- Email: [email protected]
The role of mid-band gap defect levels in persistent photoconductivity in RF sputtered SnO2 thin films
Thin Solid Films 603 (2016) 50–55
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
The role of mid-band gap defect levels in persistent photoconductivity in RF sputtered SnO2 thin films Paul Tierney ⁎, T.J. Ennis, Áine Allen, James Wright Institute of Technology Tallaght, Tallaght, Dublin 24, Ireland
a r t i c l e
i n f o
Article history: Received 1 May 2015 Received in revised form 17 December 2015 Accepted 18 December 2015 Available online 20 January 2016 Keywords: Tin dioxide Photoresponse Persistent photoconductivity RF sputtered Defects
a b s t r a c t Persistent photoconductivity (PPC) is observed in RF sputtered SnO2 thin films under illumination with a range of below band gap peak energies from 2.18 eV to 3.18 eV. This study provides new insights into the PC effect in SnO2 films. Previous studies have confined excitation exclusively to monochromatic near UV illumination or to spectral response that did not follow growth and decay patterns over time. We have found that over the range of energies used for our investigation, the photoconductivity increases up to several orders of magnitude and the level of induced photocurrent rises non-linearly with incident photon energy. The patterns of the response and the transient decay of photocurrent as a function of incident photon energy under vacuum and at atmospheric pressure at room temperature suggest that more than one mechanism accounts for PPC. These results point strongly to a mechanism that is not solely dependent on oxygen adsorption/desorption. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Tin dioxide continues to be a highly researched material for various gas sensing and optoelectronics applications. This transparent conductive oxide (TCO) is a wide band gap (≈3.6 eV) n-type semiconductor although the source of donors which can be ionised at room temperature remains disputed. Traditionally intrinsic oxygen vacancies were believed to be the primary origin of the n-type behaviour [1,2] but further contradictory arguments have been made firstly attributing the cause to interstitial tin sites [3] and then to impurity atoms [4]. Surface effects dominate the material characteristics when produced in various nano-structured forms. Further alterations of the gas sensing and photoresponse characteristics have been demonstrated by techniques such as varying deposition parameters and doping with impurity atoms [5–10]. Though acceptance exists for the general mechanisms controlling charge transport, a complex energy band structure and the adsorption of oxygen [11,12] and hydroxyls [13] affecting the SnO2 surface potential have made it difficult to gain an in-depth understanding of the participation in the process by discrete defect levels. A strong increase in the conductivity of nano-structured SnO2 and ZnO films under illumination has been previously reported [9,10, 14–21]. The persistent nature of the increased conductivity upon removal of the incident illumination in some semiconductors is described by the term Persistent Photoconductivity (PPC). Unlike materials such as AlGaAs in which the PPC effect is attributed to DX centres causing a metastable conductive state [22], studies are in agreement that oxygen ⁎ Corresponding author. E-mail address: [email protected] (P. Tierney).
http://dx.doi.org/10.1016/j.tsf.2015.12.058 0040-6090/© 2016 Elsevier B.V. All rights reserved.
chemisorption and photo-desorption on the material surface play a pivotal role in the mechanism in SnO2 and ZnO. These conclusions are based on experimentation in vacuum [14–18], environments of different gaseous compositions [14,16,18] and by surface passivation [16]. Brinzari et al. [23] have demonstrated that the induced photocurrent in In2O3 nanoscale films increases in a humid environment due to the presence of H2O and OH− groups on the surface at lower temperatures. Furthermore they have shown that the spectral photoconductivity response of SnO2 films is altered in humid air. Additional studies have addressed the rate of decay of photocurrent at different temperatures to characterise the PPC effect in various SnO2 nano-structures leading to the extraction of activation energies [18,24]. Earlier reports used above band gap photon energies to induce PPC in these metal oxides whereas some later studies demonstrated this behaviour with below band gap energies. In our study we have examined the PPC response and decay in RF sputtered SnO2 thin films when illuminated with a range of successively shorter wavelengths of below band gap energy. An investigation of the participation of defect levels within the band gap will contribute to the discussions of the origin of PPC in SnO2. 2. Experimental Twenty two devices with an active SnO2 area of 2 mm2 were deposited onto a thermally oxidised (285 nm) Si substrate at room temperature by RF sputtering a 99.95% pure SnO2 target at 10 mTorr with an argon flow rate of 30 sccm using a Kurt J. Lesker PVD75. Two photolithography lift-off processes were used to pattern the SnO2 layer and the chromium contacts which were deposited by DC sputtering a 99.95% pure Cr target.
P. Tierney et al. / Thin Solid Films 603 (2016) 50–55
A Sopralab GES-5E was used to obtain spectroscopic ellipsometry data for the tin dioxide layer in the spectral range of 900 nm to 190 nm. Regression analysis of the data was carried out using a Tauc–Lorrentz model and indicated a film thickness of 85.44 nm. Using a Bruggeman mixing layer and assuming a mixing ratio of 50% the best data fit for the surface roughness was 14.24 nm. Atomic force microscopy (AFM) revealed a porous film dominated by the inter-grain boundaries of irregularly shaped crystal grains which appear to be agglomerates of smaller grains (Fig. 1). No post-fabrication annealing processes were carried out so as to preserve defect levels in the material. Experiments under vacuum were carried out at room temperature in the process chamber of an Edwards Auto 500 deposition tool with a turbomolecular pumping system achieving a vacuum level of 10−7 Torr. A customised test rig for holding the sample and positioning the LED was mounted in the process chamber with vacuum feedthroughs for electrical connections. Atmospheric experimentation was carried out in a Weiss WT64 environmental chamber using the same test set-up at a fixed temperature of 20 °C. Current flow was measured by applying a fixed DC bias voltage of 0.1 V from a Keithley 2400 sourcemeter to the sample and logging the data in an application designed in National Instruments Labview. Illumination across a range of peak energy values from 2.18 eV to 3.18 eV was provided by light emitting diodes (LEDs) which offer a short but not always symmetrical bandwidth each side of the peak wavelength as shown in Table 1. Long exposure times were used in an attempt to reach a saturation level.
3. Results & discussion 3.1. Comparison of photoresponse with increasing photon energy After fabrication a typical current flow through the sample was observed to be 10−7 A at room temperature. The conductivity of the samples was found to decrease with time when stored without interaction with light and under atmospheric conditions. The measured current was found to fall an order of magnitude after approximately
51
Table 1 Spectral information for LEDs used. Emax is defined as the highest energy at which the output intensity becomes equal to zero. Emax is the energy at λmin. λpeak
Epeak
Emax
568 nm 520 nm 468 nm 450 nm 439 nm 410 nm 390 nm
2.18 eV 2.38 eV 2.65 eV 2.76 eV 2.82 eV 3.02 eV 3.18 eV
2.38 eV 2.56 eV 3.02 eV 3.06 eV 3.1 eV N/A 3.25 eV
8 weeks. This is likely to be related to the gradual chemisorption of oxygen onto the SnO2 surface as suggested by the following reaction: O2 þ SnO2−x þ e− ↔SnO2−x ðO2 − Þads :
ð1Þ
Much of the literature uses above band gap illumination to induce a photo-response [9,10,14–17,19,20], and some authors report no response with lower energies [10,16]. All these works suggest that the photo-generation of electron–hole pairs between the valence and conduction bands is required to induce the chemi-desorption of surface oxygen from oxygen vacancy sites giving rise to increased conductivity. Other studies [18,21] do report responses with below band gap energies suggesting at least a partial participation in the mechanism by the excitation of mid-band gap energy levels. Our study examines the effects of incident illumination with energies all below the band gap. Samples were exposed to a total of 7 different peak energies of light in the range 2.18 eV–3.18 eV while under vacuum and the level of the induced photocurrent was recorded over a 48 h period of exposure. No photoresponse was noted for the incident peak photon energy of 2.18 eV, but for higher energies the induced photo-current was found to increase up to several orders of magnitude. The level of the photocurrent reached was found to increase with the increasing energy of the incident light as shown in Fig. 2 suggesting the level of photoresponse is dependent on the incident photon energy.
