Data Storage Systems Center

Desirable memory characteristics such as fast access time, relatively low programming current and scaling capability have positioned Phase Change Random Access Memory (PCRAM) as the best candidate to replace FLASH as the next generation non-volatile memory.

Despite these impressive memory characteristics, the RESET current (defined as the current required to bring about a phase transition from the low resistance crystalline phase to high resistance amorphous phase) is still quite high and this limits the memory density. If the RESET current can be significantly reduced, the PCRAM device can be paired with a minimum width access transistor for higher memory density.

In this work, we have proposed and created a superlattice-like structure comprising of Ge2Sb2Te5 (GST) and nitrogen-doped GST (N-GST) and implemented it in a PCRAM device. These devices were electrically characterized for their RESET currents, SET current and switching endurance. It was found that the RESET current for devices with Superlattice-like structure had a lower RESET current when compared to conventional bulk GST devices and this is attributed to the reduced thermal conductivity in superlattice-like structures.

Inductors are important elements in modern RF and power circuits. However, they often require a comparatively large area on the IC. One way to make them smaller is to incorporate magnetic materials in them, the same way a solenoid’s magnetic field is increased with a magnetic core. Although magnetic materials can increase inductance, loss is introduced through eddy currents in the magnetic material. To counter this, the magnetic material must either be laminated or have a relatively high electrical resistance.

In this project, we propose the use of electroplating to deposit both the magnetic material and the electrical insulators needed to form laminations. In such a scheme, the magnetic material is deposited on the inductor structure via electroplating and then lifted out from the bath and oxidized in air to form an insulating oxide. The structure is then placed back into the bath and the process is repeated, forming laminations.

We used bonding wire as inductor structures and plated them with Ni. Thicker plated wires showed a higher low frequency inductance than those thinly plated. Experiments with the oxidation and lamination process described above were also conducted using thin films. Resistive oxides were found to form on exposure to air but they were etched when the structures were placed back into the acidic Ni bath.

In this work we study the crystallization kinetics and electrical properties of Ge50Sb50. We find it to has higher crystallization temperature and higher resistance for both ON and OFF states as compared with literature value for Ge15Sb85. Since higher crystallization temperature generally indicates a ‘slower’ material, this alloy may have application where a lowering of the reset current is desired, as in PCRAM. It does not appear to be superior to Ge15Sb85 in the switch application, which requires the ON state resistance to be low. This work is part of the activities within the MISCIC center.

Application of phase change materials to reconfigurable circuits requires a high resistance contrast between the phase change materials’ crystalline phase (low electrical resistance) and amorphous phase (high electrical resistance). In this work, a multilayered structure of two different materials, stacked alternately on top of each other, is under study to create high electrical resistance interfaces between the materials. We explore the types of materials that may be used to achieve high resistance contrast. Following the selection of the materials, the band alignment between various phase change and other materials will be investigated to understand the electronic properties of these interfaces. The band bending at the interfaces will be studied through simulation and experiment to identify the type of contact formed at their interfaces. These results will enable us to engineer the multilayered stack to suit the resistance requirements of particular applications.

Found in optical rewriteable disks, phase change materials are a general class of materials that display remarkably different properties in the amorphous and crystalline phase. These changes in properties (reflectivity and electrical resistivity) are reversible and have been exploited in optical disks and phase change random access memory (PCRAM).

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.