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The influence of annealing atmosphere on the material properties of sol–gel derived SnO2:Sb films before and after annealing
Applied Surface Science 258 (2012) 5981–5986
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The influence of annealing atmosphere on the material properties of sol–gel derived SnO2 :Sb films before and after annealing Jiann-Shing Jeng ∗ Department of Materials Science and Engineering, Far East University, Tainan 744, Taiwan
a r t i c l e
i n f o
Article history: Received 24 July 2011 Received in revised form 3 February 2012 Accepted 4 February 2012 Available online 3 March 2012 Keywords: Sol–gel SnO2 :Sb Resistivity Mobility
a b s t r a c t SnO2 films with and without Sb doping were prepared by the sol–gel spin-coating method. Material properties of the SnO2 films with different Sb contents were investigated before and after annealing under O2 or N2 . When SnO2 films are annealed under N2 or O2 , the resistivity decreases with increasing annealing temperature, which may be related to the increased crystallinity and reduced film defects. The intensity of SnO2 peaks for both O2 - and N2 -annealed films increases as the annealing temperature increases. Small nodules are revealed on the surface of SnO2 films after annealing in N2 or O2 atmospheres, and some voids are present on the surface of N2 -annealed SnO2 films. After doping with Sb, the resistivity of SnO2 films after annealing in O2 is greater than that of N2 -annealed SnO2 films. The surface morphology of SnO2 films incorporating different molar ratios of Sb after annealing are similar to that of as-spun SnO2 films with adding Sb. There were no voids found on the surfaces of N2 -annealed SnO2 :Sb films. In addition, the peak intensity of SnO2 :Sb films after O2 -annealing is higher than those films after N2 -annealing. The chemical binding states and Hall mobility of the high-temperature annealed SnO2 films without and with adding Sb are also related to the annealing atmospheres. This study discusses the connection among the material properties of the SnO2 films with different Sb contents and how these properties are influenced by the Sb-doping concentration and the annealing atmospheres of SnO2 films. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Transparent conducting oxides (TCOs) have recently been employed as transparent electrodes in a variety of optoelectronic devices such as flat-panel displays, light emitting diodes, and solar cells [1,2]. This is due to their high conductivity and high transparence in the visible region. Indium tin oxide (ITO) is the most widely used transparent conductor in the industry, because of its unique characteristics of high visible transmittance (90%), high conductivity, and high infrared reflectance. However, ITO is brittle and the indium used in ITO is relatively scarce. Thus, it is important to find an adaptive material to replace ITO. Tin dioxide (SnO2 ), which has a wide band gap (Eg = 3.6 eV), exhibits good optical, electrical, and chemical properties, and so is considered as an attractive candidate for replacing ITO [3,4]. In particular, Sb doped SnO2 films (SnO2 :Sb films) exhibit excellent properties, such as low resistance, thermal stability, high transparency in the visible region, and high reflectivity for infrared radiation [5], making them more attractive
∗ Fax: +886 6 5977909. E-mail address: [email protected] 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.02.010
than other SnO2 -based films. There are many deposition techniques used to fabricate SnO2 :Sb films on glass substrates, such as pulsedlaser deposition (PLD) [6], plasma-enhanced chemical deposition (PECVD) [7], and reactive rf magnetron sputtering [8]. Compared with conventional vacuum-based techniques, the sol–gel method is relatively simple and inexpensive, and it has two main advantages. Its stoichiometry is easy to control and adjust, and it can be processed under low temperatures [9]. This study fabricated SnO2 films with and without adding Sb using the sol–gel process method. In particular, this study involved a series of studies related to resistivity, surface morphology, crystal structures, bonding configuration, and mobility of the SnO2 film with and without adding Sb. Several studies [10–12] have been conducted to investigate the variation of SnO2 properties with different amounts of Sb prepared by solution process. But, the correlation between material properties of SnO2 and the effects of adding various amounts of Sb and the annealing atmosphere is complex and still not well understood. In the present study, SnO2 films with different contents of Sb before and after annealing in oxygen and in nitrogen atmospheres were fabricated by the sol–gel spin-coating method. The characteristics of SnO2 films with and without the addition of different
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a annealed in O2
-3
(301)
(310)
(201)
-3
1.5x10
(211)
2.0x10
(101)
-3
(110)
O2 ambient N2 ambient
Intensity (a. u.)
Resistivity (ohm-cm)
2.5x10
o
500 C o
400 C
1.0x10
o
300 C
0
100
200
300
400
as-dep.
500
o
Annealing Temperature ( C) 20
30
40
Fig. 1. Resistivity of SnO2 films without adding Sb as a function of the annealing temperature in O2 and N2 ambients.
b
3. Results and discussion Fig. 1 shows the resistivity of SnO2 films without the addition of Sb as a function of the annealing temperature for O2 and N2 ambients. For SnO2 films after annealing in N2 and in O2 ambients,
70
80
annealed in N 2
(301)
(310)
(201)
(211)
(101)
Intensity (a. u.)
2. Experimental procedure SnO2 :Sb composite film with thickness of 150 nm was prepared by using a sol–gel process in combination with the spin-coating method. The tin chloride pentahydrate (SnCl4 ·5H2 O) and antimony (III) acetate [Sb(OAc)3 ] were used as precursors for preparing the SnO2 :Sb films. Then SnCl4 ·5H2 O and Sb(OAc)3 were dissolved in ethylene glycol monomethyl ether to form two pre-solutions. For the SnO2 sols solution, different amounts of SnCl4 ·5H2 O (according to the Sb/Sn molar ratio) were first mixed with 30 ml ethylene glycol monomethyl ether, and then stirred with a magnetic stirrer for 1 h at room temperature. The Sb(OAc)3 (1 mmol) was hydrolyzed in ethylene glycol monomethyl ether (30 ml) and stirred for 2 h. The pH value of the Sb(OAc)3 pre-solutions were 3, as adjusted by adding suitable amounts of HCl (0.1 N and 0.15 ml). The main function of the HCl is to act as a catalyst to accelerate the hydrolysis reaction. The two pre-solutions were mixed to various Sb/Sn molar ratios (i.e., 0%, 3%, 5%, and 10%) and stirred for 1 h. The mixed solutions were spin-coated on cleaned quartz glass substrates. After spin-coating, the films were baked at 60 ◦ C for 5 min. Finally, the coatings were annealed in flowing O2 and N2 atmospheres [150 sccm (standard cubic centimeter per minute)] in a horizontal quartz-tube furnace at 300 ◦ C, 500 ◦ C and 700 ◦ C for 30 min. The film resistivity was calculated from the sheet resistance measured by a four-point probe. The surface morphology and the film thickness of the SnO2 films with different concentrations of Sb were investigated using field emission scanning electron microscopy (FE-SEM, Philips XL-40FEG). The characteristic crystal phases of the SnO2 films with different contents of Sb were identified using Cu K␣ in glancing-incident angle X-ray diffraction (GIAXRD, Rigaku D/MAX2500) with an incident angle of 2◦ . Surface analysis was examined by X-ray photoelectron spectroscopy (XPS, VG ESCA-210) with a monochromatic Al K␣ source. The mobility of the SnO2 films with and without adding Sb was determined by Hall measurement.
