Improvement of Short Channel Mobility and Operational Stability of Pentacene Bottom-Contact Transistors with a Sulfuric
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0965-S07-08
Improvement of Short Channel Mobility and Operational Stability of Pentacene BottomContact Transistors with a Sulfuric Acid and Hydrogen Peroxide Mixture (SPM) Treatment of Au Electrodes Haruo Kawakami, Takahiko Maeda, and Hisato Kato Material Lab., Fuji Electric Advanced Technology Co, Ltd., 1, Fuji-machi, Hino-city, Tokyo, 191-8502, Japan
ABSTRACT We report a reduction in the contact resistance between pentacene and Au source/drain electrodes of organic field effect transistors (OFETs) with bottom-contact structure. By immersing the Au electrodes in a sulfuric acid and hydrogen peroxide mixture (SPM), the injection barrier between the Au electrodes and pentacene was lowered by approximately 0.2 eV and the contact resistance significantly decreased. The fabricated bottom-contact OFETs revealed a field-effect mobility of more than 0.66 cm2/Vs at a channel length ranging from 3 to 30 µm, which is comparable to that of top-contact OFETs with a 50 µm channel length. The transfer characteristics of the OFET with the SPM treatment were stable even after 44days storage in air under room illumination without any passivation. Moreover, the drain current reduction due to threshold voltage (Vth) shift under continuous application of gate voltage quickly recovered toward the original value with unloading of gate voltage. INTRODUCTION The organic field effect transistor (OFET) is one of the most promising technologies in the field of organic electronics1, 2. There are two types of OFETs. One is the so-called top-contact structure and the other is the bottom-contact structure. The bottom-contact structure has a merit that small-size OFETs can easily be fabricated because conventional lithography is applicable to the fabrication of source/drain (S/D) electrodes. However, its field-effect mobility (µ) is low compared with that of the top-contact structure. In many cases, this is due to the large contact resistance between the S/D electrodes and organic active layer3, 4. We report a reduction of the contact resistance and injection barrier at the interface of Au S/D electrodes and pentacene5. The process involves immersing in sulfuric acid and hydrogen peroxide mixture (SPM) before evaporating pentacene active layer. The SPM treatment was also effective to improve operational stability of OFET in air and under continuous application of gate voltage. EXPERIMENTAL The fabricated OFETs had a bottom-contact structure with channel width (W) of 1 mm. The channel lengths (L) were varied from 3 to 30 µm. A Ta film with a thickness of 100 nm was deposited on glass substrates (Corning 1737) by electron beam evaporation and patterned with shadow mask. The Ta film was subsequently anodized by using a 1% ammonium borate solution
with the application of 70 V at room temperature. With this anodization process, a 110 nm Ta2O5 film was formed and the thickness of Ta film reduced to approximately 50 nm. The Ta2O5 film and the Ta film acted as gate insulator and gate electrode, respectively. Then, the Au film with a thickness of 100 n
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