

The sols were coated onto KBr pellets with a pipette (Finnpippete)and glass substrates(Knitell Glaser,1=2

cm in size)through the dip-coating method at a speed rate of3.0cm min.Before the sol transfer,the glass

substrates were cleaned with‘piranha’solution and treated with base solution at758C for15min.After each coating,the samples were left to dry at room temperature in a dissicator and were held in the hori-zontal position to avoid draining of the transferred sol, which could lead to different thicknesses in the coatings.2.3.Firing of the films

The formed films were heat treated in a furnace(EDG FV-2–Brazil)for1h in the temperature range100–5008C in order to study changes in their composition and morphology.Gels formed by further addition of lactic acid to the sols were spread on glass substrates and were also fired at different temperatures.All the experiments were run in air atmosphere.

2.4.Features of the oxide films

The infrared(FTIR)spectra of the samples were registered in the transmission mode in a Nicolet5ZDX FTIR spectrophotometer.Further information on the composition of the samples during firing was obtained by differential thermal analysis(DTA)and thermogra-vimetric analysis(TGA)in a SDT2960Simultaneous DTA-TGA(TA Instruments).

All the experiments involving X-ray diffraction (XRD)were performed with a Siemens D5005diffrac-tometer with CuK line(1.54A)in the diffraction angle

range5–708.

The surface morphology of the films was studied by atomic force microscopy(AFM)using a NanoScope IIIa microscope(Digital Instruments)with;0.3nm resolution operating in the tapping mode.Silicon nitride tips were used to scan an area of1=1m m.

Energy dispersive X-ray(EDX)analysis,performed with a JEOL JSM-5800L Vmicroscope,was used to obtain qualitative information of the fired films. Luminescence measurements were carried out on a Spex Fluorolog II spectrophotometer with a xenon lamp as excitation source.Ultraviolet–visible spectra of sam-ples formed by1–5and10oxide layers were recorded in a HP8453spectrophotometer.

3.Results

During the sol preparation,it was verified that the solution formed at the end of the reflux precipitated when the system was cooled to room temperature. However,dropwise addition of lactic acid to the volume of the reflux rendered stable and translucent precursor colloidal solutions w6,19,20x.

PCS measurements showed that sols with concentra-tion of0.44mol l present particles with average size

between2and3nm and polydispersity of0.303.No signal was obtained for the concentration of0.22mol l because the particle size is probably below the y1

detection limit of the equipment.

As previously attested w25x,the homogeneity and transparency of the AZO films observed by visual inspections as well as the absence of aggregation or stratification confirm the good quality of the samples.

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86–93

Fig.1.Curves of mass variation as a function of the evaporation time of sols transferred to quartz crystals at room temperature:(a )first transfer,(b )following

transfers.

Fig.2.FTIR spectra of the precursor colloidal sols spread on KBr pellets and fired at different

temperatures.

Fig.3.DTA-TGA curves of the dried precursor sols.

However,measurements using the quartz crystal micro-balance technique showed that the procedure of succes-sive transfers of the precursor sols to solid substrate is uncapable to form multilayered films.Fig.1shows the curves of mass variation as a function of the evaporation time for sols transferred to quartz crystals at room temperature.A mass increase was obtained only for the first transfer,as seen by the positive mass variation values (D m )0).This result was confirmed by the use of an analytical balance,where the amount of sol deposited on glass substrates in the first transfer was four times greater than that of the following depositions.3.1.Qualitative analysis of the films

FTIR spectra of sols coated on KBr pellets and heat treated in the temperature range 100–4508C were

registered in the transmission mode (Fig.2).The asym-metric O–H stretching due to crystallization water in the range 3600–3000cm is seen for samples treated y 1at 1008C.We also verified the presence of bands at 1580and 1430cm due to asymmetric and symmetric y 1C–O stretching,whose intensities decreased with increasing firing temperature w 27x .The Zn–O stretching band (400cm )is already present when the samples y 1are heated at 2008C,but a significant increase in the band intensity occurs only at 3008C w 18,20x .The broad absorption band from 400to 1000cm ,typical of the y 1Al–O stretching w 28x ,was not observed in the spectra at the same temperature and this could be due to a small concentration of aluminium.At 4008C,a band at approximately 1100cm is verified,whose intensity y 1increases with temperature,but its assignment is not straightforward.The presence of aluminium in the ZnO films was confirmed by EDX analysis,the average amount of aluminium ion being approximately 3.5%(atom ).

R.F .Silva,M.E.D.Zaniquelli /Thin Solid Films 449(2004)

86–93Fig.4.X-Ray diffraction patterns of samples heat treated at (a )dif-ferent temperatures and (b )2008C.

