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Photoemission studies of the surface electronic properties of L-CVD SnO2 ultra thin films
Applied Surface Science 258 (2012) 8425–8429
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Photoemission studies of the surface electronic properties of L-CVD SnO2 ultra thin films M. Kwoka a,∗ , L. Ottaviano b , J. Szuber a a b
Institute of Electronics, Silesian University of Technology, 44-100 Gliwice, Poland CNR- SPIN & Department of Physics, University of L’Aquila, 67100, Italy
a r t i c l e
i n f o
Article history: Available online 30 April 2012 Keywords: Tin dioxide L-CVD thin films Photoemission Surface electronic properties Fermi level position Electronic band gap states
a b s t r a c t This work presents the results of systematic X-ray photoelectron spectroscopy (XPS) and photoemission yield spectroscopy (PYS) studies of the surface electronic properties of L-CVD SnO2 ultrathin films submitted to various technological treatments. The interface Fermi level position in the band gap EF – Ev has been determined from XPS analysis of the Sn3d5/2 binding energy position. Such value of the Fermi level position was in a good agreement with the value estimated from the offset of XPS valence band. The variation of interface Fermi level position, after the various technological treatments, has been compared to the change of work function obtained by PYS. Valence band XPS spectra and PYS spectra point to the presence of two different bands of filled electronic states of the L-CVD SnO2 thin films. The first one was localized in the upper part of valence band at the surface at about 6.0 eV below the Fermi level, whereas the second one was localized in the band gap at about 3.0 eV below the Fermi level. The changes of electronic properties of the space charge layer of L-CVD SnO2 ultrathin films submitted to different technological procedures were assigned to the observed variation of their surface chemistry, including stoichiometry/nonstoichiometry and to the presence of surface carbon contamination. © 2012 Published by Elsevier B.V.
1. Introduction Tin dioxide (SnO2 ), is one of the most studied metal oxide semiconductors. It has a wide band gap (3.6 eV) and, accordingly, it belongs to the class of transparent conductive oxides (TCO). Oxygen vacancies, which act as n-type donors localized below the bottom of the conduction band, lead to n-type doping in SnO2 and, accordingly produce a fairly high electrical conductivity (in the range of 102 −1 cm−1 ). Such properties lead to several technological applications, and among others, SnO2 has great relevance in the fabrication of gas sensors [1–3]. The gas-sensing mechanism of SnO2 is owed to the interaction of gas species with the surface. In particular, the surface of SnO2 shows a significant density of the electronic states in the band gap close to the Fermi level EF . Thus the interaction at the surface with gases causes a change in the concentration of free carriers in the surface space charge region, and then a change of electrical conductivity within the space charge layer, ultimately responsible for the gas sensing mechanism. In relation to this application, studies of the surface electronic properties are critical to understand the electrical and sensing properties of this material [4,5].
∗ Corresponding author. Tel.: +48 32 2372057; fax: +48 32 2372057. E-mail address: [email protected] (M. Kwoka). 0169-4332/$ – see front matter © 2012 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.apsusc.2012.03.174
Up to now, commercial SnO2 gas sensors devices (mainly used for domestic applications in the gas alarm systems) have been fabricated using the thick films, for which a fundamental limitation is a large power consumption [2]. This limitation does not concern the thin solid film gas sensors [3]. There are various physical and chemical techniques implemented recently for the preparation of SnO2 thin films, which were nicely reviewed by Comini et al. [6]. One of the most promising technique in the preparation of high quality SnO2 ultrathin films is laser-assisted chemical vapor deposition (L-CVD) method developed by group of Larciprete [7–10]. Later on, this approach has been optimized using a mixture of two precursors, i.e. tetramethyltin Sn(CH3 )4 (TMT) and molecular oxygen O2 in the presence of ArF excimer laser [11,12]. It should be underlined that, with respect to the commonly used SnO2 thin film deposition techniques, the L-CVD method exhibits some important advantages, mainly a low substrate temperature for layers growth. This is advantageous once the growth occurs on fragile substrates, a typical technological request in the fabrication of miniaturized gas sensor arrays. In our recent studies of L-CVD SnO2 ultrathin films using XPS [11–17] we focused on the determination of their surface and in-depth chemistry, with a special emphasis on the influence of technological conditions on the surface nonstoichiometry of the deposited films.
