Date of Award

12-15-2014

Document Type

Open Access Dissertation

Department

Electrical Engineering

First Advisor

Goutam Koley

Abstract

Microcantilevers are highly attractive as transducers for detecting chemicals, explosives, and biological molecules due to their high sensitivity, micro-scale dimensions, and low power consumption. Though optical transduction of the mechanical movement of the microcantilevers into an electrical signal is widely practiced, there is a continuous thrust to develop alternative transduction methods that are more conducive to the development of compact miniaturized sensors. Piezoelectric and piezoresistive transduction methods are two of the most popular ones that have been utilized to develop miniaturized sensor systems. Piezoelectric cantilevers, which are commonly made of PZT film, have demonstrated very high sensitivity; however, they suffer from incompatibility with Si based circuitry and challenges with dc and low frequency measurements due to the problem of charge leakage. On the other hand, piezoresistive microcantilever, which are mostly made of Si, can be easily integrated with existing Si based process technologies, but suffer from low sensitivity. In addition, none of the above material systems are suitable for high temperature (>300 °C) and harsh environment operation. III-V Nitride semiconductors are being extensively studied almost two decades for electronic and optoelectronic applications due to their exceptional physical and chemical properties, which include a wide bandgap, strong piezoelectric properties, high electron mobility, and chemical inertness. AlGaN/GaN heterostructures offer unique advantage over existing piezoresistive or piezoelectric materials, as it actually converts the piezoelectric response of these materials to piezoresistive response, since the two dimensional electron gas (2DEG) formed at the AlGaN/GaN interface gets modulated by the stress induced change in piezoelectric polarization. The epitaxial growth of III-V Nitride layers on a Si substrate enables direct integration of nitride microelectromechanical systems (MEMS) with mature Si based integrated circuits to develop miniaturized sensor systems. In spite of several technological advantages of III-V Nitride MEMS, of which a microcantilever is a simple example, only a handful of studies have been reported on their deflection characterization in static mode and none on dynamic bending mode. The effect of mechanical strain, on 2DEG density and output characteristics of AlGaN/GaN heterostructure field effect transistors (HFETs), have been reported earlier. High gauge factors (>100) have been reported for quasi-static and step bending response, however, the factors contributing to such high values, especially their deviation from much lower theoretical estimates, are poorly understood. Recently, very high gauge factor of -850 was reported for microcantilevers in transient condition, however, the corresponding dynamic response was not studied. Acoustic detection using microcantilevers have attracted interest in recent years, especially in photoacoustic spectroscopy, as they can offer up to two orders of higher sensitivity compared to existing acoustic sensors. III-V Nitride based ultrasonic microcantilevers sensors, offering high sensitivity, low noise, and harsh environment operation, are ideally suited for many demanding sensing applications that are not possible at present. This dissertation aims the theory and application of III-V Nitride microcantilevers and a novel electronic transduction scheme named as ‘Piezotransistive Microcantilever’ to transduce femtoscale excitation. A complete fabrication process, measurement techniques and several application aspects of this sensing technology specially acoustic wave detection generated in solid and air media with high sensitivity, have been demonstrated. This thesis reports on displacement measurement at the femtoscale level using a GaN microcantilever with an AlGaN/GaN Heterojuction Field Effect Transistor (HFET) integrated at the base that utilizes piezoelectric polarization induced changes in two dimensional electron gas (2DEG) to transduce displacement with very high sensitivity. With appropriate biasing of the HFET, an ultra-high Gauge Factor (GF) of 8700, the highest ever reported, was obtained, with an extremely low power consumption of <1 >nW, which enabled direct electrical readout of the thermal noise spectra of the cantilever. The self-sensing piezotransistor was able to transduce external excitation with a superior noise limited resolution of 12.43 fm/Hz and an outstanding responsivity of 170 nV/fm, which is three orders higher that state-of-the-art technology, supported by both analytical calculations and laser vibrometry measurements. This extraordinary deflection sensitivity enabled unique detection of nanogram quantity of analytes using photoacoustic spectroscopy.

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