SENSITIVE UNCOOLED IR DETECTORS USING GALLIUM NITRIDE RESONATORS AND SILICON NITRIDE ABSORBERS

Abstract

This paper presents the theory and measured results of a lownoise un-cooled infrared (IR) detector, which uses a combination of piezoelectric, pyroelectric, electrostrictive, and resonant effects to achieve high sensitivity. The sensor consists of a high-Q gallium nitride (GaN) micro-mechanical resonator coated with an IR absorber layer. The IR absorber converts the IR energy into heat. The generated heat causes a shift in the frequency characteristics of the GaN resonators. IR detection is achieved by sensing the shift in the resonance frequency of the exposed GaN resonator as compared to a reference resonator. A prototype GaN-based IR sensor is implemented, showing a frequency shift of 400 Hz per μW of absorbed power. The change in the beat frequency upon IR radiation is 1830 ppm/μW, making it possible to sense IR radiation in nano watts range. INTRODUCTION The detection of infrared radiation is generally accomplished using either photonic or thermal detectors, both of which have generated a large amount of research over the last two decades [1]. The relative disadvantages of thermal detectors in terms of detectivity and response time are balanced by the fact that they do not require cooling systems, reducing the system weight, power consumption, and form factor. Furthermore, thermal detectors are better suited for radiation extending into the far wavelength IR, which is extremely useful for optical astronomy. While bolometric devices have been the most popular thermal sensors commercially, there is an increasing interest in resonant IR detectors [2]-[4]. This work presents a GaN-based resonant IR sensor that utilizes the unique material properties of GaN to achieve high sensitivity. A reference resonator is included in the system for self-calibration (Fig. 1). Fig. 2 shows a representative scanning electron micrograph (SEM) of a GaN detector. Figure 1: Schematic of a resonant sensor and a reference pair with interdigitated finger electrodes. The sensor is coated with an absorber layer. The reference resonator is either not coated with the absorber or the absorber layer on the reference resonator is blocked using the top electrode.