Fig. 1. AFM image of the RF sputtered SnO2 surface.
52
P. Tierney et al. / Thin Solid Films 603 (2016) 50–55
3.2. Effects of incident light intensity
Fig. 2. Vacuum photoresponse over a 48 h period to illuminations of increasing photon energy.
The induced photocurrent was found to not rise linearly with incident photon energy. On the contrary some increases in photon energy induced large changes in the photocurrent whereas for other energy increases the change in photocurrent was relatively small. The pattern of the curves demonstrates a range of different threshold energies for the level of the induced photocurrent. In addition it suggests a cumulative effect as a photon of a particular energy appears to induce a response from defect sites with equivalent energy and lower. Considering the observed below band gap spectral response of these SnO2 thin films we have drawn some comparisons to suggested defect levels from various photoluminescence (PL) experiments [18,25–28]. The proposed positions of defect energy levels within the band structure, arrived at by fitting overlapping photoluminescence peaks, vary across the studies. This can be expected due to the variety of deposition techniques involved and widening of the optical band gap in non-stoichiometric metal oxides known as the Burstein–Moss effect [25,29–31]. The results from the reported PL studies could be grouped into three main categories. Firstly energies from ≈ 1.9 eV to 2.1 eV have been associated with tin interstitial sites. Secondly energies in the range of ≈2.1 eV to 2.9 eV have been associated with oxygen vacancies of different charge states and also structural defects. Lastly energies from ≈3.1 eV to 3.4 eV have been attributed to donor acceptor recombination and transitions from conduction band electrons to defect acceptor levels [18,25,26]. However the exact nature and positions may still draw debate as much remains to be understood about surface states on SnO2 nanostructures. For example a review of surface studies of SnO2 single crystals discusses the formation of a band gap level arising from dangling Sn-5s states due to the loss of surface lattice oxygen [32]. Our study showed no response to illumination with a peak energy of 2.18 eV which may suggest that excitation of tin interstitial sites does not induce a photoresponse. However Singh et al. [4] have suggested that tin interstitial sites in SnO2 are very low in concentration due to a high formation energy (12 eV). The next four illumination energies (2.38 eV–2.82 eV) we used cover the energy levels associated with oxygen vacancies and structural defects. Each increase in incident photon energy induced a larger photocurrent, indicating that the excitation of oxygen vacancies plays a role in the photoresponse by the possible desorption of oxygen which is generally accepted as the cause of the increased conductance under illumination [9,10,14–20]. A significant further increase in the photocurrent was observed from our final two illuminations energies above 3 eV. This increase in the photocurrent could be attributed to the formation donor acceptor pairs or interband transitions as suggested by [18,25,26] and therefore may not be directly related to oxygen desorption.
To examine any implication related to the variation in output intensities across the range of LEDs used, the effect of incident light intensity on the induced photocurrent over the 48 h period of exposure under vacuum was studied. This was done by varying the output intensity of the LED which is a function of the forward current (IF) through the device for the two LEDs with peak energy values of 2.65 eV and 3.18 eV. Using four different values of IF as shown in Fig. 3 the variation in the peak induced photocurrent for the lower energy LED was b8% and for the higher energy LED was b5% for the three highest incident light intensities. This doesn't indicate a dependence of the induced photocurrent with incident light intensity in this range. The small differences in the induced photocurrent could equally be attributed to sample variation. As also shown in Fig. 3 the illumination with IF = 10 mA at 2.65 eV peak energy induces a slightly larger photocurrent than the illumination with IF = 20 mA further suggesting a link between the induced photocurrent and incident intensity does not exist in these experiments. Only if the intensity is greatly reduced (IF = 1 mA) is there a significant reduction in the induced photocurrent suggesting a saturation of these defect sites is not reached. 3.3. Comparison of atmospheric and vacuum photocurrent A comparison was made of the induced photocurrent and its decay when exposed to below band gap illumination with peak energies of 2.38 eV, 2.65 eV and 3.18 eV under vacuum conditions and at atmospheric pressure as shown in Fig. 4. The illumination under vacuum commenced after the chamber was evacuated to 10−7 Torr. Vacuum pumping alone was found to cause a slight increase in the conductivity of the SnO2 film which we suggest is related to a reduction in the level of ambient oxygen available to adsorb onto the SnO2 surface, as the conductivity of these films at a given time is related to the dynamic equilibrium between the chemisorption and desorption of surface oxygen [9,19]. For a given incident photon energy the induced photo-current was found to be larger under vacuum than in atmosphere as previously reported [15,17,18]. The larger photocurrent has been attributed to the continual removal of desorbed oxygen from the chamber and is suggested to demonstrate the pivotal role oxygen desorption plays in the photoresponse [15,17–19]. However we note that as the incident
Fig. 3. Vacuum photoresponse under illumination with peak wavelengths of 390 nm and 468 nm at three levels of intensity over a 48 h period.
P. Tierney et al. / Thin Solid Films 603 (2016) 50–55
53
3.4. Observation of PPC by below band gap illumination As shown in Fig. 4 after removal of the illumination for each wavelength used, the conductivity was found to decay over a period of many days demonstrating the existence of PPC after illumination with incident photon energy less than the band gap in these RF sputtered SnO2 films. This suggests that the mechanism responsible for the PPC observed in these films cannot be related to the photo-generation of electron–hole pairs between the valence and conduction bands and must be attributed to mid band gap defect levels. The Kohlrausch stretched exponential function (Eq. (2)) has previously been applied to describe decay of photocurrent due to a deviation from simple exponential behaviour [18,33] Fig. 4. Comparison of the photoresponse under vacuum and at atmospheric pressure during a 48 h period of illumination with three different wavelengths and the observed PPC effect over a further 48 h period upon removal of illumination. For clarity the 520 nm response is shown in the inset. (Kohlrausch fit of photocurrent decay at atmospheric pressure (Section 3.4) is shown as the overlaid thicker line.)
photon energy was increased the saturation level of photocurrent reached at atmospheric pressure becomes proportionately closer to the level reached under vacuum conditions as highlighted in Fig. 5(A). We suggest this may demonstrate that not all of the induced photocurrent may be attributed to a mechanism based solely on oxygen desorption. Also we note that there was a significant rise in photocurrent when the incident photon energy was increased above 3 eV (Fig. 2) to energies which may be attributed to DAP excitations and interband transitions.
