Seul Ki Hong

Flexible Resistive Switching Memory Device Based on Graphene Oxide

A resistive switching memory device based on graphene oxide (GO) is presented. It is found that the resistive switching characteristic has a strong dependence on electrode material and GO thickness. In our experiment, an Al/GO/ITO structure with 30-nm-thick GO shows good switching performance with an on/off resistance ratio of 103, low set/reset voltage, and excellent data retention. The GO memory is also fabricated on a flexible substrate with no degradation in switching property, even when the substrate is bent down to 4-mm radius, indicating that the GO memory is an excellent candidate to be a memory device for future flexible electronics.

Electromagnetic interference shielding effectiveness of monolayer graphene

We report the first experimental results on the electromagnetic interference (EMI) shielding effectiveness (SE) of monolayer graphene. The monolayer CVD graphene has an average SE value of 2.27 dB, corresponding to ~40% shielding of incident waves. CVD graphene shows more than seven times (in terms of dB) greater SE than gold film. The dominant mechanism is absorption rather than reflection, and the portion of absorption decreases with an increase in the number of graphene layers. Our modeling work shows that plane-wave theory for metal shielding is also applicable to graphene. The model predicts that ideal monolayer graphene can shield as much as 97.8% of EMI. This suggests the feasibility of manufacturing an ultrathin, transparent, and flexible EMI shield by single or few-layer graphene.

Analysis on switching mechanism of graphene oxide resistive memory device

Recently, a flexible resistive switching memory device using graphene oxide was successfully demonstrated. In this work, the new findings on the switching mechanism of the graphene oxide memory are presented through a comprehensive study on the switching phenomena. It has been found that the switching operation of graphene oxide resistive switching memory (RRAM) is governed by dual mechanism of oxygen migration and Al diffusion. However, the Al diffusion into the graphene oxide is the main factor to determine the switching endurance property which limits the long term lifetime of the device. The electrode dependence on graphene oxide RRAM operation has been analyzed as well and is attributed to the difference in surface roughness of graphene oxide for the different bottom electrodes.

High-angle tilt boundary graphene domain recrystallized from mobile hot-wire-assisted chemical vapor deposition system.

Crystallization of materials has attracted research interest for a long time, and its mechanisms in three-dimensional materials have been well studied. However, crystallization of two-dimensional (2D) materials is yet to be challenged. Clarifying the dynamics underlying growth of 2D materials will provide the insight for the potential route to synthesize large and highly crystallized 2D domains with low defects. Here, we present the growth dynamics and recrystallization of 2D material graphene under a mobile hot-wire assisted chemical vapor deposition (MHW-CVD) system. Under local but sequential heating by MHW-CVD system, the initial nucleation of nanocrystalline graphenes, which was not extended into the growth stage due to the insufficient thermal energy, took a recrystallization and converted into a grand single crystal domain. During this process, the stitching-like healing of graphene was also observed. The local but sequential endowing thermal energy to nanocrystalline graphenes enabled us to simultaneously reveal the recrystallization and healing dynamics in graphene growth, which suggests an alternative route to synthesize a highly crystalline and large domain size graphene. Also, this recrystallization and healing of 2D nanocrystalline graphenes offers an interesting insight on the growth mechanism of 2D materials.

High performance graphene field effect transistors on an aluminum nitride substrate with high surface phonon energy

We demonstrate top-gate graphene field effect transistors (FETs) on an aluminum nitrite (AlN) substrate with high surface phonon energy. Electrical transport measurements reveal significant improvement of the carrier mobility of graphene FETs on AlN compared to those on SiO2. This is attributed to the suppression of surface phonon scattering due to the high surface phonon energy of the AlN substrate. The RF cut-off frequency of the graphene FET is also greatly increased when the AlN substrate is used. AlN can easily be formed on a Si or SiO2 substrate using a standard semiconductor process and thus provides a practical way to improve the performance of graphene FETs.

Free‐Standing Graphene Thermophone on a Polymer‐Mesh Substrate

A graphene thermoacoustic loudspeaker with a thin polymer mesh is fabricated using screen-printing. An experiment with substrates of various free-standing areas shows that a higher sound pressure level can be achieved as compared to previously reported graphene thermoacoustic loudspeakers. Moreover, a modified equation to predict the sound pressure level of the thermoacoustic loudspeaker with a thin and patterned substrate is proposed and verified by experimental results.

Non-volatile memory using graphene oxide for flexible electronics

A non-volatile memory device using graphene oxide (GO) as a resistive switching element is demonstrated. It is found that the electrode materials and GO thickness is critical factors to determine the s witching properties of devices. The Al/GO/ITO structure with 30 nm thick GO shows On/Off current ratio of 103. In addition, the GO memory device exhibits excellent performance when applied to flexible substrate, with good reliability and flexibility.

Resistance analysis and device design guideline for graphene RF transistors

Graphene has attracted enormous attention in recent years because of its high carrier mobility and saturation velocity. High-performance graphene transistors for radio-frequency (RF) applications are especially attractive. Synthesis of high quality graphene sheets and application of various materials for gate dielectrics and substrates have been demonstrated. However, very few studies have been performed on the effects of graphene transistor parameters, such as parasitic resistances and graphene quality, in relation to RF applications. Here we report a systematic study of those effects on electrical performance depending on the transistor structure. It is found that the access resistance and contact resistance are the dominant factors leading to degradation of the device performance, especially in deep scaled devices. A guideline for device structural parameter design for required RF performance is discussed. Furthermore, we demonstrate that the newly proposed self-aligned structure can minimize access resistance component, resulting in 6 times higher cut-off frequency compared to that of the conventional structure, when the gate length is 50 nm. The findings of this study can be used to predict the device RF performance and thus help the design of graphene transistor structures to meet specific requirements.

Graphene-based EMI shielding for vertical noise coupling reduction in 3D mixed-signal system

Vertical noise coupling caused by the near-field coupling between the RF/analog IC and logic IC is a severe problem in 3D mixed-signal systems. To reduce the vertical noise coupling, graphene is an appropriate material due to its inherent characteristics such as very low thickness, high flexibility, high mechanical strength, and EMI absorbing characteristic. Especially, the EMI absorbing characteristic is an important property as the shield to prevent the vertical noise coupling, as it reduces the re-coupling of the reflected EMI by the shield into other ICs. We measure the reduction of the vertical noise coupling by the mono-layer graphene shield in the 3D mixed-signal system composed of a low noise amplifier (LNA), an on-chip switching model DC-DC converter, and the mono-layer graphene in the frequency and time domain. Through the measurement results, we observed that the mono-layer graphene can maximally reduce the vertical noise coupling by -17 dB in the frequency domain. Additionally, the vertically coupled noise is reduced by 25% in the time domain measurement.

Vertically Formed Graphene Stripe for 3D Field-Effect Transistor Applications

A 100-nm wide, vertically formed graphene stripe (GS) is demonstrated for three-dimensional (3D) electronic applications. The GS forms along the sidewall of a thin nickel film. It is possible to further scale down the GS width by engineering the deposited thickness of the atomic layer deposition (ALD) Ni film. Unlike a conventional GS or graphene nanoribbon (GNR), the vertically formed GS is made without a graphene transfer and etching process. The process integration of the proposed GS FETs resembles that of currently commercialized vertical NAND flash memory with a design rule of less than 20 nm, implying practical usage of this formed GS for 3D advanced FET applications.