Metal-ferroelectric Interface Research Articles

The influence of polarization on properties of the potential barrier at metal-ferroelectric interface

The barrier properties of a capacitor heterostructure based on a ferroelectric lead zirconate-titanate were investigated by using the methods of current-voltage and voltage-capacitance characteristics. The variable values of the potential barrier on the PZT-Pt interface were determined by the C-V method (from 0.5 to 1.6 eV) and the I-V method (from 0.5 to 0.7 eV). The spontaneous polarization influence on the potential barrier at the PZT-Pt interface was estimated.

Negative capacitance transistors with monolayer black phosphorus

AbstractQuantum transport properties of negative capacitance transistors (NC-FETs) with monolayer black phosphorus (ML-BP) are theoretically studied. Our calculations show that atomistic thin ML-BP can enhance the amplification effect of the ferroelectric layer, and subthreshold swing is effectively reduced to 27 mV per decade in ML-BP NC-FETs. Device performance can be further improved by increasing the thickness of ferroelectric layer and using thinner or high-k insulate layer. Due to the temperature dependence of ferroelectric layer ML-BP NC-FETs have higher on-state current at low temperature, which is different from that of MOSFETs. By considering the metal–ferroelectric interface layer, our calculations show that the device performance is degraded by the interface. Compared with the International Technology Roadmap (ITRS) 2013 requirements, ML-BP NC-FETs can fulfil the ITRS requirements for high-performance logic with a reduced supply voltage. The new device can achieve very low power delay product per device width at VD=0.3 V, which is just 44% of that in ML-BP FETs.

Tuning of the depolarization field and nanodomain structure in ferroelectric thin films.

The screening efficiency of a metal-ferroelectric interface plays a critical role in determining the polarization stability and hence the functional properties of ferroelectric thin films. Imperfect screening leads to strong depolarization fields that reduce the spontaneous polarization or drive the formation of ferroelectric domains. We demonstrate that by modifying the screening at the metal-ferroelectric interface through insertion of ultrathin dielectric spacers, the strength of the depolarization field can be tuned and thus used to control the formation of nanoscale domains. Using piezoresponse force microscopy, we follow the evolution of the domain configurations as well as polarization stability as a function of depolarization field strength.

The influence of ferroelectric–electrode interface layer on the electrical characteristics of negative-capacitance ferroelectric double-gate field-effect transistors

The surface potential and subthreshold characteristics in negative capacitance (NC) double-gate ferroelectric field-effect transistor (FeFET) were investigated by considering the metal–ferroelectric interface layer. The derived results indicates that the negative capacitance regime which allows for amplified surface potential and steeper subthreshold characteristics were significantly affected by the interface layer. This imposes a severe limit to the step-up conversion capability and steeper subthreshold (<60mV/dec) obtainable in the device. These results may provide some insight into the design and performance improvement for the low power dissipation FeFETs.

Short-circuit photocurrent in epitaxial lead zirconate-titanate thin films

Photovoltaic properties of the metal-ferroelectric-metal structures, having SrRuO3 metal oxide electrodes and Pb(Zr,Ti)O3 (PZT) as ferroelectric layer, are investigated by the short-circuit photocurrent (SC-PHC) in the 200–800nm wavelength domain. The band-gap dependence on the Zr content was determined from the spectral distribution of the SC-PHC signal. It was found that the band-gap value increases linearly with the Zr content, from about 3.9eV to about 4.4eV. It is shown that the sign and the magnitude of the signal depend on the internal bias and on the spontaneous polarization direction and value. The photocurrent describes a hysteresis loop similar to that of the ferroelectric polarization and can be used as a nondestructive readout of the nonvolatile memories based on PZT films. The existence of a significant SC-PHC signal at wavelengths corresponding to subgap energies is attributed to the presence of charged, deep levels in the forbidden band. It is also shown that the epitaxial PZT films have the potential for solid-state UV detectors, with current responsivity as high as 1mA∕W. The results are not entirely consistent with a bulk photovoltaic effect and are discussed in the frame of a Schottky barrier model for the metal-ferroelectric interface.