60
Two theta (degrees)
(110)
amounts of Sb were analyzed using various methods, and the correlation of characteristics according to Sb contents and annealing atmospheres on the properties of SnO2 films is discussed.
50
o
500 C o
400 C o
300 C as-dep.
20
30
40
50
60
70
80
Two theta (degrees) Fig. 2. GIAXRD patterns of SnO2 films before and after annealing in (a) O2 and (b) N2 ambients, respectively.
the resistivity decreases with increasing annealing temperature. Similar results have been reported by Lee and Hong [13]. The crystallinity of SnO2 films generally increases with increasing annealing temperature [14]. In addition, film atoms are more mobile after annealing, which fills voids or vacancies in SnO2 films. Thus film defects and/or porosities can be eliminated with increasing annealing temperature. Therefore, the reduced resistivity of SnO2 films may be related to the increased crystallinity and diminished film defects. The reason that the resistivity of O2 -annealed samples is greater than that of N2 -annealed samples will be discussed later. To determine phases are present in the samples before and after annealing under different atmospheres, we analyzed the samples by GIAXRD. Fig. 2 shows the GIAXRD patterns of SnO2 films before and after annealing in different ambients. The GIAXRD patterns of as-deposited films do not display any characteristic peak, indicating that all as-deposited films are amorphous. After annealing at 300 ◦ C in O2 , diffraction peaks corresponding to the SnO2 phase start to appear (i.e., SnO2 phase precipitates). At the same time, the SnO2 peaks for all O2 -annealed films become more intense as the annealing temperature increases up to 500 ◦ C. On the other hand, the SnO2 phase is found in the pattern of N2 -annealed SnO2 films at 400 ◦ C. Furthermore, the intensity of SnO2 peaks for N2 -annealed samples increases with increasing annealing temperature, indicating the increased crystallinity of SnO2 after annealing. The XRD results
J.-S. Jeng / Applied Surface Science 258 (2012) 5981–5986
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Fig. 3. Surface morphology of SnO2 films: (a) as-deposited, and after annealing at (b) 300 ◦ C in O2 , (c) 300 ◦ C in N2 , (d) 400 ◦ C in O2 , (e) 400 ◦ C in N2 , (f) 500 ◦ C in O2 , and (g) 500 ◦ C in N2 .
indicate that an O2 atmosphere can enhance the precipitation of SnO2 phase in comparison with N2 ambient. Fig. 3 shows the surface morphology of SnO2 films before and after annealing in N2 and in O2 atmospheres. Before annealing, the sol–gel derived SnO2 films exhibit smooth and continuous morphology when observed by SEM. There are small nodules revealed on the surface of SnO2 films after annealing at 300 ◦ C under N2 and under O2 . The surface morphology of annealed films changes from small nodules on surface for films after annealing at 300 ◦ C to large nodules for samples after annealing up to 500 ◦ C, as shown in
Fig. 3(b)–(g). It is noteworthy that some voids begin to appear on the surface of SnO2 after annealing at 300 ◦ C in N2 , and the voids then enlarge after annealing up to 500 ◦ C. Material properties of SnO2 films after the addition of Sb are discussed below. Fig. 4 presents the resistivity of SnO2 films incorporating different molar ratios of Sb before and after annealing at 500 ◦ C in O2 and in N2 atmospheres. After annealing, the resistivity of SnO2 films incorporating different molar ratios of Sb is greater than that of as-deposited samples. Compared with SnO2 films incorporating 3% of Sb, the resistivity of SnO2 films incorporating
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Resistivity (ohm-cm)
1.8x10
As-dep. o 500 C in O2
-3
1.5x10
o
500 C in N2
-3
1.2x10
-4
9.0x10
-4
6.0x10
2
4
6
8
10
Sb-doping concentration (mol%) Fig. 4. Resistivity of SnO2 films incorporating different molar ratios of Sb before and after annealing at 500 ◦ C in O2 and N2 ambients.
5% of Sb decreases before and after annealing at 500 ◦ C in O2 and in N2 , as shown in Fig. 4. On the other hand, the resistivity of SnO2 films incorporating 10% of Sb increases before and after annealing at 500 ◦ C in O2 and in N2 . In addition, the resistivity of SnO2 films
incorporating different molar ratios of Sb after annealing in O2 is larger than that of N2 -annealed SnO2 films incorporating different molar ratios of Sb. This may be caused by elimination of oxygen vacancy due to the SnO2 films incorporating different molar ratios of Sb after annealing in O2 , which will be discussed later with the XPS results. The SnO2 films with and without different molar ratios of Sb are considered to be n-type semiconductors exhibiting oxygen vacancies that behave as donor impurities. The decrease in oxygen vacancies of SnO2 films incorporating different molar ratios of Sb after annealing in O2 is related to the larger resistivity of O2 -annealed SnO2 films with different molar ratios of Sb. Fig. 5 shows the surface morphology of SnO2 films incorporating 5% molar ratios of Sb before and after annealing in N2 and in O2 atmospheres. The surface morphology of SnO2 films incorporating Sb is similar to that of as-deposited SnO2 films without Sb incorporated. In addition, the surface morphology of SnO2 films incorporating 5% molar ratios of Sb after annealing present a morphology similar to as-deposited SnO2 films. In addition, there were no voids revealed on the N2 -annealed film surface. This indicates that the SnO2 films incorporating Sb can withstand higher temperatures to prevent void formation than SnO2 films without adding Sb.
