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SiOxCy films with X-ray photoelectron spectroscopy
Materials Letters 133 (2014) 247–250
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Evolution of element distribution at the interface of FTO/SiOxCy films with X-ray photoelectron spectroscopy J.K. Yang n, F.C. Zhang, J.J. Chen, B. Yu, Y. Gao, M.J. Zhao, B. Liang, H.L. Zhao n State Key Laboratory of Metastable Materials Science and Technology, College of Materials Science and Engineering, Yanshan University, Qinhuangdao 066004, China
art ic l e i nf o
a b s t r a c t
Article history: Received 15 January 2014 Accepted 2 July 2014 Available online 10 July 2014
X-ray photoelectron spectroscopy was carried out to investigate the element distribution along the film depth, especially at the interface of FTO/SiOxCy films as-deposited and post-treated at 700 1C for 202 s and 262 s in the tempering furnace. The results show that the middle layer may effect a little on the conductivity, while an important diffusion layer exists between the functional layer and the barrier layer. It has been proved experimentally that the exacerbated diffusion at the interface layer makes the conductivity decreased. & 2014 Elsevier B.V. All rights reserved.
Keywords: FTO/SiCxOy films Element distribution XPS Interface Diffusion
1. Introduction Doped SnO2 films, such as fluorine (F) [1–5], antimony (Sb) [6–8] and cobalt (Co) [9], have been proven to effectively increase the conductivity of SnO2 film to improve the infrared reflectivity of the low emissivity (Low-E) glass and the efficiency of the photovoltaic cells. Some researchers [10,11] have found that the grain boundary is an important factor on the properties of nanograined oxides. In our previous work, Hall mobility is limited by the ionized impurity scattering rather than the grain boundary scattering for this F-doped SnO2 (FTO) films [12] and other researchers have reported similar conclusions [13]. Therefore, the effects of the grain boundary will not be discussed in detail here. Besides the grain boundaries, most researchers [1–5] usually explained in theory the variation tendency of the resistivity from the stoichiometry and oxygen vacancies. However, there is less knowledge about the conductive mechanism explained from the experiment data to clarify the chemical states and the relative content of every element which plays an important role in the conductivity of the doped films. Fortunately, a procedure for the quantitative evaluation of the relative concentration of tin oxides was proposed [14]. Most investigations are focused on the stoichiometry and oxygen states at the surface of SnO2 films by X-ray photoelectron spectroscopy (XPS) [15–17]. They found the relative concentration [O]/[Sn] was
n
Corresponding authors. Tel.: þ 86 13903339287; fax: þ 86 335 8050727. E-mail addresses: [email protected] (J.K. Yang), [email protected] (H.L. Zhao). http://dx.doi.org/10.1016/j.matlet.2014.07.012 0167-577X/& 2014 Elsevier B.V. All rights reserved.
seriously deviant from the stoichiometry at the film surface. About F-doped SnO2 films, XPS has been carried out to detect the fluorine concentration in films [18,19] or adsorbed carbon distribution on the surface of FTO films [20]. In our previous work [21], the relative concentration [O]/[Sn] increased from 1.46 to above 2 on the surface of FTO films. However, in our opinion, the conductivity is related to not only the surface, but also the whole of FTO films. Therefore, it is essential to investigate the variations of element distribution to clarify the tendency of the electrical properties of FTO films during the post processing. 2. Experimental FTO films were deposited by atmosphere pressure chemical vapor deposition (APCVD) on a glass substrate at 650 1C, which was pre-deposited with SiOxCy as a barrier layer [12,22]. Monobutyltin trichloride (MBTC) and trifluoro acetic acid (TFA) were used as precursor and dopant, respectively. MBTC (99% purity) and TFA (99% purity) were gasified in a bubble room at 160 1C and 20 1C, respectively. The as-deposited FTO-coated glass (S1) was post-heated at 700 1C for 202 s (S2) and 262 s (S3) in the tempering furnace, respectively. In our previous work [12], the resistivity measured with the Van der Pauw method increased from 3.13 10 4 Ω cm for S1 to 4.73 10 4 Ω cm for S3, while the carrier concentration decreased from 6.98 1020 cm 3 for S1 to 1.39 1020 cm 3 for S3. X-ray photoelectron spectroscopy (XPS, PHI 5300 system) was carried out to analyze the chemical states of O1s and Sn3d in FTO films. The excitation source was the Kα radiation of an Al anode
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J.K. Yang et al. / Materials Letters 133 (2014) 247–250
Fig. 1. Element distribution with sputtering time of FTO thin films (a) relative concentration of every element in the as-deposited film; (b) [O]/[Sn] ratio with different posttreating conditions.