h i iðt Þ ¼ ið0Þ exp −ðt=τÞβ
ð2Þ
where i(0) is the saturation photocurrent, τ is the time constant and β is the stretching parameter with a value from 0 to 1. In our experiment this exponential function was applied to the photocurrent decay at atmospheric pressure (and fixed RT) as shown in Fig. 4. Modelling was not applied to the decay under vacuum conditions because constant temperature could not be maintained. Data fitting after illumination in the wavelength range of 525 nm–390 nm show a variation in the time constant and the stretching parameter as presented in Table 2. These wavelength dependant decay parameters could be consistent with a decay mechanism which is a composite of multiple processes with individual time constants. Models for the decay of the photocurrent were initially based solely on the re-adsorption of oxygen and the consequent trapping of electrons [9,10,14,19,20] whereas acceptance has now grown for a second concurrent mechanism in the decay stage [15–18]. The second mechanism is believed to be due to the recombination of electron–hole pairs spatially separated across a surface depletion region due to the prior migration of holes to a negatively charged surface. PPC occurs because paths are not available for rapid recombination and hence the lifetimes of the carriers are extended [16]. As the recombination of these carriers is not directly oxygen related we could expect to see this decay under vacuum. Considering this model, the extent of spatial separation is dependent on the surface depletion layer width which is a function of the amount of surface adsorbed oxygen. For the higher incident photon energies used oxygen desorption should be at its greatest and therefore the surface depletion layer width should approach a minimum. In this situation recombination should be faster and this could explain the greater decay we observed after higher illumination energies (Fig. 5(B)). In Fig. 2 we observed a notable increase in photocurrent when the incident photon energies were increased above 3 eV to energies which may be associated with donor acceptor pairs and interband transitions. Upon removal of the illumination recombination of DAPs should be in the nanosecond range but it can clearly be seen from Fig. 4 that there is no instantaneous decay and PPC behaviour dominates which may illustrate the extension of the photo-induced carrier recombination times due to separation across the surface depletion region. Some previous studies have reported no decay in the photocurrent under vacuum if the temperature remains constant [9,19,20]. Others
Table 2 Time constant (τ) and stretching parameter (β) values of Kohlrausch stretched exponential function used to fit photocurrent decay after illumination by light of wavelength (λ). Fig. 5. (A) Saturation photocurrent at atmosphere as a proportion of saturation photocurrent under vacuum at three decreasing wavelengths. (B) Percentage decay of the photocurrent after 48 h under vacuum (■) and at atmosphere (●) after illumination as described by the following equation:
Ið48h DecayÞ ISaturation
100%.
λ
τ (s)
β
525 nm 468 nm 390 nm
2.2 × 106 7.9 × 105 3.75 × 105
0.58 0.68 0.7
54
P. Tierney et al. / Thin Solid Films 603 (2016) 50–55
Fig. 6. Schematic diagram illustrating approximate positions and energy ranges of defects contributing to PPC and their relative association with oxygen.
have observed some decay [15,18,24], sometimes temperature dependent decay [24]. In our experiment we could not prevent some temperature fluctuation (± 8 °C) but we observed significant decay in the photocurrent over a 48 h period while the vacuum was maintained. In fact we noted no considerable difference in the percentage decay (Fig. 5(B)) of the samples maintained under vacuum and those at atmospheric pressure. SnO2 thin film conductance has been shown to be closely related to oxygen partial pressure [11,34]. Though the presence of physisorbed oxygen on the sample surface cannot be negated, the substantial deficit of oxygen at a pressure of 4 × 10−7 Torr did not unduly inhibit the decay. This is not the behaviour that might be expected from a decay mechanism that is considerably oxygen dependent. Referring to the increase in photocurrent observed for incident photon energy N 3 eV and the resulting larger proportionate decay recorded for these higher energy illuminations we suggest that a significant proportion of the decay mechanism is based on the extension of free carrier lifetimes and not on oxygen adsorption. Comparison of the maximum illumination energy of each LED as outlined in Table 1 with the induced photocurrent change for each wavelength step illustrated in Fig. 2 demonstrates that at least four adsorption centres exist mid-band gap in energy up to 3.25 eV. Excitation of defects in these different energy ranges has been demonstrated to induce photocurrent and exhibit decay which is persistent in nature. The photocurrent associated with these centres on the lower end of the energy range predominately exhibits oxygen dependence but this nature appears to diminish as incident photon energy is increased. The schematic in Fig. 6 shows the possible placement of mid-band gap sources of photoconductivity in SnO2 with a simplified distribution between oxygen dependent and non-oxygen dependent defect types. The range of energies which exhibit the greatest O2 dependence coincide with the energies reported to be associated with oxygen vacancies [18,25,28]. Because we observe decay under vacuum and at atmosphere at all illumination energies we suggest the coexistence of both defect
types throughout the band gap. Non-oxygen dependent defects appear to predominately exist at energies N 3 eV. 4. Conclusion Persistent photoconductivity was noted after exposure with incident photon energies of below band gap energy in RF sputtered SnO2 thin films. The saturation level of the induced photocurrent increases non-linearly with increasing incident photon energy suggesting the existence of energy thresholds that may be associated with mid-band gap defect levels. The reducing difference in photocurrent achieved under vacuum and at atmospheric pressure as incident photon energy is increased may suggest the presence of a second mechanism which is not related to oxygen desorption. Comparable photocurrent decay observed under vacuum and at atmospheric pressure over a 48 h period is not indicative of a decay mechanism predominately based on oxygen adsorption. Persistent photoconductivity behaviour observed for higher incident photon energies may demonstrate the extension of free carrier lifetimes. The decay of the free carriers may be controlled more by the depletion layer width than by the characteristics of the defect. References [1] C.G. Fonstad, Electrical properties of high-quality stannic oxide crystals, J. Appl. Phys. 42 (1971) 2911, http://dx.doi.org/10.1063/1.1660648. [2] S. Samson, Defect structure and electronic donor levels in stannic oxide crystals, J. Appl. Phys. 44 (1973) 4618, http://dx.doi.org/10.1063/1.1662011. [3] Ç. Kılıç, A. Zunger, Origins of coexistence of conductivity and transparency in SnO_ {2}, Phys. Rev. Lett. 88 (2002) 095501, http://dx.doi.org/10.1103/PhysRevLett.88. 095501. [4] A.K. Singh, A. Janotti, M. Scheffler, C.G. Van de Walle, Sources of electrical conductivity in SnO_{2}, Phys. Rev. Lett. 101 (2008) 055502, http://dx.doi.org/10.1103/ PhysRevLett.101.055502. [5] S. Rani, S.C. Roy, M.C. Bhatnagar, Effect of Fe doping on the gas sensing properties of nano-crystalline SnO2 thin films, Sensors Actuators B Chem. 122 (2007) 204–210, http://dx.doi.org/10.1016/j.snb.2006.05.032.
