Publication Date

5-2014

Date of Final Oral Examination (Defense)

4-3-2014

Type of Culminating Activity

Dissertation

Degree Title

Doctor of Philosophy in Electrical and Computer Engineering

Department

Electrical and Computer Engineering

Supervisory Committee Chair

Maria I. Mitkova, Ph.D.

Abstract

In recent years, Redox Conductive Bridge Memory (RCBM), which falls in the Resistive Random Access Memory (RRAM) category, has gained considerable attention as one of the promising candidates for future generation non-volatile memory due to its advantages over Flash memory as it offers high density, low operating power, fast read/write operation, and compatibility with conventional CMOS process. Currently research is being conducted to improve the reliability of the RCBM devices, which are comprised of an insulating material, also known as active layer, sandwiched between two metal electrodes. The main working mechanism of these devices is based on the resistance change induced by filament formation and dissolution through metal cations movement in the active film. The composition of these active films can vary from oxides to chalcogenides, among which we chose to work on Ge-containing chalcogenide glass films (i.e., Ge-S, Ge-Se, and Ge-Te), which are the most thermally stable among this family of materials, to get a broad perspective of the device performance as a function of the active film structures.

This work is focused on research related to new solutions for the active films to improve RCBM device performance. Application of two different deposition methods were investigated: Plasma-Enhanced Chemical Vapor Deposition (PECVD), which is not well studied for RCBM devices, and formation of nanostructures in the active film under oblique angle deposition by thermal evaporation method. Holding the sample surface at an oblique angle to the arriving vapor flux resulted in a columnar morphology within the devices’ active layer, which aided in improving the device performance by controlling the filament growth through these nanostructures. To minimize the radiant heating inside the evaporation chamber, a strict control over the evaporation current was required, which otherwise resulted in morphological changes of the nanostructure due to increased adatom diffusivity triggered by the thermal energy.

Study of the bare films provided insight into the active material average surface roughness and structural changes occurring in the layers by changing deposition temperature or deposition angle. These investigations were performed using Atomic Force Microscopy (AFM), Raman Spectroscopy, and Energy Dispersive X-ray Spectroscopy (EDS). The results demonstrated that for PECVD, low temperature deposited films with relatively low concentration of Ge had good relaxed structure. The surface roughness was also observed to be minimal with less frequency of hillocks while EDS studies yielded a compositional variation of ~1% for such films. For obliquely deposited films, Raman and EDS results revealed structural reorganization and compositional alterations with changing the deposition angle. Each of these respective results provided a partial view of the mechanisms that contribute to a reliable device performance. Since the RCBM device performance relies on silver diffusion in the chalcogenide matrix, the neutron reflectrometry method was applied to study the silver kinetics.

After studying and considering active film material analysis, three types of devices were fabricated i.e., PECVD deposited devices, thermally evaporated devices under normal deposition angle and devices with obliquely deposited films. The devices were electrically characterized by conducting current vs. voltage (IV) measurements, Read/Write voltage, the two resistive states, switching rate, and the reliability. The devices show greater than four orders of magnitude difference in the two resistive states, which can be detected using lower voltages, thus allowing RCBM devices to be used in low power memory applications.

By integrating the RCBM cells into a system, it can fulfill the essential role of memory with high storage density, precision, and access speed. As part of this research work, an array of RCBM devices were fabricated by non-conventional processing technique with individual cell addressing. The cells were electrically tested to enable application towards various memory architectures.

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