Showing papers by "Gyu Tae Kim published in 2022"

Tailoring the Electrical Characteristics of MoS2 FETs through Controllable Surface Charge Transfer Doping Using Selective Inkjet Printing.

Abstract: Surface charge transfer doping (SCTD) has been regarded as an effective approach to tailor the electrical characteristics of atomically thin transition metal dichalcogenides (TMDs) in a nondestructive manner due to their two-dimensional nature. However, the difficulty of achieving rationally controlled SCTD on TMDs via conventional doping methods, such as solution immersion and dopant vaporization, has impeded the realization of practical optoelectronic and electronic devices. Here, we demonstrate controllable SCTD of molybdenum disulfide (MoS2) field-effect transistors using inkjet-printed benzyl viologen (BV) as an n-type dopant. By adjusting the BV concentration and the areal coverage of inkjet-printed BV dopants, controllable SCTD results in BV-doped MoS2 FETs with elaborately tailored electrical performance. Specifically, the suggested solvent system creates well-defined droplets of BV ink having a volume of ∼2 pL, which allows the high spatial selectivity of SCTD onto the MoS2 channels by depositing the BV dopant on demand. Our inkjet-printed SCTD method provides a feasible solution for achieving controllable doping to modulate the electrical and optical performances of TMD-based devices.

Hidden surface channel in two-dimensional multilayers

Abstract: Numerous carrier scatterers, such as atomic defects, fixed oxide charges, impurities, chemical residues, and undesired surface adsorbates, including oxygen and water molecules, strongly degrade the carrier mobility of atomically thin two-dimensional (2D) materials. However, the effect of surface adsorbates and surface oxidation on the carrier density profile along the thickness of 2D multilayers is not well known, particularly for a substantial interruption in the formation of the top-surface channel. Here, we uncover a hidden surface channel in p-type black phosphorus and n-type rhenium disulfide multilayers originating from undesired ambient adsorbates and surface oxides that not only populate hole density (or reduce electron density) but also suppress carrier mobility. The absence of a second peak in the transconductance curve under ambient conditions indicates the disappearance of the top-surface channel inside the 2D multilayers, which is a possible indicator for the cleanliness of the top surface and can be used in gas sensor applications. Moreover, the negligible variation in the drain bias polarity-dependent turn-on voltage for the bottom channel under ambient conditions validates the exclusive contribution of surface adsorbates to the formation of the top channel in 2D multilayers. Our results provide a novel insight into the distinct carrier transport in 2D optoelectronic devices and diverse sensors.

Understanding random telegraph noise in two-dimensional BP/ReS2 heterointerface

Abstract: Black phosphorus (BP)-based broken gap heterojunctions have attracted significant attention mainly owing to its wide thickness-dependent Fermi level, offering opportunities to demonstrate various carrier transport characteristics and high performing optoelectronic applications. However, the interfacial effects on the carrier scattering mechanism of the two-dimensional (2D) broken gap heterojunctions are unclear. Herein, we discuss the origin of random telegraph noise of multilayer BP/ReS2 heterojunction diode, in particular, at the direct tunneling (DT) conduction regime. The gate-tunable diode characteristic of BP/ReS2 heterojunction allows one to unveil systematically the transition of the charge fluctuation mechanism from drift-diffusion to the DT regime. Unlike individual BP and ReS2 devices, the current noise histogram obtained from the BP/ReS2 heterojunction device exhibits exclusively two dominant peaks at the DT regime. We ascribed this distinct low-frequency noise feature representing the presence of random telegraph signal to the BP/ReS2 interfacial traps by taking into account of the inherent direct tunneling current conduction mechanism. In addition, the electrostatic bias-dependent power spectrum density manifests clearly that the dominant scattering mechanism is the carrier number fluctuation rather than tunneling barrier height fluctuation at the BP/ReS2 heterointerface. This study elucidates the carrier transport and the charge fluctuation mechanism at the 2D heterostructure interface.

Energy-Efficient III-V Tunnel FET-Based Synaptic Device with Enhanced Charge Trapping Ability Utilizing Both Hot Hole and Hot Electron Injections for Analog Neuromorphic Computing.