P. Tierney et al. / Thin Solid Films 603 (2016) 50–55 [6] G. Korotcenkov, V. Brinzari, Y. Boris, M. Ivanov, J. Schwank, J. Morante, Influence of surface Pd doping on gas sensing characteristics of SnO2 thin films deposited by spray pirolysis, Thin Solid Films 436 (2003) 119–126, http://dx.doi.org/10.1016/ S0040-6090(03)00506-6. [7] Y. Shen, T. Yamazaki, Z. Liu, C. Jin, T. Kikuta, N. Nakatani, Porous SnO2 sputtered films with high H2 sensitivity at low operation temperature, Thin Solid Films 516 (2008) 5111–5117, http://dx.doi.org/10.1016/j.tsf.2007.12.139. [8] C. Jin, T. Yamazaki, K. Ito, T. Kikuta, N. Nakatani, H2S sensing property of porous SnO2 sputtered films coated with various doping films, Vacuum 80 (2006) 723–725, http://dx.doi.org/10.1016/j.vacuum.2005.11.002. [9] E. Morais, L. Scalvi, V. Geraldo, Electro-optical properties of Er-doped SnO2 thin films, J. Eur. Ceram. Soc. 24 (2004) 1857–1860 (http://www.sciencedirect.com/ science/article/pii/S0955221903005156 accessed July 21, 2014). [10] L. Scalvi, F.R. Messias, A. Souza, Improved conductivity induced by photodesorption in SnO2 thin films grown by a sol–gel dip coating technique, J. Sol–Gel Sci. Technol. 13 (1998) 793–798. [11] T. Sahm, a. Gurlo, N. Bârsan, U. Weimar, Basics of oxygen and SnO2 interaction; work function change and conductivity measurements, Sensors Actuators B Chem. 118 (2006) 78–83, http://dx.doi.org/10.1016/j.snb.2006.04.004. [12] S. Chang, Oxygen chemisorption on tin oxide: correlation between electrical conductivity and EPR measurements, J. Vac. Sci. Technol. 17 (1980) 366, http://dx. doi.org/10.1116/1.570389. [13] G. Korotcenkov, I. Blinov, V. Brinzari, J.R. Stetter, Effect of air humidity on gas response of SnO2 thin film ozone sensors, Sensors Actuators B Chem. 122 (2007) 519–526, http://dx.doi.org/10.1016/j.snb.2006.06.025. [14] F. Prado, G. Meyer, J. Saura, Photocurrents in SnO2 films due to desorption of oxygen, J. Phys. Condens. Matter. 5 (1993) A351–A352. [15] C.-H. Lin, R.-S. Chen, T.-T. Chen, H.-Y. Chen, Y.-F. Chen, K.-H. Chen, et al., High photocurrent gain in SnO2 nanowires, Appl. Phys. Lett. 93 (2008) 112115, http:// dx.doi.org/10.1063/1.2987422. [16] J.D. Prades, F. Hernandez-Ramirez, R. Jimenez-Diaz, M. Manzanares, T. Andreu, A. Cirera, et al., The effects of electron–hole separation on the photoconductivity of individual metal oxide nanowires, Nanotechnology 19 (2008) 465501, http://dx.doi. org/10.1088/0957-4484/19/46/465501. [17] J. Bao, I. Shalish, Z. Su, R. Gurwitz, F. Capasso, X. Wang, et al., Photoinduced oxygen release and persistent photoconductivity in ZnO nanowires, Nanoscale Res. Lett. 6 (2011) 404, http://dx.doi.org/10.1186/1556-276X-6-404. [18] E. Viana, J. González, Photoluminescence and high-temperature persistent photoconductivity experiments in SnO2 nanobelts, J. Phys. Chem. 117 (2013) 7844–7849, http://dx.doi.org/10.1021/jp312191c l. [19] F.R. Messias, B.A.V. Vega, L.V.A. Scalvi, M.S. Li, C.V. Santilli, Electron scattering and effects of sources of light on photoconductivity of SnO2 coatings prepared by sol– gel, J. Non. Cryst. Solids 247 (1999) 171–175. [20] V. Geraldo, L.V.a. Scalvi, E.a. Morais, C.V. Santilli, P.B. Miranda, T.J. Pereira, Ultraviolet excitation of photoconductivity in thin films of sol–gel SnO2, J. Eur. Ceram. Soc. 25 (2005) 2825–2828, http://dx.doi.org/10.1016/j.jeurceramsoc.2005.03.149.
55
[21] E.a. Floriano, L.V.a. Scalvi, J.R. Sambrano, a.d. Andrade, Decay of photo-induced conductivity in Sb-doped SnO2 thin films, using monochromatic light of about bandgap energy, Appl. Surf. Sci. 267 (2013) 164–168, http://dx.doi.org/10.1016/j.apsusc. 2012.09.003. [22] S. Lany, A. Zunger, Anion vacancies as a source of persistent photoconductivity in II– VI and chalcopyrite semiconductors, Phys. Rev. B — Condens. Matter Mater. Phys. 72 (2005)http://dx.doi.org/10.1103/PhysRevB.72.035215. [23] V. Brinzari, M. Ivanov, B.K. Cho, M. Kamei, G. Korotcenkov, Photoconductivity in In2O3 nanoscale thin films: interrelation with chemisorbed-type conductometric response towards oxygen, Sensors Actuators B Chem. 148 (2010) 427–438, http:// dx.doi.org/10.1016/j.snb.2010.05.015. [24] E.a. Morais, L.V.a. Scalvi, Electron trapping of laser-induced carriers in Er-doped SnO2 thin films, J. Eur. Ceram. Soc. 27 (2007) 3803–3806, http://dx.doi.org/10. 1016/j.jeurceramsoc.2007.02.037. [25] S. Bansal, D.K. Pandya, S.C. Kashyap, D. Haranath, Growth ambient dependence of defects, structural disorder and photoluminescence in SnO2 films deposited by reactive magnetron sputtering, J. Alloys Compd. 583 (2014) 186–190, http://dx. doi.org/10.1016/j.jallcom.2013.08.135. [26] A. Kar, M.A. Stroscio, M. Dutta, J. Kumari, M. Meyyappan, Growth and properties of tin oxide nanowires and the effect of annealing conditions, Semicond. Sci. Technol. 25 (2010) 024012, http://dx.doi.org/10.1088/0268-1242/25/2/024012. [27] W. Yan, M. Fang, X. Tan, M. Liu, P. Liu, X. Hu, et al., Template-free fabrication of SnO2 hollow spheres with photoluminescence from Sni, Mater. Lett. 64 (2010) 2033–2035, http://dx.doi.org/10.1016/j.matlet.2010.06.070. [28] S. Rani, S.C. Roy, N. Karar, M.C. Bhatnagar, Structure, microstructure and photoluminescence properties of Fe doped SnO2 thin films, Solid State Commun. 141 (2007) 214–218, http://dx.doi.org/10.1016/j.ssc.2006.10.036. [29] S. D'Elia, N. Scaramuzza, F. Ciuchi, C. Versace, G. Strangi, R. Bartolino., Ellipsometry investigation of the effects of annealing temperature on the optical properties of indium tin oxide thin films studied by Drude–Lorentz model, Appl. Surf. Sci. 255 (2009) 7203–7211, http://dx.doi.org/10.1016/j.apsusc.2009.03.064. [30] E. Burstein, Anomalous optical absorption limit in InSb, Phys. Rev. 93 (1954) 632–633, http://dx.doi.org/10.1103/PhysRev.93.632. [31] A. Walsh, J. Da Silva, S.-H. Wei, Origins of band-gap renormalization in degenerately doped semiconductors, Phys. Rev. B 78 (2008) 075211, http://dx.doi.org/10.1103/ PhysRevB.78.075211. [32] M. Batzill, Surface science studies of gas sensing materials: SnO2, Sensors 6 (2006) 1345–1366, http://dx.doi.org/10.3390/s6101345 (Peterboroug.). [33] L.R. Covington, J.C. Moore, Photoconductivity and transient response of Al:ZnO:Al planar structures fabricated via a thermal oxidation process, Thin Solid Films 540 (2013) 106–111, http://dx.doi.org/10.1016/j.tsf.2013.05.128. [34] N. Barsan, U. Weimar, Conduction model of metal oxide gas sensors, J. Electroceramics. 7 (2001) 143–167, http://dx.doi.org/10.1023/A:1014405811371.
