Jongmyung Yoo

Threshold switching behavior of Ag-Si based selector device and hydrogen doping effect on its characteristics

The effect of hydrogen treatment on the threshold switching property in a Ag/amorphous Si based programmable metallization cells was investigated for selector device applications. Using the Ag filament formed during motion of Ag ions, a steep-slope (5 mV/dec.) for threshold switching with higher selectivity (∼105) could be achieved. Because of the faster diffusivity of Ag atoms, which are inside solid-electrolytes, the resulting Ag filament could easily be dissolved under low current regime, where the Ag filament possesses weak stability. We found that the dissolution process could be further enhanced by hydrogen treatment that facilitated the movement of the Ag atoms.

Steep Slope Field-Effect Transistors With Ag/TiO 2 -Based Threshold Switching Device

In this letter, we demonstrate a steep slope field-effect transistor (FET) using a threshold switching (TS) device. The Ag/TiO2-based TS device reported in our previous work was implemented in series with the drain region of a transistor. Since the TS device has an abrupt transition between the OFF- and ON-states and vice versa, the transistor has a 5-mV/decade subthreshold slope and a high ON/OFF-current ratio (ION/IOFF) of >107 with a low drain voltage (0.3 V). Furthermore, the threshold voltage (Vth,FET) of the transistor can be tuned by controlling the thickness of the TS device.

Excitonic diffusion dynamics in ZnO

We investigate excitonic carrier diffusion in both bulk ZnO and nanorods (NRs). Using time-resolved differential reflectivity spectroscopy, we observe a fast decaying component together with a longer exponential relaxation. In bulk ZnO, we find that the fast decay term (∼1 ps) originates from excitonic diffusion along the growth direction. By probing at both the A and B excitons, we find different diffusion coefficients for each. In ZnO nanorods, the diffusion contribution is missing. We attribute this to two effects: (1) defects in the nanorods substantially slow the diffusion process and (2) excitons in nanorods are generated more uniformly than in bulk.

Nanophotonic energy up conversion using ZnO nanorod double-quantum-well structures

We report on near-field spectroscopy of ZnO/ZnMgO nanorod double-quantum-well structures (DQWs) for a nanometer-scale photonic device. We observed energy up conversion assisted by the absorption of phonons generated in the ZnO nanorod DQWs. Theoretical calculation of the coherent excitonic excitation of the population with incoherent phonon coupling agrees well with the obtained experimental power dependence of the up conversion efficiency. It should result in an increase in the efficiency of devices, such as photodetectors, solar cells, and so on.

Monolithic integration of AgTe/TiO 2 based threshold switching device with TiN liner for steep slope field-effect transistors

AgTe/TiN/TiO2/TiN threshold switching (TS) device was monolithically integrated with silicon MOSFET to demonstrate steep subthreshold slope field-effect transistors. The TS device with AgTe top electrode showed the high on-current, since the Te allows an extraction of the Ag out of the filament. The TiN liner was also inserted at the AgTe/TiO2 interface to prevent in-diffusion of Ag into the TiO2 layer during back-end-of-line process. Finally, the transistor with TS device has a sub-5-mV/dec subthreshold slope (SS) and a high on/off current ratio (Ion/Ioff) of >108 with a low drain voltage (0.5 V) even after the 400°C annealing process.

Additional hardening in harmonic structured materials by strain partitioning and back stress

ABSTRACT Deformation behavior of a harmonic structured material (HSM), core–shell 304L stainless steel, is investigated using micro-digital image correlation (micro-DIC). High strain-partitioning between core and shell is observed. Because the grain boundaries with a grain-size gradient in HSM induce high deformation-incompatibility, strain peaks are detected near core–shell boundaries and in grain boundaries of cores. This incompatibility is compensated by geometrically necessary dislocations, generating back stress. The back stress is measured using tensile unloading-reloading testing. This investigation demonstrates higher back stress and strain hardening rate in HSMs than homogeneous materials, resulting in enhanced ductility. GRAPHICAL ABSTRACT IMPACT STATEMENT Core–shell structure with grain size gradient leads to strain partitioning and high back stress hardening in HSMs. micro-DIC was utilized to observe the local strain distribution.

Effects of Liner Thickness on the Reliability of AgTe/TiO 2 -Based Threshold Switching Devices

The effects of liner thickness on the reliability of AgTe/TiO2-based threshold switching (TS) devices were investigated. The off-state current of an AgTe/TiO2/Pt TS device was found to be significantly increased by in-diffusion of Ag into the TiO2 layer during the annealing process. Therefore, 3-, 5- and 7-nm TiN liners were introduced and compared to prevent the in-diffusion of Ag. While the 3-nm TiN liner was shown to be incapable of blocking Ag in-diffusion into the TiO2 layer, the 5- and 7-nm liners effectively suppressed in-diffusion and maintained high off-state resistance. However, the TS device with the 7-nm TiN liner exhibited wide threshold voltage distribution and poor endurance characteristics owing to a lack of Ag sources. The TS device with a 5-nm TiN liner, by contrast, was found to have an adequate amount of Ag sources and to demonstrate thermally stable and electrically reliable characteristics. The effects of TiN liner on Ag diffusion were also directly confirmed using energy dispersive spectrometry line profiles, transmission electron microscopy imaging, and mapping analyses.

Field-induced nucleation in threshold switching characteristics of electrochemical metallization devices

In this research, we investigate electrically driven threshold switching (TS) characteristics in electrochemical metallization cells by adopting the field-induced nucleation theory. For this aim, Ag/HfO2 and Ag/TiO2 based TS devices are prepared and examined. First, we carry out the field driven turn-on process to form Ag filaments created as a consequence of sequential nucleation of Ag ions from the bottom electrode. During the filament formation process, it is observed that the prepared devices show switching time exponential in voltage and temperature with different nucleation barrier energies (W0), which confirms the field-induced nucleation theory. Furthermore, we find that the device with higher W0 shows faster dissolution speed. This implies that the slow turn-off speed of the TS device can be improved by finding a material system with a higher W0 value.

Superradiance from one-dimensionally aligned ZnO nanorod multiple-quantum-well structures

Using one-dimensionally aligned ZnO nanorod multiple-quantum-well structures (MQWs), we observed a superradiance, i.e., a cooperative spontaneous emission. We confirmed that the excitation power dependence of the emissions from the MQWs originated from the coherent coupling of the QWs due to the well organization at nanoscale. We identified two QWs with cooperative emission. Additionally, we evaluated the number of coherently coupled QWs sets of four that resulted in the superradiance. Our findings provide criteria for designing nanoscale synergetic devices without the use of an external cavity.

Effect of cation amount in the electrolyte on characteristics of Ag/TiO2 based threshold switching devices

In this study, we investigate the effect of cation amount in an electrolyte on Ag/TiO2 based threshold switching (TS) devices, based on field-induced nucleation theory. For this purpose, normal Ag/TiO2, annealed Ag/TiO2, and AgTe/TiO2 based TS devices are prepared, which have different cation amounts in their electrolytes during the switching process. First, we find that all of the prepared TS devices follow the field-induced nucleation theory with different nucleation barrier energy (W0), by investigating the delay-time dependency at various voltages and temperatures. Based on the investigation, we reveal that the amount of cations in the electrolyte during the switching process is the control parameter that affects the W0 values, which are found to be inversely proportional to the turn-off speed of the TS devices. This implies that the turn-off speed of the TS devices can be modulated by controlling the amount of cations in the matrix.