Bo-Sung Kim

Effects of Plasma Process Induced Damages on Organic Gate Dielectrics of Organic Thin-Film Transistors

The effects of plasma damages on the organic gate dielectric of the organic thin film transistor (OTFT) during the fabrication process are investigated; metal deposition process on the organic gate insulator by the plasma sputtering mainly generates the process induced damage of bottom contact structured OTFT. For this study, two different deposition methods (thermal evaporation and plasma sputtering) have been tested for their damage effects onto poly(4-vinyl phenol) (PVP) as organic gate dielectric. Unlike thermal evaporation, conventional plasma sputtering process induces serious damages onto the organic layer as increasing the surface energy, changing the surface morphology and degrading OTFT performances.

Effect of Cu, CuO, and Cu–CuO Bilayer Source/Drain Electrodes on the Performance of the Pentacene Thin-Film Transistor

Cu, CuO, and Cu-CuO bilayer films were deposited with radio-frequency sputtering by controlling the O 2 /Ar gas flow ratio and the performance of the pentacene thin-film transistor (TFT) with these films as a source/drain (S/D) electrode was measured. With an O 2 /Ar gas flow ratio higher than 1, CuO film was obtained with a resistivity of ~ 10 5 μΩ cm and a work function of ∼ 5.0 eV close to the highest occupied molecular orbital energy level of pentacene. Pentacene TFT with CuO film deposited at (Ar:O 2 = 0:50 sccm) showed better performance than Cu film because the barrier height between the electrode and the semiconductor layer was smaller. The pentacene TFTs with CuO/Cu and Cu/CuO bilayer S/D electrodes were fabricated through the shadow mask patterning, and the CuO/Cu structure showed better performance than Cu or Cu/CuO because hole injection was through the CuO layer. It was confirmed that the edge effect with a shadow mask had an influence on the electrode pattern formation due to the infiltration of the film-forming species through the microgap.

Role of Adsorbed H

We investigated the variation of electrical performances of solution-processed zinc tin oxide thin-film transistors (ZTO TFTs) when their channel layer was exposed to ambient gases at room temperature. During our research, we observed that adsorption of H2O on the backchannel surface can act as an electron trap and/or donor, depending on the amount of H2O. In addition, we found that the abnormal behavior seen in the TFTs was caused by different rates of adsorption/desorption. Finally, the instability in the electrical characteristics of the ZTO TFTs caused by the ambient atmosphere can easily be reversed by vacuum seasoning.

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The a-GIZO TFTs have attracted considerable attention for a next generation display due to its high mobility and good uniformity[1,2]. The passivation on the back surface of the active layer is a important issue to protect back interface from moisture or plasma damage because the back channel may control the reliability of the device.[3] It has been reported that the moisture and oxygen are strongly associated with caracteristics of oxide transistors. Since H2O and O2 molecules are adsorbed or desorbed on the oxide semiconductor, they can easily capture electrons or release electrons.[4] During the adsorption or desorption process, electrical properties of oxide semiconductor are changed considerably. The purpose of the paper is to report for the first time that SAM(self-assembled monolayer), which consists of a functional group at the terminal and the head group at the other end, improve the back interface properties of a-IGZO TFT. The ordered SAMs with molecular dipole moment generate the local electric field and have a influence on the Vth of the oxide semiconductor. Also a hydrophobic SAM can be used in blocking moisture efficiently. In addition, SAM can suppress various plasma damages considerably as a very thin organic layer. We fabricated bottom gate a-GIZO TFTs. 200nm Mo metal is deposited on a glass substrate as a gate electrode with a 300nm SiOx insulator on top of it. A 40nm ITO metal is used as a drain and source electrodes. The a-IGZO active layer was formed by sputtering method. Each layers are patterned by photolithography and etching. Finally we deposited 3 different SAMs(3-chloropropyltriethoxysilane, Triethoxy(octyl)silane and (3-Aminopropyl)triethoxysilane) on the oxide TFT by solution method.[5] After SAM treatment, we observed the different Vth shift of oxide TFTs with 3 different SAMs treatment as shown in Table 1. In the case of Cl-SAM(3-chloropropyltriethoxysilane) and NH2-SAM((3-Aminopropyl)triethoxysilane), the Vth is shifted to a negative direction while that for CH3-SAM is shifted to a positive direction. This phenomenon can be explained from the fact that SAM has a dipole moment according to a functional group. The dipole moment is positive in Cl-, F-, and NH2functional group, while it is negative in CH3-functional group. So each SAMs can generate the built-in local electric field that accumulates or release electrons on the oxide semiconductor.[6] Cl-SAM with negative dipole moment create local electric field normal to semiconductor and this electric field provides electrons to the active layer similar to a negative back gate. Therefore Cl-SAM with a negative dipole mement forms a high conductive back channel which requires more negative gate bias in order to deplete the active channel layer. This agrees fairly well that we observed smaller negative Vth shift in NH2-SAM with less dipole moment and positive Vth shift in CH3-SAM with opposite dipole direction. Also, in order to investigate SAM as a blocking layer from moisture and oxygen, we measured the bias stability of a-IGZO TFTs. We observed similar positive Vth shift in both deviece(A) with Cl-SAM and device(B) without SAM under positive gate bias stress.(Vg=+20V, Vds=0.1V, from 0s to 1000s). In contrast, device A shows very small negative Vth shift while the Von(turn on voltage) is shifted to a positive direction including a change of swing value in device B under negative gate bias stress(Vg=-20V, Vds=10V, from 0s to 1000s) as shown in fig. 2. It was reported previously that H2O molecules can act as acceptorlike deep trap as well as electron donors on the a-IGZO surface[3]. In case of accompanying a change of swing value, H2O molecules are closely involved in it. So it can be inferred that SAM blocked moisture effectively in device A without any significant change in Vth, swing. We also investigated SAM as a plasma protecting layer at the back side of oxide TFT. After SAM deposition, the device was treated with CF4 plasma. While the device without SAM was degraded severely, there was no degradation in the device with SAM as shown in Fig 3. The proposed SAM suppressed CF4 plasma damage successfully. Our experimental result shows that by employing SAM on the back interface of the active layer, the back channel was protected from moisture for better stability and SAM suppressed various troublesome plasma damage without any additional photolithography process. We also observed different Vth shifts of a-IGZO TFT according to the dipole moment of each SAMs. Before SAM treatment After SAM treatment