Chemical Synthesis, Characterization and Gas-Sensing Properties of Thin Films in the In 2 O 3 -SnO 2 System

Transcription

1 Chemical Synthesis, Characterization and Gas-Sensing Properties of Thin Films in the In 2 -SnO 2 System Mauro Epifani 1, Raül Díaz 2, Antonella Taurino 1, Luca Francioso 1, Pietro Siciliano 1, Joan R. Morante 2 1 CNR, Istituto per la Microelettronica ed i Microsistemi, Sezione di Lecce, via Arnesano, 731 Lecce, Italy; 2 Universitat de Barcelona, Departament d Electrònica, C. Martí i Franqués 1, 828 Barcelona, Spain. ABSTRACT Thin films in the SnO 2 -In 2 system, with relative concentrations of the two oxides ranging from 2% to 98% (molar percentage of the oxide), were deposited by sol-gel and solution processes. The films for the physical characterizations were deposited onto oxidized silicon substrates, while the films for the gas-sensing tests were deposited onto alumina. The starting solutions were characterized by FTIR spectroscopy, while the films on silicon, heated at various temperatures, were characterized by X-ray diffraction and SEM observations. The interaction between the two systems is particularly evident in the case of the system described by a 5% In 2-5% SnO 2 nominal composition. The crystallization on In 2 during the film heattreatment hinders the crystallization of SnO 2, thus Sn is dispersed as an n-dopant in the In 2 lattice, indeed showing a current signal, in the gas-sensing test, two orders of magnitude higher than the pure film. The response of the mixed-oxide based device to NO 2 is better than pure In 2. On the other hand, the response (relative resistance change) of pure SnO 2 to low (from.1 to 1 ppm) NO 2 concentrations ranges from 15 to 3, a result that can be correlated with the nanostructure of the film, which, from SEM and XRD results, seems constituted by very small grains. INTRODUCTION The utilization of additive elements to the active material in gas-sensing devices is a usual procedure [1], aiming at improving the device performance, in particular as concerns its selectivity and response time. In this sense the additive acts as a catalyst, but it must be taken into account that it may also influence the intrinsic properties of the sensing layer, from the grain size to the surface morphology and to the electrical conductivity. Thus, both from a fundamental and a device-oriented perspective, it is of interest to explore the influence of the additive concentration on the sensor properties and performances. In the case of thin-films based devices, the physical deposition techniques hardly allow to control the additive concentration and its distribution in the active layer. Very recently, an increasing interest has been emerging in the chemical deposition of thin films for gas-sensing devices. The problematic of introducing varying additive amounts in the active material can be properly analyzed with such techniques. Moreover, it is possible to start from nominal compositions that are not allowed by the phase diagrams of the two components, and to investigate the possible formation of heterostructures of the two oxides on a nanometric scale. In this work, the change in the sensor properties and

2 performance were studied by focusing on In 2 and SnO 2 thin films, both pure and containing different relative concentrations of the two oxides [2-4]. We show that the precursor solutions are constituted by non-interacting species of the two elements, which results in films where the only a prevalent oxide is present while the other element is dispersed within the structure as a dopant. The electrical and gas-sensing properties of selected films are discussed with respect to the film composition. EXPERIMENT Indium oxide sols were prepared by dissolving 1 g of In(N ) 3 5H 2 O in 15 ml of methanol [5]. After the dissolution of the salt, acetylacetone (CH 3 COCH 2 COCH 3, acach) was dropped in the solution in order to chelate the In 3+ ions, with an acach/in molar ratio of 2. Finally, concentrated ammonium hydroxide (3 wt% solution in water) was added to the solution, with a NH 3 /In molar ratio of.5. As regards the SnO 2 sol, the process was based on the use of Sn(II) ethylhexanoate [6]: 1.25 g of such precursor were dissolved in 7 ml of butanol. To all the solutions a suitable amount of cetyltrimethylammonium bromide (CTAB) was added in order to improve the adhesion of the films onto the silicon substrates. For preparing the mixed films, suitable aliquots of the two sols were mixed in order to get molar relative concentrations of the two oxides ranging from 2% to 98%. Silicon (<1> orientation) with a 5 Å SiO 2 overlayer or alumina substrates were used for depositing thin films, by spin-coating the obtained sols at 2 rpm onto the pre-cleaned substrates. The films were dried for 5 minutes at 7C in air, then heat-treated for 1 hour at temperatures ranging from 15C to 5C in air atmosphere, with a heating rate of 3C/hr. The films were very uniform and crackless after the various heat-treatments. The phase composition of the thin films was determined by X-ray diffraction (XRD). The chemical structure of the starting solution was studied by Fourier Transform Infrared Spectroscopy (FTIR). The samples for such analyses were prepared by placing a drop of each solution onto a KBr pellet and then drying it at room temperature. Films for the gas-sensing tests were deposited onto alumina substrates followed by a lithographic process for depositing interdigitated electrical contacts. The experimental set-up for gas sensing measurements consists of a test chamber, which hosts the sensors, a pipeline system for the transfer of the gases from certified bottle to the chamber, a mass flow controller (MKS mod. 647B) connected to the mass flow meters, a power supply for applying the suitable voltage V H to the heating meander whose resistance gave the desired sensor s working temperature, and, finally, an electrometer (Keithley mod. 6517A) equipped with a multiplexer for continuously monitoring the electrical signals from the sensors and with an internal power supply for fixing the bias at the interdigitated electrical contacts. Dry air was used both as reference gas and as diluting gas to obtain NO 2 mixtures in air at different concentrations (.1 1 ppm). Certified bottles of synthetic air and NO 2 (5 ppm in dry air) were used. All gases were injected normally to the film surface at the same total flow rate of 1 sccm. RESULTS The final composition of the films is determined by the chemical structure of the starting sols, which was investigated by FTIR spectroscopy. In Figure 1 the spectra obtained on sols with various compositions are reported. The bottom curve refers to pure In 2 sol, while the top curve

