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Effects of sapphire substrates surface treatment on the ZnO thin films grown by magnetron sputtering
ARTICLE IN PRESS
Physica B 403 (2008) 1979–1982 www.elsevier.com/locate/physb
Effects of sapphire substrates surface treatment on the ZnO thin films grown by magnetron sputtering Yinzhen Wang, Benli Chu School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou 510631, PR China Received 6 October 2007; accepted 1 November 2007
Abstract The surface treatment effects of sapphire substrate on the ZnO thin films grown by magnetron sputtering were studied. The sapphire substrates properties have been investigated by means of atomic force microscopy (AFM) and X-ray diffraction rocking curves (XRCs). The results show that sapphire substrate surfaces have the best quality by CMP with subsequent chemical etching. The surface treatment effects of sapphire substrate on the ZnO thin films were examined by X-ray diffraction (XRD) and photoluminescence (PL) measurements. Results show that the intensity of (0 0 2) diffraction peak of ZnO thin films on sapphire substrates treated by CMP with subsequent chemical etching was strongest, FWHM of (0 0 2) diffraction peak is the narrowest and the intensity of UV peak of PL spectrum is strongest, indicating surface treatment on sapphire substrate preparation may improve ZnO thin films crystal quality and photoluminescent property. r 2007 Elsevier B.V. All rights reserved. PACS: 81.15.Gh; 68.55. a; 78.2. e Keywords: ZnO thin films; Sapphire substrate; Surface treatment; Magnetron sputtering
1. Introduction In recent years, ZnO thin films have attracted attention because of their potential applications such transparent electrodes, solar cells, gas sensors, acoustic wave devices, waveguides, varistors, LDs, LEDs, etc. [1–5]. Many different deposition methods have been used to grow ZnO thin films on various substrates such as sapphire [6,7], silicon [8], GaAs [9], and glass [10,11]. Among them, the sapphire substrate plays an important role in applying ZnO thin films to optoelectronic devices. Despite the large differences in the thermal expansion coefficient and lattice constant between the ZnO thin films and sapphire substrate, sapphire is widely used as a substrate because of its physical robustness and high-temperature stability. Although there are many reports on growing ZnO thin films on sapphire substrates, there are only a few reports on the effects of the substrate on the epitaxial ZnO thin films. Corresponding author. Tel./fax: +86 20 39310066.
E-mail address: [email protected] (Y.Z. Wang). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.11.003
It is now well recognized that the surface treatment of the substrate prior to growth has crucial effects on both the growth mechanism and the material properties [12,13]. Furthermore, there are few reports on the surface treatment effects of sapphire substrates on the epitaxial ZnO thin films. In this paper, we have first studied the effects of sapphire substrate surface treatment on the ZnO thin films grown by magnetron sputtering. 2. Experiment In this study, a c-plane (0 0 0 1)-oriented sapphire boule was grown by the temperature gradient technique (TGT). The raw material was high-purity 99.99% Al2O3 power. The sapphire boule appeared colorless and transparent after annealing. The (0 0 0 1)-oriented sapphire substrates obtained from the central region of the boule were cut perpendicularly to the growth direction and treated in three different ways in order to carry out studies in (a) mechanical polishing, (b) CMP, and (c) CMP with subsequent chemical etching. The substrates were colorless,
ARTICLE IN PRESS Y.Z. Wang, B. Chu / Physica B 403 (2008) 1979–1982
1980
