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Orientation relationship of polycrystalline Pd-doped SnO2 thin film deposits on sapphire substrates
Thin Solid Films 517 (2008) 550–553
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f
Orientation relationship of polycrystalline Pd-doped SnO2 thin film deposits on sapphire substrates Ji-Hwan Kwon, Yun-Hyuk Choi, Dai Hong Kim, Myeong Yang, Jiyoung Jang, Tae Woong Kim, Seong-Hyeon Hong, Miyoung Kim ⁎ Department of Materials Science and Engineering, Seoul National University, Shillim-Dong, Kwanak-Gu, Seoul 151-744, Republic of Korea
A R T I C L E
I N F O
Article history: Received 23 August 2007 Received in revised form 2 April 2008 Accepted 22 June 2008 Available online 27 June 2008 Keywords: SnO2 thin films X-ray pole figures In-plane relationship Transmission electron microscopy
A B S T R A C T Pd-doped SnO2 thin films of rutile structure were deposited using radio-frequency magnetron sputtering method on sapphire substrates with three different orientations: r-cut (0112), a-cut (1120), and m-cut (1010). The deposited films were characterized using transmission electron microscopy and X-ray pole figures. All the deposited films were polycrystalline but highly oriented with the interface, and also had in-plane or nearepitaxial relationships were observed for the SnO2 films grown on all three cases of sapphire substrates. In particular, (101), (101), and (001) oriented films were respectively grown on r-, a-, and m-cut sapphire substrates. The in-plane orientation relationships were determined to be [010]SnO2 || [100]Al2O3 and [101] -SnO2 || [121]Al2O3 (r-cut), [101]SnO2 || [110]Al2O3 and [010]SnO2 || [001]Al2O3 (a-cut), and [100]SnO2 || [010] Al2O3 and [010]SnO2 || [001]Al2O3 (m-cut). A highly textured structure was observed for the films grown on m-cut substrate. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor metal oxide materials have been studied extensively for their wide variety of applications such as gas sensors [1], dye-sensitized solar cells [2], ion-sensitive field effect transistors [3], and catalysts [4]. In particular, gas sensors based on SnO2 have been widely used because of their high sensitivity for various gases including H2, CO, and CH4. The hydrogen gas sensing performance of SnO2 has been greatly enhanced by increasing the surface-to-volume ratio through the formation of nanostructures or by impregnating a small amount of noble metals [5,6]. The efficiency of gas sensors depend basically on their electrical transport properties [7–10], which are related to chemical bonding states and potential barriers at the grain boundaries. In order to evaluate the efficiency of sensor materials and explore further applications, it is therefore important to analyze various microstructural features like grain size, neck size, interface, and crystal orientations [11–13]. However, the reported literature on the influence of crystal orientation was limited to whiskers and epitaxial films [14,15]. Moreover, the epitaxial relationships of SnO2 thin films on sapphire have been reported only on ‘r-cut’ (0112) and ‘c-cut’ (0001) substrates [16–21]. The radio-frequency (RF) magnetron sputtering is one of the potential methods to deposit polycrystalline or epitaxial SnO2 films on
⁎ Corresponding author. Tel.: +82 2 880 9239; fax: +82 2 884 1413. E-mail address: [email protected] (M. Kim). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.06.074
sapphire substrates [15,22]. Choi and Hong [22] used RF magnetron sputtering to deposit undoped SnO2 films on ‘r-cut’ (0112) substrate. Their deposited films were oriented in (101) plane, perpendicular to the interface and had in-plane relationship of [010]SnO2 || [100]Al2O3 -and [101]SnO2 || [121] Al2O3. Similar in-plane relationship was also reported in case of epitaxial films, grown using atomic layer deposition [16,17]. The deposited SnO2 films on the ‘a-cut’ (1120) and ‘m-cut’ (1010) Al2O3 were highly textured and appeared to have in-plain orientation, but no orientation relationship was determined. In the present study, the Pd-doped SnO2 thin films were deposited on three different sapphire substrates (r-cut, a-cut, and m-cut) using RF magnetron sputtering method and the developed interface was subjected to detailed microstructural characterization using transmission electron microscopy (TEM) and X-ray pole-figure techniques. Thus, the major objective of the present investigation is to evaluate inplane orientation relationships of Pd-doped SnO2 thin films with the substrates for all three cases; (0112) (r-cut), (1120) (a-cut), and (1010) (m-cut) Al2O3. 