E ﬀ ects of Annealing Atmosphere on Electrical Performance and Stability of High-Mobility Indium-Gallium-Tin Oxide Thin-Film Transistors

: In this study, we examined the effects of the annealing atmosphere on the electrical performance and stability of high-mobility indium-gallium-tin oxide (IGTO) thin-film transistors (TFTs). The annealing process was performed at a temperature of 180 ◦ C under N 2 , O 2 , or air atmosphere after the deposition of IGTO thin films by direct current magnetron sputtering. The field-effect mobility ( µ FE ) of the N 2 - and O 2 -annealed IGTO TFTs was 26.6 cm 2 / V · s and 25.0 cm 2 / V · s, respectively; these values were higher than that of the air-annealed IGTO TFT ( µ FE = 23.5 cm 2 / V · s). Furthermore, the stability of the N 2 - and O 2 -annealed IGTO TFTs under the application of a positive bias stress (PBS) was greater than that of the air-annealed device. However, the N 2 -annealed IGTO TFT exhibited a larger threshold voltage shift under negative bias illumination stress (NBIS) compared with the O 2 - and air-annealed IGTO TFTs. The obtained results indicate that O 2 gas is the most suitable environment for the heat treatment of IGTO TFTs to maximize their electrical properties and stability. The low electrical stability of the air-annealed IGTO TFT under PBS and the N 2 -annealed IGTO TFT under NBIS are primarily attributed to the high density of hydroxyl groups and oxygen vacancies in the channel layers, respectively. by the of oxygen atoms during N 2 The obtained results suggest that O 2 gas is suitable


Introduction
Since the inceptive report on indium-gallium-zinc oxide (IGZO) thin-film transistors (TFTs) published by Nomura et al. in 2004, IGZO TFTs have attracted significant research interest, owing to their excellent electrical properties, high uniformity, and low fabrication costs. IGZO TFTs are widely used as the backplanes of large-area flat-panel displays, including active matrix organic light-emitting diode (OLED) displays [1][2][3][4][5]. However, the field-effect mobility (µ FE ) of IGZO TFTs is approximately 10 cm 2 /V·s, which is insufficient to meet the requirements of ultra-high-resolution and high-frame-rate next-generation displays. Over the past decade, various oxide TFTs with higher field-effect mobilities than those of IGZO TFTs have been extensively studied for next-generation display applications. Among these transistors, indium-gallium-tin oxide (IGTO) TFTs are promising as high-mobility oxide TFTs because of their excellent performance, even under low-temperature annealing conditions (<200 • C). The IGTO alloy comprises Sn cations instead of the Zn cations in IGZO, where the similar electronic configuration of the Sn 4+ and In 3+ ions enhances the formation of percolation conduction paths and increases the electron mobility of the former [6][7][8].
The oxygen-related species present in oxide thin-film transistors (TFTs), such as oxygen vacancies (V O ) or hydroxyl (OH) groups, have a significant impact on the stability and electrical characteristics of the TFTs [9,10]. Thus far, extensive studies have been conducted to investigate the effects of various process conditions on the concentration of oxygen-related species in the oxide channel layer [11][12][13][14][15][16][17]. The post-deposition annealing atmosphere has a particularly strong influence on the number of oxygen species within the channel layer and also affects the electrical properties and stability of oxide TFTs with various channel materials [18][19][20][21][22][23][24]. However, in most prior studies, the results obtained under various heat-treatment environments differed depending on the selected channel material and process conditions, suggesting the need to determine the most suitable annealing atmosphere for IGTO TFTs in order to enhance the electrical characteristics of the IGTO TFTs. However, to date, the influence of the annealing environment on the electrical properties of IGTO TFTs has not been examined. In this study, we investigate the effects of various annealing environments on the electrical performance and stability of IGTO TFTs. For this purpose, post-deposition annealing is performed at a temperature of 180 • C under N 2 , O 2 , and air atmosphere. From the obtained results, it is concluded that O 2 gas is the most suitable post-deposition atmosphere for IGTO TFT fabrication.