transparent and free of pores, bubbles, and grains. Chemical etching was performed using a hot H2SO4:H3PO4 (3:1) solution. The substrates were then rinsed for 5 min each in acetone and alcohol. The above steps were repeated three times to complete the degreasing process. The ZnO thin films were grown on (0 0 0 1) sapphire substrates by dc planar magnetron sputtering using an 8.6 cm-in-diameter Zn target (99.99%) at room temperature. Argon and oxygen were used as the sputtering and reactive gases, respectively. The target-to–substrate distance was 15 cm. The cathode was mounted on a water-cooled copper plate. The chamber was pumped to a base pressure of 1 10 3 Pa before deposition. Film growth was carried out in the growth ambient with a mixture of argon (40%) and oxygen (60%) and at a constant working pressure of 0.15 Pa. The sapphire substrates were characterized by atomic force microscopy (AFM) and X-ray diffraction rocking curves (XRCs). The structure of ZnO thin films was studied by X-ray diffraction (XRD) method using Cu Ka radiation(l=0.15405 nm). Photoluminescence (PL) spectra were recorded using a xenon arc lamp as the excitation source. The excitation wavelength was 325 nm and the resolution was 0.5 nm. All measurements were carried out at room temperature. 3. Results and discussion Fig. 1 shows AFM images of the variously treated sapphire substrates surfaces. It is apparent that there are
scratches on the sample surface after mechanical polishing (Fig. 1(a)). Mechanical polishing alone, theoretically, may achieve the planarization, but it is not desirable because of the extensively associated damage of material surfaces. There are no scratches on the wafer surface after CMP treatment, and the root-mean square (RMS) roughness decreases (Fig. 1(b)). CMP technique can provide planarization without damaging the material surfaces. In the case of CMP with subsequent chemical etching (Fig. 1(c)), the sample surface is significantly smoother and the surface roughness value is only 0.230 nm RMS. The above results suggest that sapphire substrates subjected to CMP with subsequent chemical etching exhibit a very smooth morphology and are of the highest quality. Fig. 2 shows the corresponding X-ray rocking curves that clearly indicate that the substrate surface crystallinity has been also improved substantially by using the CMP and subsequent chemical etching treatment. The XRC fullwidth at half-maxima (FWHM) of samples subjected to mechanical polishing, CMP and CMP with subsequent chemical etching are 30.2, 20.3, and 11.2 arcsec, respectively. The sequential narrowing of the rocking curve width is expected, and it indicates furthermore the improvement of the substrate surface quality. The quality of epitaxial layer is related to the surface of the substrate. Fig. 3 shows the XRD of the ZnO thin films deposited on different surface treatment sapphire substrates. All the films show a preferential (0 0 2) orientation, indicative of a strong c-axis orientation, with the (0 0 2)
2.0
2.0 10.0 nM
1.0
5.0nM
5.0 nM
1.0
0.0nM
0.0 nM
0
0 2.0
1.0 µM
2.5nM
0
1.0 µM
0 2.0
2.00 2.0nM 1.00
1.0nM 0.0nM
0
1.00 µM
0 2.00
Fig. 1. AFM images of sapphire substrate surfaces after: (a) mechanical polishing, (b) CMP, and (c) CMP with subsequent chemical etching.
ARTICLE IN PRESS Y.Z. Wang, B. Chu / Physica B 403 (2008) 1979–1982
18000
12000
15000 Intensity/arb.units
Intensity/arb.units
10000 8000 6000 4000 2000 0 20.5
1981
12000 9000 6000 3000 0
20.6
20.7
20.8
20.9
20.5
20.6
Omega/ °
20.7 20.8 Omega/ °
20.9
50000
Intensity/arb.units
40000 30000 20000 10000 0 20.4 20.5 20.6 20.7 20.8 20.9 Omega/ ° Fig. 2. X-ray rocking curves of sapphire substrate after: (a) mechanical polishing, (b) CMP, and (c) CMP with subsequent chemical etching.
Fig. 3. XRD spectra of ZnO films deposited on (a) mechanical polishing sapphire substrate, (b) CMP sapphire substrate, and (c) CMP with subsequent chemical etching sapphire substrate.
plane parallel to the surface of the substrate. The intensity of the (0 0 2) ZnO thin films peak in the XRD spectra increases on CMP sapphire substrates, while the FWHM value of (0 0 2) peak decreases. The strongest peak occurs at the substrate for CMP with subsequent chemical
Fig. 4. PL spectra of ZnO films deposited on (a) mechanical polishing sapphire substrate, (b) CMP sapphire substrate, and (c) CMP with subsequent chemical etching sapphire substrate.
