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ransparent conductive Ga2O3/Cu/ITO multilayer films prepared onexible substrates at room temperature
uihui Zhuang, Jinliang Yan ∗, Chengyang Xu, Delan Mengchool of Physics and Optoelectronic Engineering, Ludong University, Hongqi Road 186, Yantai 264025, Shandong, People’s Republic of China
r t i c l e i n f o
rticle history:eceived 9 January 2014eceived in revised form 25 February 2014ccepted 3 April 2014vailable online 13 April 2014
a b s t r a c t
Transparent conductive Ga2O3/Cu/ITO films were prepared on the polyethylene terephthalate (PET) sub-strates using radio frequency (RF) and direct current (DC) magnetron sputtering without substratesheating. The effects of Ga2O3 layer thickness and Cu layer thickness on the optical and electrical propertiesof the Ga2O3/Cu/ITO films were studied. Changes in the optoelectrical properties of Ga2O3/Cu (4.2 nm)/ITO(30 nm) films were investigated with respect to the Ga2O3 layer thickness. The maximum transmission
−2 −1
eywords:ndium tin oxideoppera2O3 buffer layerlexible substrate
of 86%, the sheet resistance of 45 �/sq and the figure of merit of 8.89 × 10 � were achieved for Ga2O3
(15 nm)/Cu (4.2 nm)/ITO (30 nm) films. The optoelectrical properties of the Ga2O3/Cu/ITO films were alsosignificantly influenced by the thickness of the Cu layer. When the thickness of Cu layer was 3.7 nm, themaximum transmission of 87.6%, the sheet resistance of 50 �/sq and the figure of merit of 9.26 × 10−2 �−1
Indium tin oxide (ITO) transparent conductive films have beensed as transparent electrodes in organic light emitting diodes, liq-id crystal displays, plasma display panels and solar cells [1–3]. The
TO films deposited on glass substrates have been widely investi-ated during the last decade. Recently, there has been considerablenterest in the application of ITO films deposited on polymer sub-trates because polymer substrates are lightweight and flexible,hich overcomes the disadvantage of glass substrates such aseavy weight and brittleness [4,5]. However, it is difficult to depositigh quality ITO thin films on polymer substrates because there is aestriction on the substrate temperature during the deposition pro-ess [6]. ITO/metal/ITO (IMI) multilayer films have been studied inhe hope of increasing conductivity without significant losses inransmission [7]. The metal for the practical use includes Al, Cu andg. Ag is expensive and Al is sensitive to oxygen. So we select Cu as
he metal layer [8]. The polymer substrates have weaker mechan-cal strength and absorb moisture and gas more easily than glassubstrates [9]. Therefore, a high transmittance buffer layer is intro-
uced to make the polymer substrate surface smoother and reducehe level of vapor and oxygen diffusion. A range of materials haveeen used as buffer layers, such as TiO2, ZnO and SiO2 [10,11]. Ga2O3
has excellent chemical stability and good adhesion on substrate,has wide band gap of 4.8 eV, resulting in a transparency from thevisible into UV regions [12]. Additionally, Ga2O3 layer depositedat room temperature is amorphous [13], and the refractive indexis closed to ITO. In this work, we use the Ga2O3 instead of ITO orother buffer layers to prepare the Ga2O3/Cu/ITO multilayer filmson the polyethylene terephthalate (PET) substrates using radio fre-quency (RF) and direct current (DC) magnetron sputtering at roomtemperature. The effects of the Ga2O3 layer and Cu layer thicknesson the electrical and optical properties of Ga2O3/Cu/ITO multilayerfilms were investigated.
2. Experiment
Two groups of Ga2O3/Cu/ITO multilayer films were prepared onthe PET substrates. Firstly, we fixed the thickness of Cu (4.2 nm)/ITO(30 nm) films to investigate the effect of Ga2O3 layer on the prop-erty of the Ga2O3/Cu/ITO multilayer films. Secondly, the propertyof the Ga2O3 (15 nm)/Cu/ITO (30 nm) multilayer films was investi-gated at different Cu interlayer thicknesses. The Ga2O3 buffer layerwas deposited on PET substrates by radio frequency magnetronsputtering Ga2O3 ceramic targets (purity of 99.99%). Cu/ITO mul-tilayer films were deposited on the top of Ga2O3 buffer layer by
direct current magnetron sputtering ITO targets (purity of 99.99%,In2O3:SnO2 = 90:10 wt.%) and Cu targets (purity of 99.999%). Allthe films were prepared in pure Ar ambient at room temperatureand the sputtering chamber was evacuated to a base pressure of
× 10−4 Pa. The experimental deposition conditions are listed inable 1.