Fig. 5. Surface morphology of SnO2 incorporating 10% Sb molar ratio (a) as-deposited, and after annealing at (b) 500 ◦ C in O2 , (c) 500 ◦ C in N2 , (d) 700 ◦ C in O2 , and (e) 700 ◦ C in N2 .
J.-S. Jeng / Applied Surface Science 258 (2012) 5981–5986
o
700 C (O2) o
700 C (N2)
Intensity (cps)
(310)
(301)
16000
(211)
(201)
Intensity (a. u.)
(101)
(110)
a
Peak sum Background
12000
SnO2 SnOx
8000
H2O
o
o
o
700 C in N2 Original
500 C (O2) 500 C (N2)
5985
4000 545
as-dep.
540
535
530
525
Binding energy (eV) 20
30
40
50
60
70
80 20000
Fig. 6. GIAXRD patterns of SnO2 films with 5% molar ratio of Sb added before and after annealing in (a) O2 and (b) N2 atmospheres, respectively.
16000
Fig. 6 shows the GIAXRD patterns of SnO2 films incorporating 5% Sb before and after annealing in different ambients. The GIAXRD pattern of as-deposited film does not reveal any characteristic peak, indicating that the as-deposited film is amorphous. After annealing, however diffraction peaks of the SnO2 phase are present. Furthermore, intensity of the SnO2 peak for film increases as the annealing temperature rises to 700 ◦ C. It should be noted that the peak position of SnO2 film with a 5% molar ratio of Sb shifts after annealing from its position for SnO2 film without adding Sb. This peak shift may be due to the substitution of atoms in the lattice. In general, the effect of nitrogen annealing on SnO2 films is similar to that of oxygen annealing, which leads to crystallization of the as-deposited SnO2 films. Furthermore, the effect of nitrogen annealing on SnO2 films leads to a partial reconstruction of the local oxygen deficiency. The main difference between N2 -annealed and O2 -annealed effects on SnO2 films is the degree of phase transformations. That is, phase transformation of SnO2 films, with and without adding Sb, annealed under an O2 atmosphere at 700 ◦ C occurs more completely than SnO2 films annealed under a N2 atmosphere at the same temperature. Therefore, the peak intensity of samples after O2 -annealing is higher than that of samples after N2 -annealing. A similar result can be observed in the SnO2 films without adding Sb (see Fig. 2). Chemical binding states of the 700 ◦ C-annealed SnO2 films without adding Sb and with a 5% molar ratio of Sb were examined by XPS, and the O 1s spectra of the two types of samples are shown in Figs. 7 and 8. Before XPS, the samples were transferred to a storage box with a relative humidity of 30% and maintained for 24 h. Then the films were measured with XPS. For XPS analysis, no attempt was made to sputter clean the samples in order to avoid sputterinduced chemical changes on the surface. In addition, all binding energies of various core levels were determined for the charging effect with respect to the adventitious C 1s peak at 284.6 eV. All spectra were deconvoluted and are shown in Figs. 7 and 8. The XPS O 1s spectra of the 700 ◦ C-annealed SnO2 under N2 and O2 were deconvoluted into three components (i.e., lattice oxygen in SnO2 , oxygen inside nonstoichiometric SnOx , and oxygen in water molecules adsorbed on the film surface). The fitted results are also shown in the figures. In addition, the binding energy values of Sb 3d5/2 and O 1s peaks of SnO2 and/or SnO are very close, making the deconvolution of the individual contributions difficult, as shown in Fig. 8. The parameters used for deconvolution of all spectra include the intensity ratio of Sb3d5/2 /Sb3d3/2 = 1.5. The peaks of Sb 3d5/2 and 3d3/2 are at around 530.7 and 540 eV with a separation of about
Intensity (cps)
Two theta (degrees)
b
o
700 C in O2 Original Peak sum Background
12000
SnO2 SnOx
8000
H2O
4000 545
540
535
530
525
Binding energy (eV) Fig. 7. XPS spectra of SnO2 films after annealing at 700 ◦ C in (a) N2 and (b) O2 atmospheres.
9.3 eV (spin orbit splitting) [15]. As compared with the SnO2 films after annealing at 700 ◦ C in N2 , the ratio of the SnOx to SnO2 area for 700 ◦ C-annealed sample under O2 decreases from 3.24 to 2.16. Based on XRD results, the samples at high-temperature annealing in O2 have a greater tendency to be oxidized than those in N2 atmosphere. This result indicates the oxygen deficiency of SnO2 films without adding Sb can be eliminated after annealing in O2 atmospheres, leading to a small ratio of the SnOx to SnO2 area for the O2 -annealed sample. In addition, the peak area of O (1s) in water molecules adsorbed on the SnO2 after annealing in N2 is larger than that for samples after annealing in O2 . SEM results show some voids present on the surface of SnO2 films after annealing in N2 atmosphere. In general, water can be adsorbed by the voids in a material, which results in the enhancement of peak area of O (1s) in water molecules (Fig. 7). On the other hand, the bonding configuration of SnO2 films with a 5% molar ratio of Sb after annealing in different atmospheres is shown in Fig. 8. The binding energy of Sb 3d5/2 for samples after annealing in N2 and O2 (Fig. 8) is at 530.6–530.9 eV, indicating that all antimony detected was in a pentavalent state (Sb5+ ) [16]. Similar to the SnO2 films without adding Sb, the XPS O (1s) peak area of SnO2 films with 5% Sb added after annealing in O2 related to water adsorption is less than that of SnO2 films with 5% Sb added after annealing in N2 . At the same time, the ratio of the SnOx to SnO2 peak area for the 700 ◦ C-annealed sample in N2 continually decreases to 0.59, implying that the addition of Sb in SnO2 can eliminate oxygen deficiency in SnO2 films. After annealing at 700 ◦ C in O2 , the XPS spectra show a O (1s) peak corresponding to the SnO2 , as shown in
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J.-S. Jeng / Applied Surface Science 258 (2012) 5981–5986
20000
a
is about 0.75 cm2 V−1 s−1 . The mobility value is determined by the interaction between the various scattering centers and free carriers. The oxygen deficiency of SnO2 with Sb after annealing at 700 ◦ C in O2 disappears, implying that scattering centers decrease. Simultaneously, the crystallinity of SnO2 samples with Sb after O2 annealing is higher than that of samples after N2 -annealing (see Fig. 6). As can be seen, the mobility of SnO2 with a 5% molar ratio of Sb is related to the oxygen deficiency as well as the crystallinity of films. Thus, sol–gel derived SnO2 :Sb films with controlled morphology, crystallinity, bonding configuration, and mobility can be manufactured after post-O2 -annealing and post-N2 -annealing.