2. Experimental details In the present study, three types of sapphire substrates were used: ‘rcut’ (0112), ‘a-cut’ (1120), and ‘m-cut’ (1010). Pd (0.5 wt.%)-doped SnO2 target was used to deposit SnO2 thin films by RF magnetron sputtering. The deposition process was performed in an argon atmosphere with a RF power of 100 W at a working pressure of 3.3 Pa for 15–30 min. The temperature was maintained at 350 °C during the deposition process. The
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deposited films were subjected to subsequent heat treatment at 600 °C in air in order to enhance the crystallinity of the films. The presence of various phases and orientations of the films was examined by X-ray diffraction (XRD, Model M18XHF-SRA, Mac Science, Japan) using Cu Kα radiation (λ=0.154 nm) and X-ray pole figures (X'pert Pro, PANalytical, The Netherlands). Pole figures were measured in Schulz reflection geometry by scanning the tilt angle of goniometer, χ (chi), in the range of 0–85° and the azimuthal angle, ϕ (phi), in the range of 0–360°, using a 5° step. For the TEM investigation, cross-sectional specimens were prepared by mechanical polishing and subsequent Ar ion milling at 5 kV. The samples were examined using JEOL-3000F and TECNAI F20 transmission electron microscopes, operating at 300 kV and 200 kV, respectively. 3. Results and discussion Tin dioxide has rutile-type structure with space group of P42/mnm and lattice constants of a = 4.737 Å, c = 3.185 Å. The sapphire substrate has hexagonal structure, a = 4.763 Å, c = 13.003 Å with space group of R3̄c (No.167). Fig. 1 shows the XRD patterns of the Pd-doped SnO2 films deposited on the various sapphire substrates. Only (101), (101), and (001) diffraction peaks were observed in Pd-doped SnO2 film grown on (0112) (r-cut), (1120) (a-cut), and (1010) (m-cut) Al2O3, respectively and the deposited Pd-doped SnO2 films were highly oriented perpendicular to the substrate interface. These orientations were the same as that reported for the undoped SnO2 films [22], and a small amount of Pd doping had little influence on the structures of deposited SnO2 films [23]. Further, a closer examination of the XRD pattern in case of film deposited on ‘m-cut’ substrate indicated the additional presence of a weak peak of (301) at 66°. Therefore, it can be stated that the Pd-doped SnO2 film on the ‘m-cut’ substrate was strongly (001) oriented with a small amount of the (301) orientation. The TEM image in Fig. 2 shows a cross-sectional view of the Pddoped SnO2 film grown on the ‘r-cut’ sapphire substrate, indicating a columnar structure of the films. Each column was approximately 10 nm in width and 100 nm in height. All three types (r-cut, a-cut and m-cut) of Pd-doped SnO2 films reveal similar columnar structure. Fig. 3 shows the {101} pole figures of the Pd-doped SnO2 films. The substrate orientations are indicated next to the corresponding pole figures. The pole figures of the films on the ‘r-cut’ and ‘a-cut’ substrates were similar, where reflections from the {101} planes [(101), (011), (101), and (011)] were only observed with an additional contribution from the 180° rotation of the (101) plane, which was marked as T1 in the figures. This means that there exist in-plane orientation relationships between the films and the substrates, even though the deposited films are polycrystalline. The in-plane relationship of the Pd-doped (101) SnO2 film on the ‘r-cut’ substrate was [010]SnO2 || [100] Al2O3 and [101]SnO2 ||
Fig. 1. X-ray diffraction (XRD) patterns of the Pd-doped SnO2 films grown on the ‘r-cut’, ‘a-cut’, and ‘m-cut’ substrates.
Fig. 2. TEM image of the SnO2:Pd films grown on an ‘r-cut’ sapphire substrate.
-[121]Al2O3, which is consistent with the previously reported epitaxial relationship of undoped SnO2 [16–18,22]. The in-plane orientation relationship of Pd-doped (101) SnO2 film on the ‘a-cut’ substrate was found to be [101]SnO2 || [110]Al2O3 and [010]SnO2 || [001]Al2O3. In the
Fig. 3. X-ray pole figures from the SnO2 films deposited on the ‘r-cut’, ‘a-cut’, and ‘m-cut’ substrates.