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This work follows our preceding contributions [18] in which, among others, it was proposed the idea of comparative determination of the interface Fermi level position EF − Ev for the L-CVD SnO2 ultrathin by comparing experimental information from: (i) the Sn3d5/2 XPS peak binding energy position, (ii) the offset of the XPS valence band spectrum, (iii) the energy threshold in photoemission yield spectroscopy (PYS). In this paper we present the results of a systematic PYS and XPS study of the surface electronic properties of the space charge layer of L-CVD SnO2 ultrathin film submitted to a sequence of different technological treatments. Special emphasis is given on the positioning of the Fermi level and on the energy distribution of band gap electronic states. The obtained results are discussed in comparison to the observed variations of surface chemistry of the L-CVD SnO2 in terms of Sn/O stoichiometric ratio and of the abundance of carbon contaminants. 2. Experimental The L-CVD SnO2 ultrathin films have been obtained in ENEA (Ente Nazionale Energie Alternative) Centre, Frascati, Italy. These layers were deposited onto and atomically clean Si(1 0 0) surface prepared by in situ annealing at 940 ◦ C inUHV (10−7 Pa) a previously chemically etched Si(1 0 0) substrate. During the deposition the Si(1 0 0) substrate was kept at room temperature and TMT–O2 mixture was fluxed on the substrate with 0.2 sccm and 5 sccm fluxes respectively. The pulsed ArF (193 nm) excimer laser (Lambda Physik, LPX 100 Model) was operating with the following settings: 5 Hz, 20 mJ/cm2 beam flux density, perpendicular geometry. The thickness of the deposited SnO2 thin films was 20 nm as determined with quartz crystal microbalance (QCM). Subsequently, the deposited L-CVD SnO2 thin films were submitted to the following sequential treatments: (i) (ii) (iii) (iv)
in situ exposure to 108 L of oxygen at 500 ◦ C, ex situ exposure to dry air atmosphere, subsequent UHV (10−6 Pa) outgasing at 400 ◦ C for 1 h dry air oxidation at 400 ◦ C for 1 h.
For the in situ XPS study a “home brewed” XPS spectrometer was used equipped with a X-ray source (Al K␣ 1486.6 eV) and a doublepass cylindrical mirror analyser (DPCMA) (PHI 255G Model) [11,12]. The ex situ XPS experiments were carried out at the Department of Physics, University of L’Aquila, Italy, as well as at the CESIS Centre, Silesian University of Technology, Gliwice, Poland. The XPS system in L’Aquila was equipped with an X-ray source (Al K␣ 1486.6 eV) and a concentric hemispherical analyzer (CHA) (PHI 10360 Model). In the XPS studies in Gliwice, a SPECS spectrometer equipped with an X-ray source (Al K␣ 1486.6 eV), and a concentric hemispherical analyzer (PHOIBOS-100 Model) was used. Other experimental details have been described elsewhere [13–18]. All the reported XPS binding energy data have been calibrated to the C1s line of residual carbon present on SnO2 thin films positioned at 285.0 eV. The ex situ PYS studies have been performed at CESIS Centre, Silesian University of Technology, Gliwice, Poland using an “homebrewed” UHV experimental set-up, consisting of a high resolution SPM-2 monochromator with quartz optics, equipped with 40 W deuterium lamp D2 E. In the system, a small part of the light beam was deflected by a thin quartz plate toward a M12FQC51 photomultiplier in order to measure the spectral intensity of the incident photon flux. The photoelectrons were collected with a channeltron type detection system. The photoemission yield spectra Y(E) were
Fig. 1. Evolution of XPS Sn3d5/2 spectral lines of the L-CVD SnO2 ultrathin films after different technological processing.
taken in the photon energy range 3.5–6.2 eV and recorded by the digital counting electronics and then transferred to IBM PC type microcomputer for data storage and subsequent analysis. Other experimental details have been described elsewhere [11,12,18]. 3. Results and discussion Fig. 1 shows the set of XPS Sn3d5/2 spectral lines of the L-CVD SnO2 thin films after different technological processing. The interface Fermi level position EF = EF − Ev with respect to the top of the valence band can be determined from the binding energy position (centre of gravity of the main core level) of the XPS Sn3d5/2 line in the sample under investigation. This procedure, proposed for III–V semiconductor surfaces by Grant et al. [19], was also successfully applied by group of Semancik [20] for SnO2 thin films. As reference samples a stoichiometric SnO2 (1 1 0) surface as well as stoichiometric SnO2 thin films deposited by plasma oxidation can be used, for which the binding energy of the Sn3d5/2 line was equal to 483.0 eV [20,21]. Fig. 2 shows the variation of the binding energy of the XPS Sn3d5/2 line for the L-CVD SnO2 thin film submitted to the technological processing described above. This corresponds to the
Fig. 2. Variation of the binding energy centre of gravity of the XPS Sn3d5/2 spectral lines for the L-CVD SnO2 ultrathin films as determined from the respective spectra shown in Fig. 1.
M. Kwoka et al. / Applied Surface Science 258 (2012) 8425–8429
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Table 1 Interface Fermi level position EF − Ev estimated from the XPS Sn3d5/2 spectra, and the work function estimated from the PYS spectra for L-CVD SnO2 ultrathin films after different technological procedures. Procedure
Freshly deposited UHV oxidized (108 L, 500 ◦ C) Air (dry) exposed UHV outgased (10−6 Pa, 400 ◦ C) Air (dry) oxidation (2 h)
Interface Fermi level position EF − Ev [eV] Sn3d5/2 BE c.g.