o
700 C in N2 Original Peak sum
Intensity (cps)
16000
Background Sb2O5
12000
SnO2 SnOx
8000
H2O
4. Conclusion
4000 550
545
540
535
530
525
520
Binding energy (eV)
b
o
700 C in O2 Original
16000
Intensity (cps)
Peak sum Background
12000
Sb2O5 SnO2 H2O
8000
4000 545
540
535
530
525
520
Binding energy (eV) Fig. 8. XPS spectra of SnO2 films incorporating 5% molar ratios of Sb after annealing at 700 ◦ C in (a) N2 and (b) O2 atmospheres.
Fig. 8(b). The process of substituting of Sb ions can be considered as [17]: •
Sb2 O5 → 2SbSn + 2e + 4OO
X
1 + O2(g) 2
where SbSn • is the Sb ion occupying the site of the Sn ion with one positive charge, and OO X is the oxygen element in its position without any net charge. According to the equation, the production of oxygen gas takes place after doping with antimony in SnO2 films, which assists the oxidation reaction of SnOx to SnO2 . Thus, the ratio of the SnOx to SnO2 peak area for the 700 ◦ C-annealed sample in N2 is less than that of SnO2 films without adding Sb after annealing at 700 ◦ C in N2 . For SnO2 with a 5% molar ratio of Sb after annealing at 700 ◦ C in O2 , not only a product of oxygen gas but also the O2 atmosphere causes the film to fully transform into the SnO2 phase. To clarify the influence of annealing atmosphere on carrier mobility, we used the Hall measurement to examine the mobility of SnO2 films with a 5% molar ratio of Sb after annealing in N2 and O2 atmospheres. The carrier mobility of SnO2 film with 5% Sb after annealing at 700 ◦ C in O2 is about 3.1 cm2 V−1 s−1 , and the mobility of SnO2 film with 5% Sb after annealing at 700 ◦ C in N2
In conclusion, SnO2 :Sb transparent semiconductor films with different material properties can be prepared by optimizing the Sn/Sb molar ratios and the annealing atmosphere. Based on XRD results, the SnO2 films with and without adding Sb after annealing in O2 have a greater tendency to be oxidized than films annealed in N2 . Some voids are revealed on the surface of SnO2 films after annealing in N2 , whereas the voids disappear for high-temperature annealed SnO2 samples with the addition of Sb in the same atmosphere. Oxygen deficiency of SnO2 films without adding Sb after annealing in N2 and in O2 ambients decreases, while it vanishes for SnO2 :Sb films after annealing in O2 . This is due to the production of oxygen gas after doping antimony in SnO2 films and O2 annealing atmosphere. The elimination of oxygen vacancies in SnO2 films leads to the increased resistivity of annealed-SnO2 films after doping with Sb. At the same time, the mobility of SnO2 with a 5% molar ratio of Sb is related to both oxygen deficiency and the crystallinity of films. Acknowledgments The authors gratefully acknowledge the financial support from the National Science Council of Taiwan, R.O.C. (Grant no. 100-2221E-269-017-). References [1] X. Bulliard, S.-G. Ihn, S. Yun, Y. Kim, D. Choi, J.-Y. Choi, M. Kim, M. Sim, J.-H. Park, W. Choi, K. Cho, Adv. Funct. Mater. 20 (24) (2010) 4381. [2] Y. Liang, Z. Xu, J. Xia, S.-T. Tsai, Y. Wu, G. Li, C. Ray, L. Yu, Adv. Mater. 22 (20) (2010) E135. [3] J. Puetz, M.A. Aegerter, J. Sol–Gel Sci. Technol. 32 (2004) 125. [4] C.G. Granqvist, Thin Solid Films 193/194 (1990) 730. [5] B. Thangajaru, Thin Solid Films 402 (2002) 71. [6] H. Kim, A. Piqué, Appl. Phys. Lett. 84 (2004) 218. [7] P.Y. Liu, J.F. Chen, W.D. Sun, Vacuum 76 (2004) 7. [8] B. Stjerna, E. Olsson, C.G. Granqvist, J. Appl. Phys. 76 (1994) 3797. [9] C.J. Brinker, G.W. Scherer, Sol–Gel Science: The Physics Chemistry of Sol–Gel Processing (Hardcover), Academic Press, New York, 1990. [10] C. Terrier, J.P. Chatelon, J.A. Roger, Thin Solid Films 295 (1997) 95. [11] E.R. Leite, M. Inês, B. Bernardi, E. Longo, J.A. Varela, C.A. Paskocimas, Thin Solid Films 449 (2004) 67. [12] G. Gasparro, J. Pütz, D. Ganz, M.A. Aegerter, Solar Energy Mater. Solar Cells 54 (1998) 287. [13] S.U. Lee, B. Hong, J. Korean Phys. Soc. 55 (2009) 1915. [14] Y.-J. Choi, H.-H. Park, S. Golledge, D.C. Johnson, H.J. Chang, H. Jeon, Surf. Coat. Technol. 205 (2010) 2649. [15] F. Garbassi, Surf. Interface Anal. 2 (1980) 165. [16] F. Montilla, E. Morallón, A. De Battisti, S. Barison, S. Daolio, J.L. Vázquez, J. Phys. Chem. B 108 (2004) 15976. [17] J. Fayat, M.S. Castro, J. Eur. Ceram. Soc. 23 (2003) 1598.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The influence of annealing atmosphere on the material properties of sol–gel derived SnO2 :Sb films before and after annealing Jiann-Shing Jeng ∗ Department of Materials Science and Engineering, Far East University, Tainan 744, Taiwan
a r t i c l e
i n f o
Article history: Received 24 July 2011 Received in revised form 3 February 2012 Accepted 4 February 2012 Available online 3 March 2012 Keywords: Sol–gel SnO2 :Sb Resistivity Mobility
a b s t r a c t SnO2 films with and without Sb doping were prepared by the sol–gel spin-coating method. Material properties of the SnO2 films with different Sb contents were investigated before and after annealing under O2 or N2 . When SnO2 films are annealed under N2 or O2 , the resistivity decreases with increasing annealing temperature, which may be related to the increased crystallinity and reduced film defects. The intensity of SnO2 peaks for both O2 - and N2 -annealed films increases as the annealing temperature increases. Small nodules are revealed on the surface of SnO2 films after annealing in N2 or O2 atmospheres, and some voids are present on the surface of N2 -annealed SnO2 films. After doping with Sb, the resistivity of SnO2 films after annealing in O2 is greater than that of N2 -annealed SnO2 films. The surface morphology of SnO2 films incorporating different molar ratios of Sb after annealing are similar to that of as-spun SnO2 films with adding Sb. There were no voids found on the surfaces of N2 -annealed SnO2 :Sb films. In addition, the peak intensity of SnO2 :Sb films after O2 -annealing is higher than those films after N2 -annealing. The chemical binding states and Hall mobility of the high-temperature annealed SnO2 films without and with adding Sb are also related to the annealing atmospheres. This study discusses the connection among the material properties of the SnO2 films with different Sb contents and how these properties are influenced by the Sb-doping concentration and the annealing atmospheres of SnO2 films. