Li-Assisted Low-Temperature Phase Transitions in Solution-Processed Indium Oxide Films for High-Performance Thin Film Transistor

Lithium (Li)-assisted indium oxide (In2O3) thin films with ordered structures were prepared on solution-processed zirconium oxide (ZrO2) gate dielectrics by spin-casting and thermally annealing hydrated indium nitrate solutions with different Li nitrate loadings. It was found that the Li-assisted In precursor films on ZrO2 dielectrics could form crystalline structures even at processing temperatures (T) below 200 °C. Different In oxidation states were observed in the Li-doped films, and the development of such states was significantly affected by both temperature and the mol% of Li cations, [Li+]/([In3+] + [Li+]), in the precursor solutions. Upon annealing the Li-assisted precursor films below 200 °C, metastable indium hydroxide and/or indium oxyhydroxide phases were formed. These phases were subsequently transformed into crystalline In2O3 nanostructures after thermal dehydration and oxidation. Finally, an In2O3 film doped with 13.5 mol% Li+ and annealed at 250 °C for 1 h exhibited the highest electron mobility of 60 cm2 V−1 s−1 and an on/off current ratio above 108 when utilized in a thin film transistor.

Scientific RepoRts | 6:25079 | DOI: 10.1038/srep25079 (T A = 200 °C) on SiO 2 dielectrics and ~39.0 cm 2 V −1 s −1 (T A = 250 °C) on high-k dielectrics have been reported [5][6][7] . Such results are superior to those obtained with the best RF-sputtered In 2 O 3 TFTs, which display μ e values of approximately 15.0 cm 2 V −1 s −1 and an on/off current ratio (I ON /I OFF ) above 10 8 10 . At present, the highest Hall effect mobilities observed in single crystalline, polycrystalline, and amorphous In 2 O 3 films are 160 cm 2 V −1 s −1 , 150 cm 2 V −1 s −1 , and 51 cm 2 V −1 s −1 , respectively 18,19 . Enhancement of the lattice ordering dimension, i.e., increasing the average crystalline size, provides a fast path for carrier transport. However, two-dimensional (2D) planar defects stemming from misorientation between the crystallites may act as charge traps, resulting in the formation of a potential energy barrier that impedes the transport of free carriers. Therefore, understanding and controlling the crystalline microstructure are critical to improve charge mobility in In 2 O 3 -based TFTs. While the carrier mobility in a crystallite is relatively higher than that in an amorphous phase, there is ongoing debate as to whether MOS crystallites enhance charge carrier transport 6,7 , as grain boundaries (GBs) can serve as charge scattering sites 1,4,15,16 .
Efforts to partially crystallize oxide thin films have significantly improved the carrier mobility in materials systems such as IZO and ZnO 4,15,16 . Optimum crystallization of the oxide thin films was predicted to be dependent on the formation of GBs 4,15,16 . Recently, it has been reported that heterogeneous metallic seeds, e.g., individual or aggregated forms of assisted Li atoms, could enhance the low-T crystallization of ZnO thin films, thereby yielding high-performance TFTs 15,16,20 .
In this work, ordered In 2 O 3 films on zirconium oxide (ZrO 2 ) gate dielectrics were successfully fabricated by a low-T (≤ 250 °C) solution-based processing method. It was found that the Li-assisted precursor films on ZrO 2 dielectrics could form ordered structures even at processing temperatures below 200 °C. Different In oxidation states were observed in the Li-doped films, and the formation of such states was significantly affected by both temperature and the mol% of Li cations, [Li + ]/([In 3+ ] + [Li + ]), in the precursor solutions. Upon annealing the Li-assisted precursor films below 200 °C, metastable indium hydroxide (In(OH) 3 ) and/or indium oxyhydroxide (InOOH) phases were formed. These states were subsequently transformed into crystalline In 2 O 3 nanostructures after further thermal dehydration and oxidation. Finally, an In 2 O 3 film doped with 13.5 mol% Li + and annealed at 250 °C for 1 h exhibited the highest electron mobility of 60 cm 2 V −1 s −1 and an on/off current ratio above 10 8 when utilized in a thin film transistor.

