properties of al doped zinc oxide films prepared by
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This content was downloaded from IP address 65.21.228.167 on 07/10/2021 at 01:31
N. Yamaguchi, T. Kuroyama, Y. Okuhara and H. Matsubara
Japan Fine Ceramics Center (JFCC), 2-4-1 Mutsuno, atsuta-ku, Nagoya 456- 8587, Japan
E-mail: [email protected]
Abstract. Al doped ZnO (AZO) films were prepared on quartz substrates by the co- evaporation of ZnO and Al2O3 ingots by EB-PVD. EB power applied to the Al2O3 source was ranged from 3kW to 10kW, while EB power to the ZnO source was fixed at 3kW, at a substrate temperature of 400oC. X-ray diffraction measurement showed that the AZO films were weakly c-axis oriented. Transmittance of All the AZO films was over 80% in the visible range. Highest reflectance in the near IR range was obtained at the EB power on Al2O3 of 5kW. The lowest resistivity of 3.05 x 10-4 cm was obtained for the film deposited with the EB power on Al2O3 of 5kW with the deposition time of 300s.
1. Introduction Transparent conductive oxides (TCOs) are transparent in visible range and shows the high reflectance in the near-infrared (IR) region. Therefore, TCOs are good candidate for application as the coatings for IR-reflectors on super-insulating windows, which realize a considerable energy-saving effect in air conditioning of houses and buildings. Among TCOs, ZnO is a promising material, due to its low cost, resource availability and nontoxicity compared to indium tin oxide and SnO2. It is well known that group III elements such as Al, In and Ga act as donors in ZnO [1]. As previously reported, Al doping seems to be the most successful due to its advantages: Al ions have a similar ionic radius compared to Zn2+, which results in only small lattice distortions for high Al concentrations. At present, to our knowledge, there are few reports on the IR-reflective property of Al doped ZnO (AZO) films prepared by electron beam physical vapor deposition (EB-PVD) [2,3].
In this work, structural and optical properties of AZO films prepared by EB-PVD were investigated, addressing the effect of the EB power applied on the Al2O3 evaporation source.
2. Experimental procedure AZO layers were prepared on quartz substrates (30mm × 30mm × 1mm) by electron beam physical vapor deposition. The source material ingot used in this study was a 40mm diameter × 20mm thickness of ZnO (Hakusui Tech.) and a 63mm diameter × 50mm thickness of Al2O3 (Kyoto Thin- Film Materials Institute). The source materials were co-evaporated by a 100kW EB gun, using electron beam jumping technique. Electron beam power applied to the ZnOsource (EBZnO) was fixed at 3kW. Electron beam power applied to the Al2O3 source (EBAl2O3) was varied from 3 to 10kW. Co- deposition using electron beam jumping technique, the electron beam is scanned over one target (ZnO) for a certain amount of time and then moved to the next target (Al2O3). EBx (x=ZnO, Al2O3) was
ICC3: Symposium 6: Advances in Electro Ceramics IOP Publishing IOP Conf. Series: Materials Science and Engineering 18 (2011) 092025 doi:10.1088/1757-899X/18/9/092025
c© 2011 Ceramic Society of Japan. Published under licence by IOP Publishing Ltd1
controlled by dividing the dwell time of electron beam among the evaporation sources with the ratio of EBx to EBtotal. The substrate was placed parallel to the ingot surface at a distance of 600mm. Substrate rotation (in-plane) speed was kept at 30rpm. The vacuum chamber was evacuated to a base pressure below 5 ×10-3Pa. Substrate temperature was maintained at 400oC. 2.2 Characterization Microstructures of AZO films were investigated by field emission scanning electron microscope (FE- SEM, Hitachi S-8000). Phase composition and texture were analyzed by X-ray diffraction (XRD, Rigaku 2000V/PC). Optical transmittance and reflectance were measured in the range of 190-2500nm by UV-VIS-IR spectro-photometer (Perkin Elmer Lambda 950). The resistivity, Hall mobility and carrier density were measured using a four-point probe and Hall effect apparatus at room temperature.
