Article pubs.acs.org/cm
Low-Temperature Chemical Transformations for High-Performance Solution-Processed Oxide Transistors Rohit Abraham John,† Nguyen Anh Chien,† Sudhanshu Shukla,‡ Naveen Tiwari,† Chen Shi,§ Ng Geok Ing,∥ and Nripan Mathews*,†,⊥ †
School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 Energy Research Institute@NTU (ERI@N), Interdisciplinary Graduate School, Nanyang Technological University, Singapore 637553 § Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371 ∥ School of Electrical & Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 ⊥ Energy Research Institute@NTU (ERI@N), Nanyang Technological University, Singapore 637553 ‡
S Supporting Information *
ABSTRACT: The challenges associated with low-temperature solution-processed metal oxide network formation have hindered the realization of high-performance solution-based electronic circuitry at temperatures lower than 200 °C. Here, UV irradiation is embarked upon as a route to effectively transform the chemical precursors to semiconducting metal oxides with high electrical quality. High-performance UV-irradiated indium oxide (In2O3) and indium zinc oxide (IZO) thin film transistors with mobility greater than 30 cm2/(V s) have been obtained from nitrate-based precursors. The chemical transformation has been monitored by detailed spectroscopic studies, physical characterization, and temperature-dependent electrical transport measurements. In comparison to thermal annealing, UV annealing seems to result in higher M−O−M network formation (depicted by M−O bonds in XPS), better removal of chemical impurities (depicted by FTIR and XPS), and structural relaxation driven electron doping, transforming the oxygen vacancies to act as shallow donors (depicted by TFT characteristics, XPS, XRD, and Urbach studies). Our results provide new insight into how UV irradiation drives metal oxide network formation and passivates the subgap density of states (DOS).
1. INTRODUCTION Research on oxide-based semiconductors received immense impetus after the demonstration of an all oxide based transparent thin film transistor (TFT) with SnO2 as the active layer by Prins et al. in 1996.1,2 Hosono and co-workers took the work forward and demonstrated the quaternary oxide indium gallium zinc oxide (IGZO) to be a promising material for future flexible and transparent electronics.3 With high intrinsic carrier mobility, high optical transparency, and compatibility with large area processing techniques, post transition metal oxide semiconductors (PTMOS) of indium (In), gallium (Ga), tin (Sn), and zinc (Zn) are a superior choice over a-Si:H and polySi for applications which require transparency, flexibility, and large area uniformity.1,3−5 However, additional advantages can be unlocked if these semiconductors can be deposited by more cost effective means beyond the oft-utilized pulsed laser deposition (PLD) and sputtering processes.1,3,6−9 Printed electronics envision the deposition and patterning of electronic materials through low-temperature solution processing techni© 2016 American Chemical Society
ques such as spin coating, slot die coating, inkjet printing, and roll-to-roll printing. Although organic conjugated semiconductors are the most common material set associated with printed electronics, enabling the formation of high-quality post transition metal oxide semiconductors from solution at low temperatures would allow printed electronics to free itself from limitations such as cumbersome organic synthesis techniques and low carrier mobility.1,4,5,8,10 Although PTMOS thin films have been formed from thermal conversion of precursors deposited from solution, desired conductivities/performances are achieved only at elevated temperatures (>400 °C). The mechanisms behind these conversions range from sol−gel chemistry4,11−13 to oxidation of metal salts7,14−17 and organometallic precursors.18 Although combustion synthesis19,20 has shown tremendous promise in Received: August 20, 2016 Revised: October 24, 2016 Published: October 25, 2016 8305
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Figure 1. (a) Proposed UV exposure mechanism and the device architecture for In2O3 and IZO TFTs. (b, c) Variation in saturation mobility and subthreshold swing for In2O3 and IZO TFTs as a function of UV exposure time. (d, e) Transfer characteristics for a single batch of In2O3 and IZO TFT devices fabricated under different annealing conditions. The data compared here are for a channel width of 4000 μm and channel length of 100 μm.
