Data Storage Systems Center

Many metal-oxide-metal heterostructures are known to show non-rectifying IV characteristics (so-called Space-Charge-Limited Current) and also to exhibit resistance switching behavior. They have gained much attention as candidates for data storage. However, the physical theory that explains the behavior is still unclear. In this work, we are focused on understanding electrical transport phenomenon on such structures and suggest a possible cause for resistance switching behavior.

First, experiments were done with metal/Cr-doped SrZrO3/SrRuO3 structures. The effects of the Cr-content and the thickness of the SrZrO3 film were examined by their current-voltage characteristics (IVCs). Space-charge-limited current (SCLC) was identified as the dominating electrical transport mechanism for both resistance states. Also, a steep increase of resistance with respect to thickness was observed. For high Cr-doping levels, R∝t3 was observed, while for low Cr-doping levels an even stronger relationship was observed.

Next, IVCs of SCLC were simulated using numerical method in Matlab. Using one-dimensional geometry of semiconductor with two contacts designed to behave as carrier-injecting electrodes, providing free electrons (or holes) into the semiconductor, IVCs of SCLC are demonstrated. Simulations made with a series of doping levels and thicknesses provided a plausible explanation for experimental observations made earlier. The results suggest that the functional layer (SrZrO3) of the heterostructure is under total influence of contacts. In other words, the every location in the oxide film has not reached its bulk characteristic due to the overlap of Space-charged region of two contacts. Based the comparison between experimental IVCs and calculated IVCs, it is likely that the reason of resistance switching is due to the change of effective doping level inside the film, especially at the center position of oxide film.

This project focus on the feasibility of employing phase change materials for reconfigurable switch in RF applications such as inductors. This work is part of the activities within the MISCIC center. This specific project addresses the process for fabrication of test structures that can be reconfigured with MEMS probes and the prototype of the reconfigurable inductor is fabricated. In addition, a comparison study between GeSb and Ge2Sb2Te5 is done. GeSb is found to exhibit certain desirable characteristics for this application such as having a lower on state resistivity of 1.2 ohm m ( reduced by ~ 5.8 X) and higher activation energy of 5.13 eV (increased by ~ 2.3 X) as compared to Ge2Sb2Te5.

The current vs. voltage (I-V) behavior and its temperature dependence of Pt / (001) SrTiO3-x hetero-junction is investigated to shed light on the mechanism behind the colossal electro-resistance (CER) phenomenon. The overall features agree with the standard description of a Schottky contact. When swept to a sufficiently large forward bias (~15 V), the current of the device suddenly decreased by a factor of 10, indicating the creation of a series resistive element. However, the leakage component of the Schottky contact current for both the reverse bias and small forward bias voltage are increased. The change in I-V characteristics can be explained by the creation of a thin (~1 nm), highly resistive insulating layer at the interface. Under large forward bias where bulk series resistance limits the current, this insulating layer adds to the total resistance. At small forward bias and reverse bias, significant voltage drop across this insulating layer reduces the effective barrier height leading to a current increase that more than compensates for the current reduction arising for the need of the electrons to tunnel through the interfacial layer. Reversible ionic motion in and out of the interface is demonstrate by the shift in zero point of the I-V curve at low temperatures. The direction of oxidation and reduction of the interface and the direction of the zero point shift agrees with the motion of ionized oxygen vacancies under the electric field.

This project focuses on developing probe addressable phase change vias for reconfigurable electronics. This work is part of a larger activity at Carnegie Mellon University focused on MISCIC: memory intensive self-configuring integrated circuits. First of all, the focus is on via materials, via sizing and on the addressing using probes. Additionally, the project addresses the fabrication and demonstration of the process using conducting atomic force microscopy. Then, optimal circuits for use with these reconfigurable vias are being examined. It includes present measurements, models and designs developed over the past year for a viable via structure for reconfiguring RF inductors over a wide dynamic range. Vias designed using recently published data on GeSb are suggested to achieve off states with acceptably low shunt capacitance. Furthermore, breaking each via into one hundred sub-vias (100 nm x 100 nm) will be necessary to manage write currents. Probes appear to be well-suited to this array architecture, and we present a new three terminal design for probe addressing of these sub-vias.

This project focus on the feasibility of employing phase change materials for reconfigurable switch in RF applications such as inductors. This work is part of the activities within the MISCIS center. This specific project addresses the process for fabrication of test structures that can be reconfigured with MEMS probes.

Resistive switching in metal(M)/functional-layer(FL)/base electrode(BE) heterostructures is thought to occur, in part, by the movement of oxygen vacancies near the electrode/FL interfaces. While previous investigations have focused on the effects of the top metal electrode, this work involves a comparative investigation in which SrRuO3, LaNiO3, and La0.6Sr0.4CoO3 were used as BE materials in Ag/Pr0.7Ca0.3MnO3/BE and Ag/La0.7Sr0.3MnO3/BE heterostructures. Results indicate that switching behavior is a strong function of BE material as BE materials with higher oxygen affinities show a larger delta-R than those with lower oxygen affinities. In comparing heterostructures with FL thicknesses ranging from 20nm to 500nm, resistive switching is found to be a function of FL thickness. The total resistivities of the heterostructures are much larger than expected when compared to the bulk resistivities of the FL materials, indicating heterostructure resistivities that are dominated by interfacial resistances. Heterostructures slowly cooled to room temperature in different pressures of oxygen show different transport properties, indicating that mobile oxygen vacancies play a role in the resistive switching of manganese based oxide thin films. The work presented here discusses the interfacial nature of resistive switching and supports the theory that resistive switching is caused by mobile defects.