UCLA

The objective of this work is to leverage the unique insulator to metal transition of Lanthanum Cobaltite (LaCoO3, LCO) towards the next generation of power limiters for high temperature, wide bandwidth, and high-power operation. LaCoO3 is a semiconducting perovskite that demonstrates a spin-state transition to metallic behavior about 500 K. In this dissertation we establish a wafer-scale reactive sputtering deposition process, characterize the temperature-dependent properties of thin film LCO, and estimate the thermally driven band gap collapse by near-IR absorption measurements.

The aforementioned sputtering process was used in the fabrication of power limiters that can be triggered either by external DC bias or by self-turn on at an input power threshold around 16 dBm. We present a 2 GHz LCO-based shunt power limiter constructed on silicon carbide with continuous wave power handling capability of 40 dBm, flat leakage of 20 dBm, and insertion loss less than 1 dB over a broad operating temperature range of 10 ᵒC to 75 ᵒC. Furthermore, very high temperature limiting response is recorded up to 225 ᵒC with a flat leakage of 15 dBm and power handling up to 33 dBm. LCO limiters of varied dimensions and geometries were studied to better understand the necessary tradeoffs in design for low insertion loss and high-power handling. To that end, a 3D COMSOL Multiphysics simulation was also developed that accurately predicts limiting performance and switching speed. This model is used to propose further improvements to the power limiter design.

S-parameter testing was conducted from 0.1 to 50 GHz, verifying the broadband viability of LCO microwave devices. We present extremely low small signal losses, with a maximum insertion loss of 1.2 dB at 125 ᵒC and 50 GHz. Benchmarking against both recent research art and commercial products indicates that LCO is a strong candidate for the next generation of microwave power limiters. Finally, we conclude the thesis with an evaluation of an LCO-based series radio frequency switch fabricated on sapphire substrate. We further summarize and discuss the significant potential of LCO in a wide range of thermally or electronically driven switching applications.