Harald Bachhofer

Transient conduction in multidielectric silicon–oxide–nitride–oxide semiconductor structures

The voltage- and time-dependence of the tunneling currents in polysilicon–oxide–nitride–oxide semiconductor structures have been investigated. Electron and hole contributions were separated using a shallow junction technique. The standard tunneling model for charge injection was successfully applied to describe the observed threshold voltage shifts. For both positive and negative gate voltages, the time-dependence of the current density through the tunneling oxide is given by a simple analytical equation. This equation is characterized by an initial time constant and an asymptotic t−1-dependence. At large programming times the current density follows the t−1-dependence, independent of the tunneling oxide thickness and applied voltage. Under positive polarity (write) electrons are injected from the substrate. Under negative polarity (erase) and previous injection electron back-tunneling rather than hole injection is dominant at the beginning of erasing. At the end of erasing, steady-state conduction can be...

High-resolution x-ray reflectivity study of thin layered Pt-electrodes for integrated ferroelectric devices

The structural interface properties of layered Pt/Ti/SiO2/Si electrodes have been investigated using high-resolution specular and diffuse x-ray reflectivity under grazing angles. Currently this multilayer system represents a technological standard as bottom electrodes for ferroelectric thin-film applications. For the electronic and ferroelectric properties of integrated devices, the film-electrode interface is of crucial importance. We focused on Pt 1000 A/Ti 100 A/SiO2/Si electrodes prepared under annealing conditions as employed in industrial processing, prior to the deposition of ferroelectric films. The comparison between annealed and non-annealed electrodes clearly revealed strong interfacial effects due to interdiffusion and oxidation of Ti, especially at the Pt-Ti interface. Migration of Ti into the Pt layer results in a shift of the critical angle due to the enclosure of TiO2-x within the Pt layer. The heterogeneous distribution of TiO2-x suggests a diffusion mechanism mainly along the Pt grain boundaries. At the SiO2 interface a relatively weakly oxidized Ti layer of 20 A remained, which is most probably correlated with the remaining adhesion to the substrate.

Dielectric relaxation and charge transport mechanisms in (Ba,Sr)TiO3 thin films

Abstract Electrical properties of barium strontium titanate (BST) thin films were investigated. The transient relaxation current was measured at various temperatures for different BST thicknesses and was found to obey the Curie-von-Schweidler law. A bulk related relaxation model involving hopping of charge carriers was used to explain these results. In addition, the frequency dependence of the dielectric loss was measured and compared with the transient relaxation current. To interpret the leakage data, the temperature and field dependence were analyzed qualitatively and quantitatively. An interfacial layer with reduced static dielectric constant enhances the electric field near the electrodes. For temperatures lower than 400 K and electric fields higher than 3 MVcm−1 charge transport is dominated by Fowler-Nordheim tunneling.

Relaxation effects and steady-state conduction in non-stoichiometric SBT films

Abstract The DC current response of strontium bismuth tantalate (SBT) thin films was investigated as a function of applied voltage and temperature. The transient relaxation currents have been found to obey the universal Curie-von Schweidler law independent of the previous polarization. The steady-state current is independent of the initial polarization but depends on temperature. In the mid-voltage regime the conduction is thermally activated. At high applied voltages V the leakage current depends upon V4.

Effect of Film Composition on Low Temperature Processing of SBT Deposited by MOCVD

Bi-layered ferroelectric strontium bismuth tantalate (SBT) thin films of various film compositions were deposited on Pt/Ti/SiO 2 /Si substrates by metalorganic chemical vapor deposition (MOCVD) and crystallized at 700°C in oxygen ambient. Phase transition, orientation, second phases and remanent polarization were investigated with respect to film stoichiometry. X-ray diffraction (XRD) measurements revealed that excess Bi lowers the transition temperature from fluorite-type to ferroelectric phase. However, SBT films with Bi-excess of 15% or higher exhibit pronounced Bi-loss during crystallization and a decrease in the relative intensity of the (200) peak. Highly Sr-deficient films are not fully crystallized but support pyrochlore formation. The maximum remanent polarization is obtained at a Sr-deficiency of 15–25% and a Bi-excess of 10% (0.85/2.20/2.00).

Interfacial layers and their effect on leakage current in mocvd-deposited SBT thin films

Abstract Strontium bismuth tantalate (SBT) thin films were deposited on Pt/Ti electrodes by metalorganic chemical vapor deposition (MOCVD). Interactions at the interface of Pt and SBT and their effect on leakage current were investigated. High-resolution transmission electron micrographs (HRTEM) reveal that after annealing at 700°C, a 1–2 nm thick interfacial layer built. Auger electron spectra (AES) confirm that the constituents of SBT intermix with the Pt and vice versa. Schottky emission yields a nice linear fit to the leakage current data but the extracted values of the optical dielectric constant and the Richardson constant do not meet experimental values. Taking into account an interfacial layer with low dielectric constant and the effect of diffusion on the Schottky emission these inconsistencies can be resolved.