Indium tin oxide synthesized by a low cost route as SEGFET pH sensor

Abstract

Polycrystalline ITO films with good optoelectronics characteristics and homogeneous surface has been obtained upon annealing at 550 °C in N² atmosphere using a low-cost chemical vapor deposition (CVD) system. The films were evaluated as pH sensors in separative extended gate field-effect transistor (SEGFET) apparatus, exhibiting a sensitivity of 53 mV/pH, close to the expected Nernstian theoretical value for ion sensitive materials. The use of CVD process to synthesize ITO, as described here, may represent an alternative for fabrication of SEGFET pH sensors at low cost to be used in disposable biosensors since H+ ions are the product of several oxireductase enzymes.

ITO; CVD; pH sensor; field-effect transistor

Indium tin oxide synthesized by a low cost route as SEGFET pH sensor

Nirton Cristi Silva Vieira^{I, *}; Edson Giuliani Ramos Fernandes^I; Alvaro Antonio Alencar de Queiroz^II; Francisco Eduardo Gontijo Guimarães^I; Valtencir Zucolotto^I

^IInstituto de Física de São Carlos - IFSC, Universidade de São Paulo - USP, CP 369, CEP 13560-970, São Carlos, SP, Brazil

^IIInstituto de Ciências Exatas - ICE, Universidade Federal de Itajubá - UNIFEI, CP 50, CEP 37500-903, Itajubá, MG, Brazil

ABSTRACT

Polycrystalline ITO films with good optoelectronics characteristics and homogeneous surface has been obtained upon annealing at 550 °C in N² atmosphere using a low-cost chemical vapor deposition (CVD) system. The films were evaluated as pH sensors in separative extended gate field-effect transistor (SEGFET) apparatus, exhibiting a sensitivity of 53 mV/pH, close to the expected Nernstian theoretical value for ion sensitive materials. The use of CVD process to synthesize ITO, as described here, may represent an alternative for fabrication of SEGFET pH sensors at low cost to be used in disposable biosensors since H⁺ ions are the product of several oxireductase enzymes.

Keywords: ITO, CVD, pH sensor, field-effect transistor

1. Introduction

Indium tin oxide (ITO) has emerged as important engineering materials, since their versatile optical and electronic properties have found application in many fields including optoelectronics and photonics¹. ITO semiconductors show high transmission in the visible region, high reflectance in the infrared and high electrical conductivity. Due to the latter properties, ITO has been used as optically transparent conducting electrode in a range of applications^2-4. Recently, ITO films have been used as work electrode in the development of bioengineering devices such as ion sensors and biosensors^5-7. The emerging sensor and biosensor technology based on ITO films has emerged as an advantageous alternative for applications in medicine^8,9.

ITO can also be applied to detect H⁺ ions. In this case, two types of electronic devices can be used to pH sensing: a conventional ion sensitive field-effect transistor (ISFET)¹⁰ and a separative extended gate FET^11,12. ISFET comprises a metal oxide semiconductor field-effect transistor (MOSFET) without gate metallization, being its insulator gate in direct contact with the solution¹⁰, whereas a SEGFET comprises a chemically sensitive membrane as separative extended gate (SEG) connecting to the gate of a commercial MOSFET^11,12. The latter configuration is advantageous for sensing, since the sensor is isolated from the chemical environment and can be reused.

Synthesis of ITO has been carried out by several methods, including RF magnetron sputtering¹³, electron¹⁴ or ion beam evaporation¹⁵, pulsed laser ablation¹⁶, sol-gel process¹⁷ and chemical vapor deposition (CVD)^18,19. The relative simplicity and low-cost of ITO films synthesized by CVD have made this technique technologically advantageous.

This paper reports on the synthesis and characterization of ITO films to be applied as pH sensor in a SEGFET configuration using the inexpensive CVD technique at very low-cost effective procedure. Measurements of the drain current (I_D) as function of the drain to source voltage (V_DS) or as function of the gate to source voltage (V_GS) were used to characterize the ITO films as SEGFET pH sensor.