Fig. 2. O1s, Sn3d5/2, Si2p and C1s core level spectra in the diffusion layer between FTO films and the barrier layer of SiOxCy in the as-deposited films. (“○” curve represents the experiment result, and the smoothing curve (solid line) is obtained by means of curve fitting using a Gaussion-80% Lorentzian function.)
(hυ ¼1486.6 eV). The X-ray gun was operated at 13 kV and 250 W. In order to reduce the effects of Arþ ion etching on the binding energy of electrons, low-energy Arþ ion was used for the etching of FTO films. It is assumed that ion beam bombardment effects have played a negligible role in this case [23]. The work pressure in the analysis chamber was 1 10 7 mbar under irradiation. All the reported binding energy (BE) data had been calibrated using the C1s binding energy of residual carbon present on the FTO film surface, positioned at 285.0 eV. The core level spectra of every element were fitted using XPS Peak Fitting Program (version 4.1) [24].
3. Results and discussion The relative concentration of every element along the depth of the as-deposited FTO films (shown in Fig. 1) was quantitatively analyzed with XPS by means of the relative atomic sensitivity factor [25]. The curve can be divided into three parts: 1. From 0 s to 40 s, the oxygen content on the surface of the as-deposited films is high to 69.32%, and decreases sharply to 53.92% at 40 s (seen in Fig. 2(a)). This region could be considered as the surface layer. As seen in Fig. 1(b), with
J.K. Yang et al. / Materials Letters 133 (2014) 247–250
increasing post-heating time, significant changes happen to [O]/[Sn] (detailed discussion is present in our previous work [21]). 2. From 40 s to 490 s, the oxygen content remains almost unchanged together with the relative Sn and Si content. This region is in relatively stable states and called as the middle layer. The value of [O]/[Sn] in the middle layer of S1 is about 1.21, much lower than the stoichiometric value 2.0 for perfect SnO2 crystal and in agreement with the results of Laser-CVD asdeposited SnO2 films with [O]/[Sn] of 1.29 7 0.05 [16]. The value of [O]/[Sn] is independent on post-heating conditions, revealing that the conductivity of FTO films may be affected little by the middle layer. 3. After 490 s, Sn content decreases and Si content begins to increase, which indicates that this region is the diffusion layer between the functional layer of FTO and the barrier layer of SiOxCy. [O]/[Sn] in the diffusion layer increases with the increasing time, which is similar to that in the surface layer and may affect the conductivity of the functional layer. It can be noted that F concentration has not changed from the surface to the diffusion layer or as a result of post-treatment, so the effect of F concentration may be ignored here. Therefore, the chemical states of O1s and Sn3d5/2 in the diffusion layer (take sputtering time of 650 s for example) will be focused on to discuss the effects on the film conductivity from the specific experimental data.