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
The role of mid-band gap defect levels in persistent photoconductivity in RF sputtered SnO2 thin films Paul Tierney ⁎, T.J. Ennis, Áine Allen, James Wright Institute of Technology Tallaght, Tallaght, Dublin 24, Ireland
a r t i c l e
i n f o
Article history: Received 1 May 2015 Received in revised form 17 December 2015 Accepted 18 December 2015 Available online 20 January 2016 Keywords: Tin dioxide Photoresponse Persistent photoconductivity RF sputtered Defects
a b s t r a c t Persistent photoconductivity (PPC) is observed in RF sputtered SnO2 thin films under illumination with a range of below band gap peak energies from 2.18 eV to 3.18 eV. This study provides new insights into the PC effect in SnO2 films. Previous studies have confined excitation exclusively to monochromatic near UV illumination or to spectral response that did not follow growth and decay patterns over time. We have found that over the range of energies used for our investigation, the photoconductivity increases up to several orders of magnitude and the level of induced photocurrent rises non-linearly with incident photon energy. The patterns of the response and the transient decay of photocurrent as a function of incident photon energy under vacuum and at atmospheric pressure at room temperature suggest that more than one mechanism accounts for PPC. These results point strongly to a mechanism that is not solely dependent on oxygen adsorption/desorption. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Tin dioxide continues to be a highly researched material for various gas sensing and optoelectronics applications. This transparent conductive oxide (TCO) is a wide band gap (≈3.6 eV) n-type semiconductor although the source of donors which can be ionised at room temperature remains disputed. Traditionally intrinsic oxygen vacancies were believed to be the primary origin of the n-type behaviour [1,2] but further contradictory arguments have been made firstly attributing the cause to interstitial tin sites [3] and then to impurity atoms [4]. Surface effects dominate the material characteristics when produced in various nano-structured forms. Further alterations of the gas sensing and photoresponse characteristics have been demonstrated by techniques such as varying deposition parameters and doping with impurity atoms [5–10]. Though acceptance exists for the general mechanisms controlling charge transport, a complex energy band structure and the adsorption of oxygen [11,12] and hydroxyls [13] affecting the SnO2 surface potential have made it difficult to gain an in-depth understanding of the participation in the process by discrete defect levels. A strong increase in the conductivity of nano-structured SnO2 and ZnO films under illumination has been previously reported [9,10, 14–21]. The persistent nature of the increased conductivity upon removal of the incident illumination in some semiconductors is described by the term Persistent Photoconductivity (PPC). Unlike materials such as AlGaAs in which the PPC effect is attributed to DX centres causing a metastable conductive state [22], studies are in agreement that oxygen ⁎ Corresponding author. E-mail address: [email protected] (P. Tierney).
http://dx.doi.org/10.1016/j.tsf.2015.12.058 0040-6090/© 2016 Elsevier B.V. All rights reserved.
chemisorption and photo-desorption on the material surface play a pivotal role in the mechanism in SnO2 and ZnO. These conclusions are based on experimentation in vacuum [14–18], environments of different gaseous compositions [14,16,18] and by surface passivation [16]. Brinzari et al. [23] have demonstrated that the induced photocurrent in In2O3 nanoscale films increases in a humid environment due to the presence of H2O and OH− groups on the surface at lower temperatures. Furthermore they have shown that the spectral photoconductivity response of SnO2 films is altered in humid air. Additional studies have addressed the rate of decay of photocurrent at different temperatures to characterise the PPC effect in various SnO2 nano-structures leading to the extraction of activation energies [18,24]. Earlier reports used above band gap photon energies to induce PPC in these metal oxides whereas some later studies demonstrated this behaviour with below band gap energies. In our study we have examined the PPC response and decay in RF sputtered SnO2 thin films when illuminated with a range of successively shorter wavelengths of below band gap energy. An investigation of the participation of defect levels within the band gap will contribute to the discussions of the origin of PPC in SnO2. 2. Experimental Twenty two devices with an active SnO2 area of 2 mm2 were deposited onto a thermally oxidised (285 nm) Si substrate at room temperature by RF sputtering a 99.95% pure SnO2 target at 10 mTorr with an argon flow rate of 30 sccm using a Kurt J. Lesker PVD75. Two photolithography lift-off processes were used to pattern the SnO2 layer and the chromium contacts which were deposited by DC sputtering a 99.95% pure Cr target.
P. Tierney et al. / Thin Solid Films 603 (2016) 50–55
A Sopralab GES-5E was used to obtain spectroscopic ellipsometry data for the tin dioxide layer in the spectral range of 900 nm to 190 nm. Regression analysis of the data was carried out using a Tauc–Lorrentz model and indicated a film thickness of 85.44 nm. Using a Bruggeman mixing layer and assuming a mixing ratio of 50% the best data fit for the surface roughness was 14.24 nm. Atomic force microscopy (AFM) revealed a porous film dominated by the inter-grain boundaries of irregularly shaped crystal grains which appear to be agglomerates of smaller grains (Fig. 1). No post-fabrication annealing processes were carried out so as to preserve defect levels in the material. Experiments under vacuum were carried out at room temperature in the process chamber of an Edwards Auto 500 deposition tool with a turbomolecular pumping system achieving a vacuum level of 10−7 Torr. A customised test rig for holding the sample and positioning the LED was mounted in the process chamber with vacuum feedthroughs for electrical connections. Atmospheric experimentation was carried out in a Weiss WT64 environmental chamber using the same test set-up at a fixed temperature of 20 °C. Current flow was measured by applying a fixed DC bias voltage of 0.1 V from a Keithley 2400 sourcemeter to the sample and logging the data in an application designed in National Instruments Labview. Illumination across a range of peak energy values from 2.18 eV to 3.18 eV was provided by light emitting diodes (LEDs) which offer a short but not always symmetrical bandwidth each side of the peak wavelength as shown in Table 1. Long exposure times were used in an attempt to reach a saturation level.
3. Results & discussion 3.1. Comparison of photoresponse with increasing photon energy After fabrication a typical current flow through the sample was observed to be 10−7 A at room temperature. The conductivity of the samples was found to decrease with time when stored without interaction with light and under atmospheric conditions. The measured current was found to fall an order of magnitude after approximately
51
Table 1 Spectral information for LEDs used. Emax is defined as the highest energy at which the output intensity becomes equal to zero. Emax is the energy at λmin. λpeak
Epeak
Emax
568 nm 520 nm 468 nm 450 nm 439 nm 410 nm 390 nm
2.18 eV 2.38 eV 2.65 eV 2.76 eV 2.82 eV 3.02 eV 3.18 eV
2.38 eV 2.56 eV 3.02 eV 3.06 eV 3.1 eV N/A 3.25 eV
8 weeks. This is likely to be related to the gradual chemisorption of oxygen onto the SnO2 surface as suggested by the following reaction: O2 þ SnO2−x þ e− ↔SnO2−x ðO2 − Þads :
ð1Þ
Much of the literature uses above band gap illumination to induce a photo-response [9,10,14–17,19,20], and some authors report no response with lower energies [10,16]. All these works suggest that the photo-generation of electron–hole pairs between the valence and conduction bands is required to induce the chemi-desorption of surface oxygen from oxygen vacancy sites giving rise to increased conductivity. Other studies [18,21] do report responses with below band gap energies suggesting at least a partial participation in the mechanism by the excitation of mid-band gap energy levels. Our study examines the effects of incident illumination with energies all below the band gap. Samples were exposed to a total of 7 different peak energies of light in the range 2.18 eV–3.18 eV while under vacuum and the level of the induced photocurrent was recorded over a 48 h period of exposure. No photoresponse was noted for the incident peak photon energy of 2.18 eV, but for higher energies the induced photo-current was found to increase up to several orders of magnitude. The level of the photocurrent reached was found to increase with the increasing energy of the incident light as shown in Fig. 2 suggesting the level of photoresponse is dependent on the incident photon energy.