3. Results and discussion Figure 1 shows the XRD patterns of AZO films deposited with different EBAl2O3. All layers showed poly crystalline nature, with (002), (101) and (103) peaks of hexagonal ZnO, except the AZO film deposited at EBAl2O3 of 10kW which showed no apparent peak, indicating amorphous nature. Compared with the standard diffraction pattern, the AZO films exhibited weak (002) preferential orientation, which differs from the previous literatures. In general, it is considered that a slow deposition rate would yield well (002)-oriented ZnO layers. Therefore, this difference seems to be due to the high deposition rate, shown in Figure 1. The Al doping does not change the structure of ZnO, because an obvious peak shift was not observed. The peak intensity tended to decrease slightly with the increase of EBAl2O3.
The SEM images of AZO films, as given in Figure 2 (a) to (e), revealed a poly-crystalline structure
with grain size of 100 - 200nm. Thickness of the AZO films increased with increasing from 230nm (EBAl2O3 of 3kW) to 300nm (EBAl2O3 of 6kW). However, at 10kW, thickness of the AZO film decreased.
In co-deposition of ZnO and Al2O3 with medium EBAl2O3, the deposition rate of AZO was dominated by the evaporation rate of ZnO, because ZnO sublimes and its vapor pressure is much higher than that of Al2O3, which was in semi-melt state and the vapor pressure is very low at EBAl2O3 below 6kW. Increasing EBAl2O3 decreases the dwell time on the ZnO source and the amount of ZnO vapor generated in certain time and the effective deposition rate dropped insignificantly. In addition, at EBAl2O3 of 10kW, Al2O3 changed to molten state, and the vapor pressure of Al2O3 rose suddenly, and the rate of Al2O3 vapor in the whole vapor increased and the AZO film approached to Al2O3. The substrate temperature of 400oC is too low for Al2O3 to crystallize. As a result, amorphous phase was formed and the film thickness decreased.
Figure 1. XRD patterns of AZO films deposited with different EBAl2O3. 20 30 40 50 60 70 80
2θ (degree)
(0 02
ICC3: Symposium 6: Advances in Electro Ceramics IOP Publishing IOP Conf. Series: Materials Science and Engineering 18 (2011) 092025 doi:10.1088/1757-899X/18/9/092025
2
Optical properties of AZO films were shown in Figure 3 (a), (b). All the AZO films show high
transmittance (> 80%) in visible range (Figure 3 (a)). However, in the near-IR region, significant difference was observed. The drop of transmittance in near-IR region mainly comes from the increase of reflectance which is due to the plasma resonance of electron gas in the conduction band. IR- reflectance was observed in all the AZO films, except for the film deposited at EBAl2O3 of 10kW. Highest reflectance in near-IR region was obtained EBAl2O3 of 5kW (Figure 3 (b)). At higher EBAl2O3, IR-reflectance decreased with increasing EBAl2O3. Therefore, the effect of deposition time on optical property of AZO films deposited at 5kW of EBAl2O3 was surveyed. Increase in deposition time resulted in increase in IR-reflectance up to 68% at 2500 nm at 300s, which is much higher than that of reported films by EB-PVD [2]. As the deposition time increases, the AZO film thickens and thus the number of carriers (electrons) in the AZO film increase, resulting in increase in IR-reflectance which originates from the plasma arising from the high electron concentration.
Electrical property of the most IR-reflective AZO films (EBAl2O3 of 5kW with deposition time of
300s) was measured by Hall measurement using van der Pauw’s technique. The AZO film achieved electrical resistivity of 3.05×10-4 cm, carrier concentration of 0.86×1021 cm-3 and Hall mobility of 23.8 cm2/Vs. These values are comparable to that of the reported IR-reflective AZO films prepared by
500nm
(d) EBAl2O3=6kW
Figure 2. Cross-sectional SEM images of the AZO films deposited at EBAl2O3 of: (a)3kW, (b)4kW, (c)5kW, (d)6kW and (e)10kW.
Figure 3. Optical transmittance (a) and reflectance (b) spectra of AZO films deposited at different EBAl2O3 and deposition time as a function of wavelength.
