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Room-temperature deposition of transparent conducting Al-doped ZnO films by RF magnetron sputtering method
Applied Surface Science 255 (2009) 5669–5673
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Room-temperature deposition of transparent conducting Al-doped ZnO films by RF magnetron sputtering method Weifeng Yang a, Zhuguang Liu a, Dong-Liang Peng b, Feng Zhang a, Huolin Huang a, Yannan Xie a, Zhengyun Wu a,* a b
Department of Physics, Xiamen University, Xiamen 361005, PR China Department of Materials Science & Engineering, Xiamen University, Xiamen 361005, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 July 2008 Received in revised form 29 October 2008 Accepted 8 December 2008 Available online 13 December 2008
Transparent conductive Al-doped zinc oxide (AZO) films with highly (0 0 2)-preferred orientation were deposited on quartz substrates at room temperature by RF magnetron sputtering. Optimization of deposition parameters was based on RF power, Ar pressure in the vacuum chamber, and distance between the target and substrate. The structural, electrical, and optical properties of the AZO thin films were investigated by X-ray diffraction, Hall measurement, and optical transmission spectroscopy. The 250 nm thickness AZO films with an electrical resistivity as low as 4.62 104 V cm and an average optical transmission of 93.7% in the visible range were obtained at RF power of 300 W, Ar flow rate of 30 sccm, and target distance of 7 cm. The optical bandgap depends on the deposition condition, and was in the range of 3.75–3.86 eV. These results make the possibility for light emitting diodes (LEDs) and solar cells with AZO films as transparent electrodes, especially using lift-off process to achieve the transparent electrode pattern transfer. ß 2008 Elsevier B.V. All rights reserved.
PACS: 68.55.a 73.50.h 78.20.e 73.61.r 81.15.cd Keywords: AZO film Transparent conductive oxide RF magnetron sputtering Low temperature deposition
1. Introduction Recently, a great deal of interest has been fueled in zinc oxide (ZnO) semiconductor materials. They are transparent at visible wavelengths, have direct and wide bandgap (Eg 3.30 eV at room temperature), and have a large exciton binding energy (60 meV at room temperature). Such properties make them well suited for the realization of many optoelectronic applications including transparent conductive oxides in display devices and solar cells [1– 4]. Doped ZnO has similar electrical and optical properties to ITO, but it is a non-toxic material, has high temperature stability, and costs less to manufacture [5]. Recent research demonstrated that B-, Al-, Ga-, In-, and F-doped ZnO films reveal both low resistivity and high transmittance in the visible region [6–9].
* Corresponding author at: Department of Physics, Xiamen University, No. 422, Siming Nan Road, Xiamen 361005, PR China. Tel.: +86 592 2180522; fax: +86 592 2189426. E-mail address: [email protected] (Z. Wu). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.12.021
Different technologies such as electron beam evaporation [10], sol–gel [11], chemical spray [12], pulsed laser deposition [13], DC and RF magnetron sputtering [14] have been reported to produce thin films of Al-doped zinc oxide (AZO) with adequate performance for applications. RF magnetron sputtering technique has some advantages in comparison to the other methods. This technique is quite simple and the required setup is less expensive, and is considered to be the most available deposition method to obtain highly uniform films with high packing density and strong adhesion at a high deposition rate. Unfortunately, these deposition techniques, while yielding high-quality films, require relatively high temperatures [14,15] which are incompatible with plastic substrates or sensitive photoresist as those used in light emitting diodes (LEDs) and solar cells. The high temperature (more than 120 8C) will destroy the active performance of photoresist, leading to invalidation of lift-off process, which has been broadly utilized in semiconductor fabrication process. Many researchers have tried to produce highly conductive ZnO films at low substrate temperature. However, it became difficult to obtain a film with both low resistivity and high transmittance without substrate heating. Lee and Song [16] reported the AZO films deposited by RF
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sputtering on glass substrate with as low as 6.2 104 V cm and an average transmittance merely around 80%. Wang et al. [17] and Lin et al. [18] reported the doped ZnO films deposited at room temperature with only as low as 1.4 103 V cm and 8.43 103 V cm respectively. In this letter, high performance AZO thin films were grown by RF magnetron sputtering at room temperature, and a systematic study of structural, electrical, and optical properties of AZO thin films with different deposition condition has been discussed in detail.
