Fabrication of semiconductor nanowire multifunctional devices

Abstract

Portable multi-functional devices can play a major role in the new age society embracing internet-of-things (IoT). Being able to perform primary functions such as sensing and secondary functions such as storing information is quite critical when out of connectivity. However, such bespoke devices are almost unheard of as it is very difficult to fabricate it due to several factors such as device architecture, dimension, scalability, and parasitic effects.
This work describes the fabrication and characterization of a multi-functional device that acts an ultra-sensitive pressure sensor but is also capable of storing that information for a prolonged period. Both sensitivity and charge storage ability are attributed to the inclusion of one-dimensional (1-D) nanostructures. The alternating crystal phases in the as-grown gold (Au) catalyzed GaAs and self-induced AlGaAs/GaAs nanowires (NWs) were used in our case.
This thesis discusses the fabrication, growth, characterization, integration and electrical testing involved to produce the multi-functional device. Bespoke nanowires were grown on a template prepared using a combination of nanosphere (NSL) and nanoimprint lithography (NIL) which provided a reproducible large-area periodic array of growth site at a relatively low cost. The inclusion of these NWs in the polymer helps enhance the relative permittivity of the host polymer by a factor of 40 making it an almost-perfect dielectric for a capacitive pressure sensor. NWs also acted as charge storage nodes allowing to extend the functionality. The technique consists of creating nanoholes in silicon dioxide (SiO2) to expose the silicon Si (111) beneath where self-induced NWs can nucleate, while nanodots deposited onto the Si (111) surface serve as catalyst seeds. For Au-catalysed NWs, a monolayer of self-assembled polystyrene nanospheres (PNS 300 nm) was created on a 2 inch Si wafer by spin coating and later etched for a short time before a very thin Au-catalyst layer was deposited. In turn, for self-induced, PNS monolayer was created onto a SiO2-Si substrate. A longer etch was required to reduce PNS diameter significantly to leave relatively larger spacing where chromium is blanket deposited. PNS were lifted off by sonicating the samples in toluene produce the periodic arrays of nanodots and nanoholes, respectively. The underlying SiO2 was etched further through the nanoholes to uncover the Si below. 200 nm holes and 30-70 nm dots were demonstrated through the bespoke methods. The patterned substrates served as master templates, subsequently copied using polydimethylsiloxane (PDMS) to produce a flexible stamp for nanoimprint lithography. A bi-layer resist lift off process was developed to print the replicated nanodots or nanoholes on large-area substrates onto which GaAs NWs were subsequently grown. GaAs NWs were extracted and mixed in PMMA to produce a composite dielectric which was sandwiched between electrodes to act as a capacitor. An order of magnitude increase in relative permittivity (ϵr) is observed after the addition of the NWs allowing a high signal to noise ratio output on the application of pressure. This is due to the addition of higher permittivity nano-filler in the matrix.
Furthermore, it was demonstrated that encapsulated high aspect ratio NWs in a host (polymer in this case) can be integrated in devices to improve existing functionality. Devices were successfully fabricated for pressure sensing and memory using the above described low-cost high-volume process with high sensitivity and large memory window, respectively. This demonstration is one of the first steps in enabling low cost electronics without compromising on performance which is imperative for IoT.