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pole figure of the Pd-doped (001) SnO2 film on the ‘m-cut’ substrate, strong {101} planes were observed along with very weak reflections from the {301} planes marked as A1, A2, A3, and A4. The in-plane relationship of the main orientations in the Pd-doped (001) SnO2 film on the ‘m-cut’ substrate was determined to be [100]SnO2 || [010]Al2O3 and [010]SnO2 || [001]Al2O3. While heteroepitaxial relationships were reported previously in (101) and (001) rutile TiO2 films on ‘a-cut’ and ‘m-cut’ Al2O3 substrates [24], the in-plane orientation relationships were observed in the present investigation of Pd-doped SnO2 films deposited on ‘a-cut’ and ‘m-cut’ Al2O3 substrates. Based on these observations, the polycrystalline Pd-doped SnO2 films in this study were highly textured, close to the epitaxial films. The highly textured Pd-doped SnO2 films were further examined using high-resolution transmission electron microscopy (HRTEM). Fig. 4 shows the HRTEM images at the SnO2/Al2O3 interface as well as the electron diffraction patterns of the film and substrate in the three cases: (a) r-cut, (b) a-cut, and (c) m-cut. The HRTEM images reveal many defects and grains, particularly in the vicinity of the interface. The grains were a few tens of nanometers in diameter, and mainly formed low angles with each other. The interface between the Pddoped SnO2 thin films and Al2O3 substrates in the ‘a-cut’ was quite rough, while the ‘r-cut’ and the ‘m-cut’ films had a rather well defined interface, despite of their polycrystalline nature. The ‘m-cut’ films, in particular, had an atomistically sharp interface. The electron diffraction patterns clearly demonstrate the polycrystalline phases. Although the films were highly oriented with dominant peaks in all three cases, many additional peaks out of the main domains could be easily seen. The diffraction patterns of films on the ‘r-cut’ and ‘a-cut’ showed weak rings, indicating the presence of randomly oriented small-sized domains. The major diffraction peaks of all three types of films were elongated, confirming the existence of misorientations of a few degrees in the high-resolution images. It was difficult to identify a second phase due to Pd or the segregation of Pd into the grain boundary in the HRTEM images. However, the electron diffraction patterns indicated few weak tetragonal PdO (110) spots with d-spacing of 2.15 Å, as marked in Fig. 4(b), in all three samples. The appearance of the PdO phase in the films is consistent with previously reported results [23]. The sizes of PdO domains, however, were too small to be detected in the X-ray diffraction patterns, judging from the intensities of those peaks in the electron diffraction patterns. The in-plane relationships of the three cases were obtained on the basis of the electron diffraction patterns. In the case of the ‘r-cut’ substrate, the film was oriented in (101) plane, perpendicular to the interface and with in-plane relationship of [010]SnO2 || [100]Al2O3 and -[101]SnO2 || [121]Al2O3, which is again consistent with the results reported for the undoped SnO2 thin films [16–18,22]. On the other hand, the SnO2 films grown on the ‘a-cut’ and ‘m-cut’ Al2O3 substrates were oriented in (101) and (001) planes, perpendicular to the interface and their in-plane relationship between the films and substrates was [101]SnO2 || [110]Al2O3 and [010]SnO2 || [001]Al2O3 in the case of the ‘a-cut’, and [100]SnO2 || [010]Al2O3 and [010]SnO2 || [001]Al2O3 in the case of the ‘m-cut’. These results are consistent with the X-ray polefigure results. It should be noted that the interface of the ‘m-cut’ substrate in Fig. 4(c) appeared almost epitaxial. Considering that the X-ray pole figures of ‘m-cut’ indicated much stronger peaks than that of the other two substrates, it is presumable that this near-epitaxial relation resulted in a highly textured structure for the former. Fig. 5 illustrates in-plane atomic configurations of Pd-doped SnO2 and Al2O3, determined from the experimental results. The marked repeating rectangular unit cells in the schematic diagram are used for calculating the directional lattice mismatches. Table 1 lists the directional lattice mismatches of the ‘r-cut’, ‘a-cut’ and ‘m-cut’ along the in-plane directions. In all cases, the directional lattice mismatch in any direction is much smaller than that in orthogonal direction. In fact, this excellent lattice matching in a single direction is one of the reasons for using sapphire as a substrate.
4. Conclusions The crystallographic orientations of Pd-doped SnO2 thin films deposited by RF magnetron sputtering on three types of sapphire substrates, ‘r-cut’, ‘a-cut’ and ‘m-cut’, were examined by X-ray pole figures and TEM. Although Pd-doped SnO2 thin films were polycrystalline with a columnar structure, all the films were highly oriented; (101) SnO2 films on the ‘r-cut’ (0112) substrate; (101) SnO2 films on the ‘a-cut’ (1120) substrate; (001) SnO2 films on the ‘m-cut’ (1010) substrate, which
Fig. 4. High-resolution TEM images of the SnO2:Pd film/sapphire substrate interface and electron diffraction patterns of the SnO2:Pd film and sapphire substrate for films grown on (a) r-cut, (b) a-cut, and (c) m-cut substrates. Interfaces are indicated with arrows in (b) and (c).
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Fig. 5. Schematic diagram of the in-plane atomic configuration of SnO2:Pd films grown on (a) r-cut, (b) a-cut, and (c) m-cut Al2O3 substrates.
