Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Intense Pulsed Light Annealing Process of Indium−Gallium−Zinc− Oxide Semiconductors via Flash White Light Combined with DeepUV and Near-Infrared Drying for High-Performance Thin-Film Transistors Chang-Jin Moon† and Hak-Sung Kim*,†,‡ †
Department of Mechanical Engineering, Hanyang University, Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea Institute of Nano Science and Technology, Hanyang University, Seoul 133-791, Republic of Korea
ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 03/27/19. For personal use only.
‡
S Supporting Information *
ABSTRACT: In this study, an intense pulsed light (IPL) process for annealing an indium− gallium−zinc−oxide (IGZO) semiconductor was conducted via flash white light combined with near-infrared (NIR) and deep-ultraviolet (DUV) drying to form a thin-film transistor (TFT). The IGZO thin-film semiconductor was fabricated using a solution-based process on a doped-silicon wafer covered with silicon dioxide. In order to optimize the IPL irradiation condition for the annealing process, the flash white light irradiation energy was varied from 70 to 130 J/cm2. Drying by NIR and DUV irradiation was employed and optimized to improve the performance of the TFT during IPL annealing. A TFT with a bottom-gate and top-contact structure was formed by depositing an aluminum electrode on the source and drain on the IPL-annealed IGZO. The electrical transfer characteristic of the TFT was measured using a parameter analyzer. The field effect mobility of the saturation regime and on/off current ratio were evaluated. Changes of the metal−oxide bonds in the IGZO thin film were analyzed using X-ray photoelectron spectroscopy to verify the effect of NIR and DUV drying and IPL annealing. Also, the distributions of the carrier concentration on the IPL-annealed IGZO were measured through a hall-effect system to deeply investigate the transition of the electrical characteristic of the TFT. From the results, it was found that the bond between oxygen and the gallium compound was activated via DUV irradiation. The NIR- and DUV-assisted IPL-annealed IGZO-based TFT showed highly enhanced electrical performance with a 7.7 cm2/V·s mobility and a 3 × 106 on/off ratio. KEYWORDS: IGZO semiconductor, intense pulsed light annealing, near-infrared, deep-UV, gallium oxide, mobility, on/off ratio, thin-film transistor sputtered film was achieved.13 However, sol−gels require hightemperature annealing (above 300 °C) to remove organic ligand groups and to activate the IGZO film properties.11 To overcome the annealing temperature problem, a variety of approaches were proposed, such as modifying the precursor chemistry,13 addition of combustion processing,14 and light annealing15−17 (i.e., excimer laser, deep-ultraviolet (DUV), and intense pulsed light). Among these methods, the light annealing process has begun to attract attention because of its low process temperature and selective heating of materials without substrate damage.15−21 However, the laser annealing process has several disadvantages, such as expensive equipment and too small annealing area.15 The DUV annealing process has the drawback that it requires a long time (around 2 h) in an inert atmosphere.16 On the other hand, the IPL process using flash white light irradiation is able to anneal thin films
1. INTRODUCTION Thin-film transistors (TFTs) based on metal oxide semiconductors have received significant attention as switching devices for application in high-definition display backplanes.1−3 Various metal oxide semiconductors have been developed and amorphous In−Ga−Zn−O (IGZO) has shown outstanding electrical properties, including high mobility (exceeding 10 cm2/V·s), in spite of its purely amorphous-state structure.4,5 In addition, a uniform film was obtained using vacuum deposition techniques, such as sputtering or atomic layer deposition at room temperature on flexible substrates.6−9 Thus, IGZO TFTs emerged as an alternative to hydrogenated amorphous silicon TFTs for use in transparent and flexible TFTs on polymer substrates. However, these techniques require high vacuum, are relatively expensive, and make it difficult to control the composition of the metal oxide film. For the solution process, a sol−gel method that converts metal−organic precursor solutions to metal−oxide films by heating has widely been used to form metal−oxide thin films.10−13 Through this process, a high electrical performance equivalent to that of the © XXXX American Chemical Society