XPS VB offset
3.6 3.0 3.4 3.6 3.0
– – 3.4 3.5 3.1
change of the interface Fermi level position EF − Ev in the band gap determined for all the studied the L-CVD SnO2 ultrathin films. The experimental values are summarized in Table 1 (left panel), together with the relative concentration of main elements, as well as the abundance of carbon contamination determined from the respective XPS survey spectra shown in Fig. 3. As can be seen from Figs. 1 and 2, and Table 1, the interface Fermi level position for the almost stoichiometric L-CVD SnO2 ultrathin films (in situ UHV oxidized, and dry air oxidized) having relative concentration [O/[Sn] ≈ 2.0 is equal to about 3.0 eV. This is in a good correlation with the fact that, for these samples, an evident domination of Sn+4 component is observed for XPS Sn3d5/2 line after deconvolution, as already reported in our recent studies [12,13]. Moreover, taking into account the bulk Fermi level position for the stoichiometric SnO2 single crystal (0.6 eV) as determined by Themlin et al. [22] one can obtain the relative variation of the band bending. For the almost stoichiometric L-CVD SnO2 ultrathin films we are very close to the flat bend conditions. This is not the case for other L-CVD SnO2 ultrathin films (freshly deposited, dry air exposed, and subsequent UHV outgased), for which the interface Fermi level position is in the range 3.4–3.6 eV. This is in a good correlation with the fact that for those samples a relative concentration [O]/[[Sn] is in the range 1.3–1.5, what corresponds to the existence of strong SnO fraction. This is also in line with the observation of an evident domination of Sn+2 component in XPS Sn3d5/2 line observed after deconvolution [12,13]. For these last nonstoichiometric L-CVD SnO2 ultrathin films a weak accumulation is observed with band bending close to 0.6 eV. It should be pointed out that the general tendency in variation of the interface Fermi level position EF − Ev in the band gap for the L-CVD SnO2 thin films determined from the variation of binding
Fig. 3. Evolution of XPS survey spectra of the L-CVD SnO2 ultrathin films after different technological processing.
Work Function [eV]
– – 4.5 4.2 4.5
Relative concentration O/SnO2
C/SnO2
1.30 1.95 1.55 1.30 2.00
0.3 – 3.2 2.8 2.5
energy of the centre of gravity of XPS Sn3d5/2 line was in a good correlation with the values determined from the offset of valence band region of XPS spectra. XPS VB spectra for the L-CVD SnO2 ultrathin films after dry air exposed, then UHV annealed at 400 ◦ C, and finally after dry air oxidation at 400 ◦ C, are shown in Fig. 4. For the almost stoichiometric L-CVD SnO2 ultrathin films after dry air oxidation, the interface Fermi level position EF − Ev from the true VB offset was also equal to about 3.1 eV. On the other hand for L-CVD SnO2 ultrathin films dry air exposed, and subsequent UHV outgassed, the interface Fermi level increased by about 0.4 eV. The general tendency in variation of the interface Fermi level position EF − Ev in the band gap for the L-CVD SnO2 ultrathin films exposed to dry air and subsequently outgased, as well after dry air oxidation determined using the two above described approaches was also in a good correlation with the respective variation of work function determined as the low energy threshold of corresponding PYS spectra. Fig. 5a shows the effective density of states spectrum N(E) of the L-CVD SnO2 ultrathin films after dry air exposure, subsequent UHV outgasing at 400 ◦ C, as well as after dry air oxidation at 400 ◦ C. N(E) spectra were obtained as the first derivative of corresponding photoemission yield spectra, according to the procedure recently proposed by Sebenne [23] and widely implemented in our recent studies [11,12,18]. The work function ϕ, which gives the position of the Fermi level EF with respect to the vacuum level Evac , was determined as the threshold of the low energy tail of N(E), according to the recently described procedure [18,24].
Fig. 4. Evolution of VB region of XPS spectrum of the L-CVD SnO2 ultrathin films after dry air exposed, then UHV annealed at 400 ◦ C, and finally after dry air oxidation at 400 ◦ C.
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evident non-stoichiometry and thus a high concentration of the oxygen vacancies, this corresponds to an accumulation layer and a downward surface band bending. A similar tendency was observed by group of Semancik [26] for the oxidized and oxygen deficient SnO2 (1 1 0) surface with the (1 × 1) atomic structure. For a better recognition of the contribution of filled electronic band gap states of the L-CVD SnO2 thin films after the above mentioned technological procedures, as well as for better comparison with the respective VB XPS spectra, the effective density spectra N(E) shown in Fig. 5a were transferred into the linear scale and are shown in Fig. 5b. Mainly O2p states contribute to the valence band of SnO2 (as discussed in [22,26]) while the conduction band contains mainly 5s states, what is in a good correlation with the density of states (DOS) curve calculated for bulk SnO2 [27]. It corresponds to band localized at about 6 eV below the Fermi level as observed in our VB XPS and PYS spectra. However, there is also a small band gap state at about 3.0 eV with respect to the Fermi level. This feature is in agreement with the shape of density of states calculated by Jimenez et al. [28], which is typical for SnO. A similar XPS valence band spectrum has been obtained by Themlin et al. [22] for SnO2 (1 1 0) surface after short time sputtering with a small contribution at about 2.5 eV, which was ascribed as anti-bonding Sn5s derived electronic band gap states. This is probably related to the fact that with a small reduction of SnO2 towards the SnO phase the Sn5s states originated from Sn2+ cations in the surface region are pushed down into the valence band by the surface defects of the sputtered surface [29]. It should be also pointed out that the similar electronic band gap states were also observed in our recent PYS studies of atomically clean SnO2 (1 1 0) surface after short time sputtering [25]. This well recognized peak extended up to the Fermi level observed also for the reduced L-CVD SnO2 ultrathin films having a relative concentration [O]/[Sn] in range 1.3–1.5 that can also be attributed to the defect-type oxygen vacancy states near surface region. The significant density of the electronic states in the band gap close to the Fermi level EF was also recently observed by Maffeis et al. [30] for the SnO2 nanoparticles by scanning tunneling spectroscopy. Fig. 5. Evolution of the effective density of states spectrum N(E) (in log. scale) of the L-CVD SnO2 ultrathin films after various technological treatments (a), and evolution of the effective density of states spectrum N(E) (in linear scale) of the L-CVD SnO2 ultrathin films after various technological treatments (b).
