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Effect of Ga2O3 buffer layer thickness on the properties of Cu/ITO thin films deposited on

flexible substrates

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2014 J. Semicond. 35 053001

(http://iopscience.iop.org/1674-4926/35/5/053001)

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Vol. 35, No. 5 Journal of Semiconductors May 2014

Effect of Ga2O3 buffer layer thickness on the properties of Cu/ITO thin filmsdeposited on flexible substrates�

Zhuang Huihui(庄慧慧), Yan Jinliang(闫金良)�, Xu Chengyang(徐诚阳),and Meng Delan(孟德兰)

School of Physics and Optoelectronic Engineering, Ludong University, Yantai 264025, China

Abstract: Cu and Cu/ITO films were prepared on polyethylene terephthalate (PET) substrates with a Ga2O3 bufferlayer using radio frequency (RF) and direct current (DC) magnetron sputtering. The effect of Cu layer thicknesson the optical and electrical properties of the Cu film deposited on a PET substrate with a Ga2O3 buffer layer wasstudied, and an appropriate Cu layer thickness of 4.2 nm was obtained. Changes in the optoelectrical propertiesof Cu(4.2 nm)/ITO(30 nm) films were investigated with respect to the Ga2O3 buffer layer thickness. The opticaland electrical properties of the Cu/ITO films were significantly influenced by the thickness of the Ga2O3 bufferlayer. A maximum transmission of 86%, sheet resistance of 45 �/� and figure of merit of 3.96 � 10�3 ��1 wereachieved for Cu(4.2 nm)/ITO(30 nm) films with a Ga2O3 layer thickness of 15 nm.

Key words: transparent conductive films; Ga2O3 buffer layer; Cu/ITO films; optical property; electrical propertyDOI: 10.1088/1674-4926/35/5/053001 EEACC: 2520M; 2530N; 2530D

1. Introduction

Transparent conductive films (TCOs) have been used astransparent electrodes in organic light emitting diodes, liquidcrystal displays, plasma display panels and solar cells becausethey have high electrical conductivity and high transparencyin the visible regionŒ1�4�. Tin-doped indium oxide (ITO) is apreferred TCO material for these applications because of itsfavorable electrical and optical properties. Recently, there hasbeen considerable interest in the use of ITO films deposited onpolymer substrates for applications. However, it is difficult todeposit high quality ITO thin films on polymer substrates be-cause there is a restriction on the substrate temperature duringthe deposition processŒ5�. One way to improve the optical andelectrical properties of ITO thin films deposited on polymersubstrates is to use ITO/metal/ITO (IMI) multilayer filmsŒ6; 7�,which have lower resistivity than single layer ITO films ofthe same thickness. Furthermore, the polymer substrates haveweaker mechanical strength and more easily absorb moistureand gas than glass substratesŒ8�. Therefore, it is essential tomake a buffer layer when TCO films are deposited on poly-mer substrates, which will make the polymer substrate surfacesmoother and reduce the level of vapor and oxygen diffusion.A range of materials have been used as buffer layers, such asTiO2, ZnO and SiO2

Œ9; 10�.Ga2O3 has excellent chemical stability and good adhesion

on substrates, and has a wide band gap of 4.8 eV, resulting in atransparency from the visible into the UV regionŒ11�. Addition-ally, the Ga2O3 layer deposited at room temperature is amor-phousŒ12�, and the refractive index is close to ITO. In this pa-per, the Ga2O3 buffer layer instead of ITO is inserted betweenthe polymer substrate and Cu/ITO film to reduce diffusion of

moisture and gas, the effect of the Cu layer thickness on theproperties of the Cu film and the effect of Ga2O3 buffer layerthickness on the properties of Cu/ITO film were investigated.

2. Experimental details

The Cu films and Cu/ITO thin films were preparedon the polyethylene terephthalate (PET) substrates with theGa2O3 buffer layer using radio frequency magnetron sputter-ing Ga2O3 ceramic targets (purity of 99.99%) and direct cur-rent magnetron sputtering ITO targets (purity of 99.99%) andCu targets (purity of 99.99%) at room temperature, respec-tively. Firstly, the thickness of Cu films was varied from 1.4to 4.6 nm, and the Ga2O3 buffer layer thickness was kept at20 nm. Secondly, the thickness of the Ga2O3 buffer layer wasranging from 0 to 50 nm, and the thicknesses of Cu and ITOlayers were kept constant at 4.2 nm and 30 nm, respectively.All the films were prepared in pure Ar ambient at room tem-perature and the sputtering chamber was evacuated to a basepressure of 6 � 10�4 Pa. The experimental deposition condi-tions are listed in Table 1.

