Influence of TiO2 buffer layer and post-annealing on the quality of Ti-doped ZnO thin films

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CERAMICS

INTERNATIONAL

Ceramics International 39 (2013) 5795–5803

Inﬂuence of TiO

₂

buffer layer and post-annealing on the quality

of Ti-doped ZnO thin ﬁlms

Y.C. Lin

a

, C.Y. Hsu

b

, S.K. Hung

a

, D.C. Wen

c,n a_{Department of Mechanical Engineering, National Chiao Tung University, Taiwan, ROC} b_{Department of Mechanical Engineering, Lunghwa University of Science and Technology, Taiwan, ROC}

c_{Department of Mechanical Engineering, China University of Science and Technology, Taiwan, ROC} Received 7 November 2012; received in revised form 17 December 2012; accepted 29 December 2012

Available online 11 January 2013

Abstract

Ti-doped ZnO (TZO) thin ﬁlms were grown on soda lime glass (SLG) substrate without and with a TiO2 buffer layer by radio

frequency magnetron sputtering and then annealed under vacuum at 450 and 500 1C for 20 min. The structural, electrical, and optical properties of TZO films were investigated. XRD analysis shows that all TZO films are highly textured along the c-axis and perpendicular to the substrate. The structural properties of TZO films are improved by controlling the annealing temperature and inserting a TiO2

buffer layer. When the ﬁlms were annealed at 450 1C, the crystallinity increased, but it then decreased slightly with increase in annealing temperature from 450 to 500 1C. Due to superior crystallinity, TZO ﬁlms annealed at 450 1C exhibited lower resistivity and higher average transmittances in the visible region. The improvements in crystallinity, resistivity and transmittance are more obvious when a TiO2 buffer layer was inserted. The decrease in resistivity is mainly attributed to an increase in Hall mobility rather than carrier

concentration. When the TZO ﬁlms deposited on bare SLG substrate, the energy band gaps was decreased after annealing at 450 and 500 1C due to the decrease in carrier concentration. However, the absorption edge of TZO ﬁlms deposited on TiO2-buffered substrate

was blue shifted, and the energy band gap was increased due to the increase of carrier concentration. In this study, the TZO ﬁlm with optimal properties was grown on the TiO2-buffered substrate and post annealed at 450 1C, achieving a resistivity of 3.76 10

3

O-cm and an average transmittance above 85%. Therefore, it can be concluded that inserting a buffer layer at an early stage of ﬁlm deposition to improve crystallinity can help achieve low resistivity, high transmittance, and high energy band gap in transparent conducting TZO thin ﬁlms.

1. Introduction

Transparent conducting oxide (TCO) films with optical transmission above 80% in the visible region and resistivity below 103O-cm are extensively used in solar cells, flat panels, and light emitting diodes as well as flexible displays

[1–3]. Materials such as indium tin oxide (ITO) and zinc oxide (ZnO) films are well-known transparent conductive films. Doped ZnO films are promising alternative of ITO films due to their stable electrical and optical properties. Among the various types of doped ZnO films, Ti-doped ZnO (TZO) films, in comparison with ZnO films doped with

Group III elements, have more than one charge valence state. Ti4 þhas a radius of 0.68 ˚A, which is smaller than that of Zn2 þ, 0.74 ˚A. When titanium atoms are doped into a ZnO lattice, they act as donors by providing two free electrons, thus increasing the free carrier concentration and decreasing the resistivity. Furthermore, ZnO films doped with Ti have larger preferential c-axis orientation and better optical properties than pure ZnO films[4]. TCO thin films have been prepared by several techniques, including magnetron sputtering[5], chemical vapor deposi-tion [6], and sol–gel process [7]. Among these methods, sputtering has advantages of good uniformity, high process controllability, and large-area deposition.

Recent research has studied and reported the properties of deposited TZO ﬁlms. Chung et al. [8] indicated that the

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n

Corresponding author. Tel.: þ 886 2 2786 7048; fax: þ 886 2 2786 7253. E-mail address: dcwen@cc.cust.edu.tw (D.C. Wen).