The chemical states of O1s, Sn3d5/2, Si2p and C1s in the diffusion layer of the as-deposited FTO films are shown in Fig. 2. Compared with oxygen chemical states at the surface layer [21,26], there is no
249
absorbed oxygen Oabs (binding energy (BE) E532.2 eV) in the diffusion layer, but there are two different types of lattice oxygen: OI in the oxygen-deficient regions with higher BE of 530.95 eV, and OII in [SnO6] octahedral coordination with lower BE of 530.42 eV. The OI1s peak has a binding energy of about 0.53 eV higher than that of OII1s peak, which is comparable with the difference at the FTO film surface (0.58 eV) [21]. What is more, the third O1s with the lowest BE of 529.80 eV is corresponding to O2 bonded with Si4þ and labeled as OSi1s [27]. While silicon (Si) exists in two types (in Fig. 2c) in the diffusion layer: one with BE of 100.29 eV is bonded with C element and labeled as “SiC2p”. Delplancke et al. [28] have found that the binding energy of Si2p in SiC1.3, SiC1.5 and SiC1.7 is about 100.10 100.30 eV by XPS. The other one with BE of 98.98 eV is bonded with O element and labeled as “SiO2p”, which is in agreement with other results (98.7098.90 eV) [29]. At the same time, the binding energy of C1s in the diffusion layer is 281.90 eV, which belongs to C Si4þ (in Fig. 2d). From the asymmetric shape of Sn3d5/2 peak (in Fig. 2b), it can be indicated that Sn3d5/2 is built-up as a mixture of three components. One SnI with BE of 486.98 eV is relative to SnI O2I in the oxygen-deficient states. SnII with BE of 486.32 eV belongs to SnII O2 in [SnO6] octahedral coordination in II SnO2. The Sn3d5/2 states with the lowest BE of 485.21 eV is corresponding to the interstitial Sn atoms in the host SnO2 and labeled as “Snin”, which is agreement with other's results (485.20 eV) [30]. Therefore, in the diffusion layer of the as-deposited films, there are a certain amount of interstitial Sn atoms, and Si, C and O atoms in the barrier layer of SiCxOy have diffused into the functional FTO layer, and exist in the host SnO2 in the form of Si C bonds and Si O bonds. In order to investigate the element distribution in the diffusion layer after heat treatment at such a high temperature of 700 1C,
Fig. 3. Sn3d5/2 and Si2p core level spectra for the interface between FTO films and the barrier layer of SiOxCy when tempered at 700 1C for 202 s and 262 s. (“○” curve represents the experiment result, and the smoothing curve (solid line) is obtained by means of curve fitting.) (a) Sn3d5/2, (a-2) Si2p, (b) Sn3d5/2, (b-2) Si2p.
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J.K. Yang et al. / Materials Letters 133 (2014) 247–250
Sn3d5/2 and Si2p core level spectra will be focused on and seen in Fig. 3. Compared with that in the as-deposited film, great changes happen to the shapes of Sn3d5/2 and Si2p XPS spectra after heat treatment. The core level spectra of Sn3d5/2 and Si2p were fitted utilizing a Gaussion–Lorentzian function. It can be seen that Sn3d5/2 in the diffusion layer after heat treatment is built up as a mixture of two components: one with higher BE of 486.86 eV is relative to SnI O2I in the oxygen-deficient states, the other one with lower BE of 484.90 eV belongs to the interstitial Sn atoms Snin. It is obvious that there is no SnII states in [SnO6] octahedral coordination in SnO2, indicating that after heated at such a high temperature, the structure of the host SnO2 may change a lot due to the diffusion atoms from the barrier layer. Accordingly, Si2p in the diffusion layer heated at 700 1C is built up as a mixture of two components SiC2p and SiO2p, which is similar to that in the as-deposited films. However, the BEs of SiC2p (100.71 eV for 202 s, 100.84 eV for 262 s) and SiO2p (99.43 eV for 202 s, 99.74 eV for 262 s) are higher than the corresponding one in as-deposited films. It is because that there is a relative oxidizing surrounding in the diffusion layer after heat treatment. In addition, it can be seen clearly that the relative integrated intensity (i.e. the relative content) of Snin and SiO2p increases sharply after heat treatment, which reveals that the diffusion extent between the functional layer and the barrier layer has been aggravated after heat treatment, and the lattice mismatch increases further, leading to increasing scattering of free carriers and the decrease of the conductivity of the functional FTO layer. 4. Conclusions The element distribution along the depth of FTO films asdeposited and post-treated at 700 1C for 202 s and 262 s in the tempering furnace were discussed with XPS. FTO films consist of three parts. The value of [O]/[Sn] in the middle layer is about 1.21 and independent on post-heating conditions. In the diffusion layer, there are some interstitial Sn atoms, and Si, C and O atoms exist in the host SnO2 in the form of Si C bonds and Si O bonds. It has been proved experimentally that after heated at such a high temperature, the diffusion extent between the functional layer and the barrier layer has been aggravated, and the conductivity of the functional FTO layer decreases. Acknowledgment The authors would like to thank the financial supports from the National Natural Science Foundation of China (No. 50972126),
Key Project of Research Program on Applied Fundamentals of Hebei Province (No.13961106D), and the Science Foundation of Yanshan University for the excellent Ph.D. students (No. 201203).