etching, the FWHM value on mechanical polishing, CMP and CMP with subsequent chemical etching substrate are 0.9391, 0.8521, and 0.7011, respectively, which suggests that
ARTICLE IN PRESS 1982
Y.Z. Wang, B. Chu / Physica B 403 (2008) 1979–1982
high-quality ZnO thin film could be formed on the CMP with subsequent chemical etching substrate. These results show sapphire substrate surface treatment prior to film deposition can provide some improvements in film quality. For CMP sapphire substrates surface has no scratches and RMS decrease, which is favorable for the film to grow. CMP with subsequent chemical etching substrate surface is much smooth and flat, which decrease the stress of the substrate and the ZnO thin films to a certain extent. PL is very sensitive to the quality of crystal structure and to the presence of defects. Fig. 4 shows the room temperature PL spectra of ZnO films deposited on different surface treatment sapphire substrates. It is observed that all the ZnO thin films show a near-band-edge emission at 385 nm. The intensity of the UV emission peak increases with CMP sapphire substrates, while the FWHM value of UV peak decreases; the strongest UV peak occurs at the substrate for CMP with subsequent chemical etching and the FWHM value is least, which suggests that the CMP with subsequent chemical etching sapphire substrate may improve ZnO thin films photoluminescent property. UV emission is usually related to the crystalline quality, strong UV emission results from high crystalline quality [14]. This is in accord with XRD results. 4. Conclusion We have studied the surface treatment effects of sapphire substrate on the ZnO thin films grown by magnetron sputtering. Sapphire substrates treated by CMP with subsequent chemical etching exhibit a significantly better surface smoothness as compared to substrates treated by
mechanical polishing or CMP alone. Experimental results indicate that the film quality is strongly affected by surface treatment of the sapphire substrate surface. The ZnO thin films grown on sapphire substrates treated by CMP with subsequent chemical etching show good quality and optical properties. Surface treatment on sapphire substrate prior to ZnO films growth may improve ZnO thin films crystal quality and photoluminescent property. References [1] J.B. Lee, H.J. Kim, S.G. Kim, C.S. Hwang, S.-H. Hong, Y.H. Shin, N.H. Lee, Thin Solid Films 435 (2003) 179. [2] A. Goux, T. Pauporte, J. Chivot, D. Lincot, Electrochim. Acta 50 (2005) 2239. [3] D.R. Clarke, J. Am. Ceram. Soc. 82 (1999) 485. [4] R.F. Serice, Science 276 (1997) 895. [5] J. Narayan, K. Dovidenko, A.K. Sharma, S. Oktyabrsky, J. Appl. Phys. 84 (1998) 2597. [6] G.A. Mohamed, E.-M. Mohamed, A.A. El-Fadl, Physica B 308–310 (2001) 949. [7] B.P. Zhang, K. Wakatsuki, N.T. Binh, N. Usami, Y. Segawa, Thin Solid Films 449 (2004) 12. [8] I. Sayago, M. Aleixandre, L. Ares, M.J. Fernandez, J.P. Santos, J. Gutierrez, M.C. Horrillo, Appl. Surf. Sci. 245 (2005) 273. [9] F.K. Shan, B.C. Shin, S.W. Jang, Y.S. Yu, J. Eur. Ceram. Soc. 24 (2004) 1015. [10] F.K. Shan, G.X. Liu, W.J. Lee, G.H. Lee, I.S. Kim, B.C. Shin, Y.C. Kim, J. Cryst. Growth 277 (2005) 284. [11] A.S. Riad, S.A. Mahmoud, A.A. Ibrahim, Physica B 296 (2001) 319. [12] J.H. Kim, S.C. Choi, J.Y. Choi, K.S. Kim, G.M. Yan, C.H. Hong, K.Y. Lim, H.J. Lee, Jpn. J. Appl. Phys. 38 (1999) 2721. [13] C. Heinlein, J. Grepstad, H. Riechert, R. Averbeck, Mater. Sci. Eng. B 43 (1997) 253. [14] Y. Chen, D.M. Bagnall, H.J. Koh, K.T. Park, K. Hiraga, Z.Q. Zhu, T. Yao, J. Appl. Phys. 84 (1998) 3912.