with more widespread flexible plastic substrates such as polyethylene terephthalate (PET) and polyethylene naphtha-
realizing circuits on high-temperature plastic substrates such as AryLite, the temperatures are still too high to be compatible 8306
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electronically clean semiconductor−dielectric interface with reduced interface state density, thereby allowing a better control over channel formation. With increase in UV exposure time, S is found to increase, as seen from Figure 1b,c and Figures S2−S6. An increase in S with UV exposure indicates increased trap densities at the semiconductor−dielectric interface. This is generally attributed to the presence of electron traps because of incomplete precursor decomposition,33,34 to UV-induced semiconductor35 or dielectric damage,29 or to screening of gate effect due to increasing carrier density.36 Such UV-induced dielectric damage has been attributed to the generation of electron−hole pairs in SiO2 by high-energy UV bombardment and the trapping of holes at the channel−dielectric interface.29 Another important TFT characteristic determining reliable device operation is the on−off ratio of the drain current. The on−off ratio shows a tendency to decrease with increased UV dosage due to an increase in both the on and off currents (Figure 1d,e). This indicates increased carrier concentration with UV exposure and is also reflected in the negative shift of threshold voltage with UV exposure time. In many devices, we even observe a transition from enhancement to depletion mode with increased UV exposure time. On the whole, with UV irradiation, we are able to achieve a 10-fold increase in mobility with respect to conventional thermal annealing at effectively 100 °C lower temperature. In order to account for the dramatic increase in performance with UV exposure, we investigate the reaction mechanisms for good metal oxide thin film formation. Dissolution of indium and zinc nitrate salt in 2-methoxyethanol could initiate partial ligand exchange reactions to form indium or zinc methoxide/ hydroxide complexes in solution.4,12 With annealing, condensation reactions are triggered to form temporary In−O−In/ In−O−Zn networks, followed by densification, crystallization, and removal of residual organic and hydroxyl groups (Figure 1a).12 Incompletion of any one of these reactions has been shown to hamper the device performance drastically.29,33,37,38 The second factor that determines the electronic properties of the thin film is the relative concentration of oxygen vacancies the major source for charge carriers in such oxides.5,39,40 Typically, a high-temperature annealing step satisfies all the above requirements and ensures good semiconducting behavior. However, high device performance demonstrated with UV exposure indicates that the UV irradiation is capable of driving the aforementioned reactions to completion at lower temperatures and also modulates the oxygen vacancy concentration in a manner suitable for high-performance semiconducting behavior. The UV source used in our experiment has broad emission spectra (200−300 nm, 398.48−598.2 kJ/mol) with the major emission peak at 250 nm (477.6 kJ/mol). This corresponds to the mid-UV range capable of decomposing all possible organic bonds in our precursor system (for example, C−H = 413 kJ/mol, C−C = 348 kJ/mol, and C−O = 352 kJ/mol),12,25,29 indicating that chemical transformation of the precursors can be easily triggered by the UV irradiation. Moreover, the release of ozone during UV exposure could result in unstable oxygen radical formation that could now react with the methoxide species and aid the condensation process and also help remove unfavorable organic residues.25 In addition, the unintentional heating up to 150 °C (measured using a thermocouple placed next to the surface of the substrate) observed with UV irradiation could assist in the removal of organic residues and provide extra energy to drive the condensation and
late (PEN). Therefore, it remains challenging to achieve high device performance and stability at process temperatures below 200 °C, particularly for mixed-metal oxides. A promising approach to reduce processing temperatures for the thin film deposition is to supply the energy for chemical transformation through alternative means. Approaches such as irradiation with energetic photons, excimer laser annealing,21 microwave annealing,22,23 and UV annealing12,24−29 have been demonstrated with mixed success. However, the associated chemical reactions and structural condensation and densification mechanisms have not been understood clearly. In this work, we demonstrate high-performance UV-cured indium oxide (In2O3) and indium zinc oxide (IZO) thin film transistors (mobility >30 cm2/(V s)), with an emphasis on investigating the evolution of electronic structure of such lowtemperature solution-processed amorphous oxide semiconductors. TFT characteristics are analyzed first, followed by detailed spectroscopic studies, physical characterizations, and temperature-dependent electrical transport measurements, to investigate the carrier transport mechanism and electronic structure in such oxides. Our results provide new insight into how UV irradiation drives metal oxide network formation and passivates the subgap density of states (DOS). We also provide the first evidence of lattice structural relaxation occurring through UV irradiation in a thin film transistor configuration. It is found that passivation of subgap acceptor states and formation of shallow donor states is responsible for the high device performance achieved by UV irradiation.