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crystallinity of TZO films was improved with a low working pressure and a high substrate temperature, and the lowest film resistivity was obtained with 1.34 wt% of Ti added. Lu et al. [9,10] and Lin et al. [11] have reported using simultaneous magnetron co-sputtering technique to deposit TZO films. The surface roughness of TZO films increased with higher power applied to the Ti target. Wang et al.[12]

investigated the effects of substrate temperature on properties of TZO ﬁlms and found enhancement in TZO ﬁlm properties when the substrate temperature increased from room tem-perature to 300 1C. Chang et al. [13] and Jiang et al. [14]

studied the effects of annealing treatment on properties of TZO thin films. They reported that the film resistivity decreased and the average optical transmittance in the visible wavelength range changed slightly after annealing treatment. It is well known that TCO epitaxial thin films must be grown with low defect densities and more n-type characteristics[15]. Unfortunately, direct growth of TCO epitaxial thin film deposited on glass substrate is difficult at low growth temperature and high doping concentration due to large lattice mismatch and thermal expansion coefficient between the films and the substrates[16,17]. Several research groups have studied the growth of epitaxial Al- or Ga-doped ZnO (AZO and GZO) thin films by introducing buffer layers like Al, ZnO and SiO2 to overcome these difficulties [18–20].

However, the effects of inserting a buffer layer on properties of TZO thin ﬁlms have rarely been reported so far.

In this study, TiO2 ﬁlm was chosen as a buffer layer

since titanium has high chemical resistance, high melting point, and the same hexagonal-close-packed (HCP) struc-ture as ZnO has [21]. TZO ﬁlms were deposited on TiO2

-buffered and bare soda lime glass (SLG) substrates using radio frequency (RF) magnetron sputtering and then annealed under vacuum. The structural, electrical, and optical properties of TZO ﬁlms were investigated.

2. Experimental

TZO transparent conducting ﬁlms and TiO2buffer layer

were deposited on 25 25 1.1 mm3 SLG substrates by magnetron sputtering system. The diameter of the TZO and TiO2 targets was 50.8 mm and their thickness was

3 mm. Before deposition, the targets were pre-sputtered for

5 min to remove any contamination. The substrates were ultrasonically cleaned with acetone and de-ionized water, and then blown dry with nitrogen gas. All samples were deposited with substrate rotation in order to have a good surface morphology. The RF sputtering conditions are shown inTable 1. According to these sputtering conditions the thickness of the TZO ﬁlms was 330 nm.

Our previous study has shown that the optimal thickness of the buffer layer for AZO ﬁlms with high transmittance was about 10 nm [22]. Therefore, the deposition time of TiO2 buffer layer was adjusted to make sure that the

thickness of the layer was 10 nm. Research has found that the resistivity of TZO and AZO was improved after a vacuum post-annealing at temperatures ranging from 400 1C to 500 1C but was decreased under annealing at 600 1C [14,23]. Hence, the as-deposited samples were subsequently annealed in a tube furnace evacuated to a primary vacuum level (5.0 106Torr) at 450 1C and 500 1C for 20 min. Moreover, another set of samples were deposited on silicon substrate for the transmission electro-nic microscopy (TEM) measurement.

After deposition and annealing, the surface morphologi-cal properties were analyzed using an atomic force micro-scope (AFM, SPA-400) and ﬁeld emission scanning electron microscopy (SEM, JEOL, JSM-6500 F). The crystallo-graphic properties of the ﬁlms were determined by X-ray diffraction (Rigaku-2000 X-ray diffractometer), with Cu-Ka radiation (40 kV, 30 mA and l=0.1541 nm) and an angle incidence of 11. Electrical resistivity was measured by the four-point probe method (Mitsubishi chemical MCP-T600). Carrier concentration and Hall mobility were obtained from Hall-effect measurement by the Van der Pauw method (Ecopia, HMS-3000). Optical transmittance was measured by a UV/vis/IR spectrophotometer (Jasco, V-570) in the wavelength ranging from 300 to 800 nm. All measurements were performed at room temperature in air.

3. Results and discussion 3.1. Structural properties

Fig. 1 shows the XRD patterns of TZO ﬁlms prepared

without and with a TiO2 buffer layer as a function of Table 1

Deposition parameters of TZO ﬁlm and TiO2buffer layer.