Received: December 24, 2018 Accepted: March 18, 2019 Published: March 18, 2019 A
DOI: 10.1021/acsami.8b22458 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
and the IGO was prepared by excluding zinc (IZO: 0.108 M, In/Zn = 7:2; IGO: 0.096 M, In/Ga = 7:1). The other processes were conducted similarly to that used for IGZO. 2.2. IPL Annealing of the IGZO Film via Flash White Light Combined with DUV and NIR. As shown in Figure 1a, the
within a short time (a few milliseconds) at room temperature under ambient conditions.17−21 In several studies, metal oxide thin films were annealed using IPL by controlling the IPL energy.17,22,23 In general, the intensity of IPL determines the temperature during the annealing process. However, the thin film formed may not be dense because of its short irradiation time and temperature deviation in the layers.24−26 Because of these problems, the IGZO annealed by the IPL method showed a low mobility of 2.67 cm2/V·s, and the mobility deviation was large, even under the same annealing conditions.17 Although annealing of single-structure materials such as In2O3 and ZnO has been attempted, such a single structure has many limitations to be practically used as an active layer of a TFT. In addition, annealing has been conducted using flash lamp irradiation for more than 10 s, but this irradiation method is difficult to apply for a large-area process.22,23 In cases of annealing metal layers such as copper and silver electrodes, a two-step flash light annealing method or flash white light combined with DUV and near-infrared (NIR) was adopted to anneal metal layers effectively.27,28 However, these combined multipulsed IPL annealing processes have not yet been tried for annealing IGZO layers. The total annealing process of IGZO proceeds step by step, including (1) low-temperature drying for decomposition and hydrolysis and (2) high temperature for dehydroxylation and alloys of metal and oxygen.29 As the quality of the IGZO film has a significant effect on the performance of TFTs, there is demand for a systematic annealing process to develop physically stable and chemically uniform layers.30 In this study, an IPL annealing process of the IGZO semiconductor was developed via flash white light combined with DUV and NIR. The effect of IPL energy and NIR irradiation on the drying step was investigated. Furthermore, the DUV drying-assisted IPL annealing process was performed by changing the DUV intensity. To evaluate the electrical property of the IPL-annealed semiconductor, TFTs were fabricated on a silicon wafer using a sputtered electrode and the current characteristic versus voltage was observed. To verify the mechanism of IPL annealing of IGZO, changes of the metal−oxide bonding were measured using X-ray photoelectron spectroscopy (XPS). Also, distribution of the carrier concentration on IPL-annealed IGZO was analyzed through the hall-effect system to investigate the application of DUV on IPL annealing. Finally, the IPL annealing process using flash white light combined with DUV and NIR was optimized to anneal the IGZO semiconductor for a TFT with high mobility and on/off ratio.
Figure 1. Schematics of the IPL annealing process of IGZO via flash white light combined with NIR and DUV. (a) Schematic diagrams of the drying and annealing process by combined light sources and TFT with a bottom-gate and top-contact structure (inset image). (b) Diagram of the solution-processed IGZO conversion process from the precursor state to the amorphous oxide semiconductor state.