The AZO films were deposited on quartz substrates in a reconstructive SP-2 RF magnetron sputtering system with a base pressure of 4 104 Pa and at room temperature. A sintered ceramic sputter target with a mixture of ZnO (99.99% purity) and Al2O3 (99.99% purity) was employed as source material. The content of Al2O3 added to the sputter target was 2 wt.%. The sputtering power and Ar flow rate were varied from 100 to 500 W, and from 15 sccm to 75 sccm respectively. The distance between sputter target and substrate was varied from 5 cm to 8 cm. In order to make the films uniformity, the substrates were kept spinning during the sputtering. And all the films were deposited at room temperature and the target was 30 8C water-cooled. The structural properties of AZO films were analyzed with a Panalytical X’pert PRO powder X-ray diffractometer which uses a Cu Ka radiation (l = 0.15406 nm). The surface morphology and section shape of AZO films were examined using a SPA-400 atomic force microscope (AFM) and FESEM LEO 1530 respectively. The optical transmittance of AZO films was measured using a cary5000 spectrophotometer. The film thickness was determined using a Dektak3 surface profile measurement system. The resistivity, carrier concentration and carrier mobility of AZO films were measured using Accent HL5500 Hall System with a four-point probe. The chemical state in the films was investigated by means of X-ray photoelectron spectroscopy (XPS) using a PHI Quantum 2000 Scanning ESCA Microprobe instrument. The refractive index of the AZO films was tested by UVISELTM Ellipsometric measurements system.
cular to the substrate plane. The highest peak value of the XRD measurement was obtained from the thin AZO films deposited at power of 300 W, Ar flow rate of 30 sccm (i.e., Ar partial pressure of 0.4 Pa) and the distance of 7 cm, as shown in Fig. 1. This implies the AZO thin films deposited in this condition show the best multicrystal structure. To examine the quality of the Al-doped ZnO films, the full-width at half-maximum (FWHM) of (0 0 2) peak and the crystallite dimension are estimated. The FWHM values for AZO/ quartz varies from 0.260 to 0.325, exhibiting good crystallinity. The grain size of the films was evaluated according to Scherrer relation to be in range of 26.9–34.6 nm with the variety of sputtering power from 100 W to 500 W, and reached to a highest value of 34.6 nm at the power of 300 W. No Al2O3 phase was found, which implies that Al atoms substitute Zn in the hexagonal lattice and Al ions may occupy the interstitial sites of ZnO or probably Al segregates to the non-crystalline region in grain boundaries and forms Al–O bond. In order to illustrate the surface and section morphology of the AZO/quartz films, we measured AFM and SEM images (Fig. 2). The nanocrystalline surface of AZO films exhibits coalescence of the grains in AZO/quartz at different sputtering power. The same surface phenomena were also observed at the different Ar flow rate and different target distance (not show here). The films are composed of closely packed nanocrystallites. The roughness of the grown film was found to be in the range of 2.98–4.55 nm with an irregular variation with sputtering power, as represented in Fig. 2(a). It is very clear that grain size of the AZO films increases with increasing power from 100 W to 500 W, as Fig. 2(a) shows. That implied different morphologies were observed for AZO films grown on quartz with increasing sputtering power. In addition, the roughness of the grown film was found to be in the range of 2.98– 5.29 nm with a little variation with Ar flow rate and target distance. Fig. 2(b) shows the cross-sectional SEM micrograph of the sample deposited at the power of 300 W, the Ar flow rate of 30 sccm, and the distance of 7 cm. The thickness of the film is 250 nm and the film grows in a columnar structure vertical to the substrate, being consistent with the XRD measurement result (caxis orientation). We also obtained the same results as shown in Fig. 2(b) for AZO films prepared at different conditions.