Experimental
Experiments were conducted using bottom-gate top-source/drain electrode IGTO TFTs, where p + -Si wafers served as both the substrates and gate electrodes. A 100 nm thick SiO 2 layer was thermally grown on top of a Si wafer as the gate insulator, and a 20 nm thick IGTO thin film was deposited on top of a SiO 2 /p + -Si substrate via direct current (DC) magnetron sputtering of a 3-inch IGTO target. Sputtering was performed using a DC power of 150 W, an Ar/O 2 ratio of 35/15 (sccm/sccm), and a deposition pressure of 3 mTorr; the substrate was at room temperature (RT). The source and drain electrodes were produced from a DC magnetron-sputtered 100 nm thick indium tin oxide layer. The channel and source/drain electrode layers were patterned using photolithography and lift-off techniques. Finally, the IGTO TFTs were thermally annealed at a temperature of 180 • C and a pressure of 1 atm for 2 h under N 2 , O 2 , and air atmosphere. Figure 1 shows a schematic of the fabricated IGTO TFTs. All TFTs designed in this work had a channel width/length (W/L) of 75/100 (µm/µm). The effects of post-deposition annealing on the optical, structural, and chemical properties of the fabricated IGTO thin films were investigated by ultraviolet visible-near infrared (UV-vis-NIR) spectroscopy (V-670, JASCO, Tokyo, Japan), X-ray diffraction (XRD, D8-Advance, Bruker-AXS, Wisconsin, USA) and X-ray photoelectron spectroscopy (XPS, K-alpha+, Thermo Fisher Scientific-KR, Seoul, Korea). Electrical characterization of the produced TFTs was conducted using a semiconductor parameter analyzer (4156C, Agilent Technologies, Santa Clara, USA) at RT, in the dark, under vacuum to avoid possible effects of the ambient environment on the properties of the IGTO TFT.
Electronics 2020, 9, x FOR PEER REVIEW 2 of 11 characteristics of the TFTs [9,10]. Thus far, extensive studies have been conducted to investigate the effects of various process conditions on the concentration of oxygen-related species in the oxide channel layer [11][12][13][14][15][16][17]. The post-deposition annealing atmosphere has a particularly strong influence on the number of oxygen species within the channel layer and also affects the electrical properties and stability of oxide TFTs with various channel materials [18][19][20][21][22][23][24]. However, in most prior studies, the results obtained under various heat-treatment environments differed depending on the selected channel material and process conditions, suggesting the need to determine the most suitable annealing atmosphere for IGTO TFTs in order to enhance the electrical characteristics of the IGTO TFTs. However, to date, the influence of the annealing environment on the electrical properties of IGTO TFTs has not been examined. In this study, we investigate the effects of various annealing environments on the electrical performance and stability of IGTO TFTs. For this purpose, postdeposition annealing is performed at a temperature of 180 °C under N2, O2, and air atmosphere. From the obtained results, it is concluded that O2 gas is the most suitable post-deposition atmosphere for IGTO TFT fabrication.

Experimental
Experiments were conducted using bottom-gate top-source/drain electrode IGTO TFTs, where p + -Si wafers served as both the substrates and gate electrodes. A 100 nm thick SiO2 layer was thermally grown on top of a Si wafer as the gate insulator, and a 20 nm thick IGTO thin film was deposited on top of a SiO2/p + -Si substrate via direct current (DC) magnetron sputtering of a 3-inch IGTO target. Sputtering was performed using a DC power of 150 W, an Ar/O2 ratio of 35/15 (sccm/sccm), and a deposition pressure of 3 mTorr; the substrate was at room temperature (RT). The source and drain electrodes were produced from a DC magnetron-sputtered 100 nm thick indium tin oxide layer. The channel and source/drain electrode layers were patterned using photolithography and lift-off techniques. Finally, the IGTO TFTs were thermally annealed at a temperature of 180 °C and a pressure of 1 atm for 2 h under N2, O2, and air atmosphere. Figure 1 shows a schematic of the fabricated IGTO TFTs. All TFTs designed in this work had a channel width/length (W/L) of 75/100 (μm/μm). The effects of post-deposition annealing on the optical, structural, and chemical properties of the fabricated IGTO thin films were investigated by ultraviolet visible-near infrared (UV-vis-NIR) spectroscopy (V-670, JASCO, Tokyo, Japan), X-ray diffraction (XRD, D8-Advance, Bruker-AXS, Wisconsin, USA) and X-ray photoelectron spectroscopy (XPS, K-alpha+, Thermo Fisher Scientific-KR, Seoul, Korea). Electrical characterization of the produced TFTs was conducted using a semiconductor parameter analyzer (4156C, Agilent Technologies, Santa Clara, USA) at RT, in the dark, under vacuum to avoid possible effects of the ambient environment on the properties of the IGTO TFT.   