Effect of UV Irradiation on Threshold Voltage and Subthreshold Slope in Pentacene TFT

Abstract

We report the influence of UV irradiation on the electrical properties of pentacene thin-film transistors ͑TFTs͒ with SiO 2 gate insulator. The threshold voltage ͑V th ͒ shifted from −9.0 to −5.1 V and subthreshold slope reduced from 3.6 to 1.1 V/dec by the treatment before pentacene deposition. The UV treatment reduced the surface energy by removing the hydroxyl groups at the SiO 2 surface. Thus, molecular packing of pentacene on the UV-treated SiO 2 was improved, lowering the ͉V th ͉ value via the reduction of trap densities in the channel region. The decrease in interface state density reduced the subthreshold slope.A thin-film transistor ͑TFT͒ with device parameters of low threshold voltage ͑V th ͒, zero turn-on voltage ͑V to ͒, and small subthreshold slope ͑S͒ is desirable as a switch device for the application of active-matrix displays. In p-type pentacene ͑PEN͒ TFT with SiO 2 dielectric layers, low electric performances with high ͑V th ͒, large positive ͑V to ͒, and large S values were commonly observed. 1-4 This might be due to the hydroxyl group ͑OH − ͒ at the SiO 2 surface, which acts as an electron trapping center, leaving the mobile holes in the channel. 1-4 The surface treatment with hexamethyldisilazane changed the SiO 2 surface from a OH-terminated surface to a CH 3 -terminated one, improving the V to , V th , and S values. 2,3 The formation of built-in dipole at the SiO 2 surface using phenethylchlorosilane also showed the performance improvement of PEN devices. 4 However, it is not clear whether the device characteristics after those treatments were directly related to the reduction of the hydroxyl group at the SiO 2 surface. [2][3][4][5] UV light has been used to induce the structural modification in the SiO 2 . 6,7 It was reported that the photolysis using UV light could modify the bond structures of the SiO 2 , including the elimination of hydroxyl groups. 7 UV irradiation on the SiO 2 surface could reduce the OH − groups effectively without rendering negative charges at the SiO 2 /PEN interface. Because the reduction of OH − at the SiO 2 surface could decrease the surface energy, it was thought that lower surface energy could also improve the grain interconnectivity. 8 Thus, it was expected that UV treatment on the SiO 2 surface in a vacuum could improve the device performances.In this study, we report the effects of UV irradiation on the surface electronic states and the performance of PEN-TFT with SiO 2 dielectric layer. Chemical compositions and bonding structures of the gate insulator ͑SiO 2 ͒ were investigated using synchrotron radiation photoemission spectroscopy ͑SRPES͒. The surface morphology of PEN film was examined using atomic force microscopy ͑AFM, digital instrument͒ and scanning electron microscopy ͑SEM͒. From these, the influence of UV irradiation on the electrical performance of PEN-TFT is discussed.Devices were fabricated on a highly doped n-Si ͑ϳ5 ⍀ cm͒ substrate with thermally grown SiO 2 ͑300 nm thick, insulator capacitance C i = 10 nF/cm 2 ͒, which functioned as the gate ...

Engineered interface using a hydroxyl group‐free polymeric buffer layer onto a TiO2 nanocomposite film for improving the electrical properties in a low‐voltage operated organic transistor

“…On the basis of this study, it is definitely found that hydroxyl groups act as charge carrier trapping sites. Considering some previous works about the interfacial trap density on various dielectric layers such as SiO 2 , and self‐assembled monolayer deposited on SiO 2 , and Al 2 O 3 , our calculated interfacial trap density is in a reasonable range . Compared with the hydroxyl group‐existing polymeric buffer layer, it is found that the hydroxyl group‐free polymeric buffer layer is much more efficient in minimizing the N trap .…”

“…Considering that the magnitude of SS represents the quality about how soon the OFETs turn from off state to on state, a lower value is desirable for fast switching. The SS is determined from the following equation in the subthreshold regime at the transfer characteristic curves: …”