Quantum Conductance Probing of Oxygen Vacancies in SrTiO3 Epitaxial Thin Film Using Graphene

The quantum Hall conductance in monolayer graphene on an epitaxial SrTiO3 (STO) thin film is studied to understand the role of oxygen vacancies in determining the dielectric properties of STO. As the gate voltage sweep range is gradually increased in our device, we observe systematic generation and annihilation of oxygen vacancies evidenced from the hysteretic conductance behavior in graphene. Furthermore, based on the experimentally observed linear scaling relation between the effective capacitance and the voltage sweep range, a simple model is constructed to manifest the relationship among the dielectric properties of STO with oxygen vacancies. The inherent quantum Hall conductance in graphene can be considered as a sensitive, robust, and non-invasive probe for understanding the electronic and ionic phenomena in complex transition metal oxides without impairing the oxide layer underneath.Comment: 21 pages, 4 figures, 2 supp. figure

Large variation in Young's modulus of carbon nanotube yarns with different diameters

Twist-spun carbon nanotube (CNT) yarns are composed of numerous CNTs and their bundles with entangled and twisted structures. In this paper, we studied the mechanical properties of CNT yarns. The individual CNT, a component of yarn, is well known to have an extremely high mechanical strength. However, CNT yarns are very flexible and relatively free to transform their shapes, showing the potential for application in the design of wearable devices. Since CNT yarns have two opposing characteristics at the same time, a wide range of Young's modulus can be achieved by simply changing the geometrical structure while using the same fabrication process. We also suggest that CNT yarns can be utilized as the base material for several applications that require different stresses in a structure, such as bioimplants or foldable devices.11Nsciescopuskc

Multifunctional characterization of carbon nanotube sheets, yarns, and their composites

Carbon nanotube (CNT) based macroscopic objects such as dry-state free-standing sheets and yarns have attracted much attention during more than a decade for their multifunctional features. Thanks to their lightweight, highly conductive, mechanically strong and flexible properties, various applications had been explored so far. However, because of the difficulties in the spinnable CNT forest growth, the sample availability in the academic fields has been quite limited. In this report, various properties of CNT sheets, yarns, and their composites were experimentally presented using the samples prepared from the spinnable CNT forest grown in the newly installed, acetylene-based, chemical-vapor-deposition chamber system. Clear observation of the dimensional effect on the charge transport through CNTs, the enhancement of electro-mechanical actuation owing the volume-expandable infiltration material inside CNTs, and other exemplary evaluations proved the versatility of this macroscopic assembly as well as the good quality of our sample. © 2016 Elsevier B.V.1681sciescopuskc

Detection of hidden localized states by the quantum Hall effect in graphene

© 2021 Korean Physical Society. We fabricated a monolayer graphene transistor device in the shape of the Hall-bar structure, which produced an exactly symmetric signal following the sample geometry. During electrical characterization, the device showed the standard integer quantum Hall effect of monolayer graphene except for a broader range of several quantum Hall plateaus corresponding to small filling factors in the electron region. We investigated this anomaly on the basis of localized states owing to the presence of possible electron traps, whose energy levels were estimated to be near the Dirac point. In particular, the inequality between the filling of electrons and holes was ascribed to the requirement of excess electrons to fill the trap levels. The relations between the quantum Hall plateau, Landau level, and filling factor were carefully analyzed to reveal the details of the localized states in this graphene device.11Nsciescopu

Coulomb scattering mechanism transition in 2D layered MoTe2: effect of high-kappa passivation and Schottky barrier height

