Jui-Yuan Chen

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.

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.

Revealing Controllable Nanowire Transformation through Cationic Exchange for RRAM Application

One dimensional metal oxide nanostructures have attracted much attention owing to their fascinating functional properties. Among them, piezoelectricity and photocatalysts along with their related materials have stirred significant interests and widespread studies in recent years. In this work, we successfully transformed piezoelectric ZnO into photocatalytic TiO2 and formed TiO2/ZnO axial heterostructure nanowires with flat interfaces by solid to solid cationic exchange reactions in high vacuum (approximately 10(-8) Torr) transmission electron microscope (TEM). Kinetic behavior of the single crystalline TiO2 was systematically analyzed. The nanoscale growth rate of TiO2 has been measured using in situ TEM videos. On the basis of the rate, we can control the dimensions of the axial-nanoheterostructure. In addition, the unique Pt/ ZnO / TiO2/ ZnO /Pt heterostructures with complementary resistive switching (CRS) characteristics were designed to solve the important issue of sneak-peak current. The resistive switching behavior was attributed to the migration of oxygen and TiO2 layer served as reservoir, which was confirmed by energy dispersive spectrometry (EDS) analysis. This study not only supplied a distinct method to explore the transformation mechanisms but also exhibited the potential application of ZnO/TiO2 heterostructure in nanoscale crossbar array resistive random-access memory (RRAM).

In Situ TEM and Energy Dispersion Spectrometer Analysis of Chemical Composition Change in ZnO Nanowire Resistive Memories

Resistive random-access memory (ReRAM) has been of wide interest for its potential to replace flash memory in the next-generation nonvolatile memory roadmap. In this study, we have fabricated the Au/ZnO-nanowire/Au nanomemory device by electron beam lithography and, subsequently, utilized in situ transmission electron microscopy (TEM) to observe the atomic structure evolution from the initial state to the low-resistance state (LRS) in the ZnO nanowire. The element mapping of LRS showing that the nanowire was zinc dominant indicating that the oxygen vacancies were introduced after resistance switching. The results provided direct evidence, suggesting that the resistance change resulted from oxygen migration.

Strain relaxation induced microphotoluminescence characteristics of a single InGaN-based nanopillar fabricated by focused ion beam milling

A freestanding nanopillar with a diameter of 300nm and a height of 2μm is demonstrated by focused ion beam milling. The measured microphotoluminescence (μ-PL) from the embedded InGaN∕GaN multiple quantum wells shows a blueshift of 68meV in energy with a broadened full width at half maximum, ∼200meV. Calculations based on the valence force field method suggest that the spatial variation of the strain tensors in the nanopillar results in the observed energy shift and spectrum broadening. Moreover, the power-dependent μ-PL measurement suggests that the strain-relaxed emission region exhibits a higher radiative recombination rate than that of the strained region, indicating potential for realizing high-efficiency nanodevices in the UV/blue wavelength range.

Optical study of a-plane InGaN/GaN multiple quantum wells with different well widths grown by metal-organic chemical vapor deposition

a-plane InGaN/GaN multiple quantum wells of different widths ranging from 3 to 12 nm grown on r-plane sapphire by metal-organic chemical vapor deposition were investigated. The peak emission intensity of the photoluminescence (PL) reveals a decreasing trend as the well width increases from 3 to 12 nm. Low temperature (9 K) time-resolved PL (TRPL) study shows that the sample with 3-nm-thick wells has the best optical property with a fastest exciton decay time of 0.57 ns. The results of cathodoluminescence and micro-PL scanning images for samples of different well widths further verify that the more uniform and stronger luminescence intensity distribution are observed for the samples of thinner quantum wells. In addition, more effective capturing of excitons due to larger localization energy Eloc and shorter radiative lifetime of localized excitons are observed in thinner well width samples in the temperature dependent TRPL.

Optical characteristics of a-plane InGaN∕GaN multiple quantum wells with different well widths

a-plane InGaN∕GaN multiple quantum wells of different widths ranging from 3to12nm were grown on r-plane sapphire by metal organic chemical vapor deposition for investigation. The peak emission intensity of the photoluminescence (PL) reveals a decreasing trend as the well width increases from 3to12nm. Low temperature (9K) time-resolved PL study shows that the sample with 3-nm-thick wells has a better optical property with a fast exciton decay time of 0.57ns. The results of cathodoluminescence and micro-PL scanning images for samples of different well widths further verify the more uniform and stronger luminescence intensity distribution observed for the samples of thinner quantum wells, indicating that the important growth parameters for a-plane InGaN∕GaN multiple quantum wells could be dominated by the In fluctuation and crystal quality during the epitaxial growth.

Opto-electrical properties of Sb-doped p-type ZnO nanowires

P-type ZnO nanowires (NWs) have attracted much attention in the past years due to the potential applications for optoelectronics and piezotronics. In this study, we have synthesized Sb-doped p-type ZnO NWs on Si (100) substrates by chemical vapor deposition with Aucatalyst. The Sb-doped ZnO NWs are single crystalline with high density, grown along [1-1-2] direction. The doping percentage of Sb is about 2.49%, which has been confirmed by X-ray photoelectron spectroscopy. The ZnO NW field effect transistor demonstrated its p-type characteristics. A high responsivity to ultraviolet photodetection was also observed. In addition, compared to intrinsic ZnO NWs, the conductivity of the Sb-doped ZnO NWs exhibited ∼2 orders of magnitude higher. These properties make the p-type ZnO NWs a promising candidate for electronic and optoelectronic devices.

Direct Observation of Dual-Filament Switching Behaviors in Ta2O5-Based Memristors

The Forming phenomenon is observed via in situ transmission electron microscopy in the Ag/Ta2 O5 /Pt system. The device is switched to a low-resistance state as the dual filament is connected to the electrodes. The results of energy dispersive spectrometer and electron energy loss spectroscopy analyses demonstrate that the filament is composed by a stack of oxygen vacancies and Ag metal.

Single-crystalline CuO nanowires for resistive random access memory applications

Recently, the mechanism of resistive random access memory (RRAM) has been partly clarified and determined to be controlled by the forming and erasing of conducting filaments (CF). However, the size of the CF may restrict the application and development as devices are scaled down. In this work, we synthesized CuO nanowires (NW) (∼150 nm in diameter) to fabricate a CuO NW RRAM nanodevice that was much smaller than the filament (∼2 μm) observed in a bulk CuO RRAM device in a previous study. HRTEM indicated that the Cu2O phase was generated after operation, which demonstrated that the filament could be minimize to as small as 3.8 nm when the device is scaled down. In addition, energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) show the resistive switching of the dielectric layer resulted from the aggregated oxygen vacancies, which also match with the I-V fitting results. Those results not only verify the switching mechanism of CuO RRAM but also show RRAM has the potential to shr...