Effect of Post-Deposition Treatment on Characteristics of P-channel SnO

Transcription

1 Effect of Post-Deposition Treatment on Characteristics of P-channel SnO Thin-Film Transistors 1 Byeong-Jun Song, 2 Ho-Nyeon Lee 1, First Author Department of Electric & Robotics Engineering, Soonchunhyang University, Asan , Korea, s_bjbj@lmsglobal.com *2,Corresponding Author Department of Display and Electronic Information Engineering, Soonchunhyang University, Asan , Korea, hnlee@sch.ac.kr Abstract We have fabricated p-channel thin-film transistors (TFTs) using tin-oxide active layers deposited by a vacuum thermal evaporation method. The field-effect mobility of the TFT is 1.01 cm 2 V -1 s -1. Postdeposition treatments composed of a thermal annealing in a nitrogen environment and an oxygen plasma treatment are essential in obtaining the p-channel switching capability. The as-deposited tinoxide film is Sn rich in the entire region. The composition of tin-oxide films changes through the postdeposition treatment to be partially O rich. This composition change causes the p-channel switching capability of tin-oxide TFTs. 1. Introduction Keywords: Thin-Film Transistor, P-Channel, Tin Oxide An oxide-semiconductor thin-film transistor (TFT) (OS-TFT) is expected to be a mainstream of the TFT technology in the near future. This is because of the low-cost fabrication process and the high performance of OS-TFTs[1]. The majority of research activities for OS-TFTs have focused on n- channel TFTs because of the ease in obtaining n-type oxide semiconductors. An indium gallium zinc oxide (IGZO) TFT technology is a major n-channel OS-TFT technology. IGZO TFTs have high stability and high performance[2 4] so industries are attempting to apply the IGZO TFT technology to their commercial products. Despite the difficulties in research, p-channel OS-TFTs is required to widen the application of OS-TFTs. P-channel TFTs are better at driving organic light-emitting diodes with conventional bottom-anode structure than n-channel TFTs. In addition, both of a p-channel TFT and an n-channel TFT are required in fabricating complementary metal oxide semiconductor circuits. Therefore, p- channel OS-TFTs are essential in establishing high-quality active-matrix flat-panel displays (AMFPDs) such as active-matrix organic light-emitting diodes with integrated peripheral circuits. There have been a few researches on p-channel OS-TFTs using oxide semiconductors such as nickel oxide[5], copper oxide[6], and tin oxide[7 10]. In most oxide semiconductors, O 2p orbitals form a deep localized valence band maximum (VBM)[11]. Hence, it is difficult to obtain a p-type oxide semiconductor with high performance. However, for SnO, the spherically spread 5s orbitals of Sn 2+ contribute to the VBM[7]. Consequently, using epitaxially grown SnO films, p-channel SnO TFTs with field-effect mobility (μ FE ) of approximately 1 cm 2 V -1 s -1 were reported[7,8]. A SnO TFT is one of the best prospects for a practical p-channel OS-TFT technology. For the previous reports, a pulsed laser deposition (PLD) method was used to fabricate SnO TFTs[7,8]. However, the PLD method is not appropriate for a large-area deposition. Ability to deposit large-area films is vital for AMFPD applications. Considering this, we have deposited tin-oxide layers using a vacuum thermal evaporation method, which is adequate for large-area films. In addition, postdeposition treatments such as a thermal annealing and an oxygen plasma treatment have been conducted to improve the performance of tin-oxide films. Through this work, we have obtained p- channel OSTFTs using large-size applicable processes. This article presents a new fabrication process of p-channel tin-oxide TFTs. The main point of the process is a vacuum thermal evaporation process combined with post-deposition treatments, which are composed of a thermal annealing in nitrogen environment and an oxygen plasma treatment. 2. Experiments International Journal of Digital Content Technology and its Applications(JDCTA) Volume 7, Number 12, August