500 1000 1500 2000 25000
20
40
60
80
100
500 1000 1500 2000 2500 Wavelength (nm)
R ef
le ct
an ce
(a) (b)
ICC3: Symposium 6: Advances in Electro Ceramics IOP Publishing IOP Conf. Series: Materials Science and Engineering 18 (2011) 092025 doi:10.1088/1757-899X/18/9/092025
3
EB-PVD [3] and by sputtering [4, 5]. Further improvement in optical and electrical properties of AZO films is expected by optimizing the deposition parameters.
4. Conclusions Al doped ZnO (AZO) films were prepared on quartz substrates by the co-deposition of ZnO and Al2O3 evaporation sources by EB-PVD. The effect of EB power applied to the Al2O3 (EBAl2O3) source on the structure, morphology and optical properties of AZO film was investigated. X-ray diffraction measurement showed that the AZO films were weakly c-axis oriented. Transmittance of All the AZO films was over 80% in the visible range. Highest reflectance in the near IR range was obtained at EBAl2O3 of 5kW. The lowest resistivity of 3.05 × 10-4 cm was obtained for the film deposited with the EBAl2O3 of 5kW with the deposition time of 300s.
Acknowledgement This work was entrusted by NEDO as ‘the Project of Development of Multiceramic Film for New Thermal Insulators’.
References [1] T. Minami, H. Sato, H. Nanto and S. Tanaka, Jpn. J. Appl. Phys., 24, L781-784 (1985) [2] H. M. Ali, M. M. Abd El-Raheem, N. M. Megahed and H. A. Mohamed, J. Phys. Chem. Solids,
67, 1823-1829 (2006) [3] D. R. Sahu, Shin-Yuan Lin and Jow-Lay Huang, Microelectronics Journal, 38, 245-250 (2007) [4] D. Song, A.G. Aberle and J. Xia, Appl. Surf. Sci., 195, 291-296 (2002) [5] W. W. Wang, X. G. Diao, Z.Wang, M. Yang, T.M. Wang and Z. Wu, Thin Solid Films, 491, 54-
60 (2005)
ICC3: Symposium 6: Advances in Electro Ceramics IOP Publishing IOP Conf. Series: Materials Science and Engineering 18 (2011) 092025 doi:10.1088/1757-899X/18/9/092025
4
View the article online for updates and enhancements.
-
This content was downloaded from IP address 65.21.228.167 on 07/10/2021 at 01:31
N. Yamaguchi, T. Kuroyama, Y. Okuhara and H. Matsubara
Japan Fine Ceramics Center (JFCC), 2-4-1 Mutsuno, atsuta-ku, Nagoya 456- 8587, Japan
E-mail: [email protected]
Abstract. Al doped ZnO (AZO) films were prepared on quartz substrates by the co- evaporation of ZnO and Al2O3 ingots by EB-PVD. EB power applied to the Al2O3 source was ranged from 3kW to 10kW, while EB power to the ZnO source was fixed at 3kW, at a substrate temperature of 400oC. X-ray diffraction measurement showed that the AZO films were weakly c-axis oriented. Transmittance of All the AZO films was over 80% in the visible range. Highest reflectance in the near IR range was obtained at the EB power on Al2O3 of 5kW. The lowest resistivity of 3.05 x 10-4 cm was obtained for the film deposited with the EB power on Al2O3 of 5kW with the deposition time of 300s.
1. Introduction Transparent conductive oxides (TCOs) are transparent in visible range and shows the high reflectance in the near-infrared (IR) region. Therefore, TCOs are good candidate for application as the coatings for IR-reflectors on super-insulating windows, which realize a considerable energy-saving effect in air conditioning of houses and buildings. Among TCOs, ZnO is a promising material, due to its low cost, resource availability and nontoxicity compared to indium tin oxide and SnO2. It is well known that group III elements such as Al, In and Ga act as donors in ZnO [1]. As previously reported, Al doping seems to be the most successful due to its advantages: Al ions have a similar ionic radius compared to Zn2+, which results in only small lattice distortions for high Al concentrations. At present, to our knowledge, there are few reports on the IR-reflective property of Al doped ZnO (AZO) films prepared by electron beam physical vapor deposition (EB-PVD) [2,3].
In this work, structural and optical properties of AZO films prepared by EB-PVD were investigated, addressing the effect of the EB power applied on the Al2O3 evaporation source.
