Jaehyun Lee

Enhanced diode characteristics of organic solar cells using titanium suboxide electron transport layer

The (dark) diode characteristics of the organic bulk heterojunction solar cell based on the phase separated blend of poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thenyl-2′,1′,3′-benzothiadiazole)] with [6,6]-phenyl C70-butyric acid methyl ester have been analyzed with a focus on the effect of the titanium suboxide (TiOx) electron transport layer. The addition of the TiOx layer into the device structure causes the saturation current density to decrease by a factor of 26 and the shunt resistance to increase by a factor of 12. The diode ideality factor and series resistance are, respectively, almost the same for diodes made with and without the TiOx layer. The results indicate that the TiOx layer increases the energy barrier for hole transport and reduces the minority carrier density.

Density functional theory based simulations of silicon nanowire field effect transistors

First-principles density functional theory (DFT) based, atomistic, self-consistent device simulations are performed for realistically sized Si nanowire field effect transistors (NW FETs) having tens of thousands of atoms. Through mode space transformation, DFT Hamiltonian and overlap matrices are reduced in size from a few thousands to around one hundred. Ultra-efficient quantum-mechanical transport calculations in the non-equilibrium Greens function formalism in a non-orthogonal basis are therefore made possible. The n-type and p-type Si NW FETs are simulated and found to exhibit similar device performance in the nanoscale regime.

Analysis of Drain-Induced Barrier Rising in Short-Channel Negative-Capacitance FETs and Its Applications

We investigate the performance of hysteresis-free short-channel negative-capacitance FETs (NCFETs) by combining quantum-mechanical calculations with the Landau–Khalatnikov equation. When the subthreshold swing (SS) becomes smaller than 60 mV/dec, a negative value of drain-induced barrier lowering is obtained. This behavior, drain-induced barrier rising (DIBR), causes negative differential resistance in the output characteristics of the NCFETs. We also examine the performance of an inverter composed of hysteresis-free NCFETs to assess the effects of DIBR at the circuit level. Contrary to our expectation, although hysteresis-free NCFETs are used, hysteresis behavior is observed in the transfer properties of the inverter. Furthermore, it is expected that the NCFET inverter with hysteresis behavior can be used as a Schmitt trigger inverter.

A theoretical model for predicting Schottky-barrier height of the nanostructured silicide-silicon junction

In this work, we have performed the first-principles calculations to investigate the Schottky barrier height (SBH) of various nanostructured silicide-silicon junctions. As for the silicides, PtSi, NiSi, TiSi2, and YSi2 have been used. We find that EFiF =u2009EFi – EF, where EFi and EF are the intrinsic Fermi level of the semiconductor part and the Fermi level of the junction, respectively, is unchanged by nanostructuring. From this finding, we suggest a model, a symmetric increase of the SBH (SI) model, to properly predict SBHs of nanostructured silicide-silicon junctions. We also suggest two measurable quantities for the experimental validation of our model. The effect of our SI model applied to nanostructures such as nanowires and ultra-thin-bodies is compared with that of the widely used previous SBH model.

Spin Torque Nano-Oscillators Directly Integrated on a mosfet

We propose spin torque nano-oscillators (STNOs) directly integrated on a metal-oxide-semiconductor field-effect-transistor (mosfet). In this model, we consider an array of STNOs, where the STNOs are synchronized via magnetodipolar interaction. We found that the ac voltage generated by magnetization precession of STNOs directly integrated on a mosfet can be amplified by the normally-on mosfet. We also found that the load resistance plays an important role with respect to increasing the output voltage of synchronized STNOs compared to a single STNO and in relation to the gate voltage of the mosfet. By using numerical calculation, we found that the emitted microwave power is greatly enhanced by amplifying the ac voltage generated by synchronized STNOs. It is anticipated that the results will be verified with existing experimental techniques.

Understanding Electromigration in Cu-CNT Composite Interconnects: A Multiscale Electrothermal Simulation Study

In this paper, we report a hierarchical simulation study of the electromigration (EM) problem in Cu-carbon nanotube (CNT) composite interconnects. This paper is based on the investigation of the activation energy and self-heating temperature using a multiscale electrothermal simulation framework. We first investigate the electrical and thermal properties of Cu-CNT composites, including contact resistances, using the density functional theory and reactive force field approaches, respectively. The corresponding results are employed in macroscopic electrothermal simulations taking into account the self-heating phenomenon. Our simulations show that although Cu atoms have similar activation energies in both bulk Cu and Cu-CNT composites, Cu-CNT composite interconnects are more resistant to EM thanks to the large Lorenz number of the CNTs. Moreover, we found that a large and homogenous conductivity along the transport direction in interconnects is one of the most important design rules to minimize the EM.

The impact of vacancy defects on CNT interconnects: From statistical atomistic study to circuit simulations

We have performed statistical atomistic simulations with tight-binding approach to investigate the effects of randomly distributed mono-vacancy defects in metallic single-walled carbon nanotube (SWCNT) interconnects. We also extracted defective resistances from the atomistic simulations and performed circuit- level simulations to compare the performance of interconnects with and without defects. We have found that the defects induce significant fluctuations of SWCNT resistance with a median value showing an Ohmic-like behaviour. Fortunately, the resistance depends only on the diameter of SWCNTs and not on their chirality. Moreover, our circuit simulations show that the defective resistance induces important propagation time delay ratio that should be accounted for when designing CNT interconnects.

Atoms-to-circuits simulation investigation of CNT interconnects for next generation CMOS technology

In this study, we suggest a hierarchical model to investigate the electrical performance of carbon nanotube (CNT)- based interconnects. From the density functional theory, we have obtained important physical parameters, which are used in TCAD simulators to obtain the RC netlists. We then use these RC netlists for the circuit-level simulations to optimize interconnect design in VLSI. Also, we have compared various CNT-based interconnects such as single-walled CNTs, multi-walled CNTs, doped CNTs, and Cu-CNT composites in terms of conductivity, ring oscillator delay, and propagation time delay.

Invited) Modelling and Simulation of Advanced Semiconductor Devices

This paper presents a modelling and simulation study of advanced semiconductor devices. Different Technology Computer Aided Design approaches and models, used in nowadays research are described here. Our discussions are based on numerous theoretical approaches starting from first principle methods and continuing with discussions based on more well stablished methods such as Drift-Diffusion, Monte Carlo and Non-Equilibrium Green’s Function formalism.

First principles based NEGF simulations of Si nanowire FETs

In this work, we have performed first-principles density functional theory (DFT) based, self-consistent device simulations for Si nanowire field effect transistors (NW FETs) of cross-section up to 3.24 nm. Through mode space transformation, non-orthogonal DFT Hamiltonian and overlap matrices are reduced in size from a few thousands to around one hundred. Ultra-efficient quantum-mechanical transport calculations in the non-equilibrium Greens function (NEGF) formalism in a nonorthogonal basis are therefore made possible.