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Cathodoluminescence emission study of nanocrystalline indium oxide films deposited by spray pyrolysis
Thin Solid Films 515 (2007) 8065 – 8071 www.elsevier.com/locate/tsf
Cathodoluminescence emission study of nanocrystalline indium oxide films deposited by spray pyrolysis G. Korotcenkov a,⁎, M. Nazarov a,b , M.V. Zamoryanskaya c , M. Ivanov a a b c
Technical University of Moldova, Chisinau, Republic of Moldova Gwangju Institute of Science and Technology, Republic of Korea A.F.Ioffe Physical Technical Institute, RAS, St. Petersburg, Russia
Received 18 March 2006; received in revised form 25 January 2007; accepted 28 March 2007 Available online 11 April 2007
Abstract The results of analysis of In2O3 film cathodoluminescence (CL) spectra are presented in this paper. In2O3 films, aimed for gas sensor application, were deposited by spray pyrolysis from 0.2 M InCl3–water solutions. The influence of grain size (10–60 nm), film thickness (20– 400 nm), pyrolysis temperature (Tpyr = 400–520 °C), and annealing in the air or nitrogen atmospheres (Tan = 600–1100 °C) on CL emission of In2O3 is discussed. CL spectra of as-deposited In2O3 films were characterized by a broad band centered at λ ∼ 570–600 nm. The annealing of studied films leads to a considerable increase of CL intensity. High annealing temperature of In2O3 films (Tan N 850 °C) is being accompanied by the appearance of additional bands centered at λ ∼ 400, 550, and 650 nm, which are peculiar to single-crystalline In2O3 nanobelts, or nanowires with perfect crystal structure. It was concluded that the improvement of crystal structure and the decrease of the concentration of oxygen vacancies are the main factors determining the change of CL spectra of In2O3 films and the appearance of edge luminescence. © 2007 Elsevier B.V. All rights reserved. Keywords: In2O3; Films; Spray pyrolysis; Cathodoluminescence; Annealing
1. Introduction The transformation of metal oxide structure during synthesis and following thermal treatment in many cases determines the properties of the majority of electronic devices, designed on their base. This process has been studied pretty well for most of the metal oxides, including In2O3. The results of indicated research that one can find, for example, in Ref. [1]. Increased interest to In2O3, which is one of the wide band gap semiconducting materials, is conditioned by the fact that this metal oxide can be promising for various applications due to its interesting optical, electronic and surface properties. In2O3 has been widely used in the microelectronic applications, including transparent heating elements for aircraft and car windows, solar cells, heat reflecting mirrors for glass windows, gas sensors, photovoltaic devices, biological systems and in a variety of
⁎ Corresponding author. Tel.: +373 22 235437. E-mail address: [email protected] (G. Korotcenkov). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.03.186
electro-optical devices, such as liquid-crystal flat panel displays [2–6]. Many articles, devoted to the study of physical and optical properties of In2O3 films have been published in [7–10]. However, the study of In2O3 luminescence properties remains very limited. At the same time, the importance of those researches for understanding the nature of many phenomena, observed in metal oxides and devices on their basis, do not raise doubts. They may provide information about both structural properties and energy levels of native defects, which cannot be obtained, using standard methods of studying the electrophysical properties. All mentioned above has determined the objective of the present research. In2O3 films, deposited by spray pyrolysis, have been chosen as a subject of study. In the literature it was found that indium oxide thin films could be deposited by a magnetron sputtering, reactive thermal deposition, spray pyrolysis and so on. Our choice of spray pyrolysis deposition method is conditioned by the fact that this technique is simple, cheap, and easily adaptable for various applications [11,12].
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Those advantages of spray pyrolysis technology have been especially useful for both commercial applications and preparing samples with variation of their structural properties. The results, obtained during the testing of gas sensing properties of In2O3 films, deposited by spray pyrolysis method, have been an additional stimulus for conducting discussed research. Sensors on the base of In2O3 films have shown high response to oxidizing gases [5,13], and what is the most important, they had shorter characteristic times of both response and recovery processes in comparison with studied early sensors on the base of tin and tungsten oxides [14,15]. 2. Experimental details The spray pyrolysis experimental set-ups, and the details of the procedure, applied for the deposition of the studied In2O3 thin films, have been described in Ref. [12]. Indium oxide thin films were prepared by spraying of 0.2 M InCl3–water solution on oxidized Si substrates, heated at different temperatures (Tpyr) from 400 to 520 °C. Structural parameters of In2O3 films were estimated using Xray diffraction (XRD), Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM) methods. AFM images were obtained using MultiMode Scanning Probe Microscope with Nanoscope IIIa Controller of Digital Instruments. These measurements have been carried out in contact scanning mode, using AS-0.5 scanner (scan rate 2 Hz; resolution 0.4 nm). XRD analysis was carried out by a Siemens D5000 diffractometer and Rigaku Rotaflex X-ray diffractometer with a rotating anode source, working with the Kα of the Cu. For structural characterization we used the θ/2θ mode of measurements. The size of the crystallites forming the film was evaluated by Scherrer's formula. For SEM measurements we used the scanning electronic microscopes Jeol JSM840, Philips XL30, and Stereoscan JS360 Cambridge Instruments. Typical parameters of In2O3 films, used in present research, are given in Table 1. Morphology of studied In2O3 films is shown in Fig. 1. More detailed description of the structural properties of studied In2O3 films was presented earlier in Ref. [1]. Basing on the results of X-ray diffraction, Scanning Electron Microscopy (SEM) and AFM study of In2O3 films it was established that depending from deposition temperature and thickness, In2O3 films, having cubic structure, can be either randomly oriented, or textured in direction [100]. The degree of
