Byung Jin Cho

Graphene Oxide Thin Films for Flexible Nonvolatile Memory Applications

There has been strong demand for novel nonvolatile memory technology for low-cost, large-area, and low-power flexible electronics applications. Resistive memories based on metal oxide thin films have been extensively studied for application as next-generation nonvolatile memory devices. However, although the metal oxide based resistive memories have several advantages, such as good scalability, low-power consumption, and fast switching speed, their application to large-area flexible substrates has been limited due to their material characteristics and necessity of a high-temperature fabrication process. As a promising nonvolatile memory technology for large-area flexible applications, we present a graphene oxide based memory that can be easily fabricated using a room temperature spin-casting method on flexible substrates and has reliable memory performance in terms of retention and endurance. The microscopic origin of the bipolar resistive switching behavior was elucidated and is attributed to rupture and formation of conducting filaments at the top amorphous interface layer formed between the graphene oxide film and the top Al metal electrode, via high-resolution transmission electron microscopy and in situ X-ray photoemission spectroscopy. This work provides an important step for developing understanding of the fundamental physics of bipolar resistive switching in graphene oxide films, for the application to future flexible electronics.

Direct Measurement of Adhesion Energy of Monolayer Graphene As-Grown on Copper and Its Application to Renewable Transfer Process

Direct measurement of the adhesion energy of monolayer graphene as-grown on metal substrates is important to better understand its bonding mechanism and control the mechanical release of the graphene from the substrates, but it has not been reported yet. We report the adhesion energy of large-area monolayer graphene synthesized on copper measured by double cantilever beam fracture mechanics testing. The adhesion energy of 0.72 ± 0.07 J m(-2) was found. Knowing the directly measured value, we further demonstrate the etching-free renewable transfer process of monolayer graphene that utilizes the repetition of the mechanical delamination followed by the regrowth of monolayer graphene on a copper substrate.

A wearable thermoelectric generator fabricated on a glass fabric

The conversion of body heat into electrical energy using a thermoelectric (TE) power generator is useful for wearable self-powered mobile electronic systems such as medical sensors or smart watches. We herein demonstrate a glass fabric-based flexible TE generator using a screen printing technique and the self-sustaining structure of a TE device without top and bottom substrates. With this technique it is possible to make the device thin (∼500 μm), lightweight (∼0.13 g cm−2), and flexible. In addition, the developed TE generator achieved an unprecedentedly large output power density which is several tens of times higher than that of flexible TE generators reported to date. The developed TE generator shows an allowable bending radius of as low as 20 mm and no change in performance by repeated bending for 120 cycles. This work can expedite the development of wearable self-powered mobile devices.

Charge-transfer-based Gas Sensing Using Atomic-layer MoS2

Two-dimensional (2D) molybdenum disulphide (MoS2) atomic layers have a strong potential to be used as 2D electronic sensor components. However, intrinsic synthesis challenges have made this task difficult. In addition, the detection mechanisms for gas molecules are not fully understood. Here, we report a high-performance gas sensor constructed using atomic-layered MoS2 synthesised by chemical vapour deposition (CVD). A highly sensitive and selective gas sensor based on the CVD-synthesised MoS2 was developed. In situ photoluminescence characterisation revealed the charge transfer mechanism between the gas molecules and MoS2, which was validated by theoretical calculations. First-principles density functional theory calculations indicated that NO2 and NH3 molecules have negative adsorption energies (i.e., the adsorption processes are exothermic). Thus, NO2 and NH3 molecules are likely to adsorb onto the surface of the MoS2. The in situ PL characterisation of the changes in the peaks corresponding to charged trions and neutral excitons via gas adsorption processes was used to elucidate the mechanisms of charge transfer between the MoS2 and the gas molecules.

Determination of work function of graphene under a metal electrode and its role in contact resistance.

Although the work function of graphene under a given metal electrode is critical information for the realization of high-performance graphene-based electronic devices, relatively little relevant research has been carried out to date. In this work, the work function values of graphene under various metals are accurately measured for the first time through a detailed analysis of the capacitance-voltage (C-V) characteristics of a metal-graphene-oxide-semiconductor (MGOS) capacitor structure. In contrast to the high work function of exposed graphene of 4.89-5.16 eV, the work function of graphene under a metal electrode varies depending on the metal species. With a Cr/Au or Ni contact, the work function of graphene is pinned to that of the contacted metal, whereas with a Pd or Au contact the work function assumes a value of ∼4.62 eV regardless of the work function of the contact metal. A study of the gate voltage dependence on the contact resistance shows that the latter case provides lower contact resistance.