irradiation with flash white light, NIR, and DUV was conducted to dry and anneal the IGZO thin-film semiconductor at room temperature under ambient conditions. The NIR intensity (wavelength: 800−1500 nm, Adphos L40) was fixed at 3 W/cm2 and NIR was performed for 1 min before flash white light irradiation. The flash white light system consisted of a xenon flash lamp (PerkinElmer Co.), a power supply, capacitors, a pulse controller, and a water-cooling system. The flash white light from the xenon flash lamp has a broad wavelength range (380−950 nm), as explained in a previous work.27 The IPL energy was changed from 70 to 130 J/cm2 and the pulse duration time, gap time, and number were fixed to 20 ms, 30 ms, and 5, respectively. In addition, a DUV system (100 mW, Lumatec SUV-DC) with wavelength range from 180 to 280 nm was used for drying. The DUV intensity was controlled from 30 to 90 mW/cm2. The NIR and DUV were used simultaneously to irradiate the IGZO for 1 min in the drying step. This was followed by the flash white light irradiation on the annealing process. 2.3. Fabrication and Characterization of the IGZO-Based Transistor. To fabricate the IGZO-based transistor on a Si wafer, a 50 nm thick aluminum (Al) electrode was deposited on the IPLannealed IGZO semiconductor using a dc sputtering system. To build an electron path on the semiconductor, the designed channel length and width were 50 and 500 μm, respectively (inset image of Figure 1a). A shadow mask was used to build the designed channel shape, and the channel pattern was formed on IGZO surface without a large error as shown in Figure S1. To estimate the electrical property of the IPL-annealed IGZO TFT, the current−voltage characteristic of the TFT was measured using a parameter analyzer (Keithley 4200-SCS) under ambient conditions in a dark box. The drain voltage was fixed at 30 V and gate voltage was controlled from −30 to 30 V. The TFT performance was calculated using the I(Ids) − V(Vg) curve. The saturation mobility (μsat) was obtained from the following equation.
2. EXPERIMENTAL SECTION 2.1. Fabrication of an IGZO Thin-Film Semiconductor on a Si Wafer. The IGZO precursor solutions were prepared by mixing zinc acetate dehydrate, indium nitrate(III) hydrate, and gallium nitrate(III) hydrate in 2-methoxyethanol. The total concentration of the solution was 0.12 M, and the mole ratio of the atoms was adjusted to 7 (In):1 (Ga):2 (Zn). This solution was vigorously stirred (800 rpm) for over 12 h. Meanwhile, a heavily p-type doped silicon wafer covered with 300 nm-thick, thermally grown SiO2 was sonicated in acetone, then isopropyl alcohol for 5 min. The substrates were treated with ultraviolet−ozone for 5 min. Subsequently, the IGZO solution was applied using a spin-coater (SC-200, Nano-tech) at 3000 rpm for 30 s. The thickness of the IGZO layer coated on the Si wafer was 11.93 nm, which was measured by an ellipsometer (Elli-SE-UaM8, Ellipso Technology Co. Ltd.). The IZO solution was prepared by excluding the precursor amount of gallium from the IGZO solution
Ids,sat = Ci·μsat · W ·2L−1(Vg − Vth)2 B
(1)
DOI: 10.1021/acsami.8b22458 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 2. Electrical transfer properties (a) and mobility and on/off ratio (b) of an IPL-annealed IGZO-based TFT via NIR-assisted flash white light (energy range: 70−160 J/cm2, pulse duration: 20 ms, pulse interval: 30 ms, 5 pulses, NIR intensity: 3 W/cm2).
Figure 3. IPL annealing of IGZO using flash white light combined with DUV and NIR (DUV irradiation intensity: 30 mW/cm2). (a) Transfer properties of TFTs and (b) mobility and on/off ratio of IGZO-based TFTs annealed using flash white light, DUV, and NIR.