Physica B 403 (2008) 1979–1982 www.elsevier.com/locate/physb
Effects of sapphire substrates surface treatment on the ZnO thin films grown by magnetron sputtering Yinzhen Wang, Benli Chu School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou 510631, PR China Received 6 October 2007; accepted 1 November 2007
Abstract The surface treatment effects of sapphire substrate on the ZnO thin films grown by magnetron sputtering were studied. The sapphire substrates properties have been investigated by means of atomic force microscopy (AFM) and X-ray diffraction rocking curves (XRCs). The results show that sapphire substrate surfaces have the best quality by CMP with subsequent chemical etching. The surface treatment effects of sapphire substrate on the ZnO thin films were examined by X-ray diffraction (XRD) and photoluminescence (PL) measurements. Results show that the intensity of (0 0 2) diffraction peak of ZnO thin films on sapphire substrates treated by CMP with subsequent chemical etching was strongest, FWHM of (0 0 2) diffraction peak is the narrowest and the intensity of UV peak of PL spectrum is strongest, indicating surface treatment on sapphire substrate preparation may improve ZnO thin films crystal quality and photoluminescent property. r 2007 Elsevier B.V. All rights reserved. PACS: 81.15.Gh; 68.55. a; 78.2. e Keywords: ZnO thin films; Sapphire substrate; Surface treatment; Magnetron sputtering
1. Introduction In recent years, ZnO thin films have attracted attention because of their potential applications such transparent electrodes, solar cells, gas sensors, acoustic wave devices, waveguides, varistors, LDs, LEDs, etc. [1–5]. Many different deposition methods have been used to grow ZnO thin films on various substrates such as sapphire [6,7], silicon [8], GaAs [9], and glass [10,11]. Among them, the sapphire substrate plays an important role in applying ZnO thin films to optoelectronic devices. Despite the large differences in the thermal expansion coefficient and lattice constant between the ZnO thin films and sapphire substrate, sapphire is widely used as a substrate because of its physical robustness and high-temperature stability. Although there are many reports on growing ZnO thin films on sapphire substrates, there are only a few reports on the effects of the substrate on the epitaxial ZnO thin films. Corresponding author. Tel./fax: +86 20 39310066.
E-mail address: [email protected] (Y.Z. Wang). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.11.003
It is now well recognized that the surface treatment of the substrate prior to growth has crucial effects on both the growth mechanism and the material properties [12,13]. Furthermore, there are few reports on the surface treatment effects of sapphire substrates on the epitaxial ZnO thin films. In this paper, we have first studied the effects of sapphire substrate surface treatment on the ZnO thin films grown by magnetron sputtering. 2. Experiment In this study, a c-plane (0 0 0 1)-oriented sapphire boule was grown by the temperature gradient technique (TGT). The raw material was high-purity 99.99% Al2O3 power. The sapphire boule appeared colorless and transparent after annealing. The (0 0 0 1)-oriented sapphire substrates obtained from the central region of the boule were cut perpendicularly to the growth direction and treated in three different ways in order to carry out studies in (a) mechanical polishing, (b) CMP, and (c) CMP with subsequent chemical etching. The substrates were colorless,
ARTICLE IN PRESS Y.Z. Wang, B. Chu / Physica B 403 (2008) 1979–1982
1980
