Preparation and properties of AZO thin films on different substrates

Preparation and properties of AZO thin films on different substrates Hong-lie SHEN, Hui ZHANG, Lin-feng LU, Feng JIANG, Chao YANG College of Materials Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China Received 23 August 2010; accepted 23 October 2010 Abstract: Al doped ZnO (AZO) films were deposited on glass, polyethylene naphthalate (PEN) and polyethylene terephthalate (PET) at room temperature by radio frequency magnetron sputtering. The structural, surface morphology, electrical and optical properties of the AZO films were characterized by X-ray diffractometry (XRD), atomic force microscopy (AFM), Hall effect measurement and ultraviolet-visible spectrometer, respectively. The films exhibit highly c-axis preferred orientation, and AZO film on glass exhibits compressive stress while that on plastic substrates presents tensile stress. The electrical property of the AZO film on glass is the best among the films on three kinds of substrates. The value of the figure of merit for the AZO film on PEN is much better than that of the AZO film on PET and is close to that of AZO on glass in the wavelength range of 500−900 nm. Key words: AZO; magnetron sputtering; electrical and optical property; plastic substrate; glass substrate

1 Introduction Al doped ZnO (AZO) film is one of the most promising transparent and conductive oxide (TCO) thin films for its high optical transmittance, relatively low resistivity, non-toxicity, low material cost and good stability in hydrogen plasma compared with SnO2 doped In2O3 films (ITO) and F doped SnO2 (FTO) films[1]. These excellent properties make it suitable for solar cells and other applications. Nowadays, AZO films on flexible substrates[2−4] have attracted more and more attention because they can meet the expanding needs of modern photoelectrical devices, due to their special merits including lightweight, non-friability, small volume. They can also make the fabricated devices folded, easy to carry[5]. They are widely used in flexible tandem solar cells, plastic liquid crystal, unbreakable heat-reflecting mirrors[6], etc. The comprehensive properties of polyethylene naphthalate (PEN) and polyethylene terephthalate (PET) are better than other plastic substrates[7], and the cost of these two substrates is relatively low. These merits make PET and PEN suitable substrates to deposit AZO films. Many techniques have been used to deposit AZO films, such as metal-organic chemical vapor deposition (MOCVD), sol-gel processing, pulsed laser deposition (PLD), magnetron sputtering. Among these film Corresponding author: Hong-lie SHEN; E-mail: [email protected]

deposition techniques, magnetron sputtering has characteristics of low deposition temperature, good film adhesion, high reproducibility and easy preparation of large area. Therefore, magnetron sputtering is a good candidate for depositing AZO films at room temperature. In this paper, AZO films were deposited on glass, PEN and PET at room temperature by radio frequency (RF) magnetron sputtering. AZO films with good optoelectronic properties were obtained on the glass and plastic substrates. The properties of the AZO films on different substrates were analyzed comparatively.

2 Experimental The AZO films of about 600 nm were deposited on glass, PEN and PET at room temperature by RF magnetron sputtering using a ceramic oxide target ZnO/Al2O3 (purity of 99.99%, mass ratio of 98:2; diameter of 6 cm). The thicknesses of PEN and PET were 188 μm and 75 μm, respectively. Before being loaded into the chamber, all the substrates with dimensions of 2 cm×2 cm were ultrasonically cleaned in acetone, alcohol, deionized water and finally dried by nitrogen gas. The distance between the substrate and the target was about 14 cm. The base pressure of the chamber was lower than 0.4 mPa. During the film deposition, the sputtering power was 150 W. The flow rate of the Ar (purity of 99.999%) was maintained at 20

Hong-lie SHEN, et al/Progress in Natural Science: Materials International 20(2010) 44−48

mL/min and the working pressure was 0.05 Pa. All these process parameters were optimized for obtaining AZO films with good optoelectronic properties. Whether on the glass substrate or on the flexible substrates, AZO film was physically stable and showed good adherence to the substrates. No break or peel-off was observed even after the flexible films were bent many times. The AZO films were analyzed with a Rigaku D/Max-RA X-ray diffractometer (XRD) with CuKα radiation. The thickness of the films was measured by a surface profilometer (AMBIOS XP−1). The surface morphology of the samples was measured by an atomic force microscope (AFM, Mutilmode NanoScope 30). The resistivity (ρ), free carrier concentration (n) and Hall mobility (μ) were inferred by Hall effect measurements using van der Pauw technique at room temperature. The optical transmittance spectra were recorded using a Shimadzu UV−2550 spectrophotometer.