3 is related to a pure SnO 2 sol. In the pure In 2 sol the main features are the In-OH stretching at about 14 cm -1 and the bands of the indium acetylacetonato complex above 15 cm -1 [7], Absorbance (a.u.) Decreasing In 2 molar percentage Wavenumbers (cm -1 ) Figure 1. FTIR spectra measured on solutions whose composition is indicated by the nominal In 2 molar percentage in the right part of the figure. 5C 98% 95% 9% Intensity (a.u.) 7% 5% 3% 1% 5% 2 % % Θ (degrees) Figure 2. XRD patterns measured on thin films whose composition is indicated by the nominal In 2 molar percentage in the right part of the figure. The stars indicate the cubic phase of In 2, while the circles indicate tetragonal SnO 2. indicating that In 3+ are effectively chelated by acetylacetone. Nevertheless, peaks at 323 and 1389 cm -1, which we have assigned to In-OH, reveal that the In 3+ ions are not completely prevented by acach from reacting with the base. The low frequency flank of the curve, which for

4 many inorganic oxides contains typical features due to the metal-o-metal vibrations, only displays a very broad band. It is concluded that, upon base addition, the indium complex is partially hydrolyzed but extensive inorganic polymerization is hindered. On the other hand the curve related to the SnO 2 precursor solution is identical to that of the Sn precursor (not shown), apart for the O-H stretching of butanol. Thus the mixed solutions are formed starting from weakly polymerized In 2 sols and Sn(II) 2-ethylhexanoate molecules dispersed in butanol. The result of the mixing are shown in the remaining curves in Figure 1. It is clear that for the various compositions the curves appear as a simple sum of the curves related to the parent solutions, with relative intensities dictated by the starting proportions of the two components in the solutions. No obvious differences can be seen, in particular as concerns the possible formation of In-O-Sn heterobonds. This result can be interpreted as a direct consequence of the chemistry of the Sn solution, where the precursor is simply dispersed in butanol without any solvolysis. The important consequences of these results on the composition of the final films, as determined by their XRD patterns, are shown in Figure 2. The pattern for pure In 2 is identical to that with 2% SnO 2 and is not shown for clarity. Despite the two systems are not miscible in all proportions, there is no evidence of phase separations or new phases, but only pure In 2 or SnO 2 are observed, depending on the composition of the starting sol. This result can be explained referring to the FTIR results. Since no mixed bonds appear in the solution, the as-deposited films is constituted by: a) when the nominal In 2 concentration is prevalent, an amorphous In-based layer in which the molecules of the Sn precursor are embedded; b) when the nominal SnO 2 concentration is prevalent, a layer of SnO 2 precursor embedding In-based species. 5.x1-6 In 2 1E-4 4.x1-6 SnO 2 3.x1-6 1E-5 I(A) 2.x1-6,1 ppm I (A) 1E-6,1 ppm 1.x1-6,5 ppm 1 ppm Time (min),5 ppm 1 ppm 1E Time (min) 6.x1-4 5% In2 O3-5% SnO2 5.x1-4 4.x1-4 I (A) 3.x1-4 2.x1-4,1 ppm 1.x1-4,5 ppm. 1 ppm Time (min) Figure 3. Dynamic responses to the indicated NO 2 concentrations of pure In 2 and SnO 2 films and a mixed film with the indicated composition.

