Chanwoo Yang

Bending-stress-driven phase transitions in pentacene thin films for flexible organic field-effect transistors

The effects of bending strain on the structure and electrical characteristics of pentacene films in flexible devices were investigated. It was found that the volume fraction of bulk phase in the pentacene film increases from 10.7% to 27.7% under 1.1% of tensile strain but decreases to 3.5% under 1.0% of compressive strain. These bending-stress-driven phase transitions between the bulk phase and the thin-film phase in the pentacene film resulted in the changes in field-effect mobility, and were driven by the differences between the in-plane dimensions of the crystal unit cells of the two phases to reduce the external bending stress.

Universal Ti-rich termination of atomically flat SrTiO3 (001), (110), and (111) surfaces

We have studied the surface termination of atomically flat SrTiO3 surfaces treated by chemical etching and subsequent thermal annealing, for all commercially available orientations (001), (110), and (111). Atomic force microscopy confirms that our treatment processes produce unit cell steps with flat terrace structures. We have also determined the topmost atomic layer of SrTiO3 surfaces through time-of-flight mass spectroscopy. We found that all three orientations exhibit a Ti-rich surface. Our observation opens doors for interface engineering along the [110] and [111] directions in addition to a well known [100] case, which widens the range of functional heterostructures and interfaces.

Effect of the hydrophobicity and thickness of polymer gate dielectrics on the hysteresis behavior of pentacene-based field-effect transistors

We demonstrate the origin and mechanism of the hysteresis behavior that is frequently observed during the operation of organic field-effect transistors (OFETs) based on polymer gate dielectrics. Although polar functionalities, such as hydroxyl groups, present in the polymer gate dielectrics are known to induce hysteresis, there have only been a few detailed investigations examining how the presence of such end functionalities both at the polymer surface—forming an interface with the semiconductor layer—and in the bulk influences the hysteresis. In this study, we control the hydrophobicity of the polymer by varying the number of hydroxyl groups, and use an ultrathin polymer/SiO2 bilayer and a thick single polymer as the gate dielectric structure so that the hysteresis behavior is divided into contributions from hydroxyl groups present at the polymer surface and in the bulk, respectively. Electrical characterizations of the OFETs, performed both in vacuum (≈10−3 Torr) and in ambient air (relative humidity o...

Effects of polymer gate dielectrics roughness on pentacene field-effect transistors

The effects of the surface roughness of the polymer gate dielectrics on pentacene field-effect transistors were investigated. Using a poly(methylmethacrylate)/anodized Al2O3 dual-layer gate dielectric, the root-mean-square roughness of the gate dielectrics varied from 0.45 to 1.51nm, independently of other gate dielectric properties such as the capacitance and surface energy. This range of root-mean-square roughnesses had little effect on the carrier mobility. X-ray diffraction analyses further revealed that the roughness of poly(methylmethacrylate) neither decreased the degree of crystallinity nor distorted the crystalline structure of pentacene.

Origin of high mobility within an amorphous polymeric semiconductor: Space-charge-limited current and trap distribution

To elucidate the origin of the high field-effect mobility (≈0.02cm2∕Vs) of amorphous poly[(1,2-bis-(2′-thienyl)vinyl-5′,5″-diyl)-alt-(9,9-dioctyldecylfluorene-2,7-diyl], we investigated the current density–voltage (J-V) and mobility–voltage (μ-V) relationships as a function of temperature. By using the power law model and the Gaussian hopping model, we determined a characteristic trap energy of 67meV, an energetic disorder parameter of 64meV, and a total trap density of 2.5×1016cm−3, comparable to that of poly(3-hexylthiophene). We conclude that the relatively low trap density, which originates from the grain-boundary-free amorphous nature of the semiconductor, enables this high field-effect mobility.

High-performance solution-processed triisopropylsilylethynyl pentacene transistors and inverters fabricated by using the selective self-organization technique

To obtain selectively self-organized active layers of organic field-effect transistors (OFETs) and inverters from a solution-phased triisopropylsilylethynyl pentacene (TIPS-PEN) semiconductor, we locally patterned an oxide dielectric, covered with a hydrophobic fluoropolymer, using O2 plasma etching. Drop-cast TIPS-PEN molecules were selectively crystallized on the O2-plasma-etched area, where a hydrophilic oxide surface was produced. Modification of the patterned oxide dielectrics with hexamethyldisilazane led to a field-effect mobility of the TIPS-PEN OFETs of 0.185 cm2 V−1 s−1, a substhreshold swing of 0.738 V/decade and an on/off ratio of 107. Moreover, an inverter composed of two of these OFETs showed good device operation and inverter gain of 5.6.

