Yasuo Wada

Origin of characteristics differences between top and bottom contact organic thin film transistors

The differences in drain current and drain voltage (Id-Vd) characteristics of top and bottom contact organic thin film transistors (OTFTs) are analyzed by an OTFT devices simulator, which makes it possible to derive Id-Vd characteristics, potential distribution, and hole concentration distribution by solving Poisson’s equation and current continuity equation. It is found that the intrinsic characteristics of top contact devices are superior to those of the bottom contact ones, which is usually believed to be due to poor contact characteristics and poor semiconductor quality of bottom contact OTFTs. The mechanism behind the intrinsic characteristics differences is the deficiency of carriers at the source-channel interface, resulting to a very high potential drop, which the bottom contact devices suffer more. Remarkable improvements in drain current are expected by only inserting high carrier concentration region around the source/drain contact area, which totally eliminates the potential drop.

Graphoepitaxy of sexithiophene and orientation control by surface treatment

The factors influencing the graphoepitaxy of organic semiconductor α-sexithiophene (6T) on thermally oxidized silicon substrates were studied and it was discovered that a wider pitch in the microgrooves decreased the degree of graphoepitaxy. A more significant finding was that in-plane orientation could be changed by simple surface treatment. On UV∕ozone-treated substrates (hydrophilic condition), the b-axis of 6T was parallel to the grooves. Further surface treatment with hexamethyl-disiloxane (under hydrophobic conditions) changed this in-plane orientation by 90°. This change is due to the interaction between the topmost chemical species (functional groups) of the groove walls and organic molecules, a behavior peculiar to organic graphoepitaxy and exploitable for optimal orientation control in device processing. The nucleation and growth processes that cause the graphoepitaxy are discussed, based on the experimental results.

Molecular doping effect in bottom-gate, bottom-contact pentacene thin-film transistors

A bottom-gate, bottom-contact (BGBC) organic thin-film transistor (OTFT) with carrier-doped regions over source-drain electrodes was investigated. Device simulation with our originally developed device simulator demonstrates that heavily doped layers (p+ layers) on top of the source-drain contact region can compensate the deficiency of charge carriers at the source-channel interface during transistor operation, leading to the increase of the drain current and the apparent field-effect mobility. The phenomena expected with the device simulation were experimentally confirmed in typical BGBC pentacene thin-film transistors. The 5-nm-thick p+ layers, located 10 nm (or 20 nm) over the source-drain electrodes, were prepared by coevaporation of pentacene and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane as an acceptor dopant. Since the molecular doping in this study can increase the drain current without positive shift of threshold voltage, p+ layers were formed precisely on top of the source-drain region...

Numerical investigation of organic thin-film transistors using a thermionic field emission model

Charge injection barriers at the electrode/semiconductor interface were considered for numerical investigation of electrical characteristics in realistic organic thin-film transistors (OTFTs). A new thermionic field emission (TFE) model, which addresses tunneling of thermally excited electrons, was used as the carrier injection model of OTFTs in this study. Device simulation for an n-channel OTFT with a bottom-gate, bottom-contact configuration was performed with a thin-film organic transistor advanced simulator (TOTAS). Typical electrical characteristics of realistic OTFTs such as severe nonlinearity in output characteristics were reproduced by this simulation using the TFE model. The effects of contact-area-limited doping for n-channel OTFTs were also examined from the simulation results, suggesting that highly doped semiconducting layers prepared over contact electrodes can neutralize the effect of a Schottky energy barrier. Both the simulation technique with the TFE model and the contact-area-limited doping are promising in designing and developing high-performance OTFTs.

Current Enhancement with Contact-Area-Limited Doping for Bottom-Gate, Bottom-Contact Organic Thin-Film Transistors

The current enhancement mechanism in contact-area-limited doping for bottom-gate, bottom-contact (BGBC) p-channel organic thin-film transistors (OTFTs) was investigated both by simulation and experiment. Simulation results suggest that carrier shortage and large potential drop occur in the source-electrode/channel interface region in a conventional BGBC OTFT during operation, which results in a decrease in the effective field-effect mobility. These phenomena are attributed to the low carrier concentration of active semiconductor layers in OTFTs and can be alleviated by contact-area-limited doping, where highly doped layers are prepared over source–drain electrodes. According to two-dimensional current distribution obtained from the device simulation, a current flow from the source electrode to the channel region via highly doped layers is generated in addition to the direct carrier injection from the source electrode to the channel, leading to the enhancement of the drain current and effective field-effect mobility. The expected current enhancement mechanism in contact-area-limited doping was experimentally confirmed in typical α-sexithiophene (α-6T) BGBC thin-film transistors.

Characterization of submicron-scale periodic grooves by grazing incidence ultra-small-angle X-ray scattering

A grazing incidence ultra-small-angle X-ray scattering measurement system using a laboratory X-ray source has been developed. Submicron-scale artificial periodic grooves on thermally oxidized silicon wafers are characterized using this system. They are fabricated by electron beam lithography. The average pitch widths of the grooves are determined accurately with a low standard uncertainty of less than 0.02%. The cross-sectional profiles are also analyzed by intensity ratios of higher-order diffraction peaks from the periodic structures and X-ray reflectivity measurements. The obtained cross-sectional profiles are in good agreement with those obtained by atomic force microscopy.

Fabrication and Characterization of Decanedithiol Molecular Junction Using Nanogap Electrodes

We have fabricated and characterized the decanedithiol molecular junctions using planar Au nanogap electrodes in solution. The molecular junctions were formed by immersing the nanogap electrodes in decanedithiol solution. The number of molecules bridging the Au electrodes was precisely evaluated using two independent methods. First, the number of molecules was estimated from the area of the nanogap electrodes. The area of the nanogap electrodes was evaluated from the current–voltage (I–V) curve of the nanogap electrodes before the immersion process. Second, the number of molecules was estimated by comparing the conductance of the molecular junction with the previously reported conductance value of the single molecular junction. We also repeatedly form and break the molecular junctions by an electrochemical process.

Amorphous Si waveguides with high-quality stacked gratings for multi-layer Si optical circuits

Amorphous Si wire waveguides with stacked gratings are successfully fabricated by carefully controlling the SOG coating thickness. The peak energy of the light from the waveguide is controlled to be just 1.55 μm as designed.

Oriented Film Growth of Organic Semiconductor Sexithiophene on Artificial Periodic Grooves and Electrical Conduction Properties of the Films

Film growth of organic semiconductor α-sexithiophene (6T) was studied using substrates with artificial periodic grooves. The grooves were fabricated on thermally oxidized silicon substrates by electron beam lithography. Based on the orientation analysis by atomic force microscopy and grazing-incidence x-ray diffraction, part of the 6T grains grew having the in-plane orientational relationship with the artificial grooves. This phenomenon corresponds to “ggraphoepitaxy” which has been well known in inorganic materials research field. As well as the in-plane preferred orientation, one-dimensional structure consisting of connected 6T grains was observed. The electrical conduction properties of the films were also evaluated.