Clean interface and low contact resistance are crucial requirements in two-dimensional (2D) materials to preserve their intrinsic carrier mobility. However, atomically thin 2D materials are sensitive to undesired Coulomb scatterers such as surface/interface adsorbates, metal-tosemiconductor Schottky barrier (SB), and ionic charges in the gate oxides, which often limits the understanding of the charge scattering mechanism in 2D electronic systems. Here, we present the effects of hafnium dioxide (HfO2) high-kappa passivation and SB height on the low-frequency (LF) noise characteristics of multilayer molybdenum ditelluride (MoTe2) transistors. The passivated HfO2 passivation layer significantly suppresses the surface reaction and enhances dielectric screening effect, resulting in an excess electron n-doping, zero hysteresis, and substantial improvement in carrier mobility. After the high-kappa HfO2 passivation, the obtained LF noise data appropriately demonstrates the transition of the Coulomb scattering mechanism from the SB contact to the channel, revealing the significant SB noise contribution to the 1/f noise. The substantial excess LF noise in the subthreshold regime is mainly attributed to the excess metal-to-MoTe2 SB noise and is fully eliminated at the high drain bias regime. This study provides a clear insight into the origin of electronic signal perturbation in 2D electronic systems © 2018 IOP Publishing Ltd Printed in the U

Low-Voltage-Operated Highly Sensitive Graphene Hall Elements by Ionic Gating

The advanced Hall magnetic sensor using an ion-gated graphene field-effect transistor demonstrates a high current-normalized sensitivity larger than 3000 V/AT and low operation voltages smaller than 0.5 V. From commercially available grapheneon-SiO2 wafers, large-area arrays of ion-gated graphene Hall element (ig-GHE) samples are prepared through complementary metal-oxide-semiconductor-compatible fabrication processes except the final addition of ionic liquid electrolyte covering the exposed graphene channel and the separate gate-electrode area. The enhanced carrier tunability by ionic gating enables this ig-GHE device to be extremely sensitive to magnetic fields in low-voltage-operation regimes. Further electrical characterization indicates that the operation window is limited by the nonuniform carrier concentration over the channel under high bias conditions. The drain-current-normalized magnetic resolution of the device measured using the low-frequency noise technique is comparable to the previously reported values despite its significant low power consumption. © 2019 American Chemical Society

Passivation effect on gate-bias stress instability of carbon nanotube thin film transistors

A prior requirement of any developed transistor for practical use is the stability test. Random network carbon nanotube-thin film transistor (CNT-TFT) was fabricated on SiO2/Si. Gate bias stress stability was investigated with various passivation layers of HfO2 and Al 2O3. Compared to the threshold voltage shift without passivation layer, the measured values in the presence of passivation layers were reduced independent of gate bias polarity except HfO2 under positive gate bias stress (PGBS). Al2O3 capping layer was found to be the best passivation layer to prevent ambient gas adsorption, while gas adsorption on HfO2 layer was unavoidable, inducing surface charges to increase threshold voltage shift in particular for PGBS. This high performance in the gate bias stress test of CNT-TFT even superior to that of amorphous silicon opens potential applications to active TFT industry for soft electronics. © 2014 AIP Publishing LLC.1681sciescopu

MoTe2 on ferroelectric single-crystal substrate in the dual-gate field-effect transistor operation

© 2021 Korean Physical Society. An exfoliated MoTe2 flake in contact with a ferroelectric single-crystal substrate was studied to examine its charge carrier modulation by neighboring ferroelectric polarization. A MoTe2 field-effect transistor was fabricated, having a hexagonal-BN (hBN) flake and a ferroelectric substrate employed as top and bottom gate dielectrics. In the dual-gate operation, the charge conduction exhibited an ambipolar behavior with large hysteresis during the gate voltage sweep. It mainly originates from the ferroelectric nature in combination with the charge trap phenomena at the interfaces. Interestingly, we found out that holes are more easily trapped than electrons, and charge carriers in MoTe2 are easily modulated through the top hBN gate when the electron conduction is predominantly set by the bottom ferroelectric field. However, the controllability becomes much weaker under opposing ferroelectric polarizations. This unbalanced controllability reveals the interfacial hole-trap effect resulting from ferroelectric polarization.11Nsciescopuskc