M. S. Rodgers

Bio: M. S. Rodgers is an academic researcher from Sandia National Laboratories. The author has contributed to research in topics: Capacitor & Ferroelectricity. The author has an hindex of 6, co-authored 7 publications receiving 670 citations.

Device modeling of ferroelectric capacitors

Abstract: A physically based methodology is developed for modeling the behavior of electrical circuits containing nonideal ferroelectric capacitors. The methodology is illustrated by modeling the discrete ferroelectric capacitor as a stacked dielectric structure, with switching ferroelectric and nonswitching dielectric layers. Electrical properties of a modified Sawyer–Tower circuit are predicted by the model. Distortions of hysteresis loops due to resistive losses as a function of input signal frequency are accurately predicted by the model. The effect of signal amplitude variations predicted by the model also agree with experimental data. The model is used as a diagnostic tool to demonstrate that cycling degradation, at least for the sample investigated, cannot be modeled by the formation of nonswitching dielectric layer(s) or the formation of conductive regions near the electrodes, but is consistent with a spatially uniform reduction in the number of switching dipoles.

Modeling ferroelectric capacitor switching with asymmetric nonperiodic input signals and arbitrary initial conditions

Abstract: The switching behavior of ferroelectric capacitors experiencing arbitrary time‐dependent electric fields and arbitrary initial conditions is investigated both theoretically and experimentally. A general approach for modeling incomplete dipole switching in ferroelectric capacitors is used to derive equations describing the electrical behavior of a simple characterization circuit with arbitrary initial conditions and arbitrary time‐dependent applied voltages. The equations include four experimentally determined parameters: the remanent and spontaneous polarizations, the coercive field, and the ferroelectric dielectric constant. Once these model parameters are determined from a single high‐frequency sinusoidal hysteresis loop, model predictions are made with no adjustable parameters. The circuit behavior for both sinusoidal and trapezoidal input signals is computed, including asymmetric and nonperiodic signals as well as several different initial conditions. The accuracy of the model predictions is quantitatively verified with experimental data. The approach is also utilized to model the switching behavior of a ferroelectric capacitor containing a sheet of space charge. It is found that hysteresis loop distortions resulting from ionizing radiation resemble those caused by a sheet of space charge. This quantitative modeling capability facilitates the optimization of the design of ferroelectric memory circuits by minimizing the amount of required electrical testing and characterization. It can also be used to facilitate the identification and understanding of degradation mechanisms occurring in ferroelectric thin films.

Total-dose radiation-induced degradation of thin film ferroelectric capacitors

Abstract: The radiation hardness of thin-film PbZr/sub y/Ti/sub 1-y/O/sub 3/ (PZT) ferroelectric capacitors is explored. Ferroelectric capacitors were irradiated using X-ray and Co-60 sources to dose levels up to 16 Mrad(Si). The capacitors were characterized for their memory properties both before and after irradiation. The radiation hardness was process dependent. Three out of four processes resulted in capacitors that showed less than 30% radiation-induced degradation in retained polarization charge and remnant polarization after irradiating to 16 Mrad(Si). On the other hand, one of the processes showed significant radiation-induced degradation in retained polarization charge and remanent polarization at dose levels above 1 Mrad(Si). A model for simulating the observed degradation is developed. The model indicates that the data are consistent with trapping of radiation-induced charge in the ferroelectric material. The radiation hardness levels indicate that ferroelectric devices can be fabricated that can survive radiation exposures well in excess of 10 Mrad(Si). >

Temperature effects on the radiation response of MOS devices

Abstract: The effect of device temperature during irradiation was investigated for MOS transistors and integrated circuits over the MIL-STD temperature range, -55 to 125 degrees C. Large, qualitative changes in transistor and 16k SRAM response are observed at both high and low temperatures. Although changes in transistor response are generally consistent with expectations, it is not always straightforward to predict circuit response as a function of temperature. For low-dose-rate (0.27 rad/s) irradiations of 16k SRAMs, the failure dose at 25 degrees C is at least twice that at 125 degrees C. This reduction is probably a result of a large increase in the SRAM cell imbalance. The IC failures consistently occurred at dose levels where the difference in threshold voltage between the transistors irradiated 'on' and 'off' became greater than 1.7 V. At high dose rates, large shifts in the parasitic field oxide threshold voltage caused large increases in the transistor leakage current at low temperatures. >