2. RESULTS AND DISCUSSION Nitrate-based precursors have been chosen over their chloride and acetate counterparts owing to their lower decomposition temperature and higher UV absorptivity (as indicated in Figure S1a,b in the Supporting Information). The nitrate-based precursor solutions are spin-coated onto SiO2 substrates and subjected to UV exposure in air without any intentional application of heat. In contrast to the N2 atmosphere employed in many cases,12,21,30 all of our experiments have been performed in air to better simulate practical fabrication conditions. Figure 1a depicts the device architecture and schematic of the UV exposure treatment used in this study. Figure 1b−e and Figures S2−S6 in the Supporting Information show variation in saturation mobility, subthreshold swing, threshold voltage, and on−off ratio across the entire batch of devices. The thermally annealed samples In2O3 (250 °C) and IZO (300 °C), having saturation mobilities of 1.21 and 0.27 cm2/(V s), respectively, are used for reference here. In comparison, the saturation mobility of UV-irradiated samples increases with UV exposure time and reaches a maximum of 34.44 cm2/(V s) for In2O3 and 33.14 cm2/(V s) for IZO thin films for an annealing time of 15 min. This is one of the highest reported mobilities for In2O3 and IZO TFT for comparable SiO2 dielectrics and channel lengths.6,7,19,29,31−33 The ultrathin nature of our films (15 nm) enables maximum UV penetration, thereby facilitating a more efficient annealing process. It is interesting to note that the TFTs become active even after a short 15 s of UV exposure for the In2O3 thin films and 1 min for the IZO films. Further increase in UV exposure duration (>15 min) did not increase the mobility explicitly, indicating possible saturation of the underlying reactions with 15 min of UV exposure. In addition to high saturation mobility, a low subthreshold swing (S) is a prerequisite for reliable performance with low power consumption. A low S value indicates an 8307
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Figure 2. (a, b) Detailed FTIR scan of In2O3 and IZO thin films over the range 1300−4000 cm−1. (c, d) Variation in the M-O, Ovac, and M−OH peaks observed in XPS as a function of UV annealing time for In2O3 and IZO respectively. Note that data corresponding to a UV annealing time of 0 min represent thermally annealed samples.
condensation of the metal oxide gel films by chelating through coordination bonding with the metal cations.33,42 The broad absorption at 3600−3100 cm−1 corresponds to the O−H stretching vibrations (Figure 2a,b).25,33 In-plane deformation vibrations arising from C−H residues (possibly from indium methoxyethanol complexes) at around 1350 cm−1 and around 1630 cm−1 accompany this.37 We observe from Figure 2a,b that the organic residues are removed effectively by UV irradiation. With 15 min of UV irradiation, practically all of the impurities are decomposed completely. Complete precursor decomposition ensures removal of a large number of electron trap states that hinder transport. Since the conduction pathways in these oxides are predominantly dictated by vacant spatially dispersed ns orbitals, we analyzed the O 1s peak via XPS to estimate the formation of the M−O−M oxide network (Figure 2c,d and Figures S7 and S8 in the Supporting Information). In all samples, the nitrogen contribution was at the limit of XPS resolution, suggesting that nitrogen species are quickly eliminated from the samples even at 250 °C. The possibility of adventitious carbon contamination was eliminated by analyzing the oxygen atoms bonded to carbon from the carbon 1s spectra. It was found that the amounts of oxygen atoms bonded to carbon were less than 5% and 2.5% relative to the oxygen atoms bonded to the metal in
densification reactions to completion. To decouple the major factor propelling the aforementioned reactions and to understand the contribution of this temperature rise, a few devices were subjected to thermal annealing at 150 °C. These devices displayed very low currents with no transistor behavior. This indicates that the thermal energy provided by the unintentional temperature rise is not sufficient to drive these reactions to completion and form good metal oxide networks. Thus, UV irradiation is the primary driving force behind the aforementioned chemical reactions and is mandatory for obtaining highperformance devices at low temperatures. However, achieving high-performance TFTs requires not just chemical transformation of the precursors; it is also necessary to have a continuous charge carrier path (metal oxide network) formation, a source of sufficient charge carriers (oxygen vacancies), and elimination of trap centers (remnant organic and inorganic ligands). In order to probe the effectiveness of UV treatment in forming a dense metal oxide network and removing the remnant impurities, an FTIR spectroscopic technique was utilized. Major features were observed at 1384 and 3000−3700 cm−1. The sharp peak observed at 1384 cm−1 can be assigned to an NO3− deformation vibration, while the features between 700 and 1600 cm −1 signify O−H deformation25,33,41 (Figure 2a,c). These groups suppress the 8308
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Figure 3. (a) XRD spectra of In2O3 thin films zoomed in to show the ⟨222⟩ peak corresponding to cubic In2O3. (b) XRD spectra of IZO thin films exhibiting its amorphous nature under the extreme annealing conditions.