References [1] Yates HM, Evans P, Sheel DW, Nicolay S, Ding L, Ballif C. Surf Coat Technol 2012;213:167. [2] Garcés FA, Budini N, Koropecki RR, Arce RD. Thin Solid Films 2013;531:172. [3] Gao Q, Li M, Li X, Liu Y, Song CL, Wang JX, et al. J Alloys Compd 2013;550:144. [4] Remes Z, Vanecek M, Yates HM, Evans P, Sheel DW. Thin Solid Films 2009;517: 6287. [5] Yang R, Zhao GY, Chen YQ. Appl Surf Sci 2011;258:914. [6] Montero J, Guillén C, Herrero J. Surf Coat Technol 2012;211:37. [7] Wang CM, Huang CC, Kuo JC, Huang JL. Surf Coat Technol 2013;231:374. [8] Boltz J, Koehl D, Wuttig M. Surf Coat Technol 2010;205:2455. [9] Mazloom J, Ghodsi FE. Mater Res Bull 2013;48:1468. [10] Protasova SG, Straumal BB, Mazilkin A A, Stakhanova SV, Straumal PB, Baretzky B. J Mater Sci 2014;49:4490. [11] Straumal BB, Protasova SG, Mazilkin A A, Tietze T, Goering E, Schutz G, et al. Beilstein J Nanotechnol 2013;4:361. [12] Yang JK, Liu WC, Dong LZ, Li YX, Li C, Zhao HL, et al. Appl Surf Sci 2011;257: 10499. [13] Lee SY, Park BO. Thin Solid Films 2006;510:154. [14] Themlin JM, Chtaib M, Henrard L, Lambin P, Darville J, Gilles JM. Phys Rev B 1992;46:2460. [15] Chenari HM, Weinhardt L, Lastra NSR, Ernst MA, Reinert F, Golzan MM, et al. Mater Lett 2012;85:168. [16] Kwoka M, Ottaviano L, Passacantando M, Santucci S, Czempik G, Szuber J. Thin Solid Films 2005;490:36. [17] Szuber J, Czempik G, Larciprete R, Koziej D, Adamowicz B. Thin Solid Films 2001;391:198. [18] Shaltout AA, Afify HH, Ali SA. J Electron Spectrosc 2012;185:140. [19] Martel A, Caballero-Briones F, Bartolo-Perez P, Iribarren A, Castro-Rodriguez R, Zapata-Navarro A, et al. Surf Coat Technol 2001;148:103. [20] Minaai T, Nara A, Kumagai M, Tanemura S, Tanemura M. Vacuum 2006;80: 957. [21] Yang JK, Zhao HL, Zhang FC. Mater Lett 2013;90:37. [22] Zhao HL, Liu QY, Cai YX, Zhang FC. Mater Lett 2008;62:1294. [23] Chuang TJ, Brundle CR, Wandelt K. Thin Solid Films 1978;53:19. [24] Kwok RWM, XPS Peak Fitting Program for WIN95/98 XPSPEAK Version 4.1, Department of Chemistry, The Chinese University of Hong Kong. [25] Wagner CD, Riggs WM, Davis LE, Moulder JF, Mnilenberger GE. Handbook of x-ray photoelectron spectroscopy. Eden Prairie, MN: Perkin-Elmer; 1979. [26] Yea B, Sasaki H, Osaki T, Sughara K, Konishi R. Jpn J Appl Phys 1999;38:2103. [27] Paparazzo E. J Phys D 1987;20:1091. [28] Delplancke MP, Powers JM, Vandentop GJ, Salmeron M, Somorjai GA. J Vac Sci Technol A 1991;9:450. [29] Yu X, Hantsche H. Surf Interface Anal 1993;20:555. [30] Okamoto Y, Carter WJ, Hercules DM. Appl Spectrosc 1979;33:287.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Evolution of element distribution at the interface of FTO/SiOxCy films with X-ray photoelectron spectroscopy J.K. Yang n, F.C. Zhang, J.J. Chen, B. Yu, Y. Gao, M.J. Zhao, B. Liang, H.L. Zhao n State Key Laboratory of Metastable Materials Science and Technology, College of Materials Science and Engineering, Yanshan University, Qinhuangdao 066004, China
art ic l e i nf o
a b s t r a c t
Article history: Received 15 January 2014 Accepted 2 July 2014 Available online 10 July 2014
X-ray photoelectron spectroscopy was carried out to investigate the element distribution along the film depth, especially at the interface of FTO/SiOxCy films as-deposited and post-treated at 700 1C for 202 s and 262 s in the tempering furnace. The results show that the middle layer may effect a little on the conductivity, while an important diffusion layer exists between the functional layer and the barrier layer. It has been proved experimentally that the exacerbated diffusion at the interface layer makes the conductivity decreased. & 2014 Elsevier B.V. All rights reserved.
