Deposition of transparent and conductive ZnO films by an atmospheric pressure plasma-jet-assisted process

TSF-33249; No of Pages 6 Thin Solid Films xxx (2014) xxx–xxx

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Deposition of transparent and conductive ZnO films by an atmospheric pressure plasma-jet-assisted process C.M. Hsu a, S.T. Lien b, Y.J. Yang a, J.Z. Chen b,⁎, I.C. Cheng c,⁎, C.C. Hsu a,⁎ a b c

Department of Chemical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei 106, Taiwan Graduate Institute of Applied Mechanics, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei 106, Taiwan Graduate Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei 106, Taiwan

a r t i c l e

i n f o

Available online xxxx Keywords: Transparent and conductive coatings ZnO TCO Atmospheric pressure plasmas APPJ

a b s t r a c t In this work, deposition of transparent and conductive ZnO thin films using an atmospheric pressure plasma jet (APPJ) is presented. The APPJ is sustained by a pulsed power source with a repetitive frequency up to 25 kHz using N2 as the plasma gas. ZnCl2 solution is used as the precursor and is nebulized and then sprayed into the downstream of the APPJ. With sufficiently long retention time and high jet temperature, a nearly full conversion of the precursor to ZnO occurs. Zinc hydroxyl chloride, with sheet-like structures, is formed as the intermediate when the precursor is not fully converted to ZnO. With an optimal condition, ZnO films with a resistivity of 1.4 Ωcm and average transmittance between 400 and 800 nm of greater than 80% can be obtained with the root-meansquare surface roughness approximately 10 nm. This demonstrates a one-step and fast process without the need of post-annealing steps and intentionally doping to fabricate transparent and conductive ZnO films. © 2014 Elsevier B.V. All rights reserved.

1. Introduction ZnO is a semiconductor and has drawn considerable attention for decades due to its wide band gap energy of 3.37 eV, a large exciton binding energy of 60 meV at room temperature, transparent in the visible wavelength range, and low cost. With the existence of intrinsic defects such as oxygen deficiency or intentional doping, ZnO thin film can be highly conductive. It is therefore a promising transparent conductive oxide. Therefore, ZnO thin films are extensively studied and actively implemented into various applications such as solar cells [1–5], gas sensors [6–9], and light emitting diodes [10,11]. Many techniques have been employed to fabricate ZnO thin films such as sputter [12–16], molecular beam epitaxy [17–19], chemical vapor deposition [20–23], sol gel processes [24–28], electrochemical deposition [29–31], and spray pyrolysis [32–37]. Vacuum processes require the use of high vacuum systems and are therefore expensive and lack of process flexibility. Other processes either require high usage of wet chemicals, or yield low deposition rate, and are not cost effective. Atmospheric pressure plasmas (APPs) are plasmas operated at one atmosphere. Considerable attention is paid to APPs due to its process flexibility, low cost, and the potential for large-area applications such as roll-to-roll processes. APP-assisted processes are broadly applied to thin film deposition [38–40]. Several processes are reported using ⁎ Corresponding authors at: No. 1, Sec. 4, Roosevelt Rd. Taipei, 106, Taiwan. E-mail addresses: [email protected] (J.Z. Chen), [email protected] (I.C. Cheng), [email protected] (C.C. Hsu).

APP-assisted processes to fabricate ZnO thin films. Chang et al. [41] reported an APPJ-assisted process to deposit transparent and conductive In-doped ZnO films. For processes reported by Ito et al. [42] and Maruyama et al. [43], electrical conductivity was not reported. In this work, we report a one-step APP jet (APPJ)-assisted process without the need of post-annealing steps to deposit transparent and conductive ZnO thin films without intentional doping. The deposited film properties, namely UV–vis transmittance, crystallinity, morphology, and electrical conductivity, are reported. How the process parameter influences the film properties will be discussed. 2. Experimental The plasma system under investigation is an APPJ sustained using a pulse power source. It undergoes a glow-to-arc transition within each pulse power period. The pulsed power supply uses a solid state switching device and has a current rise time of 50 A μs−1 with 20 μH output impedance. The detailed description of the APPJ has been described previously [44–46], and its schematic is shown in Fig. 1. The power source delivers DC pulse voltage up to 275 V with a repetitive frequency up to 25 kHz, followed by a transformer that raises the voltage up to 16 kV. The applied voltage described in the following section is the voltage at the primary side (the low voltage side) of the transformer. The plasma jet consists of cylindrical-type electrodes, made by stainless steel, with a powered inner electrode and a ground outer electrode. The diameter of the powered electrode is 1.5 cm. The ground electrode is a converging nozzle with 3.5 cm of inner-diameter in the upstream and 0.4 cm of inner-diameter at the jet exit. The x–y moving stage allows