2. Experimental

2.1. ITO synthesis and characterization

ITO films were deposited on glass substrates (Corning 7059) using a commercial atomizer (HuaYi Sprayer) according to the literature¹⁸. A methanol solution (0.1 M) of indium chloride (InCl₃ . 3.5 H₂O, 99.99%, Merck) and tin chloride (SnCl₄ . 2 H₂O, 99.99%, Merck ) was prepared with 5 wt% of Sn concentration. Substrate temperature was fixed in 300 C and the solution was sprayed manually on glass substrates. After deposition, ITO films were thermally treated at 550 C in N₂ atmosphere for 1 hour. Film thickness can be controlled by the amount of sprayed solution.

Surface morphology and thickness of ITO films were examined using a scanning electron microscope (SEM, model: Phillips XL 30) set at an operating voltage of 20.0 kV. The optical transmission of the ITO films on the glass substrates were measured using a spectrophotometer (UV-Vis, Varian 634) in the wavelength range of 300 to 800 nm. The study of crystalline ITO films phases were investigated by the X-ray diffraction technique (XRD) (D/MAX-200). The X-ray patterns were taken from monochromatic CuK radiation source (λ = 1.5418 Å). Film resistivity was measured using the van der Pauw method²⁰.

2.2. ITO as pH sensor: measurement system

SEGFET configuration comprises two parts^11,12: the chemically sensitive membrane, formed by the ITO film (contact area of 35 mm²) connected to the gate terminal of a commercial MOSFET (CD4007UB) with its gate replaced by a reference electrode of silver/silver chloride (Ag/AgCl/Sat. KCl). A programmable curve tracer (Tektronix-370A) was used to record the data. Measurements of the drain current (I_D) as function of the variable voltages (V_DS or V_GS) in different pH buffer solutions (5 min after immersion) were carried out to determine the pH sensitivity of the ITO films. Figure 1 shows the scheme of the SEGFET measurement system.

Figure 1.
Scheme of the SEGFET architecture and the measurement system.

3. Results and Discussion

ITO films with thickness about 200 nm (estimated by profilometry) and resistivity in the order of 10 ^{- 4} W.cm (measured using the van der Pauw method)²⁰ was deposited on glass substrate by CVD. In the case of ITO conducting thin film, indium oxide (In₂O₃) based material have been doped with Sn to improve the electrical conductivity. Sn acts as an n-dopant material in the In₂O₃ lattice, in substitution to indium atoms, since In has valence 3⁺ and Sn has valence 4⁺, thus adding electrons in the conduction band. Furthermore, oxygen vacancies can donor two electrons, i.e., both oxygen vacancies and the donor atoms (Sn) contribute to improve ITO conductivity^1,21.

Figure 2 shows the optical transmission spectrum of the synthesized ITO film. Optical transmittance changes significantly by increasing annealing temperature and reaches a maximum transmission for ITO films thermally treated at 550 °C, in agreement to what has been reported.²² The optical absorption spectrum (Figure 2, inset) gives an estimated value for the ITO film band gap when an abrupt change in the slope of the absorbance curve is observed²³. The associated band gap energy of 3.9 eV characterizes the semiconductor properties of the material.

Figure 2.
Transmission spectrum of the synthesized ITO film. Inset: Absorption spectrum of the same film.

Figure 3 shows the XRD spectrum of the ITO film deposited onto glass substrate (for comparison, the results of In₂O₃ powders are also shown). XRD diffractograms revealed that ITO films become polycrystalline when deposited at higher substrate temperature and crystallize in a cubic bixbyite structure (In₂O₃)^24,25. The peaks observed for 2θ angle associated to the planes (222) and (400) may be observed. The preferential growth of the ITO films is the (222) plane and this orientation should be dependent on the deposition conditions²⁵.

Figure 3.
X-ray diffraction of ITO film deposited onto glass substrate. The peaks of the In₂O₃ powders are shown for reference.

Surface morphology of the ITO films deposited onto glass substrates was examined by SEM as shown in Figure 4. After annealing at 550 °C, films with a rough polycrystalline formation with grains and any voids of about 1 µm in diameter were obtained. This represents a large grain misalignment, which is a very crucial requirement in thin film for electronic devices involving oxide materials.

Figure 4.
SEM image of the synthesized ITO film.