texturing is increased for films with thickness more than 30– 40 nm deposited at higher temperature. The specificity of the influence of structural parameters of indium oxide films on their gas sensing characteristics was described in Refs. [5,13]. Annealing of In2O3 films was carried out in a flow-type reactor in an atmosphere of air or N2 + (1–2) % O2. The temperature was varied from 600 to 1100 °C. The duration of annealing was equal to 1 h. The selection of these temperatures is conditioned by the fact that at those very temperatures the most considerable structural changes in the films have been taking place [1]. Room temperature cathodoluminescence (CL) measurements have been conducted, using spectrometer, recording radiation in the range of 300–850 nm. The CL spectra were excited in vacuum in the camera of a standard microprobe analyzer Camebax-4. The luminescence emitted was collected through the quartz window of the camera of the unit. The sample was excited by the electron beam with a diameter up to 1 μm. The conditions of excitation have been selected under the stipulation, that at the maximum of CL intensity the electron beam did not interact with substrate. In particular, for studied objects optimal conditions of measurement have been observed at excitation energies about 10–15 kV and electron current from 30 to 100 nA. 3. Results and discussion 3.1. Influence of deposition parameters on cathodoluminescence spectra Typical CL spectra of In2O3 films, grown at various technological parameters, are shown in Figs. 2 and 3. The electron beam energy was 10 keV. Though a considerable difference in pyrolysis temperatures and grain size, the shape of the CL spectra was similar for all In2O3 as-deposited samples. The CL spectra looked like broad peaks with maximum centered at 570–600 nm (2.0–2.1 eV) and halfwidth (FWHM) equaled to ∼ 170–180 nm. It means that we do not have edge luminescence. According to [16,17] a typical value of In2O3 optical direct band gap is 3.6–3.78 eV (λ ∼ 345 nm). The increase of pyrolysis temperature was accompanied only by small red-shift of the emission band, by the increase of its intensity, and by some narrowing of the emission band. For example, as it is shown in Figs. 2–4, the increase of Tpyr from
Table 1 Structural parameters of some studied In2O3 films estimated on the base of AFM, SEM and XRD measurements Sample
1 2 3 4
Tpyr, °C
400 400 520 520
Thickness, nm
∼50–60 ∼300 ∼50–60 ∼200
Structure
Average crystallite size, nm As-deposited
Randomly oriented Randomly oriented Textured Highly textured
Tan = 800 °C
Tan = 1100 °C
XRD
XRD
AFM
SEM
XRD (222)
(400)
(400)
(400)
11 ± 3 – 12 ± 4 18 ± 8
– 48–70 ∼ 17 ∼ 21
∼ 21 ∼ 44 ∼ 15 ∼ 19
∼35 ∼57 ∼33 ∼56
∼ 55 ∼ 60 ∼ 52 ∼ 72
∼ 55 N100 ∼ 54 N100
During XRD characterization the average grain sizes were determined in (222) and (400) crystallographic directions.
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Fig. 1. AFM images of In2O3 films with thickness ∼ 50 nm and ∼ 200 nm after different thermal treatments: (a) as-deposited; (b) Tan = 800 °C; (c) Tan = 1100 °C.
400 to 520 °C during the deposition of In2O3 films with thickness of 50–60 nm was accompanied by the shift of the CL band from 580 to 600 nm. It was also established that the CL spectra of as-deposited films did not depend on film morphology (see Fig. 1). The same shape of the CL spectra was observed for both randomly oriented In2O3 films, and highly textured ones. As it was shown in Ref. [1] and indicated in Table 1, if In2O3 films, deposited at low pyrolysis temperatures, are randomly oriented, In2O3 films deposited at high pyrolysis temperatures would be textured or highly textured in direction [100]. Regarding the influence of In2O3 film thickness on the CL spectra, here it is necessary to note such peculiarities, as the drop of the CL intensity, and the broadening of the band of radiation with the thickness decrease (see Fig. 2, curves a and b).
Similar spectra with dominant luminescence in the orange spectral region, peaking at λ ∼ 634 nm, were reported earlier in Ref. [18] for In2O3 films, formed by thermal oxidation of indium thin film. The broad band had an FWHM of 0.4 eV. Strong and broad photoluminescence (PL) emission spectra with peak, centered at λ ∼ 570 nm, and shoulder at 630 nm, were observed in [19] for In2O3 nanobelts synthesized by chemical vapor deposition method as well. Analyzing the CL and PL spectra of In2O3, the authors of the paper [16] have supposed that the origin of this luminescence was connected either with structural defects, or oxygen deficiencies, formed in these structures. Even more, in [3,19] it was supposed that those oxygen vacancies generally act as deep defect donors in semiconductor, and would cause the formation of new energy levels in the band gap. Thus,
Fig. 2. Typical CL spectra of various In2O3 films deposited by spray pyrolysis: (a) Tpyr = 400 °C, d ∼ 50–60 nm; (b) Tpyr = 400 °C, d ∼ 300 nm; (c) Tpyr = 520 °C, d ∼ 50–60 nm.
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Fig. 3. Influence of annealing in air on CL spectra of In2O3 films deposited at Tpyr = 520 °C (d ∼ 200 nm): (1) as-deposited; (2) Tan = 800 °C.
the emission results from the radiative recombination of photogenerated holes with electrons, which belong to ionized oxygen vacancies. It's clear, that this statement is too indefinite and maybe inaccurate. It is known that in In2O3, besides oxygen vacancies, the generation of other defects is also possible. For example, in Ref. [20] it was concluded that interstitial indium ions are probably the predominant defects in unstoichiometric In2O3. While in Ref. [21], F-centers and In2+ ions in two types of coordination neighborhood were found in In2O3 after annealing in air. Besides, impurities and point defects could form different complexes and associates [22]. It is also necessary to take into account that oxygen vacancies, responsible for electron character of indium oxide conductivity, form shallow energy levels [10], and therefore they cannot be responsible for the appearance of the emission bands with energies, corresponding to the midgap of In2O3.
Fig. 4. CL spectra of In2O3 films deposited at Tpyr = 400 °C (d∼50–60 nm) after different thermal treatments: (1) as-deposited; (2) Tan = 800 °C; (3) Tan = 1000 °C.
It's necessary to note that it was not conducted by any systematical research aimed at the estimation of energy levels of native defects in In2O3. Therefore at present we are not able to make any well-defined conclusion about the nature of the centers, responsible for observed CL bands. We can only suppose that probably observed CL emission was conditioned by donor–acceptor pair recombination [23], where oxygen vacancies can be considered only as possible participants of this process. It is known that In2 O3 is related to oxides, characterized by natural unstoichiometry. Selected method of deposition is an additional reason for increased concentration of both structural and point defects in studied In2O3 films. In metal oxide films, deposited in non-equilibrium conditions, the probability of both incomplete metal oxidation, and structural defects' accumulation during process of crystallite growth is high. It could be a consequence of high rate of film growth, peculiar to spray pyrolysis deposition. Structural study of tin oxide powders, synthesized at low temperatures, has shown that as-deposited nano-size grains of tin dioxide contained both crystalline core and amorphous or highly distorted unstoichiometric surface layer [24]. One can suppose that In2O3 grains synthesized at low temperatures had the same structure. The data of In2O3 surface characterization, conducted using synchrotron radiation photoemission study of films, deposited by spray pyrolysis (Tpyr − 350–500 °C), confirm this fact [25]. So, thin unstoichiometric layer, formed at the surface of In2O3 grains, can be also a source of oxygen vacancies in the bulk of In2O3 grains. 3.2. Influence of post deposition annealing on cathodoluminescence spectra Cathodoluminescence spectra of In2O3 films, subjected to various thermal treatments, are given in Figs. 3–5. Analyzing the influence of annealing on the CL spectra, we have determined that the annealing at temperatures smaller than 600–800 °C showed weak influence on their shape. Only after annealing at temperatures higher than 800 °C, we could observe
Fig. 5. CL spectrum of thick In2O3 film deposited at Tpyr = 400 °C (d ∼ 300 nm) after annealing at 1000 °C.