Results
Conventional High-T Film Fabrication of Solution-Processed In 2 O 3 . It is known that oxide formation from MOS precursors based on metal acetates, nitrates, and halides is an endothermic process in which a massive external energy input is needed to form metal-O-metal lattices 5 . In many cases, a phase transition requires an elevated T, typically higher than 400 °C, to completely decompose the precursor and avoid undesirable organic contamination within the resulting MOS films. Consequently, most solution-based methods developed for the fabrication of oxide films have been incompatible with the use of plastic substrates, which have poor thermal stability and a higher thermal expansion coefficient.
In order to investigate the thermal dehydration, decomposition, and crystallization behaviors of the spun-cast films during the annealing treatment, thermogravimetric differential thermal analysis (TG-DTA) was first conducted for a powder dried from a 9 mol% In(NO 3 ) 3 ·xH 2 O solution. It should be noted that 9 mol% In(NO 3 ) 3 ·xH 2 O solutions were also used in the fabrication procedure for all thin films. Figure 1 shows representative TG-DTA profiles of the dried In(NO 3 ) 3 ·xH 2 O powder, including typical weight loss behavior and heat flux variations as a function of T. Based on the TG-DTA results, 5% of the water residue  3 with a weight loss of 10%, (4) conversion of InOOH to In 2 O with 7% weight loss, and (5) residual decomposition and crystallization with a 16% weight loss.
in the dried MOS precursor was eliminated before decomposition. In the temperature range of 110 to 150 °C (the second heating zone in Fig. 1), a weight loss of about 45% was observed due to the hydrothermal reaction. Here, In(NO 3 ) 3 started to decompose and change into In(OH) 3 ; this was confirmed by grazing-incidence X-ray diffraction (GIXD) analysis (as will be discussed later). In the third heating zone, endothermic melting of the In(OH) 3 occurred. It has been reported that In(OH) 3 can be transformed into orthorhombic InOOH, an intermediate product [21][22][23] . Such a reaction could be responsible for the additional 10% weight loss above 187 °C due to thermal dehydroxylation. As the temperature was raised above 275 °C, the final In 2 O 3 product started to form via decomposition of the InOOH. Crystallization of the In 2 O 3 was ultimately achieved above 400 °C 22 .
The morphologies of films annealed at a given T corresponding to each heating zone in Fig. 1 were systematically examined by atomic force microscopy (AFM). MOS precursor layers were spun-cast onto ZrO 2 /Si substrates from a 9 mol% In(NO 3 ) 3 ·xH 2 O solution and then thermally annealed at various temperatures for 1 h. Figure 2 shows typical AFM topographies of heat-treated precursor layers with clearly discernible phases. For the 130 °C-annealed film, flower-like agglomerates with an average diameter of 2.7 μ m and height of 40− 50 nm were observed. In contrast, the 170 °C-annealed film was composed of large InOOH grains. These spherulitic grains disappeared almost entirely in the 250 °C-annealed film. After annealing above 250 °C, micron-sized grains were completely absent, and either smooth surfaces or nano-sized aggregates were observed. The nano-sized aggregates in the 500 °C-annealed film (Fig. 2f) were confirmed to be In 2 O 3 crystals by GIXD analysis (Fig. 3) 24 .
To determine the structure of the metal precursor films on ZrO 2 after annealing at various T, GIXD data were acquired. The 1D GIXD profiles of indium oxide layers annealed at different temperatures are displayed in Fig. 3; reflections from both ordered indium oxide phases and amorphous structures are evident. The 130 °C-annealed film with flower-like crystals showed intense X-ray reflections, specifically at a scattering vector (Q) of 1.58 Å −1 , which corresponded to an inter-plane distance (d hkl = 2π /Q) of 3.98 Å between the (200) planes of In(OH) 3 crystals. The In(OH) 3 had a cubic Pn3m(224) structure with a lattice distance of 7.958 Å (ICDD PDF # 17-0549). For the 170 °C-annealed film, the intensities of the (200) reflections decreased considerably. The 1D X-ray profile of the 250 °C-annealed film did not show any clear X-ray reflections, and it was subsequently found that the film was primarily composed of InOOH (as determined by XPS analysis), suggesting that In(OH) 3 crystallites were transformed to a less-ordered InOOH phase by melting and dehydration. Gurlo et al. reported that an aggregate form of In 2 O 3 could be synthesized by annealing InOOH under ambient pressure 23 . In contrast to the other specimens, the 400 °C-and 500 °C-annealed films showed intense X-ray reflections at Q = 1.521 and 2.151 Å −1 , respectively, which corresponded to the (222) and (211) crystal planes of In 2 O 3 crystallites with an Ia-3(206) structure and a lattice distance of 10.12 Å (ICDD PDF # 06-0416). Based on the GIXD results, it was concluded that In 2 O 3 films could be formed from solution-processed In(NO 3 ) 3 ·xH 2 O precursor layers via a solid-solid phase transition induced by high-T dehydration and oxidation.