Improved Retention Characteristics of Metal-Ferroelectric-Insulator-Semiconductor Structure Using a Post-Oxygen-Annealing Treatment

An oxygen-annealing process after SrBi2Ta2O9 (SBT) ferroelectric film deposition has effectively improved retention properties of the metal-ferroelectric-insulator-semiconductor (MFIS) structure for FET-type ferroelectric nonvolatile memory. The annealing treatment 1) reduces the surface roughness of the SBT film, 2) improves crystallinity, 3) effectively decreases the leakage current through the MFM structure, 4) increases the barrier height at the metal-ferroelectric interface, 5) decreases the carrier trap density in SBT film, and 6) improves the retained polarization of SBT film itself. Finally, the retention time of the memorized state in the MFIS diode was successfully increased to 2×104 s or more by the postannealing treatment.

X-ray photoelectron spectroscopy and high resolution electron microscopy studies of Aurivillius compounds: Bi4−xLaxTi3O12(x=0, 0.5, 0.75, 1.0, 1.5, and 2.0)

In order to determine the fatigue-free origin of the ferroelectric Bi3.25La0.75Ti3O12, a series of x-ray photoelectron spectroscopy and high resolution electron microscopy studies were performed on the polycrystalline Bi4−xLaxTi3O12 (BLTx, x=0, 0.5, 0.75, 1.0, 1.5, and 2.0) powders. From the XPS study, the surfaces of all La-containing compounds are found to consist of one outermost Bi-rich region and followed by a La-rich region, instead of the chemical stoichiometry in the bulk. An HREM study on Bi3.25La0.75Ti3O12 further confirms that this surface configuration arises from some intergrowth defects with a thickness of 5 nm appearing on the crystal edge, which is also observed in compounds with x=1.0 and 2.0, but not in the poor fatigue-resistant Bi4Ti3O12 (x=0). This La-induced defect locally changes the chemical composition of the crystal surface, which probably possesses a different physical characteristic as compared to the bulk. Consequently, it could can be the physical nature of the interface, when in contact with the metal electrode, Pt, in ferroelectric nonvolatile memory. Since the fatigue phenomenon on a ferroelectric capacitor arises predominately from the pinning of domain walls on the metal-ferroelectric interface, the surface configurations of La-containing Bi4Ti3O12 compounds should be considered as one of the fatigue-free factors as well as the self-regulation of the Bi2O2 layer and the chemical stability of the perovskite slabs. In addition, the band gap of Bi3.25La0.75Ti3O12 is also estimated by UV absorption spectrum to be 3.9±0.1 eV.

Ferroelectric Schottky diode.

A Schottky contact consisting of a semiconducting ferroelectric material and a high work function metal shows a bistable conduction characteristic. An on/off ratio of about 2 orders of magnitude was obtained in a structure consisting of a 0.2 \ensuremath{\mu}m ferroelectric PbTi${\mathrm{O}}_{3}$ film, a Au Schottky contact, and a ${\mathrm{La}}_{0.5}$${\mathrm{Sr}}_{0.5}$Co${\mathrm{O}}_{3}$ Ohmic bottom electrode. The observations are explained by a model in which the depletion width of the ferroelectric Schottky diode is determined by the polarization dependence of the internal electric field at the metal-ferroelectric interface.

A circuit model for a thin film ferroelectric memory device

Thin film ferroelectric memory devices have displayed ideal memory characteristics such as nonvolatility, radiation hardness and fast switching times. The simulation of the digital circuits using thin film ferroelectric memory devices necessitates an equivalent circuit model for these thin film device structures. The basic switching characteristics of a device is nonlinear in nature and requires a generating source in the equivalent circuit model. This current source is related to the polarization current as a function of time.The capacitance-voltage and hysteresis data on capacitor type device structures suggest the existance of an interfacial layer at the metal-ferroelectric interface. The interfacial layers are characterized by appropriate RC networks. The bulk ferroelectric material is modeled as a linear capacitance (dielectric) in parallel with a bulk resistance in parallel with the current generating source. This equivalent circuit model is simulated on the IBM 4381 mainframe computer using SPICE. The simulation results are compared with the actual switching characteristics observed on the oscilloscope. There appears to be an excellent agreement between the simulated results and experimental results.