http://dx.doi.org/10.1016/j.tsf.2014.02.102 0040-6090/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: C.M. Hsu, et al., Deposition of transparent and conductive ZnO films by an atmospheric pressure plasma-jet-assisted process, Thin Solid Films (2014), http://dx.doi.org/10.1016/j.tsf.2014.02.102

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Fig. 1. The schematic of the APPJ system.

for programmable 2-axis movement. In this work, the plasma gas is nitrogen (99.999%). The on/off duration of the power is 7 μs/33 μs. In the ZnO thin film deposition process, a Pyrex™ glass tube with 3cm of the inner diameter is set up at the jet exit to provide a better contact between the precursor and the plasma. The precursor solution, 3 wt.% ZnCl2 (98 + %, Acros Organic), is atomized by an ultrasonic nebulizer. The precursor droplets are then carried to the downstream of the plasma jet using 0.5 slm nitrogen as the carrier gas. Corning™ glass is used as the substrate. The substrate is scanned at the speed of 2 cm/s during the deposition process to achieve better uniformity [47]. During the process, the substrate is heated by the APPJ and no external heater is used. The crystalline structures of the ZnO thin films are characterized by grazing-incidence X-ray diffraction (XRD, X'pert 5000, Philips) with Cu Kα X-ray radiation 40 mA 45 kV and wavelength λ = 1.54 Å. The morphology of the thin film is examined by using scanning electron microscope (SEM, Nova NanoSEM 230, FEI) under 5 kV and atomic force microscope (AFM, SPI3800N, SEIKO) in the tapping mode. The transmittance is quantified using a UV–vis spectrometer (JASCO V-670 UV–VIS-NIR). This spectrometer is equipped with an integrating sphere.

Fig. 2. SEM images (top and cross section views) of deposited ZnO films under various gas flow rates (30 to 60 slm) and the applied voltage of 275 V. The scale bar is 500 nm.

Fig. 3. The SEM top-view images of the ZnO films deposited under 30 slm gas flow rate with an applied voltage from 225 to 275 V. The scale bar is 500 nm.

Please cite this article as: C.M. Hsu, et al., Deposition of transparent and conductive ZnO films by an atmospheric pressure plasma-jet-assisted process, Thin Solid Films (2014), http://dx.doi.org/10.1016/j.tsf.2014.02.102

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3. Result and discussion 3.1. Microstructure and crystallinity The top view and cross-section view of the microstructure of the ZnO films are first examined using SEM. Fig. 2 shows the images for samples deposited with an applied voltage of 275 V and plasma gas flow rates from 30 to 60 slm. It is shown that with a 30 slm gas flow rate, a dense and uniform film is obtained with a film thickness approximately 115 nm. With an increase in the flow rate to 60 slm, the sheet-like structure appears on the deposited films and the cross-section views show that the films become rough and less dense. These sheet-like structures are hexagonal plates and have an appearance similar to that of zinc hydroxide chloride (Zn5(OH)8Cl2, ZHC) reported in the literature [48]. The effect of the applied voltage on the film microstructure is then examined and Fig. 3 shows the results. With an applied voltage of 225 V, a sheet-like structure is observed. An increase in the applied voltage from 225 to 275 V leads to a smooth film and the disappearance of such a sheet-like structure. The surface morphology is then characterized using AFM to further quantify the surface root-mean-square roughness and to clarify how the APPJ operating conditions influence the surface morphology of the ZnO films. Fig. 4(a) and (b) shows the AFM surface scan of the film deposited under various applied voltages and gas flow rates. The roughness for each condition is tabulated in Table 1. It is shown clearly that under the condition of 30 slm gas flow rate and 275 V applied voltage, the roughness is 10.4 nm. Both the increase in the gas flow rate and the decrease in the applied voltage leads to a significantly increase in the roughness. For example, the roughness under 60 slm gas flow rate and 275 V applied voltage is 56.1 nm. The sudden increase in the roughness occurs in the same conditions as that the sheet-like structures appear in the SEM images. Fig. 5(a) shows the XRD patterns of the ZnO films deposited under different gas flow rate. We first note that under 30 slm, the diffraction pattern clearly shows that the deposited film is ZnO. The patterns show (101) and (100) preferred orientations with a rather weak (002) peak. Znaidi et al. [49] reported that in a wet-based thin film deposition process, the precursor concentration and the effective surface coverage greatly influence the preferred orientation of the film due to the inter-precursor and precursor-substrate interactions. The peak intensities decrease with the increase in the gas flow rate. As noted above, an increase in the gas flow leads to a decrease both in the surface temperature (Table 1) and the retention time. Such low retention time and temperature are not sufficient to fully convert ZnCl2 solution droplets to ZnO. With the highest flow rate under investigation (60 slm), films with rather poor crystallinity are obtained and the component is identified as ZHC. Such an observation supports the hypothesis proposed based on the SEM image: the sheet-like hexagonal plates is ZHC. Fig. 5(b) shows the XRD pattern of the ZnO film deposited at different applied voltages. It also shows that a decrease in the applied voltage leads to poor crystallinity. In addition, the appearance of ZHC is observed with a low applied voltage. The average grain size is also estimated by XRD patterns using Scherrer's equation [50]: D ¼ 0:9λ=W cosθ; Fig. 4. AFM images of the ZnO films deposited under (a) various gas flow rates (30 to 60 slm) and the applied voltage of 275 V and (b) various applied voltage (225 to 275 V) and the flow rate of 30 slm.