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both the increase of CL intensity, and the appearance of new peaks (see Figs. 3–5). As a rule, after annealing at Tan = 1000 °C, the CL intensity was increased in more than 5– 6 times in comparison with the as-deposited samples. It was also established that the appearance of additional emission bands depended from annealing temperature. If after treatment at Tan = 800 °C the main band was split on two peaks, located at λ ∼ 550 nm and λ ∼ 650 nm, after thermal treatments at Tan = 1000–1100 °C, one more band, centered at λ ∼ 400 nm, started developing on the CL spectra. The authors of Refs. [3,4,26,27] had also observed radiation bands at λ ∼430–460 nm. But they connected their appearance either with quantum-size effects, or with oxygen deficiencies. After thermal treatments, contributing to grain's growth, the CL peaks, observed in [27], had red-shift; and after annealing at 850 °C the photoluminescence spectra had broad band centered at λ ∼560 nm. We believe, that in our case the appearance of the band at λ ∼400 nm after high temperature annealing is not connected with quantum-size effect, because this band appeared only after thermal treatments, when the crystallites reached maximum size [1]. Earlier it was established that the quantum-size effect, i.e. the blue-shift of luminescence spectra of metal oxides, is being observed for metal oxides with grain size less than 2–3 nm [27]. As it is shown in Table 1, the grain size in In2O3 films after annealing at Tan N 850 °C is in the range of 50–100 nm. According to [1], the annealing of In2O3 films already at Tan N 600 °C first of all leads to coalescence of the smallest grains. Therefore, we can conclude that in In2O3 films annealed at Tan 800 °C one can neglect quantum-size effects. It means that in In2O3 films after annealing, especially at Tan N800 °C, both the broadening and the shifting of the CL peaks' positions, conditioned by quantum-size effect, does not exist, and the positions of the CL peaks for such films exactly correspond to a real energetic structure of defects, responsible for radiative recombination. At the same time we do not exclude the presence of quantum-size effects in our as-deposited films, which had broad peak, centered at λ ∼ 570–600 nm. Research, made in [1], has shown that grain sizes in as-deposited In2O3 films had big dispersion, and fine-dispersed fraction can have grains with size equaled 1–2 nm. Basing on the results of research, showing that blue-shift, caused by quantum-size effects, depended on grain size [27], it becomes clear that at big dispersion of grain sizes we have to get broad CL band due to superposition of CL peaks, located at different wavelengths. In the framework of such explanation it is also understandably described before the influence of both Tpyr, and film thickness on the position of the CL peak of as-deposited In2O3 films. It is in full accordance with the results, presented in Ref. [1], where it was shown that the lowering of both the pyrolysis temperature and the film thickness are, the bigger is the contents of fine-dispersed fraction with small grain size in the structure of In2O3 films. It means, that in such films the blue-shift of the CL band should be stronger and the CL peak should be wider. This very influence we have observed in our experiments. The absence of broad luminescence band in [27] could be explained by high homogeneity of grain size of In2O3 particles, synthesized in meso-porous silica.
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As we mentioned earlier, the In2O3 surface of as-deposited films is unstoichiometric. According to [25], high density of the surface states within In2O3 band gap is characteristic for such surface. However, as it was established in Ref. [25], thermal treatment in oxygen atmosphere already at Tan N150–180 °C leads to sharp decrease of those states' density. At the same time, as present research has shown, we did not observe any considerable changes in the CL spectra after thermal treatments at indicated temperatures. It means that the state of the surface plays a secondary role in the processes, controlling both intensity and the shape of the CL spectra. Most likely both the shape and the CL intensity are being controlled by bulk properties of In2O3, i.e. defects, responsible for radiative recombination, have bulk nature. We believe that taking place transformation of the CL spectra and therefore structure of the defects, are a consequence of the improvement of In2O3 films' crystallographic structure. The results, obtained during In2O3 nanobelts research, presented in Refs. [19,28–30], are basis for our conclusion. Exactly in the PL spectra of In2O3 nanobelts, there were found emission bands, observed in the CL spectra of our films after high temperature annealing. For example, in Ref. [29] for In2O3 nanowires with diameter of ∼ 10–100 nm, a strong and broad blue-green emission band was observed centered at λ ∼ 465 nm. For In2O3 nanowires with a diameter of about 20–200 nm (Tsynthesis = 980 °C) [20], the peaks of PL were centered at 416–475 nm. As it's known, In2O3 nanobelts are characterized by high-quality single-crystalline structure [19,28], and they have minimal concentration of structural defects. It means that the appearance of the band at λ ∼ 400 nm is not a consequence of high concentration of oxygen vacancies. We believe that the appearance of this peak is a result of this concentration's decrease. The authors of [16] have also supposed that the CL intensity is directly correlated to the film crystallinity. Therefore, the strong increase of CL intensity and the appearance of peaks, centered at λ ∼400 nm after annealing at Tan ≥1000 °C (see Fig. 5), indicates that only after annealing at Tan ≥1000 °C the crystal lattice of indium oxide approaches its structural perfection. Because of this fact, in the CL spectra one can observe the luminescence bands, close to the edge emission [16]. One can suppose that the change of the CL spectra is a consequence of the decrease of states density near either the bottom of conduction band, or top of valence band. It is necessary to note that our results are good confirmation of the conclusions, made in Ref. [31]. In [31] it was found out that In2O3 had native unstoichiometry, and the crystallographic structure of In2O3 approached to stoichiometric composition only after annealing in oxygen atmosphere at temperatures higher than 1000 °C. Results of research, testifying that CL intensity of In2O3 after annealing in the air became higher than after annealing in nitrogen atmosphere, are in the frame of suggested explanations. As In2O3 grain size in both cases was approximately the same [1], the above mentioned difference was a consequence of distinction in the rates of non-radiating recombination. Taking into account the influence of oxygen partial pressure on the concentration of oxygen vacancies, one can conclude that discussed effect is conditioned by the difference in both bulk
