Thesis defense of Yurii Kutovyi

Single-trap phenomena (STP) in nanoscale transistor devices possess outstanding properties that are promising for many useful and important applications including information technologies and biosensing. In this thesis, a novel biosensing approach based on monitoring of STP parameters as a function of target biomolecules on the surface of liquid-gated (LG) silicon (Si) nanowire (NW) field-effect transistor (FET) biosensors was proposed and demonstrated. To enhance STP dynamics and improve the efficiency of the approach, unique two-layer (TL) NW FETs with NW channels consisting of two Si layers with different concentrations of dopants were designed and fabricated. A stable and leakage-free operation in liquid confirms the high quality of TL NW devices. At the same time, fabricated TL nanostructures are conceptually different from the conventional uniformly doped Si NWs and demonstrate more statistically pronounced STP with considerably stronger capture time dependencies on drain current compared to that predicted by classical Shockley-Read-Hall theory. A comprehensive analysis of the experimental data measured at low temperatures allowed the identification of the origin of single traps in TL NWs as a vacancy-boron complex. Several important effects enabling the advancement of sensing capabilities of STP-based devices were revealed using fabricated TL NW FET biosensors. First, a significant effect of channel doping on the quantum tunneling dynamics of charge carriers to/from a single trap was registered in TL nanostructures, analyzed, and explained within the framework of the proposed analytical model. Second, a distinct fine-tuning effect of STP parameters by applying a back-gate potential to LG TL NW FETs was experimentally revealed and supported by numerical simulations. Such a unique feature of STP in TL NWs allows the sensitivity of STP-based biosensors to be enhanced in a well-controllable way. Furthermore, STP in NW FETs offer a great opportunity for the suppression of low-frequency noise. Considering a trap occupancy probability (g-factor) as a signal, a new method for the estimation of g-factor noise was proposed and utilized. As a result, the effective suppression of the low-frequency noise even beyond the thermal noise limit was experimentally and numerically demonstrated. The derived analytical model showed an excellent agreement with the obtained results underlining the importance of STP for biosensing applications. Utilizing the unique advantages of STP in fabricated TL NW FET biosensors, several proof-of-concept applications including high-sensitive detection of target chemical and biological analytes: mono and divalent ions, ascorbate molecules, and amyloid-beta peptides were demonstrated. Thus, the performed experiments together with the developed analytical models represent a major advance in the field of biosensors and pave the way for the next generation of novel ultrasensitive bioelectronic sensors exploiting single-trap phenomena.

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