Abstract
The dielectric constant of ferroelectric material, Ba(Zr,Ti)O3, (BZT) changes with external dc biasing voltage. Based on this tunable characteristic, it is useful to fabricate the varactors, tunable filters and phase shifter devices. In this research, two kinds of ferroelectric varactors were investigated, i.e. structures made with parallel and coplanar electrodes, and the study includes three topics: (1) the influences of bottom electrodes in parallel structure, (2) the influences of interfacial layer in the ferroelectric varactors fabricated on high-resistivity silicon (HR-Si), (3) the influences of high-k and low loss buffered layer and annealing condition in the ferroelectric varactors fabricated on high-resistivity silicon. In the first topic, three kinds of bottom electrode with different thickness were studied; including Pt, LaNiO3/Pt and LaNiO3/Au/Pt. Increasing the Pt thickness reduces effectively the total loss and raises the figure-of-merit (FOM) value. The conductive oxide, LaNiO3 (LNO) layer, enhances the textured crystallinity and increases the breakdown voltage, resulting in higher tunability of capacitance and FOM value. In the second topic, the BZT films deposited on HR-Si with or without tetraethylorthosilane (TEOS) SiO2 as buffer layer were studied. The influences of electrical characteristics were measured by impedance and network analyzer at high and microwave frequencies, respectively. The results at high frequency show that the tunability of coplanar varactors results from the depletion and accumulation of carriers in Si, which can be modeled by two back-to-back MOS capacitors in series connection. At microwave frequencies, positive tunability, i.e. [C(V)-C(0)]/C(0) > 0, was observed and can be explained by an equivalent circuit in relation to the formation of positively-charged defects at the interfacial layer and the modulation of carrier accumulation at the interface in Si. In the third topic, the Ta2O5 and (Ta2O5)1-x(TiO2)x layers were used to investigate the influences of high-k and low loss buffer layer. The results show that the thicker buffer layer and lower dielectric constant result in lower tunability. Moreover, the reduction of Ta5+ and formation of oxygen vacancies induce the change of carrier distribution at interface in Si, giving rise to different tunability at microwave frequencies. The positively-charged defects in the interfacial layer attract the electrons accumulated on the surface of silicon which screen the microwave from penetrating into the HR-Si, and affect the capacitive effect of HR-Si in series connection to the varactor. In addition, annealing in O2 annihilates the oxygen vacancies and oxidize the cations of BZT films causing the reduction of carrier accumulation and tunability. Annealing in N2 increases the oxygen vacancies and the reduction of cations in BZT films causing the raise of charged defects and injected carriers giving rise to hystersis behavior.