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Final Report Summary - MPIS-FET (Metal-Piezoelectric-Insulator-Semiconductor Field-Effect-Transistor for high temperature pressure sensing applications)

Robust and high-sensitivity pressure sensors are strongly in demand in order to operate in harsh environments (high temperatures, corrosive chemicals, etc) such as in the hot section of a turbine engine. Conventional pressure sensors made from metal foils suffer from limited sensitivity, large temperature dependence, and high-power consumption. Piezoelectric materials alone can only be used under ac condition.
In order to perform pressure sensing at high temperatures, we propose a self-integrated metal-piezoelectric-insulator -semiconductor (MPIS) field-effect transistor (FET) device concept for high-sensitivity and high-temperature pressure detection based on microelectromechanical system (MEMS) technology. The MPIS-FET is fabricated on a robust semiconductor with freestanding or suspended structure.
To achieve the objective, we propose the application of diamond as the semiconductor and the structure material (cantilevers or bridges) for high temperature applications. The reasons to select diamond are: (i) diamond is the best semiconductor for high-temperature applications since it has a wide bandgap (5. 5 eV), a high breakdown electric field, a high carrier mobility, and the highest thermal conductivity, and (ii) diamond is the ideal MEMS material due to its outstanding properties such as the highest Young’s modulus, the highest hardness, a hydrophobic surface, low mass density, and high corrosion resistance upon caustic chemicals. On the other hand, high-Curie temperature ferroelectrics are required as the gate insulator on semiconductor. BixTiyOz (BTO) has a high Curie temperature above 500 o C and also shows high ferroelectric polarization.
To fabricate the ultimate MPIS-FET, the interface properties between the gate oxide and the semiconductor should be understood and controlled. Due to the low bandgap of BTO, wide bandgap oxide such as Al2O3 is firstly selected as the gate oxide on diamond to reduce the leakage current of the MIPS-FET. The Al2O3 layer can also act as a buffer layer for BTO growth on diamond. We developed for the first time the impedance spectroscopy for the characterization the interface of the MOS structure based on p-type diamond. An Al2O3 layer was used as the gate oxide on diamond with a p-type channel due to surface hydrogenation. The advantage of the impedance spectroscopy (IS) over the normal capacitance-voltage technique is that the series resistance effect in the device and measurement system can be avoided, so that the origin of the frequency dispersion effect can be distinguished. Based on the IS data analysis with a combination of the C-V technique, the interface states of the MOS structure can also be understood. Therefore, a precise gate capacitance can be obtained in principle. By using the IS technique, we obtained a dielectric value close to 9, which was larger than that obtained by C-V method.
In addition, we developed an energy-efficient low-temperature method for the fabrication of gate oxides with high dielectric constants on p-type diamond. As an example, we selected TiOx with high dielectric constant as the initial candidate. As shown in Fig. 1 of X-ray photoemission spectroscopy (XPS), the metallic Ti with a thickness of 10 nm was fully oxidized at 110 o C in air. We used this TiOx layer successfully for the fabrication of diamond MOSFET. Fig. 2 is the FET structure and properties of the MOSFET containing source, drain and gate. The transistor action was successfully demonstrated with good gate controlled drain current behaviour.

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