Chunhum Cho

Flexible organic solar cells composed of P3HT:PCBM using chemically doped graphene electrodes

Flexible organic solar cells (OSCs) composed of blended films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were fabricated and investigated with chemically doped multilayer graphene films as transparent and conducting electrodes on plastic substrates. The sheet resistance of the chemically doped graphene film was reduced to half of its original value, resulting in a significant performance enhancement of OSCs featuring doped graphene electrodes. Moreover, there was no substantial variation observed in the fill factor and power conversion efficiency values of the flexible OSCs under bending conditions. A power conversion efficiency of ~2.5% for flexible OSCs with doped graphene electrodes was observed under bending conditions, even up to a 5.2 mm bending radius.

Effects of multi-layer graphene capping on Cu interconnects

The benefits of multi-layer graphene (MLG) capping on Cu interconnects have been experimentally demonstrated. The resistance of MLG capped Cu wires improved by 2-7% compared to Cu wires. The breakdown current density increased by 18%, suggesting that the MLG can act as an excellent capping material for Cu interconnects, improving the reliability characteristics. With a proper process optimization, MLG capped Cu interconnects could become a promising technology for high density back end-of-line interconnects.

Low-temperature-grown continuous graphene films from benzene by chemical vapor deposition at ambient pressure.

There is significant interest in synthesizing large-area graphene films at low temperatures by chemical vapor deposition (CVD) for nanoelectronic and flexible device applications. However, to date, low-temperature CVD methods have suffered from lower surface coverage because micro-sized graphene flakes are produced. Here, we demonstrate a modified CVD technique for the production of large-area, continuous monolayer graphene films from benzene on Cu at 100–300 °C at ambient pressure. In this method, we extended the graphene growth step in the absence of residual oxidizing species by introducing pumping and purging cycles prior to growth. This led to continuous monolayer graphene films with full surface coverage and excellent quality, which were comparable to those achieved with high-temperature CVD; for example, the surface coverage, transmittance, and carrier mobilities of the graphene grown at 300 °C were 100%, 97.6%, and 1,900–2,500 cm2 V−1 s−1, respectively. In addition, the growth temperature was substantially reduced to as low as 100 °C, which is the lowest temperature reported to date for pristine graphene produced by CVD. Our modified CVD method is expected to allow the direct growth of graphene in device manufacturing processes for practical applications while keeping underlying devices intact.

Enhanced Current Drivability of CVD Graphene Interconnect in Oxygen-Deficient Environment

Graphene has been considered as a candidate for interconnect metal due to its high carrier mobility and current drivability. In this letter, the breakdown mechanism of single-layer chemical-vapor-deposited (CVD) graphene and triple-layer CVD graphene has been investigated at three different conditions (air exposed, vacuum, and dielectric capped) to identify a failure mechanism. In vacuum, both single- and triple-layer graphenes demonstrated a breakdown current density as high as ~108 A/cm2, which is similar to that of exfoliated graphene. On the other hand, the breakdown current of graphene exposed to air was degraded by one order of magnitude from that of graphene tested in vacuum. Thus, oxidation initiated at the defect sites of CVD graphene was suggested as a major failure mechanism in air, while Joule heating was more dominant with dielectric capping and in vacuum.

Characteristics of a pressure sensitive touch sensor using a piezoelectric PVDF-TrFE/MoS2 stack

A new touch sensor device has been demonstrated with molybdenum disulfide (MoS2) field effect transistors stacked with a piezoelectric polymer, polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE). The performance of two device stack structures, metal/PVDF-TrFE/MoS2 (MPM) and metal/PVDF-TrFE/Al2O3/MoS2 (MPAM), were compared as a function of the thickness of PVDF-TrFE and Al2O3. The sensitivity of the touch sensor has been improved by two orders of magnitude by reducing the charge scattering and enhancing the passivation effects using a thin Al2O3 interfacial layer. Reliable switching behavior has been demonstrated up to 120 touch press cycles.

Excellent resistive switching in nitrogen-doped Ge2Sb2Te5 devices for field-programmable gate array configurations

The nitrogen-doped Ge2Sb2Te5 (GST) programmable metallization cell is investigated to address the low switching voltage and need for a high resistance ratio, which are critical issues for the filed-programmable gate array (FPGA) configuration. Nitrogen doping of GST yields Ge–N covalent bonds, as confirmed by x-ray photoelectron spectroscopy and Raman spectroscopy; this increases the resistivity of GST. Consequently, an excellent resistance ratio (∼107) with appropriate operating voltage and stable retention properties more than for 104 s at 85 °C are achieved. The results indicate that the film is a suitable alternative candidate for the logic switch in static-random-access-memory-based FPGA technology.

Influence of extrinsic factors on accuracy of mobility extraction in graphene metal-oxide-semiconductor field effect transistors

Graphene has attracted attention because of its extraordinarily high mobility. However, procedures to extract mobility from graphene metal-oxide semiconductor transistors have not been systematically established because the accuracy of mobility value is affected by many extrinsic parameters. In this work, the influence of extrinsic parameters, such as contact resistance, transient charging effect, measurement temperature, and ambient on mobility are examined in order to provide a protocol capable of accurately assessing the mobility of graphene metal-oxide-semiconductor field effect transistors. Using a well controlled test protocol, the mobility of graphene is found to be temperature independent up to 450 K.

Electrical characteristics of wrinkle-free graphene formed by laser graphitization of 4H-SiC

Wrinkle free few layer graphene was demonstrated by a graphitization of 4H-SiC substrates using a high power pulsed KrF laser. Wrinkles often observed after thermal graphitization were eliminated with a short heat cycle using a pulse laser anneal. Few layer graphene formed by the laser graphitization appears to have a non-Bernal stack, which leads to on-off ratio of ∼2 even at a few layer graphene. Drive current of 143 μA/μm was obtained at Vd = 100 mV and field effect mobility was 374 cm2/Vs.

Correlation between the hysteresis and the initial defect density of graphene

The role of the initial defects of graphene characterized by Raman spectroscopy is correlated with the physical mechanisms causing the hysteretic device characteristics of graphene field effect transistors (FETs). Fast charging related to the tunneling-induced charge exchange is found to be closely correlated with the initial defect density, while slow charging related to environmental influences such as the water redox reaction showed a weak correlation. It can be concluded that the intrinsic quality of graphene should be improved to minimize the hysteresis of graphene FETs even in an air-tight environment.

Contact resistance reduction using Fermi level de-pinning layer for MoS 2 FETs

Achieving a low contact resistance for 2D materials is a critical challenge for device applications. In this work, the contact resistance of MoS2 FETs has been drastically reduced by five times from the reference data using an optimized TiO2 Fermi level de-pinning layer which reduced the effective Schottky barrier height to 0.1 eV. As a result, a very low contact resistance ~5.4 kΩ·μm was achieved without any doping technique.