The I_D-V_DS characteristics curves of the ITO film as SEGFET pH sensor are shown in Figure 5a. A SEGFET operates similarly to MOSFET except that, for the former, a voltage in the reference electrode (V_Ref) replaces the gate-source voltage (V_GS)^11,12. The drain current is now a function of pH value, since the potential on ITO surface changes due to concentration of H⁺ in the solution. Based on MOSFET equations, in the non-linear region I_D is expressed as:

Figure 5. I_D-V_DS characteristics for constant V_GS of the ITO film as separative extend gate-FET in the pH range from 2 to 12: (a) I_D-V_DS characteristics, and (b) square root of I_D.

and in the linear region:

where β is a conduction parameter, V_DS is the drain - source voltage, and V_T the threshold voltage, i.e. is defined as the minimum voltage required to make the transistor ON, which is dependent on the pH value. ^{11, 12} Based on equation 1, I_D^1/2 presents a linear pH response and can be expressed as a function of pH value, as seen in Figure 5b.

Figure 6a shows the corresponding I_D-V_GS characteristics curves of the ITO as SEGFET pH sensor. As shown in Figure 6b, a linear response of V_GS in the pH range of 2-12 may be observed. The sensitivities of the ITO SEGFET was calculated from the slope in the Figure 6b for fixed I_D = 200 µA. ITO films presented a sensitivity about 53 mV/pH unity, close to the expected Nernstian theoretical value (59,15 mV/pH)²⁶ to pH sensors and in and in good agreement with commercial ITO films pH sensors⁵, which presented 58 mV/pH of sensitivity⁵. Comparing with other synthetized ITO films, a sensitivity of 50 mV/pH was found by Lue et al.²⁷ using ITO deposited on flexible polyethylene terephthalate (PET) by RF sputtering²⁷. ITO films fabricated via anodic oxidation exhibited a sensitivity of ca. 54 mV/pH in a pH range from 2 to 12²⁸. Therefore, our results indicate that the CVD technique is an alternative route to construct pH-sensitive materials in a simple and low-cost way. In the other words, the results indicate that active sites in ITO films are involved in the formation of a charged double layer with the distribution of the potential between the film and the glass surface.

Figure 6. I_D-V_GS characteristics for constant V_DS of the ITO film as separative extend gate-FET in the pH range from 2 to 12: (a) I_D-V_GS characteristics, and (b) sensitivity calculated when I_D was fixed in 200 µA.

The ITO sensitivity may be explained by the well-known site-binding model theory²⁹. According to this theory, three sites can be found on ITO surface, viz., negatively charged ITO^- groups, neutral ITOH groups and positively charged ITOH⁺ groups. The total surface charge can be altered by complex formation on ITO surface. Upon changing electrolyte pH, a change in the protons concentration occurs at ITO surface, modulating the drain-source current in the SEGFET device.

4. Conclusions

This study presented the ITO film deposited on glass substrates by low cost CVD spray system. Homogeneous ITO films with low resistivity and high optical transmittance were obtained. The pH sensing characteristics of the films were also analyzed using a SEGFET configuration. ITO films presented a Nerstian pH response sensitivity of ca. 53 mV/pH. The use of CVD process in junction with SEGFET configuration represents a low cost alternative and appears to be beneficial for real applications in the fabrication of membranes for pH sensors and as a sensitive substrate for biosensors since H⁺ is the product of several oxireductase enzymes.

Acknowledgements

The authors are grateful to CNPq, CAPES, FAPEMIG and FAPESP for the financial support.