Keywords: FTO/SiCxOy films Element distribution XPS Interface Diffusion
1. Introduction Doped SnO2 films, such as fluorine (F) [1–5], antimony (Sb) [6–8] and cobalt (Co) [9], have been proven to effectively increase the conductivity of SnO2 film to improve the infrared reflectivity of the low emissivity (Low-E) glass and the efficiency of the photovoltaic cells. Some researchers [10,11] have found that the grain boundary is an important factor on the properties of nanograined oxides. In our previous work, Hall mobility is limited by the ionized impurity scattering rather than the grain boundary scattering for this F-doped SnO2 (FTO) films [12] and other researchers have reported similar conclusions [13]. Therefore, the effects of the grain boundary will not be discussed in detail here. Besides the grain boundaries, most researchers [1–5] usually explained in theory the variation tendency of the resistivity from the stoichiometry and oxygen vacancies. However, there is less knowledge about the conductive mechanism explained from the experiment data to clarify the chemical states and the relative content of every element which plays an important role in the conductivity of the doped films. Fortunately, a procedure for the quantitative evaluation of the relative concentration of tin oxides was proposed [14]. Most investigations are focused on the stoichiometry and oxygen states at the surface of SnO2 films by X-ray photoelectron spectroscopy (XPS) [15–17]. They found the relative concentration [O]/[Sn] was
n
Corresponding authors. Tel.: þ 86 13903339287; fax: þ 86 335 8050727. E-mail addresses: [email protected] (J.K. Yang), [email protected] (H.L. Zhao). http://dx.doi.org/10.1016/j.matlet.2014.07.012 0167-577X/& 2014 Elsevier B.V. All rights reserved.
seriously deviant from the stoichiometry at the film surface. About F-doped SnO2 films, XPS has been carried out to detect the fluorine concentration in films [18,19] or adsorbed carbon distribution on the surface of FTO films [20]. In our previous work [21], the relative concentration [O]/[Sn] increased from 1.46 to above 2 on the surface of FTO films. However, in our opinion, the conductivity is related to not only the surface, but also the whole of FTO films. Therefore, it is essential to investigate the variations of element distribution to clarify the tendency of the electrical properties of FTO films during the post processing. 2. Experimental FTO films were deposited by atmosphere pressure chemical vapor deposition (APCVD) on a glass substrate at 650 1C, which was pre-deposited with SiOxCy as a barrier layer [12,22]. Monobutyltin trichloride (MBTC) and trifluoro acetic acid (TFA) were used as precursor and dopant, respectively. MBTC (99% purity) and TFA (99% purity) were gasified in a bubble room at 160 1C and 20 1C, respectively. The as-deposited FTO-coated glass (S1) was post-heated at 700 1C for 202 s (S2) and 262 s (S3) in the tempering furnace, respectively. In our previous work [12], the resistivity measured with the Van der Pauw method increased from 3.13 10 4 Ω cm for S1 to 4.73 10 4 Ω cm for S3, while the carrier concentration decreased from 6.98 1020 cm 3 for S1 to 1.39 1020 cm 3 for S3. X-ray photoelectron spectroscopy (XPS, PHI 5300 system) was carried out to analyze the chemical states of O1s and Sn3d in FTO films. The excitation source was the Kα radiation of an Al anode
248
J.K. Yang et al. / Materials Letters 133 (2014) 247–250
Fig. 1. Element distribution with sputtering time of FTO thin films (a) relative concentration of every element in the as-deposited film; (b) [O]/[Sn] ratio with different posttreating conditions.