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Transparent conducting oxides (TCOs) have recently been employed as transparent electrodes in a variety of optoelectronic devices such as flat-panel displays, light emitting diodes, and solar cells [1,2]. This is due to their high conductivity and high transparence in the visible region. Indium tin oxide (ITO) is the most widely used transparent conductor in the industry, because of its unique characteristics of high visible transmittance (90%), high conductivity, and high infrared reflectance. However, ITO is brittle and the indium used in ITO is relatively scarce. Thus, it is important to find an adaptive material to replace ITO. Tin dioxide (SnO2 ), which has a wide band gap (Eg = 3.6 eV), exhibits good optical, electrical, and chemical properties, and so is considered as an attractive candidate for replacing ITO [3,4]. In particular, Sb doped SnO2 films (SnO2 :Sb films) exhibit excellent properties, such as low resistance, thermal stability, high transparency in the visible region, and high reflectivity for infrared radiation [5], making them more attractive
∗ Fax: +886 6 5977909. E-mail address: [email protected] 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.02.010
than other SnO2 -based films. There are many deposition techniques used to fabricate SnO2 :Sb films on glass substrates, such as pulsedlaser deposition (PLD) [6], plasma-enhanced chemical deposition (PECVD) [7], and reactive rf magnetron sputtering [8]. Compared with conventional vacuum-based techniques, the sol–gel method is relatively simple and inexpensive, and it has two main advantages. Its stoichiometry is easy to control and adjust, and it can be processed under low temperatures [9]. This study fabricated SnO2 films with and without adding Sb using the sol–gel process method. In particular, this study involved a series of studies related to resistivity, surface morphology, crystal structures, bonding configuration, and mobility of the SnO2 film with and without adding Sb. Several studies [10–12] have been conducted to investigate the variation of SnO2 properties with different amounts of Sb prepared by solution process. But, the correlation between material properties of SnO2 and the effects of adding various amounts of Sb and the annealing atmosphere is complex and still not well understood. In the present study, SnO2 films with different contents of Sb before and after annealing in oxygen and in nitrogen atmospheres were fabricated by the sol–gel spin-coating method. The characteristics of SnO2 films with and without the addition of different
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J.-S. Jeng / Applied Surface Science 258 (2012) 5981–5986 -3
a annealed in O2
-3
(301)
(310)
(201)
-3
1.5x10
(211)
2.0x10
(101)
-3
(110)
O2 ambient N2 ambient
Intensity (a. u.)
Resistivity (ohm-cm)
2.5x10
o
500 C o
400 C
1.0x10
o
300 C
0
100
200
300
400
as-dep.
500
o
Annealing Temperature ( C) 20
30
40
Fig. 1. Resistivity of SnO2 films without adding Sb as a function of the annealing temperature in O2 and N2 ambients.
b
3. Results and discussion Fig. 1 shows the resistivity of SnO2 films without the addition of Sb as a function of the annealing temperature for O2 and N2 ambients. For SnO2 films after annealing in N2 and in O2 ambients,
70
80
annealed in N 2
(301)
(310)
(201)
(211)
(101)
Intensity (a. u.)
2. Experimental procedure SnO2 :Sb composite film with thickness of 150 nm was prepared by using a sol–gel process in combination with the spin-coating method. The tin chloride pentahydrate (SnCl4 ·5H2 O) and antimony (III) acetate [Sb(OAc)3 ] were used as precursors for preparing the SnO2 :Sb films. Then SnCl4 ·5H2 O and Sb(OAc)3 were dissolved in ethylene glycol monomethyl ether to form two pre-solutions. For the SnO2 sols solution, different amounts of SnCl4 ·5H2 O (according to the Sb/Sn molar ratio) were first mixed with 30 ml ethylene glycol monomethyl ether, and then stirred with a magnetic stirrer for 1 h at room temperature. The Sb(OAc)3 (1 mmol) was hydrolyzed in ethylene glycol monomethyl ether (30 ml) and stirred for 2 h. The pH value of the Sb(OAc)3 pre-solutions were 3, as adjusted by adding suitable amounts of HCl (0.1 N and 0.15 ml). The main function of the HCl is to act as a catalyst to accelerate the hydrolysis reaction. The two pre-solutions were mixed to various Sb/Sn molar ratios (i.e., 0%, 3%, 5%, and 10%) and stirred for 1 h. The mixed solutions were spin-coated on cleaned quartz glass substrates. After spin-coating, the films were baked at 60 ◦ C for 5 min. Finally, the coatings were annealed in flowing O2 and N2 atmospheres [150 sccm (standard cubic centimeter per minute)] in a horizontal quartz-tube furnace at 300 ◦ C, 500 ◦ C and 700 ◦ C for 30 min. The film resistivity was calculated from the sheet resistance measured by a four-point probe. The surface morphology and the film thickness of the SnO2 films with different concentrations of Sb were investigated using field emission scanning electron microscopy (FE-SEM, Philips XL-40FEG). The characteristic crystal phases of the SnO2 films with different contents of Sb were identified using Cu K␣ in glancing-incident angle X-ray diffraction (GIAXRD, Rigaku D/MAX2500) with an incident angle of 2◦ . Surface analysis was examined by X-ray photoelectron spectroscopy (XPS, VG ESCA-210) with a monochromatic Al K␣ source. The mobility of the SnO2 films with and without adding Sb was determined by Hall measurement.
