Acta Chim. Slov. 2004, 51, 507-527. 507 Scientific Paper STUDY OF ELECTRONIC STRUCTURE OF TIN-DOPED ln203 (ITO) FILM DEPOSITED ON GLASS Ali Hassanzadeh, Mohammad Hossein Habibi,* and Asghar Zeini-Isfahani Department of Chemistry, University oflsfahan, 81745, Isfahan, Iran Received 31-03-2004 Abstract Electronic properties of lowly tin doped ln203 (ITO) electrode with nanoscale particles (ca. 36 nm) prepared by electron-beam evaporation on glass substrate in contact with 0.1 mol drn3 H2S04 solution under visible light was investigated by means of linear sweep voltammetry and electrochemical impedance spectroscopy techniques. Differential capacitance results showed that in the high frequency region of EIS data, ITO and impurity element oxides may develops a p-n junction (Esaki tunnel diode) with low charge carrier concentration (ND = 2.07xl09 cm3). In this EIS data region, a reverse semicircle with a negative resistance was appeared. In contrast, in the low frequency region of EIS data, these semiconductors were converted into only n-type one. The flat-band potential of ITO film at low frequency region as determined from capacitance measurements was found to be around -0.0344 volt vs. SCE. Also, from the slope of linear portion of Mott-Schottky plot, charge carrier concentration was calculated to be about ND = 7.1501 x 1021 cm3. Key words: EIS, Mott-Schottky plot, electronic structure, electron beam evaporation, ITO Introduction Nanoparticles are generally categorized as the class of materials that fall between the molecular and bulk solid limits, with an average size between 1-50 nm. Nanoparticles exhibit physical and chemical properties different from either the individual molecules or the extended solid.1'2 Electrochemical and photoelectrochemical properties of nanostructured semiconductor ftlms have attracted considerable interest in recent years due to the quantum-sized effect in comparison with those of the compact polycrystalline or single-crystal ones. Also, the small size of the particles and the presence of semiconductor-electrolyte interface over the whole nanostructured film up to the back contact, reduce the role of a built-in electron field within the particles and increase the importance of the interfacial kinetics.3 A number of nanocrystalline film electrodes such as Ti02, ZnO, Sn02, W03, Fe203, CdSe, CdS (references cited in the refi 3) and ln2034 have been studied. It is worth to note that in the čase of ln203 nanostructured ftlms only optical and/or electrical properties have mostly been investigated. However, the electrochemical properties of indium oxide and/or ITO in A. Hassanzadeh, M. H. Habibi, A. Zeini-Isfahani: Study of Electronic Structure of Tin-Doped In203... 508 Acta Chim. Slov. 2004, 51, 507-527. electrolyte solution are rather poorly known. For instance, Metikos-Hukovic and his co-workers5 have reported the indium oxide thin film growth as potentiodynamically on the polycrystalline indium substrate and its electrochemical properties have studied in contact with a 0.1 mol dnT3 Na-borate buffer (pH=10) solution. Since, the electronic properties (flat band potential and charge carrier concentration) of semiconducting oxides like ln203 depends strongly on different factors such as crystallinity6 (single or polycrystal), doped material and its amount,7 the substrate onto which the film was deposited and temperature,8"10 annealing temperature and atmosphere,11 and defect density created by external doping or disturbed stoichiometry12 as well as their growth conditions.13 Therefore, we can not use the difference of ca. 60 mV/pH unit for flat band potential shift of films which doped with tin (Sn) and prepared by electron beam evaporation technique in solutions with different pH. In order to solve this problem, it is necessary to study the electronic structure of new materials by different techniques. Among the effective techniques for determination of electronic properties, the electrochemical impedance spectroscopy (EIS) is most valuable. To the best of our knowledge according to literatures, less