T. Kamata

Surface Potential Control of an Insulator Layer for the High Performance Organic FET

It has been indicated that the pentacene crystal growth near the channel region is the most essential factor for determining pentacene FET properties and strongly depends on the surface energy of the substrate. In this study, surface modification effect of the SiO 2 /Si substrate by several kinds of polymers was investigated for the purpose of controlling surface potential and improve the FET properties. Matching the surface energy between the insulator and semiconductor layers using a polymer surface modifier gave improvement of the pentacene crystal growth and its FET properties. It was revealed that the molecular diffusion on a solid substrate and crystal growth mechanism were essential to give the effects.

High Performance Organic FET with Double-Semiconductor Layers

The over-coating effect of a second semiconductor layer on a pentacene field-effect transistor (FET) was investigated. Me-PTC (n-type) coating gave improvements of the field-effect mobility and I DS for pentacene (p-type) FET. Such remarkable improvements were not observed in the TiOPC (p-type) coated pentacene (p-type) FET. These phenomena indicate that n-type semiconductor coating on a p-type semiconductor layer reduce the effect of trapped electrons at the crystal grain boundary. Besides, the over-coated layer also prevents the effect of water molecules adsorbed on pentacene crystals. Consequently, it was revealed that the p/n type hetero double-layered structure is effective to improve FET properties.

Memory effects of pentacene MFS-FET

The metal-ferroelectrics-semiconductor field effect transistor (MFS-FET) was fabricated on Si substrate by using semiconductive pentacene thin film and ferroelectrics lead titanate zirconate Pb(Zr,Ti)O 3 (PZT) thin film prepared by the sol-gel technique. The memory effects were estimated by the cyclic I d -V g characteristic measurements. MFS-FET with an intermediate insulator layer between the gate electrode and the PZT layer gave the clear square hysteresis loop. The shape of the hysteresis loop was much dependent on the preparation condition of the PZT thin film. The surface morphology of the PZT layer contributes to the difference of the shape of the hysteresis loop.

Highly Sensitive Organic Photo-FET Using Photosensitive Polymer Insulator

We fabricated a photo field effect transistor (photo FET) having a poly (N-vinyl carbazole) (PVK) insulator layer. PVK was employed as a photosensitive insulator material for the Photo FET. In this photo FET, illumination of a blue light to the PVK insulator drastically improved the field effect mobility and the gate-switching ratio. This result would be due to that the photo-generated carriers in the PVK layer were effectively accumulated in the gate capacitor of the photo FET.

Interfacial control for developing organic rewritable optical memory using organic photo-FET having photosensitive gate dielectric

We have already reported on the pentacene photo FET having a photo-sensitive gate dielectric layer (poly(Nvinylcarbazole): PVK). This photo FET showed excellent photo-switching properties upon illuminating the gate dielectric layer, because the gate capacitor was rapidly charged up by providing a large amount of photo-generated charges in the dielectric layer to the gate capacitor. When the gate bias and photo-illumination were turned off, accumulated charges diffuse from the channel region to the source electrode and the channel immediately becomes OFF state. Therefore, this photo FET can switch ON-OFF states rapidly, however, memory property is poor. In this study, we have tried to give a memory function to the photo FET. For keeping the accumulated charges at the channel region even after turning off the gate bias and photo-illumination, we intentionally formed Schottky barriers at the interface between the semiconductor and the DS electrodes by using aluminum as a low work function metal instead of gold. As a result, the photo FET did not show any gate voltage modulations of drain currents under dark condition. On the other hand, the photo FET showed the drastic increase in drain current upon illuminating the gate dielectric layer. Further, this high-drain current state was kept even after turning off the gate bias and photo-illumination (written state), probably because the accumulated charges at the channel region could not escape from the source electrode owing to the Schottky barrier at the semiconductor/DS electrodes interface. From these results, we have concluded that we could develop the novel organic rewritable optical memory.

Improving photo-switching property of organic photo-FET having photosensitive gate dielectric

We fabricated a novel type photo-FET using poly(N-vinylcarbazole) (PVK) as a photosensitive gate dielectric. For the photo-FET, photo-illumination to the PVK insulator layer makes the field-effect mobility μFET two orders of magnitude higher than dark condition. In particular, under blue-light illumination condition the on-off ratio was also a few ten times higher than dark condition. Furthermore, by introducing blocking layer between semiconductor layer and insulator layer lead. We concluded that the improvement of the transistor properties resulted from effective charge accumulation at the conductive channel by photo illuminations.

Novel organic photo FET using photo-sensitive gate dielectric layer

We fabricated a novel type photo-FET using poly(N-vinylcarbazole) as a photosensitive gate dielectric. For the photo-FET, photo-illumination to the PVK insulator layer make the field-effect mobility μFET two order of magnitude higher than dark condition. In particular, under blue-light illumination condition the on-off ratio was also a few ten times higher than dark condition. We concluded that the improvement of the transistor properties resulted from effective charge accumulation at the conductive channel by photo illuminations.

Optimization of p/n multilayer structure for organic photoreceptor device

Organic photoreceptor devices fabricated with titanyl phthalocyanine as a p-type semiconductor and perylene dye as a n-type semiconductor were fabricated with various p/n multilayer structures. Fabrication of the device including p/n junction gave one-order increase in on/off ratio as compared with that without the junction. When the device has multiple p/n junctions, the decrease of driving voltage was also observed. The multilayer p/n structure shows the improvement of the device performance.