The sheet resistance Rs, free carrier concentration n, and Hallobility �H were determined from Hall effect measurements using
he Vander Pauw method (Accent HL5500 Hall System) at a con-tant magnetic field of 0.517 T. Optical transmittance was measuredsing a double beam spectrophotometer (TU1901) without an
ntegrating sphere. Optical transmittance means specular transmit-ance in this paper except special explanation.
. Results and discussion
.1. Effect of Ga2O3 layer thickness
In order to investigate the properties of the Ga2O3/Cu/ITO filmseposited on PET substrates with different Ga2O3 layer thick-esses, we fixed the Cu layer thickness at 4.2 nm and the ITO layerhickness at 30 nm. Fig. 1 shows surface morphologies of Cu/ITO,a2O3 (15 nm)/Cu/ITO and Ga2O3 (20 nm)/Cu/ITO with different
hickness of Ga2O3 layers. The surface roughness of Cu/ITO films,a2O3 (15 nm)/Cu/ITO films and Ga2O3 (20 nm)/Cu/ITO films is.31 nm, 1.4 nm and 1.82 nm, respectively. As can be seen frombove, the surface morphology of the Cu/ITO films is improvedy the insertion of the Ga2O3 layers, and the surface roughnessf the Ga2O3 (15 nm)/Cu/ITO films is lower than that of the Ga2O320 nm)/Cu/ITO films.
Fig. 2 shows the transmittance of Ga2O3/Cu/ITO films withifferent Ga2O3 layer thicknesses in the wavelength range from00 to 850 nm. The transmittance of the Ga2O3/Cu/ITO films is
ncreased with the increase of the Ga2O3 layer thickness (≤15 nm).hen the thickness of Ga2O3 layer is 15 nm, the maximum trans-ittance of Ga2O3/Cu/ITO films is 86%. The transmittance of thea2O3/Cu/ITO films is decreased with the further increase of thea2O3 layer thickness. The decrease of the transmittance is related
o the increasing absorption of the Ga2O3 layers [1] or the increasef surface roughness results in more diffuse transmittance, whichauses the reduction of specular transmittance [14].
Fig. 3 shows the sheet resistance (RS), Hall mobility (�H) andarrier concentration (n) of the Ga2O3/Cu/ITO films as a functionf the Ga2O3 layer thickness. When the Cu layer and ITO layer arenterfaced, the Fermi level aligns across the interface after transferf electrons from the lower work function Cu to the higher workunction ITO, thereby resulting in the accumulation of electrons in aery thin region near the interface, until reaching thermodynamicquilibrium [15]. That is why we can detect the lower sheet resis-ance of Ga2O3/Cu/ITO films than the Ga2O3/ITO films. The totalesistance of a Ga2O3/Cu/ITO film is a combinative resistance of thea2O3 layer, Cu layer and ITO layer in parallel as follows:
1RTotal
= 1RGa2O3
+ 1RITO
+ 1RCu
(1)
Since RCu and RITO are much less than RGa2O3 , the total resis-ance RTotal would mainly depend on the Cu layer and ITO layer. A
moother surface has a positive effect on the decrease of the sheetesistance. The sheet resistance of Ga2O3/Cu/ITO films decreasesnitially with increasing Ga2O3 layer thickness (≤15 nm) and thenhe sheet resistance increases with the further increase of the
Fig. 1. AFM images of samples: (a) Cu/ITO, (b) Ga2O3 (15 nm)/Cu/ITO and (c) Ga2O3
(20 nm)/Cu/ITO.
