Effects of Substrate-Target Distance and Si Co-Doping on the Properties of Al-Doped ZnO Films Deposited by Magnetron Sputtering

XU Hao LU Fang FU Zheng-Wen

(1Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials,Surface Physics Laboratory&Department of Physics,Fudan University,Shanghai 200433,P.R.China;2Department of Chemistry&Laser Chemistry Institute,Fudan University,Shanghai 200433,P.R.China)

Effects of Substrate-Target Distance and Si Co-Doping on the Properties of Al-Doped ZnO Films Deposited by Magnetron Sputtering

XU Hao1,2LU Fang1FU Zheng-Wen2,*

(1Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials,Surface Physics Laboratory&Department of Physics,Fudan University,Shanghai 200433,P.R.China;2Department of Chemistry&Laser Chemistry Institute,Fudan University,Shanghai 200433,P.R.China)

Abstract: Transparent conductive Al-doped ZnO(AZO)and Si-codoped AZO(AZO:Si)films were deposited on square quartz substrates by radio frequency(RF)magnetron sputtering.The effect of distance between the substrate and target(Dst)and the effect of co-doping Si on the electrical and optical properties of theAZO films were systematically investigated.The resistivity,carrier concentration,and mobility were found to be strongly dependent on theDstvalues.With a decrease inDst,the carrier concentration and mobility increased significantly,which resulted in improved conductivity.The lowest resistivity of 4.94×10-4Ω·cm was obtained at aDstof 4.5 cm,and this was associated with a carrier concentration of 3.75×1020cm-3and a mobility of 33.7 cm2·V-1·s-1.X-ray photoelectron spectroscopy(XPS),X-ray diffraction(XRD)spectroscopy,and grain boundary scattering models were used to analyze the relationship between the carrier concentration and the mobility at different deposition(Dst)values.Transmittance spectra showed an average transmittance of>93%in the visible-near infrared range for all the samples and a blue shift of the absorption edge with a decrease inDst.AZO:Si films had high-conductance and high-transmittance optical properties compared with AZO films,and they had better resistivity stability than the AZO films when exposed to a hot and damp atmosphere,which is practically meaningful.

Key Words:AZO;AZO:Si;Substrate-target distance;Radio frequency magnetron sputtering

1 Introduction

Aluminum-doped zinc oxide(AZO)is a promising transparent conductive oxide material for application as transparent electrode in thin film solar cells,flat panel display,and optoelectronic devices.Besides high conductivity and optical transmittance in visible region,AZO film has a lot of advantages,such as non-toxicity,low cost,material abundance,relatively low deposition temperature,and high stability against hydrogen plasma compared to ITO and SnO2films.1For the fabrication of AZO thin films,there are many deposition techniques currently in use,for example,magnetron sputtering,2-5pulsed laser deposition,6,7metal organic chemical vapor deposition,8and sol-gel process.9Among all these methods,conventional RF magnetron sputtering has become widely recognized as a promising versatile technique for the fabrication of metal oxides.Its advantages over other deposition methods are its ability to obtain high quality films even at low substrate temperature with the low cost,high deposition efficiency,and large deposition area.Previous results showed that the electrical and optical properties of AZO films were strongly dependent on the growth parameters,such as work pressure,2,10RF power,5,11impurity percent,2,6substrate temperature,3,10,11and annealing.4,12In fact,the distance between the target and substrate(Dst)is also one of important parameters for depositing high quality films during sputtering process.However,up to now,there were only a few papers concerning on the relationship between the properties of AZO films and Dst.Jeong et al.13suggested that the resistivity of AZO films was related to the Dstand increased rapidly with the increase of Dst,using Al(OH)3doped ZnO targets,they got the lowest resistivity of 9.8×10-2Ω·cm at 4.5 cm.Recently,Yang et al.14reported the lowest resistivity of 4.62×10-4Ω·cm and the highest Hall mobility of 15.6 cm2·V-1·s-1obtained at the Dstof 7 cm.Apparently,the relationship between the electrical properties of AZO films and Dstis complicated and data is still scarce.More work must be done to elucidate the electrical and physical intrinsic of AZO films associated with growth condition of Dst.In addition,it has been expected that the co-doping Si into AZO is effective in improving its chemical stability and resistivity stability,15,16but do not significantly influence the high-conductance and high-transmittance properties ofAZO films.