3. Results and discussion
3.2. Electrical properties
3.1. Structural properties
The dependence of electrical resistivity (r), carrier concentration (n), and mobility (m) on various power, Ar flow rate, and distance between target and substrate are shown in Fig. 3. The results showed that all the films are degenerate doped n-type semiconductor. The plot in Fig. 3(a) indicates the carrier concentration of AZO films increased with the power increased from 100 W to 500 W and shows the lowest resistivity obtained at the power of 300 W. As the sputtering power increases from 100 W to 300 W, Hall mobility increases accordingly, reach the maximum at the power of 300 W and then decrease. However, when the
2. Experimental
XRD patterns of the films are shown in Fig. 1 for films grown on quartz at different sputtering power, different Ar flow rate and different distance between target and quartz. The AZO films were 250 nm thick. X-ray diffraction measurements showed that AZO films deposited on different condition all gave strong (0 0 2) diffraction peaks. Other peaks (0 0 4) with much less intensity were observed, indicating that the films are oriented with their c axes perpendi-
Fig. 1. X-ray diffraction patterns of AZO films deposited under (a) different power, (b) different Ar flow rate, and (c) different distance between target and substrate.
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Fig. 2. (a) Atomic force microscopic images of films surface (scan size 1 mm 1 mm) for various growth sputtering power. (b) SEM images of the cross-section of the AZO films deposited at the power of 300 W, the Ar flow rate of 30 sccm, and the distance of 7 cm.
Fig. 3. Resistivity, Hall mobility, and carrier concentrations as a function of (a) sputtering power, (b) Ar flow rate, and (c) distance between target and substrate.
power reaches >300 W, the films will be damaged by too high energy ion, which leads to the limitation of crystal quality. Therefore, the window of 250–300 W RF power was appropriate and selected in the following experiments. As Ar flow rate were changed from 15 sccm to 75 sccm, the plot in Fig. 3(b) indicates that the lowest resistivity obtained at the Ar flow rate of 30 sccm. The plot in Fig. 3(c) indicates the lowest resistivity of 4.62 104 V cm, the highest Hall mobility of 15.6 cm2/(V s) and the carrier concentration of 8.68 1020 cm3 obtained at the distance of 7 cm. Therefore, the results implied the best AZO films with the lowest resistivity were obtained at the room temperature condition with the power of 250–300 W, Ar flow rate of 30 sccm, and target distance of 7 cm. And the change of the mobility and the resistivity can be attributed to the change of the crystallinity, which supports the analysis of XRD results. At the previous appreciate condition, the film has the highest peak value of XRD and the narrowest value of FWHM, that is, it has the best crystal quality. Minami [19] reported the impurity scattering will show dominant influence on mobility, due to the tightness of the films deposited at high temperature. The films deposited at low temperature in this experiment show similar electrical and structure properties to those deposited at high temperature
[14]. Therefore, impurity scattering is main factor of the films mobility. By XPS analysis, it was found that AZO films are almost stoichiometric, i.e., O/Zn 1.03. The Zn, O, and Al amount in the film was 47.5 at.%, 49.0 at.%, and 3.5 at.% respectively. Fig. 4 shows XPS spectra Zn 2p3/2 (a) and O 1s (b) in the depth of 100 nm of the AZO film. The core line of Zn 2p exhibited high symmetry and the binding energy of Zn 2p3/2 remains at 1021.80 0.10 eV, which confirms that the majority of Zn atoms remain in the same formal valence state of Zn2+ within the ZnO matrix. The O 1s peak can be consistently fitted by two nearly Gaussian, centered at 530.25 0.15 eV and 531.20 0.15 eV respectively. The component on the low binding energy side of the O 1s spectrum at 530.25 0.15 eV is attributed to O2 ions on wurtzite structure of hexagonal Zn2+ ion array, surrounded by Zn atoms [20]. The other binding energy component, centered at 531.20 0.15 eV, is associated with O2 ions in the oxygen regions within the matrix of ZnO. 3.3. Optical properties Fig. 5 shows the transmission spectra in the wavelength range of 200–800 nm for AZO thin films. All films exhibit an average
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Fig. 4. XPS spectra of Zn 2p3/2 (a) and O 1s (b) in the depth of 100 nm of the AZO.
