Self-Standing Silicon Nanostructures Fabricated Using Chemical/Electrochemical Technique: Application in Gas Field Ionization Tunneling Sensor

The air surrounding us may contain different amount of hazardous gases or atmospheric pollutants. As the public health and safety is the prime priority, therefore it is very essential to have a reliable detector to distinguish the presence of these pollutions. So far, different gas sensing technologies have been invented. However none of the traditional sensors, which are mostly based on chemical reaction between the gas and detector material, could fulfill all the requirements such as sensitivity, selectivity, reversibility, durability and fast recovery time. Therefore, it is worth investigating contemporary sensors for the detection of different target gases. An ingenious approach would be to use the ionization characteristics of distinct gases to uniquely identify the gas type. Gas ionization sensors contain two parallel electrodes separated by a narrow gap. Applied electric field between the electrodes ionizes the target gas. Ionization energy is a unique amount of energy required for the gaseous atoms to discharge electrons. In these sensors the discharge characteristics are used as a calibrating data, which provides excellent selectivity. In addition, because this technique does not involve adsorption or desorption of the gases, the sensor exhibits fast response and high reversibility and recovery. In planar parallel electrode structures the applied electric field is uniform. Applying nanostructures as one of the electrodes creates nonlinear electric field, enhanced at the vicinity of the tips of the structures. In these structures the ionization initiates at lower voltages. There are two different types of gas ionization sensors. One is based on measuring the breakdown voltages of the gases, which is extensively discussed in the literature. The second one is an innovative concept for nanostructure-based gas ionization sensors. These devices, which are studied in this work, is based on tunneling field ionization characteristics of the gases. In this phenomenon an electron from the gas tunnels into the vacant energy levels of a semiconductor employed as the anode. The fabrication technique of the anode involves the growth of p-type silicon nanostructures through a mask-less chemical/electrochemical etching technique. The technique and the fabrication of the silicon nanowires used in the structure of the device are described in detail. Prior to the fabrication process, the mechanism of nanowires formations was modeled for 2D and 3D structures and simulated using COMSOL multiphysics. Our studies showed that the positive carriers (holes) current greatly influences the etching rate of the silicon during the electrochemical etching process. Our experiments showed that growth process of the nanostructures fits very well with our analytical model and helped us to adjust the fabrication parameters to control the characteristic as well as the density and concentration of the nanostructure arrays. Finally, a miniaturized gas sensors based on the grown nanostructured was fabricated. I-V characteristics of the sensor was measured for various gases like Ar, N2, O2, and He. Charge transport mechanisms are explained based upon the gathered data. The sensor was tested for 200 μm and 50 μm inter-electrode distances. Charge transport mechanism is explained based upon the gathered data for both devices. The fabricated devices are capable to characterize several gas species including inert gases. Gas ionization voltages were much lower compared to those, reported in the literature using devices fabricated by metallic nanostructures