Optical transmittance was measured using a double beamspectrophotometer (TU1901). The sheet resistance RS, free

Table 1. Deposition conditions of ITO, Ga2O3 and Cu layers.Parameter ITO Ga2O3 CuDeposition pressure (Pa) 0.51 0.5 0.3Power density (W/cm2/ DC, 1.66 RF, 2.48 DC,1.47Deposition rate (nm/s) 0.48 0.08 0.46Gas flow rate (Ar, sccm) 20 20 20

* Project supported by the National Natural Science Foundation of China (No. 10974077), the National Science Foundation of ShandongProvince, China (No. 2009ZRB01702), and the Shandong Province Higher Educational Science and Technology Program, China (No.J10LA08).

† Corresponding author. Email: yanjinliang@yahoo.cnReceived 13 October 2013, revised manuscript received 29 November 2013 © 2014 Chinese Institute of Electronics

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Fig. 1. Electrical properties of the Cu films deposited on PET sub-strates with a 20 nm Ga2O3 layer.

carrier concentration n, and Hall mobility �H were determinedfrom Hall effect measurements using the Vander Pauw method(Accent HL5500 Hall System) at a constant magnetic field of0.517 T. The surface morphology was examined by an atomicforce microscope (NanoScope IIIa).

3. Results and discussion

3.1. Effect of the Cu layer thickness on the properties of theCu film

The Cu films are deposited on PET substrates with 20 nmGa2O3 buffer layers, the sheet resistance, carrier concentrationand Hall mobility of the Cu films as a function of the Cu layerthickness are shown in Fig. 1. The sheet resistance is decreasedwith the increase of the Cu layer thickness. When the Cu layerthickness is ranged from 1.4 to 4.2 nm, the sheet resistance de-creases from 622 to 80 �/�. With further increasing the Culayer thickness, the sheet resistance has a slight decrease. Thesheet resistance is defined as RS D �/t , where the resistivity �

is proportional to the reciprocal of the product of carrier con-centration n and mobility �. The higher product of the carrierconcentration n and mobility � leads to the lower electrical re-sistivity �.

When the Cu layer thickness is 1.4 nm, the carrierconcentration is 1 � 1021 cm�3 and the Hall mobility is0.06 cm2/(V�s). The carrier concentration and Hall mobility ofthe Cu films are increased with the increase of the Cu layerthickness. When the Cu layer thickness is 4.2 nm, the carrierconcentration is 2.9 � 1022 cm�3 and the Hall mobility is 6.6cm2/(V�s). However, as the Cu layer thickness increases from4.2 to 4.6 nm, it can be seen that the carrier concentration in-creases slightly from 2.9 � 1022 to 3 � 1022 cm�3, and Hallmobility decreases slightly from 6.6 to 6.5 cm2/(V�s).

Figure 2 shows the transmittance of the Cu films with dif-ferent Cu layer thicknesses in the wavelength range from 300to 850 nm, using PET as the reference. The transmittance ofthe films decreases as the Cu layer thickness increases. Whenthe Cu layer thickness is 1.4 nm, the maximum transmission ofthe Cu film is 85%. But when the Cu layer thickness is 4.6 nm,the maximum transmission of the Cu film is about 77%. The

Fig. 2. Transmittance spectra of the Cu films with different Cu layerthicknesses.

transmittance is defined as

T DI

I0

D e�˛t ; (1)

where I0 and I are the intensity of the incident light and emer-gent light, ˛ is the absorption coefficient, and t is the thicknessof the thin film. So the optical transmittance of the Cu filmsdecreases with the increase of the Cu layer thickness. In addi-tion, the surface roughness results in surface scattering, whichdecreases the optical transmittance of the Cu filmsŒ13�.

The figure of merit (�TC/ is an important index for evalu-ating the performance of transparent conducting oxide (TCO)films. We estimated a figure of merit �TC for the multilayers asfollows:

�TC DT 10

RS; (2)

where T is the transmittance at the peak value and RS is thesheet resistance. The figures of merit are 3.12 � 10�4 ��1,6.48 � 10�4 ��1, 6.76 � 10�4 ��1, 8.69 � 10�4 ��1, 1.02 �

10�3 ��1, 1.15 � 10�3 ��1, and 1.02 � 10�3 ��1 for the Cufilm thicknesses of 1.4 nm, 2.3 nm, 2.8 nm, 3.2 nm, 3.7 nm,4.2 nm, and 4.6 nm, respectively. The best figure of merit isobtained when the thickness of the Cu layer is 4.2 nm.