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and surface concentrations of oxygen vacancies in In2O3. After annealing in oxygen atmosphere this concentration is considerably lower. One should note that observed improvement of crystal structure of metal oxide grains during high temperature treatment most likely could serve as an important factor for the achievement of optimal gas sensing characteristics of resistive type sensor [11]. For example, in Ref. [32] it was established that undoped SnO2 calcinated at 1000 °C showed very good sensitivity to nitrogen oxide (NO2), with a negligible cross-sensitivity to CO, together with a high long term stability, as a consequence of its structural parameters improvement. In Ref. [13] it was found that the increase of pyrolysis temperature during In2O3 deposition had the same effect and leaded to a considerable increase of sensitivity to ozone; and to a decrease of sensor response to reducing gases, such as CO and H2. We assume that crystals with more perfect crystallographic structure should have smaller concentration of native bulk defects, lesser concentration of native surface states, and stronger bond of bridging oxygen with In2O3 lattice. According to Refs. [33,34], the decrease of surface states density, which may assist in the pinning of surface Fermi level, expands possible range of surface potential change; and, as a result, promotes the increase of sensor response to ozone. It is important to note here, that a short-wave peaks in the range of λ ∼400 nm have been observed just for In2O3 nanobelts or nanowires, synthesized at high temperatures; or while using special catalysts. In2O3 nanowires, synthesized at low temperatures (b800 °C), like our films, annealed at T ∼ 800 °C, had PL with emission bands, centered at λ ∼ 570 nm, and ∼ 670 nm. It testifies that In2O3 nanowires synthesized at low temperatures are not characterized by such high crystallographic perfection, as In2O3 nanowires, synthesized at high temperatures. It means that in In2O3 nanowires, synthesized at low temperatures, the concentration of point defects didn't decrease to the level, when either band–band or shallow levelband recombination starts dominating in CL spectra. With regard of the CL band, centered at λ ∼ 425–460 nm, observed in Ref. [35] for In2O3 nanowires with diameter ∼ 40 nm, it is early to say something about its nature. Present nanowires were assembled into ordered nanochannels of anodic alumina membranes (AAM), using specific method, based on indium electro-deposition with following thermal oxidation. Besides, it was found that AAM had PL emission in the same spectral range. Moreover, this band was observed in the PL spectra of as-deposited nanowires, which, according to their composition, should correspond to metallic indium, but not indium oxide. 4. Conclusion The initial shape of the CL spectra and the regularities of their transformation during high temperature annealing allow to assume that as-deposited In2O3 films (Tpyr − 350–520 °C) are highly defective ones with lattice disorder and high concentration of point defects independently of either pyrolysis temperature and film thickness. Exactly because of this reason
the short-wave emission band is absent in the experimental spectra of In2O3 films. The CL spectra of these films were characterized by one broad band, centered at λ ∼ 650 nm. The In2O3 film's annealing leads to a sufficient increase of CL intensity. At that the annealing is accompanied by the appearance of additional bands, centered at λ ∼ 400, 550, and 650 nm, which are peculiar to high-quality single-crystalline In2O3 nanobelts, or nanowires with perfect crystallographic structure. It was made a conclusion that the changes of the CL spectra shape and the appearance of short-wave band at λ ∼400 nm are connected with improvement of crystallographic structure of In2O3 grains. This process becomes more intensive after annealing at temperatures higher, than 800 °C. Materials, annealed at temperatures lower than 600–800 °C, retain high concentration of bulk point defects, conditioned by lattice disordering and unstoichiometry of deposited films. It was established that the after-effects of annealing depend on annealing atmosphere. Short-wave band centered at λ ∼400 nm after annealing in oxygen atmosphere is more intensive, than after annealing in nitrogen atmosphere. Indicated difference was explained by the influence of atmosphere on the concentration of oxygen vacancies in In2O3 films. Acknowledgements This work was supported by the Civilian Research and Development Foundation (CRDF) and Moldovan Research and Development Association (MRDA) in the framework of CGP Program (Grant MO-E2-3054-CS-03) and partially supported by Korean Program BK21. Authors are thankful also to the Supreme Council of Science and Advanced Technology of the Republic of Moldova for financial support, and to Prof. A. Cornet, J.R. Morante, J. Schwank, V. Matolin and Dr. V. Brinzari for helping with structural and surface characterization of studied films. References [1] G. Korotcenkov, V. Brinzari, M. Ivanov, A. Cerneavschi, J. Rodriguez, A. Cirera, A. Cornet, J.R. Morante, Thin Solid Films 479 (2005) 38. [2] G. Korotcenkov, I. Boris, V. Brinzari, Yu. Luchkovsky, G. Karkotsky, V. Golovanov, A. Cornet, E. Rossinyol, J. Rodriguez, A. Cirera, Sens. Actuators, B, Chem. 103 (2004) 13. [3] C.H. Liang, G.W. Meng, Y. Lei, F. Fhillipp, L.D. Zhang, Adv. Mater. 13 (2001) 1330. [4] X. Li, M.W. Wanlass, T.A. Gessert, K.A. Emery, T.J. Coutts, Appl. Phys. Lett. 54 (1989) 2674. [5] G. Korotcenkov, A. Cerneavschi, V. Brinzari, A. Vasiliev, A. Cornet, J. Morante, A. Cabot, J. Arbiol, Sens. Actuators, B, Chem. 99 (2004) 304. [6] C.G. Granqvist, Appl. Phys., A 57 (1993) 19. [7] E. Benamar, M. Rami, C. Messoudi, D. Sayah, A. Enaaoui, Sol. Energy Mater. Sol. Cells 56 (1999) 125. [8] H. Bisht, H.T. Eun, A. Mehrtens, M.A. Aegerter, Thin Solid Films 351 (1999) 109. [9] Z.B. Zhou, R.Q. Cui, Q.J. Pang, Y.D. Wang, F.Y. Meng, T.T. Sun, Z.M. Ding, X.B. Yu, Appl. Surf. Sci. 172 (2001) 245. [10] I. Hamberg, C.G. Granqvist, J. Appl. Phys. 60 (1986) R123. [11] G. Korotcenkov, Sens. Actuators, B, Chem. 107 (2005) 209. [12] G. Korotcenkov, V. Brinzari, J. Schwank, A. Cerneavschi, Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 19 (2001) 73.
