Piezoresistive Effect in 4H Silicon Carbide towards Mechanical Sensing in Harsh Environments

Abstract

The fast-growing demand for mechanical sensors in harsh environments (e.g. mining/deep oil explorations, power/chemical plants and space explorations) urges the development of advanced materials which can replace silicon to work in these conditions. The superior mechanical properties of 4H silicon carbide (4H-SiC) combined with the physical stability at high temperatures offer new capabilities to develop MEMS sensors for those challenging situations. The piezoresistive effect is positioned as one of the most significant sensing mechanisms used in MEMS/NEMS sensors to detect or monitor mechanical signals, such as pressure, inertia, acceleration and deflection. Additionally, the use of micromachined sensors enables the miniaturization and integration capabilities while requiring low power consumption and simple readout circuitries. The main goals of this thesis are to investigate the piezoresistive effect in p-type 4H-SiC and to develop 4H-SiC based sensors which can be utilised for mechanical sensing in harsh environments. First, a literature review of developments and current research interests in the piezoresistive effect and silicon carbide materials for mechanical sensing are presented. Next, theoretical analyses on the strain induced effect in the silicon carbide energy band structure are thoroughly conducted. Moreover, the calculation of the coordinate transformation is performed to determine the fundamental piezoresistive coefficients in the (0001) plane of 4H-SiC. To verify the theoretical results, the fabrication of 4H-SiC sensing devices and experimental measurements are carried out. As such, the piezoresistive effect in p-type 4H-SiC at room and high temperatures is discovered by measuring the effect in longitudinal and transverse piezoresistor configurations. Additionally, the piezoresistive coefficients in the (0001) plane of 4H-SiC are investigated, providing insight into the orientation dependence of the piezoresistive effect in p-type 4H-SiC for the optimization of the sensing design. Subsequently, 4H-SiC based sensing devices are fabricated and characterised including a 4H-SiC based van der Pauw strain sensor and a 4H-SiC based pressure sensor. The excellent linearity, good repeatability, and stability of these sensors are favourable for mechanical sensing applications. Additionally, the 4H-SiC based pressure sensor exhibits high sensitivity and good reliability in cryogenic and high temperatures, indicating the potential for hostile environmental sensing.