is consistent with the previously reported results for undoped SnO2 [22]. Furthermore, all the films had highly preferred in-plane relationships; -[010]SnO2 || [100]Al2O3 and [101]SnO2 || [121]Al2O3 in the case of the ‘rcut’; [101]SnO2 || [110]Al2O3 and [010]SnO2 || [001]Al2O3 in the case of the ‘a-cut’; [100]SnO2 || [010]Al2O3 and [010]SnO2 || [001]Al2O3 in the case of the ‘m-cut’. While, the in-plane relationships of the Pd-doped SnO2 film on the ‘r-cut’ Al2O3 substrate were the same as those of the undoped epitaxial single crystal [16–18,22], the in-plane relationships for the other two substrates have been reported in the present investigation. Many grains and defects were found in films of all three cases, while the interface between the film and the substrate of the ‘mcut’ was exceptionally sharp and well defined. Acknowledgements This work was supported by grant No. (R01-2006-000-11071-0) from the Basic Research Program and by grant No (ROA-2007-000Table 1 In-plane relationship and lattice mismatch for the investigated SnO2 films grown on Al2O3 substrates In-plane d (Å) of rectangular relationship unit cell in the film [010]SnO2 || 4.74 [100]Al2O3 [101]SnO2 || 5.71 -[121]Al2O3 a-cut [101]SnO2 || 5.71 (101)||(112̄0) [110]Al2O3 [010]SnO2 || 14.22 [001]Al2O3 m-cut [100]SnO2 || 4.74 (001)||(101̄0) [010]Al2O3 [010]SnO2 || 14.22 [001]Al2O3 r-cut (101)||(011̄2)
d (Å) of rectangular unit cell in the substrate
Mismatch (%)
4.76
−0.42%
5.14
11.09%
5.78
−1.21%
13.01
9.30%
4.76
−0.42%
13.01
9.30%
10014-0) of the Korea Science and Engineering Foundation, Korea. SHH was supported by the NANO Systems Institute-National Core Research Center (NSI-NCRC) program of Korea Science and Engineering Foundation (KOSEF), Korea. References [1] A. Rosental, A. Tarre, A. Gerst, J. Sundqvist, A. Hårsta, A. Aidla, J. Aarik, V. Sammelselg, T. Uustare, Sens. Actuators B 93 (2003) 552. [2] Y. Tachibana, K. Hara, S. Takano, K. Sayama, H. Arakawa, Chem. Phys. Lett. 364 (2002) 297. [3] Y.-L. Chin, J.-C. Chou, T.-P. Sun, H.-K. Liao, W.-Y. Chung, S.-K. Hsiung, Sens. Actuators B 75 (2001) 36. [4] A. Nilgün Akın, G. Kılaz, A. İnci İşli, Z. İlsen Önsan, Eng. Sci. 56 (2001) 88. [5] N. Yamazoe, Sens. Actuators B 5 (1991) 7. [6] A. Kolmakov, D.O. Klenov, Y. Lilach, S. Stemmer, M. Moskovits, Nano Lett. 5 (2005) 667. [7] X.Q. Pan, L. Fu, J.E. Dominguez, J. Appl. Phys. 89 (2001) 6056. [8] H. Windischmann, P. Mark, J. Electrochem. Soc. 126 (1979) 627. [9] P.K. Clifford, D.T. Tuma, Sens. Actuators 3 (1983) 233. [10] S.R. Morrison, Sens. Actuators 11 (1987) 283. [11] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Sens. Actuators B 3 (1991) 147. [12] V. Demarne, A. Grisel, R. Sanjinés, D. Rosenfeld, F. Lévy, Sens. Actuators B 7 (1992) 704. [13] W.-T. Moon, K.-S. Lee, Y.-K. Jun, H.-S. Kim, S.-H. Hong, Sens. Actuators B 115 (2006) 123. [14] M. Egashira, Y. Shimizu, in: N. Yamazoe (Ed.), Chemical Sensor Technology, vol. 3, Kodansha, Tokyo, 1991, p. 1. [15] S. Semancik, R.E. Cavicchi, Thin Solid Films 206 (1991) 81. [16] J. Lu, J. Sundqvist, M. Ottosson, A. Tarre, A. Rosental, J. Aarik, A. Hårsta, J. Cryst. Growth 260 (2004) 191. [17] J. Sundqvist, J. Lu, M. Ottosson, A. Hårsta, Thin Solid Films 514 (2006) 63. [18] J. Sundqvist, M. Ottosson, A. Hårsta, Chem. Vap. Deposition 10 (2004) 77. [19] J.E. Dominguez, X.Q. Pan, L. Fu, P.A. Van Rompay, Z. Zhang, J.A. Nees, P.P. Pronk, J. Appl. Phys. 91 (2002) 1060. [20] J.E. Dominguez, L. Fu, X.Q. Pan, Appl. Phys. Lett. 79 (2001) 614. [21] R.E. Cavicchi, S. Semancik, M.D. Antonik, R.J. Lad, Appl. Phys. Lett. 61 (1992) 1921. [22] Y.-H. Choi, S.-H. Hong, Sens. Actuators B 125 (2007) 504. [23] Y. Suda, H. Kawasaki, J. Namba, K. Iwa tsuji, K. Doi, K. Wada, Surf. Coat. Technol. 174 (2003) 1293. [24] P.A. Morris Hotsenpiller, A. Roshko, J.B. Lowekamp, G.S. Rohrer, J. Cryst. Growth 174 (1997) 424.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f
Orientation relationship of polycrystalline Pd-doped SnO2 thin film deposits on sapphire substrates Ji-Hwan Kwon, Yun-Hyuk Choi, Dai Hong Kim, Myeong Yang, Jiyoung Jang, Tae Woong Kim, Seong-Hyeon Hong, Miyoung Kim ⁎ Department of Materials Science and Engineering, Seoul National University, Shillim-Dong, Kwanak-Gu, Seoul 151-744, Republic of Korea
