Date of Award

1-1-2013

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

College/School/Department

Department of Nanoscale Science and Engineering

Program

Nanoscale Engineering

Content Description

1 online resource (ix, 153 pages) : illustrations (some color)

Dissertation/Thesis Chair

Robert E Geer

Committee Members

Mengbing Huang, Nathaniel Cady, Shadi Shahedipour-Sandvik, Rebecca Cortez

Keywords

Hafnium Oxide, nonvolatile memory, Radiation effect, Resistive switching model, Nonvolatile random-access memory, Hafnium oxide

Subject Categories

Materials Science and Engineering | Nanoscience and Nanotechnology

Abstract

The impacts of ion irradiation on so-called vacancy-change mechanism (VCM) and electrochemical-metallization mechanism (ECM) ReRAM devices based on HfO2 are investigated using various ion sources: H+ (1 MeV), He+ (1 MeV), N+ (1MeV), Ne+ (1.6 MeV) and Ar+ (2.75 MeV) over a range of total doses (105 - 1011 rad(Si)) and fluences (1012 - 1015 cm-2). VCM-ReRAM devices show robust resistive switching function after all irradiation experiments. VCM resistive switching parameters including set voltage (Vset), reset voltage (Vreset), on-state resistance (Ron) and off-state resistance (Roff) exhibited, in most cases, modest changes after irradiation. Decreases in forming voltage (Vf) and initial resistance (Rfresh) of fresh devices were observed after all irradiation experiments on VCM-ReRAM devices with the exception of Ar+ irradiation at the highest fluence (1015 cm-2). In that case Rfresh increased by an order of magnitude. For VCM-ReRAM devices it was also observed that irradiation beyond a dose threshold of approximately 5 Grad(Si) could induce off-to-on state transition events. This behavior could lead to errors in a VCM-ReRAM memory system. ECM-ReRAM devices (based on HfO2) were also subjected to ion irradiation. Under proton irradiation ECM-ReRAM devices remained functional, but with relatively large positive variations (20-40%) in Vset, Vreset and Ron and large negative variations (~ -60%) in Roff. In contrast to VCM HfO2-ReRAMs, ECM-based devices exhibited increased Vf after irradiation, and no off-to-on transitions were observed. Interestingly, for ECM-ReRAM devices, high-fluence Ar irradiation resulted in a transition of the electrical conduction mechanism associated with the conductive filament forming process from a Poole-Frenkel conduction mechanism (pre-irradiation) to ionic conduction (post-Ar irradiation). ECM-ReRAM devices irradiated with lighter ions did not exhibit this effect. The different ion irradiation responses of the two types of HfO2-ReRAMs studied originate from their distinct switching mechanisms - vacancy filament switching for VCM-ReRAMs and metal filament switching for ECM-ReRAMs - which respond differently to the irradiation-induced changes in the vacancy/defect densities and crystallite structures in HfO2. SRIM (Stopping and Range of Ions in Matter) modeling was used to roughly estimate the density of irradiation-induced vacancies. These model results correlated well with experimental observations in terms of vacancy defect density thresholds sufficient to impact ReRAM switching behavior. Physical characterization of pre- and post-irradiation ReRAM devices using techniques including XRD, AES, SEM, EDS, and SIMS were also employed to support the modeling and electrical measurements. This work suggests that HfO2-based ReRAM devices are a promising candidate for space and nuclear applications requiring a `radiation-hard' memory technology.

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