Capacitive structures based on barium-strontium titanate films on silicon carbide

Ferroelectric (FE) films have received much attention as promising material for microwave (MW) applications. However, tuning under high power can lead to the nonlinear distortion of the operating signal, due to the overheating of the FE material. The MW devices based on the FE film deposited on a high heat conducting substrate should radically increase the power handling capability, i.e. the capability to process the high power MW signal without the deviation of working parameters, as compared to semiconductor elements. In this context, the present work considers the possibility of creating tunable structures “ferroelectric film/silicon carbide” based on barium-strontium titanate (BST) films on semi-insulating silicon carbide (SiC) substrates for microwave applications at elevated level of MW power.

Thin BST films on SiC substrates were obtained by RF magnetron sputtering of Ba_0.4Sr_0.6TiO3 target. The structure and microwave properties of the BST films on silicon carbide were substantially improved by using the intermediate annealing of layers during the deposition.

Tunable planar capacitors and slot line phase shifters were designed and manufactured based on BST/SiC heterostructures. The capacitor based on the highly oriented BST/SiC heterostructure demonstrate a tunability more than two times at an electric field strength of 50 V/µm, and a Q-factor of not lower than 50 at a frequency of 2 GHz, giving the best combination of tunability and losses for BST/SiC structures at microwaves.

Microwave investigations of BST/SiC tunable capacitors and phase shifters were performed at the elevated MW signal up to 10 W of the reactive power on the element. It was demonstrated that the power handling capability of BST/SiC tunable structures was limited by the electric non-linearity, not the overheating. Thus, the integration of ferroelectric BST films with semi-insulating SiC would be a promising route for the development of tunable high-power microwave devices.