This proposal addresses critical needs in the field of infrared (IR) detection and imaging and proposes a new approach to sense the IR radiation with ultra-high precision. In recent years, there has been a growing interest in high-precision IR detectors for application ranging from military and security to automotive and consumer market. Ideal IR detectors need to be agile, un-cooled, and exhibit ultra-high sensitivity. In the proposed sensor, both piezoelectric and pyroelectric effects will be exploited in a single resonant sensor element to achieve noise equivalent delta temperature of 5mK. Such performance makes it possible to replace cryogenically cooled photonic detectors with the low-cost uncooled system proposed herein. The sensitivity of the proposed IR detector is independent of area thanks to the simultaneous employment of piezo and pyroelectric effect. The use of the resonant effect increases the signal to noise ratio and eliminates the need for mechanical choppers that are needed for pyroelectric detectors, further reducing the overall size of the system. Applications of these low-noise un-cooled IR detectors in fingerprint sensors, night vision, hand-held imaging, and proximity sensors can be envisaged. The novelties of the proposed research include: (1) The combination of piezoelectricity and pyroelectricity in a single sensor element using GaN as the structural material to implement an un-cooled IR detector with sensitivity comparable to that of photonic detectors;(2) The use of resonant sensor array and a reference resonator to improve signal to noise ratio; (3) The employment of low-stress and highly polarized GaN grown using selective area epitaxy; (4) The application of thin film CNT-polymer nanocomposites with IR absorptivity > 0.99 as the absorbing layer for improved IR sensitivity; (5) The exploration into the use of two-dimensional electric gas (2DEG) as a metal-less electrode.
Intellectual merit: This research eliminates the disadvantages of thermal detectors, namely large area, slow response time and relatively small sensitivity. Simultaneous employment of piezo and pyroelectric effect in single resonant element leads to >4.5× improvement in sensitivity and response time. In addition to infrared detection, the proposed microsystem sensor array is capable of sensing, with ultra-low power, multiple measurands, namely magnetic field, inertial, and gas spectra. Therefore, the proposed platform technology makes the implementation of multi-sensor fusion possible. Unique properties of GaN, such as high electron mobility and large piezo and pyroelectric coefficient make it possible to achieve a high signal to noise ratio and low power consumption. Furthermore, the high chemical stability of GaN and the ability of the 2DEG to withstand high and low temperatures can lead to a broad range of applications. In addition to their application in sensing systems, high-Q GaN resonators are envisioned to have farreaching applications in frequency synthesizers and high-performance filters integrated with emerging high-performance GaN electronics. Therefore, the proposed platform has a great potential and is of great interest to the MEMS community.
Broader Impact: The proposed high-performance and small-size resonant sensors could have broad applications in diverse areas such as environmental sensors, biomedical sensors, intelligent sensors, and sensor networks. In addition to the outlined research effort, an integrated educational program will be established which aims to educate and motivate students through direct participation in the research activities. The study of microelectromechanical resonant microsystems is of particular value for students as it encompasses a variety of topics ranging from micro and nanofabrication, material characterization, structural analysis, physics of loss mechanisms, thermal and radiation effects, modeling and high frequency interface electronics. Therefore, it has an unprecedented multidisciplinary educational value at the fundamental engineering science. The goal of the educational plan is to educate and motivate students by (1) creating a multi-disciplinary scientific learning environment for students and directly training two doctoral students,(2) summer educational outreach program to expose several high school students to the field of MEMS, and microsystems by involving them directly in the proposed research activities, and (3) Involvement of undergraduate students from underrepresented groups in PIs' research and educational activities.
This NSF project aimed to achieve an uncooled infrared sensor by exploiting the piezo- and pyro-electric properties in wurtzite GaN materials. Conventional infrared photodetectors require cryogenic cooling and hence are difficult to be miniaturized. Uncooled detectors are compact, lightweight, and can considerably lower the cost. In this research, both piezoelectric and pyroelectric effects are exploited in a single resonant sensor element. As a result, a noise equivalent delta temperature (NEDT) of 5 mK is possible. During the first year of this project, the PI has demonstrated the acoustoelectric Q amplification in GaN bulk acoustic resonators and filters for the first time. Acoustoelectric effect is used to improve the quality factor of the fabricated resonators, and thus improve the sensitivity of GaN detectors. In addition, the PI group could obtain some of the highest-Q GaN resonators reported to-date. In the second year of the project, the PI demonstrated a GaN-based resonant IR detector. This was the first ever demonstration of resonant un-cooled IR detectors that exploit piezo, pyro, and electrostrictive effects in the resonator material. The proof-of concept demonstration shows promising performance. In the third year of the program, the PI and her students have demonstrated the 1st micromachined resonant IR detector array using GaN-based micromechanical resonators and silicon nitride based IR absorbers. The performance of the detector was analyzed and the resonator design was optimized to meets the 5 mK NEDT and 1 msec response time. The devices show improved sensitivity over other uncooled detectors and while the detectivity of these devices will not be as high as photonic detectors, the broad band response, low noise, and uncooled operation provide an attractive alternative. The GaN sensor prototype was fabricated on a GaN on silicon-on-insulator or GaN-on-silicon substrate. In the final year of the project, the team further analyzed the acoustoelectric effect and also demonstrated piezo/pyro resonant detectors with wavelength selective plasmonic absorbers. The educational component of this project involved substantial participation of graduate, undergraduate, and also high school students in research. Four graduate students as well as several undergraduate students participated in this research through NSF-NNIN REU program and University of Michigan Undergraduate Research Opportunity Program (UROP) and Summer Undergraduate Research Experience (SURE). Their training has involved materials growth, device fabrication, analytical tools (e.g. atomic force microscopy), and optical characterizations (e.g. photoluminescence). The educational component also included the development of a new graduate level course by the PI and also been constantly incorporated by the co-PI into the existing curriculum.