Parameters TZO ﬁlm TiO2buffer layer

Target 98 wt% ZnO, 2 wt% TiO2(99.995% purity) TiO2(99.995%, purity)

Sputtering power 130 W, 13.56 MHz 100 W, 13.56 MHz

Base pressure 5.0 106Torr 5.0 106Torr

Working gas Argon (99.995%) Argon (99.995%)

Deposition pressure 15 103_Torr _{15 10}3_Torr

Substrate-to-target distance 85 mm 85 mm

Substrate temperature 300 1C 100 1C

Substrate rotate vertical axis 10 rpm 10 rpm

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annealing temperature. As can be seen, all films exhibited a significant (002) peak at 2y value of 34–351, indicating that the TZO films prepared by RF magnetron sputtering had a hexagonal wurtzite structure and showed a good c-axis orientation perpendicular to the substrate [24]. Fig. 2shows SEM micrograph of cross-section of as-deposed TZO film prepared without TiO2buffer layer. The cross-section shows

the ﬁlm to have a dense columnar structure which is perpendicular to the glass substrate. This result is similar to ﬁndings previously reported[9–13]. In addition, there was no TiO2 phase found from the XRD spectra, implying that

titanium may replace zinc in the hexagonal lattice or segregate to the non-crystalline region in grain boundaries. The relative intensity of the (002) diffraction peak increased when the films were annealed at 450 1C, indicating stronger c-axis orientation than that of the as-deposited TZO films. The smaller intensity for the (002) diffraction peak of TZO film grown on TiO2-buffered glass and annealed at 500 1C

in comparison to that annealed at 450 1C is attributed to film deterioration (Fig. 1b). This result indicates that preferred orientation changes for a few grains in the film and the structure of TZO film is becoming more random. Thus, various grains with (002) and (102) preferred orienta-tions normal to substrate were observed.

Lin et al.[11]mentioned that more Ti atoms substituted into Zn sites led to more compressive stress in the lattice of TZO ﬁlms in a direction parallel to the surface due to an increase in substrate temperature, which could affect the lattice spacing perpendicular to the surface and cause the position of the (002) peak to shift to a lower diffraction angle than that of the ZnO bulk position ( 34.451). On the contrary, during the annealing treatment, the atoms receive energy and migrate to relative equilibrium positions, which induces a series of effects; for example, reducing the lattice strain, showing a more perfect crystal-lite, weakening grain boundary scattering and increasing the number of current carriers [25]. Han et al. [26]

indicated that the strain of the as-grown AZO film decreased when the film was annealed due to the relaxation of planes. The (002) peak position of non-buffered TZO films was changed and shifted to the normal peak of the ZnO bulk position by annealing, as shown in Fig. 3. However, the influence of annealing on the position of the (002) peak for the films prepared with a TiO2buffer layer

is quite distinct. It seems that TZO film deposited directly on glass is more sensitive to annealing treatment. It is supposed that the defects, induced by the lattice mismatch and thermal expansion coefficient, can get thermal energy from annealing treatment and move to the film surface, therefore, the strain in film is effectively relaxed. However, buffer layer in TZO films can preclude this kind of movement and confine most of the defects in itself. Thus the position of the (002) peak was almost unchanged.

Fig. 4shows the full width at half-maximum (FWHM) and

grain size with annealing temperature of TZO ﬁlms prepared

2θ(degree) Intensity (a.u.) 30 35 40 45 50 As-deposited Annealing at 450 oC Annealing at 500 oC ( 0 0 2 ) Without buffer layer

2θ(degree) Intensity (a.u.) 30 35 40 45 50 As-deposited Annealing at 450 oC Annealing at 500 oC (0 0 2 ) ( 1 0 2 ) With buffer layer

Fig. 1. XRD patterns of TZO ﬁlms prepared (a) without and (b) with TiO2buffer layer.

TZO film

Glass Substrate 10 kV 100 nm

Fig. 2. SEM micrograph cross-section of as-deposed TZO ﬁlm prepared without TiO2buffer layer.

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without and with a TiO2buffer layer. The grain size (D) was

calculated according to Scherrer’s formula: D ¼ 0:9l

bcos y ð1Þ

where X-ray wave length l ¼ 1.54 ˚A, b is the FWHM, and y is the Bragg angle of (002) peak. FWHM in all films are in range of 0.31–0.37, and grain sizes are in range of 23–28 nm. The FWHM of annealed TZO films decreased and their grain size increased as compared with the as-deposited film. The films annealed at 450 1C had larger grain size. The increase in grain size after insertion of a TiO2buffer layer is obvious.