IGZO film, thereby inducing more efficient annealing.25 As more heat is generated in the IGZO film, more M−O−M bonds form, which induces semiconductor characteristics. Up to an IPL energy of 100 J/cm2, the on-current level of the TFT was improved with increasing IPL energy up to 10−4 A. This is the typical semiconductor characteristic of IGZO (see Figure 2b). However, with flash light irradiation of 130 J/cm2, the current level was increased to as high as 10−3 A, and its offcurrent also increased more than that of the lower IPL energy, as shown by the green line of Figure 2a. With this trend, it could be seen that as the on-current rises, the mobility of the TFT increases, as shown in Figure 2b. With irradiation of NIRassisted flash white light of 100 J/cm2, mobility was 8.12 cm2/ V·s, and with irradiation of 130 J/cm2, it exceeded 10 cm2/V·s and was calculated to be 13.5 cm2/V·s. This result implies that an IGZO with high mobility could be formed through NIR drying followed by IPL annealing without a conventional thermal drying process, and in a shorter process time. However, as the flash white light energy increases, the TFT on/off ratio decreases, whereas the mobility tends to increase. When an IPL energy of 130 J/cm2 was used, the on/off ratio became much lower than 104, which is insufficient performance for TFTs. Therefore, it was found that using only NIR drying followed by IPL annealing could not enhance the TFT on/off ratio sufficiently. This electrical behavior of TFTs was also reported when IGZO was processed with excimer laser annealing (ELA), high-temperature annealing, and annealing under nitrogen or vacuum atmosphere.31−34 As the parameters such as laser energy and temperature on annealing of IGZO increase, the carrier concentration in IGZO changes and the off-current rises as transfer property of IPL-annealed IGZO.
where Ci, W, L, and Vth denote the gate capacitance, channel width, length, and threshold gate voltage, respectively. Depending on the IPL irradiation energy and DUV irradiation, the carrier concentration of the annealed IGZO was observed using a hall-effect system (HL5500PC, Bio-Rad Co.). The metal−oxide bonding distribution on the IGZO was analyzed using XPS (K-alpha, Thermo Fisher Scientific Co.).
3. RESULTS AND DISCUSSION 3.1. NIR-Assisted IPL Annealing of IGZO. As shown in Figure 1b, the process for annealing the solution IGZO semiconductor was conducted by the formation of M−OH groups through hydrolysis of the precursors in the solvent, followed by the formation of metal−oxide−metal (M−O−M) bonds and crystalline at high temperature (above 300 °C).29 Thus, low-temperature drying using NIR and high-temperature annealing using IPL were performed in this work. In a conventional drying process, a heater was used.22 In this study, the drying process by a heater was replaced by NIR irradiation, which is more appropriate for the continuous light drying− annealing process (IPL, NIR, and DUV). The IGZO film was dried by irradiation with NIR light of 3 W/cm2 intensity for 1 min, followed by IPL annealing with different IPL energies (results are shown in Figure 2a). In the case of the sample dried by NIR irradiation without IPL, the current level was low and semiconductor characteristics, such as current amplification at a specific voltage, were not observed. This might be because the IGZO was hydrolyzed only during NIR drying and its metal−oxide bond had not formed yet. It was also found that the on-current level of the TFT increases with rising IPL energy, a result similar to that in previous research.17 This is because the higher IPL energy generated more heat in the C
DOI: 10.1021/acsami.8b22458 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 4. Effect of DUV irradiation intensity on IPL annealing of IGZO. (a) Change of transfer property (I−V curve) on increasing the DUV intensity from 30 to 90 mW/cm2 (IPL energy: 100 J/cm2). (b) Mobility and on/off ratio of IGZO-based TFTs according to the intensity of DUV.