5 In the first case, since pure In 2 crystallizes at lower temperatures than pure SnO 2 (XRD series not shown here), upon the heat-treatment a crystalline In 2 film is formed, in which Sn is dispersed upon the pyrolysis of its precursor. The shift to lower angles of the XRD peaks indicates a substitution of Sn ions for In ions in the In 2 lattice, an hypothesis that will be reinforced by the gas-sensing tests, shown below. The same holds in the case b), but here the presence of larger, polymerized In 2 species generated in the related sol hinders the crystallization of the SnO 2 layer when the In 2 concentration exceeds a 1% value, probably by decreasing the diffusion of Sn species during the heat-treatment. These results help in understanding the electrical and gas-sensing properties of the films, which were measured for the pure films and the 5% In 2-5% SnO 2 film. The dynamic responses to various concentration of NO 2 are shown in Figure 3. The films display a response time between 3 s (.1 ppm NO 2 ) and 8 s (1 ppm NO 2 ) in the case of the SnO 2 and the mixed films, and between 1 and 15 s for the In 2 film. The recovery times are all above 3 s (.1 ppm NO 2 ) or 6 s (1 ppm NO 2 ), which is a typical effect of NO 2. In relationship with the previous results, it is important to observe that the baseline current values are in the order In 2 < SnO 2 < In 2 -SnO 2. The last relationship is a strong support to the hypothesis that Sn is dispersed in the In 2 lattice in substitutional position, since in this case an n-doping effect would be expected and, then, an increase of the conductivity, in agreement with the observed results. Response ( R/R) In 2,1 ppm NO 2,5 ppm NO 2 1 ppm NO 2 25% RH Response ( R/R) SnO 2,1 ppm NO 2,5 ppm NO 2 1 ppm NO 2 25% RH Temperature (C) Temperature (C) Response ( R/R) % In 2-5% SnO 2,1 ppm NO 2,5 ppm NO 2 1 ppm NO 2 25% RH Temperature (C) Figure 4. Calibration curves to NO 2 of pure In 2 and SnO 2 films and a mixed film with the indicated composition. The humidity effect is also shown. The calibration curves to NO 2 for the three films are shown in Figure 4. Appreciable response is observed for the mixed film, but it is remarkable the behavior of pure SnO 2, displaying very high response even to.1 ppm of NO 2. This result can be partly explained referring to the film morphology and structure: while the XRD result evidence the formation of smaller grains (7-8

6 nm) if compared to pure In 2 or to the mixed films, the SEM observations (not shown) show that the film has a granular structure, constituted by very small particles in a compact arrangement. The more efficient charge depletion in such small grains upon interaction with oxygen could be responsible for the large response when the material is exposed to NO 2. Another important result is the low effect of humidity, which usual acts as an interferent in gas-sensing tests. This result can be related to the clean surface of the films, in particular as concerns the surface coverage by OH groups, which is absent, as shown by FTIR studies (not shown). CONCLUSIONS The use of chemical processing has allowed the preparation of thin films in the In 2 -SnO 2 system, starting from solutions with unusual relative percentages of the two oxides. These compositions, due to the lack of interaction of the precursors of the two oxides in the sol stage, result in films where only one oxide crystallizes while the other produces a heavy doping effect. Structural studies by XRD have clearly evidenced this effect, which has then been confirmed by the electrical characterization of the films. A more detailed characterization of the films by Auger spectroscopy is under way, which should allow a more careful determination of the composition of the films and a better understanding of the gas-sensing properties. ACKNOWLEDGMENTS This work was supported in the frame of the NANOS4 (Grant NMP4-CT ) project. One of the authors (M. E.) acknowledges the financial support from the National Council of Research (CNR) for a short-term stay at the University of Barcelona (Spain). REFERENCES 1. D. Kohl, J. Phys. D: Appl. Phys. 34, R125 R149 (21). 2. A. L. Swint, P. W. Bohn, Langmuir 2, 476, (24). 3. A.Salehi, M. Gholizadeh, Sens. Act. B 89, 173 (23). 4. S. M. Lee, Y. S. Lee, C. H. Shim, N. J. Choi, B. S. Joo, K. D. Song, J. S. Huh, D. D. Lee, Sens. Act. B 93, 31 (23). 5. M. Epifani, S. Capone, R. Rella, P. Siciliano, L. Vasanelli, G. Faglia, P. Nelli and G. Sberveglieri, J. Sol-Gel. Sci. Techn. 26, 741 (23). 6. C. Savaniu, A. Arnautu, C. Cobianu, G. Craciun, C. Flueraru, M. Zaharescu, C. Parlog, F. Paszti, A. Van den Berg, Thin Solid Films 349, 29, (1999). 7. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds. Part B: Applications in Coordination, Organometallic and Bioinorganic Chemistry, 5 th ed. (Wiley, New York, 1997), pp