Fig. 1. AFM image of the RF sputtered SnO2 surface.
52
P. Tierney et al. / Thin Solid Films 603 (2016) 50–55
3.2. Effects of incident light intensity
Fig. 2. Vacuum photoresponse over a 48 h period to illuminations of increasing photon energy.
The induced photocurrent was found to not rise linearly with incident photon energy. On the contrary some increases in photon energy induced large changes in the photocurrent whereas for other energy increases the change in photocurrent was relatively small. The pattern of the curves demonstrates a range of different threshold energies for the level of the induced photocurrent. In addition it suggests a cumulative effect as a photon of a particular energy appears to induce a response from defect sites with equivalent energy and lower. Considering the observed below band gap spectral response of these SnO2 thin films we have drawn some comparisons to suggested defect levels from various photoluminescence (PL) experiments [18,25–28]. The proposed positions of defect energy levels within the band structure, arrived at by fitting overlapping photoluminescence peaks, vary across the studies. This can be expected due to the variety of deposition techniques involved and widening of the optical band gap in non-stoichiometric metal oxides known as the Burstein–Moss effect [25,29–31]. The results from the reported PL studies could be grouped into three main categories. Firstly energies from ≈ 1.9 eV to 2.1 eV have been associated with tin interstitial sites. Secondly energies in the range of ≈2.1 eV to 2.9 eV have been associated with oxygen vacancies of different charge states and also structural defects. Lastly energies from ≈3.1 eV to 3.4 eV have been attributed to donor acceptor recombination and transitions from conduction band electrons to defect acceptor levels [18,25,26]. However the exact nature and positions may still draw debate as much remains to be understood about surface states on SnO2 nanostructures. For example a review of surface studies of SnO2 single crystals discusses the formation of a band gap level arising from dangling Sn-5s states due to the loss of surface lattice oxygen [32]. Our study showed no response to illumination with a peak energy of 2.18 eV which may suggest that excitation of tin interstitial sites does not induce a photoresponse. However Singh et al. [4] have suggested that tin interstitial sites in SnO2 are very low in concentration due to a high formation energy (12 eV). The next four illumination energies (2.38 eV–2.82 eV) we used cover the energy levels associated with oxygen vacancies and structural defects. Each increase in incident photon energy induced a larger photocurrent, indicating that the excitation of oxygen vacancies plays a role in the photoresponse by the possible desorption of oxygen which is generally accepted as the cause of the increased conductance under illumination [9,10,14–20]. A significant further increase in the photocurrent was observed from our final two illuminations energies above 3 eV. This increase in the photocurrent could be attributed to the formation donor acceptor pairs or interband transitions as suggested by [18,25,26] and therefore may not be directly related to oxygen desorption.
To examine any implication related to the variation in output intensities across the range of LEDs used, the effect of incident light intensity on the induced photocurrent over the 48 h period of exposure under vacuum was studied. This was done by varying the output intensity of the LED which is a function of the forward current (IF) through the device for the two LEDs with peak energy values of 2.65 eV and 3.18 eV. Using four different values of IF as shown in Fig. 3 the variation in the peak induced photocurrent for the lower energy LED was b8% and for the higher energy LED was b5% for the three highest incident light intensities. This doesn't indicate a dependence of the induced photocurrent with incident light intensity in this range. The small differences in the induced photocurrent could equally be attributed to sample variation. As also shown in Fig. 3 the illumination with IF = 10 mA at 2.65 eV peak energy induces a slightly larger photocurrent than the illumination with IF = 20 mA further suggesting a link between the induced photocurrent and incident intensity does not exist in these experiments. Only if the intensity is greatly reduced (IF = 1 mA) is there a significant reduction in the induced photocurrent suggesting a saturation of these defect sites is not reached. 3.3. Comparison of atmospheric and vacuum photocurrent A comparison was made of the induced photocurrent and its decay when exposed to below band gap illumination with peak energies of 2.38 eV, 2.65 eV and 3.18 eV under vacuum conditions and at atmospheric pressure as shown in Fig. 4. The illumination under vacuum commenced after the chamber was evacuated to 10−7 Torr. Vacuum pumping alone was found to cause a slight increase in the conductivity of the SnO2 film which we suggest is related to a reduction in the level of ambient oxygen available to adsorb onto the SnO2 surface, as the conductivity of these films at a given time is related to the dynamic equilibrium between the chemisorption and desorption of surface oxygen [9,19]. For a given incident photon energy the induced photo-current was found to be larger under vacuum than in atmosphere as previously reported [15,17,18]. The larger photocurrent has been attributed to the continual removal of desorbed oxygen from the chamber and is suggested to demonstrate the pivotal role oxygen desorption plays in the photoresponse [15,17–19]. However we note that as the incident
Fig. 3. Vacuum photoresponse under illumination with peak wavelengths of 390 nm and 468 nm at three levels of intensity over a 48 h period.
P. Tierney et al. / Thin Solid Films 603 (2016) 50–55
53
3.4. Observation of PPC by below band gap illumination As shown in Fig. 4 after removal of the illumination for each wavelength used, the conductivity was found to decay over a period of many days demonstrating the existence of PPC after illumination with incident photon energy less than the band gap in these RF sputtered SnO2 films. This suggests that the mechanism responsible for the PPC observed in these films cannot be related to the photo-generation of electron–hole pairs between the valence and conduction bands and must be attributed to mid band gap defect levels. The Kohlrausch stretched exponential function (Eq. (2)) has previously been applied to describe decay of photocurrent due to a deviation from simple exponential behaviour [18,33] Fig. 4. Comparison of the photoresponse under vacuum and at atmospheric pressure during a 48 h period of illumination with three different wavelengths and the observed PPC effect over a further 48 h period upon removal of illumination. For clarity the 520 nm response is shown in the inset. (Kohlrausch fit of photocurrent decay at atmospheric pressure (Section 3.4) is shown as the overlaid thicker line.)
photon energy was increased the saturation level of photocurrent reached at atmospheric pressure becomes proportionately closer to the level reached under vacuum conditions as highlighted in Fig. 5(A). We suggest this may demonstrate that not all of the induced photocurrent may be attributed to a mechanism based solely on oxygen desorption. Also we note that there was a significant rise in photocurrent when the incident photon energy was increased above 3 eV (Fig. 2) to energies which may be attributed to DAP excitations and interband transitions.