Fig. 5 shows AFM images of TZO ﬁlms prepared

without and with a TiO2 buffer layer and then annealed

at various temperatures. Fig. 5a shows that the morphol-ogy of as-deposited films was in the shape of cobblestone and island, indicating that the growth has taken place by nucleation and coalescence during the deposition of TZO film by sputtering onto substrates. Randomly distributed nuclei may have first formed and these nuclei then grow to form observable islands. When the TZO films were

annealed at 450 1C, the islands formed came closer to each other, with the larger ones appearing to grow by coalescence of smaller ones. Thus, the surface structure of ﬁlms became denser and the grains grew larger, as shown

in Fig. 5b. This enhancement was more obvious when a

TiO2buffer layer was inserted. However, micro voids were

observed at the grain boundaries and the surface morphol-ogy of the films became loose with open grain boundaries when the annealing temperature increased from 450 1C to 500 1C, as shown in Fig. 5c. The results confirm that the structural properties of TZO films can be improved by controlling the annealing temperature and inserting a TiO2

buffer layer.

Surface roughness (Ra) was also calculated from AFM. The Ra value of TZO ﬁlms deposited without and with a TiO2 buffer layer is 4.224 and 5.717 nm, respectively.

Relatively large Ra value appeared for the TZO ﬁlm deposited on the TiO2-buffered substrate. Pre-deposition

of TiO2buffer layer increased the surface roughness of the

substrate, resulted in the increase of the Ra value of the TZO films. Ra value increases when the TZO films were annealed and as the temperature increased from 450 1C to 500 1C, the surface of the TZO films became rougher. Ra values of the TZO films increased with increasing tem-perature due to the three-dimensional island growth during thermal annealing.

The bright-ﬁeld TEM micrograph obtained from the cross-section of TZO ﬁlm prepared with a TiO2 buffer

layer and post-annealed at 450 1C is shown inFig. 6. The cross-section of the TZO/ TiO2 junction reveals a good

contact between the TiO2 buffer layer and the TZO ﬁlm.

The thickness of the TiO2buffer layer is about 10 nm. The

microstructure shows that the TZO ﬁlm and the TiO2layer

are both very dense and uniform. The spacing between the planes in the atomic lattice (d) of TZO film was measured by the TEM image, and the d spacing of the (002) plane was about 0.2604 nm. This value is very close to the spacing between the TZO (002) plane calculated by Bragg’s law (0.2602 nm). Moreover, a grain boundary in the TZO film was clearly observed, indicating a polycrystalline structure but with a preferential orientation along the c-axis. With increasing annealing temperature, the polycrys-tallic character of TZO film became more obvious hence a low intensity of (102) peak was observed in the TZO film prepared with a TiO2 buffer layer and then annealed at

500 1C, as shown inFig. 1b. 3.2. Electrical properties

The resistivity (r), Hall mobility (m) and carrier concen-tration (N) for TZO ﬁlms prepared without and with a TiO2 buffer layer as a function of annealing temperature

are shown in Fig. 7. The optimal annealing temperature was 450 1C and it caused the resistivity to decrease by 30% (from 7.98 103 to 5.62 103O-cm) and 45% (from 6.81 103 to 3.76 103O-cm) for the ﬁlms prepared without and with a TiO2 buffer layer, respectively.

Annealing temperature 2(degree) 34.25 34.29 34.33 34.37 34.41 34.45

As-deposited 450oC 500oC Without buffer layer With buffer layer

Fig. 3. Dependence of diffraction angle of (002) reﬂection peak on the annealing temperature. Annealing temperature FWHM,(degree) 0.31 0.33 0.35 0.37 23 24 25 26 27 28

As-deposited 450oC 500oC With buffer layer Without buffer layer Grain size, D (nm)

Fig. 4. FWHM and grain size as a function of annealing temperature for TZO thin ﬁlms prepared without (open) and with (solid) TiO2 buffer layer.

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However, with increase in annealing temperature to 500 1C, resistivity increased.