2. Experimental procedure AZO layers were prepared on quartz substrates (30mm × 30mm × 1mm) by electron beam physical vapor deposition. The source material ingot used in this study was a 40mm diameter × 20mm thickness of ZnO (Hakusui Tech.) and a 63mm diameter × 50mm thickness of Al2O3 (Kyoto Thin- Film Materials Institute). The source materials were co-evaporated by a 100kW EB gun, using electron beam jumping technique. Electron beam power applied to the ZnOsource (EBZnO) was fixed at 3kW. Electron beam power applied to the Al2O3 source (EBAl2O3) was varied from 3 to 10kW. Co- deposition using electron beam jumping technique, the electron beam is scanned over one target (ZnO) for a certain amount of time and then moved to the next target (Al2O3). EBx (x=ZnO, Al2O3) was
ICC3: Symposium 6: Advances in Electro Ceramics IOP Publishing IOP Conf. Series: Materials Science and Engineering 18 (2011) 092025 doi:10.1088/1757-899X/18/9/092025
c© 2011 Ceramic Society of Japan. Published under licence by IOP Publishing Ltd1
controlled by dividing the dwell time of electron beam among the evaporation sources with the ratio of EBx to EBtotal. The substrate was placed parallel to the ingot surface at a distance of 600mm. Substrate rotation (in-plane) speed was kept at 30rpm. The vacuum chamber was evacuated to a base pressure below 5 ×10-3Pa. Substrate temperature was maintained at 400oC. 2.2 Characterization Microstructures of AZO films were investigated by field emission scanning electron microscope (FE- SEM, Hitachi S-8000). Phase composition and texture were analyzed by X-ray diffraction (XRD, Rigaku 2000V/PC). Optical transmittance and reflectance were measured in the range of 190-2500nm by UV-VIS-IR spectro-photometer (Perkin Elmer Lambda 950). The resistivity, Hall mobility and carrier density were measured using a four-point probe and Hall effect apparatus at room temperature.
3. Results and discussion Figure 1 shows the XRD patterns of AZO films deposited with different EBAl2O3. All layers showed poly crystalline nature, with (002), (101) and (103) peaks of hexagonal ZnO, except the AZO film deposited at EBAl2O3 of 10kW which showed no apparent peak, indicating amorphous nature. Compared with the standard diffraction pattern, the AZO films exhibited weak (002) preferential orientation, which differs from the previous literatures. In general, it is considered that a slow deposition rate would yield well (002)-oriented ZnO layers. Therefore, this difference seems to be due to the high deposition rate, shown in Figure 1. The Al doping does not change the structure of ZnO, because an obvious peak shift was not observed. The peak intensity tended to decrease slightly with the increase of EBAl2O3.
The SEM images of AZO films, as given in Figure 2 (a) to (e), revealed a poly-crystalline structure
with grain size of 100 - 200nm. Thickness of the AZO films increased with increasing from 230nm (EBAl2O3 of 3kW) to 300nm (EBAl2O3 of 6kW). However, at 10kW, thickness of the AZO film decreased.
In co-deposition of ZnO and Al2O3 with medium EBAl2O3, the deposition rate of AZO was dominated by the evaporation rate of ZnO, because ZnO sublimes and its vapor pressure is much higher than that of Al2O3, which was in semi-melt state and the vapor pressure is very low at EBAl2O3 below 6kW. Increasing EBAl2O3 decreases the dwell time on the ZnO source and the amount of ZnO vapor generated in certain time and the effective deposition rate dropped insignificantly. In addition, at EBAl2O3 of 10kW, Al2O3 changed to molten state, and the vapor pressure of Al2O3 rose suddenly, and the rate of Al2O3 vapor in the whole vapor increased and the AZO film approached to Al2O3. The substrate temperature of 400oC is too low for Al2O3 to crystallize. As a result, amorphous phase was formed and the film thickness decreased.