To examine closely the effect of DUV, NIR−DUV drying followed by IPL annealing was performed by adjusting the intensity of DUV from 30 to 90 mW/cm2, with the NIR intensity and IPL energy fixed at 3 W/cm2 and 100 J/cm2, respectively. Figure 4a shows the IGZO-based TFT characteristics depending on the DUV intensity. The off-current varied according to the DUV intensity. In the case of 60 mW/cm2 DUV irradiation, the on-current was the same as that of irradiation with 30 mW/cm2 DUV. In addition, the off-current decreased about 102 times. In the case of irradiation with 90 mW/cm2, the on-current increased again more than for other conditions; however, the off-current also increased in this case. The on/off ratio increased until DUV energy reached 60 mW/ cm2 and then decreased again about 103 times when DUV intensity reached 90 mW/cm2 (see Figure 4b). The mobility decreases slightly as the intensity of DUV increases, and increases sharply with a DUV of 90 mW/cm2. Under this intensity condition, it was found that the state of indium− oxygen bond is abruptly changed and the TFT characteristics are deteriorated, whereas Zn−O, Ga−O including In−O were stable at the 60 mW/cm2 condition (Figure S2). The application of DUV could improve the TFT characteristics, but the intensity of the DUV needs to be controlled in the annealing process, considering the bond state change of the materials on the IGZO (Figure S2). In conclusion, the optimal annealing conditions chosen for IGZO were DUV intensity of 60 mW/cm2 and IPL irradiation of 100 J/cm2. This combination enabled the realization of a TFT with superior performance for both mobility and on/off ratio. To review, when compared with the annealing of IGZO for 2 h with deepUV only, IGZO with a high mobility of 5 cm2/V·s could be formed on the polymer film or glass substrate using DUV. However, it showed low mobility (2.3 cm2/V·s) and inadequate on/off ratio (104) on the Si/SiO2 substrate. In ambient air, semiconductor characteristics could not be obtained with IGZO during only DUV annealing despite 2 h of annealing.16 Through optimized NIR−DUV drying with IPL annealing, IGZO could be annealed under ambient conditions and the optimally dried, then IPL annealed TFT showed good performance for mobility (7.7 cm2/V·s) and on/ off ratio (3 × 106). These performances were comparable to the standard characteristics of IGZO produced by conventional deposition (∼10 cm2/V·s, >105 on/off ratio).35 3.3. Effect of DUV Drying on IPL Annealing of IGZO and Its Characterization. To investigate more deeply the role of DUV and IPL on the annealing of IGZO, IZO (indium−zinc−oxide) and IGO (indium−gallium−oxide)
Above a certain annealing parameter level, IGZO did not show the switching characteristic and it changed into a conductive channel.32−34 This phenomenon is closely related to the oxygen vacancy and carrier concentration on IGZO, which will be discussed further below. 3.2. IPL Annealing Combined with DUV. In order to improve the on/off ratio characteristics of the TFT, DUV was added for the NIR-assisted IPL annealing process. DUV (30 mW/cm2) was used to irradiate the surface of IGZO for 1 min simultaneously with NIR (3 W/cm2) irradiation, followed by IPL irradiation. In Figure 3a, the off-current level was reduced more than 10 times at the same IPL energy, whereas the oncurrent was reduced slightly, compared to the result with only NIR drying and IPL annealing (see Figure 2a). It is noteworthy that the threshold voltage (Vth) was close to 0 V, forming a state called enhancement mode (Figure 3a), whereas it was negative with only NIR drying and IPL annealing. As shown in Figure 3b, as the IPL energy increases, the on/off ratio decreases similarly to the case with only NIR drying and IPL annealing. It can be found that, in the range 70−100 J/cm2 of IPL energy, the on/off ratio of the IGZO with NIR−DUV drying and IPL annealing (105 to 108) is much higher than in the case with only NIR drying and IPL annealing (104 to 107). At the same time, its mobility value decreases slightly, similar to that with only NIR drying and IPL annealing. With the IPL energy of 130 J/cm2, the on/off ratio decreases to 104 (as with NIR drying and IPL annealing). From that result, it was concluded that the DUV irradiation during NIR drying has a great effect on improving the on/off ratio of the IGZO TFT. In addition, one of the switching parameters of TFT, subthreshold slop (SS), could be seen to be improved. Regardless of the drying condition, the SS value tended to decrease as the IPL energy used for annealing increased. On the other hand, when DUV was applied to the drying process, it was found that the SS value was improved more than that of NIR drying followed by IPL-annealed IGZO (Table S1). This might be because the IGZO thin film shows a strong light-absorption peak in the UV region of 300 nm or less, and the DUV at this wavelength band can easily activate chemical reactions on the IGZO film.16 However, note again that with an annealing process for IGZO using only DUV, an inert gas atmosphere (Ar, N2) and a long irradiation time (around 2 h) are required.16 In our newly proposed process (NIR−DUV drying followed by IPL) annealing can be accomplished within a much shorter irradiation time of 1 min and the on/off ratio of TFTs can be improved without complicated equipment, such as an inert atmosphere and a special chamber. D
DOI: 10.1021/acsami.8b22458 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 5. Effect of DUV irradiation on indium- and gallium-based semiconductors. The transfer property curves of an (a) indium−zinc−oxide (IZO) semiconductor and an (b) indium−gallium−oxide (IGO) semiconductor (IPL energy range from 70 to 100 J/cm2, DUV with intensity of 60 mW/cm2).