60
Two theta (degrees)
(110)
amounts of Sb were analyzed using various methods, and the correlation of characteristics according to Sb contents and annealing atmospheres on the properties of SnO2 films is discussed.
50
o
500 C o
400 C o
300 C as-dep.
20
30
40
50
60
70
80
Two theta (degrees) Fig. 2. GIAXRD patterns of SnO2 films before and after annealing in (a) O2 and (b) N2 ambients, respectively.
the resistivity decreases with increasing annealing temperature. Similar results have been reported by Lee and Hong [13]. The crystallinity of SnO2 films generally increases with increasing annealing temperature [14]. In addition, film atoms are more mobile after annealing, which fills voids or vacancies in SnO2 films. Thus film defects and/or porosities can be eliminated with increasing annealing temperature. Therefore, the reduced resistivity of SnO2 films may be related to the increased crystallinity and diminished film defects. The reason that the resistivity of O2 -annealed samples is greater than that of N2 -annealed samples will be discussed later. To determine phases are present in the samples before and after annealing under different atmospheres, we analyzed the samples by GIAXRD. Fig. 2 shows the GIAXRD patterns of SnO2 films before and after annealing in different ambients. The GIAXRD patterns of as-deposited films do not display any characteristic peak, indicating that all as-deposited films are amorphous. After annealing at 300 ◦ C in O2 , diffraction peaks corresponding to the SnO2 phase start to appear (i.e., SnO2 phase precipitates). At the same time, the SnO2 peaks for all O2 -annealed films become more intense as the annealing temperature increases up to 500 ◦ C. On the other hand, the SnO2 phase is found in the pattern of N2 -annealed SnO2 films at 400 ◦ C. Furthermore, the intensity of SnO2 peaks for N2 -annealed samples increases with increasing annealing temperature, indicating the increased crystallinity of SnO2 after annealing. The XRD results
J.-S. Jeng / Applied Surface Science 258 (2012) 5981–5986
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Fig. 3. Surface morphology of SnO2 films: (a) as-deposited, and after annealing at (b) 300 ◦ C in O2 , (c) 300 ◦ C in N2 , (d) 400 ◦ C in O2 , (e) 400 ◦ C in N2 , (f) 500 ◦ C in O2 , and (g) 500 ◦ C in N2 .
indicate that an O2 atmosphere can enhance the precipitation of SnO2 phase in comparison with N2 ambient. Fig. 3 shows the surface morphology of SnO2 films before and after annealing in N2 and in O2 atmospheres. Before annealing, the sol–gel derived SnO2 films exhibit smooth and continuous morphology when observed by SEM. There are small nodules revealed on the surface of SnO2 films after annealing at 300 ◦ C under N2 and under O2 . The surface morphology of annealed films changes from small nodules on surface for films after annealing at 300 ◦ C to large nodules for samples after annealing up to 500 ◦ C, as shown in
Fig. 3(b)–(g). It is noteworthy that some voids begin to appear on the surface of SnO2 after annealing at 300 ◦ C in N2 , and the voids then enlarge after annealing up to 500 ◦ C. Material properties of SnO2 films after the addition of Sb are discussed below. Fig. 4 presents the resistivity of SnO2 films incorporating different molar ratios of Sb before and after annealing at 500 ◦ C in O2 and in N2 atmospheres. After annealing, the resistivity of SnO2 films incorporating different molar ratios of Sb is greater than that of as-deposited samples. Compared with SnO2 films incorporating 3% of Sb, the resistivity of SnO2 films incorporating
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Resistivity (ohm-cm)
1.8x10
As-dep. o 500 C in O2
-3
1.5x10
o
500 C in N2
-3
1.2x10
-4
9.0x10
-4
6.0x10
2
4
6
8
10
Sb-doping concentration (mol%) Fig. 4. Resistivity of SnO2 films incorporating different molar ratios of Sb before and after annealing at 500 ◦ C in O2 and N2 ambients.
5% of Sb decreases before and after annealing at 500 ◦ C in O2 and in N2 , as shown in Fig. 4. On the other hand, the resistivity of SnO2 films incorporating 10% of Sb increases before and after annealing at 500 ◦ C in O2 and in N2 . In addition, the resistivity of SnO2 films
incorporating different molar ratios of Sb after annealing in O2 is larger than that of N2 -annealed SnO2 films incorporating different molar ratios of Sb. This may be caused by elimination of oxygen vacancy due to the SnO2 films incorporating different molar ratios of Sb after annealing in O2 , which will be discussed later with the XPS results. The SnO2 films with and without different molar ratios of Sb are considered to be n-type semiconductors exhibiting oxygen vacancies that behave as donor impurities. The decrease in oxygen vacancies of SnO2 films incorporating different molar ratios of Sb after annealing in O2 is related to the larger resistivity of O2 -annealed SnO2 films with different molar ratios of Sb. Fig. 5 shows the surface morphology of SnO2 films incorporating 5% molar ratios of Sb before and after annealing in N2 and in O2 atmospheres. The surface morphology of SnO2 films incorporating Sb is similar to that of as-deposited SnO2 films without Sb incorporated. In addition, the surface morphology of SnO2 films incorporating 5% molar ratios of Sb after annealing present a morphology similar to as-deposited SnO2 films. In addition, there were no voids revealed on the N2 -annealed film surface. This indicates that the SnO2 films incorporating Sb can withstand higher temperatures to prevent void formation than SnO2 films without adding Sb.