h i iðt Þ ¼ ið0Þ exp −ðt=τÞβ
ð2Þ
where i(0) is the saturation photocurrent, τ is the time constant and β is the stretching parameter with a value from 0 to 1. In our experiment this exponential function was applied to the photocurrent decay at atmospheric pressure (and fixed RT) as shown in Fig. 4. Modelling was not applied to the decay under vacuum conditions because constant temperature could not be maintained. Data fitting after illumination in the wavelength range of 525 nm–390 nm show a variation in the time constant and the stretching parameter as presented in Table 2. These wavelength dependant decay parameters could be consistent with a decay mechanism which is a composite of multiple processes with individual time constants. Models for the decay of the photocurrent were initially based solely on the re-adsorption of oxygen and the consequent trapping of electrons [9,10,14,19,20] whereas acceptance has now grown for a second concurrent mechanism in the decay stage [15–18]. The second mechanism is believed to be due to the recombination of electron–hole pairs spatially separated across a surface depletion region due to the prior migration of holes to a negatively charged surface. PPC occurs because paths are not available for rapid recombination and hence the lifetimes of the carriers are extended [16]. As the recombination of these carriers is not directly oxygen related we could expect to see this decay under vacuum. Considering this model, the extent of spatial separation is dependent on the surface depletion layer width which is a function of the amount of surface adsorbed oxygen. For the higher incident photon energies used oxygen desorption should be at its greatest and therefore the surface depletion layer width should approach a minimum. In this situation recombination should be faster and this could explain the greater decay we observed after higher illumination energies (Fig. 5(B)). In Fig. 2 we observed a notable increase in photocurrent when the incident photon energies were increased above 3 eV to energies which may be associated with donor acceptor pairs and interband transitions. Upon removal of the illumination recombination of DAPs should be in the nanosecond range but it can clearly be seen from Fig. 4 that there is no instantaneous decay and PPC behaviour dominates which may illustrate the extension of the photo-induced carrier recombination times due to separation across the surface depletion region. Some previous studies have reported no decay in the photocurrent under vacuum if the temperature remains constant [9,19,20]. Others
Table 2 Time constant (τ) and stretching parameter (β) values of Kohlrausch stretched exponential function used to fit photocurrent decay after illumination by light of wavelength (λ). Fig. 5. (A) Saturation photocurrent at atmosphere as a proportion of saturation photocurrent under vacuum at three decreasing wavelengths. (B) Percentage decay of the photocurrent after 48 h under vacuum (■) and at atmosphere (●) after illumination as described by the following equation:
Ið48h DecayÞ ISaturation
100%.
λ
τ (s)
β
525 nm 468 nm 390 nm
2.2 × 106 7.9 × 105 3.75 × 105
0.58 0.68 0.7
54
P. Tierney et al. / Thin Solid Films 603 (2016) 50–55
Fig. 6. Schematic diagram illustrating approximate positions and energy ranges of defects contributing to PPC and their relative association with oxygen.
have observed some decay [15,18,24], sometimes temperature dependent decay [24]. In our experiment we could not prevent some temperature fluctuation (± 8 °C) but we observed significant decay in the photocurrent over a 48 h period while the vacuum was maintained. In fact we noted no considerable difference in the percentage decay (Fig. 5(B)) of the samples maintained under vacuum and those at atmospheric pressure. SnO2 thin film conductance has been shown to be closely related to oxygen partial pressure [11,34]. Though the presence of physisorbed oxygen on the sample surface cannot be negated, the substantial deficit of oxygen at a pressure of 4 × 10−7 Torr did not unduly inhibit the decay. This is not the behaviour that might be expected from a decay mechanism that is considerably oxygen dependent. Referring to the increase in photocurrent observed for incident photon energy N 3 eV and the resulting larger proportionate decay recorded for these higher energy illuminations we suggest that a significant proportion of the decay mechanism is based on the extension of free carrier lifetimes and not on oxygen adsorption. Comparison of the maximum illumination energy of each LED as outlined in Table 1 with the induced photocurrent change for each wavelength step illustrated in Fig. 2 demonstrates that at least four adsorption centres exist mid-band gap in energy up to 3.25 eV. Excitation of defects in these different energy ranges has been demonstrated to induce photocurrent and exhibit decay which is persistent in nature. The photocurrent associated with these centres on the lower end of the energy range predominately exhibits oxygen dependence but this nature appears to diminish as incident photon energy is increased. The schematic in Fig. 6 shows the possible placement of mid-band gap sources of photoconductivity in SnO2 with a simplified distribution between oxygen dependent and non-oxygen dependent defect types. The range of energies which exhibit the greatest O2 dependence coincide with the energies reported to be associated with oxygen vacancies [18,25,28]. Because we observe decay under vacuum and at atmosphere at all illumination energies we suggest the coexistence of both defect
types throughout the band gap. Non-oxygen dependent defects appear to predominately exist at energies N 3 eV. 4. Conclusion Persistent photoconductivity was noted after exposure with incident photon energies of below band gap energy in RF sputtered SnO2 thin films. The saturation level of the induced photocurrent increases non-linearly with increasing incident photon energy suggesting the existence of energy thresholds that may be associated with mid-band gap defect levels. The reducing difference in photocurrent achieved under vacuum and at atmospheric pressure as incident photon energy is increased may suggest the presence of a second mechanism which is not related to oxygen desorption. Comparable photocurrent decay observed under vacuum and at atmospheric pressure over a 48 h period is not indicative of a decay mechanism predominately based on oxygen adsorption. Persistent photoconductivity behaviour observed for higher incident photon energies may demonstrate the extension of free carrier lifetimes. The decay of the free carriers may be controlled more by the depletion layer width than by the characteristics of the defect. References [1] C.G. Fonstad, Electrical properties of high-quality stannic oxide crystals, J. Appl. Phys. 42 (1971) 2911, http://dx.doi.org/10.1063/1.1660648. [2] S. Samson, Defect structure and electronic donor levels in stannic oxide crystals, J. Appl. Phys. 44 (1973) 4618, http://dx.doi.org/10.1063/1.1662011. [3] Ç. Kılıç, A. Zunger, Origins of coexistence of conductivity and transparency in SnO_ {2}, Phys. Rev. Lett. 88 (2002) 095501, http://dx.doi.org/10.1103/PhysRevLett.88. 095501. [4] A.K. Singh, A. Janotti, M. Scheffler, C.G. Van de Walle, Sources of electrical conductivity in SnO_{2}, Phys. Rev. Lett. 101 (2008) 055502, http://dx.doi.org/10.1103/ PhysRevLett.101.055502. [5] S. Rani, S.C. Roy, M.C. Bhatnagar, Effect of Fe doping on the gas sensing properties of nano-crystalline SnO2 thin films, Sensors Actuators B Chem. 122 (2007) 204–210, http://dx.doi.org/10.1016/j.snb.2006.05.032.