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Cathodoluminescence emission study of nanocrystalline indium oxide films deposited by spray pyrolysis G. Korotcenkov a,⁎, M. Nazarov a,b , M.V. Zamoryanskaya c , M. Ivanov a a b c
Technical University of Moldova, Chisinau, Republic of Moldova Gwangju Institute of Science and Technology, Republic of Korea A.F.Ioffe Physical Technical Institute, RAS, St. Petersburg, Russia
Received 18 March 2006; received in revised form 25 January 2007; accepted 28 March 2007 Available online 11 April 2007
Abstract The results of analysis of In2O3 film cathodoluminescence (CL) spectra are presented in this paper. In2O3 films, aimed for gas sensor application, were deposited by spray pyrolysis from 0.2 M InCl3–water solutions. The influence of grain size (10–60 nm), film thickness (20– 400 nm), pyrolysis temperature (Tpyr = 400–520 °C), and annealing in the air or nitrogen atmospheres (Tan = 600–1100 °C) on CL emission of In2O3 is discussed. CL spectra of as-deposited In2O3 films were characterized by a broad band centered at λ ∼ 570–600 nm. The annealing of studied films leads to a considerable increase of CL intensity. High annealing temperature of In2O3 films (Tan N 850 °C) is being accompanied by the appearance of additional bands centered at λ ∼ 400, 550, and 650 nm, which are peculiar to single-crystalline In2O3 nanobelts, or nanowires with perfect crystal structure. It was concluded that the improvement of crystal structure and the decrease of the concentration of oxygen vacancies are the main factors determining the change of CL spectra of In2O3 films and the appearance of edge luminescence. © 2007 Elsevier B.V. All rights reserved. Keywords: In2O3; Films; Spray pyrolysis; Cathodoluminescence; Annealing
1. Introduction The transformation of metal oxide structure during synthesis and following thermal treatment in many cases determines the properties of the majority of electronic devices, designed on their base. This process has been studied pretty well for most of the metal oxides, including In2O3. The results of indicated research that one can find, for example, in Ref. [1]. Increased interest to In2O3, which is one of the wide band gap semiconducting materials, is conditioned by the fact that this metal oxide can be promising for various applications due to its interesting optical, electronic and surface properties. In2O3 has been widely used in the microelectronic applications, including transparent heating elements for aircraft and car windows, solar cells, heat reflecting mirrors for glass windows, gas sensors, photovoltaic devices, biological systems and in a variety of
⁎ Corresponding author. Tel.: +373 22 235437. E-mail address: [email protected] (G. Korotcenkov). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.03.186
electro-optical devices, such as liquid-crystal flat panel displays [2–6]. Many articles, devoted to the study of physical and optical properties of In2O3 films have been published in [7–10]. However, the study of In2O3 luminescence properties remains very limited. At the same time, the importance of those researches for understanding the nature of many phenomena, observed in metal oxides and devices on their basis, do not raise doubts. They may provide information about both structural properties and energy levels of native defects, which cannot be obtained, using standard methods of studying the electrophysical properties. All mentioned above has determined the objective of the present research. In2O3 films, deposited by spray pyrolysis, have been chosen as a subject of study. In the literature it was found that indium oxide thin films could be deposited by a magnetron sputtering, reactive thermal deposition, spray pyrolysis and so on. Our choice of spray pyrolysis deposition method is conditioned by the fact that this technique is simple, cheap, and easily adaptable for various applications [11,12].
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Those advantages of spray pyrolysis technology have been especially useful for both commercial applications and preparing samples with variation of their structural properties. The results, obtained during the testing of gas sensing properties of In2O3 films, deposited by spray pyrolysis method, have been an additional stimulus for conducting discussed research. Sensors on the base of In2O3 films have shown high response to oxidizing gases [5,13], and what is the most important, they had shorter characteristic times of both response and recovery processes in comparison with studied early sensors on the base of tin and tungsten oxides [14,15]. 2. Experimental details The spray pyrolysis experimental set-ups, and the details of the procedure, applied for the deposition of the studied In2O3 thin films, have been described in Ref. [12]. Indium oxide thin films were prepared by spraying of 0.2 M InCl3–water solution on oxidized Si substrates, heated at different temperatures (Tpyr) from 400 to 520 °C. Structural parameters of In2O3 films were estimated using Xray diffraction (XRD), Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM) methods. AFM images were obtained using MultiMode Scanning Probe Microscope with Nanoscope IIIa Controller of Digital Instruments. These measurements have been carried out in contact scanning mode, using AS-0.5 scanner (scan rate 2 Hz; resolution 0.4 nm). XRD analysis was carried out by a Siemens D5000 diffractometer and Rigaku Rotaflex X-ray diffractometer with a rotating anode source, working with the Kα of the Cu. For structural characterization we used the θ/2θ mode of measurements. The size of the crystallites forming the film was evaluated by Scherrer's formula. For SEM measurements we used the scanning electronic microscopes Jeol JSM840, Philips XL30, and Stereoscan JS360 Cambridge Instruments. Typical parameters of In2O3 films, used in present research, are given in Table 1. Morphology of studied In2O3 films is shown in Fig. 1. More detailed description of the structural properties of studied In2O3 films was presented earlier in Ref. [1]. Basing on the results of X-ray diffraction, Scanning Electron Microscopy (SEM) and AFM study of In2O3 films it was established that depending from deposition temperature and thickness, In2O3 films, having cubic structure, can be either randomly oriented, or textured in direction [100]. The degree of
