Carnegie Mellon University

Currently (2021) planned Master of Science projects.

Increasing the density of amorphous chalcogenides for memory devices
As deposited amorphous chalcogenides such as Ge2Sb2Te5 have significantly lower density than the polycrystalline form of this material. This leads to formation of voids and failure during device operation. The goal of the project is to explore the deposition processing window to identify the optimum deposition parameters giving the highest density. The project will also attempt to answer a fundamental question of what is an ideal amorphous material. Ge2Sb2Te5 layers will be deposited co-sputtering and evaluated by x-ray reflectivity, grazing angle x-ray diffraction, and small angle x-ray scattering.

Imaging of partial SET states in GeTe mushroom-type memory cells
The most commonly used functional material in Phase Change Memories is the Ge2Sb2Te5 alloy. This material has a very high nucleation rate with crystallization proceeding by nucleation followed by growth. The cycle-to-cycle resistance variability can be reduced by using material with higher growth rate as compared to the nucleation rate. One of these is GeTe. The project will use mushroom-type cells fabricated in the group and will focus on electrical testing and electron microscopy. The specific goal will be to assess the resistance variability upon partial SET pulses and determine the extent of nucleation versus growth.

Controlling the electrical properties of TaOx for applications in memristors
The project aims at improved control of electrical conductivity of TaOx layers by adjusting the deposition temperature and bias during sputtering. The primary evaluation techniques will be the measurements of conductivity and mobility by Transfer Line Method and van der Pauw techniques as well as quasi static I-V characterization of TiN / TaOx / TiN nanodevices. Auxiliary measurements can include x-ray reflectivity and diffraction.

Atom Probe Tomography of filaments in TaOx
The Holy Grail of resistive switching device technology is imaging the SET and RESET states of the device with atomic resolution. We have explored the limits of TEM imaging and many features of the filament could not be observed. This project will take a different route by using an Atom Probe Tomography technique. The project will fabricate and test the switching devices and will focus on preparation of APT needles using the Focused Ion Beam technique. The samples will be analyzed by APT.

Gradual switching and electro-forming of TaOx resistive switches
The primary application of resistive switching devices is for solid state memories with the low and high resistance states of the device. The switching process is self-accelerating which helps defining the two states with large resistance contrast. For neuromorphic computing applications, however, one needs be able to stop the runaway process in order to gradually adjust the resistance of the device mimicking the function of the brain synapse. The approach used in this project will be to use ultrashort pulses applied to a nanoscale TiN / TaOx /TiN devices.


updated 26apr21