Solution-processed flexible ZnO transparent thin-film transistors with a polymer gate dielectric fabricated by microwave heating

We report the development of solution-processed zinc oxide (ZnO) transparent thin-film transistors (TFTs) with a poly(2-hydroxyethyl methacrylate) (PHEMA) gate dielectric on a plastic substrate. The ZnO nanorod film active layer, prepared by microwave heating, showed a highly uniform and densely packed array of large crystal size (58 nm) in the [002] direction of ZnO nanorods on the plasma-treated PHEMA. The flexible ZnO TFTs with the plasma-treated PHEMA gate dielectric exhibited an electron mobility of 1.1 cm(2) V(-1) s(-1), which was higher by a factor of approximately 8.5 than that of ZnO TFTs based on the bare PHEMA gate dielectric.

Hysteresis behaviour of low-voltage organic field-effect transistors employing high dielectric constant polymer gate dielectrics

Here, we report on the fabrication of low-voltage-operating pentacene-based organic field-effect transistors (OFETs) that utilize crosslinked cyanoethylated poly(vinyl alcohol) (CR-V) gate dielectrics. The crosslinked CR-V-based OFET could be operated successfully at low voltages (below 4 V), but abnormal behaviour during device operation, such as uncertainty in the field-effect mobility (μ) and hysteresis, was induced by the slow polarization of moieties embedded in the gate dielectric (e.g. polar functionalities, ionic impurities, water and solvent molecules). In an effort to improve the stability of OFET operation, we measured the dependence of μ and hysteresis on dielectric thickness, CR-V crosslinking conditions and sweep rate of the gate bias. The influence of the CR-V surface properties on μ, hysteresis, and the structural and morphological features of the pentacene layer grown on the gate dielectric was characterized and compared with the properties of pentacene grown on a polystyrene surface.

Dependence of Pentacene Crystal Growth on Dielectric Roughness for Fabrication of Flexible Field-Effect Transistors

The dependence of pentacene nanostructures on gate dielectric surfaces were investigated for flexible organic field-effect transistor (OFET) applications. Two bilayer types of polymer/aluminum oxide (Al(2)O(3)) gate dielectrics were fabricated on commercial Al foils laminated onto a polymer back plate. Some Al foils were directly used as gate electrodes, and others were smoothly polished by an electrolytic etching. These Al surfaces were then anodized and coated with poly(alpha-methyl styrene) (PAMS). For PAMS/Al(2)O(3) dielectrics onto etched Al foils, surface roughness up to approximately 1 nm could be reached, although isolated dimples with a lateral diameter of several micrometers were still present. On PAMS/Al(2)O(3) dielectrics (surface roughness >40 nm) containing mechanical grooves of Al foil, average hole mobility (mu(FET)) of 50 nm thick pentacene-FETs under the low operating voltages (|V| < 6 V) was approximately 0.15 cm(2) V(-1) s(-1). In contrast, pentacene-FETs employing the etched Al gates exhibited mu(FET) of approximately 0.39 cm(2) V(-1) s(-1), which was comparable to that of reference samples with PAMS/Al(2)O(3) dielectrics onto flat sputtered Al gates. Conducting-probe atomic force microscopy and two-dimensional X-ray diffraction of pentacene films with various thicknesses revealed different out-of-plane and in-plane crystal orderings of pentacene, depending on the surface roughness of the gate dielectrics.

Hysteresis-free organic field-effect transistors and inverters using photocrosslinkable poly(vinyl cinnamate) as a gate dielectric

We have fabricated organic field-effect transistors (OFETs) and inverters using photocrosslinkable poly(vinyl cinnamate) (PVCN) as a gate dielectric. The photocrosslinked PVCN dielectric film has superior insulating properties and does not require thermal curing. The high water resistance of the dielectric, which arises because PVCN is hydroxyl group-free, means that the devices were found to be hysteresis-free in all operations. The OFETs with the PVCN dielectric were found to exhibit a carrier mobility of 0.51cm2∕Vs, an on/off ratio of 106, and a subthreshold swing of 0.913V/decade. An organic inverter consisting of two OFETs exhibited a high inverter gain of 17.9.