T. Schneller

The interface screening model as origin of imprint in PbZrxTi1−xO3 thin films. I. Dopant, illumination, and bias dependence

Comprehensive imprint measurements on PbZrxTi1−xO3 (PZT) thin films were carried out. Different models, which were proposed in literature to explain imprint in ferroelectric thin films or a similar aging effect (internal bias) in ferroelectric bulk material, are reviewed. Discrepancies between the experimental results obtained on the PZT films in this work and the prediction of the literature models indicate that these models do not describe the dominant imprint mechanism in PZT thin films. Hence, in this work a model is proposed which suggests imprint to be caused by a strong electric field within a thin surface layer in which the ferroelectric polarization is smaller or even absent compared to the bulk of the film. With the proposed imprint model the influence of important experimental parameters like dopant, illumination, and bias dependence can be qualitatively explained.

Piezoresponse force microscopy of lead titanate nanograins possibly reaching the limit of ferroelectricity

Single ferroelectric lead titanate (PTO) grains down to 15 nm were fabricated by chemical solution deposition. Varying the dilution of the precursor solution leads to different grain sizes between 15 and 200 nm. The grain-size-dependent domain configuration was studied using three-dimensional piezoresponse force microscopy (PFM). It is found that the PTO grains in a dense film contain laminar 90° domain walls, whereas separated PTO grains show more complicated structures of mainly 180° domain walls. For grains smaller than 20 nm, no piezoresponse was observed and we suppose this could be due to the transition from the ferroelectric to the superparaelectric phase which has no spontaneous polarization. Recent calculations showed that the ferroelectricity of fine ferroelectric particles decrease with decreasing particle size. From these experiments the extrapolated critical size of PTO particles was found to be around 4–14 nm.

Towards the limit of ferroelectric nanosized grains

Ferroelectric random access memories are non-volatile, low voltage, high read/write speed devices which have been introduced into the market in recent years and which show the clear potential of future gigabit scale universal non-volatile memories. The ultimate limit of this concept will depend on the ferroelectric limit (synonymous superparaelectric limit), i.e. the size limit below which the ferroelectricity is quenched. While there are clear indications that 2D ferroelectric oxide films may sustain their ferroelectric polarization below 4 nm in thickness (Tybell T, Ahn C H and Triscone J M 1999 Appl. Phys. Lett. 75 856), the limit will be quite different for isolated 3D nanostructures (nanograins, nanoclusters). To investigate scaling effects of ferroelectric nanograins on Si wafers, we studied PbTiO3 (PTO) and Pb(ZrxTi1−x)O3 grown by a self-assembly chemical solution deposition method. Preparing highly diluted precursor solutions we achieved single separated ferroelectric grains with grain sizes ranging from 200 nm down to less than 20 nm. For grains smaller than 20 nm, no piezoresponse was observed and we suppose this could be due to the transition from the ferroelectric to the paraelectric phase which has no spontaneous polarization. Recent calculations (Zhong W L, Wang Y G, Zhang P L and Qu B D 1994 Phys. Rev. B 50 698) and experiments (Jiang B, Peng J L, Zhong W L and Bursill L A 2000 J. Appl. Phys. 87 3462) showed that the ferroelectricity of fine ferroelectric particles decrease with decreasing particle size. From these experiments the extrapolated critical size of PTO particles was found to be around 4.2–20 nm.

Advanced chemical deposition techniques - from research to production

Abstract Chemical deposition techniques show distinct advantages for research tasks as well as for microelectronic production lines. This review will summarize the state of the art of the Chemical Solution Deposition (CSD) and of the Metal-Organic Chemical Vapor Deposition (MOCVD) processes for high-permittivity, ferroelectric, and other complex electronic oxide thin films as well as their applications in integrated capacitors, FeRAMs, etc Emphasis will be laid on the selection and the properties of the precursors, the microscopic mechanisms of film deposition, and the tailoring of the microstructure by the deposition parameters. In the main part of the review, topics of current interest will be specifically discussed: (1) the deposition of ultrathin films, (2) device integration issues, and (3) lateral nanostructuring and the formation of single grains. The techniques will be evaluated for their potential use in a future nanotechnology environment. Besides these research-oriented topics, both CSD and MOCVD show features which render the techniques highly suitable for large-wafer, high through-put production. The migration path from R&D systems to production tools will be outlined.

