Defect and electrical properties of high-K̳ dielectric Gd₂O₃ for magneto-ionic and memristive memory devices

Abstract

While high-[subscript K] dielectrics utilized in CMOS technology are noted for their highly insulating characteristics, they have demonstrated surprising electrolytic behavior as key components in a variety of thin film memory devices, including those based on magneto-ionic and memristive behavior. In this work, we focus on the rare earth sesquioxide, Gd₂O₃, a well-known high-κ dielectric that has exhibited a variety of electrolytic properties during the development and operation of the first magneto-ionic devices developed at MIT. Specifically, we focused our investigation on the defect chemistry and electrical properties of Gd₂O₃ in order to better understand the relationship between the structure, chemistry, processing conditions, and operating environment and the material's low-temperature ionic and electronic transport properties and the means for their optimization vis-à-vis memory device operation.

Phase (monoclinic and cubic) and dopant controlled (Ca, Ce, Sr, Zr) polycrystalline pellets of 8 different Gd₂O₃ systems were prepared to investigate various defect regimes in consideration of this material's polymorphism. We considered intrinsic anion-Frenkel disorder and electronic disorder, equilibration with the gas phase, water incorporation, and dopant incorporation in the defect modeling, taking into account the roles of crystallographic structure as well as oxygen ion defect and protonic generation. The primary method utilized to characterize the defect chemistry and transport properties of Gd₂O₃ was the analysis of the dopant, p0₂ and temperature dependencies of the electrical conductivity extracted from complex impedance spectra obtained over the p0₂ range of 1 to 10⁻¹⁵ atm, for 5 isotherms between 700 and 900 °C with 50 °C steps and for a range of acceptor and donor dopants.

Based on the p0₂ dependency of conductivities, in light of the defect modeling, the majority point defects in each system were identified. Electronic and ionic migration energies and thermodynamic parameters were extracted via the defect modeling and temperature dependencies of conductivities. In nearly all cases, the predominant charge carrier under oxidizing conditions at elevated temperatures was identified as the p-type electron-hole, largely due to oxygen excess non-stoichiometry in these systems. With decreasing p0₂, transport tended to switch from semiconducting towards ionic. Depending on phase, dopant type & concentration, temperature, and relative humidity, the predominant ionic conductivity was found to be via oxygen interstitials, oxygen vacancies, and/or protons, the latter given by the propensity of Gd₂O₃ to take up water in solid solution from the environment by the formation of OH[superscript .]species.

Unexpectedly, the ionic mobilities of defects in the denser and less symmetric monoclinic system exhibited higher ionic mobilities than the more open bixbyite structure. The hole electronic species in the investigated systems were found to migrate via the small polaron hopping mechanism with rather large hopping energies. This resulted in an inversion of hole and proton mobility magnitudes at reduced temperatures in the monoclinic system. Extrapolation of ionic and electronic defect conductivities to near room temperature, based on our derived defect and transport models, was not able to explain, on its own, the observed electrolytic properties of the Gd₂O₃ thin films utilized in magneto-ionic devices.

In an attempt to connect the transport properties obtained under equilibrium conditions at elevated temperatures with the behavior of Gd₂O₃ near room temperature, selected thin films Gd₂O₃, prepared by pulsed laser deposition or sputtering, were investigated by complex impedance spectroscopy over the temperature range of 20 - 170°C. While films prepared under dry conditions were indeed found to be highly electrically insulating, films exposed to water vapor exhibited dramatically higher proton conductivities (more than ~10⁸ x) than values extrapolated from high temperature. Parallel thermogravimetric analysis on Gd₂O₃ powder specimens, as a function of temperature, under high humidity conditions, demonstrated a correlation between uptake/loss of incorporated water and conductivity upon cooling and heating, respectively.

We can therefore conclude that the large disconnect between the electrical and electrolytic properties observed between high-κ dielectrics used in CMOS devices such as Gd₂O₃, and their much more highly conductive counterparts used in thin film memory devices, depends strategically on the thin film processing conditions. High-κ dielectrics are fabricated in carefully controlled environments with low relative humidity, while research on, for example, Gd₂O₃ - based magneto-ionic memory devices, is performed under ambient laboratory conditions, where significant water uptake becomes possible at surfaces and grain boundaries. The results and insights obtained in this study can be expected to be applied in achieving further progress in the understanding and optimization of magneto-ionic, memristive, and other devices that rely on proton gating.