Fig. 5. Dependence of the transmission of the AZO thin films prepared at (a) different sputtering power: 100–500 W and (b) different Ar flow rate and distance between target and substrate.
optical transmittance of >90% in the visible range and a sharp fundamental absorption edge. The average optical transmittance of the AZO films with the lowest resistivity of 4.62 104 V cm was 93.7%. With the sputtering power increasing from 100 W to 500 W, as Fig. 5(a) shows, a sharp UV-off shifts to shorter wavelength with an increase of carrier concentration, known as the Burstein–Moss shift [21]. It is known that AZO film with electron carrier concentration above 1020 cm3 is degenerate and the Fermi energy penetrates into the conduction band. The optical bandgap (Eg) of the films can be obtained by plotting a2 vs. hn (a is the absorption coefficient and hn the photon energy) and extrapolating the linear portion of this plot to the energy axis. The Eg of the AZO films with 250 nm thickness are varied among 3.75–3.86 eV, which is much larger than those reported by Kima et al. (3.46– 3.54 eV) [22] and Ma et al. (3.58–3.74 eV) [23]. Additionally, it has been well established that the refractive index is closely correlated with film density. The relation between the packing density (P) and the corresponding refractive index of the material is given as [24,25]: P¼
rf ðn2f 1Þðn2b þ 2Þ ¼ rb ðn2f þ 2Þðn2b 1Þ
(1)
where nf and nb are the refractive index of the film and the bulk material of ZnO, and rf and rb are the density of the film and bulk material respectively. The packing density of the films can be calculated from Eq. (1) and by using the refractive index values, and a higher refractive index implies a denser film. The measured refractive index of the AZO films with the lowest resistivity is 1.87 (l = 600 nm), which is close to the single crystal value of 2.0.
Using the obtained refractive index value, we calculated the Pvalue to be 0.91 for the optimal sample. This result indicates that the films deposited in this experiment are denser, which supports the analysis of AFM results. 4. Conclusion In summary, high-quality transparent conducting AZO films were grown on quartz substrates at room temperature by the RF magnetron sputtering deposition technique, showing strong c-axis orientation perpendicular to the substrate. The surface morphology of the films shows different characteristics under variable deposition condition. The films demonstrate the optical transparency of >93% in the visible region and show conductivity with a very low electrical resistivity of 4.62 104 V cm, which have similar electrical and optical properties to AZO films deposited on high temperature substrates. The optical bandgap of the AZO films is in the range of 3.75–3.86 eV. The films deposited at room temperature have satisfactory properties of low resistance and high transmittance for application as transparent conductive electrodes in LEDs and solar cells, especially in lift-off fabrication technology. References [1] E. Fortunato, P. Barquinha, A. Pimentel, A. Gonc¸alves, A. Marques, L. Pereira, R. Martins, Thin Solid Films 487 (2005) 205. [2] K. Iwata, T. Sakemi, A. Yamada, P. Fons, K. Awa, T. Yamamoto, S. Shirakata, K. Matsubara, H. Tampo, K. Sakurai, S. Ishizuka, S. Niki, Thin Solid Films 480–481 (2005) 199. [3] J. Yoo, J. Lee, S. Kim, K. Yoon, I. Jun Park, S.K. Dhungel, B. Karunagaran, D. Mangalaraj, J. Yi, Thin Solid Films 480 (2005) 213. [4] M.A. Martı´nez, J. Herrero, M.T. Gutie´rrez, Sol. Energy Mater. Sol. Cells 45 (1997) 75.