Fig. 5. Surface morphology of SnO2 incorporating 10% Sb molar ratio (a) as-deposited, and after annealing at (b) 500 ◦ C in O2 , (c) 500 ◦ C in N2 , (d) 700 ◦ C in O2 , and (e) 700 ◦ C in N2 .
J.-S. Jeng / Applied Surface Science 258 (2012) 5981–5986
o
700 C (O2) o
700 C (N2)
Intensity (cps)
(310)
(301)
16000
(211)
(201)
Intensity (a. u.)
(101)
(110)
a
Peak sum Background
12000
SnO2 SnOx
8000
H2O
o
o
o
700 C in N2 Original
500 C (O2) 500 C (N2)
5985
4000 545
as-dep.
540
535
530
525
Binding energy (eV) 20
30
40
50
60
70
80 20000
Fig. 6. GIAXRD patterns of SnO2 films with 5% molar ratio of Sb added before and after annealing in (a) O2 and (b) N2 atmospheres, respectively.
16000
Fig. 6 shows the GIAXRD patterns of SnO2 films incorporating 5% Sb before and after annealing in different ambients. The GIAXRD pattern of as-deposited film does not reveal any characteristic peak, indicating that the as-deposited film is amorphous. After annealing, however diffraction peaks of the SnO2 phase are present. Furthermore, intensity of the SnO2 peak for film increases as the annealing temperature rises to 700 ◦ C. It should be noted that the peak position of SnO2 film with a 5% molar ratio of Sb shifts after annealing from its position for SnO2 film without adding Sb. This peak shift may be due to the substitution of atoms in the lattice. In general, the effect of nitrogen annealing on SnO2 films is similar to that of oxygen annealing, which leads to crystallization of the as-deposited SnO2 films. Furthermore, the effect of nitrogen annealing on SnO2 films leads to a partial reconstruction of the local oxygen deficiency. The main difference between N2 -annealed and O2 -annealed effects on SnO2 films is the degree of phase transformations. That is, phase transformation of SnO2 films, with and without adding Sb, annealed under an O2 atmosphere at 700 ◦ C occurs more completely than SnO2 films annealed under a N2 atmosphere at the same temperature. Therefore, the peak intensity of samples after O2 -annealing is higher than that of samples after N2 -annealing. A similar result can be observed in the SnO2 films without adding Sb (see Fig. 2). Chemical binding states of the 700 ◦ C-annealed SnO2 films without adding Sb and with a 5% molar ratio of Sb were examined by XPS, and the O 1s spectra of the two types of samples are shown in Figs. 7 and 8. Before XPS, the samples were transferred to a storage box with a relative humidity of 30% and maintained for 24 h. Then the films were measured with XPS. For XPS analysis, no attempt was made to sputter clean the samples in order to avoid sputterinduced chemical changes on the surface. In addition, all binding energies of various core levels were determined for the charging effect with respect to the adventitious C 1s peak at 284.6 eV. All spectra were deconvoluted and are shown in Figs. 7 and 8. The XPS O 1s spectra of the 700 ◦ C-annealed SnO2 under N2 and O2 were deconvoluted into three components (i.e., lattice oxygen in SnO2 , oxygen inside nonstoichiometric SnOx , and oxygen in water molecules adsorbed on the film surface). The fitted results are also shown in the figures. In addition, the binding energy values of Sb 3d5/2 and O 1s peaks of SnO2 and/or SnO are very close, making the deconvolution of the individual contributions difficult, as shown in Fig. 8. The parameters used for deconvolution of all spectra include the intensity ratio of Sb3d5/2 /Sb3d3/2 = 1.5. The peaks of Sb 3d5/2 and 3d3/2 are at around 530.7 and 540 eV with a separation of about
Intensity (cps)
Two theta (degrees)
b
o
700 C in O2 Original Peak sum Background
12000
SnO2 SnOx
8000
H2O
4000 545
540
535
530
525
Binding energy (eV) Fig. 7. XPS spectra of SnO2 films after annealing at 700 ◦ C in (a) N2 and (b) O2 atmospheres.
9.3 eV (spin orbit splitting) [15]. As compared with the SnO2 films after annealing at 700 ◦ C in N2 , the ratio of the SnOx to SnO2 area for 700 ◦ C-annealed sample under O2 decreases from 3.24 to 2.16. Based on XRD results, the samples at high-temperature annealing in O2 have a greater tendency to be oxidized than those in N2 atmosphere. This result indicates the oxygen deficiency of SnO2 films without adding Sb can be eliminated after annealing in O2 atmospheres, leading to a small ratio of the SnOx to SnO2 area for the O2 -annealed sample. In addition, the peak area of O (1s) in water molecules adsorbed on the SnO2 after annealing in N2 is larger than that for samples after annealing in O2 . SEM results show some voids present on the surface of SnO2 films after annealing in N2 atmosphere. In general, water can be adsorbed by the voids in a material, which results in the enhancement of peak area of O (1s) in water molecules (Fig. 7). On the other hand, the bonding configuration of SnO2 films with a 5% molar ratio of Sb after annealing in different atmospheres is shown in Fig. 8. The binding energy of Sb 3d5/2 for samples after annealing in N2 and O2 (Fig. 8) is at 530.6–530.9 eV, indicating that all antimony detected was in a pentavalent state (Sb5+ ) [16]. Similar to the SnO2 films without adding Sb, the XPS O (1s) peak area of SnO2 films with 5% Sb added after annealing in O2 related to water adsorption is less than that of SnO2 films with 5% Sb added after annealing in N2 . At the same time, the ratio of the SnOx to SnO2 peak area for the 700 ◦ C-annealed sample in N2 continually decreases to 0.59, implying that the addition of Sb in SnO2 can eliminate oxygen deficiency in SnO2 films. After annealing at 700 ◦ C in O2 , the XPS spectra show a O (1s) peak corresponding to the SnO2 , as shown in
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20000
a
is about 0.75 cm2 V−1 s−1 . The mobility value is determined by the interaction between the various scattering centers and free carriers. The oxygen deficiency of SnO2 with Sb after annealing at 700 ◦ C in O2 disappears, implying that scattering centers decrease. Simultaneously, the crystallinity of SnO2 samples with Sb after O2 annealing is higher than that of samples after N2 -annealing (see Fig. 6). As can be seen, the mobility of SnO2 with a 5% molar ratio of Sb is related to the oxygen deficiency as well as the crystallinity of films. Thus, sol–gel derived SnO2 :Sb films with controlled morphology, crystallinity, bonding configuration, and mobility can be manufactured after post-O2 -annealing and post-N2 -annealing.
