Role of surfaces and dopants in quantum devices and nanowire transistors

Abstract

Miniaturisation of electronic devices has driven development of high speed, high density processors and memory elements. This process has required extensive optimisation of semiconductor materials and interfaces as the random nature of doping increasingly affects device performance and the influence of non-ideal surfaces and interfaces need to be counteracted. As Moore's law for silicon may soon reach its limit, there is a desire to harness electrically efficient III-V semiconductor materials in an economically viable way. There is also a desire to utilise new functionalities brought by quantum mechanics, thermoelectrics and organic materials.

This thesis explores the role of p-type AlGaAs/GaAs heterostructures, self-assembled semiconductor nanowires and organic polymer electrolytes in this broad research programme. My research investigated the impact of background potentials generated by doping and surface states for quantum devices. I developed new wrap-gating techniques for InAs semiconductor nanowires towards economically viable arrays of III-V transistors on silicon substrates. This involved both conventional metal/oxide wrap-gates as well as nanoscale patterning of polymer electrolyte films to both improve the compatibility of organics with nanostructures and seek new functionalities for nanowire transistors. I then used polymer electrolytes to both act as an external dopant, and set the background potential for nanowire thermoelectrics. I also developed proof-of-principle complementary n- and p-type proton-to-electron transducers.

Throughout, I highlight the importance of dopants and surfaces. I show how these non-ideal aspects of semiconductor devices affect performance and attempt to find solutions where possible by, e.g., using sulfur-based surface passivation solutions or polymer electrolytes as an external dopant. Using these examples I illustrate that the drive to develop new technologies leads to new physics on both ends. Imperfections in research devices lead to new understanding of material physics, and once these are overcome, the new functionalities embodied by the devices can be used to study new aspects of nature.