Here,AZO and AZO:Si films were deposited by RF magnetron sputtering.The structural,chemical composition,optical transmittance,electrical properties,and resistivity stability of deposited films were examined by X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),UV-Vis spectrophotometer,Hall measurements,and hot humidity environment test(air at relative humidity of 90%and temperature of 60°C).Our purpose is to clarify the effect of Dstand Si addition on the properties of Al-doped ZnO films prepared by magnetron sputtering.

2 Experimental

The sputtering targetsused in thisexperimentwere home-made with high purity ZnO(99.9%,Alfa Aesar),Al2O3(99.9%,Alfa Aesar),and SiO2(99.9%,Alfa Aesar)powders.Three ceramic targets with different mass fractions of Al2O3(1%,2%,3%)were prepared for the deposition of AZO films.In addition,two mixtures of ZnO,Al2O3,and SiO2powder targets were also prepared for the deposition of AZO:Si films,which contents are(1%Al2O3,1%SiO2)and(2%Al2O3,0.5%SiO2),respectively.All targets have a common diameter of 5 cm.Thin films were all deposited on flat quartz substrates(1 cm×1 cm×1 mm),which had been ultrasonically cleaned in de-ionized water,acetone,alcohol,and de-ionized water,sequentially,and finally dried with argon flow.

Before deposition,chamber was evacuated to an ultimate background pressure of 10-4Pa using a turbo molecular pump.Then high-purity(99.99%)Ar gas was introduced into the chamber at a fixed flow rate of 28 cm3·min-1,working pressure was maintained on 1.2 Pa.Then 5 min pre-sputtering with the quartz substrate covered by a closely mounted shutter was employed to clean contamination on the target surface,followed by true sputtering.During deposition,RF power is fixed at 100 W,and substrate temperature was maintained at ca 150°C.Dstis varied from 4.5 to 7 cm,so sputtering time was different(from 6 to 15 min)for different Dst(different deposition rates),which makes sure that all films have thickness of 380-400 nm.Atlast,allfilmswereannealedinvacuumat500°Cfor 2h.

The thicknesses of films were measured by a conventional surface-roughness detector with stylus(Vecoo Dektak 150),and the crystal structure was examined by XRD using a Bruker D8 advance diffractometer equipped with Cu Kαradiation(λ=0.1540562 nm).XPS experiment,which was carried out on a PHI-5000C ESCA system(Perkin Elmer)with Al Kαradiation(hν=1486.6 eV),was utilized to analyze the composition and the chemical states of the AZO films.Electrical properties,such as resistivity,carrier concentration,and mobility,were measured by Hall-measurement system(ECOPIA HMS-5000),4 Au pins were pressed on 4 corners of films after 4 indium points were added to make sure ohmic contact,then constant current of 10 mA and magnetron field of 0.55 T were applied.The optical transmittance of films was measured by ultravioletvisible(UV-Vis)spectrophotometer,and the transmittance wascalibrated against a bare quartz glass as reference sample.Resistivity stability tests were carried out in a hot humidity environment,in which relative humidity of air was 90%and temperature was fixed at 60°C.

3 Results and discussion

Fig.1(a)shows typical XRD patterns of AZO(2%Al2O3)films deposited at different Dst.All measurements were taken on the central region of the films.Only strong(002)peak is observed at 2θ ≈34.47°,which indicates a hexagonal wurtzite structure and an oriented film growth with c-axis perpendicular to the substrate surface.As shown in Fig.1(b),the full width at half maximum(FWHM)decreases from 0.323°to 0.264°with the decrease of Dst.The observed increase in the XRD intensity and decrease in FWHM with the decrease of Dstreveals that the crystallinity is improved with decreasing Dst.AZO films of other mass percents(1%Al2O3,3%Al2O3)have also shown similar characteristic.Using FWHM value,the grain size(g)can also be estimated by Scherrer formula17

where λ=0.1540562 nm,is the wavelength of the X-ray,θ is the Bragg diffraction angle at the peak position,and Δ(2θ)is the FWHM in radian.An estimated grain size in the range of 26-33 nm can be obtained with the corresponding FWHM value in Fig.1(b).