Received: January 10, 2013

Revised: April 19, 2013

• 1. Tahar RBH, Ban T, Ohya Y and Takahashi Y. Tin doped indium oxide thin films: electrical properties. Journal of Applied Physics 1998; 83(5):2631-2645. http://dx.doi.org/10.1063/1.367025
• 2. Saim HB, Campbell DS and Avaritsiotis JA. Indium tin oxides (ITO) thick-films for solar-cells. Solar Energy Materials 1986; 13(2):85-96. http://dx.doi.org/10.1016/0165-1633(86)90037-7
• 3. Zhu F, Zhang K, Guenther E and Jin CS. Optimized indium tin oxide contact for organic light emitting diode applications. Thin Solid Films 2000; 363(1-2):314-317. http://dx.doi.org/10.1016/S0040-6090(99)01003-2
• 4. Betz U, Olsson MK, Marthy J, Escola MF and Atamny F. Thin films engineering of indium tin oxide: Large area flat panel displays application. Surface & Coatings Technology 2006; 200(20-21):5751-5759. http://dx.doi.org/10.1016/j.surfcoat.2005.08.144
• 5. Yin LT, Chou JC, Chung WY, Sun TP and Hsiung SK. Study of indium tin oxide thin film for separative extended gate ISFET. Materials Chemistry and Physics 2001; 70(1):12-16. http://dx.doi.org/10.1016/S0254-0584(00)00373-4
• 6. Crespilho FN, Ghica ME, Gouveia-Caridade C, Oliveira ON Jr and Brett CMA. Enzyme immobilisation on electroactive nanostructured membranes (ENM): Optimised architectures for biosensing. Talanta 2008; 76(4):922-928. PMid:18656679. http://dx.doi.org/10.1016/j.talanta.2008.04.054
• 7. Crespilho FN, Iost RM, Travain SA, Oliveira ON Jr and Zucolotto V. Enzyme immobilization on Ag nanoparticles/polyaniline nanocomposites. Biosensors and Bioelectronics 2009; 24(10):3073-3077. PMid:19427191. http://dx.doi.org/10.1016/j.bios.2009.03.026
• 8. Matharu Z, Sumana G, Arya SK, Singh SP, Gupta V and Malhotra BD. Polyaniline Langmuir - Blodgett Film Based Cholesterol Biosensor. Langmuir 2007; 23(26):13188-13192. PMid:18001068. http://dx.doi.org/10.1021/la702123a
• 9. Choi CK, English AE, Jun S-I, Kihm KD and Rack PD. An endothelial cell compatible biosensor fabricated using optically thin indium tin oxide silicon nitride electrodes. Biosensors and Bioelectronics 2007; 22(11):2585-2590. PMid:17113768. http://dx.doi.org/10.1016/j.bios.2006.10.006
• 10. Bergveld P. Thirty years of ISFETOLOGY - what happened in the past 30 years and what may happen in the next 30 years. Sensors and Actuators B: Chemical. 2003; 88(1):1-20. http://dx.doi.org/10.1016/S0925-4005(02)00301-5
• 11. Chi LL, Chou JC, Chung WY, Sun TP and Hsiung SK. Study on extended gate field effect transistor with tin oxide sensing membrane. Materials Chemistry and Physics 2000; 63(1):19-23. http://dx.doi.org/10.1016/S0254-0584(99)00184-4
• 12. Fernandes EGR, Vieira NCS, De Queiroz AAA, Guimarães FEG and Zucolotto V. Immobilization of Poly(propylene imine) Dendrimer/Nickel Phtalocyanine as Nanostructured Multilayer Films To Be Used as Gate Membranes for SEGFET pH Sensors. Journal of Physical Chemistry C 2010; 114(14):6478-6483. http://dx.doi.org/10.1021/jp9106052
• 13. Zhang K, Zhu F, Huan CHA and Wee ATS. Indium tin oxide films prepared by radio frequency magnetron sputtering method at a low processing temperature. Thin Solid Films 2000; 376(1-2):255-263. http://dx.doi.org/10.1016/S0040-6090(00)01418-8
• 14. Wan N, Wang T, Sun H, Chen G, Geng L, Gan X et al. Indium tin oxide thin films for silicon-based electro-luminescence devices prepared by electron beam evaporation method. Journal of Non-Crystalline Solids 2010; 356(18-19):911-916. http://dx.doi.org/10.1016/j.jnoncrysol.2009.12.026