attention has been paid to the study of electronic properties of nanostructured tin doped indium oxide films which have been prepared by electron beam evaporation on glass substrate using EIS technique. Furthermore, it is not clear that the space charge layer capacitance (Mott-Schottky) theory would be apply to these nanocrystalline electrodes in electrochemical cells and quantum efficiency could be obtained from nanocrystalline samples comparable with those of the polycrystalline and single crystal samples. To help answer these questions, we report here capacitance and transmittance data of lowly tin doped ln203 (ITO) electrode with nanoscale particles (ca. 36 nm) which has been deposited using electron-beam evaporation technique on glass substrate. Experimental Materials and Apparatus Ali materials were of analytical reagent grade and were purchased from Merck and used as received without further purifications. Water used in solution preparation was double distilled. The UV-visible optical transmission spectra were recorded by a double-beam spectrophotometer Cary 500 Scan. A. Hassanzadeh, M. H. Habibi, A. Zeini-Isfahani: Study of Electronic Structure of Tin-Doped In203... Acta Chim. Slov. 2004, 51, 507-527. 509 X-ray diffraction study The phase composition of ITO film deposited on glass was characterized using XRD technique with a D8 Advanced Bruker X-ray diffractometer at room temperature, with monochromated CuKa (A,=1.54 A) in the scan range of 29 between 10° to 100° with a step size of 0.03 (20/s). Measurements were taken under beam-acceleration conditions of 40 kV/35 mA. The grain size, D, was determined from the FWHM of the strongest line of the XRD peak which correspond to the reflection from the (222) plane at 20=30.5° (Figure 1) using the Debye-Scherrer formula.1 0.9/1 D (1) BxCos0B where X=1.54 A and B (was 0.239 degree or 3.983xl0"3 radian in our čase) is the measured broadening of the diffraction line peak at an angle of 20, at half its maximum intensity (FWHM) in radian and the obtained grain size value for the investigated sample was about 36 nm. 600 500 400 - 300 200 - 100 10 20 30 40 50 60 20 (°) Hfy?y»*^^ 70 80 90 100 Figure 1. High angle-XRD pattern of lowly tin doped ln203 sample after annealing in air at ca. 500 °C for 24 h. The bar graphs represents XRD pattern of ln203 sample. _ 0 A. Hassanzadeh, M. H. Habibi, A. Zeini-Isfahani: Study of Electronic Structure of Tin-Doped In203... 510 Acta Chim. Slov. 2004, 51, 507-527. Electrochemical measurements The electrochemical measurements were performed in a conventional three-electrode celi under visible light. The solution volume was ca. 50 ml. A lowly tin doped ln203 (ITO) film electrode on glass substrate with nanoscale particles which has been made by electron-beam evaporation was used as working electrode. The target material used to make the working electrode was an ITO pellet which has been analyzed with XRF technique with composition in wt. %: ln203, 89.75; Sn02, 9.42; CaO, 0.12; K20, 0.096; Fe203, 0.069; CuO, 0.069; U02, 0.026; ZnO, 0.025 and A1203, 0.025. The thickness of film was determined by quartz crystal microbalance measurements and the obtained value for the investigated sample was about 45 nm. After deposition, electrode was annealed in air at ca. 500 °C for 24 h to ensure good electrical contact between particles and then quenched in air at the room temperature. The geometric surface area of working electrode exposed to solution was 9.6 cm2 (1.90 cmx5.05 cm). A cylindrical vessel was used as electrochemical celi in which the whole size of working electrode was immersed in the solution. The counter electrode was a large-area platinum gauze, which is particularly important for impedance measurements (impedance of a counter electrode must be negligible in comparison with a working electrode) and an Hg|Hg2Cl2, (saturated KC1) (SCE) as the reference electrode. Linear sweep voltammetry and cyclic voltammetry are performed using Autolab GPES (Eco Chimi B.V.) with a scan rate of 100 mV s"1 and electrochemical impedance spectra were measured with