vacancies together with the decrease in M−OH bonds again reinforces the effectiveness of UV exposure in forming the metal oxide network and decomposing the remnant impurities in the film. Detailed analysis of the In 3d peak in these films shows a sharper and narrower peak with UV 15 min annealing in comparison to thermally annealed samples, as shown in Figure S9 in the Supporting Information. The peak with the narrower fwhm indicates better In−O−In network formation with negligible impurities. On the other hand, the broad In peak associated with the thermally annealed sample suggests the existence of different species, possibly due to incomplete decomposition of organic and hydroxyl residues.34 The above compositional/chemical analyses provide ample evidence of the role of UV irradiation in realizing highperformance devices at low temperatures. Additional physical characterizations to gain insight into the lattice order/atomic packing and surface morphology were performed (Figure 3 and Figures S10 and S11 in the Supporting Information). In2O3 shows a tendency to become crystalline with UV dosage duration greater than 5 min. The major peak observed at 31° is assigned to the ⟨222⟩ cubic phase of In2O3. This increase in lattice order with In2O3 films forces us to believe that the UV irradiation engages itself in reordering and rearranging the M− O−M network, after triggering and driving the condensation and densification mechanisms to completion. This increase in packing order of In2O3 with increased UV exposure time is summarized in Table S1 in the Supporting Information. With increased UV dosage, the tendency for structural relaxation (reduction of disorder by transformation from amorphous to crystalline) is found to increase. This is supported by the increase in grain size and decrease in lattice strain calculated in Table S1. Very recently, structural relaxation induced electron doping in amorphous oxide semiconductors was proposed by Joo et al.43 To the best of our knowledge, we are the first to report transistors based on a UV-induced structural relaxation mechanism modulating the electronic structure in metal oxide semiconductors. Increasing crystallinity with high-temperature thermal annealing is not a surprise and has been well documented.8,44 However, an increase in lattice order with UV irradiation has not been reported so far. Moreover, the fact that such structural relaxation could possibly electronically dope
the thermally annealed sample and UV 15 min treated sample, respectively. Thus, it is unlikely that C−O contamination affects the subsequent analysis. The O 1s peak is deconvoluted to three individual peaks located around 530.5, 532, and 533 eV. The peak at the lowest binding energy (∼530.5 eV) is assigned to the oxygen atoms in the fully oxidized indium environment (lattice oxygen; M−O−M). The midpeak at ∼532 eV is assigned to oxygen ions in the oxygen-deficient region (indicative of oxygen vacancy concentration), and the peak at high binding energy (∼533 eV) is assigned to the presence of loosely bound oxygens associated with H2O and OH groups on the surface.12,19 All of the annealed films display signatures consistent with an oxide lattice and the presence of oxygen vacancies and surface hydroxyl groups. The effectiveness of UV treatment in removing reactant residues can be scrutinized by estimating the relative amount of M−OH bonding in the films after annealing. The unannealed film, dominated by OH species, gives a clear indication of the incomplete oxide skeleton formation in the as-deposited stage (data not shown). At this stage, the film is a mixture of different alkoxide/hydroxide complexes, and hence, the deconvolution becomes ambiguous. As annealing proceeds, these evolve into fully coordinated oxide structures through condensation and densification reactions (Figures S7 and S8 in the Supporting Information). These structures are evident from the increased M−O peak intensity located at ∼530.5 eV and the diminished M−OH peak intensity located at ∼533 eV. In comparison to the thermally annealed sample, the signature of the M−O−M is almost double for the 15 min UV-treated sample. In addition, the signature associated with oxygen vacancies (∼532 eV) is also found to increase with UV exposure and far exceeds that in thermally annealed samples (Figure 2c,d and Figures S7 and S8 in the Supporting Information). Oxygen vacancies typically act as shallow donors.5,43 This manifests itself in increased off currents and a negative shift of threshold voltages, both reflecting an increase in overall carrier concentration in the thin films with increased UV exposure. Thus, UV exposure provides sufficient shallow donors (and therefore electrons) to fill any possible electron trap states that could arise from residual unreacted precursors. The increase in M−O bonds and oxygen 8309
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Figure 4. (a) Urbach tail of In2O3 and IZO thin films subjected to thermal annealing and 15 min UV irradiation. (b) Low-temperature conductivity measurements and the extracted activation energy values for In2O3 and IZO thin films annealed under boundary conditions.
