Young Gug Seol

Atomic layer deposition ZnO:N flexible thin film transistors and the effects of bending on device properties

ZnO:N flexible thin film transistors were fabricated by atomic layer deposition on polyethylene naphthalate substrates and the effects of bending on the device properties investigated. The threshold voltage and saturation mobility were observed to change with respect to the amount of substrate bending. These modulations can be explained in terms of piezoelectric nature of in ZnO. In comparison with the previously reported single crystal nanowires ZnO field effect transistors, the amount of the electrical property modulation under bent condition is significantly reduced and our report shows a much improved stability for ZnO:N as a flexible device material.

Mechanically flexible low-leakage multilayer gate dielectrics for flexible organic thin film transistors

The incorporation of an ultrathin, atomic layer deposited HfO2 layer in between the spin-coated poly-4-vinyl phenol (PVP) organic layers in the laminated multilayer gate dielectric for pentacene organic thin film transistors on a flexible substrate reduced the gate leakage current by three to four orders of magnitude and thereby significantly enhanced the current on/off ratio up to ≅104-fold. Cyclic bending testing indicated that the electrical characteristics of the device with the PVP∕HfO2∕PVP trilayer gate dielectric stack were superior to those of the device with the single PVP gate dielectrics due to the improved mechanical and electrical stabilities of the gate dielectric.

Nanocomposites of reduced graphene oxide nanosheets and conducting polymer for stretchable transparent conducting electrodes

Nanocomposites of functionalized reduced graphene oxide (FR-GO) nanosheets and a conducting polymer, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), were investigated for their applicability as stretchable transparent conducting electrode (TCE) materials. Adding FR-GO nanosheets functionalized by a surfactant, phenyl isocyanate, through the reduction of functionalized GO nanosheets into the PEDOT:PSS solution was an effective way of increasing the dispersibility of R-GO and, in turn, the electrical conductivity of the nanocomposite thin layers. A sheet resistance as low as ∼68 Ω □−1 and optical transmittance of ∼86% at the wavelength of 550 nm was obtained for the nanocomposite layer with FR-GO well-dispersed in the PEDOT:PSS matrix. A patterned nanocomposite electrode formed on a pre-stretched polydimethylsiloxane substrate showed potential applicability as an excellent TCE for transparent and stretchable electronics.

Reduction of Electrical Hysteresis in Cyclically Bent Organic Field Effect Transistors by Incorporating Multistack Hybrid Gate Dielectrics

We have fabricated flexible organic field effect transistors (OFETs) on polyimide substrate with low hysteresis and low leakage current under repetitive bending. The insertion of an ultrathin atomic-layer-deposited Al 2 O 3 layer in between spin-coated poly-4-vinyl phenol organic layers in a multistack hybrid gate dielectric for OFETs significantly improved stability in the electrical hysteresis during cyclic bending. The observed hysteresis stability for cyclically bent multistack hybrid OFET devices was attributed to efficient blocking of charges injected from the gate electrode due to improved mechanical stability. Cyclically bent samples showed no cracking for thinner Al 2 O 3 layers in the multistack hybrid gate dielectrics.

Improved electrical stability in cyclically bent organic thin film transistors with nanocomposite gate dielectrics and surface passivation

This paper investigates reliability improvements in cyclically bent bottom-gated pentacene organic thin film transistors by employing surface passivation as well as nanocomposite gate dielectrics. The variation in the hysteresis of the cyclically bent nanocomposite devices with increased Al2O3 nanoparticle fractions without surface passivation decreased. This was primarily attributed to the absence of change in charge trapping sites such as hydroxyl (OH) groups in the gate dielectrics due to reduced susceptibility of the nanocomposite dielectrics to penetration of water molecules. Furthermore, surface passivation of the devices by depositing organic thin films effectively improved their stability in the off-state current due to protection of the pentacene, which prevented penetration of ambient water.

Study of the Characteristics of Organic Thin Film Transistors with Plasma-Polymer Gate Dielectrics

The effects of gate dielectrics material in organic thin film transistors (OTFTs) were investigated. The gate dielectrics were deposited by plasma enhanced chemical vapor deposition (PECVD) with cyclohexane and tetraethylorthosilane (TEOS) respectively used as organic and inorganic precursors. The gate dielectrics (gate insulators) were deposited as either organic plasma-polymer or organic–inorganic hybrid plasma-polymer thin film by using cyclohexane or cyclohexane with TEOS, respectively. Additionally, hydrogen and argon were used as precursor bubbler gases. A polyimide (PI) substrate was used in the fabrication of pentacene OTFTs with a plasma-polymer gate insulator, an Au source–drain (S/D), and Cu gate electrodes. Different gate dielectrics were investigated. The as-grown plasma-polymer thin films were first analyzed using Fouriertransform infrared (FT-IR) spectroscopy. Also, they were analyzed by nano-indentation and capacitance measurements. The electrical properties, such as mobility and threshold voltage of the pentacene field-effect transistors with the plasma-polymer gate-dielectrics were investigated. Transistor with cyclohexane gate dielectric had a higher field-effect mobility, � FET ¼ 0:84cm 2 V � 1 s � 1 , and a smaller threshold voltage, VT ¼� 6:8V, than the transistor with the hybrid gate-dielectric. # 2011 The Japan Society of Applied Physics

NUMERICAL ANALYSIS ON THE MECHANICAL PROPERTIES OF ORGANIC THIN FILM TRANSISTOR

The organic thin film transistor (OTFT) on flexible substrate electroplated electrodes has many advantages as in the fabrication of low cost sensors, e-paper, smart cards, and flexible displays. In this study, we simulated the mechanical and electrical characteristics of the OTFT with various voltage conditions by using COMSOL. The model consisting of a channel, source and drain was employed to investigate the temperature distribution and thermal stress concentration. The channel length is 40 µm and the voltage ranged between -20V and -40V. The OTFT was fabricated using pentacene as a semiconducting layer and electroplated Ni as a gate electrode. Mechanical properties of the fabricated OTFT were characterized by thermal stress which was predicted with the result of stress distribution.