transparent and free of pores, bubbles, and grains. Chemical etching was performed using a hot H2SO4:H3PO4 (3:1) solution. The substrates were then rinsed for 5 min each in acetone and alcohol. The above steps were repeated three times to complete the degreasing process. The ZnO thin films were grown on (0 0 0 1) sapphire substrates by dc planar magnetron sputtering using an 8.6 cm-in-diameter Zn target (99.99%) at room temperature. Argon and oxygen were used as the sputtering and reactive gases, respectively. The target-to–substrate distance was 15 cm. The cathode was mounted on a water-cooled copper plate. The chamber was pumped to a base pressure of 1 10 3 Pa before deposition. Film growth was carried out in the growth ambient with a mixture of argon (40%) and oxygen (60%) and at a constant working pressure of 0.15 Pa. The sapphire substrates were characterized by atomic force microscopy (AFM) and X-ray diffraction rocking curves (XRCs). The structure of ZnO thin films was studied by X-ray diffraction (XRD) method using Cu Ka radiation(l=0.15405 nm). Photoluminescence (PL) spectra were recorded using a xenon arc lamp as the excitation source. The excitation wavelength was 325 nm and the resolution was 0.5 nm. All measurements were carried out at room temperature. 3. Results and discussion Fig. 1 shows AFM images of the variously treated sapphire substrates surfaces. It is apparent that there are
scratches on the sample surface after mechanical polishing (Fig. 1(a)). Mechanical polishing alone, theoretically, may achieve the planarization, but it is not desirable because of the extensively associated damage of material surfaces. There are no scratches on the wafer surface after CMP treatment, and the root-mean square (RMS) roughness decreases (Fig. 1(b)). CMP technique can provide planarization without damaging the material surfaces. In the case of CMP with subsequent chemical etching (Fig. 1(c)), the sample surface is significantly smoother and the surface roughness value is only 0.230 nm RMS. The above results suggest that sapphire substrates subjected to CMP with subsequent chemical etching exhibit a very smooth morphology and are of the highest quality. Fig. 2 shows the corresponding X-ray rocking curves that clearly indicate that the substrate surface crystallinity has been also improved substantially by using the CMP and subsequent chemical etching treatment. The XRC fullwidth at half-maxima (FWHM) of samples subjected to mechanical polishing, CMP and CMP with subsequent chemical etching are 30.2, 20.3, and 11.2 arcsec, respectively. The sequential narrowing of the rocking curve width is expected, and it indicates furthermore the improvement of the substrate surface quality. The quality of epitaxial layer is related to the surface of the substrate. Fig. 3 shows the XRD of the ZnO thin films deposited on different surface treatment sapphire substrates. All the films show a preferential (0 0 2) orientation, indicative of a strong c-axis orientation, with the (0 0 2)
2.0
2.0 10.0 nM
1.0
5.0nM
5.0 nM
1.0
0.0nM
0.0 nM
0
0 2.0
1.0 µM
2.5nM
0
1.0 µM
0 2.0
2.00 2.0nM 1.00
1.0nM 0.0nM
0
1.00 µM
0 2.00
Fig. 1. AFM images of sapphire substrate surfaces after: (a) mechanical polishing, (b) CMP, and (c) CMP with subsequent chemical etching.
ARTICLE IN PRESS Y.Z. Wang, B. Chu / Physica B 403 (2008) 1979–1982
18000
12000
15000 Intensity/arb.units
Intensity/arb.units
10000 8000 6000 4000 2000 0 20.5
1981
12000 9000 6000 3000 0
20.6
20.7
20.8
20.9
20.5
20.6
Omega/ °
20.7 20.8 Omega/ °
20.9
50000
Intensity/arb.units
40000 30000 20000 10000 0 20.4 20.5 20.6 20.7 20.8 20.9 Omega/ ° Fig. 2. X-ray rocking curves of sapphire substrate after: (a) mechanical polishing, (b) CMP, and (c) CMP with subsequent chemical etching.
Fig. 3. XRD spectra of ZnO films deposited on (a) mechanical polishing sapphire substrate, (b) CMP sapphire substrate, and (c) CMP with subsequent chemical etching sapphire substrate.
plane parallel to the surface of the substrate. The intensity of the (0 0 2) ZnO thin films peak in the XRD spectra increases on CMP sapphire substrates, while the FWHM value of (0 0 2) peak decreases. The strongest peak occurs at the substrate for CMP with subsequent chemical
Fig. 4. PL spectra of ZnO films deposited on (a) mechanical polishing sapphire substrate, (b) CMP sapphire substrate, and (c) CMP with subsequent chemical etching sapphire substrate.