Figure 2 shows the transfer characteristics of the N 2 -, O 2 -, and air-annealed IGTO TFTs plotted on a semi-logarithmic scale, where I D , V GS , and V DS are the drain current, gate-to-source voltage, and drain-to-source voltage, respectively. Measurements were conducted by sweeping V GS from −30 to 30 V at V DS = 0.5 V for all TFTs. Table 1 lists the electrical parameters of the three fabricated TFTs. The field-effect mobility (µ FE ) was calculated from the maximum transconductance at a V DS of 0.5 V, and the threshold voltage (V TH ) was obtained from the V GS value, assuming that I D = W/L × 10 −9 (A). The subthreshold swing (SS) was determined as the dV GS /dlogI D value in the range of 10 −10 < I D < 10 −9 A. Electronics 2020, 9, x FOR PEER REVIEW 3 of 11 Figure 2 shows the transfer characteristics of the N2-, O2-, and air-annealed IGTO TFTs plotted on a semi-logarithmic scale, where ID, VGS, and VDS are the drain current, gate-to-source voltage, and drain-to-source voltage, respectively. Measurements were conducted by sweeping VGS from −30 to 30 V at VDS = 0.5 V for all TFTs. Table 1 lists the electrical parameters of the three fabricated TFTs. The field-effect mobility (μFE) was calculated from the maximum transconductance at a VDS of 0.5 V, and the threshold voltage (VTH) was obtained from the VGS value, assuming that ID = W/L × 10 −9 (A). The subthreshold swing (SS) was determined as the dVGS/dlogID value in the range of 10 −10 < ID < 10 −9 A.  The results presented in Figure 2 and Table 1 indicate that the highest μFE (26.6 cm 2 /V·s) and lowest VTH (= −6.0 V) were achieved with the N2-annealed IGTO TFT. In contrast, the lowest μFE (= 23.5 cm 2 /V·s) and highest VTH (= −2.0 V) were obtained with the air-annealed IGTO TFT. Finally, the corresponding values for the O2-annealed IGTO TFT were μFE = 25.0 cm 2 /V·s and VTH = −3.2 V, which lie between the corresponding values obtained for the N2-and air-annealed IGTO TFTs. The O2annealed IGTO TFT afforded the lowest SS of 0.48 V/dec. as compared with those of the IGTO TFTs annealed in other environments. The obtained results clearly show that the post-deposition annealing atmosphere significantly affects the electrical performance of the IGTO TFTs.  Figure 3d displays the VTH shifts (ΔVTH) determined for the three IGTO TFTs at various stress times. Analysis of the transfer characteristics of the respective TFTs showed a shift of VTH in the positive direction with an increase in the stress time, and the largest ΔVTH was observed for the air annealed IGTO TFT. However, the ΔVTH was significantly lower for the N2and O2-annealed IGTO TFTs than for the air-annealed IGTO TFT after subjection to PBS for the same duration.  The results presented in Figure 2 and Table 1 indicate that the highest µ FE (26.6 cm 2 /V·s) and lowest V TH (= −6.0 V) were achieved with the N 2 -annealed IGTO TFT. In contrast, the lowest µ FE (= 23.5 cm 2 /V·s) and highest V TH (= −2.0 V) were obtained with the air-annealed IGTO TFT.   Figure 3d displays the V TH shifts (∆V TH ) determined for the three IGTO TFTs at various stress times. Analysis of the transfer characteristics of the respective TFTs showed a shift of V TH in the positive direction with an increase in the stress time, and the largest ∆V TH was observed for the air annealed IGTO TFT. However, the ∆V TH was significantly lower for the N 2 -and O 2 -annealed IGTO TFTs than for the air-annealed IGTO TFT after subjection to PBS for the same duration. The SS value remains nearly unchanged during the PBS. Therefore, we did not consider additional defect generation in the active region during the PBS [25,26].    The SS value remains nearly unchanged during the PBS. Therefore, we did not consider additional defect generation in the active region during the PBS [25,26].    The transfer curves of all IGTO TFTs shifted in the negative direction with an increase in the stress time. The largest V TH shift was observed for the N 2 -annealed IGTO TFT; however, for the O 2 -and air annealed IGTO TFTs, ∆V TH was lower than that of the N 2 -annealed IGTO TFT after subjection to the negative bias illumination stress (NBIS). The SS value remained nearly unchanged during the NBIS. Figures 3 and 4 demonstrate that the post-deposition annealing environment affectrf not only the electrical properties of the IGTO TFTs, but also their stability under PBS and NBIS [27][28][29]. To elucidate the physical mechanism responsible for the processes illustrated in Figures 3 and 4, the IGTO thin films annealed under different atmospheres were characterized by XRD, UV-vis-NIR spectroscopy, and XPS. Figure 5 shows the XRD patterns of the N 2 -, O 2 -, and air annealed 20 nm thick IGTO thin films deposited on glass substrates. The obtained diffraction patterns contained only halo peaks at approximately 23 • and 45 • , originating from the glass substrates [30]; this suggests that the IGTO thin films comprised an amorphous phase, regardless of the annealing environment.  The transfer curves of all IGTO TFTs shifted in the negative direction with an increase in the stress time. The largest VTH shift was observed for the N2-annealed IGTO TFT; however, for the O2and air annealed IGTO TFTs, ΔVTH was lower than that of the N2-annealed IGTO TFT after subjection to the negative bias illumination stress (NBIS). The SS value remained nearly unchanged during the NBIS. Figures 3 and 4 demonstrate that the post-deposition annealing environment affectrf not only the electrical properties of the IGTO TFTs, but also their stability under PBS and NBIS [27][28][29]. To elucidate the physical mechanism responsible for the processes illustrated in Figures 3 and 4, the IGTO thin films annealed under different atmospheres were characterized by XRD, UV-vis-NIR spectroscopy, and XPS. Figure 5 shows the XRD patterns of the N2-, O2-, and air annealed 20 nm thick IGTO thin films deposited on glass substrates. The obtained diffraction patterns contained only halo peaks at approximately 23° and 45°, originating from the glass substrates [30]; this suggests that the IGTO thin films comprised an amorphous phase, regardless of the annealing environment.  