Peter Darmawan

Highly enhanced charge injection in thienoacene-based organic field-effect transistors with chemically doped contact

We describe the mechanism of contact resistance reduction and improvement in device performance of organic field-effect transistors by chemical doping at the contact interface. Insertion of iron(III)trichloride into the contact interface significantly reduced the contact resistance from 200 to 8.8 kΩ cm at a gate voltage of −40 V, and a field-effect mobility of 7.0 cm2/V s was achieved in devices based on dioctylbenzothienobenzothiophene. The improved charge injection is attributable to a reduction in the depletion layer thickness at the contact interface and occupation of trap states in the access region due to the generation of charge carriers by contact doping.

Solution-processed, Self-organized Organic Single Crystal Arrays with Controlled Crystal Orientation

A facile solution process for the fabrication of organic single crystal semiconductor devices which meets the demand for low-cost and large-area fabrication of high performance electronic devices is demonstrated. In this paper, we develop a bottom-up method which enables direct formation of organic semiconductor single crystals at selected locations with desired orientations. Here oriented growth of one-dimensional organic crystals is achieved by using self-assembly of organic molecules as the driving force to align these crystals in patterned regions. Based upon the self-organized organic single crystals, we fabricate organic field effect transistor arrays which exhibit an average field-effect mobility of 1.1 cm2V−1s−1. This method can be carried out under ambient atmosphere at room temperature, thus particularly promising for production of future plastic electronics.

Thin-film transistors fabricated by low-temperature process based on Ga- and Zn-free amorphous oxide semiconductor

We propose the use of indium tungsten oxide (IWO) as a channel material for thin-film transistors (TFTs). In the present study, an IWO film was deposited at room temperature by means of DC magnetron sputtering and then annealed at 100 °C in N2 prior to formation of Au source and drain electrodes. Analysis using X-ray diffraction and transmission electron microscopy revealed that the film remained amorphous even after the post-deposition annealing treatment. TFTs fabricated using a Si substrate as a back-gate electrode showed good performance, with a saturation field-effect mobility of 19.3 cm2 · V−1 · s−1, an on/off current ratio of 8.9 × 109.

On Practical Charge Injection at the Metal/Organic Semiconductor Interface

We have revealed practical charge injection at metal and organic semiconductor interface in organic field effect transistor configurations. We have developed a facile interface structure that consisted of double-layer electrodes in order to investigate the efficiency through contact metal dependence. The metal interlayer with few nanometers thickness between electrode and organic semiconductor drastically reduces the contact resistance at the interface. The improvement has clearly obtained when the interlayer is a metal with lower standard electrode potential of contact metals than large work function of the contact metals. The electrode potential also implies that the most dominant effect on the mechanism at the contact interface is induced by charge transfer. This mechanism represents a step forward towards understanding the fundamental physics of intrinsic charge injection in all organic devices.

Patterning solution-processed organic single-crystal transistors with high device performance

We report on the patterning of organic single-crystal transistors with high device performance fabricated via a solution process under ambient conditions. The semiconductor was patterned on substrates via surface selective deposition. Subsequently, solvent-vapor annealing was performed to reorganize the semiconductor into single crystals. The transistors exhibited field-effect mobility (μFET) of up to 3.5 cm2/V s. Good reliability under bias-stress conditions indicates low density of intrinsic defects in crystals and low density of traps at the active interfaces. Furthermore, the Y function method clearly suggests that the variation of μFET of organic crystal transistors was caused by contact resistance. Further improvement of the device with higher μFET with smaller variation can be expected when lower and more uniform contact resistance is achieved.

Reduction of charge injection barrier by 1-nm contact oxide interlayer in organic field effect transistors

The enhancement of the charge injection process by the insertion of an ultrathin (∼1 nm) contact oxide interlayer (COI) at the metal/organic material interface in organic field effect transistors (OFETs) is reported. Six different oxides were used as COI, and Al2O3 was found to exhibit the highest OFET mobility with a reduction in the average contact resistance (Rc) from 19.9 to 1.9 kΩ·cm. Photoelectron yield spectroscopy analysis revealed that the insertion of COI increases the work function of an Au contact and reduces the charge injection barrier at the interface, which lowers Rc and, therefore, results in enhanced device performance.

Thermal stability of rare-earth based ultrathin Lu2O3 for high-k dielectrics

Lu2O3 thin film was deposited on n-type (100) Si substrates using pulsed laser deposition. A k value of 15.95 with an equivalent oxide thickness (EOT) of 1.10nm and a current density of 2.6×10−5A∕cm2 at +1V accumulation bias is achievable for the 4.5nm thick Lu2O3 thin film deposited at room temperature after postdeposition annealing at 600°C in oxygen ambient. Annealing a similar sample at 900°C caused the EOT and leakage current density to increase to 1.68nm and 1×10−4A∕cm2, respectively. High resolution transmission electron microscopy analysis has shown that Lu2O3 film remains amorphous at high temperature annealing at 900°C. An x-ray reflectivity analysis on a separately prepared sample with lower annealing temperature (800°C) suggested a formation of Lu-based silicate layer. It is believed that the formation of low-k silicate layer may have contributed to the observed increase in EOT and the reduction in the k value.

Joule's law for organic transistors exploration: Case of contact resistance

Joules law opens a straightforward way to explore the operating mechanism of organic field-effect transistors, from the angle of inner transported heating. The microscopic dissipated power is calculated from the local conductivity and electric field, which solves the widespread difficulties in building a macroscopic model to determine the contact resistance. The result quantitatively discloses the correlation between the contact resistance and the charge transport properties covering energetic disorder, band-like and hopping transport as well as carrier mobility anisotropy. It turns out that in a staggered configuration with ideal Ohmic contact, the contact resistance is highly affected by the charge transport.

Effect of low fluence laser annealing on ultrathin Lu2O3 high-k dielectric

The effect of low fluence single pulse laser annealing on a pulsed laser deposited high-k dielectric, Lu2O3 is reported. With low fluence laser irradiation, high “k” of 45 is achieved with an equivalent oxide thickness of 0.39nm, without taking into account the quantum mechanical tunneling effect. High-resolution transmission electron microscopy micrograph revealed well-ordered epitaxial-like interfacial layer. High-resolution Rutherford backscattering confirmed the presence of Lu-based silicate layer at the interface. It was proposed that the high dielectric constant was caused by the increased ionic polarizability in the film, thereby increasing the ionic contribution of the dielectric constant.

Leakage conduction mechanism of amorphous Lu2O3 high-k dielectric films fabricated by pulsed laser ablation

Amorphous Lu2O3 thin films have been deposited on p-type (111) Si substrates by pulsed laser deposition (PLD). A equivalent oxide thickness (EOT) of 1.16 nm with a leakage current density of 4×10−5 A/cm2 at 1 V accumulation bias was obtained for 4.5 nm thick Lu2O3 thin film deposited at room temperature followed by post-deposition anneal (PDA) at 600 °C in oxygen ambient. The leakage conduction mechanisms of amorphous Lu2O3 films were investigated. It was found that the Poole-Frenkel (P-F) emission is the dominant conduction mechanism at high electric field region.