The transmittance measured includes scattered light. The electrical conductivity of the film is measured by 2-probe measurement (Keithley 2636A) with indium tin oxide (ITO) pads as the contact electrodes. The use of the ITO contact allows for overcoming the Schottky barrier in the contact.

where D is the grain size, λ is the X-ray wavelength (λ = 1.54 Å for CuKα), and W is the full-width at half-maximum in radian. The calculated grain sizes are shown in Table 1. It clearly shows that a high applied voltage and a low flow rate lead to films with a large grain size, which can be well explained by the high temperature and long retention time under such a condition [51,52]. Based on the above observations, we believe that for the conversion from ZnCl2 solution droplet to ZnO, ZHC is an intermediate product. As reported in the literature, the ZnCl 2 solution reacts

Please cite this article as: C.M. Hsu, et al., Deposition of transparent and conductive ZnO films by an atmospheric pressure plasma-jet-assisted process, Thin Solid Films (2014), http://dx.doi.org/10.1016/j.tsf.2014.02.102

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Table 1 The substrate temperature and film characteristics obtained in the different conditions. Applied voltage (volt)

Gas flow rate

Substrate temperature (°C)

Grain size (nm)

Roughness (nm)

Band gap (eV)

275 275 275 275 250 225

30 slm 40 slm 50 slm 60 slm 30 slm 30 slm

620–600 530–545 490–505 450–460 580–565 540–530

37 30 19 N.A. 33 19

10.4 9.1 41.6 56.1 13.5 27.2

3.24 3.25 3.29 3.29 3.25 3.25

with H2O and form sheet-like structure ZHC [53]. By thermal annealing, ZHC can be converted to ZnO [48]. A fully conversion to ZnO requires sufficiently high temperature and long contact between the droplet and the jet. In such a conversion process, the evaporation of the solvent (water) and the conversion of the precursor to ZnO take place. With an increase in the gas flow rate, the gas temperature at the plasma jet downstream decreases as reported previously [44, 45]. It leads to a decrease in the substrate temperature, as tabulated in Table 1. In addition, the resident time of the precursor droplets staying in the APPJ downstream decreases with the increase in the gas flow rate. When the temperature is not sufficiently high and the retention time is too short, the observation of ZHC is a result of incomplete conversion of the solution droplets.

3.2. The resistivity of the ZnO thin films Fig. 6 shows the resistivity of the ZnO thin films deposition under various applied voltages and gas flow rates. Under the optimal condition (275 V and 30 slm), the resistivity of the ZnO film is 1.4 Ω-cm. It clearly shows that the resistivity increases with the increase in the gas flow rate and the decrease in the applied voltage. Such an increase in the resistivity is a result of the decrease in the grain size, the increase in the roughness, and the formation of ZHC. Such a range of the film resistivity is clearly higher than those reported in a number of literatures, especially in comparison with films deposited by sputtering, processes with postannealing steps, or conditions with intentionally doping [4,12,24,41]. We note that the key features of the process presented in this work, namely non vacuum, continuous process, no intentional doping, and no need of post annealing treatment, allows for this process to be utilized in a great number of applications.