Ga2O3 layer thickness. The lowest sheet resistance of 45 �/sq isobtained when the thickness of Ga2O3 layer is 15 nm. This result issuperior to the sheet resistance (143 �/sq) of ITO/Cu/ITO depositedon PET substrates with a 40 nm thick SiO2 buffer layer [16]. Thedecrease of sheet resistance can be interpreted by the followingtwo reasons: (1) the barrier effect of the Ga2O3 layer to diffu-
sion of the impurities from PET substrate has a positive role inimproving the quality of Cu/ITO films [17], and (2) the smooth sub-strate surface induced by Ga2O3 layer tends to reduce the defectsin deposited Cu/ITO films, which enhances the carrier mobility
H. Zhuang et al. / Applied Surface Science 307 (2014) 241–245 243
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wtn8Hdit
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excitation leading to a greater drop in transmittance. However, in
ig. 2. Transmittance spectra of Ga2O3/Cu/ITO films with different Ga2O3 layerhickness.
nd thus decreases the sheet resistance of Ga2O3/Cu/ITO films18,19]. When the Ga2O3 thickness was further increased, the sheetesistance increased, which is caused by the decrease of carrieroncentration.
As can be seen, the carrier concentration is initially increasedith the increase of the Ga2O3 layer thickness (≤15 nm) and
hen decreased with further increasing the Ga2O3 layer thick-ess. The carrier concentration achieves the maximum value of.6 × 1021 cm−3 when the thickness of Ga2O3 layer is 15 nm. Theall mobility of the Ga2O3/Cu/ITO films increases at first and thenecreases with the increase of Ga2O3 layer thickness, whose max-
mum value of 9.4 cm2/V/s is obtained at a 20 nm Ga2O3 layerhickness.
An effective transparent conducting oxide (TCO) films shouldave high electrical conductivity combined with low absorptionf visible light. Thus an appropriate quantitative measure of theerformance of TCO films is the ratio of the electrical conductivity
to the visible absorption coefficient ̨ [20]
�
˛= −
{RS ln (T + R)
}−1(2)
here RS is the sheet resistance, T is the visible transmittance at50 nm, and R is the visible reflectance at 550 nm. Thus �/ ̨ is a fig-re of merit for rating TCO films. A larger value of �/ ̨ indicates
etter performance of the TCO films. The figure of merit of thea2O3/Cu/ITO films with different Ga2O3 layers is shown in Fig. 4.s can be seen, the figure of merit is firstly increased, and thenecreased with the increase of the Ga2O3 layer thickness. When
ig. 3. Sheet resistance (Rs), Hall mobility (�H) and carrier concentration (n) of thea2O3/Cu/ITO films as a function of Ga2O3 layer thickness.
Fig. 4. Figure of merits of Ga2O3/Cu/ITO films with different Ga2O3 layers thickness.
the thickness of Ga2O3 layer is 15 nm, the figure of merit reaches amaximum value of 8.89 × 10−2 �−1.
3.2. Effect of Cu layer thickness
According to the above results, we obtain that the Ga2O3/Cu/ITOfilm with an appropriate Ga2O3 layer thickness has better opticaltransmittance and conductivity. In order to investigate the proper-ties of the Ga2O3/Cu/ITO films deposited on PET substrates withdifferent Cu layer thicknesses, we choose the Ga2O3 layer withthickness of 15 nm and the ITO layer with thickness of 30 nm.
The transmission spectra in the wavelength range 300–850 nmwas measured for Ga2O3/Cu/ITO films at various Cu layer thick-nesses using PET as reference, as shown in Fig. 5. After the insertionof the Cu layer, the transmittance of the Ga2O3/Cu/ITO films drops.The variation in transmittance with copper thickness is different inthe short wavelengths and long wavelengths.
In the short wavelengths, as the thickness of the copper layerincreases, the transmittance decreases. The optical transmittanceis mainly affected by light absorption, due to interband electronictransitions, in particular, due to the excitation of electrons fromthe d-band to the Fermi surface [5,21]. With the increase of cop-per layer thickness, there are more bound electrons available for
the longer wavelengths, the transmittance of Ga2O3/Cu/ITO filmsis firstly increased and then decreased with the increase of Cu layerthickness. The transmittance of Ga2O3/Cu/ITO films with a 2.3 nm
Fig. 5. Transmittance spectra of Ga2O3/Cu/ITO films with different Cu interlayerthickness.