For the almost stoichiometric L-CVD SnO2 ultrathin films after dry air exposure and dry air oxidation work function was almost equal to 4.5 eV. This value was almost 0.3 eV higher than for the case of L-CVD SnO2 ultrathin films after air and subsequently UHV outgassed. This is due to oxide reduction, being the relative concentration [O]/[Sn] = 1.2, as determined in [13]. This last value of work function was almost the same as for the atomically clean SnO2 (1 1 0) surface cleaned by the IBA cleaning procedure and having the evident non-stoichiometry, as observed in our recent PYS studies [25]. It should be pointed out that, almost the observation of almost the same value of work function of L-CVD SnO2 ultrathin films after dry air exposure and after dry air oxidation is probably related to the attenuation of photoemission by the carbon contamination (∼2–3 nm) caused by the limited escape depth of photoelectrons. This occurrence was not observed for in situ L-CVD SnO2 ultrathin films freshly deposited and UHV oxidized (see respected spectra in Fig. 3, and Table 1 for comparison). Apart from this effect one can conclude that all the observed variations of the interface Fermi level position and work function for the L-CVD SnO2 thin films after different technological procedures are mainly related to the relative changes of concentration of the oxygen vacancies near the surface region. For the L-CVD SnO2 ultrathin films exhibiting
4. Conclusions We have shown the results of a systematic core level and valence band XPS studies of the surface electronic properties of L-CVD SnO2 ultrathin films submitted to various technological treatments. The XPS data have been compared with PYS spectra, in order to determine from the various techniques the same information like the Fermi level position in the band gap EF − Ev . The obtained data are in a very good mutual agreement. Moreover, the variation of interface Fermi level position, after the various technological treatments, has been compared to the change of work function obtained by PYS. Valence band XPS spectra and PYS spectra point to the presence of two different types of filled electronic states of the L-CVD SnO2 thin films. The energies of these two states are at 6.0 eV and 3.0 eV below the Fermi level. These findings have been put in correlation with the observed changes in the sample surface chemistry: i.e. the Sn/O stoichiometric ratio, and the abundance of surface carbon contamination. Acknowledgements Dr. Monika Kwoka thanks for the fellowship for long term stay in CASTI Lab, University L’Aquila, Italy, sponsored by the V FPEC Project of Centre of Excellence in Physics and Technology of Semiconductor Interfaces and Sensors (CESIS), under the Contract: G6MA-CT-2002-04042.
M. Kwoka et al. / Applied Surface Science 258 (2012) 8425–8429
This work was partially sponsored by the statutory funding of the Institute of Electronics, Silesian University of Technology, Gliwice, Poland, and performed within the Operational Programme of Innovation Economy – project InTechFun – UDA-POIG.01.03.0100-159/08. References [1] K. Takahata, in: T. Seiyama (Ed.), Chemical Sensors Technology, vol. 1, Elsevier, Amsterdam, 1988, p. 39. [2] K. Ihokura, J. Watson, The Stannic Oxide Gas Sensor: Principles and Applications, CRC Press, Boca Raton, FL, 1994. [3] G. Sberveglieri, Sensors and Actuators B23 (1995) 103. [4] W. Göpel, K.-D. Schierbaum, Sensors and ActuatorsV B26-27 (1995) 1. [5] N. Barsan, M. Schweitzer-Barberich, W. Göpel, Fresenius Journal of Analytical Chemistry 365 (1999) 287. [6] E. Comini, G. Faglia, G. Sberveglieri (Eds.), Solid State Gas Sensing, Springer, New York, 2009, p. 47. [7] R. Larciprete, E. Borsella, P. DePadova, M. Fanfoni, M. Mangiantini, P. Perfetti, Applied Surface Science 69 (1993) 59. [8] R. Larciprete, E. Borsella, P. DePadova, M. Mangiantini, P. Perfetti, M. Fanfoni, Journal of Vacuum Science & Technology A11 (1993) 336. [9] R. Larciprete, E. Borsella, P. DePadova, P. Perfetti, C. Crotti, Journal of Vacuum Science & Technology A15 (1997) 2492. [10] R. Larciprete, E. Borsella, P. DePadova, P. Perfetti, G. Faglia, G. Sberveglieri, Thin Solid Films 323 (1998) 291. [11] J. Szuber, G. Czempik, R. Larciprete, B. Adamowicz, Sensors &Actuators B70 (2000) 177. [12] J. Szuber, G. Czempik, R. Larciprete, D. Koziej, B. Adamowicz, Thin Solid Films 391 (2001) 198.