Figure 6a shows the optical transmittance spectra of the N2-, O2-, and air annealed 20 nm-thick IGTO thin films on the glass substrates, which were recorded in the wavelength range of 300-1400 nm. The optical transmittance of the glass substrate was subtracted from the obtained spectra to determine the actual optical transmittance of the deposited IGTO thin films. Figure 6b shows the Tauc plot constructed from the spectra presented in Figure 6a. The optical bandgap (Eg) of the IGTO thin films was approximately 3.88 eV regardless of the annealing environment, which indicates that the annealing environment had no effect on the Eg value.  Figure 6a shows the optical transmittance spectra of the N 2 -, O 2 -, and air annealed 20 nm-thick IGTO thin films on the glass substrates, which were recorded in the wavelength range of 300-1400 nm. The optical transmittance of the glass substrate was subtracted from the obtained spectra to determine the actual optical transmittance of the deposited IGTO thin films. Figure 6b shows the Tauc plot constructed from the spectra presented in Figure 6a. The optical bandgap (E g ) of the IGTO thin films was approximately 3.88 eV regardless of the annealing environment, which indicates that the annealing environment had no effect on the E g value. Figure 7a-c displays the XPS O 1s spectra of the N 2 -, O 2 -, and air annealed IGTO thin films, respectively, recorded for the middle of the thin films. The obtained XPS profiles were deconvoluted into three sub-peaks originating from the lattice oxygen (O I ), V O (O II ), and impurity-related oxygen (O III ), respectively, using the Gaussian function. The binding energies of these components were fixed at 529.8 ± 0.1 eV, 530.7 ± 0.1 eV, and 531.8 ± 0.1 eV, respectively [7,31]. Figure 7d shows the relative areas of the O I , O II , and O III peaks, which were obtained for the IGTO thin films, annealed under different atmospheres. The XPS data presented in Figure 7 indicate that the relative area of the O III component of the air-annealed IGTO thin film was much larger than that of the N 2 -and O 2 -annealed films. In previous studies on oxide thin films and TFTs, O III was primarily attributed to the oxygen bonds in OH functional groups, which generate acceptor-like states near the conduction band (CB) edge and enhance electron trapping during PBS application because of their polar nature in oxide semiconductors, such as IGZO and IGTO [32][33][34][35]. Therefore, the small value of µ FE , large value of SS, and poor PBS stability of the air-annealed IGTO TFT can be attributed to the high concentration of OH groups within the channel layer that originated from the H 2 O species present in air. Because an n-type TFT with a higher density of acceptor-like states near the CB edge requires a larger V GS to switch on and fill up the states, the highest V TH value obtained for the air-annealed IGTO TFT can also be ascribed to the large number of OH groups within the IGTO channel. Figure 7a-c displays the XPS O 1s spectra of the N2-, O2-, and air annealed IGTO thin films, respectively, recorded for the middle of the thin films. The obtained XPS profiles were deconvoluted into three sub-peaks originating from the lattice oxygen (OI), VO (OII), and impurity-related oxygen (OIII), respectively, using the Gaussian function. The binding energies of these components were fixed at 529.8 ± 0.1 eV, 530.7 ± 0.1 eV, and 531.8 ± 0.1 eV, respectively [7,31]. Figure 7d shows the relative areas of the OI, OII, and OIII peaks, which were obtained for the IGTO thin films, annealed under different atmospheres. The XPS data presented in Figure 7 indicate that the relative area of the OIII component of the air-annealed IGTO thin film was much larger than that of the N2-and O2-annealed films. In previous studies on oxide thin films and TFTs, OIII was primarily attributed to the oxygen bonds in OH functional groups, which generate acceptor-like states near the conduction band (CB) edge and enhance electron trapping during PBS application because of their polar nature in oxide semiconductors, such as IGZO and IGTO [32][33][34][35]. Therefore, the small value of μFE, large value of SS, and poor PBS stability of the air-annealed IGTO TFT can be attributed to the high concentration of OH groups within the channel layer that originated from the H2O species present in air. Because an n-type TFT with a higher density of acceptor-like states near the CB edge requires a larger VGS to switch on and fill up the states, the highest VTH value obtained for the air-annealed IGTO TFT can also be ascribed to the large number of OH groups within the IGTO channel.