Figure 6. Effect of DUV drying on IPL annealing of IGZO. (a) Change of the metal−oxide bond by the drying method on the O 1s peak of XPS analysis. (b) Gallium−oxide peak of Ga 2p depending on the drying method. (c) Mechanism of metal−oxide bond consolidation on the IGZO semiconductor by gallium−oxide bond activation from DUV irradiation.
were fabricated. The IPL annealing was carried out using flash white light with energy from 70 to 100 J/cm2 after a 1 min drying process using 60 mW/cm2 of DUV and 3 W/cm2 of NIR. Figure 5a shows the I−V characteristics of IZO according to the IPL energy and DUV−NIR drying conditions. In the case of 70 J/cm2 of IPL irradiation, the curve shifts to the left (the threshold voltage decreases) from the DUV irradiation during the drying process (compare NIR → IPL 70 J/cm2 case and (NIR + DUV) → IPL 70 J/cm2). In this condition, the threshold voltage could be shifted because of the increased carrier concentration of IZO by the application of DUV as shown in Figure S3. At higher IPL energy conditions, the change in the carrier concentration was not significant. Under relatively low IPL annealing conditions, a noticeable carrier concentration change on IZO could be observed with the application of DUV. It is noteworthy that in this case, the offcurrent level does not change in spite of the DUV irradiation. With irradiation of 100 J/cm2, the I−V curve is saturated upward with a high off-current value. In this case, DUV
irradiation during drying did not make any difference in the off-current level and on/off ratio of the TFT. Unlike IZO, in the case of IGO, the use of DUV results in significant changes in the TFT characteristics, as shown in Figure 5b. With irradiation of 70 J/cm2, the overall curve for NIR−DUV drying with IPL annealing can be observed to be slightly lower than that for only NIR drying with IPL. With IPL of 100 J/cm2, this difference is even more pronounced. In the case of NIR−DUV drying followed by IPL annealing, the on-current levels were similar to those of NIR-assisted IPL annealing, but the offcurrent level remained low compared to the case of only NIR drying with IPL annealing. That is, by applying DUV in the drying process, not only electrical conductivity such as mobility of IPL-annealed IGZO and IGO can be developed, but switching characteristics such as on/off ratio can be improved. Especially, gallium among the metal oxide components affects these properties as a carrier suppressor, and this tendency was shown in the application of DUV on IPL annealing. On the basis of the analysis of the carrier concentration and metal− E
DOI: 10.1021/acsami.8b22458 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 7. XPS analysis of the metal−oxygen bonding state on the IPL-annealed IGZO semiconductor depending on drying and annealing conditions. The distribution of (a) M−O, M−OH and (b) Vo on IPL-annealed IGZO. (c) Schematics of the bonding conversion of IGZO from a semiconductor to a conductor with the IPL energy level.
