A. Ranjan

Analysis of quantum conductance, read disturb and switching statistics in HfO2 RRAM using conductive AFM

Abstract Most studies on resistance switching have been carried out at the device level with the standard electrical characterization setup, which allows for effective automated reliability test and extensive characterization of the lifetime of an RRAM device. However, it is equally important to be able to probe the switching phenomenon at the nanoscale so as to improve insight on the bias-dependent kinetic behavior of the filament during multiple reversible breakdown and recovery cycles. This study aims to do just that by probing HfO2 blanket films (~xa04xa0nm) with a W bottom electrode using an ultra-sharp Pt-wire conductive AFM (CAFM) tip with an areal resolution of ~10–20xa0nm at ambient conditions. The use of the CAFM allows for a more reliable assessment of single filament evolution behavior as possible multiple filamentation events (common at the device level) are rare for such small probing areas. The role of oxygen vacancy induced filaments is studied here by using low compliance setting and moderate voltage levels, ensuring operation in the sub-quantum conductance regime. Our results show good repeatable switching trends and also provide insight on the quantum conductance phenomenon in oxygen vacancy based filaments. The read disturb trends in switching are investigated for the high resistance state (HRS) and the impact of tip-induced mechanical stresses on forming lifetime is also presented, which could serve as a motivator for further studies on non-volatile memory (NVM) reliability for flexible electronics devices and system on chip (SoC) applications.

Conductive Atomic Force Microscope Study of Bipolar and Threshold Resistive Switching in 2D Hexagonal Boron Nitride Films

This study investigates the resistive switching characteristics and underlying mechanism in 2D layered hexagonal boron nitride (h-BN) dielectricxa0films using conductive atomic force microscopy. A combination of bipolar and threshold resistive switching is observed consistently on multi-layer h-BN/Cu stacks in the low power regime withxa0current compliance (Icomp) of less than 100u2009nA. Standard random telegraph noise signatures were observed in the low resistance state (LRS), similar to the trends in oxygen vacancy-based RRAM devices. While h-BN appears to be a good candidate in terms of switching performance and endurance, it performs poorly in terms of retention lifetime due to the self-recovery of LRS state (similar to recovery of soft breakdown in oxide-based dielectrics) that is consistently observed at all locations without requiring any change in the voltage polarity for Icomp ~1–100u2009nA.

Conductive filament formation at grain boundary locations in polycrystalline HfO2 -based MIM stacks: Computational and physical insight

Abstract Resistive switching in high-κ (HK) dielectric based metal-insulator-metal (MIM) devices occurs locally and is accompanied by dynamic changes in the structural and electrical properties of the HK dielectric. In polycrystalline HfO 2 HK dielectric based MIM devices, grain boundaries (GBs) play a significant role in the formation of a percolation path for the resistive switching as the GB regions contain a large number of defects and favor the formation of conductive/low resistive paths. In this work, we present a multi-physics based combined Kinetic Monte Carlo-Finite element model (KMC-FEM) 3D percolation framework to simulate the resistive switching (high resistive state (HRS) to low resistive state (LRS)) process in TiN/HfO 2 (5xa0nm)/Pt MIM stacks. The KMC-FEM model describes the effect of GBs on the formation of conductive path during the HRS to LRS resistive switching. In addition, this model is used to find the statistical distribution of conductive filament/path formation in amorphous and polycrystalline HfO 2 dielectrics. Conductive atomic force microscopy and transmission electron microscopy observations on the characteristics of the HfO 2 dielectrics at the nanometer scale complement the simulation results. The results clearly show that the HRS to LRS resistive switching occurs preferably at the GB regions in polycrystalline HfO 2 and at random locations in amorphous HfO 2 -based MIM stacks.