A R T I C L E
I N F O
Article history: Received 23 August 2007 Received in revised form 2 April 2008 Accepted 22 June 2008 Available online 27 June 2008 Keywords: SnO2 thin films X-ray pole figures In-plane relationship Transmission electron microscopy
A B S T R A C T Pd-doped SnO2 thin films of rutile structure were deposited using radio-frequency magnetron sputtering method on sapphire substrates with three different orientations: r-cut (0112), a-cut (1120), and m-cut (1010). The deposited films were characterized using transmission electron microscopy and X-ray pole figures. All the deposited films were polycrystalline but highly oriented with the interface, and also had in-plane or nearepitaxial relationships were observed for the SnO2 films grown on all three cases of sapphire substrates. In particular, (101), (101), and (001) oriented films were respectively grown on r-, a-, and m-cut sapphire substrates. The in-plane orientation relationships were determined to be [010]SnO2 || [100]Al2O3 and [101] -SnO2 || [121]Al2O3 (r-cut), [101]SnO2 || [110]Al2O3 and [010]SnO2 || [001]Al2O3 (a-cut), and [100]SnO2 || [010] Al2O3 and [010]SnO2 || [001]Al2O3 (m-cut). A highly textured structure was observed for the films grown on m-cut substrate. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor metal oxide materials have been studied extensively for their wide variety of applications such as gas sensors [1], dye-sensitized solar cells [2], ion-sensitive field effect transistors [3], and catalysts [4]. In particular, gas sensors based on SnO2 have been widely used because of their high sensitivity for various gases including H2, CO, and CH4. The hydrogen gas sensing performance of SnO2 has been greatly enhanced by increasing the surface-to-volume ratio through the formation of nanostructures or by impregnating a small amount of noble metals [5,6]. The efficiency of gas sensors depend basically on their electrical transport properties [7–10], which are related to chemical bonding states and potential barriers at the grain boundaries. In order to evaluate the efficiency of sensor materials and explore further applications, it is therefore important to analyze various microstructural features like grain size, neck size, interface, and crystal orientations [11–13]. However, the reported literature on the influence of crystal orientation was limited to whiskers and epitaxial films [14,15]. Moreover, the epitaxial relationships of SnO2 thin films on sapphire have been reported only on ‘r-cut’ (0112) and ‘c-cut’ (0001) substrates [16–21]. The radio-frequency (RF) magnetron sputtering is one of the potential methods to deposit polycrystalline or epitaxial SnO2 films on
⁎ Corresponding author. Tel.: +82 2 880 9239; fax: +82 2 884 1413. E-mail address: [email protected] (M. Kim). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.06.074
sapphire substrates [15,22]. Choi and Hong [22] used RF magnetron sputtering to deposit undoped SnO2 films on ‘r-cut’ (0112) substrate. Their deposited films were oriented in (101) plane, perpendicular to the interface and had in-plane relationship of [010]SnO2 || [100]Al2O3 -and [101]SnO2 || [121] Al2O3. Similar in-plane relationship was also reported in case of epitaxial films, grown using atomic layer deposition [16,17]. The deposited SnO2 films on the ‘a-cut’ (1120) and ‘m-cut’ (1010) Al2O3 were highly textured and appeared to have in-plain orientation, but no orientation relationship was determined. In the present study, the Pd-doped SnO2 thin films were deposited on three different sapphire substrates (r-cut, a-cut, and m-cut) using RF magnetron sputtering method and the developed interface was subjected to detailed microstructural characterization using transmission electron microscopy (TEM) and X-ray pole-figure techniques. Thus, the major objective of the present investigation is to evaluate inplane orientation relationships of Pd-doped SnO2 thin films with the substrates for all three cases; (0112) (r-cut), (1120) (a-cut), and (1010) (m-cut) Al2O3. 2. Experimental details In the present study, three types of sapphire substrates were used: ‘rcut’ (0112), ‘a-cut’ (1120), and ‘m-cut’ (1010). Pd (0.5 wt.%)-doped SnO2 target was used to deposit SnO2 thin films by RF magnetron sputtering. The deposition process was performed in an argon atmosphere with a RF power of 100 W at a working pressure of 3.3 Pa for 15–30 min. The temperature was maintained at 350 °C during the deposition process. The
