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igure 1. (a) Magnitude of the impedance for a series RLC circuit, (b) Phase component of the impedance for a series RLC circuit, (c) Measured capacitance using an LCR meter This frequency dependent behavior limits the applications of the component, especially when the component is used in wide-band circuits. To overcome this problem, one may choose a working frequency in which the component has the smallest capacitance variation. The frequency ω1, shown in fig 1(c) as a local minimum of the capacitance-frequency curve, is a potential candidate for this purpose. This frequency is defined as a point in which the sign of the capacitance derivative changes from negative to positive. The negative derivative at lower frequencies is due to the frequency dependent dielectric constant of the material while the positive derivative at higher frequencies is due to the RLC resonance. Thus, the position of the ω1 is affected by the frequency dependent dielectric constant of the material and parameters affecting the resonance frequency of the RLC circuit (total capacitance and parasitic inductance). Neglecting the parasitic effects, for each dielectric material only one identical ω1 at each capacitance value exists. Hence, the idea of using the capacitance minima for frequency stability is limited to specific capacitance-frequency points. Adding a series capacitor with a different type of dielectric material gives more freedom in adjusting the position of ω1. Practically, a ferroelectric/dielectric heterostructure can be used for this purpose. In this novel design, ω1 is tunable by changing the ratio of ferroelectric/dielectric capacitors or changing the total capacitance. Thus for an arbitrary capacitance-frequency value, we can design a varicap by selecting the proper thickness of the ferroelectric/dielectric layers in the heterostructure. Materials with high dielectric constant are also potential candidates for energy storage in addition to miniaturized capacitors in wideband electronic systems. CCTO, because of its large dielectric constant and high temperature stability, has recently attracted much research attention for such capacitor applications [9]. However, changes in capacitance over a large frequency range limit its application. In this study, the CCTO/LNB heterostructure was deposited on silicon substrates using a cost-effective sol-gel process to produce a frequency-stable varicap integrated on Si. The CCTO and LNB thin films were fabricated on HF terminated Si using chemical solution deposition. The characterization of these heterostructures and demonstrated an improvement in the frequency stability of the capacitor. Impedance and capacitance-voltage (CV) analysis were used for electrical characterization of Si/CCTO/LNB and Si/LNB/CCTO heterostructures, while X-Ray diffraction (XRD), scanning electron microscopy (SEM), electron dispersion spectroscopy (EDS), and spectroscopic ellipsometry (SE) measurements were used for crystal structure, chemical composition, interface, and optical properties of the layers.

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