Systematic tuning of the conduction mechanisms in ferroelectric thin films

Abstract

We have investigated the macroscopic and microscopic properties of large sets of Ba0.7Sr0.3TiO3 thin films including several substitution rates of manganese. Thanks to a high degree of control of the processing parameters at each stage we have been able to find a link between the dc leakage current and the low and high frequency dielectric permittivity and losses. We supplemented these macroscopic observations with in depth investigations of the defect states through x-ray photoelectron spectroscopy. We found that both the leakage current and the extrinsic dielectric parameters arise from a large density of charged point defects related to oxygen vacancies. At the outer surfaces of the films, the density of such charged defects is so high that it can raise the Fermi level to close to the conduction band. Such degradation of the films' performance can be relieved by appropriate manganese substitution for the titanium host ions. Such doping is able to move back the Fermi level to close to the center of the bandgap thus changing the conduction process from interfacial Schottky to bulk Poole Frenkel and decreasing the extrinsic losses. This beneficial effect was already inferred in ceramics and thin films but we have established a clear link between the macroscopic parameters and the microscopic defect state. This model can be transferred to many high permittivity oxides.

Temperature stable BaSrTiO3 thin films suitable for microwave applications

“…The increase at high temperatures, more visible for low frequencies, comes from the diffusion of charge carriers which increases when the temperature increases [26,27]. In the case of ferroelectrics, the low frequency diffusion is generaly attributed to an extrinsic contribution like electronic conduction or oxygen vacancy migration [16,27,28]. In the case of acceptor-doped ferroelectrics (manganese in our case), the charge carrier diffusion is generally attributed to oxygen vacancy migration [16,27].…”

“…In the case of ferroelectrics, the low frequency diffusion is generaly attributed to an extrinsic contribution like electronic conduction or oxygen vacancy migration [16,27,28]. In the case of acceptor-doped ferroelectrics (manganese in our case), the charge carrier diffusion is generally attributed to oxygen vacancy migration [16,27]. The charge carrier diffusion is not visible at high frequencies (100 kHz).…”

“…1% manganese doping in substitution of the Ti-ions has been employed in order to reduce the low frequency diffusion and high frequency losses [14]. Electron acceptor doping permits us to compensate for oxygen vacancies which are the main defects that contribute to the conduction losses in ferroelectric thin films [14][15][16]. Moreover, manganese doping helps reduce the domain wall motion contribution to the permittivity which is very dissipative [14].…”

Decomposition of the different contributions to permittivity, losses, and tunability in BaSrTiO3 thin films using the hyperbolic law

“…This 1% dopant rate has been found to be optimum 7 for reducing the leakage current, the dielectric losses, and the low frequency charge carrier diffusion. 7,10,11 Platinum top electrodes have been deposited by RF sputtering in order to realize a MIM capacitor. The capacitance and the dielectric loss factor (tan d) have been measured using an Agilent 4294A impedance meter at 100 kHz.…”

Thermo-mechanical simulation to optimize the integration of a BST stacked MIMIM capacitor

“…Besides, BST material is known to present a low level of dielectric losses. Moreover, the doping of BST material can drastically improve the dielectric losses and leakage current of the capacitors [1]. Consequently, BST thin films represents a good candidate to perform tunable capacitors for RF and microwave applications like antenna tuner, tunable filter, and phase shifter [2].…”