J.-H. Kwon et al. / Thin Solid Films 517 (2008) 550–553
551
deposited films were subjected to subsequent heat treatment at 600 °C in air in order to enhance the crystallinity of the films. The presence of various phases and orientations of the films was examined by X-ray diffraction (XRD, Model M18XHF-SRA, Mac Science, Japan) using Cu Kα radiation (λ=0.154 nm) and X-ray pole figures (X'pert Pro, PANalytical, The Netherlands). Pole figures were measured in Schulz reflection geometry by scanning the tilt angle of goniometer, χ (chi), in the range of 0–85° and the azimuthal angle, ϕ (phi), in the range of 0–360°, using a 5° step. For the TEM investigation, cross-sectional specimens were prepared by mechanical polishing and subsequent Ar ion milling at 5 kV. The samples were examined using JEOL-3000F and TECNAI F20 transmission electron microscopes, operating at 300 kV and 200 kV, respectively. 3. Results and discussion Tin dioxide has rutile-type structure with space group of P42/mnm and lattice constants of a = 4.737 Å, c = 3.185 Å. The sapphire substrate has hexagonal structure, a = 4.763 Å, c = 13.003 Å with space group of R3̄c (No.167). Fig. 1 shows the XRD patterns of the Pd-doped SnO2 films deposited on the various sapphire substrates. Only (101), (101), and (001) diffraction peaks were observed in Pd-doped SnO2 film grown on (0112) (r-cut), (1120) (a-cut), and (1010) (m-cut) Al2O3, respectively and the deposited Pd-doped SnO2 films were highly oriented perpendicular to the substrate interface. These orientations were the same as that reported for the undoped SnO2 films [22], and a small amount of Pd doping had little influence on the structures of deposited SnO2 films [23]. Further, a closer examination of the XRD pattern in case of film deposited on ‘m-cut’ substrate indicated the additional presence of a weak peak of (301) at 66°. Therefore, it can be stated that the Pd-doped SnO2 film on the ‘m-cut’ substrate was strongly (001) oriented with a small amount of the (301) orientation. The TEM image in Fig. 2 shows a cross-sectional view of the Pddoped SnO2 film grown on the ‘r-cut’ sapphire substrate, indicating a columnar structure of the films. Each column was approximately 10 nm in width and 100 nm in height. All three types (r-cut, a-cut and m-cut) of Pd-doped SnO2 films reveal similar columnar structure. Fig. 3 shows the {101} pole figures of the Pd-doped SnO2 films. The substrate orientations are indicated next to the corresponding pole figures. The pole figures of the films on the ‘r-cut’ and ‘a-cut’ substrates were similar, where reflections from the {101} planes [(101), (011), (101), and (011)] were only observed with an additional contribution from the 180° rotation of the (101) plane, which was marked as T1 in the figures. This means that there exist in-plane orientation relationships between the films and the substrates, even though the deposited films are polycrystalline. The in-plane relationship of the Pd-doped (101) SnO2 film on the ‘r-cut’ substrate was [010]SnO2 || [100] Al2O3 and [101]SnO2 ||
Fig. 1. X-ray diffraction (XRD) patterns of the Pd-doped SnO2 films grown on the ‘r-cut’, ‘a-cut’, and ‘m-cut’ substrates.
Fig. 2. TEM image of the SnO2:Pd films grown on an ‘r-cut’ sapphire substrate.
-[121]Al2O3, which is consistent with the previously reported epitaxial relationship of undoped SnO2 [16–18,22]. The in-plane orientation relationship of Pd-doped (101) SnO2 film on the ‘a-cut’ substrate was found to be [101]SnO2 || [110]Al2O3 and [010]SnO2 || [001]Al2O3. In the
Fig. 3. X-ray pole figures from the SnO2 films deposited on the ‘r-cut’, ‘a-cut’, and ‘m-cut’ substrates.