Fig.2(a)shows the relationship between resistivity and Dstin three different Al contents obtained by Hall measurement.All films were deposited in a small area of 1 cm×1 cm,in order to avoid the spatial resistivity distribution.5It is clear that,film resistivity decreases significantly with the decrease of Dstfrom 1.63×10-3to 4.94×10-4Ω·cm(2%Al2O3).Fig.2(b,c)show the relationship of free electron carrier concentration and mobility versus Dst.Obviously,it is the larger carrier concentration and mobility at closer deposition distance that produced lower resistivity of film observed in Fig.2(a).With 2%as a case,carrier concentration and mobility at Dst=4.5 cm are 3.75×1020cm-3and 33.7 cm2·V-1·s-1respectively,but they drop to 2.35 × 1020cm-3and 16.3 cm2·V-1·s-1in Dst=7 cm.Yang et al.14reported similar results,as the decrease of Dst,the resistivity decreased to the lowest point,while further decrease of Dstgave negative effects on resistivity.In our experiments,the minimum and optimal Dstis 4.5 cm,which is limited by the position of pre-sputtering shutter,maybe further closed Dstwill show the same phenomenon as that of Yang.In addition,the highest concentration in our films is only 3.75×1020cm-3,which is less than ～8×1020cm-3in other literature.2,3,11,12This may be due to part of powder loss in target-made process and improper Al2O3content,the optimal impurity percent may be between 2%and 2.5%.

According to the data from Fig.1,closer distance means relatively higher energy of the sputtered particles,which lead to an improvement of crystallinity and lattice structure,thus producing higher lattice limited mobility,and closer Dst(smaller FWHM value)yielded larger grain size.According to a classical model suggested by Seto18in ploycrystalline,depending on the charge carrier trap density at the boundary(Qt)and carrier density(N),the trap can be partly or completely filled,leading to variation barrier heights Eb.Electrons can surmount these barriers by thermionic emission.In the case of AZO(L×N>Qt),the grain boundary limited mobility(μs)is given by

μs=μ0·exp(-Eb/kT)

whereμ0can be defined as the mobility inside a grain,εε0is the static dielectric constant,eiselementary charge,kis Boltzmann constant,m*is effective mass in ZnO,Tis Kelvin temperature,Nis the carrier density in the bulk of the grain andLis the grain size,which had already been estimated by Scherrer formula using FWHM value.Since AZO films in closerDsthave relatively larger grain size,less scattering frequency and relatively larger grain boundary limited mobility will be obtained.Since lattice structure and grain boundary scattering are two important factors in determining the mobility of AZO films,thus,the AZO films grown at closerDstwill have higher mobility than that gown at longerDst,which is consistent with what we observed in Fig.2.However,Yanget al.14reported that,over higher energy(further closerDst)may cause degradation of crystallinity and lattice structure,which can be supported by their XRD patterns,thus producing lower mobility.This was also not observed in present case because of limited distance between target and pre-sputtering shutter.

In order to explain the relationship between carrier concentration andDst,XPS spectra were taken to detect the content of O and Al in AZO films.As reported,2,13Al on substitutional site of Zn and O vacancy are two important donors in AZO film.Minamiet al.3,15got a electron concentration of ca 2×1020cm-3with pure ZnO target by magnetron sputtering,implying that deposited ZnO films were degenerated even without Al dopant.Kimet al.2also got concentration of ca 8.7×1019cm-3in deposited ZnO films.Briefly,we can get the conclusion that,O vacancy is also a non-ignorable donor of AZO in generating the free carriers.Figs.3-5 shows our XPS spectra of three AZO films(2%Al2O3)deposited at differentDst.Using Gaussian fitting,the observed O 1speak can be devoluted into two components.The lower binding energy peak(OI)located at(530.20±0.10)eV is attributed to stoichiometric O2-within wurzite structure of ZnO.While the higher binding energy peak(OII)at(531.83±0.10)eV can be assigned to chemisorbed oxygen atgrain boundaries and surface,such as O2.1,14The stoichiometric atomic ratio of O/Zn can be gotten by integrating the peak area divided by their sensitivity factors(O:0.733,Zn:2.768,Al:0.256).Our results show that,the O/Zn ratio in film(2%,Dst=4.5 cm)is 0.87,while the ratios are 0.92 and 1.02 in film(2%,Dst=6 cm)and film(2%,Dst=7 cm),respectively.This indicates that the AZO film grown at closer Dsthas higher oxygen vacancy than that grown at farther Dst.From XPS spectra,it can also be calculated the atomic ratio of Al/Zn is 0.0308 in the film deposited at 4.5 cm,which is almost equal to 0.0311 in the film deposited at 7 cm.Apparently,AZO film grown at shorter Dst(higher growth rate)may have higher oxygen vacancy rather than that grown at longer Dst(lower growth rate),suggesting that free electron carrier concentration in the AZO film of shorter Dstis higher than that grown at longer Dst,which is consistent with the variation of free electron concentration in Fig.2.

