Kayla A. Cooley

NSF Graduate Research Fellow, Ph.D. Candidate, Penn State
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Presented: October 15 and 16, 2020

“The Effect of Metal Incorporation on Properties and Critical Interfaces in GeTe-Based Devices”

Germanium telluride, a phase change material, has been the focus of numerous studies to develop next generation non-volatile memory cells, photonic devices, and radio frequency (RF) switches. To provide critical information for future GeTe-based devices, we synergistically combine experiment, materials computation, and device modeling to investigate interactions between metals and GeTe thin films and determine how these interactions affect electronic properties and contact resistance at the metal/GeTe interface.

The geometry of GeTe-based devices for RF switches and memory technologies often places GeTe thin films in contact with metal thin films, but few studies have addressed thermal stability between elemental metals and GeTe.  To investigate reactivity, we calculated ternary phase diagrams of metal-Ge-Te systems and performed transmission electron microscopy (TEM) on select metal/GeTe systems. Solid-state reactions between contact metals and GeTe produce an unexpected trend between metal work function and metal/GeTe contact resistance (Rc), which is opposite to what is projected by the Schottky-Mott Law. For a p-type semiconductor like GeTe, high work function metals should provide the lowest Rc values, even if the variation among metals is small due to Fermi level pinning. However, the high work function metals tended to react with GeTe, resulting in markedly increased Rc values.

We also studied the effect that metal alloying/doping would have on critical properties of GeTe. From density-functional theory calculations of the formation energy of the ternary solid (GeTe doped with 2-6 at. % of select metals), all metals favored substitution into the Ge site over the Te site. Certain metals clearly favored dopant atom clustering, and metal dopants distorted the GeTe crystal structure and projected density of states in various ways. We compared computational results to observed solubility trends in cross-sectional TEM studies of metal/GeTe thin film systems and newly characterized co-sputtered GeTe:Fe films.

In addition to the discussion of metal/GeTe interactions, we will also provide another example of theory and microscopy unraveling the behavior of contacts to chalcogenide semiconductors by discussing room temperature epitaxy of metals on the layered semiconductor WSe2.

Kayla A. Cooley

Kayla A. Cooley, NSF Graduate Research Fellow, Ph.D. Candidate, Penn State

As a member of Dr. Suzanne E. Mohney’s research group, Kayla has authored or co-authored 16 peer-reviewed journal articles and has been recognized at national meetings with best poster (AVS, Electronic Materials and Photonics Division) and young scientist (PCSI) awards. Prior to her graduate studies, Kayla graduated summa cum laude with a B.S. in applied physics from Kettering University at Flint, MI in 2012. She completed an undergraduate thesis entitled, “3D Reconstruction of Microstructure by Focused Ion Beam – Scanning Electron Microscopy (FIB-SEM) for Qualitative and Quantitative Evaluation of Materials Performance” while working as a cooperative education student at the Electron Microscopy Center in the Materials Science Division of Argonne National Laboratory and was passed with distinction. Between undergraduate and graduate studies, Kayla was employed as a defense contractor, serving as a staff scientist at the Edgewood Chemical Biological Center at Aberdeen Proving Ground, MD. Kayla plans to defend a Ph.D. thesis entitled “The Effect of Metal Incorporation on Properties and Critical Interfaces in GeTe-based Devices” in Fall 2020, and she has been awarded a National Research Council postdoctoral fellowship at the Naval Research Laboratory in Washington D.C., which will begin in January 2021.