P. Tierney et al. / Thin Solid Films 603 (2016) 50–55 [6] G. Korotcenkov, V. Brinzari, Y. Boris, M. Ivanov, J. Schwank, J. Morante, Influence of surface Pd doping on gas sensing characteristics of SnO2 thin films deposited by spray pirolysis, Thin Solid Films 436 (2003) 119–126, http://dx.doi.org/10.1016/ S0040-6090(03)00506-6. [7] Y. Shen, T. Yamazaki, Z. Liu, C. Jin, T. Kikuta, N. Nakatani, Porous SnO2 sputtered films with high H2 sensitivity at low operation temperature, Thin Solid Films 516 (2008) 5111–5117, http://dx.doi.org/10.1016/j.tsf.2007.12.139. [8] C. Jin, T. Yamazaki, K. Ito, T. Kikuta, N. Nakatani, H2S sensing property of porous SnO2 sputtered films coated with various doping films, Vacuum 80 (2006) 723–725, http://dx.doi.org/10.1016/j.vacuum.2005.11.002. [9] E. Morais, L. Scalvi, V. Geraldo, Electro-optical properties of Er-doped SnO2 thin films, J. Eur. Ceram. Soc. 24 (2004) 1857–1860 (http://www.sciencedirect.com/ science/article/pii/S0955221903005156 accessed July 21, 2014). [10] L. Scalvi, F.R. Messias, A. Souza, Improved conductivity induced by photodesorption in SnO2 thin films grown by a sol–gel dip coating technique, J. Sol–Gel Sci. Technol. 13 (1998) 793–798. [11] T. Sahm, a. Gurlo, N. Bârsan, U. Weimar, Basics of oxygen and SnO2 interaction; work function change and conductivity measurements, Sensors Actuators B Chem. 118 (2006) 78–83, http://dx.doi.org/10.1016/j.snb.2006.04.004. [12] S. Chang, Oxygen chemisorption on tin oxide: correlation between electrical conductivity and EPR measurements, J. Vac. Sci. Technol. 17 (1980) 366, http://dx. doi.org/10.1116/1.570389. [13] G. Korotcenkov, I. Blinov, V. Brinzari, J.R. Stetter, Effect of air humidity on gas response of SnO2 thin film ozone sensors, Sensors Actuators B Chem. 122 (2007) 519–526, http://dx.doi.org/10.1016/j.snb.2006.06.025. [14] F. Prado, G. Meyer, J. Saura, Photocurrents in SnO2 films due to desorption of oxygen, J. Phys. Condens. Matter. 5 (1993) A351–A352. [15] C.-H. Lin, R.-S. Chen, T.-T. Chen, H.-Y. Chen, Y.-F. Chen, K.-H. Chen, et al., High photocurrent gain in SnO2 nanowires, Appl. Phys. Lett. 93 (2008) 112115, http:// dx.doi.org/10.1063/1.2987422. [16] J.D. Prades, F. Hernandez-Ramirez, R. Jimenez-Diaz, M. Manzanares, T. Andreu, A. Cirera, et al., The effects of electron–hole separation on the photoconductivity of individual metal oxide nanowires, Nanotechnology 19 (2008) 465501, http://dx.doi. org/10.1088/0957-4484/19/46/465501. [17] J. Bao, I. Shalish, Z. Su, R. Gurwitz, F. Capasso, X. Wang, et al., Photoinduced oxygen release and persistent photoconductivity in ZnO nanowires, Nanoscale Res. Lett. 6 (2011) 404, http://dx.doi.org/10.1186/1556-276X-6-404. [18] E. Viana, J. González, Photoluminescence and high-temperature persistent photoconductivity experiments in SnO2 nanobelts, J. Phys. Chem. 117 (2013) 7844–7849, http://dx.doi.org/10.1021/jp312191c l. [19] F.R. Messias, B.A.V. Vega, L.V.A. Scalvi, M.S. Li, C.V. Santilli, Electron scattering and effects of sources of light on photoconductivity of SnO2 coatings prepared by sol– gel, J. Non. Cryst. Solids 247 (1999) 171–175. [20] V. Geraldo, L.V.a. Scalvi, E.a. Morais, C.V. Santilli, P.B. Miranda, T.J. Pereira, Ultraviolet excitation of photoconductivity in thin films of sol–gel SnO2, J. Eur. Ceram. Soc. 25 (2005) 2825–2828, http://dx.doi.org/10.1016/j.jeurceramsoc.2005.03.149.
55
[21] E.a. Floriano, L.V.a. Scalvi, J.R. Sambrano, a.d. Andrade, Decay of photo-induced conductivity in Sb-doped SnO2 thin films, using monochromatic light of about bandgap energy, Appl. Surf. Sci. 267 (2013) 164–168, http://dx.doi.org/10.1016/j.apsusc. 2012.09.003. [22] S. Lany, A. Zunger, Anion vacancies as a source of persistent photoconductivity in II– VI and chalcopyrite semiconductors, Phys. Rev. B — Condens. Matter Mater. Phys. 72 (2005)http://dx.doi.org/10.1103/PhysRevB.72.035215. [23] V. Brinzari, M. Ivanov, B.K. Cho, M. Kamei, G. Korotcenkov, Photoconductivity in In2O3 nanoscale thin films: interrelation with chemisorbed-type conductometric response towards oxygen, Sensors Actuators B Chem. 148 (2010) 427–438, http:// dx.doi.org/10.1016/j.snb.2010.05.015. [24] E.a. Morais, L.V.a. Scalvi, Electron trapping of laser-induced carriers in Er-doped SnO2 thin films, J. Eur. Ceram. Soc. 27 (2007) 3803–3806, http://dx.doi.org/10. 1016/j.jeurceramsoc.2007.02.037. [25] S. Bansal, D.K. Pandya, S.C. Kashyap, D. Haranath, Growth ambient dependence of defects, structural disorder and photoluminescence in SnO2 films deposited by reactive magnetron sputtering, J. Alloys Compd. 583 (2014) 186–190, http://dx. doi.org/10.1016/j.jallcom.2013.08.135. [26] A. Kar, M.A. Stroscio, M. Dutta, J. Kumari, M. Meyyappan, Growth and properties of tin oxide nanowires and the effect of annealing conditions, Semicond. Sci. Technol. 25 (2010) 024012, http://dx.doi.org/10.1088/0268-1242/25/2/024012. [27] W. Yan, M. Fang, X. Tan, M. Liu, P. Liu, X. Hu, et al., Template-free fabrication of SnO2 hollow spheres with photoluminescence from Sni, Mater. Lett. 64 (2010) 2033–2035, http://dx.doi.org/10.1016/j.matlet.2010.06.070. [28] S. Rani, S.C. Roy, N. Karar, M.C. Bhatnagar, Structure, microstructure and photoluminescence properties of Fe doped SnO2 thin films, Solid State Commun. 141 (2007) 214–218, http://dx.doi.org/10.1016/j.ssc.2006.10.036. [29] S. D'Elia, N. Scaramuzza, F. Ciuchi, C. Versace, G. Strangi, R. Bartolino., Ellipsometry investigation of the effects of annealing temperature on the optical properties of indium tin oxide thin films studied by Drude–Lorentz model, Appl. Surf. Sci. 255 (2009) 7203–7211, http://dx.doi.org/10.1016/j.apsusc.2009.03.064. [30] E. Burstein, Anomalous optical absorption limit in InSb, Phys. Rev. 93 (1954) 632–633, http://dx.doi.org/10.1103/PhysRev.93.632. [31] A. Walsh, J. Da Silva, S.-H. Wei, Origins of band-gap renormalization in degenerately doped semiconductors, Phys. Rev. B 78 (2008) 075211, http://dx.doi.org/10.1103/ PhysRevB.78.075211. [32] M. Batzill, Surface science studies of gas sensing materials: SnO2, Sensors 6 (2006) 1345–1366, http://dx.doi.org/10.3390/s6101345 (Peterboroug.). [33] L.R. Covington, J.C. Moore, Photoconductivity and transient response of Al:ZnO:Al planar structures fabricated via a thermal oxidation process, Thin Solid Films 540 (2013) 106–111, http://dx.doi.org/10.1016/j.tsf.2013.05.128. [34] N. Barsan, U. Weimar, Conduction model of metal oxide gas sensors, J. Electroceramics. 7 (2001) 143–167, http://dx.doi.org/10.1023/A:1014405811371.