W. Yang et al. / Applied Surface Science 255 (2009) 5669–5673 [5] C.J. Tun, J.K. Sheu, B.J. Pong, M.L. Lee, M.Y. Lee, C.K. Hsieh, C.C. Hu, G.C. Chi, IEEE Photon. Technol. Lett. 18 (2006) 274. [6] B.N. Pawar, S.R. Jadkar, M.G. Takwale, J. Phys. Chem. Solids 66 (2005) 1779. [7] W.W. Wang, X.G. Diao, Z. Wang, M. Yang, T.M. Wang, Z. Wu, Thin Solid Films 491 (2005) 54. [8] M. Lorenz, E.M. Kaidashev, H.V. Wenchstern, V. Riede, C. Bundesmann, D. Spemann, G. Benndorf, H. Hochmuth, A. Rahm, H.C. Semmelhack, M. Grundmann, Solid-State Electron. 47 (2003) 2205. [9] A. Maldonado, S. Tirado-Guerra, M. Mele´ndez-Lira, M. de la L. Olvera, Sol. Energy Mater. Sol. Cells 90 (2006) 742. [10] J. Ma, F. Ji, D.H. Zhang, H.L. Ma, S.Y. Li, Thin Solid Films 357 (1999) 98. [11] V. Musat, B. Teixeira, E. Fortunato, R.C.C. Monteiro, P. Vilarinho, Surf. Coat. Technol. 180–181 (2004) 659. [12] H.M. Suarez, A. Maldonado, M.D.L. Olvera, A. Reyes, R.C. Perez, G.T. Delgado, R. Asomoza, Appl. Surf. Sci. 193 (2002) 52.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Room-temperature deposition of transparent conducting Al-doped ZnO films by RF magnetron sputtering method Weifeng Yang a, Zhuguang Liu a, Dong-Liang Peng b, Feng Zhang a, Huolin Huang a, Yannan Xie a, Zhengyun Wu a,* a b
Department of Physics, Xiamen University, Xiamen 361005, PR China Department of Materials Science & Engineering, Xiamen University, Xiamen 361005, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 July 2008 Received in revised form 29 October 2008 Accepted 8 December 2008 Available online 13 December 2008
Transparent conductive Al-doped zinc oxide (AZO) films with highly (0 0 2)-preferred orientation were deposited on quartz substrates at room temperature by RF magnetron sputtering. Optimization of deposition parameters was based on RF power, Ar pressure in the vacuum chamber, and distance between the target and substrate. The structural, electrical, and optical properties of the AZO thin films were investigated by X-ray diffraction, Hall measurement, and optical transmission spectroscopy. The 250 nm thickness AZO films with an electrical resistivity as low as 4.62 104 V cm and an average optical transmission of 93.7% in the visible range were obtained at RF power of 300 W, Ar flow rate of 30 sccm, and target distance of 7 cm. The optical bandgap depends on the deposition condition, and was in the range of 3.75–3.86 eV. These results make the possibility for light emitting diodes (LEDs) and solar cells with AZO films as transparent electrodes, especially using lift-off process to achieve the transparent electrode pattern transfer. ß 2008 Elsevier B.V. All rights reserved.
PACS: 68.55.a 73.50.h 78.20.e 73.61.r 81.15.cd Keywords: AZO film Transparent conductive oxide RF magnetron sputtering Low temperature deposition
1. Introduction Recently, a great deal of interest has been fueled in zinc oxide (ZnO) semiconductor materials. They are transparent at visible wavelengths, have direct and wide bandgap (Eg 3.30 eV at room temperature), and have a large exciton binding energy (60 meV at room temperature). Such properties make them well suited for the realization of many optoelectronic applications including transparent conductive oxides in display devices and solar cells [1– 4]. Doped ZnO has similar electrical and optical properties to ITO, but it is a non-toxic material, has high temperature stability, and costs less to manufacture [5]. Recent research demonstrated that B-, Al-, Ga-, In-, and F-doped ZnO films reveal both low resistivity and high transmittance in the visible region [6–9].