Results and Discussion
Further, the largest OII relative area was obtained for the N2-annealed IGTO thin film (Figure 7). Note that VO generates shallow and deep donor states within the oxide channel layer. The shallow donor states supply electrons to the CB; thus, the electron concentration increases with an increase in the number of VO sites within the channel layer. Furthermore, the higher electron concentration promotes the formation of percolation conduction paths in oxide semiconductors, such as IGZO and IGTO, making it very difficult to turn off the transistor [36][37][38]. Therefore, the low value of VTH and the high values of μFE and SS obtained for the N2-annealed IGTO TFT can be attributed to the large VO concentration within the channel layer caused by the desorption of oxygen atoms during N2 annealing. The poor NBIS stability of the N2-annealed IGTO TFT is likely caused by the high density of VO within the IGTO channel, because VO generates VO 2+ species, which subsequently diffuse toward the gate insulator/channel interface under NBIS [39].   Figure 7. X-ray photoelectron spectroscopy (XPS) O 1s spectra of (a) N2-, (b) O2-, and (c) air annealed IGTO thin films, respectively, recorded for the middle of the thin films. (d) Relative areas of the OI, OII, and OIII peak regions that were obtained for the IGTO thin films annealed under different atmospheres.

Conclusions
The effects of various post-deposition annealing environments on the electrical characteristics and stability of high-mobility IGTO TFTs were evaluated herein. The post-deposition annealing process was conducted at 180 °C under N2, O2, or air atmosphere. The lowest μFE and highest SS and VTH were obtained with the air annealed IGTO TFT, along with the lowest PBS stability. This  Further, the largest O II relative area was obtained for the N 2 -annealed IGTO thin film (Figure 7). Note that V O generates shallow and deep donor states within the oxide channel layer. The shallow donor states supply electrons to the CB; thus, the electron concentration increases with an increase in the number of V O sites within the channel layer. Furthermore, the higher electron concentration promotes the formation of percolation conduction paths in oxide semiconductors, such as IGZO and IGTO, making it very difficult to turn off the transistor [36][37][38]. Therefore, the low value of V TH and the high values of µ FE and SS obtained for the N 2 -annealed IGTO TFT can be attributed to the large V O concentration within the channel layer caused by the desorption of oxygen atoms during N 2 annealing. The poor NBIS stability of the N 2 -annealed IGTO TFT is likely caused by the high density of V O within the IGTO channel, because V O generates V O 2+ species, which subsequently diffuse toward the gate insulator/channel interface under NBIS [39].

Conclusions
The effects of various post-deposition annealing environments on the electrical characteristics and stability of high-mobility IGTO TFTs were evaluated herein. The post-deposition annealing process was conducted at 180 • C under N 2 , O 2 , or air atmosphere. The lowest µ FE and highest SS and V TH were obtained with the air annealed IGTO TFT, along with the lowest PBS stability. This phenomenon is attributed primarily to the large number of OH groups within the IGTO channel layer that originated from the H 2 O molecules in air. The µ FE and PBS stability of the N 2 -and O 2 -annealed IGTO TFTs are higher than those of the air-annealed IGTO TFT. However, the NBIS stability of the N 2 -annealed IGTO TFTs is lower, accompanied by a larger negative shift of the V TH values compared with the corresponding parameters for the O 2 -annealed IGTO TFTs because of the large V O concentration within the channel layer, caused by the desorption of oxygen atoms during N 2 annealing. The obtained results suggest that O 2 gas is the most suitable annealing environment for optimizing the electrical properties and stability of IGTO TFTs.