244 H. Zhuang et al. / Applied Surface S
Fm
CwdWtm[
Gsnttfit
GfiCsfnfigecat
F
ig. 6. Dependence of the sheet resistance (Rs), carrier concentration (n) and Hallobility (�H) of Ga2O3/Cu/ITO films with different Cu layer thickness.
u layer thickness is lower than that of the Ga2O3/Cu/ITO filmsith a 3.2 nm Cu layer thickness. That is because the presence ofiscontinuous copper islands leads to significant scattering losses.hen the Cu layer thickness is larger than 3.2 nm, the transmit-
ance is decreased due to higher light reflectance, which is theain factor to decrease the transmittance in the longer wavelength
5].Fig. 6 displays the dependence of electrical properties for
a2O3/Cu/ITO films on the Cu layer thickness. As we can see, theheet resistance is decreased with the increase of the Cu layer thick-ess. When the Cu layer thickness is ranged from 0 nm to 3.7 nm,he sheet resistance decreases from 212 �/sq to 50 �/sq. With fur-her increase in the thickness, the Cu film becomes a continuouslm from the channel structure, which leads to a slight decrease ofhe sheet resistance.
When the Cu layer thickness is 2.3 nm, the Hall mobility of thea2O3/Cu/ITO films (2 cm2/V/s) is lower than that of the Ga2O3/ITOlms (3.7 cm2/V/s). That is because, at this Cu layer thickness, theu layer is discontinuous, which acts as discontinuous scatteringites reducing the Hall mobility. As the Cu layer thickness increasesurther, the Hall mobility increases. With increasing Cu layer thick-ess, the grains of thicker Cu films are larger than that of thinner Culms [22], and the larger grains result in a lower concentration ofrain boundaries, which behave as traps for free carriers and barri-
rs for carrier transport in the film. Hence, the increase in grain sizean cause a decrease in grain boundary scattering, which leads ton increase in the mobility. When the Cu layer thickness is 4.6 nm,he Hall mobility gets the highest value of 9.3 cm2/V/s, the carrier
ig. 7. Figure of merits of Ga2O3/Cu/ITO films with different Cu layer thicknesses.
[
[
[
[
cience 307 (2014) 241–245
concentration of the Ga2O3/ITO films is 1.2 × 1020 cm−3. After inser-tion of the Cu layer, the carrier concentration of the Ga2O3/Cu/ITOfilms is increased with the increase of the Cu layer thickness. As theCu layer thickness increases from 3.7 nm to 4.6 nm, the carrier con-centration increases slightly from 8 × 1021 cm−3 to 9 × 1021 cm−3.
The figure of merit of the Ga2O3/Cu/ITO films with Cu layer thick-nesses is shown in Fig. 7. As can be seen, the figure of merit ofthe Ga2O3/Cu (2.3 nm)/ITO films is 3.63 × 10−2 �−1, which is lowerthan the figure of merit of the Ga2O3/ITO films (4.38 × 10−2 �−1).The figure of merit reaches a maximum value of 9.26 × 10−2 �−1
when the thickness of Cu layer is 3.7 nm, which is higher than thefigure of merit of the Ga2O3/ITO films without the Cu layer.
4. Conclusions
Transparent conductive Ga2O3/Cu/ITO films were prepared onthe polyethylene terephthalate (PET) substrates using radio fre-quency (RF) and direct current (DC) magnetron sputtering withoutsubstrate heating. The optical and electrical properties of theGa2O3/Cu/ITO films deposited on PET substrates were improvedwith an appropriate Ga2O3 layer thickness. The maximum trans-mittance of 87.6%, the sheet resistance of 50 �/sq and the figure ofmerit of 9.26 × 10−2 �−1 were obtained for the Ga2O3 (15 nm)/Cu(3.7 nm)/ITO (30 nm) films.
Acknowledgements
This work is supported by the National Natural Science Foun-dation of China (Grant no. 10974077), the Innovation Project ofShandong Graduate Education, China (Grant no. SDYY13093), theNatural Science Foundation of Shandong Province, China (Grant no.ZR2010AL026).
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