2 Figure 1 shows the schematic diagram of the TFT structure of for this work. For the TFT fabrication, wet-oxidized p-type Si wafers (4 inch diameter, 1~10 Ωcm resistivity, 1 μm thick SiO 2 ) were used as substrates. The p-type doped layer and the SiO 2 layer were used as a gate electrode and a gate insulator, respectively. After an initial cleaning process, a gate contact region was opened by a wet etching method using a buffered oxide etchant. Then, a 50 nm thick active layer was deposited by a vacuum thermal evaporation method using a SnO powder source. The working pressure, the evaporation-source temperature, the deposition rate and the substrate temperature of the tin-oxide deposition were Torr, 900, 0.1 A /s and room temperature, respectively. Active patterns were fabricated using a metal shadow mask during the deposition. After the deposition, a thermal annealing was conducted at 300 for 2 h in a nitrogen environment. Then, an oxygen plasma treatment was conducted using an inductively coupled plasma equipment. For the oxygen plasma treatment, the working process pressure was Torr and the radio frequency power was 250 W. Then, source/drain electrodes and a gate contact were fabricated using Ag patterns. An Ag layer was deposited using a vacuum thermal evaporation method. The evaporation source temperature, the working pressure and the deposition rate were 700, Torr and 1 nm/s, respectively. The Ag layer was patterned using a metal shadow mask. The channel length and width of the TFT was 0.8 mm and 4 mm, respectively. The characteristics of the fabricated TFTs were measured using a semiconductor parameter analyzer in a dark shielding box at room temperature. Tin-oxide unit layers were fabricated on glass substrates (Samsung Corning Precision Materials, Eagle XG). The deposition and post-deposition treatment conditions were the same as those of the TFT fabrication process. Using these samples, we conducted x-ray photoemission spectroscopy (XPS) depth-profile analysis. Figure 1. Schematic diagram for the structure of thin-film transistors for this work 3. Results and Discussion Figure 2 shows the transfer curves of the OS-TFTs as a function of gate bias voltage according to the oxygen-plasma time. A thermal annealing in a nitrogen environment was conducted for 2 h prior to the oxygen plasma treatment. The drain bias voltage and the source bias voltage were -10 V and 0 V, respectively. Table 1 shows the μ FE, the threshold voltage (V TH ) and the switching ratio extracted from the data of Fig. 2. The μ FE and V TH are extracted using a linear-mode gradual-channel approximation. The switching ratio is the ratio of the maximum current value to the minimum current value of each transfer. In Fig. 2, all curves seem to increase more for more negative gate-bias-voltage values beyond -40 V and curves except the 120 min treated curve seem to decrease more for more positive gate-biasvoltage values beyond 40 V. Hence, larger switching ratios will be obtained for a larger gate-bias sweep range than the values in Table 1. As the oxygen plasma time increases, the drain current and the μ FE decrease. The as-deposited tin oxide is a mixture of Sn 0, Sn 2+ and Sn 4+ components[12,13]. The oxidation state of Sn increases according to the supply of thermal energy and oxygen atoms. Therefore, the p-type SnO to n-type SnO 2 ratio decreases as the oxygen plasma time increases. Consequently, the longer oxygen-plasma time causes the lower net hole-carrier density. The lower carrier density causes 439

3 the lower carrier mobility for oxide semiconductors[14]. Therefore, a tin-oxide TFT with the longer oxygen plasma time has the lower μ FE. For the 30 min treated OS-TFT, the μ FE is approximately 1 cm 2 V -1 s -1. This value is compatible to that of SnO TFTs fabricated using a PLD method[7,8] and is far higher than that of reactively evaporated tin-oxide TFTs[9]. In addition to the tin-oxide TFTs described above, various tin-oxide TFTs were fabricated using an as-deposited tin oxide, a thermally annealed tin-oxide, and a tin oxide treated only by oxygen plasma. However, these tin-oxide TFTs could not show switching capability. Figure 2. Drain currents as a function of gate bias voltage according to the oxygen-plasma treatment time; the inset is the graph with a log scale y-axis Table 1. Device parameters of OS-TFTs according to the oxygen-plasma treatment time Oxygen-plasma time (min) Field-effect mobility (cm 2 V -1 s -1 ) Threshold voltage (V) Switching ratio Figure 3 shows results of the XPS depth-profile analysis for tin-oxide films. The process conditions for the sample fabrication were the same as those of active-layer fabrication of the OS-TFTs of Figure 2. Figure 3(a), (b), and (c) are the results from an as-deposited tin oxide, a thermally annealed tin oxide, and a thermally annealed and oxygen plasma treated tin oxide, respectively. The thermal annealing was conducted at 300 for 2 h in a nitrogen environment. The oxygen-plasma treatment time was 2 h. The atomic composition ratios of Sn and O in the figures mean the percent ratios of Sn and O atomic concentration, respectively, to the sum of Sn and O atomic concentration. For the as-deposited tin oxide and the thermally annealed tin oxide, Sn has a larger concentration than O through all depths of the samples. For the thermal annealed and oxygen plasma treated tin oxide, the O concentration 440