Fig. 2. O1s, Sn3d5/2, Si2p and C1s core level spectra in the diffusion layer between FTO films and the barrier layer of SiOxCy in the as-deposited films. (“○” curve represents the experiment result, and the smoothing curve (solid line) is obtained by means of curve fitting using a Gaussion-80% Lorentzian function.)
(hυ ¼1486.6 eV). The X-ray gun was operated at 13 kV and 250 W. In order to reduce the effects of Arþ ion etching on the binding energy of electrons, low-energy Arþ ion was used for the etching of FTO films. It is assumed that ion beam bombardment effects have played a negligible role in this case [23]. The work pressure in the analysis chamber was 1 10 7 mbar under irradiation. All the reported binding energy (BE) data had been calibrated using the C1s binding energy of residual carbon present on the FTO film surface, positioned at 285.0 eV. The core level spectra of every element were fitted using XPS Peak Fitting Program (version 4.1) [24].
3. Results and discussion The relative concentration of every element along the depth of the as-deposited FTO films (shown in Fig. 1) was quantitatively analyzed with XPS by means of the relative atomic sensitivity factor [25]. The curve can be divided into three parts: 1. From 0 s to 40 s, the oxygen content on the surface of the as-deposited films is high to 69.32%, and decreases sharply to 53.92% at 40 s (seen in Fig. 2(a)). This region could be considered as the surface layer. As seen in Fig. 1(b), with
J.K. Yang et al. / Materials Letters 133 (2014) 247–250
increasing post-heating time, significant changes happen to [O]/[Sn] (detailed discussion is present in our previous work [21]). 2. From 40 s to 490 s, the oxygen content remains almost unchanged together with the relative Sn and Si content. This region is in relatively stable states and called as the middle layer. The value of [O]/[Sn] in the middle layer of S1 is about 1.21, much lower than the stoichiometric value 2.0 for perfect SnO2 crystal and in agreement with the results of Laser-CVD asdeposited SnO2 films with [O]/[Sn] of 1.29 7 0.05 [16]. The value of [O]/[Sn] is independent on post-heating conditions, revealing that the conductivity of FTO films may be affected little by the middle layer. 3. After 490 s, Sn content decreases and Si content begins to increase, which indicates that this region is the diffusion layer between the functional layer of FTO and the barrier layer of SiOxCy. [O]/[Sn] in the diffusion layer increases with the increasing time, which is similar to that in the surface layer and may affect the conductivity of the functional layer. It can be noted that F concentration has not changed from the surface to the diffusion layer or as a result of post-treatment, so the effect of F concentration may be ignored here. Therefore, the chemical states of O1s and Sn3d5/2 in the diffusion layer (take sputtering time of 650 s for example) will be focused on to discuss the effects on the film conductivity from the specific experimental data.