texturing is increased for films with thickness more than 30– 40 nm deposited at higher temperature. The specificity of the influence of structural parameters of indium oxide films on their gas sensing characteristics was described in Refs. [5,13]. Annealing of In2O3 films was carried out in a flow-type reactor in an atmosphere of air or N2 + (1–2) % O2. The temperature was varied from 600 to 1100 °C. The duration of annealing was equal to 1 h. The selection of these temperatures is conditioned by the fact that at those very temperatures the most considerable structural changes in the films have been taking place [1]. Room temperature cathodoluminescence (CL) measurements have been conducted, using spectrometer, recording radiation in the range of 300–850 nm. The CL spectra were excited in vacuum in the camera of a standard microprobe analyzer Camebax-4. The luminescence emitted was collected through the quartz window of the camera of the unit. The sample was excited by the electron beam with a diameter up to 1 μm. The conditions of excitation have been selected under the stipulation, that at the maximum of CL intensity the electron beam did not interact with substrate. In particular, for studied objects optimal conditions of measurement have been observed at excitation energies about 10–15 kV and electron current from 30 to 100 nA. 3. Results and discussion 3.1. Influence of deposition parameters on cathodoluminescence spectra Typical CL spectra of In2O3 films, grown at various technological parameters, are shown in Figs. 2 and 3. The electron beam energy was 10 keV. Though a considerable difference in pyrolysis temperatures and grain size, the shape of the CL spectra was similar for all In2O3 as-deposited samples. The CL spectra looked like broad peaks with maximum centered at 570–600 nm (2.0–2.1 eV) and halfwidth (FWHM) equaled to ∼ 170–180 nm. It means that we do not have edge luminescence. According to [16,17] a typical value of In2O3 optical direct band gap is 3.6–3.78 eV (λ ∼ 345 nm). The increase of pyrolysis temperature was accompanied only by small red-shift of the emission band, by the increase of its intensity, and by some narrowing of the emission band. For example, as it is shown in Figs. 2–4, the increase of Tpyr from
Table 1 Structural parameters of some studied In2O3 films estimated on the base of AFM, SEM and XRD measurements Sample
1 2 3 4
Tpyr, °C
400 400 520 520
Thickness, nm
∼50–60 ∼300 ∼50–60 ∼200
Structure
Average crystallite size, nm As-deposited
Randomly oriented Randomly oriented Textured Highly textured
Tan = 800 °C
Tan = 1100 °C
XRD
XRD
AFM
SEM
XRD (222)
(400)
(400)
(400)
11 ± 3 – 12 ± 4 18 ± 8
– 48–70 ∼ 17 ∼ 21
∼ 21 ∼ 44 ∼ 15 ∼ 19
∼35 ∼57 ∼33 ∼56
∼ 55 ∼ 60 ∼ 52 ∼ 72
∼ 55 N100 ∼ 54 N100
During XRD characterization the average grain sizes were determined in (222) and (400) crystallographic directions.
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Fig. 1. AFM images of In2O3 films with thickness ∼ 50 nm and ∼ 200 nm after different thermal treatments: (a) as-deposited; (b) Tan = 800 °C; (c) Tan = 1100 °C.
400 to 520 °C during the deposition of In2O3 films with thickness of 50–60 nm was accompanied by the shift of the CL band from 580 to 600 nm. It was also established that the CL spectra of as-deposited films did not depend on film morphology (see Fig. 1). The same shape of the CL spectra was observed for both randomly oriented In2O3 films, and highly textured ones. As it was shown in Ref. [1] and indicated in Table 1, if In2O3 films, deposited at low pyrolysis temperatures, are randomly oriented, In2O3 films deposited at high pyrolysis temperatures would be textured or highly textured in direction [100]. Regarding the influence of In2O3 film thickness on the CL spectra, here it is necessary to note such peculiarities, as the drop of the CL intensity, and the broadening of the band of radiation with the thickness decrease (see Fig. 2, curves a and b).
Similar spectra with dominant luminescence in the orange spectral region, peaking at λ ∼ 634 nm, were reported earlier in Ref. [18] for In2O3 films, formed by thermal oxidation of indium thin film. The broad band had an FWHM of 0.4 eV. Strong and broad photoluminescence (PL) emission spectra with peak, centered at λ ∼ 570 nm, and shoulder at 630 nm, were observed in [19] for In2O3 nanobelts synthesized by chemical vapor deposition method as well. Analyzing the CL and PL spectra of In2O3, the authors of the paper [16] have supposed that the origin of this luminescence was connected either with structural defects, or oxygen deficiencies, formed in these structures. Even more, in [3,19] it was supposed that those oxygen vacancies generally act as deep defect donors in semiconductor, and would cause the formation of new energy levels in the band gap. Thus,
Fig. 2. Typical CL spectra of various In2O3 films deposited by spray pyrolysis: (a) Tpyr = 400 °C, d ∼ 50–60 nm; (b) Tpyr = 400 °C, d ∼ 300 nm; (c) Tpyr = 520 °C, d ∼ 50–60 nm.
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Fig. 3. Influence of annealing in air on CL spectra of In2O3 films deposited at Tpyr = 520 °C (d ∼ 200 nm): (1) as-deposited; (2) Tan = 800 °C.
the emission results from the radiative recombination of photogenerated holes with electrons, which belong to ionized oxygen vacancies. It's clear, that this statement is too indefinite and maybe inaccurate. It is known that in In2O3, besides oxygen vacancies, the generation of other defects is also possible. For example, in Ref. [20] it was concluded that interstitial indium ions are probably the predominant defects in unstoichiometric In2O3. While in Ref. [21], F-centers and In2+ ions in two types of coordination neighborhood were found in In2O3 after annealing in air. Besides, impurities and point defects could form different complexes and associates [22]. It is also necessary to take into account that oxygen vacancies, responsible for electron character of indium oxide conductivity, form shallow energy levels [10], and therefore they cannot be responsible for the appearance of the emission bands with energies, corresponding to the midgap of In2O3.
Fig. 4. CL spectra of In2O3 films deposited at Tpyr = 400 °C (d∼50–60 nm) after different thermal treatments: (1) as-deposited; (2) Tan = 800 °C; (3) Tan = 1000 °C.
It's necessary to note that it was not conducted by any systematical research aimed at the estimation of energy levels of native defects in In2O3. Therefore at present we are not able to make any well-defined conclusion about the nature of the centers, responsible for observed CL bands. We can only suppose that probably observed CL emission was conditioned by donor–acceptor pair recombination [23], where oxygen vacancies can be considered only as possible participants of this process. It is known that In2 O3 is related to oxides, characterized by natural unstoichiometry. Selected method of deposition is an additional reason for increased concentration of both structural and point defects in studied In2O3 films. In metal oxide films, deposited in non-equilibrium conditions, the probability of both incomplete metal oxidation, and structural defects' accumulation during process of crystallite growth is high. It could be a consequence of high rate of film growth, peculiar to spray pyrolysis deposition. Structural study of tin oxide powders, synthesized at low temperatures, has shown that as-deposited nano-size grains of tin dioxide contained both crystalline core and amorphous or highly distorted unstoichiometric surface layer [24]. One can suppose that In2O3 grains synthesized at low temperatures had the same structure. The data of In2O3 surface characterization, conducted using synchrotron radiation photoemission study of films, deposited by spray pyrolysis (Tpyr − 350–500 °C), confirm this fact [25]. So, thin unstoichiometric layer, formed at the surface of In2O3 grains, can be also a source of oxygen vacancies in the bulk of In2O3 grains. 3.2. Influence of post deposition annealing on cathodoluminescence spectra Cathodoluminescence spectra of In2O3 films, subjected to various thermal treatments, are given in Figs. 3–5. Analyzing the influence of annealing on the CL spectra, we have determined that the annealing at temperatures smaller than 600–800 °C showed weak influence on their shape. Only after annealing at temperatures higher than 800 °C, we could observe
Fig. 5. CL spectrum of thick In2O3 film deposited at Tpyr = 400 °C (d ∼ 300 nm) after annealing at 1000 °C.