An SEM/STM based nanoprobing and TEM study of breakdown locations in HfO 2 /SiO x dielectric stacks for failure analysis

Abstract The formation of conductive percolation path in high-κ (HK)/interfacial layer (IL) dielectric stack is accompanied by dynamic changes in the electrical and chemical properties at the nanometer length scale. It is therefore essential to study these breakdown (BD) events using high-precision nanoscale characterization tools to investigate the physical mechanisms of failure for advanced HK dielectric based devices. In this work, we carry out a new method for electrical nanoprobing of HfO2/SiOx (x

Random telegraph noise in 2D hexagonal boron nitride dielectric films

This study reports the observation of low frequency random telegraph noise (RTN) in a 2D layered hexagonal boron nitride dielectric film in the pre- and post-soft breakdown phases using conductive atomic force microscopy as a nanoscale spectroscopy tool. The RTN traces of the virgin and electrically stressed dielectric (after percolation breakdown) were compared, and the signal features were statistically analyzed using the Factorial Hidden Markov Model technique. We observe a combination of both two-level and multi-level RTN signals in h-BN, akin to the trends commonly observed for bulk oxides such as SiO2 and HfO2. Experimental evidence suggests frequent occurrence of unstable and anomalous RTN traces in 2D dielectrics which makes extraction of defect energetics challenging.

Sb2Te3 and Its Superlattices: Optimization by Statistical Design

The objective of this work is to demonstrate the usefulness of fractional factorial design for optimizing the crystal quality of chalcogenide van der Waals (vdW) crystals. We statistically analyze the growth parameters of highly c axis oriented Sb2Te3 crystals and Sb2Te3-GeTe phase change vdW heterostructured superlattices. The statistical significance of the growth parameters of temperature, pressure, power, buffer materials, and buffer layer thickness was found by fractional factorial design and response surface analysis. Temperature, pressure, power, and their second-order interactions are the major factors that significantly influence the quality of the crystals. Additionally, using tungsten rather than molybdenum as a buffer layer significantly enhances the crystal quality. Fractional factorial design minimizes the number of experiments that are necessary to find the optimal growth conditions, resulting in an order of magnitude improvement in the crystal quality. We highlight that statistical design of experiment methods, which is more commonly used in product design, should be considered more broadly by those designing and optimizing materials.

Nanoscale investigations of soft breakdown events in few layered fluorinated graphene

In this study, we perform scanning tunneling spectroscopy (STS) on bi/tri-layered fluorinated graphene (FG) dielectrics, enabling investigation of the degradation and the breakdown phenomenon at the sub-nanometer scale. Our characterization results show that the energy gap can be tailored by surface functionalization of graphene with fluorine ions. Experimental evidence of electrical stress induced degradation and breakdown trends at localized spots across bi/tri-layered FG films is presented. Statistical analysis on bi-layered FG film breakdown voltage data reveals a tri-modal Weibull distribution trend possibly due to variations in the effective FG thickness due to imperfect fluorine incorporation at all C-sites during the fluorine diffusion process. Although preliminary, the results presented provide insight into the kinetics of degradation in graphene based 2-D dielectric materials.

CAFM based spectroscopy of stress-induced defects in HfO 2 with experimental evidence of the clustering model and metastable vacancy defect state

In this study, we perform random telegraph noise (RTN) spectroscopy on ultra-thin HfÜ2 dielectric films using a conductive atomic force microscope (CAFM), enabling accurate assessment of single or cluster defect kinetics in very small area regions with an ultra-sharp tip having radius of 15±5 nm. Our characterization results show that bias-dependent RTN trends can be clearly detected at high spatial resolution using CAFM technique. Experimental evidence of the metastable nature of oxygen vacancy defects is presented and the nanoscale breakdown results provide further support to the time-dependent defect clustering model that is recently proposed for oxide breakdown [1,2]. Statistical plots of the CAFM breakdown voltage show a trimodal distribution that corresponds to evolution of percolation cores at the grain (G), grain boundary/triple point (GB/TP) sites and G-GB interface regions.