3.2. Effect of the Ga2O3 layer thickness on the propertiesof Cu/ITO films

It is known that the optical properties of the IMI multilayerfilms are mainly dependent on the metal layerŒ14�. For the Cufilms deposited on the Ga2O3 buffer layer, the Cu layer is ex-posed to the atmosphere without a protective layer, and is easilyoxidized to copper oxide or basic copper carbonate. This maynot only deteriorate the conductivity of the Cu layer itself, butalso increase the contact resistance with the probe electrodes.So we deposited the ITO layer on the Cu layer to protect theCu layer from chemical and mechanical exposuresŒ15�. In orderto investigate the properties of the Cu/ITO films deposited onPET substrates with different Ga2O3 buffer layer thicknesses,we fixed the Cu layer thickness at 4.2 nm and the ITO layerthickness at 30 nm.

Figure 3 shows surface morphologies of Cu/ITO filmswith different thicknesses of Ga2O3 buffer layers. The sur-

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Fig. 3. AFM images of samples. (a) Cu/ITO. (b) Ga2O3 (15 nm)/Cu/ITO. (c) Ga2O3 (20 nm)/Cu/ITO.

Fig. 4. Transmittance spectra of Cu/ITO films with different Ga2O3

buffer layer thicknesses.

face roughness of Cu/ITO films, Ga2O3(15 nm)/Cu/ITO filmsand Ga2O3(20 nm)/Cu/ITO films is 2.31 nm, 1.4 nm and1.82 nm, respectively. As can be seen from above, the sur-face morphology of the Cu/ITO films is improved by the in-sertion of the Ga2O3 buffer layers, and the surface roughnessof the Ga2O3(15 nm) /Cu/ITO films is lower than that of theGa2O3(20 nm)/Cu/ITO films.

Figure 4 shows the transmittance of the Cu/ITO films withdifferent Ga2O3 buffer layers in the wavelength range from300 to 850 nm, using PET as the reference. The maximumtransmission of the Cu/ITO films without a Ga2O3 buffer layeris 81%, while the maximum transmission of Cu/ITO films witha 15 nm Ga2O3 buffer layer is 86%, which is higher than thatof the Cu/ITO films without a Ga2O3 buffer layer. When thethickness of the Ga2O3 buffer layer is beyond 15 nm, the trans-mittance of Cu/ITO films deposited on PET substrates withGa2O3 buffer layers decreases, indicating that the increase ofsurface roughness with the increase of Ga2O3 thickness resultsin more diffuse scattering of the incident light.

Figure 5 shows the sheet resistance of Cu/ITO films withdifferent Ga2O3 buffer layer thicknesses. When the Cu andITO layers are interfaced, the Fermi level aligns across the in-terface after transfer of electrons from the lower work functionCu to the higher work function ITO, thereby resulting in theaccumulation of electrons in a very thin region near the inter-face, until the thermodynamic equilibriumŒ16�. This is why we

Fig. 5. Sheet resistance (RS/, Hall mobility (�) and carrier concen-tration (n/ of the Cu/ ITO films as a function of Ga2O3 buffer layerthickness.

can detect the lower sheet resistance of the Cu/ITO films. Thetotal resistance of a Cu/ITO film is a combinative resistance ofthe Cu and ITO layers in parallel as follows:

1

RTotalD

1

RITOC

1

RCu: (3)

Since RCu is much less than the other resistances, the to-tal resistance RTotal would mainly depend on the Cu layer.The metal resistance originates from the collision of free elec-trons which results in the loss of the directional speed ac-celerated from the electric field. Collisions may take placein electron-lattice, electron-impurity, electron-grain boundaryand electron-surface. For a 4.2 nm thick Cu film, the resistiv-ity is dominated by electron-surface collisions. A smoother sur-face has a positive effect on the decrease of the sheet resistance.