Chemical solution deposition of ferroelectric yttrium-doped hafnium oxide films on platinum electrodes

Ferroelectric hafnium oxide films were fabricated by chemical solution deposition with a remnant polarization of >13 μC/cm2. The samples were prepared with 5.2 mol. % yttrium-doping and the thickness varied from 18 nm to 70 nm. The hafnium oxide layer was integrated into a metal-insulator-metal capacitor using platinum electrodes. Due to the processing procedure, no thickness dependence of the ferroelectric properties was observed. To confirm the ferroelectric nature of the deposited samples, polarization, capacitance, and piezoelectric displacement measurements were performed. However, no evidence of the orthorhombic phase was found which has been proposed to be the non-centrosymmetric, ferroelectric phase in HfO2.

Interface-related decrease of the permittivity in PbZrxTi1−xO3 thin films

In ferroelectric thin films, a decrease of the permittivity is observed obeying a logarithmic time dependence. In the literature, a similar effect has been reported for ferroelectric single crystals and ceramics, which is referred to as ferroelectric aging, and different models have been proposed to explain this phenomenon. In this letter, ferroelectric aging of PbZrxTi1−xO3 thin films is studied as a function of dopant types and concentrations as well as the temperature. The results clearly show that the traditional models for the aging mechanism of ferroelectric single crystals and ceramics are not applicable. Based on these results, a mechanism is proposed which explains the decrease of the dielectric constant in ferroelectric thin films by the growth of a thin surface layer with suppressed ferroelectric properties in the course of aging.

Structural investigations of Pt/TiOx electrode stacks for ferroelectric thin film devices

Effects of the thermal treatment and the fabrication process of Pb(Zr0.3Ti0.7)O3 (PZT) thin films using chemical solution deposition on Pt∕TiOx electrode stacks were investigated using complementary analytical techniques including atomic force microscopy (AFM), x-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and grazing incidence x-ray reflectivity of synchrotron radiation. The surface and interface structures of the Pt∕TiOx electrode stacks with different thermal treatments, and the PZT∕Pt∕TiOx sample were examined. The propagation of Pt hillocks on the bare Pt∕TiOx electrode stacks upon the annealing was observed. AFM observations also revealed that the upper surface of the Pt bottom electrode under PZT thin film became rougher than that of the bare Pt electrode with the same thermal history. Global structural information including the density, surface or interface root-mean-square roughness, and thickness of each constituent layer in the samples were determined using...

Morphology and electrical properties of SrTiO3-films on conductive oxide films

Abstract The effect of various electrically conductive oxides on the morphology and the electrical properties of SrTiO 3 films (STO) was investigated by means of scanning electron microscopy, impedance analysis and leakage current measurement (DC). LaNiO 3 (LNO), La 0.5 Sr 0.5 CoO 3 (LSCO) and La 0.7 Sr 0.3 MnO 3 (LSMO) as electrically conductive oxides were deposited on platinized silicon wafers by chemical solution deposition (CSD) via the propionate route. Subsequently a film of STO was deposited by CSD of an acetate based precursor solution. Platinum contacts were sputtered onto the STO film to act as top electrodes. The STO films in these multilayer structures showed significant differences to STO films grown directly on the platinized substrate with respect to morphology as well as to the electrical properties.

Basic investigations on a piezoelectric bending actuator for micro-electro-mechanical applications

Abstract This article deals with basic investigations into the development and manufacturing process of a piezoelectric animated micro actuator. The concept of the fabrication of a piezoelectric microactuator will be introduced and first fundamental investigations on materials characterization and process technology for the Pt bottom electrode and piezoelectric PZT layer will be presented. PZT thin film have been deposited with the chemical solution deposition (CSD) technique and have been characterized with dielectric and ferroelectric measurements. For optimization of actuator properties an analytical approach and a simulation with finite element method was carried out. This shows that the cantilever must have a length above 300 μm to reach a tip deflection higher than 10 μm at voltages comparable to those used in integrated circuits (IC). Additionally, stress measurements of each layer have been used to characterize the films mechanically.

Direct electrical characterization of embedded ferroelectric lead titanate nanoislands

We report on the fabrication and characterization of lead titanate nanoislands on platinized silicon substrates embedded into a low-k dielectric. Our findings with Pt and Au as collective top electrodes are compared to previous results and thin films, and we discuss the coercive field and the remanent polarization with special care devoted to capacitive and leakage contributions of the nonpolar matrix. A direct electrical characterization of sub-100-nm ferroelectric nanoislands becomes feasible if they are measured in parallel, providing that the thin film material parameters of both ferroelectric and spin-on glass are independently determined.