Autolab FRA module (Eco Chimi B.V.) by applying sinusoidal voltage with amplitude of ±10 mV and frequency ranges from 100 kHz to 0.1 Hz. Ali EIS spectra were fitted to the electrical equivalent circuits using Zveiw program developed by J. R. Mcdonald.14 Results and discussion Linear sweep voltammetry The results of linear sweep voltammetry and cyclic voltammetry experiments of lowly tin doped ln203 film with a scan rate of 100 mV/s in 0.1 mol dnT3 H2S04 (pH=0.98) solution are shown in Figure 2A,B. Although, the overvoltage of hydrogen and oxygen evolution on the ln203 surface is relatively high (i.e. ca. 1 and 0.5 volt) and corresponding peaks appear at ca. -0.8 and +1.5 volt vs. SCE in pH=0.98, respectively,5 to prevent side reactions and evolution of H2 and/or 02 gasses, the range of applied A. Hassanzadeh, M. H. Habibi, A. Zeini-Isfahani: Study of Electronic Structure of Tin-Doped In203... Acta Chim. Slov. 2004, 51, 507-527. 511 potential was from -0.5 to 1 volt vs. SCE. During anodic polarization, a cathodic current is showed over potential range between -0.5 and -0.2 volt vs. SCE. This cathodic current is probably related to the reduction of ln203 or with evolution of hydrogen. It can also be seen that there is an anodic peak around -0.2 volts vs. SCE. Since, the processes of the indium oxide formation is characterized by a well defined peak5 at a potential around -0.5 volt vs. SCE in pH=0.98 and the formation of Sn(II)-oxide/hydroxide and the formation of Sn(IV)-oxide/hydroxide layers take plače at potential -0.494 and -0.394 volt vs. SCE, respectively in pH=0.98,15 the assignment of this anodic peak is very difficult. It seemed that the impurities may cause to develop a p-n junction that controls their electronic structure. According to the physics of semiconductors,16 this feature can be related to the formation of a tunnel (Esaki) diode as an n-type and a p-type junction which can easily be detected by Mott-Schottky plot of high frequency EIS data (will be discussed later). A tunnel diode is a semiconductor device with a negative resistance which is made very much like an ordinary alloyed p-n rectifier, except that both p- and n-regions as heavily doped as possible; a typical tunnel diode may have doping densities of 1018 or 1019 carrier per cm3 in both p- and n-regions.17 It operation depends upon a quantum mechanic principle known as "tunneling" wherein the intrinsic voltage barrier is reduced due to doping levels which enhance tunneling. The negative resistance region is the important characteristic for the tunnel diode and did not appear in I-E plot of conventional diodes. In this region, with increasing voltage the current decreased just the opposite of a conventional diode. The most important specifications for the tunnel diode are the peak voltage (Ep), peak current (Ip), valley voltage (Ev), and valley current (Iv) (Figure 2). The anodic current region of Figure 2 was scale-expanded as an inset to represent these features. Clearly, in the potential region more positive than the -0.2 volt to 0.2 a negative polarization resistance appeared which is the characteristic of tunnel diodes and decaying current is due to band-to-band tunneling effect. Then, the current is limited to a very low level and this potential region is called "valley". For a voltage larger than valley potential (ca. 1 volt vs. SCE in our čase) the current increases exponentially with voltage (not shown here). The observed anodic current from point b to d is a summation of tunnel current, excess current due to trapping impurities or structural imperfections and majority carrier diffusion current.17 A. Hassanzadeh, M. H. Habibi, A. Zeini-Isfahani: Study of Electronic Structure of Tin-Doped In203... 512 Acta Chim. Slov. 2004, 51, 507-527. 0.4 -0.2 -0.0 --0.2 --0.4 --0.6 --0.8 --1.0 --1.2 --1.4 --1.6 - A c 0.30 0.25 0.20 0.15 0.10 0.05 0.00 d —-0- ^•^c Tunnel / Negative resistance region current / b, -0.6 -0.4 -0.2 0.0 Excess current region Tv d ^4 bserved rrent-voltabe curve Y-----'-------*?