Starting from Gallium Nitride (GaN) epitaxially grown on silicon, pre-stressed micro-resonators with integrated piezoelectric transducers have been designed, fabricated, and characterized. In clamped-clamped beams, it is well known that tensile stress can be used to increase the resonant frequency. Here we calculate the mode shape functions of out-of-plane flexural modes in prestressed beams and we derive a model to predict both the resonant frequency and the piezoelectric actuation factor. We show that a good agreement between theory and experimental results can be obtained and we derive the optimal design for the electromechanical transduction. Finally, our model predicts an increase of the quality factor due to the tensile stress, which is confirmed by experimental measurements under vacuum. This study demonstrates how to take advantage from the material quality and initial stress resulting of the epitaxial process.

We present the first results about microelectromechanical (MEMS) resonators fabricated on epitaxial nitride semiconductors with thin buffers engineered for MEMS and NEMS applications. These results assess the use of thin buffers for GaN MEMS fabrication. On a 700 nm thick AlGaN/GaN epilayer, a high tensile stress is observed to increase the resonant frequency. The electromechanical coupling efficiencies of integrated transducers are assessed and compared with previously obtained results on commercially available 2-µm thick epilayers used for power transistor applications. A 28 nm/V actuation efficiency is measured on the 700-nm thick structure which is slightly better than the one measured on the 2-µm buffer. The electrical response of a gate-less detector designed as a piezoresistance was carried out and a gauge factor of 60 was estimated. These results show that material issues can be unlocked to exploit the potentialities of III-nitrides for NEMS applications.

The characteristics of piezoelectric micro-resonators based on vibrating beams essentially depend on two basic physical coefficients, an effective Young’s modulus (Ye) and a piezoelectric coupling factor (e31e). An improved analytic model is proposed with newly derived expressions of Ye and e31e that account for the anisotropic properties of the III-nitride materials and beam width, W. The analytic model applicable to the only axial stress is completed by finite element (FE) simulations that allow any spatial patterns of pre-stress in wafers to be studied. The value of e31e for wider beams is analytically demonstrated to be much higher than the usual e31, and a strong dependence of e31e on W is also confirmed by FE simulations. Resonance frequency (fr) and actuation efficiency (η) are numerically studied for several pre-stress patterns and beam dimensions. The fr is found to be sensitive to the beam width only for resonators under 2D pre-stress while the η to the stress magnitude regardless of stress pattern. Compared with measurements published for some fabricated resonators, both analytic and FE approaches agree well quantitatively for the resonance frequency and qualitatively for the dynamic amplitude. The results of this study can help design optimization, such as appropriate electrode length and suitable beam width, to gain better performance for this type of resonators.

Starting from Gallium Nitride (GaN) epitaxially grown on silicon, pre-stressed micro-resonators with integrated piezoelectric transducers have been designed, fabricated, and characterized. In clamped-clamped beams, it is well known that tensile stress can be used to increase the resonant frequency. Here we calculate the mode shape functions of out-of-plane flexural modes in prestressed beams and we derive a model to predict both the resonant frequency and the piezoelectric actuation factor. We show that a good agreement between theory and experimental results can be obtained and we derive the optimal design for the electromechanical transduction. Finally, our model predicts an increase of the quality factor due to the tensile stress, which is confirmed by experimental measurements under vacuum. This study demonstrates how to take advantage from the material quality and initial stress resulting of the epitaxial process.

We report on an accelerometer micro-sensor based on epitaxial gallium nitride and silicon. The device is a vibrating beam accelerometer fabricated with a micro-electro-mechanical-system technology starting from an AlGaN/GaN heterostructure grown on silicon. The vibrating GaN beam has integrated high electron mobility transducers, whereas a high aspect ratio proof mass is engineered in the silicon substrate. The sensor response was investigated for several modes and features a scale factor up to 160 Hz/g, with unconventional dependence vs the mode number. To account for this, we propose an analytical model of the accelerometer scale factor that takes into account the built-in stress during epitaxy. This proof-of-concept device opens perspectives for inertial sensors taking advantage of GaN properties.