Abstract: A charge trap device based on field-effect transistors (FET) is a promising candidate for artificial synapses because of its high reliability and mature fabrication technology. However, conventional MOSFET-based charge trap synapses require a strong stimulus for synaptic update because of their inefficient hot-carrier injection into the charge trapping layer, consequently causing a slow speed operation and large power consumption. Here, we propose a highly efficient charge trap synapse using III-V materials-based tunnel field-effect transistor (TFET). Our synaptic TFETs present superior subthreshold swing and improved charge trapping ability utilizing both carriers as charge trapping sources: hot holes created by impact ionization in the narrow bandgap InGaAs after being provided from the p+-source, and band-to-band tunneling hot electrons (BBHEs) generated at the abrupt p+n junctions in the TFETs. Thanks to these advances, our devices achieved outstanding efficiency in synaptic characteristics with a 5750 times faster synaptic update speed and 51 times lower sub-fJ/um2 energy consumption per single synaptic update in comparison to the MOSFET-based synapse. An artificial neural network (ANN) simulation also confirmed a high recognition accuracy of handwritten digits up to ∼90% in a multilayer perceptron neural network based on our synaptic devices.

Deep Understanding of Electron Beam Effects on 2D Layered Semiconducting Devices Under Bias Applications

Abstract: In this study, the radiation effects of electron beam (e-beam) on field-effect transistors (FETs) using transition-metal dichalcogenides (TMD) as a channel are carefully investigated. Electron-hole pairs (EHPs) in SiO2 generated by e-beam irradiation induce additional traps, which change the surface potential of the TMD channel, resulting in strong negative shifts of transfer characteristics. These negative shifts, which remind one of n-doping effects, are highly affected not only by the condition of e-beam irradiation, but also by the gate bias condition during irradiating. As a result of the e-beam irradiation effect, band bending and contact resistance are affected, and the degree of formation of oxide traps and interface traps varies depending on the gate bias conditions. In the case of VG > 0 V application during e-beam irradiation, the negative shifts in the transfer characteristics are fully recovered after ambient exposure. However, the interface traps increase significantly, resulting in variations of low-frequency (LF) noise and time-dependent current fluctuations.

SPICE Study of STDP Characteristics in a Drift and Diffusive Memristor-Based Synapse for Neuromorphic Computing

Abstract: Neuromorphic hardware is a system with massive potential to enable efficient computing by mimicking the human brain. The novel system processes information using neuron spikes (Action Potentials) and the synaptic connections between neurons are trained using biologically plausible methods like spike-timing-dependent plasticity (STDP). Memristor is one of the promising candidates to implement such neuromorphic hardware. Two types of memristors, diffusive and drift, have been proposed to form a synapse showing faithful emulation of STDP, where the diffusion effect is used to trace the spike timing history crucial for STDP and the drift memristor keeps the weight information in a longer time scale. The purpose of this paper is to systematically investigate STDP characteristics in such a synapse with serially connected two memristors using SPICE models. The results show that STDP properties are strongly dependent on device parameters and even the shape of STDP curves is modified. Different shapes of the STDP curve were identified. The results and analysis could support the design of emerging device-based synapses, which can faithfully mimic biological STDP characteristics for future neuromorphic systems.

Bias Temperature Instability of Multilayer ReS2 FET with α‐MoO3 Passivation

Abstract: 2D semiconductors are regarded as promising candidates for channel applications in the next generation of field effect transistors (FETs) with sub‐5 nm pitch designs. Among 2D transition metal dichalcogenides (TMDs), rhenium disulfide (ReS2) is reported to have weak interaction among neighboring layers, resulting in more susceptibility to the functionalization of the surface channel. The bias temperature instability (BTI) of ReS2 FETs with α‐molybdenum trioxide (α‐MoO3) passivation that functions as a charge buffer layer is investigated. The transconductance (gm) with a passivation layer shows the saturation behavior under the critical gate voltage even with cumulative electric stress. In addition, unintentional shifts of threshold voltages (VTH) are significantly reduced, which is attributed to the effects of the α‐MoO3 passivation. The electron transfer with the passivation effect suggests a way of surface engineering for controlling the 2D devices with enhanced stabilities.