Wen-Wei Wu

Observation of atomic diffusion at twin-modified grain boundaries in copper.

Grain boundaries affect the migration of atoms and electrons in polycrystalline solids, thus influencing many of the mechanical and electrical properties. By introducing nanometer-scale twin defects into copper grains, we show that we can change the grain-boundary structure and atomic-diffusion behavior along the boundary. Using in situ ultrahigh-vacuum and high-resolution transmission electron microscopy, we observed electromigration-induced atomic diffusion in the twin-modified grain boundaries. The triple point where a twin boundary meets a grain boundary was found to slow down grain-boundary and surface electromigration by one order of magnitude. We propose that this occurs because of the incubation time of nucleation of a new step at the triple points. The long incubation time slows down the overall rate of atomic transport.

Dynamic evolution of conducting nanofilament in resistive switching memories.

Resistive random access memory (ReRAM) has been considered the most promising next-generation nonvolatile memory. In recent years, the switching behavior has been widely reported, and understanding the switching mechanism can improve the stability and scalability of devices. We designed an innovative sample structure for in situ transmission electron microscopy (TEM) to observe the formation of conductive filaments in the Pt/ZnO/Pt structure in real time. The corresponding current-voltage measurements help us to understand the switching mechanism of ZnO film. In addition, high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) have been used to identify the atomic structure and components of the filament/disrupted region, determining that the conducting paths are caused by the conglomeration of zinc atoms. The behavior of resistive switching is due to the migration of oxygen ions, leading to transformation between Zn-dominated ZnO(1-x) and ZnO.

Interface Engineering for High‐Performance Top‐Gated MoS2 Field‐Effect Transistors

In recent years, due to the intriguing electrical and optical characteristics, two dimensional layered transition metal dichalcogenides such as MoS2 have attracted tremendous research attention. In a distinct contrast to the bandgap issue of graphene, MoS2 is semiconducting with a satisfied thickness-dependent bandgap of 1.2 to 1.8 eV, which can enable lots of fascinating device applications. However, until now, majority of the efforts have been focused on the integration of MoS2 devices in the back- or dual-gated geometry due to the difficulty of compact and conformal top-gated dielectric deposition directly onto the 2-D channel for the realization of high-performance top-gated FETs. In this regard, interface or dielectric engineering is an important step towards the practical implementation of MoS2 devices with the optimized performance.

Single crystalline PtSi nanowires, PtSi/Si/PtSi nanowire heterostructures, and nanodevices.

We report the formation of PtSi nanowires, PtSi/Si/PtSi nanowire heterostructures, and nanodevices from such heterostructures. Scanning electron microscopy studies show that silicon nanowires can be converted into PtSi nanowires through controlled reactions between lithographically defined platinum pads and silicon nanowires. High-resolution transmission electron microscopy studies show that PtSi/Si/PtSi heterostructure has an atomically sharp interface with epitaxial relationships of Si[110]//PtSi[010] and Si(111)//PtSi(101). Electrical measurements show that the pure PtSi nanowires have low resistivities approximately 28.6 microOmega.cm and high breakdown current densities>1x10(8) A/cm2. Furthermore, using single crystal PtSi/Si/PtSi nanowire heterostructures with atomically sharp interfaces, we have fabricated high-performance nanoscale field-effect transistors from intrinsic silicon nanowires, in which the source and drain contacts are defined by the metallic PtSi nanowire regions, and the gate length is defined by the Si nanowire region. Electrical measurements show nearly perfect p-channel enhancement mode transistor behavior with a normalized transconductance of 0.3 mS/microm, field-effect hole mobility of 168 cm2/V.s, and on/off ratio>10(7), demonstrating the best performing device from intrinsic silicon nanowires.

In-situ TEM observation of repeating events of nucleation in epitaxial growth of nano CoSi2 in nanowires of Si.