Li-assisted Solid-Solid Transformation of In(NO 3 ) 3 at Low Temperature. The charge carrier mobility in inorganic semiconductor films is sufficient for TFT applications, provided that a suitable fabrication method is employed 25 . Sputtered MOS-based TFTs generally exhibit the highest μ e values up to 100 cm 2 V −1 s −1 . However, difficulties in terms of optimization have been encountered for most solution-processed metal precursor systems when the corresponding films were treated at low T 1 . As mentioned earlier, solution-processed layers from MOS precursors require a high-T treatment to remove impurities as charge trap sites and induce crystallization of the MOS phases for high-performance TFTs 1 .
Adamapoulos et al. reported that Li-doped ZnO films fabricated by an ambient solution-spray technique and annealed at 400 °C possessed a high μ e of 85 cm 2 V −1 s −1 in TFTs 16 . Recently, many studies have focused on the low-T processing of solution-based MOS films. In research by Marks and co-workers, In 2 O 3 , ZTO, and IZO TFTs were fabricated by solution-casting metal precursors and subsequently developing the structures at temperatures as low as 200 °C. Such work highlighted the benefits of exploiting self-sustaining combustion reactions to prepare MOS TFTs 5 . Laser irradiation has also been utilized to generate strong, localized exothermic heat so as to produce ordered MOS phases from precursor films at a given T (≤ 200 °C). The resulting In 2 O 3 TFT was manipulated on transparent polymer substrates, and μ e values of up to 6 cm 2 V −1 s −1 were obtained. However, the device exhibited a poor I ON /I OFF ratio of about 10 3 26 .
To investigate the effects of Li incorporation on the phase transition from In(NO 3 ) 3 ·xH 2 O to In 2 O 3 , LiNO 3 -loaded In(NO 3 ) 3 ·xH 2 O solutions were prepared and spun-cast onto ZrO 2 /Si substrates. AFM was carried out for the LiNO 3 -assisted In(NO 3 ) 3 ·xH 2 O layers before and after thermal annealing at each annealing temperature. Figure 4 shows the AFM topographies of the 130 °C-annealed films; discernible phase morphologies are evident depending on the mol% of Li + in the casting solutions. The existence of LiNO 3 in these precursor layers seemed to reduce the formation of In(OH) 3 flower-like aggregates in the annealed LiNO 3 -assisted films. The 6.7 mol% Li + -loaded film appeared to be smooth with percolated grains, while the specimen prepared with a Li + content of 8.7 mol% contained nano-sized aggregates as shown in Fig. 4b,c. Interestingly, the introduction of 13.5 mol% Li + into the previously mixed solution produced a densely-packed layer with a height of 45− 50 nm on the ZrO 2 surface after a thermal treatment at 130 °C for 1 h (Fig. 4d). However, as the Li + fraction was increased above 20 mol% in the solutions (or films), the LiNO 3 and In(NO 3 ) 3 mixtures may be phase-separated during film processing, thereby causing an increase in average surface roughness (R q ), as shown in Fig. 4e,f. For films prepared with 21 and 30 mol% Li + , the surface roughness was found to be 21.2 and 26.0 nm, respectively, which was much higher than those of the lower LiNO 3 -assisted films. Based on the AFM morphologies of the 130 °C-annealed films with different Li + loadings, it is believed that, even at 130 °C, an optimized process for the incorporation of metal ions or their complexes could generate nuclei so as to facilitate the development of ordered and uniform metal oxide structures.