The sheet resistance of the Cu/ITO films decreases initiallywith increasing Ga2O3 buffer layer thickness (6 15 nm) andthen the sheet resistance increases with the further increase ofthe buffer layer thickness. The sheet resistance of the Cu/ITOfilms without a Ga2O3 buffer layer is 52 �/�, and the sheetresistance of 45 �/� is obtained when the thickness of theGa2O3 buffer layer is 15 nm. This result is superior to the sheetresistance (143 �/�) of ITO/Cu/ITO deposited on PET sub-strates with a 40 nm thick SiO2 buffer layerŒ17�. The decreaseof sheet resistance can be interpreted by the following two rea-sons: (1) the barrier effect of the buffer layer to diffusion ofthe impurities from the PET substrate has a positive role in im-

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Fig. 6. Figure of merits of Cu/ ITO films with different Ga2O3 bufferlayers.

proving the quality of Cu/ITO filmsŒ18�, and (2) the smoothsubstrate surface induced by the buffer layer tends to reducethe defects in deposited Cu/ITO films and improve the crystal-lization of Cu/ITO films, which enhances the carrier mobilityand thus decreases the sheet resistance of Cu/ITO filmsŒ19; 20�.When the Ga2O3 thickness is further increased, the sheet resis-tance increased. The increase in sheet resistance can be under-stood by the change in carrier concentration and Hall mobilityas shown in Fig. 5.

As can be seen, the carrier concentration is initially in-creased with the increase of the Ga2O3 buffer layer thick-ness (6 15 nm) and then decreased with further increasingthe Ga2O3 buffer layer thickness. The carrier concentrationachieves the maximum value of 8.6 � 1021 cm�3 when thethickness of the Ga2O3 buffer layer is 15 nm. The variationof the Ga2O3 buffer layer has an effect on the structure ofCu/ITO multilayer films, which results in the variation of thecarrier concentration. The Hall mobility of the Cu/ ITO filmsincreases at first and then decreases with the increase of Ga2O3

layer thickness, whose maximum value of 9.4 cm2/(V�s) is ob-tained at a 20 nm Ga2O3 buffer layer. This is the reason whywe choose the Ga2O3 buffer layer thickness of 20 nm whenwe studied the effect of Cu layer thickness on the propertiesof the Cu films deposited on PET substrates. The Hall mobil-ity of the Cu/ITO film is mainly related to the crystallinity, thedefects and the surface properties. When the thickness of theGa2O3 buffer layer exceeds 20 nm, the Hall mobility of theCu/ITO films starts to degrade. The Ga2O3 buffer layer affectsthe initial growth of Cu/ITO crystallites and the surface rough-ness of Cu/ITO films, which result in different Hall mobilitywith different buffer layer thicknesses.

The figure of merits (�TC/ of the Cu/ITO films with differ-ent Ga2O3 buffer layers are shown in Fig. 6. As can be seen, the�TC reaches a maximum of 3.96 � 10�3 ��1when the thick-ness of Ga2O3 buffer layer is 15 nm, which is higher than the�TC (2.3 � 10�3 ��1/ of the Cu/ITO films without a Ga2O3

buffer layer.The electrical stability of Ga2O3 (15 nm)/Cu/ITO films in

humid environment is investigated. The resistance variation ofCu/ITO films and Ga2O3 (15 nm)/Cu/ITO films stored in 95%relative humidity (RH) at 20 ıC is shown in Fig. 7. After storing

Fig. 7. Dependence of sheet resistance of Cu/ITO films and Ga2O3

(15 nm)/Cu/ITO films on the storage time.

the samples for 30 days, the sheet resistance of Cu/ITO filmswithout a Ga2O3 buffer layer has a rapid increase from 52�/�to 97 �/�, but the sheet resistance of Ga2O3 (15 nm)/Cu/ITOfilms increases slowly from 45 to 49 �/�. The Ga2O3 layeracts as a barrier layer to restrain the diffusion of moisture andgas. We can see that the electrical stability of Cu/ITO films wasimproved greatly after the insertion of the Ga2O3 buffer layer.

4. Conclusions

The Cu films and Cu/ITO films were deposited on thepolyethylene terephthalate (PET) substrates with the Ga2O3

buffer layer using radio frequency (RF) and direct current (DC)magnetron sputtering. The best figure ofmerit is obtainedwhenthe thickness of the Cu layer is 4.2 nm. The optical and electri-cal properties of the Cu/ITO films deposited on PET substrateswere improved with an appropriate Ga2O3 buffer layer thick-ness. When the thickness of the Ga2O3 buffer layer was 15 nm,the optical transmittance of 86%, sheet resistance of 45 �/�and the figure of merit of 3.96 � 10�3 ��1 were achieved forthe Cu/ITO films deposited on PET substrates.

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