Browsing by Author "Yoo, Won Jong"

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Achieving Near-Ideal Subthreshold Swing in P-Type WSe2 Field-Effect Transistors
(2024-09) Ali, Fida; Choi, Hyungyu; Ali, Nasir; Hassan, Yasir; Ngo, Tien Dat; Ahmed, Faisal; Park, Won Kyu; Sun, Zhipei; Yoo, Won Jong
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The pursuit of near-ideal subthreshold swing (SS) ≈ 60 mV dec−1 is a primary driving force to realize the power-efficient field-effect transistors (FETs). This challenge is particularly pronounced in 2D material-based FETs, where the presence of a large interface trap density (Dit) imposes limitations on electrostatic control, consequently escalating power consumption. In this study, the gate controllability of 2D FETs is systematically analyzed by fabricating pre-patterned van der Waals (vdW)-contacted p-FETs, varying the WSe2 channel thickness from monolayer to ten-layer. As a result, the channel thickness is optimized to achieve efficient gate controllability while minimizing Dit. The findings demonstrate negligible hysteresis and excellent subthreshold swing (SSmin) close to the thermal limit (≈60 mV dec−1), with a corresponding Dit of ≈1010 cm−2 eV−1, comparable to Dit values observed in state-of-the-art Si transistors, when utilizing WSe2 channel thicknesses ≥ five-layer. However, reducing the WSe2 channel thickness below the trilayer, SSmin (≈91 mV dec−1) deviates from the thermal limit, attributed to a comparatively higher Dit (≈1011 cm−2 eV−1), despite the still lower than values reported for surface-contacted 2D transistors. Furthermore, all devices exhibit consistent p-type characteristics, featuring a high ION/IOFF ratio, high mobility, and excellent electrical stability confirmed over several months.
Link between T-Linear Resistivity and Quantum Criticality in Ambipolar Black Phosphorus
(2024-05-07) Ali, Nasir; Singh, Budhi; Srivastava, Pawan Kumar; Ali, Fida; Lee, Myeongjin; Park, Hyokwang; Shin, Hoseong; Lee, Kwangro; Choi, Hyungyu; Lee, Sungwon; Ngo, Tien Dat; Hassan, Yasir; Watanabe, Kenji; Taniguchi, Takashi; Lee, Changgu; Yoo, Won Jong
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The interplay between strong Coulomb interactions and kinetic energy leads to intricate many-body competing ground states owing to quantum fluctuations in 2D electron and hole gases. However, the simultaneous observation of quantum critical phenomena in both electron and hole regimes remains elusive. Here, we utilize anisotropic black phosphorus (BP) to show density-driven metal-insulator transition with a critical conductance ∼e2/h which highlights the significant role of quantum fluctuations in both hole and electron regimes. We observe a T-linear resistivity from the deep metallic phase to the metal-insulator boundary at moderate temperatures, while it turns to Fermi liquid behavior in the deep metallic phase at low temperatures in both regimes. An analysis of the resistivity suggests that disorder-dominated transport leads to T-linear behavior in the hole regime, while in the electron regime, the T-linear resistivity results from strong Coulomb interactions, suggestive of strange-metal behavior. Successful scaling collapse of the resistivity in the T-linear region demonstrates the link between quantum criticality and the T-linear resistivity in both regimes. Our study provides compelling evidence that ambipolar BP could serve as an exciting testbed for investigating exotic states and quantum critical phenomena in hole and electron regimes of 2D semiconductors.
Modulation of Contact Resistance of Dual-Gated MoS2 FETs Using Fermi-Level Pinning-Free Antimony Semi-Metal Contacts
(2023-07-27) Ngo, Tien Dat; Huynh, Tuyen; Jung, Hanggyo; Ali, Fida; Jeon, Jongwook; Choi, Min Sup; Yoo, Won Jong
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Achieving low contact resistance (RC) is one of the major challenges in producing 2D FETs for future CMOS technology applications. In this work, the electrical characteristics for semimetal (Sb) and normal metal (Ti) contacted MoS2 devices are systematically analyzed as a function of top and bottom gate-voltages (VTG and VBG). The semimetal contacts not only significantly reduce RC but also induce a strong dependence of RC on VTG, in sharp contrast to Ti contacts that only modulate RC by varying VBG. The anomalous behavior is attributed to the strongly modulated pseudo-junction resistance (Rjun) by VTG, resulting from weak Fermi level pinning (FLP) of Sb contacts. In contrast, the resistances under both metallic contacts remain unchanged by VTG as metal screens the electric field from the applied VTG. Technology computer aided design simulations further confirm the contribution of VTG to Rjun, which improves overall RC of Sb-contacted MoS2 devices. Consequently, the Sb contact has a distinctive merit in dual-gated (DG) device structure, as it greatly reduces RC and enables effective gate control by both VBG and VTG. The results offer new insight into the development of DG 2D FETs with enhanced contact properties realized by using semimetals.