Figure 1. XRD patterns of AZO films deposited with different EBAl2O3. 20 30 40 50 60 70 80
2θ (degree)
(0 02
ICC3: Symposium 6: Advances in Electro Ceramics IOP Publishing IOP Conf. Series: Materials Science and Engineering 18 (2011) 092025 doi:10.1088/1757-899X/18/9/092025
2
Optical properties of AZO films were shown in Figure 3 (a), (b). All the AZO films show high
transmittance (> 80%) in visible range (Figure 3 (a)). However, in the near-IR region, significant difference was observed. The drop of transmittance in near-IR region mainly comes from the increase of reflectance which is due to the plasma resonance of electron gas in the conduction band. IR- reflectance was observed in all the AZO films, except for the film deposited at EBAl2O3 of 10kW. Highest reflectance in near-IR region was obtained EBAl2O3 of 5kW (Figure 3 (b)). At higher EBAl2O3, IR-reflectance decreased with increasing EBAl2O3. Therefore, the effect of deposition time on optical property of AZO films deposited at 5kW of EBAl2O3 was surveyed. Increase in deposition time resulted in increase in IR-reflectance up to 68% at 2500 nm at 300s, which is much higher than that of reported films by EB-PVD [2]. As the deposition time increases, the AZO film thickens and thus the number of carriers (electrons) in the AZO film increase, resulting in increase in IR-reflectance which originates from the plasma arising from the high electron concentration.
Electrical property of the most IR-reflective AZO films (EBAl2O3 of 5kW with deposition time of
300s) was measured by Hall measurement using van der Pauw’s technique. The AZO film achieved electrical resistivity of 3.05×10-4 cm, carrier concentration of 0.86×1021 cm-3 and Hall mobility of 23.8 cm2/Vs. These values are comparable to that of the reported IR-reflective AZO films prepared by
500nm
(d) EBAl2O3=6kW
Figure 2. Cross-sectional SEM images of the AZO films deposited at EBAl2O3 of: (a)3kW, (b)4kW, (c)5kW, (d)6kW and (e)10kW.
Figure 3. Optical transmittance (a) and reflectance (b) spectra of AZO films deposited at different EBAl2O3 and deposition time as a function of wavelength.
500 1000 1500 2000 25000
20
40
60
80
100
500 1000 1500 2000 2500 Wavelength (nm)
R ef
le ct
an ce
(a) (b)
ICC3: Symposium 6: Advances in Electro Ceramics IOP Publishing IOP Conf. Series: Materials Science and Engineering 18 (2011) 092025 doi:10.1088/1757-899X/18/9/092025
3
EB-PVD [3] and by sputtering [4, 5]. Further improvement in optical and electrical properties of AZO films is expected by optimizing the deposition parameters.
4. Conclusions Al doped ZnO (AZO) films were prepared on quartz substrates by the co-deposition of ZnO and Al2O3 evaporation sources by EB-PVD. The effect of EB power applied to the Al2O3 (EBAl2O3) source on the structure, morphology and optical properties of AZO film was investigated. X-ray diffraction measurement showed that the AZO films were weakly c-axis oriented. Transmittance of All the AZO films was over 80% in the visible range. Highest reflectance in the near IR range was obtained at EBAl2O3 of 5kW. The lowest resistivity of 3.05 × 10-4 cm was obtained for the film deposited with the EBAl2O3 of 5kW with the deposition time of 300s.
Acknowledgement This work was entrusted by NEDO as ‘the Project of Development of Multiceramic Film for New Thermal Insulators’.
References [1] T. Minami, H. Sato, H. Nanto and S. Tanaka, Jpn. J. Appl. Phys., 24, L781-784 (1985) [2] H. M. Ali, M. M. Abd El-Raheem, N. M. Megahed and H. A. Mohamed, J. Phys. Chem. Solids,
67, 1823-1829 (2006) [3] D. R. Sahu, Shin-Yuan Lin and Jow-Lay Huang, Microelectronics Journal, 38, 245-250 (2007) [4] D. Song, A.G. Aberle and J. Xia, Appl. Surf. Sci., 195, 291-296 (2002) [5] W. W. Wang, X. G. Diao, Z.Wang, M. Yang, T.M. Wang and Z. Wu, Thin Solid Films, 491, 54-
60 (2005)
ICC3: Symposium 6: Advances in Electro Ceramics IOP Publishing IOP Conf. Series: Materials Science and Engineering 18 (2011) 092025 doi:10.1088/1757-899X/18/9/092025
4