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[13] M. Kwoka, L. Ottaviano, M. Passacantando, S. Santucci, G. Czempik, J. Szuber, Thin Solid Films 490 (2005) 36. [14] M. Kwoka, L. Ottaviano, M. Passacantando, S. Santucci, J. Szuber, Applied Surface Science 252 (2006) 7730. [15] M. Kwoka, L. Ottaviano, J. Szuber, Thin Solid Films 515 (2007) 8328. [16] M. Kwoka, L. Ottaviano, M. Passacantando, G. Czempik, S. Santucci, J. Szuber, Applied Surface Science 254 (2008) 8089. [17] M. Kwoka, L. Ottaviano, N. Waczynska, S. Santucci, J. Szuber, Applied Surface Science 256 (2010) 5771. [18] M. Kwoka, L. Ottaviano, M. Passacantando, G. Czempik, S. Santucci, J. Szuber, Applied Surface Science 252 (2006) 7734. [19] R.W. Grant, J.R. Waldrop, S.P. Kowalczyk, E.A. Kraut, Journal of Vacuum Science & Technology 19 (1981) 477. [20] R. Cavicchi, M. Tarlov, S. Semancik, Journal of Vacuum Science & Technology A8 (1990) 2347. [21] L. Kover, G. Moreti, Z. Kovacs, R. Sanjines, I. Cserny, G. Margaritondo, J. Palinkas, H. Adachi, Journal of Vacuum Science and Technology A 13 (1995) 1382. [22] J.M. Themlin, M. Chtaib, L. Henrard, P. Lambin, J. Darville, J.M. Gilles, Physical Review B 46 (1992) 2460. [23] C.A. Sebenne, Il Nuovo Cimento 39B (1977) 768. [24] J. Szuber, W. Göpel, Electron Technology 33 (2000) 261. [25] J. Szuber, G. Czempik, Vacuum 48 (1997) 289. [26] S. Semancik, D.F. Cox, Sensors and Actuators 12 (1987) 101. [27] S. Munnix, M. Schmeits, Physical Review B 33 (1986) 4136. [28] V.M. Jimenez, G. Lassaletta, A. Fernandez, J.E. Espinos, F. Yubero, A.R. GonzalesElipe, Physical Review B 60 (1990) 11171. [29] M. Sinner-Hettenbach, M. Gothelid, T. Weisz, N. Barsan, U. Weimar, H. von Schenck, L. Giovanelli, G. Le Lay, Surface Science 499 (2001) 85. [30] T.G.G. Maffeis, G.T. Owen, C. Malagu, G. Martinelli, M.K. Kennedy, F.E. Kruis, S.P. Wilks, Surface Science 550 (2004) 21.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Photoemission studies of the surface electronic properties of L-CVD SnO2 ultra thin films M. Kwoka a,∗ , L. Ottaviano b , J. Szuber a a b
Institute of Electronics, Silesian University of Technology, 44-100 Gliwice, Poland CNR- SPIN & Department of Physics, University of L’Aquila, 67100, Italy
a r t i c l e
i n f o
Article history: Available online 30 April 2012 Keywords: Tin dioxide L-CVD thin films Photoemission Surface electronic properties Fermi level position Electronic band gap states
a b s t r a c t This work presents the results of systematic X-ray photoelectron spectroscopy (XPS) and photoemission yield spectroscopy (PYS) studies of the surface electronic properties of L-CVD SnO2 ultrathin films submitted to various technological treatments. The interface Fermi level position in the band gap EF – Ev has been determined from XPS analysis of the Sn3d5/2 binding energy position. Such value of the Fermi level position was in a good agreement with the value estimated from the offset of XPS valence band. The variation of interface Fermi level position, after the various technological treatments, has been compared to the change of work function obtained by PYS. Valence band XPS spectra and PYS spectra point to the presence of two different bands of filled electronic states of the L-CVD SnO2 thin films. The first one was localized in the upper part of valence band at the surface at about 6.0 eV below the Fermi level, whereas the second one was localized in the band gap at about 3.0 eV below the Fermi level. The changes of electronic properties of the space charge layer of L-CVD SnO2 ultrathin films submitted to different technological procedures were assigned to the observed variation of their surface chemistry, including stoichiometry/nonstoichiometry and to the presence of surface carbon contamination. © 2012 Published by Elsevier B.V.
1. Introduction Tin dioxide (SnO2 ), is one of the most studied metal oxide semiconductors. It has a wide band gap (3.6 eV) and, accordingly, it belongs to the class of transparent conductive oxides (TCO). Oxygen vacancies, which act as n-type donors localized below the bottom of the conduction band, lead to n-type doping in SnO2 and, accordingly produce a fairly high electrical conductivity (in the range of 102 −1 cm−1 ). Such properties lead to several technological applications, and among others, SnO2 has great relevance in the fabrication of gas sensors [1–3]. The gas-sensing mechanism of SnO2 is owed to the interaction of gas species with the surface. In particular, the surface of SnO2 shows a significant density of the electronic states in the band gap close to the Fermi level EF . Thus the interaction at the surface with gases causes a change in the concentration of free carriers in the surface space charge region, and then a change of electrical conductivity within the space charge layer, ultimately responsible for the gas sensing mechanism. In relation to this application, studies of the surface electronic properties are critical to understand the electrical and sensing properties of this material [4,5].