Fig.6 presents all data for different Dstand different Al contents in a plot of the mobility versus carrier concentration.Thick solid line is a semi-empirical model in ZnO single crystal presented by Masetti et al.,19and the thin solid line is grain boundary scattering limited transport model(formula 2)of Seto.18Dash line is the fitting curve for combined single crystal and grain boundary scattering model,1,4yielding the grain boundary trap density Qt=2.09×1013cm-2.This is in good agreement with data reported by other scholars,for example,1.3×1013cm-2by Ellmer et al.1,4and 1.75×1013cm-2by Cornelius et al..20

Fig.7(a)shows transmittance spectra of AZO films prepared with targets having 2%Al2O3content with different Dst.The average transmittance in the range of visible-near infrared is above 93%for all samples regardless of Dst.As the decrease of Dst,the absorption edge shifts to the shorter wavelength region,and this movement of the absorption edge is known as the Burstein-Moss shift.21,22It is known that AZO films with carrier concentration above 1020cm-3degenerate and Fermi energy level penetrates into the conduction band,thus the optical band gap Egwill increase with the increased carrier concentration.The optical band gap(Eg)and absorption coefficient(α)in direct transition semiconductor are related by following equation2,23

where h is Planck′s constant,and ν is the frequency of the incident photon,α is the optical absorption coefficient defined by

where I is the intensity of transmitted light,I0is the intensity of incident light,and t is the thickness of the AZO films.Fig.7(b)is the plot of α2versus hν,the extrapolation line gives Egfor samples prepared at different Dst.The absorption edge moves from 3.52 to 3.55 eV when Dstvaries from 7 to 4.5 cm,which is also consistent with the variation of free electron carrier concentration in Fig.2.

Based on the optimal conditions for the deposition of AZO films at Dstof 4.5 cm,the same distance between target and the substrate is employed for the preparation of Si-doped ZnO films.It has been reported that Si atom in Si-doped ZnO films can act as effective donors in the same manner as Al15,16.Fig.8 shows electrical properties of AZO:Si films,in which the data from AZO films are used for comparison.It can be seen that the additional 1%SiO2cause the decrease of resistivity from 7.256×10-4to 4.977×10-4Ω·cm,while 0.5%SiO2added into the target of 2.%Al2O3cause the resistivity increasing from 4.938×10-4to 8.333×10-4Ω·cm.The variety of conductivity can be attributable to varieties in both carrier concentration andmobility as also shown in Fig.8,the carrier concentrations of two AZO:Si films are 6.121×1020and 4.813×1020cm-3,respectively,doped SiO2induces significant increase in carrier concentration,which can be attributed to an increase in additional effective donor.However,compared with AZO films,AZO:Si films have much lower mobilities 20.513 and 15.584 cm2·V-1·s-1,doped-Si may cause enhanced impurity scattering,which results in a certain decrease of mobility.

Fig.9(a)shows transmittance spectra of AZO:Si films.It can be seen that the added Si does not significantly reduce the optical transmittance in region of 190-900 nm.Comparing with AZO films(2%),the absorption edges of AZO:Si films show slight blue-shift to shorter wavelength.As shown in Fig.9(b),this phenomenon can also be explained by the increased carrier concentration in Fig.8 with the theory of Burstein-Moss.21,22

Fig.10 shows resistivity as a function of exposure time for AZO:Si films and AZO films.Such tests were carried out in high hot humidity environment(air at 90%relative humidity and 60°C).It is clearly that,after 1000 h exposure in atmosphere,the resistivities of all films increase to different extent,but AZO:Si films show better resistivity stability than AZO films.The resistivity of AZO:Si(1%Al2O3,1%SiO2)slightly rises from 4.977×10-4to 7.013×10-4Ω·cm.However the resistivity of AZO(2%Al2O3)films rise from 4.938×10-4to 9.689×10-4Ω·cm.This increase of resistivity,when films are exposed in atmosphere,may mainly be due to the decrease in carrier concentration,which should be attributed to the oxygen chemisorptions and oxidation of oxygen vacancy.Thus,the resistivity stability of AZO films can be improved by co-doping Si impurity,similar studies have also been reported by other scholars,such as co-doping Si by Nomoto et al.16and co-doping V by Minami et al.24The improvement in resistivity stability of AZO:Si films may mainly be related to that of carrier concentration stability.In AZO films,Al and oxygen vacancy can act as donors,but oxygen vacancy is easy to be oxidized,while in AZO:Si films,co-doped Si acting as additional effective donors increases carrier concentration and can not be oxidized.As practical requirement,transparent electrodes used in optoelectronic devices must be a stable enough when exposured in atmosphere.Co-doping silicon may be a feasible and promising method to improve stability of conductivity,and at the same time do not significantly influence the high-conductance electrical and high-transmittance optical properties of AZO films.

