Date of Award

5-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical and Computer Engineering

Committee Chair/Advisor

Dr. Goutam Koley, Committee Chair

Committee Member

Dr. William R. Harrell

Committee Member

Dr. Pingshan Wang

Committee Member

Dr. Apparao M. Rao

Abstract

Microcantilevers are an important form of microelectromechanical systems (MEMS) and have been widely used in notable applications including bio sensing, chemical detection, and imaging. Traditionally, laser based optical read-out systems have been used to measure amplitudes of the microcantilever tip oscillations (i.e. in an atomic force microscope (AFM)). However, this technique has several drawbacks including high power requirements and bulkiness that prevent commercializing of microcantilevers based miniaturized sensors. Monitoring oscillations of a cantilever array then also becomes immensely challenging. To overcome these problems, microcantilevers with integrated piezoresistive and piezotransistive deflection transducers have been developed. Because of issues related to piezoresistive read-out methods including low sensitivities and high thermal/electrical noise, reliable and highly sensitive microcantilever based sensor development is challenging. Recently, it has been demonstrated that AlGaN/GaN heterojunction field effect transistors (HFET) embedded on piezotransistive GaN microcantilevers have higher deflection sensitivities compared to silicon based piezoresistive cantilevers. However, the applications utilizing GaN microcantilevers with embedded AlGaN/GaN HFETs operated in linear dynamic regime have been remained limited. Moreover, operations in a nonlinear regime for oscillatory systems have gained strong interest because the nonlinearity could improve the detection performance of the systems and result in novel microelectromechanical devices. Nonlinear dynamics arising from geometric and inertial nonlinearities due to large deformations and rotations under strong driving forces has been extensively studied for many MEMS and NEMS resonators. However, systematic theoretical and experimental investigation on nonlinearity in GaN microcantilevers has not been performed.

In this work, first we have demonstrated Kelvin probe measurements utilizing piezotransistive GaN microcantilevers, with integrated AlGaN/GaN HFET at their base for highly sensitive deflection transduction, eliminating the need for external deflection detection mechanism. The Kelvin probe based surface work function (SWF) measurements employing GaN microcantilevers also eliminate the need for sensing layer coating on the cantilever, and thus providing an effective mechanism for detection of chemical and biomolecules. To demonstrate capability of this readily miniaturizable measurement technique, changes in SWF due to NO2 adsorption on graphene and In2O3 were measured, which agreed well with measurements performed previously using laser-photodetector based system.

In addition to linear dynamic regime based NO2 sensing measurements utilizing GaN microcantilevers, we have investigated nonlinear resonance characteristics of piezotransistive GaN microcantilevers with various dimensions. A comprehensive investigation involving 18 microcantilevers revealed both hardening and softening type nonlinearities in the first flexural mode in the GaN microcantilevers, at sufficiently high excitation levels, exhibiting clear hysteresis, bifurcation and jump frequencies. The dimensions of the cantilevers were found to significantly affect the nonlinear behavior, with variation in width having a much stronger effect on the nonlinear behavior compared to length over the range of dimensions studied. The experimentally observed nonlinear iv behavior was modeled using a Duffing equation which produced excellent match for both hardening and softening behavior for both frequency sweep directions.

Using the bistable states of intrinsic nonlinear regime in piezotransistive GaN microcantilevers, we have demonstrated dynamic memory operations. A novel multimodal excitation scheme involving a piezoactuator and a laser, acting as primary and secondary excitation sources, was used to perform constructive and destructive interference based on their phase relationships. Such combined excitation method was then used to demonstrate hardening and softening nonlinearities in these cantilevers, and switch between the high and low oscillatory states in the bistable regime. Minimum energy required for reliable memory operations utilizing photoacoustic excitation was determined to be less than a pico-joule, utilizing plasmonic amplification of the signal by Au nanoparticles deposited near the cantilever, which resulted in several times increase in the photoacoustic signal magnitude. The switching energy is estimated to be one of the lowest reported so far for oscillators with various dimensions, excitation sources, and readout schemes.

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