Towards the mass fabrication of single electron transistors for biosensing applications

• Main Author: Fry-Bouriaux, Louis
• Other Authors: Walti, Christoph ; Giles, A. G.
• Published: University of Leeds 2017
• Subjects: 621.3
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.727267

The development of ultra sensitive charge sensing devices such as single-electron transistors (SETs) for next-generation biomedical applications has received considerable attention in the past few years. In this thesis, a potential approach for the mass-fabrication of metallic SETs for ultra-sensitive biosensing applications --- an important prerequisit for early diagnosis of many serious diseases --- is investigated. Using the orthodox theory of Coulomb blockade it is shown that it is possible to engineer an SET system that can satisfy the requirements for a highly sensitive charge sensor operating at room temperature while using metallic electrodes rather than semiconductor structures. In this configuration, the SET design and fabrication process is simplified greatly by lifting the dependence of the system on the confinement energy of electrons in the quantum dots (QDs), as is the case in semiconductor SETs. In return, this makes the tunnel junction properties and the geometrical arrangement of the islands and electrodes far more critical in determining the maximum operating temperature of the device. Here, the geometrical requirements for such a sensitive device are studied theoretically whilst the tunnel junction properties are studied experimentally and then theoretically to provide a thorough assessment of the abilities of the proposed SET system. Atomic-layer deposition (ALD) has proven to be a highly reliable technique for depositing uniform thickness and reproducible thin metal-oxide films and particularly the Al2O3 ALD process is known to be `ideal' with highly reproducible properties. Here, a systematic study of the electronic properties of ALD deposited Al2O3 thin films in MIM structures was performed to assess the ALD techniques applicability to the mass fabrication of quantum tunneling junctions for metallic SET structures. The two most crucial material parameters relevant to the design of metallic SET tunnel barriers are studied in detail; the dielectric constant of the film that determines the junction capacitance, and the properties of the potential barrier that mediates electron tunneling. Photolithographic techniques were used to create electrodes with a wide range of characteristic lengths as to provide a wide range of impedances. Measurements and subsequent analysis show that a high consistency can be attained over large surface areas in the film properties, and that electrode coverage is very effective, showing promise for mass-fabrication applications. Further analysis of the measurements shows that small static distortions in the barrier can affect the symmetry of MIM diode IV characteristics operating in the direct tunneling regime and that under certain circumstances the effect of surface states can be observed in the tunneling conductance.