* Corresponding author at: Department of Physics, Xiamen University, No. 422, Siming Nan Road, Xiamen 361005, PR China. Tel.: +86 592 2180522; fax: +86 592 2189426. E-mail address: [email protected] (Z. Wu). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.12.021
Different technologies such as electron beam evaporation [10], sol–gel [11], chemical spray [12], pulsed laser deposition [13], DC and RF magnetron sputtering [14] have been reported to produce thin films of Al-doped zinc oxide (AZO) with adequate performance for applications. RF magnetron sputtering technique has some advantages in comparison to the other methods. This technique is quite simple and the required setup is less expensive, and is considered to be the most available deposition method to obtain highly uniform films with high packing density and strong adhesion at a high deposition rate. Unfortunately, these deposition techniques, while yielding high-quality films, require relatively high temperatures [14,15] which are incompatible with plastic substrates or sensitive photoresist as those used in light emitting diodes (LEDs) and solar cells. The high temperature (more than 120 8C) will destroy the active performance of photoresist, leading to invalidation of lift-off process, which has been broadly utilized in semiconductor fabrication process. Many researchers have tried to produce highly conductive ZnO films at low substrate temperature. However, it became difficult to obtain a film with both low resistivity and high transmittance without substrate heating. Lee and Song [16] reported the AZO films deposited by RF
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sputtering on glass substrate with as low as 6.2 104 V cm and an average transmittance merely around 80%. Wang et al. [17] and Lin et al. [18] reported the doped ZnO films deposited at room temperature with only as low as 1.4 103 V cm and 8.43 103 V cm respectively. In this letter, high performance AZO thin films were grown by RF magnetron sputtering at room temperature, and a systematic study of structural, electrical, and optical properties of AZO thin films with different deposition condition has been discussed in detail.
The AZO films were deposited on quartz substrates in a reconstructive SP-2 RF magnetron sputtering system with a base pressure of 4 104 Pa and at room temperature. A sintered ceramic sputter target with a mixture of ZnO (99.99% purity) and Al2O3 (99.99% purity) was employed as source material. The content of Al2O3 added to the sputter target was 2 wt.%. The sputtering power and Ar flow rate were varied from 100 to 500 W, and from 15 sccm to 75 sccm respectively. The distance between sputter target and substrate was varied from 5 cm to 8 cm. In order to make the films uniformity, the substrates were kept spinning during the sputtering. And all the films were deposited at room temperature and the target was 30 8C water-cooled. The structural properties of AZO films were analyzed with a Panalytical X’pert PRO powder X-ray diffractometer which uses a Cu Ka radiation (l = 0.15406 nm). The surface morphology and section shape of AZO films were examined using a SPA-400 atomic force microscope (AFM) and FESEM LEO 1530 respectively. The optical transmittance of AZO films was measured using a cary5000 spectrophotometer. The film thickness was determined using a Dektak3 surface profile measurement system. The resistivity, carrier concentration and carrier mobility of AZO films were measured using Accent HL5500 Hall System with a four-point probe. The chemical state in the films was investigated by means of X-ray photoelectron spectroscopy (XPS) using a PHI Quantum 2000 Scanning ESCA Microprobe instrument. The refractive index of the AZO films was tested by UVISELTM Ellipsometric measurements system.
cular to the substrate plane. The highest peak value of the XRD measurement was obtained from the thin AZO films deposited at power of 300 W, Ar flow rate of 30 sccm (i.e., Ar partial pressure of 0.4 Pa) and the distance of 7 cm, as shown in Fig. 1. This implies the AZO thin films deposited in this condition show the best multicrystal structure. To examine the quality of the Al-doped ZnO films, the full-width at half-maximum (FWHM) of (0 0 2) peak and the crystallite dimension are estimated. The FWHM values for AZO/ quartz varies from 0.260 to 0.325, exhibiting good crystallinity. The grain size of the films was evaluated according to Scherrer relation to be in range of 26.9–34.6 nm with the variety of sputtering power from 100 W to 500 W, and reached to a highest value of 34.6 nm at the power of 300 W. No Al2O3 phase was found, which implies that Al atoms substitute Zn in the hexagonal lattice and Al ions may occupy the interstitial sites of ZnO or probably Al segregates to the non-crystalline region in grain boundaries and forms Al–O bond. In order to illustrate the surface and section morphology of the AZO/quartz films, we measured AFM and SEM images (Fig. 2). The nanocrystalline surface of AZO films exhibits coalescence of the grains in AZO/quartz at different sputtering power. The same surface phenomena were also observed at the different Ar flow rate and different target distance (not show here). The films are composed of closely packed nanocrystallites. The roughness of the grown film was found to be in the range of 2.98–4.55 nm with an irregular variation with sputtering power, as represented in Fig. 2(a). It is very clear that grain size of the AZO films increases with increasing power from 100 W to 500 W, as Fig. 2(a) shows. That implied different morphologies were observed for AZO films grown on quartz with increasing sputtering power. In addition, the roughness of the grown film was found to be in the range of 2.98– 5.29 nm with a little variation with Ar flow rate and target distance. Fig. 2(b) shows the cross-sectional SEM micrograph of the sample deposited at the power of 300 W, the Ar flow rate of 30 sccm, and the distance of 7 cm. The thickness of the film is 250 nm and the film grows in a columnar structure vertical to the substrate, being consistent with the XRD measurement result (caxis orientation). We also obtained the same results as shown in Fig. 2(b) for AZO films prepared at different conditions.