The chemical processes that take place during the decomposition of the precursor sols and the formation of ZnO:Al were also followed by thermogravimetry.DTA-TGA curves of powder samples previously evap-orated from sols (Fig.3)show an endothermic peak at 1158C due to the loss of crystallization water.Another endothermic peak of lower intensity at 2458C suggests the melting of the anidrous compound.The melt ani-drous compound is then decomposed into organic resi-dues and AZO through an endothermic reaction (3408C ).An exothermic peak at approximately 4328C is attributed to the combustion of the remaining organic material.Finally,at temperatures above 4658C,the mass remained constant,as previously reported w 25x .No significant changes were observed with the addition of acetylacetone or diethanolamine to the sols.3.2.Crystallinity and morphology of the films

Fig.4a shows the XRD patterns of gels spread on glass substrates and fired at 200,300,400and 5008C.The (100),(002),(101),(102),(110),(103)and (112)diffraction peaks are observed when the samples are heated at 3008C.They are coincident with the JCPDS data of hexagonal wurtzite and had their intensities increased as the firing temperature was further increased.Apparently,no peaks are observed in the XRD patterns at 2008C.By shortening the scale of the same diffrac-togram (Fig.4b ),however,the three most intense peaks of ZnO can be seen.These results suggest that crystalline ZnO is already formed at 2008C,although organic compounds are still present.

Tapping mode AFM micrographs were registered for 1-layer samples treated at different firing temperatures (Fig.5).It can be seen that samples fired at 1008C present particles with dendritic-like shape.As the heat treatment reaches 3008C,a spherical feature in the particle shape with mean size of 25nm is verified.The same morphology is also observed for samples treated at 400,450and 5008C.

As regards the coating features,all the samples showed good homogeneity and no cracks were noted w 25x .The mean particle sizes obtained from the AFM data and the Scherrer–Warren formula for 10-layer AZO,AZO-aca and AZO-dea films formed at 4508C are summarized in Table 1.3.3.Optical features of the films

Luminescence spectra in the UVand visible regions are shown in Fig.6for AZO films prepared without additives and for AZO-aca and AZO-dea films.The excitation spectra show two maxima at 290and 365nm for the emission centered at 540nm.The wavelength of 250nm was used to record emissions between 280and 400nm (Fig.6a ).The maxima of the UVemission

wavelengths are observed at 365,373and 377for AZO-dea,AZO-aca and AZO films,respectively.The char-acteristic green emission of the ZnO films is seen at 521,527and 537nm for AZO-aca,AZO-dea and AZO samples,respectively,when the wavelength used to register the visible emission spectra from 450to 650nm was 365nm.

AZO films with different number of coatings (1–5,10)formed with or without additives are transparent.Transmittances of approximately 90%at l s 600nm were obtained for samples formed by up to 5layers (Fig.7).On the contrary,large absorption at wave-lengths between 300and 400nm due to the optical bandgap absorption were observed.An estimation of the

90R.F.Silva,M.E.D.Zaniquelli/Thin Solid Films449(2004)86–93

Fig.5.Atomic force micrographs of the precursor sols transferred to glass substrates and fired at different temperatures:(a)1008C;(b)2008C;

Table1

Mean particle size estimation of the10-layer AZO films formed at4508C from different precursor sols

XRD(Scherrer–Warren)y nm AFM y nm Sol without additive?AZO2730

Sol with aca?AZO-aca4740

Sol with dea?AZO-dea2320

R.F .Silva,M.E.D.Zaniquelli /Thin Solid Films 449(2004)

86–93Fig.6.Emission spectra of the AZO,AZO-aca and AZO-dea films (5layers )in the (a )UV (l s 250nm )and (b )visible (l s 365exc exc nm )

regions.

Fig.7.Transmittance spectra of the AZO films formed by different numbers of coatings.

optical bandgap value of the AZO films was made based on a procedure from the literature w 8,29x :the optical absorption coefficient (a )and the optical bandgap (E )g are related by a s (h n y E );E may be determined 1y 2g g from the plot of a vs.h n by the linear extrapolation of 2a to 0.Two-layer AZO films presented larger optical 2bandgap values (;4.60eV )than those of the 10-layer AZO samples:3.57eV (AZO-dea ),3.62eV (AZO )and 3.75eV (AZO-aca ).4.Discussion

Conversely to reports cited in the literature w 30–33x that point out to the fabrication of ZnO colloidal particles formed by the hydrolysis of organic salts in alcoholic medium,the methodology utilized in this work did not produce inorganic particles;instead,an organic precursor with colloidal dimension was prepared.The presence of lactic acid in the reflux is essential for the preparation of stable and translucent sols,and led us to

conclude that the organic acid not only stabilizes the sol,but is also incorporated into the sol particles.

The dip-coating system used in this work was capable to produce homogeneous films.However,the role paid by the withdrawal speed of the substrates is of para-mount importance to avoid the stratification of the films caused by solvent evaporation.Moreover,a dependence of film homogeneity on the cleaning process and the substrate hydrophilization was observed.

