Ciaran J. Brennan

Characterization and modelling of thin-film ferroelectric capacitors using C-V analysis

Advances have been made in the electronic characterization and analysis of thin-film ferroelectric (FE) memory capacitors using capacitance vs. voltage (C-V) measurements. A mathematical model of the small-signal electrical behavior of the FE capacitor has been developed. This analysis shows that the small-signal characteristics of the FE capacitor are largely determined by the space charge concentrations at the ferroelectric to contact interface. These space charge regions have an adverse effect on the permittivity, coercivity and switching characteristics of the ferroelectric capacitor. These results will contribute to improving ferroelectric processing, appraising the quality of ferroelectric devices, and developing device models of the ferroelectric memory capacitors for use in circuit design.

A physical model for the electrical hysteresis of thin-film ferroelectric capacitors

Abstract A mathematical model for the electrical hysteresis of ferroelectric thin-film capacitors based on the Landau free energy theory is presented. The hysteresis of a single ferroelectric crystallite is derived from free energy considerations. The hysteresis of a polycrystalline material is obtained by accounting for the spatial distribution and the structural phase of the crystallites. The hysteresis functions for the tetragonal and rhombohedral phases are derived. The model is a true expression of the polarization of a ferroelectric material based on physical principles, not a curve-fitting algorithm. The electrical history of the ferroelectric is explicit in the calculation of the polarization. It is therefore capable of simulating complex behavior such as arbitrary waveforms and sub-hysteresis loops.

Temperature dependent fatigue rates in thin-film ferroelectric capacitors

Abstract A series of fatigue measurements have been performed on PZT capacitors over temperatures ranging from 78K to 423K. The polarization loss is approximately proportional to the logarithm of the number of fatigue cycles, but the fatigue rate shows a strong temperature dependence, particularly at low temperatures. The capacitors fatigue very little at 78K but fatigue rapidly at room temperature. The temperature dependence of the fatigue rate fits an Arrhenius function with an activation energy of 0.043 eV. This data does not support the common interpretation that logarithmic decay results from an aggregate of exponentially decaying species uniformly distributed in activation energy. An alternative model is proposed that attributes fatigue to the formation of opposing domains to relieve the Coulombic and stress energy of charged defects. The result is stable defect/domain structures that resist electrical switching. The observed activation energy is presumed to pertain to the transport of electronic ch...

Defect chemistry model of the ferroelectricelectrode interface

Abstract The interactions between the electrodes and titanate ceramic materials are examined using numerical defect chemistry models. A time independent model solves for the steady-state concentrations of electrons and holes, ionized acceptors, and oxygen vacancy concentrations throughout the ferroelectric film as a function of electrical potential, temperature, and oxygen partial pressure. These calculations reveal a basic mechanism by which electrically active space charge regions may form in the ferroelectric material. The combination of atmospheric oxygen partial pressure with ionization energies of naturally occurring acceptor impurities of around one electron-volt prevents full ionization of the acceptors. However, the acceptors respond strongly to an electric field, and the percentage of ionized acceptors in the interface region may range from 0% to 100% with the application of a few tenths of a volt. The response of the partially ionized impurities to the electric field is the proposed mechanism f...

Landau theory of thin ferroelectric films

Abstract Landau-Devonshire theory1 is a useful phenomenological model to describe the properties of ferroelectric phase transitions. Below the transition temperature, the Landau model can be generalized to describe the thermodynamic stability of a ferroelectric crystallite in a bistable polarized configuration, and to predict the response of the crystallite to external fields and charges. The three primary elements to be considered in modeling thin-film ferroelectric devices are the polar response of the ferroelectric itself, the contribution of electrode interfaces, and the interaction of mobile and immobile charged defects and carriers with the ferroelectric and the electrodes. First, the hysteresis properties of a single domain or crystallite are derived. This result is generalized to find the polar response in a polycrystalline film where there may be variations in the size and orientation of the crystallites and in the coercivity, remanence and offset of the domains. After postulating that metal elec...

Ferroelectric Space Charge Non-Destructive Read-Out memory

Abstract The Ferroelectric Space Charge (FESC) Memory is a technique to achieve Non-Destructive Read-Out (NDRO) of a ferroelectric memory that employs standard metal-ferroelectric-metal memory capacitors. The NDRO technique allows the polarization state of the capacitor to be read without altering the polarization. This provides the advantage that the memory capacitor is not subjected to the destructive read/rewrite cycle commonly employed in ferroelectric memories, so the lifetime of the device is limited only by the number of times the memory is written. In addition, the NDRO avoids the period of data volatility in the time interval between the read pulse and the subsequent rewrite of the data. The FESC NDRO approach utilizes changes in the dielectric response of the ferroelectric due to interactions between the internal space charges, the spontaneous polarization, and internal and external electrical potentials. The NDRO is implemented entirely in circuitry and does not require specialized ferroelectri...