III-V Nitride Micro- and Nano-Scale Cantilevers For Multimodal Sensing Applications

Recent research trends in chemical and biological sensing have been geared toward developing molecular sensor devices that are fast, label free, miniaturized and portable. The performance of these devices can be dramatically improved by utilizing multimodal detection techniques, new materials and micro-/nanofabrication technologies. This is especially true for micro-/nanoscale cantilever sensors, which undergo changes in mechanical or electrical properties upon the specific binding of molecules. To develop the sensor devices with the above attributes, we utilized III-V nitride materials: InN nanowires for realizing nanoscale cantilevers and AlGaN/GaN heterostuctures with or without embedded HFETs, for developing microcantilevers. There are mainly two approaches of fabricating these sensor devices:bottom-up approach for nanocantilevers, and top-down approach for microcantilevers. InN NWs, which exhibit interesting properties such as high carrier density, superior electron mobility, strong surface charge accumulation, and chemical inertness, were synthesized using Chemical Vapor Deposition (CVD) technique by Vapor-Liquid-Solid (VLS) mechanism. The synthesis process was optimized to obtain growth direction modulation and enhanced performance of the devices, largely avoiding the complexity of nanofabrication/ etching typically involved in the realization of nanoscale sensors. With dimensions much smaller than conventional cantilevers, the nanocantilevers are expected to have dramatically improved physical, chemical, and biological sensitivity for sensor applications. The piezoresistive and piezoelectric properties of AlGaN/GaN heterostructures, their wide bandgap, and chemical inertness make the microcantilevers very attractive for developing highly sensitive sensors suitable for harsh environment applications. The large variation in 2-dimensional electron gas (2DEG) at the interface with mechanical strain makes these microcantilevers much more sensitive than conventional Si based piezoresistive microcantilevers. A process was developed to fabricate free standing AlGaN/GaN microcantilevers on Si(111) substrate using various processing steps involving photolithography, GaN and through wafer Si etching, and dielectric and metal deposition. The detection performance of these cantilevers is largely improved by the utilization of a multimodal detection technique

Growth direction modulation and diameter-dependent mobility in InN nanowires

Diameter-dependent electrical properties of InN nanowires (NWs) grown by chemical vapor deposition have been investigated. The NWs exhibited interesting properties of coplanar deflection at specific angles, either spontaneously, or when induced by other NWs or lithographically patterned barriers. InN NW-based back-gated field effect transistors (FETs) showed excellent gate control and drain current saturation behaviors. Both NW conductance and carrier mobility calculated from the FET characteristics were found to increase regularly with a decrease in NW diameter. The observed mobility and conductivity variations have been modeled by considering NW surface and core conduction paths