Solution Processed Silver Sulfide Thin Films for Filament Memory Applications
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Solution Processed Silver Sulfide Thin Films for Filament Memory Applications Shong Yin Vivek Subramanian, Ed. Electrical Engineering and Computer Sciences University of California at Berkeley Technical Report No. UCB/EECS-2010-166 http://www.eecs.berkeley.edu/Pubs/TechRpts/2010/EECS-2010-166.html December 17, 2010 Copyright © 2010, by the author(s). All rights reserved. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission. Solution Processed Silver Sulfide Thin Films for Filament Memory Applications by Shong Yin A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Engineering-Electrical Engineering and Computer Sciences in the Graduate Division of the University of California, Berkeley Committee in Charge: Professor Vivek Subramanian, Chair Professor Tsu-Jae King Liu Professor Ronald Gronsky Fall 2010 Solution Processed Silver Sulfide Thin Films for Filament Memory Applications © Copyright 2010 by Shong Yin Abstract Solution Processed Silver Sulfide Thin Films for Filament Memory Applications by Shong Yin Doctor of Philosophy in Engineering – Electrical Engineering and Computer Sciences University of California, Berkeley Professor Vivek Subramanian, Chair Filament Memories based on resistive switching have been attracting attention in recent years as a potential replacement for flash memory in CMOS technology and as a potential candidate memory for low-cost, large-area electronics. These memories operate at low voltages with fast switching speeds. These devices are based on ionic conduction through an electrolyte layer and differ fundamentally in operation from conventional flash memory, which is based on the field effect transistor. To facilitate development of this technology, effects of film structure on ionic and electronic conducting properties and the filament formation processes must be studied. In this work, silver sulfide, a mixed ionic-electronic conductor, is used as a model material for studying the solution processing of filament memories, and to study the impact of film structure on conducting and switching properties. Three different solution processing methods are investigated for depositing silver sulfide: sulfidation of elemental silver films, and sintering of two types of silver sulfide nanoparticles. Effects of nanoparticle sintering conditions on electrolyte structured and mixed conducting properties are investigated by a combination of X- ray diffraction, electrical impedance spectroscopy and thermo-gravimetric analysis. Impact of forming voltage and time on filament morphology is examined to provide an overall view of the impact of electrical and material parameters on device operation. 1 Table of Contents Chapter 1: Introduction .............................................................................................................. 1 1.1: Non-Volatile Memory Background .................................................................................. 1 1.1.1: Flash Memory Background and Scaling Challenges .................................................. 1 1.1.2: Printable Non-Volatile Memory Background ........................................................... 6 1.2: Resistive Switching Random Access Memory Background .............................................. 9 1.3: Cationic Electrochemical Resistive Switching Random Access Memory ....................... 12 1.4: Limitations of Leakage Sneak Paths in Crossbar Arrays ................................................. 17 1.5: Oxide Based RRAMs ....................................................................................................... 18 1.6: Application of ECM RRAMs to Reconfigurable Logic ..................................................... 21 1.7: Objectives ....................................................................................................................... 23 1.8: Silver Sulfide as a Model Material .................................................................................. 25 1.9: Organization ................................................................................................................... 26 1.9.1: Independent Contributions .................................................................................... 26 1.9.2: Overview ................................................................................................................. 27 Chapter 2: Solid State Ionic Conduction and Characterization ................................................ 28 2.1: Defect Chemistry ............................................................................................................ 28 2.1.1: Schottky Notation and Kroeger-Vink Notation ....................................................... 28 2.1.2: Point Defect Types .................................................................................................. 30 2.1.3: Mass Action Laws for Intrinsic Disorder ................................................................. 32 2.2: Expression of Electrochemical Potential for Defects ..................................................... 33 2.3: Ionic Conduction ............................................................................................................ 36 2.4: Interfacial Effects on Ionic Conductors .......................................................................... 36 2.4.1: Heterogeneous Doping ........................................................................................... 39 2.5: Characterization Method: Electrical Impedance Spectroscopy ..................................... 40 2.6: Grain Boundary Models ................................................................................................. 42 2.7: Summary ........................................................................................................................ 44 Chapter 3: Sulfidation of Elemental Silver Films ...................................................................... 45 3.1: Background ..................................................................................................................... 45 i 3.1.1: Blanket Thin Film Deposition Methods .................................................................. 45 3.1.2: Sulfidation Method ................................................................................................. 46 3.2: Sulfidation Rate Study .................................................................................................... 48 3.3: Materials Characterization ............................................................................................. 50 3.3.1: X-Ray Photoelectron Spectroscopy (XPS) ............................................................... 50 3.3.2: X-Ray Diffraction (XRD) ........................................................................................... 51 3.3.3: Atomic Force Microscopy & Scanning Electron Microscopy .................................. 53 3.3.4: Discussion................................................................................................................ 54 3.4: Electrical Characterization ............................................................................................. 57 3.5: Conclusions..................................................................................................................... 59 Chapter 4: Sintered Octadecylamine Capped Ag2S Nanoparticle Films ................................... 60 4.1: Nanoparticle Synthesis ................................................................................................... 60 4.1.1: Background on Nanoparticle Synthetic Methods ................................................... 60 4.1.2: Synthesis of Octadecylamine Encapsulated Ag2S Nanoparticles ............................ 63 4.1: Sintering Study: Materials Characterization .................................................................. 65 4.1.1: TGA/DSC .................................................................................................................. 66 4.1.2: XRD .......................................................................................................................... 67 4.1.3: AFM ......................................................................................................................... 68 4.2: Sintering Study: Electrical Characterization ................................................................... 69 4.2.1: Electrical Impedance Spectroscopy ........................................................................ 70 4.2.2: Filament Formation Tests ....................................................................................... 71 4.3: Discussion ....................................................................................................................... 71 4.4: Conclusions..................................................................................................................... 75 Chapter 5: Sintered Thioglycerol Encapsulated Ag2S Nanoparticle Films ................................ 76 5.1: Nanoparticle Synthesis ................................................................................................... 76 5.2: Sintering Study: Materials