our thin films makes this an interesting observation. Surprisingly, IZO films remain amorphous throughout all the annealing conditions tested during the study. In the case of IZO, UV irradiation induced local atomic ordering and structural relaxation may be impeded by competition between the differently sized cations, frustrating grain growth. AFM studies back up this assertion, with In2O3 films displaying a roughening transition associated with crystallization, while IZO films remain predominantly smooth despite annealing (Figures S10 and S11). Detailed optical absorption studies and low-temperature electrical transport measurements have been further conducted to get a better acumen of the sub band gap density of states under different annealing conditions. Urbach energy (a convolution of both conduction and valence band tail states) measurements were utilized to measure the sub gap disorder (Figure 4a). We expect the valence band tail states to dominate here, since the characteristic energy width of the conduction band tail states is usually considered negligible. A decreasing trend with UV irradiation suggests a cleaner band tail and can be correlated to increased M−O--M network formation and reduction of M−OH groups as observed from XPS and FTIR. This reduction in band tail states near the valence band maxima (VBM) should not ideally affect electron transport. However, enhanced electron transport with UV irradiation, as seen from the TFT characteristics, may imply that some of the deep-level oxygen vacancies could be transforming into a shallow donor state via a structural relaxation mechanism induced by UV irradiation. This structural relaxation induced doping has been observed before, but the exact origin still remains unclear and requires further investigation.43,45 Increased oxygen vacancies (XPS), decreased sub gap states (Urbach), reduced charge transport activation energies (low-temperature conductivity measurements), increased TFT saturation mobility, and finally, increased lattice order (evident in XRD of indium oxide) suggest a doping mechanism induced by such structural reordering. Low-temperature electrical conductivity measurements are used to extract the mechanism governing the charge transport in these amorphous oxide semiconductors and determine how UV irradiation achieves better charge transport with respect to conventional thermal annealing. The nonlinear nature of the curve at lower temperatures suggests the possibility of variable
range hopping governing the transport mechanism at these ranges. A good fit with T−1/4 indicates congruence with Mott’s model of variable range hopping (Mott-VRH) transport (Figure S12 in the Supporting Information).46,47 However, a more detailed low-temperature electrical data analysis is required to draw a comprehensive conclusion regarding the transport mechanism. Various models based on hopping, percolation conduction, and band transport have been proposed in the literature to explain the charge transport in such disordered ionic metal oxides. However, a large variation of the fit parameters with carrier concentration, electric field, and temperature makes it difficult to accurately derive a specific transport phenomenon.48−50 Since the purpose of our measurements is to compare thermally annealed and UVirradiated samples, we fit the low-temperature (80−200 K) data to a basic Arrhenius equation. The activation energy for charge transport (as calculated from the Arrhenius equation σ = σ0 exp(−Ea/KbT)) is found to be much smaller for UV-irradiated films (Figure 4b), indicating a more favorable transport for the UV-irradiated films. Amorphous oxide semiconductors are known to be highly disordered systems, with the Fermi energy position located within the band tail formed by electron traps or acceptor states. With annealing, these traps become filled, allowing a small portion of the carriers to be thermally activated to the extended states through multiple trap and release events. Thermal annealing is found to be ineffective in this regard at low temperatures. The large number of trap states observed from Urbach energy measurements and the high activation energy barrier together with low field effect mobility confirm this. However, with UV annealing, a major portion of these trap states is eliminated by effective removal of chemical impurities, good M−O−M network formation, and the creation of shallow donor oxygen vacancies. This raises the Fermi level up closer to the mobility edge, thereby allowing more electrons to jump into the delocalized/extended states at lower temperatures. This explains the high-mobility TFTs obtained with UV annealing. Studies have also suggested that interstitial hydrogen could be a likely contributor to n-type conductivity.51,52 The possibility of hydrogen playing a role in the present system arises from the tendency of nitrate salts generating hydrogen ions during hydrolysis34 and the possibility of direct H generation by UV scission of OH− groups.53 Accurate quantification through experimental measurements of such 8310
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oxygen vacancies to act as shallow donors (depicted by TFT characteristics and XPS, XRD, and Urbach studies). Our study also reveals the downside of UV annealing, by highlighting the increase in subthreshold swing and bias stress induced threshold voltage shifts caused by UV-induced dielectric damage. On the whole, our study provides a necessary understanding of the underlying UV curing mechanism and provides promising insight into designing alternate annealing treatments to achieve high-performance TFTs at low temperatures for flexible electronics applications. In this study, we have focused solely on the semiconducting active layer. Future efforts should address the development of low-temperature processed robust dielectrics and encapsulant materials, possibly through the adaptation of similar synthesis principles.
