InAs-on-Insulator Fin Nanostructures for Integrated Computation and Sensing Functions

Field-effect transistors (FETs) have established themselves as a leading platform for electrical detection of chemical and biological species. Their advantages over other optical, mechanical sensing platforms are attributed to being miniaturizable, mechanically durable, label-free, responsive in real-time, compatible with scalable CMOS technology and their cheap manufacturing cost. 3D nanostructures such as nanowires (NWs) show great promise for improving sensitivity and limit of detection due to their high surface-to-volume ratio and non-planar geometry. However, when seen in literature the material of choice has predominantly been Silicon because of its mastery over economies of scale in the electronic industry. Few and far in between, alternative materials are being explored and quite noticeable among them is Indium Arsenide (InAs). InAs is interesting for not only for its superior electronic properties and but also its integration capabilities. Firstly, with its higher electron mobility, the supply voltage can be scaled down in line with transistor miniaturization trends. Secondly, its small, direct bandgap enables novel heterojunction devices such as Tunnel FETs. Finally, InAs integration on Silicon has seen major advances over other materials giving it an edge for highly scalable platforms. Indeed, the properties mentioned are interesting from a computation point of view but are also highly favourable for sensing functions. Rather than the typical InAs NW geometry for sensing explored in literature, in this thesis we focus on 3D fin geometry. From a computation standpoint, a tall fin geometry is interesting for better electrostatic gate control to have a high signal-to-noise ratio as well as enabling higher ON currents in future implementations of tunneling FETs. From a sensing standpoint, the 3D dimensionality favours detection at ultra-low concentrations. We demonstrate n-type, InAs-on-insulator fin nanostructures with a unique vertical aspect ratio as single and multiple-fin arrays. Using nanofabrication and characterization tools, highly crystalline InAs fins of 130 nm fin height and fin width down to 30 nm are built (i.e. aspect ratio nearly 4:1). For a tight pitch layout of multiple fin array, where the effective device width exceeds the actual device width and for even tens of millivolts applied voltage, high current levels in microamperes range are read-out. The ion sensitive functioning is validated for hydrogen ions. A sensivity of 41.2 mV/pH is extracted at 6 microampere drain current. What differentiates this work from other InAs sensor devices in literature is not only the unique 3D fin geometry but also integration method of InAs module on Silicon that is catalyst-free, monolithically integrated on Silicon, highly scalable and CMOS compatible. Finally, further study of the devices built for sensing is carried out using metal gating from focussed ion beam (FIB) deposition and ionic liquid (IL) gating. To benchmark the performance of Ionic liquid gating technique, it has been applied to another set of devices SOI Ribbon FETs and compared with its pH response.