Publications

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Ferroelectric hafnium zirconium oxide holds great promise for a broad spectrum of complementary metal-oxide-semiconductor (CMOS) compatible and scaled microelectronic applications, including memory, low-voltage transistors, and infrared sensors, among others. An outstanding challenge hindering the implementation of this material is polarization instability during field cycling. In this study, the nanoscale phenomena contributing to both polarization fatigue and wake-up are reported. Using synchrotron X-ray diffraction, the conversion of non-polar tetragonal and polar orthorhombic phases to a non-polar monoclinic phase while field cycling devices comprising noble metal contacts is observed. This phase exchange accompanies a diminishing ferroelectric remanent polarization and provides device-scale crystallographic evidence of phase exchange leading to ferroelectric fatigue in these structures. A reduction in the full width at half-maximum of the superimposed tetragonal (101) and orthorhombic (111) diffraction reflections is observed to accompany wake-up in structures comprising tantalum nitride and tungsten electrodes. Combined with polarization and relative permittivity measurements, the observed peak narrowing and a shift in position to lower angles is attributed, in part, to a phase exchange of the non-polar tetragonal to the polar orthorhombic phase during wake-up. These results provide insight into the role of electrodes in the performance of hafnium oxide-based ferroelectrics and mechanisms driving wake-up and fatigue, and demonstrate a non-destructive means to characterize the phase changes accompanying polarization instabilities.

The creation of microelectromechanical systems (MEMS) that can operate through elevated temperatures would enable systems diagnostics and controls that are not possible with conventional-off-the-shelf components. The integration of silicon carbide (SiC) with aluminum nitride (AlN) has led to the fabrication of devices that can withstand elevated temperature anneals >935 °C. The results from a piezoelectric micromachined ultrasonic transducer (PMUT) and a microresonator are reported as demonstrations of the fabrication process. Testing the PMUT response before and after annealing at 935 °C led to a change in resonant frequency of less than 1%, which is attributable to a shift in film stress. The response of the microresonator was RF tested in situ up to 500 °C and showed no degradation in its electromechanical coupling coefficient. The resonant frequency decreased with temperature due to the temperature coefficient of Young's modulus, and the quality factor decreased with temperature and remained unrecoverable upon cooling. The degradation in the quality factor is suspected to be a result of oxidation of the titatium nitride (TiN) top electrode, which increases the resistivity and leads to an unrecoverable reduction in the quality factor. The robust piezoelectric response of AlN at these temperatures show that AlN is a very promising candidate for elevated temperature applications.

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