Temperature Dependence of Multimode Gallium Nitride/Aluminum Nitride (GaN/AlN) Heterostructure String Resonator

We report on the first experimental characterization and analysis of the temperature coefficient of resonance frequency (TCf) of gallium nitride/aluminum nitride (GaN/AlN) heterostructure doubly-clamped micro-string resonators with length L = 100 μm, thickness t = 700 nm, and width w = 5 μm. We observe three distinct resonances within the frequency range of 1.5 to 5 MHz at room temperature. Assisted by finite element method (FEM) simulations, we determine that the first two modes are out-of-plane flexural modes and the third one is an in-plane flexural mode, with the former two dominated by built-in stress and the latter one determined by both stress and flexural rigidity. We examine these resonances with varying temperature (-10 ºC to 105 ºC) and obtain TCfs of -336 ppm/K, -316 ppm/K, and -83 ppm/K for the three modes, respectively. The investigation provides essential information for thorough understanding of elastic behavior of GaN/AlN heterostructures upon temperature change.