3 Results and discussion Fig.1 shows the XRD patterns of the AZO films on different substrates. It can be seen that only the diffraction peaks (002) and (004) are observed for all the AZO thin films on different substrates. Some additional diffraction peaks of AZO films on PEN and PET substrates are observed for their semi-crystalline properties. All those AZO thin films have an obvious c-axis orientation.

Fig.1 XRD patterns of AZO films on glass, PEN and PET

The comparison of the diffraction peak (002) of AZO films on glass, PEN and PET is shown in Fig.2. In Fig.2, the vertical dot line indicates the peak location of the standard powder ZnO specimen at 34.47°. It is observed that the diffraction angle of the AZO (002) on glass locates at a lower diffraction angle compared with the value of standard ZnO powder. According to Ref.[8], the stress along the horizontal direction in the AZO film is related to the location of the diffraction angle of peak (002). AZO film exhibits compressive stress when the

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Fig.2 Comparison of diffraction peak (002) of AZO films deposited on glass, PEN and PET

peak (002) diffraction angle is lower than 34.47° while the film exhibits tensile stress when the diffraction angle is larger than 34.47°. It must be mentioned that the average thermal expand coefficient of the glass substrate (9.0×10−6 K−1), which is close to the AZO film, is far lower than that of PET or PEN, and the glass substrate is at low temperature during the film growth. So, the compressive stress in the AZO films on glass is likely to be intrinsic rather than thermal origin. On the contrary, the tensile stress in the AZO film on PEN or PET may be resulted from the plastic deformation of the PEN and PET substrates, since the plastic material is easy to extend by the bombardment of the high energetic particles at a high sputtering power, suggesting that the bombardment of the high energetic particles is an essential effect to control the stress of the AZO films on plastic substrates deposited by magnetron sputtering at room temperature. It is also seen that the peak (002) of the AZO film on glass is stronger and sharper than that on the plastic substrate. This is implied that the crystalline quality of the AZO film on glass is better than that on the plastic substrate. The thermal stability of the organic substrate is poorer than that of glass. Under the bombardment of high energetic particles, some undesirable impurities may escape from the plastic substrate, resulting in a degradation of the crystalline quality of the film. The peak (002) of the AZO film on PEN is stronger than that on PET, indicating that the crystal quality of the AZO film on PEN is better than that on PET. And this can be attributed to the poor thermal stability of PET compared with PEN. The surface morphologies of the samples measured by an atomic force microscope were analyzed in details. From the AFM images, it is found that AZO films on different substrates are all composed of closely packed nanocrystalline. Some grooves are also observed in the AZO films on PET and PEN. Those grooves may be related to the original morphology of the substrates


46

because the plastic substrates are easy to be scratched during the processes of production and operation. The root mean square (RMS) values of the AZO films are 6.94 nm on glass, 5.44 nm on PEN and 7.08 nm on PET, while the grain sizes are 54.69 nm, 97.66 nm, 94.66 nm respectively. It is obvious that the grain sizes of the AZO films on PEN and PET are both larger than that on glass. This may be due to the different surface properties of the plastic and glass substrates, which can affect the crystallization of AZO film. Fig.3 shows the resistivity, carrier concentration and Hall mobility of the AZO films deposited on different substrates. The values of resistivity are 7.60×10−4 Ω·cm on glass, 1.11×10−3 Ω·cm on PEN and 1.57×10−3 Ω·cm on PET. The AZO film on glass has the highest carrier concentration of 4.62×1020 cm−3and Hall mobility of 17.81 cm2/(V·s). The AZO film on PET has lower values of carrier concentration and Hall mobility than those on PEN. The carrier mobility is determined by some scattering mechanisms, such as ionized impurity scattering, neutral impurity scattering, grain boundary scattering, and lattice vibration scattering. At room temperature, the lattice vibration scattering is not important[9]. The grain-boundary scattering is dominant only when its grain sizes are comparable to the mean free path of the carriers. The free-electron mean free path[10] in the films is calculated using the following formula: L=(

h 3ne 1/ 3 )( ) μ 2e π

(1)

where L is the free-electron mean free path (nm); h is the Plank constant (6.63×10−34 J⋅s); e is the electron charge (1.60×10−19 C); ne is the carrier concentration (cm−3); μ is the mobility (cm2/(V⋅s)). According to Formula (1), L values for the AZO films on glass, PEN and PET are 2.8, 2.1 and 1.5 nm, respectively. All the values of L are much smaller than the grain sizes in the deposited films, indicating that the grain-boundary scattering is not dominant in the films on different substrates. For highly degenerate semiconductors, the ionized impurity scattering is the main scattering mechanism, independent of temperature in the low temperature region including room temperature[11]. However, it is noticed from Fig. 3 that the carrier concentrations of the films on glass, PEN and PET decrease successively while the mobilities also decrease. If the ionized impurity scattering is the only important scattering mechanism in the AZO films on the plastic substrates, the lower carrier concentration will result in a high mobility. So, the neutral impurity scattering may be also an important factor influencing the mobility of AZO film on the plastic substrate. It is reasonable to say that the number of Al atoms in the AZO films on different substrates is equivalent to each other. Therefore, the lower carrier concentration of the