In order to better understand the effects of Li incorporation on the solid-solid phase transition in LiNO 3 /I n(NO 3 ) 3 layers spun-cast onto ZrO 2 surfaces, 2D GIXD data were obtained after annealing. Figure 5 shows the GIXD patterns of annealed films with different Li doping levels (the GIXD patterns of films prepared with no Li doping were already presented in Fig. 3). For all GIXD patterns, there was an absence of peaks associated with crystalline polymorphs induced by Li + dopants, suggesting that Li may not be active in the In(OH) 3 , InOOH, and It is important to note that the X-ray reflections from In(OH) 3 in the 6.7 mol% Li + -assisted film annealed at 130 °C were much more intense than those observed for samples with no Li doping (Fig. 5a). Based on the GIXD analysis, it is summarized that the intermediate compounds formed during In oxidation, i.e., In(OH) 3 and InOOH, quickly decayed in the annealed films, even at relatively higher Li + loadings (see Fig. 5c), which will be addressed when discussing the X-ray photoelectron spectroscopy (XPS) findings). As shown in Fig. 5, the X-ray reflections corresponding to In(OH) 3 for all films annealed at 130 °C and 170 °C, including those with Li dopants, tended to become weaker with an increase in the Li + mol%. Furthermore, the patterns obtained for the 250 °C-annealed samples did not contain any reflections from In(OH) 3 phases; such a trend was similar to that observed for the specimen prepared with no Li doping. The particular In oxidation states present in the XPS spectra acquired for the 250 °C-annealed films (Fig. 6) strongly support the notion that these samples are comprised of In(OH) 3 , InOOH, and In 2 O 3 . The composition ratios of these compounds changed significantly with different Li loadings. In contrast, the 1D GIXD profiles of the 400 °C-and 500 °C-annealed films clearly showed X-ray reflections at Q = 1.521 and 2.151 Å −1 , which correspond to (211) and (222) crystal planes, respectively, in the nano-sized In 2 O 3 crystallites (see Fig. 2e,f). The average grain sizes of Li doping-dependent In 2 O 3 films annealed at 400 °C and 500 °C were calculated using X-ray diffraction profiles and Scherrer equation 27 and summarized in Figure S1. The enhancement in average grain size of In 2 O 3 crystal was clearly seen with increasing Li + loadings, indicating that the incorporated Li can act as a catalyst for rearrangement of In-O bonds at the elevated temperature.
The oxidation states of In in the Li-assisted In 2 O 3 thin films annealed at 250 °C for 1 h were systematically determined from In 3d 5/2 and O 1s XPS spectra (see Fig. 6). Over a binding energy range of 442− 446 eV, the In 3d 5/2 spectra were found to contain contributions from In 0 , In 2 O 3 , InOOH, and In(OH) 3 with maximum intensities at 443.2, 443.8, 444.3, and 444.8 eV, respectively 28 . Each contribution calculated from the XPS data is summarized in Table 1. As expected, the film prepared with no Li doping contained the highest InOOH fraction (0.77). As shown in Fig. 6b,c, the fraction of InOOH in the Li-assisted films decreased, while that of In 2 O 3 at 443.8 eV increased with a rise in the Li + mol%. Such findings indicate that Li incorporation can significantly enhance the oxidation of In in InOOH so as to form the desired In 2 O 3 product for high charge carrier mobility in TFTs.
Although    (Table 1). As the Li loading in the 250 °C-annealed films was increased from 0 to 13.5 mol%, the ratio of the V o -related signals to the entire XPS profile decreased from 0.167 to 0.095. The obtained results suggest that the assisted Li efficiently improved the coordination of In-O bonding so as to form energetically stable configurations. Such a scenario presumably occurred because the smaller ionic radius of Li allowed for improved oxygen diffusivity in the indium oxide network. Notably, the fraction of OH in the Li-assisted films was substantially lower than that in the undoped film, as shown in Fig. 6d-f and Table 1. In addition, undesirable impurities such as nitrogen and carbon were not detected in the XPS spectra (see Figure S2 in Supporting Information) of any 250 °C-treated film, regardless of the Li loading.