a carrier generation suppressor on IGZO, so IGZO can be formed with a high on/off ratio by combining appropriate gallium contents, thereby lowering the off-current level.38−40 The bond between indium and oxygen is relatively weak. Through the breaking of this bond, two electrons are obtained from the oxide element, and these electrons act as charge carriers. In contrast, gallium and oxygen have the strongest bonds and prevent the creation of carriers by oxygen separation.35 On this basis, the bonds between gallium and oxygen formed by DUV drying can prevent excess carrier generation in the IGZO because of the separation of oxygen during the IPL annealing, thus lowering the off-current level. As shown in Figure 6c, by using DUV in the drying step, the separation of oxygen can be controlled and the carriers can be maintained at an appropriate level. In this way, not only high mobility, but also a high on/off ratio can be realized through IPL annealing. Figure 7a shows the distribution of the O 1s peak on annealed IGZO, according to annealing conditions. Because 70 J/cm2 of IPL energy is irradiated, most of the bond states on the IGZO are converted to M−O bonds. When IGZO is annealed at 100 J/cm2, the M−O bonds decrease. The distribution of oxygen vacancies (Vo) increases slightly as the IPL energy increases, as shown in Figure 7b. This means that with increased IPL energy, the formation of oxygen vacancies because of the separation of metal and oxygen increases sharply, rather than the more stable bond structure between metal and oxygen as shown in Figure 7c. It is noteworthy that in the case of NIR−DUV drying and IPL-annealed IGZO, the reduction ratio of M−O bonding is relatively smaller and the degree of oxygen vacancy formation lower than in the case of only NIR drying and IPL-annealed IGZO. The oxygen vacancies can promote charge carriers to form high-mobility semiconductors, but too many oxygen vacancies can make the IGZO convert from a semiconductor to a conductor because of the excessive charge carrier. Therefore, the oxygen vacancy should be controlled at a certain level in the active layer of the TFT.39,41 The control of oxygen vacancy via DUV drying can
oxide bonding state, the effect of DUV on IGZO annealing will be discussed further below. Figure 6a shows the XPS data of IGZO before and after NIR or DUV−NIR drying of IGZO and chemical compositions of the metal atom on IGZO is shown in Table S2 (In: 69.16%, Ga: 9.15%, Zn: 21.69%). Depending on the drying conditions, the distribution of O 1s peaks on the IGZO changes as shown in Figure 6a. In general, the O 1s peak of IGZO occurs mainly at 530 eV (O1 peak); 531.2 eV (O2 peak), and 532 eV (O3 peak). The O1 peak is attributed to metal−oxide bonding (M−O); the O2 peak located at 531.2 eV is reflecting the oxygen vacancies (Vo); and the O3 peak located at 532 eV corresponds to the existence of weakly bound oxygen species on the film surface such as −OH.36 Because DUV and NIR are used simultaneously in the drying step, the M−O bond was activated. In the case of drying with NIR, before drying, the M−OH peak of 532 eV was dominant, but the M−O peak was largest when drying with DUV and NIR as shown in Figure 6a. With the activation of these chemical bonds, on drying conditions, physical changes such as thickness are shown in Figure S4 (as-coated: 11.93 ± 1.25 nm, NIR drying: 8.98 ± 0.34 nm, NIR + DUV drying: 7.78 ± 0.288 nm). On the Ga 2p peak, the binding energy of gallium and oxygen (Ga−O) was changed as shown in Figure 6b. When the distribution of the other components is assumed to be the same, NIR drying causes the degree of binding to be activated and the amplitude of the peak increased. By NIR−DUV drying, the peak of Ga− O shifts from 1117.9 to 1117.7 eV with lower binding energy. This means that the binding state of Ga−O reaches a more stable binding state than under other conditions (the changes in In−O and Zn−O on drying conditions included in Figure S5). In connection with Figure 6a, it can be seen that Ga−O is made more active through DUV irradiation before IPL annealing. Generally, the semiconductor characteristics of IGZO are determined by controlling the mole fraction of indium, gallium, and zinc components, or the content of oxide.35,37 Among the components of IGZO, the gallium element determines the off-current of the TFT. Gallium acts as F
DOI: 10.1021/acsami.8b22458 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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13.5 cm2/V·s could be obtained. However, to improve IGZO with low on/off ratio and high mobility, additional drying with DUV was carried out. By using NIR and DUV during the drying process, the on/off ratio of IGZO could be changed and the performance of the TFT was greatly improved by optimizing the DUV intensity. This IPL method is effective for annealing semiconducting materials mainly composed of indium and gallium. It was proven that the use of DUV in the drying step promotes the bonding of metal and oxygen, particularly gallium and oxygen. Through analysis of the metal−oxide bond distribution, depending on the use of DUV, the bonding state of metal and oxygen in the IGZO could be controlled, and the TFT characteristics were greatly improved through this control. By optimizing the NIR−DUV drying− IPL annealing conditions, IGZO with a high mobility of 7.7 cm2/V·s and a high on/off ratio of 3 × 106 could be realized by maintaining the carrier concentration at an appropriate level. Therefore, it is expected that IPL annealing combined with the NIR−DUV drying process will offer a strong alternative for realizing IGZO-based TFTs with a high performance.
