Sensors and Actuators A 299 (2019) 111627 Contents lists available at ScienceDirect Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna Development of low voltage gas ionization tunneling sensor based on p-type ZnO nanostructures Armin Agharazy Dormeny ∗ , Parsoua Abedini Sohi, Dmytry Grudin, Mojtaba Kahrizi Department of Electrical and Computer Engineering, Concordia University, Montreal, QC, H3G 1M8, Canada a r t i c l e i n f o Article history: Received 8 June 2019 Received in revised form 16 August 2019 Accepted 17 September 2019 Available online 18 September 2019 Keywords: ZnO nanowires Gas field ionization sensor Tunneling current Electrochemical deposition Semiconductor a b s t r a c t In this article, we report design, fabrication, and characterization of a gas field ionization-tunneling sensor (GFITS) based on zinc oxide (ZnO) nanowires. The device that operates at very low voltages is made up of two parallel plates separated by a narrow gap. ZnO nanowires are grown on one of the plates and used as the anode of this capacitive device. The nanowires that were synthesized using electrochemical technique on silicon or gold substrates, amplify the electric field between the two plates and reduce the ionization voltage of the gas molecules. Electrons from the gas atoms tunnel through the potential barrier of the gas atoms into the tips of nanowires. The generated tunneling current can be used to identify unknown gases. Nanowires with different aspect ratios and various morphologies were used to assemble the device, which was then tested for several gases. Distinct I–V characteristics for gases like Ar, He, and N 2 at low pressures were achieved. Our observations show that nanowires grown on gold substrates do not have vertically parallel structures, rather they grow in the form of flower shapes and the devices made of those samples operate at much lower voltages compared to those made of parallel nanowires grown on semiconductor substrates. To investigate the effect of geometrical field enhancement on the operating voltage of the sensor, the electric field enhancement of nanowires has been simulated using COMSOL Multiphysics. The results show that the enhancement factor of flower-like nanostructures of ZnO is much higher than those of freestanding nanowires. © 2019 Elsevier B.V. All rights reserved. 1. Introduction and background The nanostructures can be used as fundamental elements in manufacturing of many devices. Nanostructures such as metal- lic and semiconductor nanowires are investigated by many researchers, and they have already found their way in fabrica- tions of many novel devices like gas ionization sensors [1,2], field emission devices (because of high field-enhancement factor) [3], switches (because of the lack of resistance in the channel along the nanowires) [4], and in biosensors due to large sensitivity [5,6]. The structures are successfully applied in temperature sensors, solar cells, nanolasers, and light-emitting diodes (LEDs) [7–9]. Ferromag- netic nanowires are used in bio applications, such as drug delivery systems, and as microwave resonators [10]. Among semiconduc- tor materials, ZnO with significant physical properties has been the center of researches for many applications. This semiconductor ∗ Corresponding author. E-mail addresses: a aghar@encs.concordia.ca (A. Agharazy Dormeny), p abedin@encs.concordia.ca (P. Abedini Sohi), d grudin@encs.concordia.ca (D. Grudin), mojtaba.kahrizi@concordia.ca (M. Kahrizi). material with a direct wide bandgap of 3.37 eV is nontoxic, chemi- cally stable, and biocompatible. Recently, ZnO nanowires have been extensively investigated to be used in sensing applications, includ- ing gas sensors. Gas ionization sensors (GIS) are made up of two conducting oppositely charged parallel plates. The breakdowns of gases occur due to the applied voltage between the two plates, usually in the range of a few thousand volts and are used as a fingerprint to identify gases. Placing grown nanowires between the two paral- lel electrodes will significantly increase the local electric field at the tips of the nanowires, resulting in a considerable decrease of the externally applied voltages (breakdown voltages of gases) between the two plates. GISs have the capability of identifying a wide range of gases based on the ionization energy and the discharge current of the target gas, and they are calibrated based on either breakdown voltages (gas ionization breakdown sensors, GIBS) or tunneling cur- rents (gas field ionization tunneling sensors, GFITS) [11–13]. Here, we report the development of a gas field ionization tunneling sen- sor (GFITS) based on p-type semiconductor ZnO nanowires. Gas atoms due to the applied voltages across the parallel plates are polarized near the positively biased nanowires’ tips, and by the field gradient, electrons from gas atoms tunnel into the semiconduc- https://doi.org/10.1016/j.sna.2019.111627 0924-4247/© 2019 Elsevier B.V. All rights reserved.