3. Results and discussion
3.2. Electrical properties
3.1. Structural properties
The dependence of electrical resistivity (r), carrier concentration (n), and mobility (m) on various power, Ar flow rate, and distance between target and substrate are shown in Fig. 3. The results showed that all the films are degenerate doped n-type semiconductor. The plot in Fig. 3(a) indicates the carrier concentration of AZO films increased with the power increased from 100 W to 500 W and shows the lowest resistivity obtained at the power of 300 W. As the sputtering power increases from 100 W to 300 W, Hall mobility increases accordingly, reach the maximum at the power of 300 W and then decrease. However, when the
2. Experimental
XRD patterns of the films are shown in Fig. 1 for films grown on quartz at different sputtering power, different Ar flow rate and different distance between target and quartz. The AZO films were 250 nm thick. X-ray diffraction measurements showed that AZO films deposited on different condition all gave strong (0 0 2) diffraction peaks. Other peaks (0 0 4) with much less intensity were observed, indicating that the films are oriented with their c axes perpendi-
Fig. 1. X-ray diffraction patterns of AZO films deposited under (a) different power, (b) different Ar flow rate, and (c) different distance between target and substrate.
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Fig. 2. (a) Atomic force microscopic images of films surface (scan size 1 mm 1 mm) for various growth sputtering power. (b) SEM images of the cross-section of the AZO films deposited at the power of 300 W, the Ar flow rate of 30 sccm, and the distance of 7 cm.
Fig. 3. Resistivity, Hall mobility, and carrier concentrations as a function of (a) sputtering power, (b) Ar flow rate, and (c) distance between target and substrate.
power reaches >300 W, the films will be damaged by too high energy ion, which leads to the limitation of crystal quality. Therefore, the window of 250–300 W RF power was appropriate and selected in the following experiments. As Ar flow rate were changed from 15 sccm to 75 sccm, the plot in Fig. 3(b) indicates that the lowest resistivity obtained at the Ar flow rate of 30 sccm. The plot in Fig. 3(c) indicates the lowest resistivity of 4.62 104 V cm, the highest Hall mobility of 15.6 cm2/(V s) and the carrier concentration of 8.68 1020 cm3 obtained at the distance of 7 cm. Therefore, the results implied the best AZO films with the lowest resistivity were obtained at the room temperature condition with the power of 250–300 W, Ar flow rate of 30 sccm, and target distance of 7 cm. And the change of the mobility and the resistivity can be attributed to the change of the crystallinity, which supports the analysis of XRD results. At the previous appreciate condition, the film has the highest peak value of XRD and the narrowest value of FWHM, that is, it has the best crystal quality. Minami [19] reported the impurity scattering will show dominant influence on mobility, due to the tightness of the films deposited at high temperature. The films deposited at low temperature in this experiment show similar electrical and structure properties to those deposited at high temperature
[14]. Therefore, impurity scattering is main factor of the films mobility. By XPS analysis, it was found that AZO films are almost stoichiometric, i.e., O/Zn 1.03. The Zn, O, and Al amount in the film was 47.5 at.%, 49.0 at.%, and 3.5 at.% respectively. Fig. 4 shows XPS spectra Zn 2p3/2 (a) and O 1s (b) in the depth of 100 nm of the AZO film. The core line of Zn 2p exhibited high symmetry and the binding energy of Zn 2p3/2 remains at 1021.80 0.10 eV, which confirms that the majority of Zn atoms remain in the same formal valence state of Zn2+ within the ZnO matrix. The O 1s peak can be consistently fitted by two nearly Gaussian, centered at 530.25 0.15 eV and 531.20 0.15 eV respectively. The component on the low binding energy side of the O 1s spectrum at 530.25 0.15 eV is attributed to O2 ions on wurtzite structure of hexagonal Zn2+ ion array, surrounded by Zn atoms [20]. The other binding energy component, centered at 531.20 0.15 eV, is associated with O2 ions in the oxygen regions within the matrix of ZnO. 3.3. Optical properties Fig. 5 shows the transmission spectra in the wavelength range of 200–800 nm for AZO thin films. All films exhibit an average
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Fig. 4. XPS spectra of Zn 2p3/2 (a) and O 1s (b) in the depth of 100 nm of the AZO.
