简介概要

Preparation and characterization of ZnO/Cu/ZnO transparent conductive films

来源期刊：Rare Metals2013年第3期

论文作者：Wen-Ying Li Lai-Xin Jiang Gui-Lin Yin Yuan-Yuan Wang Zhen Yu Dan-Nong He

文章页码：273 - 277

摘要：ZnO/Cu/ZnO transparent conductive thin films were prepared by RF sputtering deposition of ZnO target and DC sputtering deposition of Cu target on n-type （001） Si and glass substrates at room temperature. The morphology, structure, optical, and electrical properties of the multilayer films were characterized by field emission scanning electron microscope （FESEM）, X-ray diffraction （XRD）, UV/Vis spectrophotometer, and Hall effect measurement system. The influence of Cu layer thickness and the oxygen pressure in sputtering atmosphere on the film properties were studied. ZnO/Cu/ZnO transparent conductive film fabricated in pure Ar atmosphere with 10 nm Cu layer thickness has the best performance: resistivity of 2.3×10-4 Ω·cm, carrier concentration of 6.44×1016cm-2 , mobility of 4.51cm2·（V·s）-1 , and acceptable average transmittance of 80 % in the visible range. The transmittance and conductivity of the films fabricated with oxygen are lower than those of the films fabricated without oxygen, which indicates that oxygen atmosphere does not improve the optical and electrical properties of ZnO/Cu/ ZnO films.

Rare Metals 2013,32(03),273-277

Preparation and characterization of ZnO/Cu/ZnO transparent conductive films

Wen-Ying Li Lai-Xin Jiang Gui-Lin Yin Yuan-Yuan Wang Zhen Yu Dan-Nong He

College of Materials Science and Engineering, Shanghai Jiao Tong University

National Engineering Research Center for Nanotechnology

作者简介：Dan-Nong He e-mail:hdnbill@163.com;

收稿日期：27 September 2012

基金：financially supported by the National Nature Science Foundation of China (No. 21071098);the Project of International Cooperation of the Ministry of Science and Technology of China (No. 2011DFA50530);the Nanotechnology Program of Shanghai Science & Technology Committee (No. 12nm0504800);

Preparation and characterization of ZnO/Cu/ZnO transparent conductive films

Abstract：

ZnO/Cu/ZnO transparent conductive thin films were prepared by RF sputtering deposition of ZnO target and DC sputtering deposition of Cu target on n-type (001) Si and glass substrates at room temperature. The morphology, structure, optical, and electrical properties of the multilayer films were characterized by field emission scanning electron microscope (FESEM), X-ray diffraction (XRD), UV/Vis spectrophotometer, and Hall effect measurement system. The influence of Cu layer thickness and the oxygen pressure in sputtering atmosphere on the film properties were studied. ZnO/Cu/ZnO transparent conductive film fabricated in pure Ar atmosphere with 10 nm Cu layer thickness has the best performance: resistivity of 2.3×10-4 Ω·cm, carrier concentration of 6.44×1016cm-2 , mobility of 4.51cm2·(V·s)-1 , and acceptable average transmittance of 80 % in the visible range. The transmittance and conductivity of the films fabricated with oxygen are lower than those of the films fabricated without oxygen, which indicates that oxygen atmosphere does not improve the optical and electrical properties of ZnO/Cu/ ZnO films.

Keyword：

ZnO; Cu; Transparent conductive films; Magnetic sputtering;

Received： 27 September 2012

1 Introduction

Nowadays,the solar energy has attracted much interest due to its wide resources and environmental protection,but solar cells are limited by necessity of fabrication of lowcost transparent conductive electrodes thin film.Indiumtin-oxide(ITO)is currently used as transparent conducting electrode.However,indium is expensive and toxic,which limits its application.Zinc oxide(Zn O)is a II–VI compound semiconductor with a large direct band-gap of3.37 e V[1,2]at room temperature,and has high optical transmittance in visible range.It has potential to be applied on solar cells and flat panel displays as a transparent conducting oxide(TCO)[3–6].TCOs with high transmittance in visible range and low resistance are preferred to be applied on these applications.Pure Zn O has high optical transmittance and high resistance.In order to meet the challenge of TCOs,extrinsic doping with Group-III elements,such as Al[7],Ga[6,8],and In[9],are always used to increase the carrier concentration.However,some problems,such as high resistivity in some cases,are still needed to be solved for improving practical applications[10,11].

Zn O/metal/Zn O multilayer is a special kind of TCO and has been studied for improving the conductivity of Zn O film Metals like Au,Ag,and Cu were introduced in this structure because of their high conductivity[12–14].Cu is the first choice as the metal layer because of its lower cost and comparable resistivity(approximately 1.7 9 10^-6X?cm)with Ag(approximately 1.6 9 10^-6X?cm).On the other hand,the Cu^2?has the comparable ion radius with Zn^2?so that Cu^2?can substitute Zn^2?in Zn O lattice,and there will be less distortion on the interface between Zn O and Cu layer which will reduce the photon scattering on the interface Hence,Cu was chosen as the interlayer metal in our study.

