Nanoscale Solid Polymer Electrolytes in Metal–Insulator–Conductor Systems for Logic and Neuromorphic Devices

This research aims at using ions to add new functionality to semiconductor devices. Electrically-insulating, ion-conducting polymers, such as poly(ethylene oxide) (PEO), with ion salts like cesium perchlorate (CsClO₄), are known as Solid Polymer Electrolytes (SPEs). These can be coated on a semiconductor and internal or applied electric fields can move the ions to the semiconductor interface, where they attract charge carriers of opposite polarity to form an electric double layer (EDL). This can be used to reversibly configure the semiconductor surface charge carrier profiles, e.g. to form p-n and tunneling junctions and to convert metal-oxide-semiconductor field-effect transistors (MOSFETs) from n-FETs to p-FETs to tunnel FETs (TFETs). SPEs can also dope 2D crystals and carbon nanotubes (CNTs), which are difficult to dope substitutionally. SPE capacitors with an added charge retention layer could lead to new forms of digital and analog memory at the scaling limits of the polymer. Additionally, capacitor or transistor structures may be used as weight storage devices to accelerate the training of deep learning systems, as well as in other neuromorphic devices based on coupling ions and electrons. However, EDL devices have various fundamental and technological limitations. It is important to understand the EDL formation and dissipation speeds, polymer thickness limitations, and forms of ion encapsulation to enable further lithography.

This research is intended to deepen the understanding of the static and dynamic response of ultrathin PEO in metal insulator semiconductor (MIS) and metal insulator metal (MIM) structures with and without CsClO₄. While SPEs are typically studied in micron-thick films, this dissertation explores decananometer thicknesses for their potential to increase the ion response time, since certain bulk properties, e.g. PEO’s ionic conductivity, are known to change in polymers confined in one dimension.

Record-thin films of PEO:CsClO₄ are demonstrated on commercial Si-on-insulator wafers and carbon nanotubes (CNTs) on quartz. Spin-coated PEO:CsClO₄ as thin as 7.8 nm, a physical thickness relevant to nanoscale device applications, is characterized in cross–sectional images by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS), to reveal the coating of structures with nanometer features. Through EELS analysis, the polymer’s amorphous carbon character is distinguished from the sp²–hybridized carbon of the CNT. With SiO₂evaporated over the PEO, Cs is observed to be contained within PEO, indicating encapsulation by the SiO₂, which can serve as a capping layer for further lithography. EELS mapping reveals the unbiased accumulation of Cs at the PEO/SiO₂interface due to the built-in electric field. In the coating of 2-nm-diameter CNTs, EELS mapping shows that PEO:CsClO₄closely surrounds the CNT wall, with no indication of interlayers, as desired for electric double layer formation used in transistors and memory.

These nanometric SPE films are modeled in COMSOL Multiphysics, using the Poisson and Nernst-Planck equations modified to account for a finite ion size. The films’ static and dynamic behavior in response to a constant bias, a voltage step, and a bipolar triangular pulse train is simulated and compared to measurements in ultrathin PEO:CsClO₄MIM capacitors.

The impedance–frequency and capacitance–voltage characteristics of PEO MIS capacitors at thicknesses relevant to transistor technology are measured with and without CsClO₄. Basic understanding of the impedance frequency and voltage characteristics of this MIS system is established in spin-coated films in the thickness range from 6 nm to 19 nm, as determined from capacitance and TEM measurements. Estimates of the dielectric constant, energy band diagram, charge trap density, and conductivity in ultrathin PEO are obtained. Simple equivalent circuits based on resistive and capacitive elements that reflect the physical system are used to model the measured impedance frequency trends and compare films with and without CsClO₄ in the polymer matrix. This study reveals the physical characterization and the simulated and measured electrical properties of PEO near the limits of thickness scaling, toward memory and neuromorphic device applications.