∗ Corresponding author. Tel.: +48 32 2372057; fax: +48 32 2372057. E-mail address: [email protected] (M. Kwoka). 0169-4332/$ – see front matter © 2012 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.apsusc.2012.03.174
Up to now, commercial SnO2 gas sensors devices (mainly used for domestic applications in the gas alarm systems) have been fabricated using the thick films, for which a fundamental limitation is a large power consumption [2]. This limitation does not concern the thin solid film gas sensors [3]. There are various physical and chemical techniques implemented recently for the preparation of SnO2 thin films, which were nicely reviewed by Comini et al. [6]. One of the most promising technique in the preparation of high quality SnO2 ultrathin films is laser-assisted chemical vapor deposition (L-CVD) method developed by group of Larciprete [7–10]. Later on, this approach has been optimized using a mixture of two precursors, i.e. tetramethyltin Sn(CH3 )4 (TMT) and molecular oxygen O2 in the presence of ArF excimer laser [11,12]. It should be underlined that, with respect to the commonly used SnO2 thin film deposition techniques, the L-CVD method exhibits some important advantages, mainly a low substrate temperature for layers growth. This is advantageous once the growth occurs on fragile substrates, a typical technological request in the fabrication of miniaturized gas sensor arrays. In our recent studies of L-CVD SnO2 ultrathin films using XPS [11–17] we focused on the determination of their surface and in-depth chemistry, with a special emphasis on the influence of technological conditions on the surface nonstoichiometry of the deposited films.
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M. Kwoka et al. / Applied Surface Science 258 (2012) 8425–8429
This work follows our preceding contributions [18] in which, among others, it was proposed the idea of comparative determination of the interface Fermi level position EF − Ev for the L-CVD SnO2 ultrathin by comparing experimental information from: (i) the Sn3d5/2 XPS peak binding energy position, (ii) the offset of the XPS valence band spectrum, (iii) the energy threshold in photoemission yield spectroscopy (PYS). In this paper we present the results of a systematic PYS and XPS study of the surface electronic properties of the space charge layer of L-CVD SnO2 ultrathin film submitted to a sequence of different technological treatments. Special emphasis is given on the positioning of the Fermi level and on the energy distribution of band gap electronic states. The obtained results are discussed in comparison to the observed variations of surface chemistry of the L-CVD SnO2 in terms of Sn/O stoichiometric ratio and of the abundance of carbon contaminants. 2. Experimental The L-CVD SnO2 ultrathin films have been obtained in ENEA (Ente Nazionale Energie Alternative) Centre, Frascati, Italy. These layers were deposited onto and atomically clean Si(1 0 0) surface prepared by in situ annealing at 940 ◦ C inUHV (10−7 Pa) a previously chemically etched Si(1 0 0) substrate. During the deposition the Si(1 0 0) substrate was kept at room temperature and TMT–O2 mixture was fluxed on the substrate with 0.2 sccm and 5 sccm fluxes respectively. The pulsed ArF (193 nm) excimer laser (Lambda Physik, LPX 100 Model) was operating with the following settings: 5 Hz, 20 mJ/cm2 beam flux density, perpendicular geometry. The thickness of the deposited SnO2 thin films was 20 nm as determined with quartz crystal microbalance (QCM). Subsequently, the deposited L-CVD SnO2 thin films were submitted to the following sequential treatments: (i) (ii) (iii) (iv)
in situ exposure to 108 L of oxygen at 500 ◦ C, ex situ exposure to dry air atmosphere, subsequent UHV (10−6 Pa) outgasing at 400 ◦ C for 1 h dry air oxidation at 400 ◦ C for 1 h.
For the in situ XPS study a “home brewed” XPS spectrometer was used equipped with a X-ray source (Al K␣ 1486.6 eV) and a doublepass cylindrical mirror analyser (DPCMA) (PHI 255G Model) [11,12]. The ex situ XPS experiments were carried out at the Department of Physics, University of L’Aquila, Italy, as well as at the CESIS Centre, Silesian University of Technology, Gliwice, Poland. The XPS system in L’Aquila was equipped with an X-ray source (Al K␣ 1486.6 eV) and a concentric hemispherical analyzer (CHA) (PHI 10360 Model). In the XPS studies in Gliwice, a SPECS spectrometer equipped with an X-ray source (Al K␣ 1486.6 eV), and a concentric hemispherical analyzer (PHOIBOS-100 Model) was used. Other experimental details have been described elsewhere [13–18]. All the reported XPS binding energy data have been calibrated to the C1s line of residual carbon present on SnO2 thin films positioned at 285.0 eV. The ex situ PYS studies have been performed at CESIS Centre, Silesian University of Technology, Gliwice, Poland using an “homebrewed” UHV experimental set-up, consisting of a high resolution SPM-2 monochromator with quartz optics, equipped with 40 W deuterium lamp D2 E. In the system, a small part of the light beam was deflected by a thin quartz plate toward a M12FQC51 photomultiplier in order to measure the spectral intensity of the incident photon flux. The photoelectrons were collected with a channeltron type detection system. The photoemission yield spectra Y(E) were
Fig. 1. Evolution of XPS Sn3d5/2 spectral lines of the L-CVD SnO2 ultrathin films after different technological processing.