o
700 C in N2 Original Peak sum
Intensity (cps)
16000
Background Sb2O5
12000
SnO2 SnOx
8000
H2O
4. Conclusion
4000 550
545
540
535
530
525
520
Binding energy (eV)
b
o
700 C in O2 Original
16000
Intensity (cps)
Peak sum Background
12000
Sb2O5 SnO2 H2O
8000
4000 545
540
535
530
525
520
Binding energy (eV) Fig. 8. XPS spectra of SnO2 films incorporating 5% molar ratios of Sb after annealing at 700 ◦ C in (a) N2 and (b) O2 atmospheres.
Fig. 8(b). The process of substituting of Sb ions can be considered as [17]: •
Sb2 O5 → 2SbSn + 2e + 4OO
X
1 + O2(g) 2
where SbSn • is the Sb ion occupying the site of the Sn ion with one positive charge, and OO X is the oxygen element in its position without any net charge. According to the equation, the production of oxygen gas takes place after doping with antimony in SnO2 films, which assists the oxidation reaction of SnOx to SnO2 . Thus, the ratio of the SnOx to SnO2 peak area for the 700 ◦ C-annealed sample in N2 is less than that of SnO2 films without adding Sb after annealing at 700 ◦ C in N2 . For SnO2 with a 5% molar ratio of Sb after annealing at 700 ◦ C in O2 , not only a product of oxygen gas but also the O2 atmosphere causes the film to fully transform into the SnO2 phase. To clarify the influence of annealing atmosphere on carrier mobility, we used the Hall measurement to examine the mobility of SnO2 films with a 5% molar ratio of Sb after annealing in N2 and O2 atmospheres. The carrier mobility of SnO2 film with 5% Sb after annealing at 700 ◦ C in O2 is about 3.1 cm2 V−1 s−1 , and the mobility of SnO2 film with 5% Sb after annealing at 700 ◦ C in N2
In conclusion, SnO2 :Sb transparent semiconductor films with different material properties can be prepared by optimizing the Sn/Sb molar ratios and the annealing atmosphere. Based on XRD results, the SnO2 films with and without adding Sb after annealing in O2 have a greater tendency to be oxidized than films annealed in N2 . Some voids are revealed on the surface of SnO2 films after annealing in N2 , whereas the voids disappear for high-temperature annealed SnO2 samples with the addition of Sb in the same atmosphere. Oxygen deficiency of SnO2 films without adding Sb after annealing in N2 and in O2 ambients decreases, while it vanishes for SnO2 :Sb films after annealing in O2 . This is due to the production of oxygen gas after doping antimony in SnO2 films and O2 annealing atmosphere. The elimination of oxygen vacancies in SnO2 films leads to the increased resistivity of annealed-SnO2 films after doping with Sb. At the same time, the mobility of SnO2 with a 5% molar ratio of Sb is related to both oxygen deficiency and the crystallinity of films. Acknowledgments The authors gratefully acknowledge the financial support from the National Science Council of Taiwan, R.O.C. (Grant no. 100-2221E-269-017-). References [1] X. Bulliard, S.-G. Ihn, S. Yun, Y. Kim, D. Choi, J.-Y. Choi, M. Kim, M. Sim, J.-H. Park, W. Choi, K. Cho, Adv. Funct. Mater. 20 (24) (2010) 4381. [2] Y. Liang, Z. Xu, J. Xia, S.-T. Tsai, Y. Wu, G. Li, C. Ray, L. Yu, Adv. Mater. 22 (20) (2010) E135. [3] J. Puetz, M.A. Aegerter, J. Sol–Gel Sci. Technol. 32 (2004) 125. [4] C.G. Granqvist, Thin Solid Films 193/194 (1990) 730. [5] B. Thangajaru, Thin Solid Films 402 (2002) 71. [6] H. Kim, A. Piqué, Appl. Phys. Lett. 84 (2004) 218. [7] P.Y. Liu, J.F. Chen, W.D. Sun, Vacuum 76 (2004) 7. [8] B. Stjerna, E. Olsson, C.G. Granqvist, J. Appl. Phys. 76 (1994) 3797. [9] C.J. Brinker, G.W. Scherer, Sol–Gel Science: The Physics Chemistry of Sol–Gel Processing (Hardcover), Academic Press, New York, 1990. [10] C. Terrier, J.P. Chatelon, J.A. Roger, Thin Solid Films 295 (1997) 95. [11] E.R. Leite, M. Inês, B. Bernardi, E. Longo, J.A. Varela, C.A. Paskocimas, Thin Solid Films 449 (2004) 67. [12] G. Gasparro, J. Pütz, D. Ganz, M.A. Aegerter, Solar Energy Mater. Solar Cells 54 (1998) 287. [13] S.U. Lee, B. Hong, J. Korean Phys. Soc. 55 (2009) 1915. [14] Y.-J. Choi, H.-H. Park, S. Golledge, D.C. Johnson, H.J. Chang, H. Jeon, Surf. Coat. Technol. 205 (2010) 2649. [15] F. Garbassi, Surf. Interface Anal. 2 (1980) 165. [16] F. Montilla, E. Morallón, A. De Battisti, S. Barison, S. Daolio, J.L. Vázquez, J. Phys. Chem. B 108 (2004) 15976. [17] J. Fayat, M.S. Castro, J. Eur. Ceram. Soc. 23 (2003) 1598.