2 Experimental

2.1 Fabrication of Zn O/Cu/Zn O films in pure Ar gas atmosphere

Zn O/Cu/Zn O transparent conductive thin films were prepared by RF sputtering depositions of Zn O target(99.99%purity)and DC sputtering deposition of Cu target(99.99%purity)on n-type(001)Si and glass substrates at room temperature.The substrates were ultrasonically cleaned in acetone,absolute ethyl alcohol and deionized water for10 min,and subsequently dried in flowing nitrogen gas.High purity Ar(99.999%purity)was used as sputtering gas,the sputtering pressure was 0.8 Pa,and the background pressure of the system was 5 9 10^-4Pa.The sputtering system was controlled by the computer.The Zn O layers were sputtered in 40 ml?min^-1Ar gas,and at a constant power of 30 W.The Cu layers were sputtered in50 ml?min^-1Ar gas,and at a constant power of 80 W.The thickness of the bottom-layer and top-layer Zn O was about50 nm,while those of interlayer Cu was 6–16 nm.

2.2 Fabrication of Zn O/Cu/Zn O in Ar/O2mixture gas atmosphere

Zn O films were fabricated in Ar/O₂mixture gas atmosphere,Cu layer was fabricated in pure Ar gas atmosphere,and its thickness was 10 nm.The constant flow rate of Ar gas was 40 ml?min^-1,and the flow rate of O₂was 5,10,and 20 ml?min^-1.Other experimental conditions were in accordance with that in Sect.2.1.

2.3 Characterization techniques

To determine the thickness,structure and morphology,ellipsometer,X-ray diffraction(XRD),and field emission scanning electron microscope(FE-SEM)were used.The diffraction patterns were obtained using Cu Ka radiation(k=0.154056 nm)with the h–2h model.

The optical properties of the multilayer films were studied by a Perkin Elmer Lambda 950 ultraviolet and visible(UV–Vis)spectrometer.The resistance of the films was measured by Hall effect measurement system.

3 Results and discussion

3.1 Influence of Cu layer thickness on the structure of Zn O/Cu/Zn O films

To investigate the effect of Cu layer on Zn O structure,X-ray diffraction(XRD)was used.Figure 1 shows the XRD spectra of Zn O and Zn O/Cu/Zn O multilayer films with different Cu layer thicknesses.The strong Zn O(002)peaks in all of the films indicates that Zn O has a c-axis preferential orientation despite of the existence of the Cu interlayer,which can be explained by the low surface free energy for Zn O(002)plane[15],and this phenomenon can also exist in Ag doping Zn O[10].The peaks at*43.2°corresponding to the Cu(111)phases become apparent with the increase in Cu thicknesses.

Despite the existence of Zn O(002)peaks in all the films,the relative intensities of(002)peak are different The intensities of Zn O(002)peak show a trend of fluctuation with the increase of the thicknesses of Cu layer Compared with the pure Zn O film,the intensity decreases suddenly when 6 nm Cu layer is introduced.Then,the intensities increase with the increase of Cu layer thicknesses up to 10 nm,and the further increase of Cu leads to weaker peaks.This may be explained as follows:during the initial stage of Cu film deposition,Cu metallic nuclei are formed on the bottom Zn O layer,and the nuclei grow into islands with further deposition.These islands do not benefit the preferential growth of top-layer Zn O.As the Cu layer thickness increases,the isolated islands continue to grow and merge with adjacent islands,and then,a continuous layer is formed,e.g.,Cu thickness=10 nm Because of the similar lattice structure and constant between Zn O and Cu[16],there are good lattice matches and less strain in the interface,which benefits the crystallinity of the top-layer Zn O.However,the thicker the Cu layer is,the more lattice defects are formed in the Cu layer which leads to mismatch between Zn O and Cu.Therefore the intensity decreases when Cu thickness ranging from 10to 16 nm.

3.2 Influence of Cu thickness on optical and electrical of Zn O/Cu/Zn O layer

The transmittance and resistivity of Zn O/Cu/Zn O films are affected by the Cu layer thickness,as shown in Figs.2 and 3Compared with pure Zn O film,the Zn O/Cu/Zn O films in the visible range shows a lower transmittance.In addition,the optical transmittance of the Zn O/Cu/Zn O films increases with the increase of Cu layer thickness up to 10 nm,and then decreases with further increase of Cu layer thickness(Fig.2).This can also be explained by the evolution of Cu layer.The morphology of Cu layer on the Zn O layer may experience from an island-like structure to a film-like structure with the increase of Cu layer deposition time.The island structure of Cu film in the initial stage(the thickness of Cu layer\10 nm)can scatter the incident light,which will reduce the transmittance.When the thickness of Cu layer is more than 10 nm,a film-like structure is formed.The thicker the Cu layer is,the less visible light transmits through the Zn O/Cu/Zn O film.It is found that the optimum value of Cu layer thickness is 10 nm with a film’s peak transmittance of 88%,and an acceptable average transmittance of 80%.