taken in the photon energy range 3.5–6.2 eV and recorded by the digital counting electronics and then transferred to IBM PC type microcomputer for data storage and subsequent analysis. Other experimental details have been described elsewhere [11,12,18]. 3. Results and discussion Fig. 1 shows the set of XPS Sn3d5/2 spectral lines of the L-CVD SnO2 thin films after different technological processing. The interface Fermi level position EF = EF − Ev with respect to the top of the valence band can be determined from the binding energy position (centre of gravity of the main core level) of the XPS Sn3d5/2 line in the sample under investigation. This procedure, proposed for III–V semiconductor surfaces by Grant et al. [19], was also successfully applied by group of Semancik [20] for SnO2 thin films. As reference samples a stoichiometric SnO2 (1 1 0) surface as well as stoichiometric SnO2 thin films deposited by plasma oxidation can be used, for which the binding energy of the Sn3d5/2 line was equal to 483.0 eV [20,21]. Fig. 2 shows the variation of the binding energy of the XPS Sn3d5/2 line for the L-CVD SnO2 thin film submitted to the technological processing described above. This corresponds to the
Fig. 2. Variation of the binding energy centre of gravity of the XPS Sn3d5/2 spectral lines for the L-CVD SnO2 ultrathin films as determined from the respective spectra shown in Fig. 1.
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Table 1 Interface Fermi level position EF − Ev estimated from the XPS Sn3d5/2 spectra, and the work function estimated from the PYS spectra for L-CVD SnO2 ultrathin films after different technological procedures. Procedure
Freshly deposited UHV oxidized (108 L, 500 ◦ C) Air (dry) exposed UHV outgased (10−6 Pa, 400 ◦ C) Air (dry) oxidation (2 h)
Interface Fermi level position EF − Ev [eV] Sn3d5/2 BE c.g.
XPS VB offset
3.6 3.0 3.4 3.6 3.0
– – 3.4 3.5 3.1
change of the interface Fermi level position EF − Ev in the band gap determined for all the studied the L-CVD SnO2 ultrathin films. The experimental values are summarized in Table 1 (left panel), together with the relative concentration of main elements, as well as the abundance of carbon contamination determined from the respective XPS survey spectra shown in Fig. 3. As can be seen from Figs. 1 and 2, and Table 1, the interface Fermi level position for the almost stoichiometric L-CVD SnO2 ultrathin films (in situ UHV oxidized, and dry air oxidized) having relative concentration [O/[Sn] ≈ 2.0 is equal to about 3.0 eV. This is in a good correlation with the fact that, for these samples, an evident domination of Sn+4 component is observed for XPS Sn3d5/2 line after deconvolution, as already reported in our recent studies [12,13]. Moreover, taking into account the bulk Fermi level position for the stoichiometric SnO2 single crystal (0.6 eV) as determined by Themlin et al. [22] one can obtain the relative variation of the band bending. For the almost stoichiometric L-CVD SnO2 ultrathin films we are very close to the flat bend conditions. This is not the case for other L-CVD SnO2 ultrathin films (freshly deposited, dry air exposed, and subsequent UHV outgased), for which the interface Fermi level position is in the range 3.4–3.6 eV. This is in a good correlation with the fact that for those samples a relative concentration [O]/[[Sn] is in the range 1.3–1.5, what corresponds to the existence of strong SnO fraction. This is also in line with the observation of an evident domination of Sn+2 component in XPS Sn3d5/2 line observed after deconvolution [12,13]. For these last nonstoichiometric L-CVD SnO2 ultrathin films a weak accumulation is observed with band bending close to 0.6 eV. It should be pointed out that the general tendency in variation of the interface Fermi level position EF − Ev in the band gap for the L-CVD SnO2 thin films determined from the variation of binding
Fig. 3. Evolution of XPS survey spectra of the L-CVD SnO2 ultrathin films after different technological processing.
Work Function [eV]
– – 4.5 4.2 4.5
Relative concentration O/SnO2
C/SnO2
1.30 1.95 1.55 1.30 2.00
0.3 – 3.2 2.8 2.5
energy of the centre of gravity of XPS Sn3d5/2 line was in a good correlation with the values determined from the offset of valence band region of XPS spectra. XPS VB spectra for the L-CVD SnO2 ultrathin films after dry air exposed, then UHV annealed at 400 ◦ C, and finally after dry air oxidation at 400 ◦ C, are shown in Fig. 4. For the almost stoichiometric L-CVD SnO2 ultrathin films after dry air oxidation, the interface Fermi level position EF − Ev from the true VB offset was also equal to about 3.1 eV. On the other hand for L-CVD SnO2 ultrathin films dry air exposed, and subsequent UHV outgassed, the interface Fermi level increased by about 0.4 eV. The general tendency in variation of the interface Fermi level position EF − Ev in the band gap for the L-CVD SnO2 ultrathin films exposed to dry air and subsequently outgased, as well after dry air oxidation determined using the two above described approaches was also in a good correlation with the respective variation of work function determined as the low energy threshold of corresponding PYS spectra. Fig. 5a shows the effective density of states spectrum N(E) of the L-CVD SnO2 ultrathin films after dry air exposure, subsequent UHV outgasing at 400 ◦ C, as well as after dry air oxidation at 400 ◦ C. N(E) spectra were obtained as the first derivative of corresponding photoemission yield spectra, according to the procedure recently proposed by Sebenne [23] and widely implemented in our recent studies [11,12,18]. The work function ϕ, which gives the position of the Fermi level EF with respect to the vacuum level Evac , was determined as the threshold of the low energy tail of N(E), according to the recently described procedure [18,24].
