ZnO thin films applied as pH sensor in EGFET devices
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ZnO thin films applied as pH sensor in EGFET devices Jessica C. Fernandes1 and Marcelo Mulato1 1 Department of Physics, FFCLRP/University of São Paulo, Av. Bandeirantes, 3900 – Ribeirão Preto, São Paulo - Brazil ABSTRACT Zinc oxide thin films prepared by sol-gel and doped with different aluminum percentages were deposited by spray-pyrolysis deposition techniques on FTO and ITO substrates. The films were applied as the sensitive layer of pH-sensors EGFET devices. ZnO:Al films on FTO substrates shows a decrease in sensor quality with the dopant percentage increase. The opposite occurs when the substrate was ITO. The best quality film to be used as pH-sensor on FTO substrates was the ZnO:Al (3%) with 29 mV/pH sensitivity value and linear adjust of 0.99. For ITO substrates the best quality was achieved for ZnO:Al (10%), with sensitivity about 30 mV/pH and 0.99 linear adjust. INTRODUCTION Oxide thin films have been hardly studied due to their catalytic, electrical and optical properties [1]. Besides, deposited oxides present a large variety of applications in sensors devices [2]. Thin films applied as pH sensor have been largely studied in current literature to be applied in several fields, as chemical analyses, clinical examinations, and environment detections [3]. In medical fields, the pH detection plays an important rule, as well as industrial fields. With that, the development of alternative electrodes such as pH sensor based on field effect transistors, or solid-state, brings huge advantages [4]. Zinc oxide is commonly used nowadays as pH sensor due to its large range in pH sensitivity and low cost. Low resistivity ZnO thin films can be obtained by the addition of donor impurities, such as aluminum or indium. Aluminum presents a lower cost in comparison with indium and also it presents non-toxicity [5]. The extended-gate field effect transistor (EGFET) device allows the electrical characterization of ionic sensors. EXPERIMENT The ZnO:Al sol-gel routine was prepared using zinc acetate dihydrate as a precursor salt. An ethanolic solution containing 0.44 M of Zn(AcAc)2.2H20 and different aluminum percentages (using Al(NO3)3.9H2O salt) was stirred under reflux for 3 hours. At the same time, four 350 µL lactic acid aliquot were dropped into solution ever 45 minutes. The sol-gel was deposited on fluorine-doped tin oxide (FTO) or indium-doped tin oxide (ITO) substrates using spray-pyrolysis deposition technique. The substrates were kept at constant temperature of 360°C during the deposition. The distance between the spray and the substrates was 18 cm and the annealing of all films was about 1 hour with a 540°C constant temperature. Nitrogen was used during the deposition.
Electric Characterization - EGFET The electric characterization was performed using a Field Effect Transistor device with an Extended Gate. It consists of a Saturated Calomel reference Electrode (SCE) connected to the drain of a commercial MOSFET (CD4007UB, Texas Instruments), which source terminal was grounded. The ZnO:Al thin films were connected to
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