Fig.3 Resistivity, carrier concentration and Hall mobility of AZO films deposited on glass, PEN and PET

AZO film on the plastic substrate means that more electrically inactive Al atoms exist in the film. These Al atoms may form the neutral Al-based defect complexs in the film which not only contribute the carriers in the materials but also act as the neutral impurity scattering center. Although the exact composition of the defect complex has not been determined, the poorer crystal quality of the film on the plastic substrate should be the reflection of the larger number of the Al-based defect complexes in the film. From the analysis of the dependence of the mobility on the carrier concentration about the AZO films on the different substrates, it can not be determined whether the neutral impurity scattering is a major scattering mechanism of the AZO film on glass. But it can be concluded that both of the ionized impurity scattering and neutral impurity scattering are the major scattering mechanisms in the AZO film on the plastic substrate. The influence of neutral impurity scattering should be decreased with increasing crystal quality of the films. RADHOUANE[12] also reported that the mobility of the AZO films can be affected not only by the ionized impurity scattering but also by the neutral impurity scattering. Fig.4 shows the optical transmittance spectra of the AZO films on different substrates. It is observed that each transmittance spectrum has a sharp absorption edge in the range of 330−400 nm, with different wavelengths due to effects of the substrates. The average transmittance values of the films in visible range (500− 900 nm) are 79.2 % on glass, 73.3 % on PEN and 74.3 % on PET. The difference of the transmittance is attributed to the transmittance of the different substrates. The calculated absolute average transmittance values of the films are 87.5 % on glass, 90.3 % on PEN and 87.0 % on PET, by eliminating the substrates effect via the expression Tabsolute=Tfilm/Tsubstrate[13], where Tfilm and Tsubstrate are the transmittances values of the film and substrate, respectively. For comparing the comprehensive property of conductivity and transmittance, the


Fig.4 Optical transmittance spectra of AZO thin films on glass, PEN and PET. (The insert shows (αhv)2 versus hv based on transmittance spectra)

performance of the AZO films on different substrates can be determined by the figure of merit (FOM), defined by the expression as follows[14]: FOM =−1/ (ρ ln(T))

(2)

where ρ is the resistivity; T is the absolute average transmittance in the wavelength range of 500−900 nm. The FOM values of the AZO on glass, PEN and PET are 9 853, 8 829 and 4 574 Ω−1·cm−1, respectively. The optoelectronic property of the AZO film on PEN is much better than that of the AZO film on PET and is close to that of the AZO film on glass. Since AZO is a direct band gap semiconductor, the absorption coefficient (α) can be estimated using the following relation ship: (αhν)2=A(hν−Eg)

α=1/dln(1/T)

of substrates deposited at room temperature by RF magnetron sputtering exhibit highly (002) preferred orientation. The AZO film on glass shows compressive stress while that on the plastic substrates presents tensile stress. The electrical property of the AZO film on glass is the best among the films on the three kinds of substrates. The resistivity (ρ), free carrier concentration (n) and Hall mobility (μ) are 7.60×10−4 Ω·cm, 4.63×1020 cm−3, and 17.81 cm2/(V·s), respectively for the AZO films on glass, in contrast with 1.11×10−3 Ω·cm 4.14×1020 cm−3, 13.60 cm2/(V·s) on PEN, and 1.57×10−3 Ω·cm, 3.80×1020 cm−3, 10.47 cm2/(V·s) on PET. The values of the figure of merit are 9 853, 8 829 and 4 574 Ω−1·cm−1 for the films on glass, PEN and PET, respectively. The obtained value of Eg for the AZO film on PEN is smaller than that of the AZO film on PET due to the narrower Eg (3.26 eV) of PEN substrate itself. While the AZO film on glass shows a larger optical band gap than that on PET. Acknowledgements This work is performed with financial support from the National High-tech Research and Development Program of China(2006AA03Z219).

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