Fig. 4. Evolution of VB region of XPS spectrum of the L-CVD SnO2 ultrathin films after dry air exposed, then UHV annealed at 400 ◦ C, and finally after dry air oxidation at 400 ◦ C.
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evident non-stoichiometry and thus a high concentration of the oxygen vacancies, this corresponds to an accumulation layer and a downward surface band bending. A similar tendency was observed by group of Semancik [26] for the oxidized and oxygen deficient SnO2 (1 1 0) surface with the (1 × 1) atomic structure. For a better recognition of the contribution of filled electronic band gap states of the L-CVD SnO2 thin films after the above mentioned technological procedures, as well as for better comparison with the respective VB XPS spectra, the effective density spectra N(E) shown in Fig. 5a were transferred into the linear scale and are shown in Fig. 5b. Mainly O2p states contribute to the valence band of SnO2 (as discussed in [22,26]) while the conduction band contains mainly 5s states, what is in a good correlation with the density of states (DOS) curve calculated for bulk SnO2 [27]. It corresponds to band localized at about 6 eV below the Fermi level as observed in our VB XPS and PYS spectra. However, there is also a small band gap state at about 3.0 eV with respect to the Fermi level. This feature is in agreement with the shape of density of states calculated by Jimenez et al. [28], which is typical for SnO. A similar XPS valence band spectrum has been obtained by Themlin et al. [22] for SnO2 (1 1 0) surface after short time sputtering with a small contribution at about 2.5 eV, which was ascribed as anti-bonding Sn5s derived electronic band gap states. This is probably related to the fact that with a small reduction of SnO2 towards the SnO phase the Sn5s states originated from Sn2+ cations in the surface region are pushed down into the valence band by the surface defects of the sputtered surface [29]. It should be also pointed out that the similar electronic band gap states were also observed in our recent PYS studies of atomically clean SnO2 (1 1 0) surface after short time sputtering [25]. This well recognized peak extended up to the Fermi level observed also for the reduced L-CVD SnO2 ultrathin films having a relative concentration [O]/[Sn] in range 1.3–1.5 that can also be attributed to the defect-type oxygen vacancy states near surface region. The significant density of the electronic states in the band gap close to the Fermi level EF was also recently observed by Maffeis et al. [30] for the SnO2 nanoparticles by scanning tunneling spectroscopy. Fig. 5. Evolution of the effective density of states spectrum N(E) (in log. scale) of the L-CVD SnO2 ultrathin films after various technological treatments (a), and evolution of the effective density of states spectrum N(E) (in linear scale) of the L-CVD SnO2 ultrathin films after various technological treatments (b).
For the almost stoichiometric L-CVD SnO2 ultrathin films after dry air exposure and dry air oxidation work function was almost equal to 4.5 eV. This value was almost 0.3 eV higher than for the case of L-CVD SnO2 ultrathin films after air and subsequently UHV outgassed. This is due to oxide reduction, being the relative concentration [O]/[Sn] = 1.2, as determined in [13]. This last value of work function was almost the same as for the atomically clean SnO2 (1 1 0) surface cleaned by the IBA cleaning procedure and having the evident non-stoichiometry, as observed in our recent PYS studies [25]. It should be pointed out that, almost the observation of almost the same value of work function of L-CVD SnO2 ultrathin films after dry air exposure and after dry air oxidation is probably related to the attenuation of photoemission by the carbon contamination (∼2–3 nm) caused by the limited escape depth of photoelectrons. This occurrence was not observed for in situ L-CVD SnO2 ultrathin films freshly deposited and UHV oxidized (see respected spectra in Fig. 3, and Table 1 for comparison). Apart from this effect one can conclude that all the observed variations of the interface Fermi level position and work function for the L-CVD SnO2 thin films after different technological procedures are mainly related to the relative changes of concentration of the oxygen vacancies near the surface region. For the L-CVD SnO2 ultrathin films exhibiting
4. Conclusions We have shown the results of a systematic core level and valence band XPS studies of the surface electronic properties of L-CVD SnO2 ultrathin films submitted to various technological treatments. The XPS data have been compared with PYS spectra, in order to determine from the various techniques the same information like the Fermi level position in the band gap EF − Ev . The obtained data are in a very good mutual agreement. Moreover, the variation of interface Fermi level position, after the various technological treatments, has been compared to the change of work function obtained by PYS. Valence band XPS spectra and PYS spectra point to the presence of two different types of filled electronic states of the L-CVD SnO2 thin films. The energies of these two states are at 6.0 eV and 3.0 eV below the Fermi level. These findings have been put in correlation with the observed changes in the sample surface chemistry: i.e. the Sn/O stoichiometric ratio, and the abundance of surface carbon contamination. Acknowledgements Dr. Monika Kwoka thanks for the fellowship for long term stay in CASTI Lab, University L’Aquila, Italy, sponsored by the V FPEC Project of Centre of Excellence in Physics and Technology of Semiconductor Interfaces and Sensors (CESIS), under the Contract: G6MA-CT-2002-04042.
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