— .2 0.4 0.6: 0.8 1.0 1.2 Diffusion current -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 E / volt vs. SCE 0.8 1.0 1.2 5e-5 -5e-5 -1e-4 -2e-4 -2e-4 4 -0.6 -0.4 -0.2 0.0 0.2 0.4 E /v o lt vs. S C E 0.6 0.8 1 .0 1 .2 Figure 2. A) Linear sweep and B) Cyclic voltammetiy curves of lowly tin doped ln203 film electrode with a scan rate of 100 mV/s in contact with 0.1 mol drn3 H2S04 solution. The inset is the detail of tunnel diode currents contributions. I o A. Hassanzadeh, M. H. Habibi, A. Zeini-Isfahani: Study of Electronic Structure of Tin-Doped In203... Acta Chim. Slov. 2004, 51, 507-527. 513 Impedance Studies The complex plane plots and Bode plots of lowly tin doped ln203 film electrode in contact with 0.1 mol dnT3 H2S04 solution were shown in Figures 3 and 4. The high-frequency EIS response of complex plane plots was scale-expanded, as presented in the inset of Figure 3. Each of these impedance spectrum in complex plane plots can be divided into three distinct frequency regions; (i) In the high frequency region (between 100 kHz and ca. 4 kHz), a tirne constant as a dispersed reveres semicircle indicating a negative polarization resistance has been emerged which has little significant variation with electrode potential changes. It is well known that the negative resistance is a direct consequence of the negative slope in the current/voltage curve just above the passivation voltage and is mostly observed in the corroding cases. Besides, it is also well documented that the most common source of negative resistance in electrochemical systems results from an interplay between adsorption and electrodissolution, giving rise to a current that decreases with increasing voltage for 0^00, even though the interfacial differential capacitance is positive.18 One of the other čase where this feature could be seen is an Esaki or tunnel diode. We attributed this feature to the development of a tunnel diode and band-to-band majority carrier tunneling. 18000 16000 14000 12000 10000 8000 6000 4000 2000 « 1 ^^~~\ H— -U.J> I I I I I I I I 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 ZVohm Figure 3. Complex plane plots of lowly tin doped ln203 film electrode in contact with 0.1 mol drn3 H2S04 solution under visible light at different potentials from -0.5 to 0.5 volt vs. SCE. The inset is high-frequency EIS response with scale-expanded. A. Hassanzadeh, M. H. Habibi, A. Zeini-Isfahani: Study of Electronic Structure of Tin-Doped In203... E=+0.5 E=+0.4 E=+0.3 ,E=-0.3 E=-0.4 1000 800 600 400 200 0 1000 1200 1400 1600 1800 200( ZVohm E=-0.2 514 Acta Chim. Slov. 2004, 51, 507-527. 100 80 60 40 20 4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 0 123 45 6 log(f/Hz) 012345 6 log(f/Hz) Figure 4. Bode: A) phase and B) magnitude plots of lowly tin doped In2O3 film electrode in contact with 0.1 mol dm-3 H2SO4 solution under visible light at different potentials from -0.5 to 0.5 volt vs. SCE. (ii) The medium frequency range of impedance spectra (between 4 kHz and ca. 20 Hz), corresponds to a straight line with an angle of about 45° in the whole spectra. This linear dependence of the imaginary and real part of impedance is related to the diffusion o A. Hassanzadeh, M. H. Habibi, A. Zeini-Isfahani: Study of Electronic Structure of Tin-Doped In203... Acta Chim. Slov. 2004, 51, 507-527. 515 processes, which called "Warburg impedance". But, the fitting procedure showed better agreement between theoretical and experimental data if a frequency dependent constant phase element (CPE) in parallel with a resistance is introduced instead of Warburg impedance. Generally, the appearance of a CPE is due to a distribution of the relaxation times as a result of inhomogenities presented at the microscopic level of the oxide phase and at the oxide | electrolyte interface contributions from static disorder such as porosity, a random mixture of conductor and insulator that can be described by the effective medium approximation as percolation or an interface that can be described by fractal geometry or a RC transmission line.5 The impedance of a constant phase element is defined as: ZCPE=[T(JayYl vith -\