Development of piezoelectric AlGaN/Si-nanoresonators for high frequency electroceramic sensors which can be integrated into semi-conductor devices
Final Report Abstract
We have successfully fabricated resonant MEMS and NEMS devices using functional layers of SiC, AlN and AlGaN/GaN heterostructures. Whereas silicon substrates were used for the SiC and AlN devices, the realization of free-standing AlGaN/GaN-based devices was achieved on 3C-SiC/Si pseudo substrates as well as on highly chemical inert substrates, namely SiC and sapphire. For the first time, we demonstrated the growth of AlGaN/GaN layers on 3C-SiC/Si pseudo substrates of low temperature LP-CVD grown 3C-SiC, and, in case of sapphire substrates, involved the use of sputtered AlN as sacrificial layer. Furthermore, isotropic and selective dry etch processes suitable for the three-dimensional micro-structuring of AlGaN/GaN-based MEMS and NEMS devices on either AlN sacrificial layers or SiC substrates were developed. Special attention has been laid upon the 2DEG at the AlGaN/GaN interface to remain unaffected by the etch processes. A magnetomotive coupling scheme was implemented for the experimental characterization of the SiC and AlN resonators. Electric actuation and sensing of their fundamental flexural vibration modes was accomplished by adopting a pulse response technique in the time domain which, for magnetomotively coupled MEMS resonators, has not been reported earlier. Based on the well-established differential measurement technique, we developed a specific electrical circuit setup to characterize the SiC and AlN resonators in the frequency domain, too. The piezoelectric transduction realized for the AlGaN/GaN resonators advantageously employs the 2DEG as counter electrode which has not been demonstrated previously for a free-standing AlGaN/GaN device. It provides an integrated electromechanical coupling scheme which is also suitable for further device miniaturization. By electrical and optical techniques, the AlGaN/GaN bridge resonators were characterized in the frequency domain. Thus, for the first time, the detection of piezoelectrically excited fundamental and higher order resonant modes of both flexural and longitudinal vibrations could be reported for such devices. The investigated materials yielded devices with high resonant frequencies and quality factors under vacuum. For the flexural vibration modes measured for the bridge resonators, the residual layer stress provided an even further upward tuning of the resonant frequencies up to ~10 MHz and, hence, of the ambient Q-factors up to ~450. The longitudinal vibration modes of the piezoelectric AlGaN/GaN bridge resonators achieved resonant frequencies up to ~63MHz and ambient Q-factors up to ~1500. Appropriate analytical modelling techniques have been reviewed and, supported by additional FE simulations, applied to the realized devices in order to relate resonant frequencies, viscous losses and electromechanical transduction factors to geometry, material and environmental parameters. Based on the derived dependencies, potential sensor applications for the measurement of material properties, the detection of small masses and the determination of the ambient pressure were demonstrated. With respect to sensor applications in gaseous or even liquid environments, future developments will be directed towards the improvement of the electrical readout of the piezoelectric AlGaN/GaN resonators including a reduction of the background signal. Finally, the implementation of sensor applications is pursued that additionally utilize the sensitivity of the 2DEG to environmental parameters.