4 Conclusions

AZO and AZO:Si films were prepared by RF magnetron sputtering with the home-made ZnO targets containing different contents of Al2O3and SiO2powders as doping source.Alltransparent conductive films were grown on 1 cm×1 cm flat quartz substrates in order to avoid the phenomenon of spatial resistivity distribution.Two parameters of substrate-target distance(Dst)and co-doping Si that can influence the property of AZO films were investigated.The structural properties,electrical properties,and optical properties of AZO films are strongly dependent on the deposited distance between substrate and target.As the decrease of Dst,carrier concentration and mobility all show a significant increase,which result an improved conductivity in AZO films.Optical experiment shows average transmittance>93%in visible-near infrared range for all samples and blue shift of absorption edge with the decrease of Dst.In addition,our experiments have demonstrated that AZO:Si films have comparable electrical and optical properties with AZO films,but better resistivity stability than AZO films in hot and damp atmosphere,which is meaningful in practical use.In summary,Dstand co-doping Si are two important but always easily neglected parameters in deposition process,proper Dstand co-doping Si may help us fabricate high-quality AZO films.

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磁控濺射中靶-基底距離與Si共摻對ZnO:Al薄膜性質(zhì)的影響

徐 浩1,2陸 昉1傅正文2,*

(1上海市分子催化和功能材料重點實驗室,表面物理實驗室和物理系,復(fù)旦大學(xué),上海200433;2化學(xué)系和激光化學(xué)研究所,復(fù)旦大學(xué),上海200433)

使用射頻磁控濺射,在正方形石英襯底上沉積透明導(dǎo)電摻Al的ZnO(AZO)和Si共摻AZO(AZO:Si)薄膜.系統(tǒng)研究了靶-基底距離(Dst)和Si共摻對AZO薄膜電學(xué)、光學(xué)性質(zhì)的影響.電阻率、載流子濃度和遷移率都強烈地依賴于靶-基底距離,隨著靶-基底距離的減少,載流子濃度和遷移率都有顯著的增加,電導(dǎo)率也隨之提高.在靶-基底距離為4.5 cm處,得到最低電阻率4.94×10-4Ω·cm,此時的載流子濃度和遷移率分別是3.75×1020cm-3和33.7 cm2·V-1·s-1.X射線光電子能譜(XPS)、X射線衍射(XRD)和邊界散射模型被用于分析載流子濃度、遷移率和靶-基底距離的關(guān)系.透射譜顯示,在可見-近紅外范圍內(nèi)所有樣品均有大于93%的平均透射率,同時隨著靶基距離的減少,吸收邊藍移.AZO:Si表現(xiàn)出可與AZO相比擬的高電導(dǎo)和高透射光學(xué)特性,但在熱濕環(huán)境中卻有著更好的電阻穩(wěn)定性,這在實際使用中很有意義.

AZO;AZO:Si; 靶-基底距離; 射頻磁控濺射

O649

Received:December 20,2010;Revised:March 4,2011;Published on Web:March 10,2011.

*Corresponding author.Email:zhengwen@sh163.net;Tel:+86-21-65642522.

The project was supported by the Science&Technology Commission of Shanghai Municipality(08DZ2270500,09JC1401300),National Natural Science Foundation of China(20773031),National Key Basic Research Program of China(973)(2007CB209702),and National High-Tech Research and Development Program of China(863)(2007AA03Z322).

上海科學(xué)技術(shù)委員會(08DZ2270500,09JC1401300),國家自然科學(xué)基金(20773031),國家重點基礎(chǔ)研究發(fā)展規(guī)劃(973)(2007CB209702),國家高技術(shù)研究發(fā)展計劃(863)(2007AA03Z322)資助項目