The formation of CoSi and CoSi2 in Si nanowires at 700 and 800 degrees C, respectively, by point contact reactions between nanodots of Co and nanowires of Si have been investigated in situ in a ultrahigh vacuum high-resolution transmission electron microscope. The CoSi2 has undergone an axial epitaxial growth in the Si nanowire and a stepwise growth mode was found. We observed that the stepwise growth occurs repeatedly in the form of an atomic step sweeping across the CoSi2/Si interface. It appears that the growth of a new step or a new silicide layer requires an independent event of nucleation. We are able to resolve the nucleation stage and the growth stage of each layer of the epitaxial growth in video images. In the nucleation stage, the incubation period is measured, which is much longer than the period needed to grow the layer across the silicide/Si interface. So the epitaxial growth consists of a repeating nucleation and a rapid stepwise growth across the epitaxial interface. This is a general behavior of epitaxial growth in nanowires. The axial heterostructure of CoSi2/Si/CoSi2 with sharp epitaxial interfaces has been obtained. A discussion of the kinetics of supply limited and source-limited reaction in nanowire case by point contact reaction is given. The heterostructures are promising as high performance transistors based on intrinsic Si nanowires.

Synthesis of taperlike Si nanowires with strong field emission

Taperlike Si nanowires (SiNWs) have been synthesized by annealing of high-density FeSi2 nanodots on (001)Si at 1200°C in a N2 ambient. The tip regions of SiNWs are about 5–10nm in diameter. The average length of the SiNWs is about 6μm with aspect ratios as high as 150–170. A growth model based on oxide-assisted growth is proposed. The taperlike morphology may be caused by the passivation of the SiO2 coating layer, which results in the different levels of absorption of SiO along the length of the nanowires. The SiNWs exhibit a turn-on field of 6.3–7.3V∕μm and a threshold field of 9–10V∕μm. The excellent field emission characteristics are attributed to the taperlike geometry of the crystalline Si nanowires.

Switching Kinetic of VCM-Based Memristor: Evolution and Positioning of Nanofilament

The filament in aAu/Ta2 O5 /Au system is analyzed and determined to be a nanoscaled TaO2-x filament. A shrunken anode localizes the filament formation and the defect boundary leads to faster accumulation of oxygen vacancies. The defect changes the switching domination between electron transport and oxygen-vacancy migration. The migration of oxygen vacancies limits the filament dynamics, indicating the crucial role played by oxygen defects.

High Mobility MoS2 Transistor with Low Schottky Barrier Contact by Using Atomic Thick h-BN as a Tunneling Layer.

High-performance MoS2 transistors are developed using atomic hexagonal boron nitride as a tunneling layer to reduce the Schottky barrier and achieve low contact resistance between metal and MoS2 . Benefiting from the ultrathin tunneling layer within 0.6 nm, the Schottky barrier is significantly reduced from 158 to 31 meV with small tunneling resistance.

Homogeneous nucleation of epitaxial CoSi2 and NiSi in Si nanowires.

Homogeneous nucleation is rare except in theory. We observed repeating events of homogeneous nucleation in epitaxial growth of CoSi(2) and NiSi silicides in nanowires of silicon by using high resolution TEM. The growth of every single atomic layer requires nucleation. Heterogeneous nucleation is prevented because of non-microreversibility between the oxide/Si and oxide/silicide interfaces. We determined the incubation time of homogeneous nucleation. The calculated and the measured nucleation rates are in good agreement. We used Zeldovich factor to estimate the number of molecules in the critical nucleus; it is about 10 and reasonable. A very high supersaturation is found for the homogeneous nucleation.

Point contact reactions between Ni and Si nanowires and reactive epitaxial growth of axial nano-NiSi∕Si

Point contact reactions between a Si nanowire and a Ni nanowire are reported in which the Si nanowire is transformed into a single crystal NiSi with an epitaxial interface which has no misfit dislocation. The reactions were carried out in situ in an ultrahigh vacuum transmission electron microscope. The growth of the NiSi occurs by the dissolution of Ni into the Si nanowire and by interstitial diffusion from the point of contact to the epitaxial interface. The point contact reactions have enabled the authors to fabricate single crystal NiSi∕Si∕NiSi heterostructures of atomically sharp interfaces for nanoscale devices.