Fig. 5. Dependence of the transmission of the AZO thin films prepared at (a) different sputtering power: 100–500 W and (b) different Ar flow rate and distance between target and substrate.
optical transmittance of >90% in the visible range and a sharp fundamental absorption edge. The average optical transmittance of the AZO films with the lowest resistivity of 4.62 104 V cm was 93.7%. With the sputtering power increasing from 100 W to 500 W, as Fig. 5(a) shows, a sharp UV-off shifts to shorter wavelength with an increase of carrier concentration, known as the Burstein–Moss shift [21]. It is known that AZO film with electron carrier concentration above 1020 cm3 is degenerate and the Fermi energy penetrates into the conduction band. The optical bandgap (Eg) of the films can be obtained by plotting a2 vs. hn (a is the absorption coefficient and hn the photon energy) and extrapolating the linear portion of this plot to the energy axis. The Eg of the AZO films with 250 nm thickness are varied among 3.75–3.86 eV, which is much larger than those reported by Kima et al. (3.46– 3.54 eV) [22] and Ma et al. (3.58–3.74 eV) [23]. Additionally, it has been well established that the refractive index is closely correlated with film density. The relation between the packing density (P) and the corresponding refractive index of the material is given as [24,25]: P¼
rf ðn2f 1Þðn2b þ 2Þ ¼ rb ðn2f þ 2Þðn2b 1Þ
(1)
where nf and nb are the refractive index of the film and the bulk material of ZnO, and rf and rb are the density of the film and bulk material respectively. The packing density of the films can be calculated from Eq. (1) and by using the refractive index values, and a higher refractive index implies a denser film. The measured refractive index of the AZO films with the lowest resistivity is 1.87 (l = 600 nm), which is close to the single crystal value of 2.0.
Using the obtained refractive index value, we calculated the Pvalue to be 0.91 for the optimal sample. This result indicates that the films deposited in this experiment are denser, which supports the analysis of AFM results. 4. Conclusion In summary, high-quality transparent conducting AZO films were grown on quartz substrates at room temperature by the RF magnetron sputtering deposition technique, showing strong c-axis orientation perpendicular to the substrate. The surface morphology of the films shows different characteristics under variable deposition condition. The films demonstrate the optical transparency of >93% in the visible region and show conductivity with a very low electrical resistivity of 4.62 104 V cm, which have similar electrical and optical properties to AZO films deposited on high temperature substrates. The optical bandgap of the AZO films is in the range of 3.75–3.86 eV. The films deposited at room temperature have satisfactory properties of low resistance and high transmittance for application as transparent conductive electrodes in LEDs and solar cells, especially in lift-off fabrication technology. References [1] E. Fortunato, P. Barquinha, A. Pimentel, A. Gonc¸alves, A. Marques, L. Pereira, R. Martins, Thin Solid Films 487 (2005) 205. [2] K. Iwata, T. Sakemi, A. Yamada, P. Fons, K. Awa, T. Yamamoto, S. Shirakata, K. Matsubara, H. Tampo, K. Sakurai, S. Ishizuka, S. Niki, Thin Solid Films 480–481 (2005) 199. [3] J. Yoo, J. Lee, S. Kim, K. Yoon, I. Jun Park, S.K. Dhungel, B. Karunagaran, D. Mangalaraj, J. Yi, Thin Solid Films 480 (2005) 213. [4] M.A. Martı´nez, J. Herrero, M.T. Gutie´rrez, Sol. Energy Mater. Sol. Cells 45 (1997) 75.
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