Development of micro-electromechanical systems in GaN

Abstract

This thesis is focused on the development of micro-electromechanical systems (MEMS) in III-V nitride semiconductors, with a primary emphasis on gallium-nitride (GaN). Though GaN exhibits unique properties that make it an effective platform for MEMS devices, to date, this field of study has received almost no attention in the nitride community. As a result, the research outlined in this thesis represents the very first steps in the development of GaN for this application. A critical issue for the advancement of GaN MEMS is the development of transducers to actuate and sense motion in deformable microstructures. In the second chapter of this text, we present two classes (and four instances) of strain transducers, native to GaN, which take advantage of its large piezoelectric constants. Unlike in conventional insulating crystals, the presence of free charge carriers in the semiconductor has a strong bearing on its response to strain. The action of piezoelectricity within a semiconductor allows for a family of versatile and sensitive transducers in GaN. Another basic issue facing this new field is the establishment of a fabrication technology to create suspended structures on the micron and nanometer scale. In Chapter 3, we describe two processes that were developed to fashion a wide range of MEMS devices in both p-type and n-type GaN. Each process exploits a distinct electrochemical etch which is dopant selective, the two etches being complementary. In one process, a photo-electrochemical method was adapted to undercut p-GaN epilayers that were grown on top of n-type sacrifical layers. For the other, a novel anodic etch was developed to undercut n-GaN layers. Both methods feature high dopant selectivity, rapid undercutting rates, and lateral etch control. The final chapter brings together both major research thrusts in the study of resonant cantilevers with integrated piezoelectric transducers. These devices are evaluated in terms of two important benchmarks: (i) the sensitivity to detect endpoint displacement, and (ii) the quality factor of the resonance. In the former case, the devices met up with our theoretical expectations; in the latter, no fundamental material limitation was found.