etching, the FWHM value on mechanical polishing, CMP and CMP with subsequent chemical etching substrate are 0.9391, 0.8521, and 0.7011, respectively, which suggests that
ARTICLE IN PRESS 1982
Y.Z. Wang, B. Chu / Physica B 403 (2008) 1979–1982
high-quality ZnO thin film could be formed on the CMP with subsequent chemical etching substrate. These results show sapphire substrate surface treatment prior to film deposition can provide some improvements in film quality. For CMP sapphire substrates surface has no scratches and RMS decrease, which is favorable for the film to grow. CMP with subsequent chemical etching substrate surface is much smooth and flat, which decrease the stress of the substrate and the ZnO thin films to a certain extent. PL is very sensitive to the quality of crystal structure and to the presence of defects. Fig. 4 shows the room temperature PL spectra of ZnO films deposited on different surface treatment sapphire substrates. It is observed that all the ZnO thin films show a near-band-edge emission at 385 nm. The intensity of the UV emission peak increases with CMP sapphire substrates, while the FWHM value of UV peak decreases; the strongest UV peak occurs at the substrate for CMP with subsequent chemical etching and the FWHM value is least, which suggests that the CMP with subsequent chemical etching sapphire substrate may improve ZnO thin films photoluminescent property. UV emission is usually related to the crystalline quality, strong UV emission results from high crystalline quality [14]. This is in accord with XRD results. 4. Conclusion We have studied the surface treatment effects of sapphire substrate on the ZnO thin films grown by magnetron sputtering. Sapphire substrates treated by CMP with subsequent chemical etching exhibit a significantly better surface smoothness as compared to substrates treated by
mechanical polishing or CMP alone. Experimental results indicate that the film quality is strongly affected by surface treatment of the sapphire substrate surface. The ZnO thin films grown on sapphire substrates treated by CMP with subsequent chemical etching show good quality and optical properties. Surface treatment on sapphire substrate prior to ZnO films growth may improve ZnO thin films crystal quality and photoluminescent property. References [1] J.B. Lee, H.J. Kim, S.G. Kim, C.S. Hwang, S.-H. Hong, Y.H. Shin, N.H. Lee, Thin Solid Films 435 (2003) 179. [2] A. Goux, T. Pauporte, J. Chivot, D. Lincot, Electrochim. Acta 50 (2005) 2239. [3] D.R. Clarke, J. Am. Ceram. Soc. 82 (1999) 485. [4] R.F. Serice, Science 276 (1997) 895. [5] J. Narayan, K. Dovidenko, A.K. Sharma, S. Oktyabrsky, J. Appl. Phys. 84 (1998) 2597. [6] G.A. Mohamed, E.-M. Mohamed, A.A. El-Fadl, Physica B 308–310 (2001) 949. [7] B.P. Zhang, K. Wakatsuki, N.T. Binh, N. Usami, Y. Segawa, Thin Solid Films 449 (2004) 12. [8] I. Sayago, M. Aleixandre, L. Ares, M.J. Fernandez, J.P. Santos, J. Gutierrez, M.C. Horrillo, Appl. Surf. Sci. 245 (2005) 273. [9] F.K. Shan, B.C. Shin, S.W. Jang, Y.S. Yu, J. Eur. Ceram. Soc. 24 (2004) 1015. [10] F.K. Shan, G.X. Liu, W.J. Lee, G.H. Lee, I.S. Kim, B.C. Shin, Y.C. Kim, J. Cryst. Growth 277 (2005) 284. [11] A.S. Riad, S.A. Mahmoud, A.A. Ibrahim, Physica B 296 (2001) 319. [12] J.H. Kim, S.C. Choi, J.Y. Choi, K.S. Kim, G.M. Yan, C.H. Hong, K.Y. Lim, H.J. Lee, Jpn. J. Appl. Phys. 38 (1999) 2721. [13] C. Heinlein, J. Grepstad, H. Riechert, R. Averbeck, Mater. Sci. Eng. B 43 (1997) 253. [14] Y. Chen, D.M. Bagnall, H.J. Koh, K.T. Park, K. Hiraga, Z.Q. Zhu, T. Yao, J. Appl. Phys. 84 (1998) 3912.