The QCM technique showed that the step of succes-sive withdrawals of the glass substrates from the sol volume at room temperature and even at lower temper-atures does not promote an effective transfer of a large number of layers.The need for intermediary heat treat-ments was confirmed by measurements performed with an analytical balance.

The analysis of FTIR spectra showed that the Zn–O stretching band is first observed for samples fired at 2008C.At the same temperature,the C–O asymmetric and symmetric stretching bands had their intensities dimin-ished due to the melting of the organic material,as also attested by an endothermic peak at 2458C in the DTA curve.A mass decrease was verified in the TGA curve in the range 140–2808C and is due to the evaporation of the melt compound.The intensity of the Zn–O stretching band increased when the samples were heat treated at 3008C and the TGA curves presented a larger mass loss (approx.33%),which evidences for the total decomposition of the organic residues in the interval from 280to 4008C.These results are in agreement with data from the literature w 27x ,where the decomposition of the organic compounds and the crystallization of oxides take place concurrently.As previously reported w 25,26x ,no organic residues remained in the samples above 4658C.In addition,the characteristic Al O 23absorption band in the range from 400to 1000cm y 1

92R.F.Silva,M.E.D.Zaniquelli/Thin Solid Films449(2004)86–93

w28x is not seen in the spectra of the samples and this might be due to the small concentration of aluminium. From EDX analysis,the homogeneous distribution of aluminium in different regions of samples treated at450 8C was attested and an average Al amount of 3.5% (atom)was found.

The nanostructure of the transferred films was altered with increasing firing temperature.AFM analysis of the samples formed by one layer of the precursor shows dendritic-like particles after heat treatment at1008C. At approximately2008C,an intermediate particle fea-ture between dendritic and spherical is observed,prob-ably associated with the melting of the precursor compound,as confirmed by the DTA-TGA curves. Finally,particles formed in the temperature range300–5008C presents a spherical-like shape due to the posterior decomposition of the organic residues and further crystallization of ZnO.No considerable changes in the particle size and roughness were observed in this temperature range,leading to the conclusion that the particulate film is stable at higher temperatures.The stability of the ZnO films explains the great interest in AZO for use in solar cells w6x.Changes in the particle size can be achieved with the addition of acetylacetone and diethanolamine to the precursor sols before the transfer step w26x.Particle size calculations from the Scherrer–Warren formula and AFM images indicated that AZO-aca presents particles with mean sizes of47 nm and40nm,respectively,which are larger than those of the AZO films without additives(see Table1). However,the presence of diethanolamine promoted a decrease in the mean particle size of the samples to approximately20nm.

Experimental evidences for the shift in the emission band of ZnO colloidal dispersions attributed to particle size differences have been cited elsewhere w29x.In the present work,we have observed that the emission of AZO-dea samples in the UVregion is blue shifted(365 nm)compared with the emission maxima of AZO-aca and AZO films:373nm and377nm,respectively.The characteristic green emission of ZnO due to intrinsic deffect centers w7x was observed in our samples and is most likely due to an excess of zinc within the solid w34x.The position of the emission maxima depends again on the additive.

The high transmittance values of the5-layer AZO films and the optical interference observed in the10-layer AZO films indicate that our samples have quality comparable to those formed by more sophisticated tech-niques like chemical vapor deposition w9x and r.f.mag-netron sputtering w12x.Since all the formed films had the same concentration of Al ions,the movement of the absorption edge of thinner AZO samples to a shorter wavelength region cannot be attributed to the Burstein–Moss shift w8,35x.The discrepancy in the optical band-gap values of AZO films formed by2and10layers is instead understood to be due to differences in the densification of the samples,which promote changes in the refraction index of the material.

5.Conclusion

Aluminium-doped zinc oxide films were succesfully prepared by an inorganic sol–gel route.The presence of lactic acid confers long-term stability to the precursor colloidal sols.FTIR and thermal analyses of the samples indicated that the organic precursor melts at;2508C and undergo total decomposition at temperatures above 3008C.The need for intermediary heat treatments of the transferred sols was evidenced by the QCM tech-nique and confirmed by analytical balance measure-ments.Residue-free ZnO films have spherical particles, as attested by AFM,and the presence of additives during the sol preparation plays an important role in the particle size of the final material.The high transmittance values demonstrate the high quality of the AZO films prepared in this work.

Acknowledgments

R.F.Silva wishes to express his gratitude to Prof Dr Helmuth Mohwald(Max Planck Institute of Colloids ¨

and Interfaces,Golm y Potsdam,Germany)for the use of the atomic force microscope,Dr Paulo S.Calefi for the acquisition of the emission spectra and Mr Carlos A.Brunello for the acquisition of the diffractograms. CAPES and FAPESP are kindly acknowledged for the financial support.

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