4 increases and the Sn concentration decreases markedly at surficial region. In the deep region, the O and Sn concentrations are almost the same as those of the as-deposited tin oxide. Figure 3. Results of XPS depth-profile analysis, (a) the result from an as-deposited tin oxide, (b) the result from a thermally annealed tin oxide, and (c) the result from a thermally annealed and oxygen plasma treated tin oxide. A higher concentration of Sn than that of O of as deposited and thermally annealed tin oxide films implies that the films have a mixed phase of metallic tin and tin oxide. The metallic Sn can be a source for electron carriers, which hinder the p-channel operation of tin-oxide TFTs. Hence, tin-oxide TFTs with as-deposited active layers or thermally annealed active layers do not have proper switching capability. For SnO films to have p-type semiconducting properties, Sn vacancies are required because 441

5 of their role as an electron acceptor[15]. The Sn concentration is larger than the O concentration for the as-deposited and thermally annealed tin-oxide films so that the probability to form Sn vacancies is low. The oxygen plasma can supply additional oxygen atoms to the tin-oxide films as shown by the result of Fig. 3(c). As the oxygen concentration increases and Sn concentration decreases, the probability to form Sn vacancies increases. The p-channel switching capability of OS-TFTs with the thermally annealed and oxygen plasma treated tin-oxide active layers can be explained through the discussion above. Oxidation due to oxygen plasma treatment reduces the concentration of metallic Sn, which can be a leakage path for the positive gate-bias range of the p-channel OS-TFTs. Therefore, the decrease of off-state current according to the increase of the oxygen-plasma time can be explained by the decrease of the metallic Sn concentration according the increase of the oxygen-plasma time. 4. Conclusions We have fabricated p-channel tin-oxide TFTs. A vacuum thermal evaporation method using a SnO powder source has been used to deposit tin-oxide active layers; this deposition process is cost effective and appropriate for large substrates. In obtaining p-channel switching properties, post-deposition treatments are important. The post-deposition treatment is composed of a thermal annealing in a nitrogen environment and an oxygen plasma treatment. The atomic composition ratio of tin-oxide layers is changed by these post-deposition treatments. The atomic composition of as-deposited tin oxide is Sn rich through all depth. An O rich region and an O-Sn balanced region are formed partially by the post-deposition treatment; through this composition change, the p-channel switching capability of the tin-oxide TFTs has been obtained. The μ FE value of our p-channel tin-oxide TFT was 1.01 cm 2 V - 1 s -1 ; this value is compatible to that of SnO TFTs fabricated with a PLD process, which is an expensive epitaxial growth method. Therefore, our result must be a meaningful progress toward a practical p- channel OS-TFT technology. However, the switching ratio of tin-oxide TFTs of this work should be improved more to be used for practical applications. This may be due to the existence of a partial Sn rich region even after the post-deposition treatment. The switching properties are expected to be improved through the optimization of the post-deposition process and the active layer thickness. 6. Acknowledgements This work was supported by the Soonchunhyang University. 10. References [1] E. Fortunato, P. Barquinha, R. Martins, Oxide semiconductor thin-film transistors: a review of recent advances, Adv. Mater., vol. 24, no. 22, pp , [2] H.-W. Zan, C.-C. Yeh, H.-F. Meng, C.-C. Tsai, L.-H. Chen, Achieving high field-effect mobility in amorphous indium-gallium-zinc oxide by capping a strong reduction layer, Adv. Mater., vol. 24, no. 26, pp , [3] J.C. Park, H.-N. Lee, Dry etch damage and recovery of gallium indium zinc oxide thinfilm transistors with etch-back structures, Displays, vol. 33, no. 3, pp , [4] J.C. Park, H.-N. Lee, Improvement of the Performance and Stability of Oxide Semiconductor Thin-Film Transistors Using Double-Stacked Active Layers, IEEE Electron Device Lett., vol. 33, no. 6, pp , [5] H. Shimotani, H. Suzuki, K. Ueno, M. Kawasaki, Y. Iwasa, p-type field-effect transistor of NiO with electric double-layer gating, Appl. Phys. Lett., vol. 92, no. 24, pp , [6] S.-Y. Sung, S.-Y. Kim, K.-M. Jo, J.-H. Lee, J.-J. Kim, S.-G. Kim, K.-H. Chai, S.J. Pearton, D.P. Norton, Y.-W. Heo, Fabrication of p-channel thin-film transistors using CuO active layers deposited at low temperature, Appl. Phys. Lett., vol. 97, no. 22, pp ,

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