be seen clearly in Figure 8. The carrier concentration increases with rising IPL annealing energy. At 70 J/cm2, the carrier
Figure 8. Carrier concentration of the IPL-annealed IGZO semiconductor according to drying and annealing conditions (IPL energy range is from 70 to 130 J/cm2, DUV with intensity of 60 mW/cm2, NIR intensity: 3 W/cm2).
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ASSOCIATED CONTENT
S Supporting Information *
concentration on IGZO formed in the range of 1014 to 1015 and gradually increases as the IPL energy increases. At 130 J/ cm2, the concentration rises sharply to the range of 1018 to 1019 (the hall mobility and electrical resistivity of IPL-annealed IGZO according to the annealing condition available in Figure S6). This result corresponds to the electrical characteristic of the TFT depending on the IPL energy (see Figure 2a). The rapid increase of the carrier concentration because of the annealing condition was also found in the annealing of IGZO through ELA as mentioned above. Over a certain laser energy, a high-off-current property was observed, and a TFT with a conductor characteristic was observed in ELA annealing. It was suggested that the characteristic of TFT is closely related to the carrier concentration of IGZO and the carrier concentration level of IGZO should be lower than 1016 on the annealing process.31 To be specific, when DUV drying is applied to annealing of IGZO, the controlled carrier concentration of 1015 was shown under the same IPL energy conditions (blue column in Figure 8). As the abrupt carrier change of IGZO greatly effects the TFT characteristics, it was suggested that carrier concentration should be precisely controlled at the 10 15 level through specific process parameters.41 By applying DUV to IPL annealing, it could be realized to control carrier concentration as well as rapid annealing. In addition to IGZO, IGO also showed a significant change in carrier concentration because of the application of DUV (Figure S3). In particular, in IPL annealing of galliumdoped metal−oxide semiconductors, the effect of the carrier concentration through the control of oxygen vacancies could be shown because of Ga−O bond activation. On the basis of this mechanism, NIR−DUV drying followed by IPL annealing of IGZO could realize easy carrier concentration control while providing 7.7 cm2/V·s of mobility, as well as an on/off ratio of 3 × 106 (see again Figure 4b, output property of an optimized IPL-annealed IGZO-based TFT available in Figure S7).
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b22458.
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Al-based channel pattern of a TFT on the IGZO surface; subthreshold slope of the TFT by drying and in the IPL annealing condition; XPS peak analysis; carrier concentration change; chemical composition of IGZO before the drying process; thickness of the IGZO film on the drying condition; XPS peak change; hall effect property of the IPL-annealed IGZO semiconductor; and output property of the optimized annealed IGZO TFT (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hak-Sung Kim: 0000-0002-6076-6636 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean Government (MEST) (2013M2A2A9043280). This research was also supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2012R1A6A1029029 and 2018R1D1A1A09083236).
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4. CONCLUSIONS In this study, a process for annealing IGZO using NIR, DUV, and intense flashes of light was performed. Using the NIR drying−IPL annealing process, IGZO with a high mobility of G
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