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both the increase of CL intensity, and the appearance of new peaks (see Figs. 3–5). As a rule, after annealing at Tan = 1000 °C, the CL intensity was increased in more than 5– 6 times in comparison with the as-deposited samples. It was also established that the appearance of additional emission bands depended from annealing temperature. If after treatment at Tan = 800 °C the main band was split on two peaks, located at λ ∼ 550 nm and λ ∼ 650 nm, after thermal treatments at Tan = 1000–1100 °C, one more band, centered at λ ∼ 400 nm, started developing on the CL spectra. The authors of Refs. [3,4,26,27] had also observed radiation bands at λ ∼430–460 nm. But they connected their appearance either with quantum-size effects, or with oxygen deficiencies. After thermal treatments, contributing to grain's growth, the CL peaks, observed in [27], had red-shift; and after annealing at 850 °C the photoluminescence spectra had broad band centered at λ ∼560 nm. We believe, that in our case the appearance of the band at λ ∼400 nm after high temperature annealing is not connected with quantum-size effect, because this band appeared only after thermal treatments, when the crystallites reached maximum size [1]. Earlier it was established that the quantum-size effect, i.e. the blue-shift of luminescence spectra of metal oxides, is being observed for metal oxides with grain size less than 2–3 nm [27]. As it is shown in Table 1, the grain size in In2O3 films after annealing at Tan N 850 °C is in the range of 50–100 nm. According to [1], the annealing of In2O3 films already at Tan N 600 °C first of all leads to coalescence of the smallest grains. Therefore, we can conclude that in In2O3 films annealed at Tan 800 °C one can neglect quantum-size effects. It means that in In2O3 films after annealing, especially at Tan N800 °C, both the broadening and the shifting of the CL peaks' positions, conditioned by quantum-size effect, does not exist, and the positions of the CL peaks for such films exactly correspond to a real energetic structure of defects, responsible for radiative recombination. At the same time we do not exclude the presence of quantum-size effects in our as-deposited films, which had broad peak, centered at λ ∼ 570–600 nm. Research, made in [1], has shown that grain sizes in as-deposited In2O3 films had big dispersion, and fine-dispersed fraction can have grains with size equaled 1–2 nm. Basing on the results of research, showing that blue-shift, caused by quantum-size effects, depended on grain size [27], it becomes clear that at big dispersion of grain sizes we have to get broad CL band due to superposition of CL peaks, located at different wavelengths. In the framework of such explanation it is also understandably described before the influence of both Tpyr, and film thickness on the position of the CL peak of as-deposited In2O3 films. It is in full accordance with the results, presented in Ref. [1], where it was shown that the lowering of both the pyrolysis temperature and the film thickness are, the bigger is the contents of fine-dispersed fraction with small grain size in the structure of In2O3 films. It means, that in such films the blue-shift of the CL band should be stronger and the CL peak should be wider. This very influence we have observed in our experiments. The absence of broad luminescence band in [27] could be explained by high homogeneity of grain size of In2O3 particles, synthesized in meso-porous silica.
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As we mentioned earlier, the In2O3 surface of as-deposited films is unstoichiometric. According to [25], high density of the surface states within In2O3 band gap is characteristic for such surface. However, as it was established in Ref. [25], thermal treatment in oxygen atmosphere already at Tan N150–180 °C leads to sharp decrease of those states' density. At the same time, as present research has shown, we did not observe any considerable changes in the CL spectra after thermal treatments at indicated temperatures. It means that the state of the surface plays a secondary role in the processes, controlling both intensity and the shape of the CL spectra. Most likely both the shape and the CL intensity are being controlled by bulk properties of In2O3, i.e. defects, responsible for radiative recombination, have bulk nature. We believe that taking place transformation of the CL spectra and therefore structure of the defects, are a consequence of the improvement of In2O3 films' crystallographic structure. The results, obtained during In2O3 nanobelts research, presented in Refs. [19,28–30], are basis for our conclusion. Exactly in the PL spectra of In2O3 nanobelts, there were found emission bands, observed in the CL spectra of our films after high temperature annealing. For example, in Ref. [29] for In2O3 nanowires with diameter of ∼ 10–100 nm, a strong and broad blue-green emission band was observed centered at λ ∼ 465 nm. For In2O3 nanowires with a diameter of about 20–200 nm (Tsynthesis = 980 °C) [20], the peaks of PL were centered at 416–475 nm. As it's known, In2O3 nanobelts are characterized by high-quality single-crystalline structure [19,28], and they have minimal concentration of structural defects. It means that the appearance of the band at λ ∼ 400 nm is not a consequence of high concentration of oxygen vacancies. We believe that the appearance of this peak is a result of this concentration's decrease. The authors of [16] have also supposed that the CL intensity is directly correlated to the film crystallinity. Therefore, the strong increase of CL intensity and the appearance of peaks, centered at λ ∼400 nm after annealing at Tan ≥1000 °C (see Fig. 5), indicates that only after annealing at Tan ≥1000 °C the crystal lattice of indium oxide approaches its structural perfection. Because of this fact, in the CL spectra one can observe the luminescence bands, close to the edge emission [16]. One can suppose that the change of the CL spectra is a consequence of the decrease of states density near either the bottom of conduction band, or top of valence band. It is necessary to note that our results are good confirmation of the conclusions, made in Ref. [31]. In [31] it was found out that In2O3 had native unstoichiometry, and the crystallographic structure of In2O3 approached to stoichiometric composition only after annealing in oxygen atmosphere at temperatures higher than 1000 °C. Results of research, testifying that CL intensity of In2O3 after annealing in the air became higher than after annealing in nitrogen atmosphere, are in the frame of suggested explanations. As In2O3 grain size in both cases was approximately the same [1], the above mentioned difference was a consequence of distinction in the rates of non-radiating recombination. Taking into account the influence of oxygen partial pressure on the concentration of oxygen vacancies, one can conclude that discussed effect is conditioned by the difference in both bulk