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pole figure of the Pd-doped (001) SnO2 film on the ‘m-cut’ substrate, strong {101} planes were observed along with very weak reflections from the {301} planes marked as A1, A2, A3, and A4. The in-plane relationship of the main orientations in the Pd-doped (001) SnO2 film on the ‘m-cut’ substrate was determined to be [100]SnO2 || [010]Al2O3 and [010]SnO2 || [001]Al2O3. While heteroepitaxial relationships were reported previously in (101) and (001) rutile TiO2 films on ‘a-cut’ and ‘m-cut’ Al2O3 substrates [24], the in-plane orientation relationships were observed in the present investigation of Pd-doped SnO2 films deposited on ‘a-cut’ and ‘m-cut’ Al2O3 substrates. Based on these observations, the polycrystalline Pd-doped SnO2 films in this study were highly textured, close to the epitaxial films. The highly textured Pd-doped SnO2 films were further examined using high-resolution transmission electron microscopy (HRTEM). Fig. 4 shows the HRTEM images at the SnO2/Al2O3 interface as well as the electron diffraction patterns of the film and substrate in the three cases: (a) r-cut, (b) a-cut, and (c) m-cut. The HRTEM images reveal many defects and grains, particularly in the vicinity of the interface. The grains were a few tens of nanometers in diameter, and mainly formed low angles with each other. The interface between the Pddoped SnO2 thin films and Al2O3 substrates in the ‘a-cut’ was quite rough, while the ‘r-cut’ and the ‘m-cut’ films had a rather well defined interface, despite of their polycrystalline nature. The ‘m-cut’ films, in particular, had an atomistically sharp interface. The electron diffraction patterns clearly demonstrate the polycrystalline phases. Although the films were highly oriented with dominant peaks in all three cases, many additional peaks out of the main domains could be easily seen. The diffraction patterns of films on the ‘r-cut’ and ‘a-cut’ showed weak rings, indicating the presence of randomly oriented small-sized domains. The major diffraction peaks of all three types of films were elongated, confirming the existence of misorientations of a few degrees in the high-resolution images. It was difficult to identify a second phase due to Pd or the segregation of Pd into the grain boundary in the HRTEM images. However, the electron diffraction patterns indicated few weak tetragonal PdO (110) spots with d-spacing of 2.15 Å, as marked in Fig. 4(b), in all three samples. The appearance of the PdO phase in the films is consistent with previously reported results [23]. The sizes of PdO domains, however, were too small to be detected in the X-ray diffraction patterns, judging from the intensities of those peaks in the electron diffraction patterns. The in-plane relationships of the three cases were obtained on the basis of the electron diffraction patterns. In the case of the ‘r-cut’ substrate, the film was oriented in (101) plane, perpendicular to the interface and with in-plane relationship of [010]SnO2 || [100]Al2O3 and -[101]SnO2 || [121]Al2O3, which is again consistent with the results reported for the undoped SnO2 thin films [16–18,22]. On the other hand, the SnO2 films grown on the ‘a-cut’ and ‘m-cut’ Al2O3 substrates were oriented in (101) and (001) planes, perpendicular to the interface and their in-plane relationship between the films and substrates was [101]SnO2 || [110]Al2O3 and [010]SnO2 || [001]Al2O3 in the case of the ‘a-cut’, and [100]SnO2 || [010]Al2O3 and [010]SnO2 || [001]Al2O3 in the case of the ‘m-cut’. These results are consistent with the X-ray polefigure results. It should be noted that the interface of the ‘m-cut’ substrate in Fig. 4(c) appeared almost epitaxial. Considering that the X-ray pole figures of ‘m-cut’ indicated much stronger peaks than that of the other two substrates, it is presumable that this near-epitaxial relation resulted in a highly textured structure for the former. Fig. 5 illustrates in-plane atomic configurations of Pd-doped SnO2 and Al2O3, determined from the experimental results. The marked repeating rectangular unit cells in the schematic diagram are used for calculating the directional lattice mismatches. Table 1 lists the directional lattice mismatches of the ‘r-cut’, ‘a-cut’ and ‘m-cut’ along the in-plane directions. In all cases, the directional lattice mismatch in any direction is much smaller than that in orthogonal direction. In fact, this excellent lattice matching in a single direction is one of the reasons for using sapphire as a substrate.
4. Conclusions The crystallographic orientations of Pd-doped SnO2 thin films deposited by RF magnetron sputtering on three types of sapphire substrates, ‘r-cut’, ‘a-cut’ and ‘m-cut’, were examined by X-ray pole figures and TEM. Although Pd-doped SnO2 thin films were polycrystalline with a columnar structure, all the films were highly oriented; (101) SnO2 films on the ‘r-cut’ (0112) substrate; (101) SnO2 films on the ‘a-cut’ (1120) substrate; (001) SnO2 films on the ‘m-cut’ (1010) substrate, which
Fig. 4. High-resolution TEM images of the SnO2:Pd film/sapphire substrate interface and electron diffraction patterns of the SnO2:Pd film and sapphire substrate for films grown on (a) r-cut, (b) a-cut, and (c) m-cut substrates. Interfaces are indicated with arrows in (b) and (c).
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553
Fig. 5. Schematic diagram of the in-plane atomic configuration of SnO2:Pd films grown on (a) r-cut, (b) a-cut, and (c) m-cut Al2O3 substrates.
