Paper EM-MoM5
Ultraviolet-Enhanced Device Properties in Pentacene-Based Thin-Film Transistors

Monday, October 15, 2007, 9:20 am, Room 612

Pentacene thin–film transistors (TFTs) have attracted much attention due to their unique potentials. Nevertheless, there are still several issues to be considered before the TFTs are launched into real applications. Among these issues, the ultraviolet (UV)-induced degradation of pentacene is probably the most immediate and serious. However, in the present study we unexpectedly found that a lower energy UV radiation could rather be advantageous. Since this finding is quite surprising and may be applicable to the improvement of the performance of pentacene TFTs, we report on the contrasting effects of deep and shallow UV on the device performance of pentacene-based TFTs. Furthermore, we also fabricated the low-voltage high-gain pentacene-based inverter using a thin polymer/high-k dielectric through UV treatments. A substrate of 200 nm-thick SiO₂ or 450 nm-thick poly-4-vinylphenol (PVP) on p⁺-Si (~0.01 Ωcm) was adopted for the gate dielectric and gate electrode in the present TFTs. The pentacene channel layers of 50 nm thickness were then patterned on the substrate by the thermal evaporation at RT. Subsequently, Au was evaporated onto the pentacene channels through a source/drain mask at RT (top-contact mode). Under the illumination of 254 nm-UV the TFTs showed degraded mobility and lowered saturation current. It is because the high energy UV degraded the crystalline quality of the pentacene channel as confirmed by optical absorption technique. However, under 352 nm UV the pentacene devices rather exhibited the enhancement of saturation current and also a positive shift of the threshold voltage (V_T), maintaining their mobilities. We also found that these UV-induced advantages were more clearly obtainable from the TFTs with polymer dielectric where extra negative charges are generated to exist as permanent form at the pentacene/polymer interface while those charges are not necessarily fixed at the pentacene/SiO₂ interface. Through inverters fabricated by illuminating 352nm-UV onto the load-TFT to selectively adjust its V_T, we also could achieve a stable high-gain low-voltage inverting within the input voltage (V_in) range of 0 ~ -6 V. We thus conclude that illuminating a low energy UV on the TFT with pentacene/polymer dielectric interface is a simple promising way of enhancing the general performance of pentacene TFTs and controlling their V_T that enables to achieve stable pentacene-based inverters as well.