Strategies for lot acceptance testing using CMOS transistors and ICs

Abstract: Direct-correlation and simple overstress methods for estimating IC response in strategic and space environments from laboratory transistor and IC data are investigated. Transistors and ICs were irradiated at dose rates from 0.2 rad(SiO/sub 2/)/s to 10/sup 6/ rad(SiO/sub 2/)/s. Over a wide range of process conditions and hardness levels, laboratory measurements of threshold voltage shift due to oxide trapped charge correlate well with IC leakage current at high dose rates for ICs with gate-oxide-dominated response. For ICs whose response is dominated by parasitic field-oxide structures, laboratory measurements of both transistor and IC leakage currents correlate well with IC hardness at high dose rates. For dose levels up to approximately=500 krad(SiO/sub 2/), it is shown that a simple factor-of-three overtest can be used as a conservative estimate of radiation hardness for strategic applications, provided that both functional and parametric testing is performed following X-ray irradiation at a dose rate of approximately=2000 rad(SiO/sub 2/)/s. For space environments, a laboratory irradiation to 1.5 times the required system level followed by a one-week 100 degrees C biased anneal gave conservative estimates of IC hardness. >

Physics of thin-film ferroelectric oxides

Abstract: This review covers important advances in recent years in the physics of thin-film ferroelectric oxides, the strongest emphasis being on those aspects particular to ferroelectrics in thin-film form. The authors introduce the current state of development in the application of ferroelectric thin films for electronic devices and discuss the physics relevant for the performance and failure of these devices. Following this the review covers the enormous progress that has been made in the first-principles computational approach to understanding ferroelectrics. The authors then discuss in detail the important role that strain plays in determining the properties of epitaxial thin ferroelectric films. Finally, this review ends with a look at the emerging possibilities for nanoscale ferroelectrics, with particular emphasis on ferroelectrics in nonconventional nanoscale geometries.

Ferroelectric, dielectric and piezoelectric properties of ferroelectric thin films and ceramics

Abstract: Ferroelectric, dielectric and piezoelectric properties of ferroelectric thin films and ceramics are reviewed with the aim of providing an insight into different processes which may affect the behaviour of ferroelectric devices, such as ferroelectric memories and micro-electro-mechanical systems. Taking into consideration recent advances in this field, topics such as polarization switching, polarization fatigue, effects of defects, depletion layers, and depolarization fields on hysteresis loop behaviour, and contributions of domain-wall displacement to dielectric and piezoelectric properties are discussed. An introduction into dielectric, pyroelectric, piezoelectric and elastic properties of ferroelectric materials, symmetry considerations, coupling of electro-mechanical and thermal properties, and definitions of relevant ferroelectric phenomena are provided.

Polarization fatigue in ferroelectric films: Basic experimental findings, phenomenological scenarios, and microscopic features

Abstract: The reduction in switchable polarization of ferroelectric thin films due to electrical stress (polarization fatigue) is a major problem in ferroelectric nonvolatile memories. There is a large body of available experimental data and a number of existing models which address this issue, however the origin of this phenomena is still not properly understood. This work synthesizes the current experimental data, models, and approaches in order to draw conclusions on the relative importance of different macro- and microscopic scenarios of fatigue. Special attention is paid to the role of oxygen vacancy migration and electron injection into the film and it is concluded that the latter plays the predominant role. Experiments and problems for theoretical investigations, which can contribute to the further elucidation of polarization fatigue mechanisms in ferroelectric thin films, are suggested.

Ferroelectricity in yttrium-doped hafnium oxide

Abstract: Structural and electrical evidence for a ferroelectric phase in yttrium doped hafnium oxide thin films is presented. A doping series ranging from 2.3 to 12.3 mol% YO1.5 in HfO2 was deposited by a thermal atomic layer deposition process. Grazing incidence X-ray diffraction of the 10 nm thick films revealed an orthorhombic phase close to the stability region of the cubic phase. The potential ferroelectricity of this orthorhombic phase was confirmed by polarization hysteresis measurements on titanium nitride based metal-insulator-metal capacitors. For 5.2 mol% YO1.5 admixture the remanent polarization peaked at 24 μC/cm2 with a coercive field of about 1.2 MV/cm. Considering the availability of conformal deposition processes and CMOS-compatibility, ferroelectric Y:HfO2 implies high scaling potential for future, ferroelectric memories.

Imaging and control of domain structures in ferroelectric thin films via scanning force microscopy

Abstract: ▪ Abstract Scanning force microscopy (SFM) is becoming a powerful technique with great potential both for imaging and for control of domain structures in ferroelectric materials at the nanometer scale. Application of SFM to visualization of domain structures in ferroelectric thin films is described. Imaging methods of ferroelectric domains are based on the detection of surface charges in the noncontact mode of SFM and on the measurement of the piezoelectric response of a ferroelectric film to an external field applied by the tip in the SFM contact mode. This latter mode can be used for nondestructive evaluation of local ferroelectric and piezoelectric properties and for manipulation of domains of less than 50 nm in diameter. The effect of the film thickness and crystallinity on the imaging resolution is discussed. Scanning force microscopy is shown to be a technique well suited for nanoscale investigation of switching processes and electrical degradation effects in ferroelectric thin films.