is consistent with the previously reported results for undoped SnO2 [22]. Furthermore, all the films had highly preferred in-plane relationships; -[010]SnO2 || [100]Al2O3 and [101]SnO2 || [121]Al2O3 in the case of the ‘rcut’; [101]SnO2 || [110]Al2O3 and [010]SnO2 || [001]Al2O3 in the case of the ‘a-cut’; [100]SnO2 || [010]Al2O3 and [010]SnO2 || [001]Al2O3 in the case of the ‘m-cut’. While, the in-plane relationships of the Pd-doped SnO2 film on the ‘r-cut’ Al2O3 substrate were the same as those of the undoped epitaxial single crystal [16–18,22], the in-plane relationships for the other two substrates have been reported in the present investigation. Many grains and defects were found in films of all three cases, while the interface between the film and the substrate of the ‘mcut’ was exceptionally sharp and well defined. Acknowledgements This work was supported by grant No. (R01-2006-000-11071-0) from the Basic Research Program and by grant No (ROA-2007-000Table 1 In-plane relationship and lattice mismatch for the investigated SnO2 films grown on Al2O3 substrates In-plane d (Å) of rectangular relationship unit cell in the film [010]SnO2 || 4.74 [100]Al2O3 [101]SnO2 || 5.71 -[121]Al2O3 a-cut [101]SnO2 || 5.71 (101)||(112̄0) [110]Al2O3 [010]SnO2 || 14.22 [001]Al2O3 m-cut [100]SnO2 || 4.74 (001)||(101̄0) [010]Al2O3 [010]SnO2 || 14.22 [001]Al2O3 r-cut (101)||(011̄2)
d (Å) of rectangular unit cell in the substrate
Mismatch (%)
4.76
−0.42%
5.14
11.09%
5.78
−1.21%
13.01
9.30%
4.76
−0.42%
13.01
9.30%
10014-0) of the Korea Science and Engineering Foundation, Korea. SHH was supported by the NANO Systems Institute-National Core Research Center (NSI-NCRC) program of Korea Science and Engineering Foundation (KOSEF), Korea. References [1] A. Rosental, A. Tarre, A. Gerst, J. Sundqvist, A. Hårsta, A. Aidla, J. Aarik, V. Sammelselg, T. Uustare, Sens. Actuators B 93 (2003) 552. [2] Y. Tachibana, K. Hara, S. Takano, K. Sayama, H. Arakawa, Chem. Phys. Lett. 364 (2002) 297. [3] Y.-L. Chin, J.-C. Chou, T.-P. Sun, H.-K. Liao, W.-Y. Chung, S.-K. Hsiung, Sens. Actuators B 75 (2001) 36. [4] A. Nilgün Akın, G. Kılaz, A. İnci İşli, Z. İlsen Önsan, Eng. Sci. 56 (2001) 88. [5] N. Yamazoe, Sens. Actuators B 5 (1991) 7. [6] A. Kolmakov, D.O. Klenov, Y. Lilach, S. Stemmer, M. Moskovits, Nano Lett. 5 (2005) 667. [7] X.Q. Pan, L. Fu, J.E. Dominguez, J. Appl. Phys. 89 (2001) 6056. [8] H. Windischmann, P. Mark, J. Electrochem. Soc. 126 (1979) 627. [9] P.K. Clifford, D.T. Tuma, Sens. Actuators 3 (1983) 233. [10] S.R. Morrison, Sens. Actuators 11 (1987) 283. [11] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Sens. Actuators B 3 (1991) 147. [12] V. Demarne, A. Grisel, R. Sanjinés, D. Rosenfeld, F. Lévy, Sens. Actuators B 7 (1992) 704. [13] W.-T. Moon, K.-S. Lee, Y.-K. Jun, H.-S. Kim, S.-H. Hong, Sens. Actuators B 115 (2006) 123. [14] M. Egashira, Y. Shimizu, in: N. Yamazoe (Ed.), Chemical Sensor Technology, vol. 3, Kodansha, Tokyo, 1991, p. 1. [15] S. Semancik, R.E. Cavicchi, Thin Solid Films 206 (1991) 81. [16] J. Lu, J. Sundqvist, M. Ottosson, A. Tarre, A. Rosental, J. Aarik, A. Hårsta, J. Cryst. Growth 260 (2004) 191. [17] J. Sundqvist, J. Lu, M. Ottosson, A. Hårsta, Thin Solid Films 514 (2006) 63. [18] J. Sundqvist, M. Ottosson, A. Hårsta, Chem. Vap. Deposition 10 (2004) 77. [19] J.E. Dominguez, X.Q. Pan, L. Fu, P.A. Van Rompay, Z. Zhang, J.A. Nees, P.P. Pronk, J. Appl. Phys. 91 (2002) 1060. [20] J.E. Dominguez, L. Fu, X.Q. Pan, Appl. Phys. Lett. 79 (2001) 614. [21] R.E. Cavicchi, S. Semancik, M.D. Antonik, R.J. Lad, Appl. Phys. Lett. 61 (1992) 1921. [22] Y.-H. Choi, S.-H. Hong, Sens. Actuators B 125 (2007) 504. [23] Y. Suda, H. Kawasaki, J. Namba, K. Iwa tsuji, K. Doi, K. Wada, Surf. Coat. Technol. 174 (2003) 1293. [24] P.A. Morris Hotsenpiller, A. Roshko, J.B. Lowekamp, G.S. Rohrer, J. Cryst. Growth 174 (1997) 424.