• 15. Zhinong Y, Yuqiong L, Fan X, Zhiwei Z and Wei X. Properties of indium tin oxide films deposited on unheated polymer substrates by ion beam assisted deposition. Thin Solid Films 2009; 517(18):5395-5398. http://dx.doi.org/10.1016/j.tsf.2008.12.057
• 16. Adurodija FO, Izumi H, Ishihara T, Yoshioka H, Yamada K, Matsui H, et al. Highly conducting indium tin oxide (ITO) thin films deposited by pulsed laser ablation. Thin Solid Films 1999; 350(1-2):79-84. http://dx.doi.org/10.1016/S0040-6090(99)00278-3
• 17. Kundu S and Biswas PK. Synthesis and photoluminescence property of nanostructured sol-gel indium tin oxide film on glass. Chemical Physics Letters. 2005; 414(1-3):107-110. http://dx.doi.org/10.1016/j.cplett.2005.08.062
• 18. Sawada Y, Kobayashi C, Seki S and Funakubo H. Highly-conducting indium-tin-oxide transparent films fabricated by spray CVD using ethanol solution of indium (III) chloride and tin (II) chloride. Thin Solid Films 2002; 409(1):46-50. http://dx.doi.org/10.1016/S0040-6090(02)00102-5
• 19. Maki K, Komiya N and Suzuki A. Fabrication of thin films of ITO by aerosol CVD. Thin Solid Films 2003; 445(2):224-228. http://dx.doi.org/10.1016/j.tsf.2003.08.021
• 20. Ramadan AA, Gould RD and Ashour A. On the Van der Pauw method of resistivity measurements. Thin Solid Films 1994; 239(2):272-275. http://dx.doi.org/10.1016/0040-6090(94)90863-X
• 21. Han H, Adams D, Mayer JW and Alford TL. Characterization of the physical and electrical properties of indium tin oxide on polyethylene napthalate. Journal of Applied Physics 2005; 98(8):1-8. http://dx.doi.org/10.1063/1.2106013
• 22. Fallah HR, Ghasemi M and Hassanzadeh A. Influence of heat treatment on structural, electrical, impedance and optical properties of nanocrystalline ITO films grown on glass at room temperature prepared by electron beam evaporation. Physica E-Low-Dimensional Systems & Nanostructures 2007; 39(1):69-74. http://dx.doi.org/10.1016/j.physe.2007.01.003
• 23. Ibanez JG, Solorza O and Gomez-del-Campo E. Preparation of semiconducting materials in the laboratory. Production of CdS thin films and estimation of their band gap energy. Journal of Chemical Education 1991; 68(10):872-875. http://dx.doi.org/10.1021/ed068p872
• 24. Brewer SH and Franzen S. Calculation of the electronic and optical properties of indium tin oxide by density functional theory. Chemical Physics 2004; 300(1-3):285-293. http://dx.doi.org/10.1016/j.chemphys.2003.11.039
• 25. Ma HL, Zhang DH, Ma P and Win SZ, Li SY. Preparation and properties of transparent conducting indium tin oxide films deposited by reactive evaporation. Thin Solid Films 1995; 263(1):105-110. http://dx.doi.org/10.1016/0040-6090(95)06554-7
• 26. Janata J and Josowicz M. Nernstian and non-nernstian potentiometry. Solid State Ionics 1997; 94(1-4):209-215. http://dx.doi.org/10.1016/S0167-2738(96)00503-6
• 27. Lue CE, Wang IS, Huang CH, Shiao YT, Wang HC, Yang CM et al. pH sensing reliability of flexible ITO/PET electrodes on EGFETs prepared by a roll-to-roll process. Microelectronics Reliability 2012; 52 (8):1651-1654. http://dx.doi.org/10.1016/j.microrel.2011.10.026
• 28. Lin JL and Hsu HY. Study of Sodium Ion Selective Electrodes and Differential Structures with Anodized Indium Tin Oxide. Sensors 2010; 10 (3):1798-1809. PMid:22294900 PMCid:3264452. http://dx.doi.org/10.3390/s100301798
• 29. Yates DE, Levine S and Healy TW. Site-binding model of electrical double-layer at oxide-water interface. Journal of the Chemical Society-Faraday Transactions I 1974; 70:1807-1818. http://dx.doi.org/10.1039/f19747001807

Publication Dates

• Publication in this collection
  02 July 2013
• Date of issue
  Oct 2013

History

• Received
  10 Jan 2013
• Reviewed
  19 Apr 2013