The depth profile of incorporated Li in the In 2 O 3 /ZrO 2 stack was further analyzed using the time-of-flight secondary ion mass spectroscopy (TOF-SIMS). In the In 2 O 3 /ZrO 2 stack with no Li loading, the Zr cations in the ZrO 2 dielectric film diffused substantially into In 2 O 3 film during the thermal annealing at 250 °C (see Fig. 7a). The penetration of Zr cation was suppressed for the In 2 O 3 /ZrO 2 stack with 13.5 mol% Li loading (see Fig. 7b). It suggests that the Li-assisted In 2 O 3 film has the more uniform morphology and higher packing density compared to the undoped In 2 O 3 film, which will be discussed later. It is also noted that the Li cation existed uniformly in the In 2 O 3 film along depth direction. These beneficial effects of Li incorporation into the In 2 O 3 films in terms of the impurity concentration should lead to superior electrical properties for the resulting TFTs.    Fig. 9. With the exception of morphological traces left by the previously-grown spherulite, no clear texture was observed in the film prepared with no Li doping (see Fig. 9a). As a higher mol% of Li was assisted, isolated nanoparticles, closely-packed grains, phase-separated domains, and bi-continuous phases were formed after annealing at 250 °C (see Fig. 9b-f). The AFM topologies of films with 6.7 and 8.6 mol% Li + showed well-dispersed nanoparticles with sizes of 20 to 80 nm (Fig. 9b,c). These isolated nanoparticles disappeared almost entirely in the 13.5 mol% Li + -assisted film, which contained closely-packed nano-grains (Fig. 9d). With an increase in the Li loading above 13.5 mol% Li + , phase-separated domains were observed in the films, and their sizes increased with a rise in the Li mol%. The obtained results suggest that Li incorporation minimizes the concentration of localized trap states, including tail states and deep-level traps.  Fig. 10; the corresponding electrical properties are summarized in Table 2. The subthreshold swing (SS) was extracted from a linear region of the log(I D )-V G plot.    The densities of fast bulk traps (N SS ) and semiconductor-insulator interface traps (D it ) were calculated using the following expression 32 :  Fig. 10b.

Li-assisted In 2 O 3 TFTs on Solution-Derived
Based on the TG-DTA, AFM, GIXD, and XPS findings, the three-fold increase in the value of μ e for the 13.5 mol% Li-assisted TFT, when compared to that in the undoped device, is attributed to both an efficient phase transition from metastable InOOH to stable In 2 O 3 and a densely packed film morphology after annealing at 250 °C. It is interesting to compare the N SS,max and D it,max values of the undoped and Li-assisted devices because they are likely to trap free electrons and thus, impede the electric field-driven drift velocity of the free carriers. For the Li-assisted In 2 O 3 TFTs, N SS,max or D it,max were found to decrease at higher Li loadings up to 13.5 mol%. Residual V o and/or impurities such as OH generally act as trapping centers for charge carriers, and the decrease in N SS,max or D it,max values at higher Li fractions may partially be attributed to a reduction in V o and unwanted impurities after Li incorporation. However, the ~3-fold increase in μ e for the 13.5 mol% Li-assisted device when compared to that of the undoped device cannot be completely explained by a ~2-fold decrease in N SS,max or D it,max . It can be inferred that the effective mass of electrons in In 2 O 3 is smaller than that in InOOH, although the electronic band structure of metastable InOOH has not yet been explicitly calculated.
Finally, the thermal instability of Li-assisted In 2 O 3 TFTs was examined in the temperature range from 120 to 360 K. The on-state drain current for In 2 O 3 TFTs with no Li loading exhibited the thermally activated behavior with increasing measurement temperature, which resulted in the huge negative V th displacement (Δ V th = − 2.6 V) as shown in Figure S3 and Fig. 11. This behavior has been frequently reported for the metal oxide TFTs, which can be attributed to the existence of the bulk traps and semiconductor-insulator interface traps 34,35 . In contrast, the In 2 O 3 TFTs with 13.5 mol% Li + exhibited the improved thermal stability (see Figure S3 and Fig. 11), which is consistent with the fact that the incorporated Li + reduced the structural defect and impurity, leading to the reduction in N SS,max and D it,max values.