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and surface concentrations of oxygen vacancies in In2O3. After annealing in oxygen atmosphere this concentration is considerably lower. One should note that observed improvement of crystal structure of metal oxide grains during high temperature treatment most likely could serve as an important factor for the achievement of optimal gas sensing characteristics of resistive type sensor [11]. For example, in Ref. [32] it was established that undoped SnO2 calcinated at 1000 °C showed very good sensitivity to nitrogen oxide (NO2), with a negligible cross-sensitivity to CO, together with a high long term stability, as a consequence of its structural parameters improvement. In Ref. [13] it was found that the increase of pyrolysis temperature during In2O3 deposition had the same effect and leaded to a considerable increase of sensitivity to ozone; and to a decrease of sensor response to reducing gases, such as CO and H2. We assume that crystals with more perfect crystallographic structure should have smaller concentration of native bulk defects, lesser concentration of native surface states, and stronger bond of bridging oxygen with In2O3 lattice. According to Refs. [33,34], the decrease of surface states density, which may assist in the pinning of surface Fermi level, expands possible range of surface potential change; and, as a result, promotes the increase of sensor response to ozone. It is important to note here, that a short-wave peaks in the range of λ ∼400 nm have been observed just for In2O3 nanobelts or nanowires, synthesized at high temperatures; or while using special catalysts. In2O3 nanowires, synthesized at low temperatures (b800 °C), like our films, annealed at T ∼ 800 °C, had PL with emission bands, centered at λ ∼ 570 nm, and ∼ 670 nm. It testifies that In2O3 nanowires synthesized at low temperatures are not characterized by such high crystallographic perfection, as In2O3 nanowires, synthesized at high temperatures. It means that in In2O3 nanowires, synthesized at low temperatures, the concentration of point defects didn't decrease to the level, when either band–band or shallow levelband recombination starts dominating in CL spectra. With regard of the CL band, centered at λ ∼ 425–460 nm, observed in Ref. [35] for In2O3 nanowires with diameter ∼ 40 nm, it is early to say something about its nature. Present nanowires were assembled into ordered nanochannels of anodic alumina membranes (AAM), using specific method, based on indium electro-deposition with following thermal oxidation. Besides, it was found that AAM had PL emission in the same spectral range. Moreover, this band was observed in the PL spectra of as-deposited nanowires, which, according to their composition, should correspond to metallic indium, but not indium oxide. 4. Conclusion The initial shape of the CL spectra and the regularities of their transformation during high temperature annealing allow to assume that as-deposited In2O3 films (Tpyr − 350–520 °C) are highly defective ones with lattice disorder and high concentration of point defects independently of either pyrolysis temperature and film thickness. Exactly because of this reason
the short-wave emission band is absent in the experimental spectra of In2O3 films. The CL spectra of these films were characterized by one broad band, centered at λ ∼ 650 nm. The In2O3 film's annealing leads to a sufficient increase of CL intensity. At that the annealing is accompanied by the appearance of additional bands, centered at λ ∼ 400, 550, and 650 nm, which are peculiar to high-quality single-crystalline In2O3 nanobelts, or nanowires with perfect crystallographic structure. It was made a conclusion that the changes of the CL spectra shape and the appearance of short-wave band at λ ∼400 nm are connected with improvement of crystallographic structure of In2O3 grains. This process becomes more intensive after annealing at temperatures higher, than 800 °C. Materials, annealed at temperatures lower than 600–800 °C, retain high concentration of bulk point defects, conditioned by lattice disordering and unstoichiometry of deposited films. It was established that the after-effects of annealing depend on annealing atmosphere. Short-wave band centered at λ ∼400 nm after annealing in oxygen atmosphere is more intensive, than after annealing in nitrogen atmosphere. Indicated difference was explained by the influence of atmosphere on the concentration of oxygen vacancies in In2O3 films. Acknowledgements This work was supported by the Civilian Research and Development Foundation (CRDF) and Moldovan Research and Development Association (MRDA) in the framework of CGP Program (Grant MO-E2-3054-CS-03) and partially supported by Korean Program BK21. Authors are thankful also to the Supreme Council of Science and Advanced Technology of the Republic of Moldova for financial support, and to Prof. A. Cornet, J.R. Morante, J. Schwank, V. Matolin and Dr. V. Brinzari for helping with structural and surface characterization of studied films. References [1] G. Korotcenkov, V. Brinzari, M. Ivanov, A. Cerneavschi, J. Rodriguez, A. Cirera, A. Cornet, J.R. Morante, Thin Solid Films 479 (2005) 38. [2] G. Korotcenkov, I. Boris, V. Brinzari, Yu. Luchkovsky, G. Karkotsky, V. Golovanov, A. Cornet, E. Rossinyol, J. Rodriguez, A. Cirera, Sens. Actuators, B, Chem. 103 (2004) 13. [3] C.H. Liang, G.W. Meng, Y. Lei, F. Fhillipp, L.D. Zhang, Adv. Mater. 13 (2001) 1330. [4] X. Li, M.W. Wanlass, T.A. Gessert, K.A. Emery, T.J. Coutts, Appl. Phys. Lett. 54 (1989) 2674. [5] G. Korotcenkov, A. Cerneavschi, V. Brinzari, A. Vasiliev, A. Cornet, J. Morante, A. Cabot, J. Arbiol, Sens. Actuators, B, Chem. 99 (2004) 304. [6] C.G. Granqvist, Appl. Phys., A 57 (1993) 19. [7] E. Benamar, M. Rami, C. Messoudi, D. Sayah, A. Enaaoui, Sol. Energy Mater. Sol. Cells 56 (1999) 125. [8] H. Bisht, H.T. Eun, A. Mehrtens, M.A. Aegerter, Thin Solid Films 351 (1999) 109. [9] Z.B. Zhou, R.Q. Cui, Q.J. Pang, Y.D. Wang, F.Y. Meng, T.T. Sun, Z.M. Ding, X.B. Yu, Appl. Surf. Sci. 172 (2001) 245. [10] I. Hamberg, C.G. Granqvist, J. Appl. Phys. 60 (1986) R123. [11] G. Korotcenkov, Sens. Actuators, B, Chem. 107 (2005) 209. [12] G. Korotcenkov, V. Brinzari, J. Schwank, A. Cerneavschi, Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 19 (2001) 73.
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