unavoidable interstitial hydrogens, however, makes it difficult to correlate the effect of H doping on the electrical properties. Although UV annealing improves field effect mobility and electron concentration in In2O3 and IZO thin films, the subthreshold performance and bias stability of the transistors are worsened by UV annealing. Subthreshold swing (S) values and bias stress induced threshold voltage shifts (ΔVth) are related to the density of interfacial states (Dit) between the semiconductor and the gate dielectric and are a direct reflection of the electronic trap states at the semiconductor−dielectric interface. Smaller values of S and ΔVth are preferred for better TFT performance, as they indicate an electronically clean interface between the semiconductor and dielectric. Since our XRD results indicate a decrease in lattice strain and Urbach energy calculations depict reduction in subgap trap states with increased UV exposure, the increase in S with UV exposure can be attributed to the damage caused to the dielectric via UV dosage.29 Damage caused by UV irradiation to the dielectric creates additional electron trap states, which increases the S value. However, during the time scale of the sweep of our transistors, we do not see significant hysteresis (Figure S13 in the Supporting Information). As a stricter test, positive gate bias stress measurements were performed for 1 h with a constant gate voltage Vg of +30 V. Individual measurements were taken after every 10 min, and the trends are summarized in Figure S14 in the Supporting Information. No particular stability difference between indium oxide and indium zinc oxide transistors is observed. The positive Vth shift with a positive bias voltage is a result of negative charges trapped at the interface of the channel and dielectric or injected into the gate dielectric. This is once again attributed to the traps generated due to the damage induced to the dielectric by UV irradiation, despite the fact that a more ordered semiconducting thin film is obtained with UV treatment. Finally, this UV irradiation induced gate dielectric damage is also reflected in the higher gate leakage currents shown in Figure S15 in the Supporting Information. However, the dielectric constant of SiO2 in our experiments remained constant even after UV irradiation.
4. EXPERIMENTAL SECTION 4.1. Synthesis of In2O3 and IZO Thin Films. In2O3 and IZO solutions were prepared by dissolving indium nitrate hydrate (In(NO3)3·xH2O) and zinc nitrate hydrate (Zn(NO3)3·xH2O) salts in 2-methoxyethanol. The total cation concentration was kept constant at 0.2 M, and the In:Zn ratio was kept constant at 7:3 for IZO in this study. The solutions were stirred for 12 h to allow complete dissolution. A simple salt/single-solvent precursor design was utilized here without additional combustion fuels or stabilizers, so as to minimize the carbon-related residues in the resulting thin film. 4.2. Device Fabrication. p-Type silicon wafers (Addison Engineering) with 300 nm thick silicon dioxide (SiO2) were used as substrates in a bottom gate−top contact configuration, with the highly doped Si acting as the gate electrode and SiO2 serving the purpose of a gate dielectric. Substrates were cleaned by ultrasonicating in deionized (DI) water, acetone, and ethanol for 10 min each with nitrogen drying steps in between, followed by UV−ozone plasma cleaning (18 W) for 10 min. Solutions were then spin-coated onto the substrate at a speed of 3000 rpm for 1 min, baked on a hot plate at 110 °C for 5 min to remove the excess solvent, and finally subjected to deep-ultraviolet (DUV) irradiation for durations ranging from 15 s to 15 min. Films thermally annealed at 250 °C (In2O3) and 300 °C (IZO) were used as references here. The film thicknesses were measured before and after annealing and were found to remain the same from X-ray reflectivity (XRR) data (not shown here). The UV irradiation system (Heraeus Noblelight Fusion UV Inc.) used in our study has an incident power equal to 16 mW/cm2 for a substrate spacing of 1−5 cm (measured using a luxmeter). Aluminum (100 nm thick) was thermally evaporated to form source and drain electrodes (W = 4000 μm/L = 75−200 μm) using shadow masks to complete the TFT architecture (Figure 1a). Thin films for XPS measurements were fabricated by spincoating the solution on a conductive Si substrate with the native thermal oxide and then annealing accordingly. The XPS was performed on the surface of the samples, since etching resulted in contributions to the XPS signal from the underlying Si surface, as the films are