Intensity (arb. units)

a 275 V, 30 slm

3.3. Optical properties

275 V, 40 slm

The optical transmission spectra of the ZnO thin films are shown in Fig. 7. All the films show the transmission well above 80% in the visible wavelength range. Due to the fact that the transmittance is measured

275 V, 50 slm 275 V, 60 slm

a

100

ZnO

20

30

40

50

60

2 Theta (degree)

Resistivity (Ω-cm)

Zn5(OH)8Cl2.H2O

10

1

b 0.1

275 V, 30 slm

30

40

50

60

Main flow rate (slm) Intensity (arb. units)

b

225 V, 30 slm

20

30

40

50

60

2 Theta (degree)

Resistivity (Ω-cm)

250 V, 30 slm

100

10

1

0.1 275

250

225

Applied voltage (Volt) Fig. 5. The XRD patterns of ZnO films deposited under (a) 275 V applied voltage and gas flow rates of 30–60 slm as well as the JCPDS patterns for ZnO and Zn5(OH)8Cl2, and (b) 30 slm gas flow rate and applied voltages of 225–275 V.

Fig. 6. The resistivity of the ZnO films deposited under (a) 275 V applied voltage and gas flow rates of 30–60 slm and (b) 30 slm gas flow rate and applied voltages of 225–275 V.

Please cite this article as: C.M. Hsu, et al., Deposition of transparent and conductive ZnO films by an atmospheric pressure plasma-jet-assisted process, Thin Solid Films (2014), http://dx.doi.org/10.1016/j.tsf.2014.02.102

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a

opportunities in a great number of applications such as large area rollto-roll processes.

Transmission (%)

100 80

Acknowledgment 60 40

30 slm 40 slm 50 slm 60 slm

20 0 200

300

400

500

600

700

800

Wavelength (nm)

b 100

Transmission (%)

5

80 60 40

275 V 250 V 225 V

20 0 200 300

400

500

600

700

800

Wavelength (nm) Fig. 7. The UV–vis transmission spectra of ZnO deposited under (a) 275 V applied voltage and gas flow rates of 30–60 slm and (b) 30 slm gas flow rate and applied voltages of 225– 275 V.

using a spectrometer equipped with an integrating sphere, the transmission is not greatly influenced by the surface roughness. The absorption edge shows a slight shift with the change in the condition. The optical band gap is estimated by Tauc equation [49]:   n ðαhνÞ ¼ A hν−Eg ; where Eg is the optical band gap of the films and A is a constant. hν is the photo energy, α is the absorption coefficient, and n = 2 for direct band gap. The estimated band gap is calculated and shown in Table 1. It shows that all the band gaps are within the range of 3.2 to 3.3 eV. 4. Conclusion Transparent and conductive ZnO film deposition by an APPJ-assisted process is reported in this work. By utilizing the reactivity of N2 APPJ, ZnCl2 precursor solution droplets can be effective converted to ZnO with a moderate deposition rate. Experimental results show that in order to obtain a nearly full conversion of ZnCl2 solution droplets to ZnO, sufficiently high temperature and long retention time are required. The ZnO formation is confirmed by XRD analysis. An applied voltage of 275 V together with a gas flow rate of 30 slm is sufficient to meet this requirement. Conditions with the applied voltage less than 275 V and a gas flow rate greater than 30 slm leads to incomplete conversion of the droplets to ZnO. Such an incomplete conversion results in the observations of the ZHC formation, high surface roughness, and high resistivity. Under an optimal condition (275 V and 30 slm), a film with the resistivity of 1.4 Ω-cm is obtained. The transmittance of the film remains well above 80% in the visible light wavelength regardless of the operation condition. This proposed APPJ-assisted process possesses key features of non-vacuum, continuous process, no need of post-annealing steps, and no need of intentional doping. Such a process offers

This work is supported by National Science Council Taiwan (101-2221-E-002-163-MY2 and 102-3113-P-002-043).

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Please cite this article as: C.M. Hsu, et al., Deposition of transparent and conductive ZnO films by an atmospheric pressure plasma-jet-assisted process, Thin Solid Films (2014), http://dx.doi.org/10.1016/j.tsf.2014.02.102