Myung Han Yoon

High performance ZnO nanowire field effect transistors with organic gate nanodielectrics: effects of metal contacts and ozone treatment

High performance ZnO nanowire field effect transistors (NW-FETs) were fabricated using a nanoscopic self-assembled organic gate insulator and characterized in terms of conventional device performance metrics. To optimize device performance and understand the effects of interface properties, devices were fabricated with both Al and Au/Ti source/drain contacts, and device electrical properties were characterized following annealing and ozone treatment. Ozone-treated single ZnO NW-FETs with Al contacts exhibited an on-current (Ion) of ~4 µA at 0.9 Vgs and 1.0 Vds, a threshold voltage (Vth) of 0.2 V, a subthreshold slope (S) of ~130 mV/decade, an on–off current ratio (Ion:Ioff) of ~107, and a field effect mobility (μeff) of ~1175 cm2 V−1 s−1. In addition, ozone-treated ZnO NW-FETs consistently retained the enhanced device performance metrics after SiO2 passivation. A 2D device simulation was performed to explain the enhanced device performance in terms of changes in interfacial trap and fixed charge densities.

Organic field-effect transistors based on a crosslinkable polymer blend as the semiconducting layer

For fabrication of top-gate polymer-based organic field-effect transistors (OFETs), it is essential that the semiconducting layer remain intact during spin coating of the overlying dielectric layer. This requirement severely limits the applicable solvent and materials combinations. We show here that a crosslinkable polymer blend consisting of a p-type semiconducting polymer {e.g., TFB; poly[9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine]} and an electroactive crosslinkable silyl reagent {e.g., TPDSi2; 4,4′-bis[(p-trichloro-silylpropylphenyl)phenylamino]biphenyl} is effective as the semiconducting layer in a top-gate bottom-contact OFET device. The TFB+TPDSi2 semiconducting blend is prepared by spin-coating in ambient. The crosslinking process occurs during spin-coating in air and is completed by curing at 90 °C, which renders the resulting film insoluble in common organic solvents and allows subsequent deposition of dielectric layers from a wide range of organic solvents. We also show that the pr...

ZnO Nanowire Field-Effect Transistors: Ozone-Induced Threshold Voltage Shift and Multiple Nanowire Effects

ZnO nanowire field-effect transistors (NW-FETs) employing single nanowires were fabricated, using a self-assembled superlattice (SAS) as the gate insulator. Both depletion-mode and enhancement-mode ZnO NW-FETs were fabricated and characterized. An electrostatic model is proposed to describe observed threshold voltage shift upon optimum ozone treatment. Temperature-dependent current-voltage characteristics of depletion-mode ZnO NW-FETs verify this model, indicating the existence of body current through ZnO nanowires with low activation energy. In addition, NW-FETs that use multiple ZnO nanowires and a SiO2gate insulator were fabricated to achieve higher on-current without significant degradation in on-off current ratio, threshold voltage shift, and subthreshold slopes.

Electron-transporting thiophene-based semiconductors exhibiting very high field effect mobilities

Organic semiconductors exhibiting complementary n-type carrier mobility are the key components for the development of the field of “plastic electronics”. We present here a novel series of oligothiophenes designed to improve performance and stability under electron- transporting conditions. Furthermore, the key structural features of these compounds allows additional modifications of the n-type conducting core to achieve material solubility and processability. Thin film transistor (TFT) devices were fabricated employing both vacuum- and solution-deposited semiconducting layers. Field-effect transistor measurements indicate that all the members of this new series are n-type semiconductors with mobilities and I on :I off ratios approaching 1 cm 2 /(Vs) and 10 7 , respectively. This family represents a key milestone in the design, understanding, and development of the next generation of highly efficient n-type OTFT components.

Performance Enhancement of ZnO Nanowire Field-effect Transistors with Self-Assembled Organic Nanodielectrics

Conventional display circuits are built using poly-silicon thin-film transistors (poly-TFTs). However, poly-TFTs are not transparent, causing inefficiency in the aperture ratio on active matrix arrays and, correspondingly, increased power consumption. They also lack flexibility and compatibility with plastic substrates, which are two important requirements for future flexible display devices. In this sense, the development of display devices has been focused on enhancing transparency and flexibility while maintaining or enhancing other device metrics such as on-current, on-off ratio, and subthreshold slope. One promising candidate that satisfies these requirements is ZnO nanowire field-effect transistors (ZnO NW-FETs) because ZnO is a transparent material with a wide bandgap (3.37 eV) and nanowires are known to have inherent flexibility. However, the previously reported device metrics of ZnO NW-FETs were not good enough to replace poly-TFTs, especially in terms of mobility. [1,2] In this study, we report high mobility ZnO NW-FETs using a self-assembled organic superlattice (SAS) as a gate insulator, and investigations of the dependence of current-voltage characteristics of devices using SiO2 insulators on ozone and oxygen plasma treatments. FETs containing single ZnO-NWs were fabricated using a device structure (Fig. 1) in a typical backgate configuration using a heavily doped n-type Si substrate as a common gate. The 15nm SAS film used in this study consists of four interlinked layer-by-layer self-assembled organic monolayers. Fig. 2 illustrates the excellent insulating properties of SAS with a large specific capacitance, 180 nF/cm, and a low leakage current density, 1×10 A/cm. SAS-based ZnO NW-FETs exhibits excellent drain current saturation at Vds = 0.5V, threshold voltage (Vth) of -0.4V, a channel mobility of ~ 196 cm/V-sec, an on/off ratio of ~10, and a subthreshold slope of 400 mV/dec, as shown in Fig. 3. The mobility of SAS-based ZnO NW-FETs is far greater than recently reported values (8~18 cm2/V-sec) of SiO2-based ZnO NWFETs [1,2] and even comparable to that of poly-TFTs. In addition, the inherent flexibility of SAS might be an enabling technology for low-power flexible display devices. An appropriate annealing treatment is also expected to reduce interfacial trap density in the SAS, improving subthreshold slope and threshold voltage shift. [3] ZnO NW-FETs with SiO2 gate dielectrics were also fabricated, and exhibited device performance comparable to that reported in the literature. To optimize the device performance of ZnO NW-FETs, investigations on several annealing and surface treatments are in progress. It has been observed that the oncurrent of SiO2-based ZnO NW-FETs was increased by an order of magnitude, with steeper subthreshold slope, by using ozone or oxygen plasma treatments.

Materials for n-type organic electronics: synthesis and properties of fluoroarene-thiophene semiconductors

Recent progress in the field of organic electronics is due to a fruitful combination of both innovative molecular design and promising low-cost material/device assembly. Targeting the first strategy, we present here the general synthesis of fluoroarene-containing thiophene-based semiconductors and the study of their properties with respect to the corresponding fluorine-free hole-transporting analogues. The new compounds have been characterized by elemental analysis, mass spectrometry, and 1H- and 19F NMR. The dramatic influence of fluorine substitution and molecular architecture has been investigated by solution/film optical absorption, fluorescence emission, and cyclic voltammetry. Single crystal data for all of the oligomers have been obtained and will be presented. Film microstructure and morphology of this new class of materials have been studied by XRD and SEM. Particular emphasis will be posed on the solution-processable oligomers and polymers.