In summary, Li-assisted In 2 O 3 channel TFTs fabricated on ZrO 2 dielectrics by a low-temperature (250 °C) solution-based process exhibited superior mobilities and I ON /I OFF ratios. It was determined that Li incorporation played various important roles in the In 2 O 3 /ZrO 2 structures, including: 1) accelerating the decomposition of metastable In(OH) 3 and InOOH phases into In 2 O 3 , 2) reducing the bulk and interface trap density in the ZrO 2 dielectric by eliminating hydroxyl groups and oxygen vacancies, and 3) enhancing the nucleation and crystallization of In(OH) 3 and In 2 O 3 crystallites by filling interstitial sites. The use of a precursor with a high Li mol%, in excess of the optimum 13.5 mol% determined in this work, may cause phase separation and severe surface roughening of LiNO 3 and In(NO 3 ) 3 -related complexes (as inferred from the AFM findings). This in turn could increase the trap density and thus, reduce the carrier mobility.
The solution-based, low-temperature preparation procedure detailed in this report involves the simple physical blending of soluble metal and dopant precursors. As such, the devised synthesis method can expand the possibilities for the development of high-quality multi-component oxide semiconductors that can be implemented on large-area substrates.   Figure S5 in Supporting Information), AZ 9200 photoresist (PR) layers were cast onto the 100-nm-thick SiO 2 /Si substrates and patterned with lines. The PR patterned SiO 2 /Si substrates were then inserted into a buffer oxide etchant to selectively remove the exposed SiO 2 surfaces. A dielectric layer was spun-cast onto the patterned SiO 2 /Si substrates from a 0.1 M ZrO(NO 3 ) 2 ·xH 2 O solution and subsequently annealed via a two-step procedure at 100 °C for 10 min and then 250 °C for 1 h. The dielectric coating process was repeated so as to produce a ZrO 2 film with a thickness of approximately 23 nm. Indium tin oxide (ITO) source/drain (S/D) electrodes with a thickness of 150 nm were deposited on the ZrO 2 layer via sputtering of an ITO target with 90% In 2 O 3 . The ITO was then patterned by PR coating/developing and ITO etching in a dilute HCl solution. The dimensions of the patterned ITO pads in the TFTs were controlled so as to ensure a channel length (L) and width (W) of 14 μ m and 150 μ m, respectively. Different Li-assisted In 2 O 3 precursor layers were subsequently spin-cast onto the patterned ITO/ZrO 2 /Si substrates and annealed at 100 °C for 10 min. Finally, the samples were loaded into a box furnace and thermally annealed at different T from 130 °C to 600 °C for 1 h; the heating rate was 2.5 °C min −1 . It should be noted that all samples for X-ray and morphological characterization were fabricated on unpatterned ZrO 2 /Si substrates.
Characterization. The dehydration, decomposition, and crystallization kinetics of dried LiNO 3 , In(NO 3 ) 3 ·x H 2 O, and mixed powders were investigated from 25 °C to 600 °C using TG-DTA (TG 209 F3 Tarsus ® , NETZSCH) with a heating rate of 10 °C min −1 from 25 °C to 600 °C under an air ambient condition. Film thicknesses were calculated from the corresponding synchrotron-based X-ray reflectivity (XRR, beamline X9, Brookhaven National Laboratory, USA) profiles. XPS (K-Alpha Thermal Scientific) was performed with K α radiation so as to investigate the elemental chemistry and bonding in the Li-assisted In 2 O 3 thin films. AFM (Multimode 8, Bruker) was carried out to examine the nano-structural morphologies of the fabricated samples. The crystalline structure of the films was evaluated by synchrotron-based GIXD beamlines 3 C and 9 A, Pohang Acceleration Laboratory, Korea 36,37 .
The electrical characteristics of the Li-assisted In 2 O 3 TFTs were measured with a semiconductor analyzer (Agilent 4155 C). The electron mobility (μ e ) and threshold voltage (V th ) values were calculated in the saturation regime (drain voltage, V D = 1 V) using the following equation, I D = μ e C i W(2L) −1 (V G -V th ) 2 , where C i is the capacitance of the gate dielectrics and V G is the gate voltage. The C i values of the dielectrics, which were sandwiched between the ITO and highly doped p-type (100) Si substrate, were measured with an Agilent E4980A instrument.