Andreas Rüdiger

Comparison of in-plane and out-of-plane optical amplification in AFM measurements

The in-plane image of piezoresponse force microscopy (PFM) generally exhibits a higher resolution and less noise than the out-of-plane image. Geometrical considerations indicate that the optical in-plane amplification is ≈40 times larger than the out-of-plane amplification. We experimentally confirm this explanation in a dedicated setup.

Analysis of shape effects on the piezoresponse in ferroelectric nanograins with and without adsorbates

Using BaTiO3 as a piezoelectric model system we compare a finite element model with experimental data to demonstrate the impact of grain topography on the in-plane piezoelectric response at the perimeter. Our findings emphasize the need for a careful consideration of both electric field and piezoelectric tensor orientation. An analysis is given showing that the in-plane piezoresponse is a function of two directions of the electric field, whereas the out-of-plane response is a function of all three directions of the applied field. The effect of an adsorbate layer on the piezoelectric response is quantified with typical material parameters.

Integration of ferroelectric lead titanate nanoislands for direct hysteresis measurements

We report on the integration of fully functional ferroelectric PbTiO3 nanostructures of typically less than 100nm lateral extension into a low-k dielectric hydrogen silsesquioxane film. Chemical mechanical polishing of the dielectric layer down to an overall thickness below the nanoparticles height exposes the structures. After confirmation of the piezoelectricity of individual embedded grains, gold electrode pads are deposited to characterize several of these grains in parallel. Evidence of ferroelectric switching is observed and discussed within an equivalent circuit model. This paves the way to a better integration and statistical analysis of ferroelectric nanostructures.

Nano-Crossbar Arrays for Nonvolatile Resistive RAM (RRAM) Applications

We present a fast and flexible method for the fabrication of nano-crossbar arrays with a feature size of 100 nm. TiO2 is integrated and electrically characterized as the nonvolatile resistively switching material. This structure serves as a key component for the investigation of novel high density nonvolatile resistive RAM cores.

Mechanical crosstalk between vertical and lateral piezoresponse force microscopy

Piezoresponse force microscopy (PFM) provides valuable insight into the inverse lateral and vertical piezoelectric effects on the nanoscale. Ideally, these contributions are separated into vertical and lateral detections of a deflected laser beam on a quadrupole diode. In contrast to the known crosstalk by a rotated diode that causes identical signals in both channels, we report on the crosstalk due to the geometrical constraints of the cantilever that is inherent to the lateral PFM. For a BaTiO3 (001) nanograin we show that the vertical response attributable to the crosstalk is 1∕8th of the lateral response. From this value we deduce the actual mechanism responsible for the crosstalk.

Liquid Injection Atomic Layer Deposition of TiO x Films Using Ti [ OCH ( CH3 ) 2 ] 4

TiO x films were prepared by liquid injection atomic layer deposition using titanium tetraisopropoxide (TTIP), Ti[OCH(CH 3 ) 2 ] 4, dissolved in ethylcyclohexane (ECH). We analyzed the residual water content and the reaction with the TTIP for several solvents, choosing ECH for dissolving the TTIP because of the lowest residual water level and no ligand exchange reaction with the TTIP. TiO x films were deposited at 240°C with a wide range of the TTIP solution injections per cycle. However, an ideal self-regulated growth was not achieved for the TiO x films due to a slow catalytic decomposition of the TTIP molecules followed by the exchange reaction with the underling layer. The contribution of the catalytic decompositions to the deposition rates was suppressed by increasing the injection frequency of the TTIP solution into the vaporizer. A rather independent deposition rate of the input of the TTIP solution was achieved by increasing the injection frequency to 4 Hz, while TiO x films deposited with a low injection frequency of 0.25 Hz showed almost linear film growth rate to the input of the TTIP solution. The deposited TiO x films were amorphous and clearly showed both unipolar and bipolar resistive switching behaviors, which are applicable to nonvolatile memory applications.

Sample-tip interaction of piezoresponse force microscopy in ferroelectric nanostructures

We report on qualitative and quantitative implications of the sample-tip interaction in piezoresponse force microscopy. Our finite-element analysis of adsorbate effects, sample heterogeneities, and tip asymmetries is in agreement with experimental observation of ferroelectric nanostructures. Qualitative discrepancies arise from locally asymmetric tip-sample interaction. Any quantitative determination of field-related material parameters as required for the verification of semiempirical models of the ferroelectric limit typically relies on an overestimated field across the sample. Our findings indicate that adsorbates reduce the actual field across the nanograin by roughly one order of magnitude

A multilayer RRAM nanoarchitecture with resistively switching Ag-doped spin-on glass

Next generation memory materials and novel memory architectures are in the focus of investigations because CMOS based systems are supposed to reach their physical limits during the next decade. To enhance the potential for very high integration density we fabricated multilayer crossbar memory architectures with integrated spin-on glass (methyl-silsesquioxane - MSQ). UV nanoimprint lithography, a widely proposed future technology, was used to structure down to 100 nm feature sizes. Electrical measurements on silver doped MSQ devices show the potential for the use of the material composition in the field of future memory applications.

Phenomenological considerations of resistively switching TiO 2 in nano crossbar arrays

Within this paper we present the fabrication of nano crosspoint junctions and arrays with electron beam direct writing (EBDW). Reactively sputtered TiO2 was incorporated as a resistively switching thin film and electrically characterized concerning its performance. These devices are suitable for novel non-volatile storage systems in form of resistive random access memories (RRAM). All used materials as well as the fabrication processes for the functional thin film are in good accordance with current and future CMOS technology and provide a way to achieve low cost, high density non-volatile memory. The experiments were performed with 100 ⋅ 100 nm2 small single junctions and arrays with 200 nm wide wires. The results of the former prove a non-volatility for more than 105 s and a switching speed better than 10 ns for the SET- and RESET operation (from high to low resistance state and in reverse direction). The latter prove the direct addressability of junctions within an array.

Embedded ferroelectric nanostructure arrays.

Ferroelectrics hold promise for high-density non-volatile data storage device use. Their eventual performance will strongly depend on the available displacement current, which primarily scales with the area but might to some extent be enhanced by substrate-induced homogeneous strain while the interface at the same time controls the coercive field. As the lateral dimensions persistently decrease, the only way to keep track of the real figures of merit with realistic electrodes is to use a macroscopic configuration instead of scanning probe approaches with undefined interfaces. We report on a novel approach to integrating arbitrarily patterned, highly registered ferroelectric nanoislands fabricated by a template controlled chemical solution deposition approach into a matrix of a low-k dielectric spin-on glass. These structures with a narrow lateral size distribution below 100 nm are subsequently polished in a chemical-mechanical polishing step to expose their very tops, whose piezoelectrically active area depends on the polishing time. At this stage our findings indicate a full piezoelectric functionality of the locally exposed nanoislands. The structures are ready for macroscopic top electrodes to average the displacement current over hundreds of almost identical structures, to provide a nanoscale scaling behaviour of the individual ferroelectric capacitors.