Ultrasmooth, extremely deformable and shape recoverable Ag nanowire embedded transparent electrode.

Transparent electrodes have been widely used in electronic devices such as solar cells, displays, and touch screens. Highly flexible transparent electrodes are especially desired for the development of next generation flexible electronic devices. Although indium tin oxide (ITO) is the most commonly used material for the fabrication of transparent electrodes, its brittleness and growing cost limit its utility for flexible electronic devices. Therefore, the need for new transparent conductive materials with superior mechanical properties is clear and urgent. Ag nanowire (AgNW) has been attracting increasing attention because of its effective combination of electrical and optical properties. However, it still suffers from several drawbacks, including large surface roughness, instability against oxidation and moisture, and poor adhesion to substrates. These issues need to be addressed before wide spread use of metallic NW as transparent electrodes can be realized. In this study, we demonstrated the fabrication of a flexible transparent electrode with superior mechanical, electrical and optical properties by embedding a AgNW film into a transparent polymer matrix. This technique can produce electrodes with an ultrasmooth and extremely deformable transparent electrode that have sheet resistance and transmittance comparable to those of an ITO electrode.

Electrical and Interfacial Characterization of Atomic Layer Deposited High-

In this paper, electrical and interfacial properties of MOS capacitors with atomic layer deposited (ALD) Al2O3, HfO2, and HfAlO gate dielectrics on sulfur-passivated (S-passivated) GaAs substrates were investigated. HfAlO on p-type GaAs has shown superior electrical properties over Al2O3 or HfO2 on GaAs, and it is attributed to the reduction of the Ga-O formation at the interfacial layer. HfAlO on p-type GaAs exhibits the best electrical properties after postdeposition annealing (PDA) at 500degC. It is found that PDA, at above 500degC, causes a significant amount of Ga and As out-diffusion into the high-k dielectric, which degrades the interface, as well as bulk high-k properties.

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Metal-insulator-metal (MIM) capacitors with different HfO/sub 2/ thickness have been investigated. The results show that both the capacitance density and voltage coefficients of capacitance (VCCs) increase with decreasing HfO/sub 2/ thickness. In addition, it is found that the VCCs decrease logarithmically with increasing thickness. Furthermore, the MIM capacitor with 10-nm HfO/sub 2/ shows a record high capacitance density of 13 fF//spl mu/m/sup 2/ and a VCC of 607 ppm/V, which can meet the requirement of the International Technology Roadmap for Semiconductors. It can also provide a low leakage current of 5.95 /spl times/ 10/sup -8/A/cm/sup 2/ at room temperature at 1 V, low tangent values below 0.05, and a small frequency dependence. These results indicate that the devices are suitable for use in silicon integrated circuit applications.

Gate Dielectrics on GaAs for Advanced CMOS Devices

Layered structures of transition metal dichalcogenides stacked by van der Waals interactions are now attracting the attention of many researchers because they have fascinating electronic, optical, thermoelectric, and catalytic properties emerging at the monolayer limit. However, the commonly used methods for preparing monolayers have limitations of low yield and poor extendibility into large-area applications. Herein, we demonstrate the synthesis of large-area MoSe2 with high quality and uniformity by selenization of MoO3 via chemical vapor deposition on arbitrary substrates such as SiO2 and sapphire. The resultant monolayer was intrinsically doped, as evidenced by the formation of charged excitons under low-temperature photoluminescence analysis. A van der Waals heterostructure of MoSe2 on graphene was also demonstrated. Interestingly, the MoSe2/graphene heterostructures show strong quenching of the characteristic photoluminescence from MoSe2, indicating the rapid transfer of photogenerated charge carriers between MoSe2 and graphene. The development of highly controlled heterostructures of two-dimensional materials will further promote advances in the physics and chemistry of reduced dimensional systems and will provide novel applications in electronics and optoelectronics.

A high-density MIM capacitor (13 fF/μm/sup 2/) using ALD HfO 2 dielectrics

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.