The Effect of Metal Incorporation on Properties and Critical Interfaces in Germanium Telluride Based Devices
Open Access
- Author:
- Cooley, Kayla Anne
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 12, 2020
- Committee Members:
- Suzanne E Mohney, Dissertation Advisor/Co-Advisor
Suzanne E Mohney, Committee Chair/Co-Chair
Ismaila Dabo, Committee Member
Nasim Alem, Committee Member
Nitin Samarth, Outside Member
John C Mauro, Program Head/Chair - Keywords:
- Phase Change Materials
Electrical Contacts
GeTe
Solid-State Reactions
Contact Resistance
Metal-Incorporation
Ternary Phase Diagrams
Transmission Electron Microscopy
Thin Films - Abstract:
- Phase change materials (PCMs) are well-known for their crystalline-to-amorphous transitions that are both quick and reversible and offer a large contrast in electrical and optical properties of the two phases. Responsible for enabling the development of rewritable CD and DVD technology, this class of materials has greatly contributed to the development of modern data storage, entertainment, and computing; and due to so many contributions, research continues to point to applications in which chalcogenide PCMs can be used in novel solid-state devices. One such PCM, germanium telluride (GeTe), has been the focus of numerous studies to develop next generation radio frequency (RF) switches and photonic devices, and its alloys with other PCMs like Sb_2Te_3 also have been of great interest for memory applications. The geometry of PCM-based devices for radio frequency switches and non-volatile memory technologies often places GeTe thin films in contact with metal thin films. Despite the potential effect of metal/GeTe reactions on device performance, few studies have addressed the reactivity between elemental metals and GeTe or systematically approached the thermal stability of GeTe with metals. In response to this need, reactivity was determined by calculating ternary phase diagrams of metal-Ge-Te systems and performing transmission electron microscopy (TEM) both after metal deposition and after samples were annealed for 12 h at 200 °C. Germanium telluride is thermodynamically favored to react with many metals at room temperature. Nine of the 24 studied metals are not reactive with GeTe (Au, Ir, Mo, Os, Re, Ru, Ta, W, and Zn), while 15 metals have a thermodynamic driving force to react with GeTe at room temperature (Ag, Al, Cd, Co, Cu, Fe, Hf, Mn, Ni, Pd, Pt, Rh, Sc, Ti, and Y). Most of the unreactive metals, except Au and Zn, are not in thermodynamic equilibrium with GeTe at room temperature. These metals are refractory, and the lack of reactivity is ascribed to kinetic limitations. For GeTe, metal/GeTe thermal stability has a pronounced effect on current transport at an electrical contact interface. Solid-state reactions between contact metals and GeTe produce an unexpected trend between metal work function and metal/GeTe contact resistance (R_c), which is actually the opposite to what is predicted by the well-known Schottky-Mott Law. For a p-type semiconductor like GeTe, high work function metals, like Ni and Pt, would be expected to provide the lowest R_c values, even if the variation in R_c is modest due to Fermi level pinning. However, systematically comparing all contact metals from coauthored studies in the literature (Au, Ni, Mo, Sn, Ti) and new electrical measurements (Pt, Cr), Mo-based contacts (lower work function of 4.60 eV) offered the lowest contact resistance. From cross-sectional TEM analysis, only Au- and Mo-based contacts do not react with GeTe. It is likely that metal/GeTe reactions change the stoichiometry of GeTe, which reduces hole concentration. In addition to understanding metal/GeTe reactivity, understanding the effect of metal-incorporation in GeTe films provides valuable insights for engineering future PCM devices, both in terms of doping and discovery of ternary PCMs. GeTe thin films doped and/or alloyed with metals (Ge_0.5-xM_xTe_0.5) have been reported to exhibit improved device performance, like improved crystallization speed, thermal stability, and power consumption. However, these films have often been fabricated using non-equilibrium methods with high metal concentrations (>10 at.%). Since switching between the low-resistance crystalline and high-resistance amorphous states requires a heating cycle, the stability of Ge_0.5-xM_xTe_0.5 films is critical to practical implementation of these materials in electronic devices. This work presents first principles calculations of the stability of GeTe doped with select metals (Cu, Fe, Mn, Mo, and Ti), as well as the effect of increasing dopant atom concentration (2.08, 4.17, 6.25 at.%) on the crystal structure and electronic properties of GeTe. From density-functional theory calculations of the formation energy of the ternary solid (GeTe doped with 2.08-6.25 at.% of Cu, Fe, Mn, Mo, and Ti), all metals favored substitution into the Ge site over the Te site. The formation energy of the Ge0.5-xMxTe0.5 structure increases (or becomes less stable) with increasing metal incorporation for all metals. Certain metals clearly favor dopant atom clustering. In addition, different metal dopants induce various distortions of the GeTe crystal structure and projected density of states. Computational results are compared to observed solubility trends in cross-sectional TEM studies of metal/GeTe thin film systems (Cu, Mn, Mo, and Ti) and TEM and transport data from newly characterized co-sputtered Ge_0.5-xFe_xTe_0.5 films. By leveraging modelling approaches as diverse as classical thermodynamic calculations of phase diagrams, solid-state device modeling, and density functional theory with a range of experimental methods, especially nanofabrication and transmission electron microscopy, this thesis presents a holistic look at metal-Ge-Te systems and metal/GeTe interfaces beyond the scope of any prior study. Understanding the stability and reactivity between metals and GeTe is significant for the design of future PCM devices, where reactions and metal-incorporation could affect reliability or be used to engineer improved interfacial behavior.