Resistivity is a combined result of both Hall mobility and carrier concentration according to the formula of resistivity, r ¼ 1

Nem ð2Þ

where e is the electron charge. For ﬁlms deposited with a TiO2

buffer layer, the diffusion of Na atoms from SLG substrate into ﬁlm could be suppressed by the buffer layer. As the TZO

film annealed at 450 1C, the grain size increases. A larger grain size reduces the grain boundary scattering and increases the carrier lifetime, which results in an increase in conductivity, due to an increase in Hall mobility and carrier concentration. As a result, the sheet resistance of the TZO films decreases. In the case of annealing at 500 1C, the film became looser with open grain boundaries and grain size reduced. It implies an increase in the number of grain boundary defects and hence decreases the carrier concentration[27]. Although TiO2buffer

layer could prevent the diffusion of Na atoms into ﬁlm,

TZO/TiO2/glass TZO/glass Ra=5.707 nm Ra=6.245 nm Ra=6.776 nm Ra=5.717 nm Ra=6.593 nm Ra=4.224 nm

Fig. 5. AFM micrographs of TZO ﬁlms prepared without (left) and with (right) TiO2buffer layer for the (a) as-deposited and for those annealed at (b) 450 1C and (c) 500 1C.

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if annealing temperature is high, some Na atoms could be still diffused into TZO ﬁlm through the TiO2 layer. If Na atom

substitutes zinc atom, it will become acceptor, resulting in decrease of carrier concentration[28]. Another possible reason is related to segregation of metal atoms into grain boundaries, where they become inactive as donors [29]. Therefore, with increase in annealing temperature to 500 1C, the carrier concentration decreased resulting in increasing the resistivity.

On the other hand, for films deposited without buffer layer, Na atoms diffusion from SLG substrate into TZO films during annealing, and the grain size of the films is smaller than that of the films grown on the TiO2-buffered

substrates. Thus, TZO ﬁlms prepared without buffer layer demonstrate higher resistivity, as shown inFig. 7a.

It should be noted that the decrease in lowest resistivity obtained was mainly related to increase in Hall mobility, as shown in Fig. 7b. As evidenced from XRD and AFM microscopic analyses, TZO ﬁlms prepared with a TiO2

buffer layer exhibited superior crystallinity than thin ﬁlms prepared without buffer layer. Thus, increase in Hall mobility may be attributable to decrease in carrier scatter-ing by the defects and imperfections in polycrystalline

TZO, due to improved crystallinity of the buffer layer-inserted TZO film. As described above, it can be concluded that the improvement of crystallinity at an early stage of film deposition contribute to lower resistivity in transpar-ent conducting TZO thin films.

3.3. Optical properties

Fig. 8shows the optical transmittance spectra of TZO ﬁlms

prepared without and with a TiO2buffer layer. The average

transmittances in the visible region are listed in Table 2. All ﬁlms exhibited high average transmittances exceeding 83% in the visible region and enhancement in transmittance of ﬁlms by annealing treatment was observed. By introducing a TiO2

buffer layer between the SLG substrate and the TZO film, the transmittance of the as-deposited TZO film is lower than the film prepared without buffer layer. However, the transmittance was improved to 85% after annealing at 450 1C due to its superior crystallinity.

Fig. 9 plots of a2 versus hv (where a is the absorption

coefﬁcient and hv is the photon energy). The energy band gap (Eg) can be estimated by extrapolations of the

straight-line part of the plot to the photon energy axis. As shown in

Fig. 8b, for TZO ﬁlm deposited with a TiO2buffer layer and

annealed at 450 1C, the absorption edge has been blue shifted compared with the as-deposited TZO ﬁlm, and correspondingly, the energy band gap is increased from 3.262 to 3.345 eV (Fig. 9b), which is attributed to Burstein– Mott effect[30]due to the increase of carrier concentration by introducing a TiO2layer. However, when the annealing

temperature increased from 450 to 500 1C, the energy band gap decreased to 3.307 eV, which is consistent with decrease in carrier concentration. InFig. 9, the absorption coefﬁcient a was calculated using the equation[31]:

a ¼ln 100=T

t ð3Þ

where t is ﬁlm thickness and T is transmittance. The energy band gap dependence of the absorption coefﬁcient is given

n-type Si (110) Substrate TZO TiO2buffer layer

10 nm

Grain boundary

2 nm

d = 0.2604 nm

Fig. 6. Bright-ﬁeld TEM micrograph of TZO ﬁlm prepared with TiO2 buffer and annealed at 450 1C.