Fig.1 XRD patterns of Zn O/Cu/Zn O films with different thicknesses of Cu layer

Fig.2 Optical transmission spectra of Zn O/Cu/Zn O films with different thickness of Cu layer

The sheet resistance(R_sh)decreases continuously with the increase of Cu layer thickness,as shown in Fig.3a.The R_shof pure Zn O is too high to be measured,so the electrical property is not shown in Fig.3.The inset in Fig.3a exhibits the sheet resistance of the films as a function of Cu layer thickness ranging from 10 to 16 nm for better revelation.It can be seen that the rate of decrease on the sheet resistance also decrease as the Cu layer thickness increased,and Fig.3b shows the carrier concentration and mobility of Zn O/Cu/Zn O films as a function of Cu layer thickness.The carrier concentration and mobility all increase as the Cu layer thickness increases,which is similar with the previous study of Sivaramakrishnan[17]When Cu layer thickness reaches 10 nm,the film has a sheet resistance of 21.5 X?sq^-1and a resistivity of 2.30 9 10^-4X?cm,with a carrier concentration of 6.438 9 10¹⁶cm^-2and a mobility of 4.51 cm²?(V?s)^-1.

3.3 Influence of oxygen flow rate on properties of Zn O/Cu/Zn O

The properties of Zn O/Cu/Zn O films are also affected by the concentration of oxygen vacancies.Here,Zn O films were fabricated in Ar/O₂mixture gas.The constant flow rate of Ar gas was 40 ml?min^-1,and the flow rate of O₂was 5,10,and 20 ml?min^-1.In order to avoid the oxidation of Cu layer,Cu was still fabricated in Ar gas atmosphere Figure 4 is the microstructure of Zn O/Cu/Zn O films fabricated in pure Ar gas and Ar/O₂gas(40 ml?min^-1Ar?20 ml?min^-1O₂).It shows that the grain size of Zn O fabricated in Ar/O₂gas is smaller than that of Zn O fabricated in pure Ar.

Figures 5 and 6 exhibit the transmittance and sheet resistance of Zn O/Cu/Zn O films fabricated in different atmosphere.The transmittance and conductivity of the films fabricated in Ar/O₂gas all decrease,as compared with that fabricated in pure Ar gas.This can be explained as follows:because of the smaller grain size of Zn O(Fig.4),there are more grain boundaries that will increase the scatter of incident light.As a result,the transmittance decreases.For the increased sheet resistance,it is because the oxygen atmosphere does not benefit the form of oxygen vacancies,which contributes to the conductivity of Zn O.On the other hand,the increase of oxygen content in Zn O can improve the probability of Cu oxidation,which will result in the decrease of carrier concentration of Cu layer.Thereby,the sheet resistance increases,as compared with Zn O/Cu/Zn O films fabricated in pure Ar gas.

Fig.3 Effect of Cu layer thickness on a sheet resistivity and b carrier concentration and Hall mobility of Zn O/Cu/Zn O films

Fig.4 SEM images of ZnO/Cu/ZnO films fabricated in a pure Ar gas and b Ar/O2=2:1

Fig.5 Transmittance of Zn O/Cu/Zn O films fabricated in different atmosphere

Fig.6 Resistivity of Zn O/Cu/Zn O films fabricated at different oxygen flow rate

4 Conclusion

Zn O/Cu/Zn O films were prepared by RF sputtering deposition of Zn O target and DC sputtering deposition of Cu target on n-type(001)Si and glass substrates at room temperature.The influence of the Cu layer thickness and oxygen content on the Zn O/Cu/Zn O transparent conductive films was studied.XRD spectra show that Zn O has a c-axis preferential orientation,and the intensities of(002)peak show a trend of fluctuation with the increase of the thicknesses of Cu layer.The optical transmittance of the Zn O/Cu/Zn O films increases with the increase of Cu layer thickness up to 10 nm,and then decreases with further increase of Cu layer thickness.The sheet resistance decreases continuously with the increase of Cu layer thickness.When Cu layer thickness reaches 10 nm,the peak transmittance of the film reaches 88%with an acceptable average transmittance of 80%,the resistivity is2.3 9 10^-4X?cm with a carrier concentration of 6.44 910¹⁶cm^-2,and the mobility is 4.51 cm²?(V?s)^-1.The transmittance and conductivity of the films fabricated in Ar/O₂atmosphere are lower than those of the films fabricated without oxygen.The best performance for transparent conducting application is obtained with the Zn O/Cu/Zn O films fabricated in pure Ar atmosphere with 10 nm Cu layer thickness.

Acknowledgments This study was financially supported by the National Nature Science Foundation of China(No.21071098),the Project of International Cooperation of the Ministry of Science and Technology of China(No.2011DFA50530),and the Nanotechnology Program of Shanghai Science&Technology Committee(No.12nm0504800).