Alexander Kerelsky

Nanofilament Formation and Regeneration During Cu/Al2O3 Resistive Memory Switching

Conductive bridge random access memory (CBRAM) is a leading candidate to supersede flash memory, but poor understanding of its switching process impedes widespread implementation. The underlying physics and basic, unresolved issues such as the connecting filaments growth direction can be revealed with direct imaging, but the nanoscale target region is completely encased and thus difficult to access with real-time, high-resolution probes. In Pt/Al2O3/Cu CBRAM devices with a realistic topology, we find that the filament grows backward toward the source metal electrode. This observation, consistent over many cycles in different devices, corroborates the standard electrochemical metallization model of CBRAM operation. Time-resolved scanning transmission electron microscopy (STEM) reveals distinct nucleation-limited and potential-limited no-growth periods occurring before and after a connection is made, respectively. The subfemtoampere ionic currents visualized move some thousands of atoms during a switch and lag the nanoampere electronic currents.

Imaging interfacial electrical transport in graphene–MoS2 heterostructures with electron-beam-induced-currents

Heterostructure devices with specific and extraordinary properties can be fabricated by stacking two-dimensional crystals. Cleanliness at the inter-crystal interfaces within a heterostructure is crucial for maximizing device performance. However, because these interfaces are buried, characterizing their impact on device function is challenging. Here, we show that electron-beam induced current (EBIC) mapping can be used to image interfacial contamination and to characterize the quality of buried heterostructure interfaces with nanometer-scale spatial resolution. We applied EBIC and photocurrent imaging to map photo-sensitive graphene-MoS2 heterostructures. The EBIC maps, together with concurrently acquired scanning transmission electron microscopy images, reveal how a devices photocurrent collection efficiency is adversely affected by nanoscale debris invisible to optical-resolution photocurrent mapping.

Absence of a Band Gap at the Interface of a Metal and Highly Doped Monolayer MoS2

High quality electrical contact to semiconducting transition metal dichalcogenides (TMDCs) such as MoS2 is key to unlocking their unique electronic and optoelectronic properties for fundamental research and device applications. Despite extensive experimental and theoretical efforts reliable ohmic contact to doped TMDCs remains elusive and would benefit from a better understanding of the underlying physics of the metal-TMDC interface. Here we present measurements of the atomic-scale energy band diagram of junctions between various metals and heavily doped monolayer MoS2 using ultrahigh vacuum scanning tunneling microscopy (UHV-STM). Our measurements reveal that the electronic properties of these junctions are dominated by two-dimensional metal-induced gap states (MIGS). These MIGS are characterized by a spatially growing measured gap in the local density of states (L-DOS) of the MoS2 within 2 nm of the metal-semiconductor interface. Their decay lengths extend from a minimum of ∼0.55 nm near midgap to as long as 2 nm near the band edges and are nearly identical for Au, Pd, and graphite contacts, indicating that it is a universal property of the monolayer semiconductor. Our findings indicate that even in heavily doped semiconductors, the presence of MIGS sets the ultimate limit for electrical contact.

In Situ STEM of Ag and Cu Conducting Bridge Formation through Al2O3 in Nanoscale Resistive Memory Devices

Non-volatile resistive memory, specifically conducting-bridge RAM or CBRAM, is a potential successor to flash memory. CBRAM requires less power than flash memory, can switch on and off faster, and can withstand a larger number of on/off cycles [1]. In principle CBRAM devices are also expected to be scalable to only a few nanometers. In CBRAM, the memory element switches to an “on” state when a conducting bridge forms through the insulating layer separating two conducting electrodes. Despite recent intense interest in CBRAM, the specifics of the formation and breaking of this bridge is not well understood. We have fabricated horizontally-aligned CBRAM devices specifically designed for high-resolution S/TEM imaging of conducting filaments in situ.

STEM EBIC to Study 2D Materials

Since its discovery in 2004 graphene has become a very widely studied material. The electronic excitations in single layer graphene can be described as two-dimensional massless Dirac particles, resulting in desirable electronic properties for nanoelectronic devices. Adding layers allows the electronic properties to be tuned, thus multilayer graphene is better suited for some device applications. Characterizing defects in this material is critical for understanding device performance. Recently Butz et al. have used dark field TEM to image dislocations in bilayer graphene, furthering our understanding of its electronic properties and the key importance of defects [1]. Here, we report using electron beam induced current (EBIC) measurements in a scanning transmission electron microscope (STEM) to image local changes in the conductivity of multilayer graphene.

Nanoscale Mapping of Interfacial Electrical Transport in Graphene-MoS 2 Heterostructures with STEM-EBIC

1. Department of Chemistry, Imperial College London, London SW7 2AZ, UK. 2. Department of Physics & Astronomy and California NanoSystems Institute, University of California, Los Angeles, California, 90095, USA. 3. Department of Electrical Engineering, University of Southern California, Los Angeles, California, 90089, USA. 4. Center for Electron Microscopy and Microanalysis, University of Southern California, Los Angeles, California, 90089, USA.

Time-Resolved Imaging of Electrochemical Switching in Nanoscale Resistive Memory Elements

FLASH memory is reaching its scaling limit, but resistive random access memory (ReRAM) is considered a promising successor [1]. In ReRAM, metal electrodes sandwiching an insulating electrolyte form a digital memory element, where the presence or absence of a conducting path through the insulator represents one bit of information. The conducting filament is thought to form, atom-by-atom, when subject to a SET voltage applied across the electrodes, and to disintegrate when subject to a RESET voltage. We use scanning transmission electron microscopy (STEM) to image nanoscale ReRAM devices switching in situ. Operating the devices with small current limits slows the rate of filament formation and reduces confounding thermal effects, allowing us to obtain time-resolved images of filament formation and regeneration.

Nanobubbles on Electron Transparent Electrodes

The experimental observation of inhomogeneous nanobubble formation is challenging, and consequently the onset and growth of nanobubbles has yet to be fully localized or observed directly. Conventional methods, such as optical microscopy and atomic force microscopy (AFM), have been used to image small bubbles [1, 2]. However, optical images have limited spatial resolution due to the wavelength of visible light, and AFM is an invasive technique which gives data that can be difficult to interpret. Here we report using an electrical bias applied to graphene electrodes to form nanobubbles in situ for observation with scanning transmission electron microscopy (STEM) .