Annealing temperature Resistivity, ρ ( × 10-3 Ω -cm) 3 4 5 6 7 8 9 As-deposited ₄₅₀o_C ₅₀₀o_C Without buffer layer

With buffer layer

Annealing temperature Hall mobility, μ (cm 2 /V-sec) Carrier concentration, N ( × 10 20 cm -3) 1 2 3 4 5 6 7 8 1 2 3 4 5 6 As-deposited ₄₅₀o_C ₅₀₀o_C Without buffer layer

With buffer layer

Fig. 7. (a) Resistivity and (b) Hall mobility and carrier concentration as a function of annealing temperature for TZO thin ﬁlms prepared without (open) and with (solid) TiO2buffer layer.

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by the equation[32]:

ðahnÞ2¼A hnEg ð4Þ

where A is constant. As can be seen inTable 2 andFig. 7b, for the TiO2 buffered TZO ﬁlms, the variation trend of

average transmittance and carrier concentration is agree with each other. Therefore, when the carrier concentration decreases or increases the energy band gap also decreases or increases.

For TZO films deposited without buffer layer, the absorption edges do not depend on variety of annealing temperature and are almost same (Fig. 8a). However, the energy band gap was decreased from 3.274 eV to 3.265 and 3.243 eV for the films annealed at 450 and 500 1C com-pared with the case of as-deposited TZO film (Fig. 9a) since the carrier concentration was decreased due to the diffusion of Na atoms into the TZO films (Fig. 7b). 4. Conclusions

TZO ﬁlms were prepared on TiO2-buffered and

non-buffered glass substrates by RF sputtering and followed by annealing under vacuum at 450 1C and 500 1C for 20 min. The electrical and optical properties of TZO ﬁlms were strongly dependent on ﬁlm crystallinity. The structural

Wavelength (nm) Transmittance (%) 300 400 500 600 700 800 0 20 40 60 80 100 Annealing at 500oC Annealing at 450oC As-deposited Without buffer layer

300 400 500 600 700 800 0 20 40 60 80 100 Annealing at 500oC Annealing at 450oC As-deposited With buffer layer

Transmittance (%)

Wavelength (nm)

Fig. 8. Optical transmittance spectra of TZO ﬁlms prepared (a) without and (b) with TiO2buffer layer.

Table 2

Average optical transmittances of TZO ﬁlms in the visible region (%).

Sample As-deposited Annealed

at 450 1C

Annealed at 500 1C Without buffer layer 84.86 85.25 85.11

With buffer layer 83.65 85.17 84.21

Photon energy (eV)

2.5 3 3.5 0 1 2 3 4 As-deposited, E_g= 3.274 eV Annealing at 450oC, E_g= 3.265 eV Annealing at 500oC, E_g= 3.243 eV α 2/(a.u.)

Without buffer layer

Photon energy (eV)

3.5 0 1 2 3 4 3.262 eV 3.345 eV 3.307 eV α 2/(a.u.)

With buffer layer

2.5 3 As-deposited, Annealing at 450oC, Annealing at 500oC, E_g= E_g= E_g=

Fig. 9. Plots of a2 _{versus hv for TZO ﬁlms prepared (a) without and} (b) with TiO2buffer layer.

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properties of TZO ﬁlms were highly textured along the c-axis and perpendicular to the substrate and could be improved by controlling the annealing temperature and inserting a TiO2buffer layer. TZO ﬁlm prepared without

buffer layer and annealed at 450 1C exhibited lower resistivity and higher average transmittances in the visible region due to its superior crystallinity. These properties were further improved by inserting a TiO2 buffer layer,

and a resistivity of 3.76 103O-cm and an average transmittance more than 85% were achieved. The decrease in resistivity was mainly attributed to increase in Hall mobility rather than carrier concentration. When the TZO ﬁlms deposited on bare SLG substrate, the energy band gap was decreased after annealing at 450 and 500 1C due to the decrease in carrier concentration. However, the absorp-tion edge of TZO ﬁlms deposited with a TiO2buffer layer

was blue shifted, and the energy band gap was increased due to the increase of carrier concentration. The results indicate that inserting a buffer layer at an early stage of ﬁlm deposition to improve crystallinity helps obtain lower resistivity, higher transmittance, and higher energy band gap in transparent conducting TZO thin ﬁlms.

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