The chemical states of O1s, Sn3d5/2, Si2p and C1s in the diffusion layer of the as-deposited FTO films are shown in Fig. 2. Compared with oxygen chemical states at the surface layer [21,26], there is no
249
absorbed oxygen Oabs (binding energy (BE) E532.2 eV) in the diffusion layer, but there are two different types of lattice oxygen: OI in the oxygen-deficient regions with higher BE of 530.95 eV, and OII in [SnO6] octahedral coordination with lower BE of 530.42 eV. The OI1s peak has a binding energy of about 0.53 eV higher than that of OII1s peak, which is comparable with the difference at the FTO film surface (0.58 eV) [21]. What is more, the third O1s with the lowest BE of 529.80 eV is corresponding to O2 bonded with Si4þ and labeled as OSi1s [27]. While silicon (Si) exists in two types (in Fig. 2c) in the diffusion layer: one with BE of 100.29 eV is bonded with C element and labeled as “SiC2p”. Delplancke et al. [28] have found that the binding energy of Si2p in SiC1.3, SiC1.5 and SiC1.7 is about 100.10 100.30 eV by XPS. The other one with BE of 98.98 eV is bonded with O element and labeled as “SiO2p”, which is in agreement with other results (98.7098.90 eV) [29]. At the same time, the binding energy of C1s in the diffusion layer is 281.90 eV, which belongs to C Si4þ (in Fig. 2d). From the asymmetric shape of Sn3d5/2 peak (in Fig. 2b), it can be indicated that Sn3d5/2 is built-up as a mixture of three components. One SnI with BE of 486.98 eV is relative to SnI O2I in the oxygen-deficient states. SnII with BE of 486.32 eV belongs to SnII O2 in [SnO6] octahedral coordination in II SnO2. The Sn3d5/2 states with the lowest BE of 485.21 eV is corresponding to the interstitial Sn atoms in the host SnO2 and labeled as “Snin”, which is agreement with other's results (485.20 eV) [30]. Therefore, in the diffusion layer of the as-deposited films, there are a certain amount of interstitial Sn atoms, and Si, C and O atoms in the barrier layer of SiCxOy have diffused into the functional FTO layer, and exist in the host SnO2 in the form of Si C bonds and Si O bonds. In order to investigate the element distribution in the diffusion layer after heat treatment at such a high temperature of 700 1C,
Fig. 3. Sn3d5/2 and Si2p core level spectra for the interface between FTO films and the barrier layer of SiOxCy when tempered at 700 1C for 202 s and 262 s. (“○” curve represents the experiment result, and the smoothing curve (solid line) is obtained by means of curve fitting.) (a) Sn3d5/2, (a-2) Si2p, (b) Sn3d5/2, (b-2) Si2p.
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J.K. Yang et al. / Materials Letters 133 (2014) 247–250
Sn3d5/2 and Si2p core level spectra will be focused on and seen in Fig. 3. Compared with that in the as-deposited film, great changes happen to the shapes of Sn3d5/2 and Si2p XPS spectra after heat treatment. The core level spectra of Sn3d5/2 and Si2p were fitted utilizing a Gaussion–Lorentzian function. It can be seen that Sn3d5/2 in the diffusion layer after heat treatment is built up as a mixture of two components: one with higher BE of 486.86 eV is relative to SnI O2I in the oxygen-deficient states, the other one with lower BE of 484.90 eV belongs to the interstitial Sn atoms Snin. It is obvious that there is no SnII states in [SnO6] octahedral coordination in SnO2, indicating that after heated at such a high temperature, the structure of the host SnO2 may change a lot due to the diffusion atoms from the barrier layer. Accordingly, Si2p in the diffusion layer heated at 700 1C is built up as a mixture of two components SiC2p and SiO2p, which is similar to that in the as-deposited films. However, the BEs of SiC2p (100.71 eV for 202 s, 100.84 eV for 262 s) and SiO2p (99.43 eV for 202 s, 99.74 eV for 262 s) are higher than the corresponding one in as-deposited films. It is because that there is a relative oxidizing surrounding in the diffusion layer after heat treatment. In addition, it can be seen clearly that the relative integrated intensity (i.e. the relative content) of Snin and SiO2p increases sharply after heat treatment, which reveals that the diffusion extent between the functional layer and the barrier layer has been aggravated after heat treatment, and the lattice mismatch increases further, leading to increasing scattering of free carriers and the decrease of the conductivity of the functional FTO layer. 4. Conclusions The element distribution along the depth of FTO films asdeposited and post-treated at 700 1C for 202 s and 262 s in the tempering furnace were discussed with XPS. FTO films consist of three parts. The value of [O]/[Sn] in the middle layer is about 1.21 and independent on post-heating conditions. In the diffusion layer, there are some interstitial Sn atoms, and Si, C and O atoms exist in the host SnO2 in the form of Si C bonds and Si O bonds. It has been proved experimentally that after heated at such a high temperature, the diffusion extent between the functional layer and the barrier layer has been aggravated, and the conductivity of the functional FTO layer decreases. Acknowledgment The authors would like to thank the financial supports from the National Natural Science Foundation of China (No. 50972126),
Key Project of Research Program on Applied Fundamentals of Hebei Province (No.13961106D), and the Science Foundation of Yanshan University for the excellent Ph.D. students (No. 201203).
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