Improved Negative Bias Stress Stability of Sol-Gel-Processed Mg-Doped In2O3 Thin Film Transistors

We demonstrate sol–gel-processed Mg-doped In<sub>2</sub>O<sub>3</sub> thin film transistors (TFTs) with high performance and improved stability. To improve the performance of indium oxide, which is unstable at a high negative threshold voltage, magnesium is used to suppress oxygen vacancy formation. As the Mg doping concentration increases, the oxygen deficiencies and OH impurities decrease, resulting in a positive shift in the threshold voltage and improved stability in negative bias stress environments. In this experiment, Mg-doped (0 to 2 wt%) indium oxide TFTs are fabricated. Indium oxide prepared from a synthesized solution of an indium nitrate hydrate precursor and 2-methoxyethanol has a mobility of 17.2 cm<sup>2</sup>/V s. In<sub>2</sub>O<sub>3</sub> TFTs doped with 1 wt% Mg also show a high mobility of 11.02 cm<sup>2</sup>/V s and a noticeable −1.5 V threshold voltage shift under negative bias stress. Our results suggest that the sol–gel-processed Mg-doped In<sub>2</sub>O<sub>3</sub> TFTs are a promising candidate for use in high-stability and high-performance applications in transparent devices.


Introduction
Recently, the demand for oxide semiconductors has been increased due to their high usability. Oxide semiconductor-based thin-film transistors (TFTs) show higher mobility than amorphous phase Si and can achieve higher stability than low-temperature polysilicon material-based transistors [1][2][3][4][5]. In addition, their wide bandgap properties are critical factors to realize transparent electronics. These advantages are being used not only for large-area displays but also for several devices that require transparent features. The most commercially available oxide semiconductors are the indium gallium zinc oxide (IGZO) semiconductors for channel materials of TFTs. However, IGZO contains indium, which has low price competitiveness because it is a rare-earth element [6]. Therefore, many studies have been conducted to fabricate inexpensive oxide TFTs to replace IGZO, which is expensive, and ZnO, ZnSnO, and SnO 2 , excluding indium, have shown excellent performance as substitutes [7][8][9]. SnO 2 is a promising material for use as an active channel layer of high-performance oxide TFT (high intrinsic Hall mobility, transparency because of a wide bandgap, and a low melting point, which determines the sintering temperature for achieving highquality crystalline thin films [10][11][12]. To date, the expensive and time costly vacuum-based deposition technique has been used for fabricating metal-oxide-based thin films. To overcome this issue, the sol-gel process has been recently employed [13][14][15][16]. The solgel route process is cost-effective, eco-friendly, and adaptable for large area applications. Moreover, because the starting materials used in this process are in the liquid phase, they have been considered promising inks for various printing systems [17]. Although the abovementioned sol-gel process-based electronics have shown high performance, one critical disadvantage is their chemical or bias stress instability, originating from the absorbed molecules or oxygen vacancy inside metal-oxide semiconductors (MOSs), which Figure 1 shows the GIXRD spectra of the sol-gel-processed SnO 2 films with various Li contents. All the deposited TFTs showed the polycrystalline tetragonal SnO 2 phase. The diffraction peaks at 26.6, 33.8, 37.95, and 51.8 corresponded to the crystal planes of (110), (101), (200), and (211) (JCPDS card number: 41-1445), respectively. The peak at 31.7 corresponded to Sn (JCPDS card number: 82-2958), indicating that Sn was partially oxidized. The full width at half-maximum (FWHM) of each diffraction peak was associated with the crystalline size of the films. The (110) plane was primarily the direction of growth of SnO 2 films. The crystalline size of the films under the (110) crystal plane was calculated using the Scherrer equation: where D, λ, β, and θ indicate the crystalline size, CuKα wavelength (1.54 Å), FWHM of the peak, and peak position, respectively. The obtained crystalline sizes of the SnO 2 TFTs from the (110) plane were 1.33 nm, 1.47 nm, and 1.47 nm for Li-doped SnO 2 films with Li contents of 0.0 wt%, 0.5 wt%, and 1.0 wt%, respectively. The Li-doped SnO 2 flims exhibited larger crystalline sizes than the undoped SnO 2 films. The added Li could accelerate the growth process during the sol-gel process [20].
from the (110) plane were 1.33 nm, 1.47 nm, and 1.47 nm for Li-doped SnO2 films with Li contents of 0.0 wt%, 0.5 wt%, and 1.0 wt%, respectively. The Li-doped SnO2 flims exhibited larger crystalline sizes than the undoped SnO2 films. The added Li could accelerate the growth process during the sol-gel process [20]. Figure 2a-c show the representative O 1s XPS spectra of Li-doped SnO2 as a function of Li (wt%). The O 1s peak was divided into three types of peaks [21]. One peak is for the O2 -combined with the adjacent ion in Lattice (OLattice), another for the Oxygen bonds related to the oxygen vacancy (OVacancy), and the other for the amount of OH. Figure 2d shows that the variation in the composition ratio of OVacancy, OLattice, and -OH groups as a function of Li (wt%). When a 0.5 wt% Li dopant was added, the relative percentage of OVacancy gradually decreased from 27.5% to 21.7%, whereas the relative percentage of OLattice gradually increased from 65.7% to 68.1%, which meant that a small amount of Li dopant could work as a carrier inhibitor. Several factors, such as electronegativity, standard electrode potential (SEP), and ion radius, need to be assessed for dopants to be used as carrier inhibitors [22,23]. The electronegativity of dopants and oxygen should significantly differ from each other. When the SEP of the dopant is lower than that of the host, the dopantoxygen bond becomes stronger. The electronegativity and SEP of Sn hosts and Li dopants were 1.96 and −0.14 V and 0.98 and −3.04 V, respectively. Based on the aforementioned theory, the Li dopant could work as a carrier inhibitor successfully. Moreover, the radius of the dopant and host ions should not differ considerably to avoid the breakdown of the structure; the radii of Sn 4+ and Li + were 74 and 68 pm, respectively, making Li + suitable as the dopant. However, when 1.0-wt% Li was added, the relative percentage of OVacancy increased from 21.2% to 23.3%, while the relative percentage of OLattice decreased from 68.1% to 66.6%.   Figure 2d shows that the variation in the composition ratio of O Vacancy , O Lattice , and -OH groups as a function of Li (wt%). When a 0.5 wt% Li dopant was added, the relative percentage of O Vacancy gradually decreased from 27.5% to 21.7%, whereas the relative percentage of O Lattice gradually increased from 65.7% to 68.1%, which meant that a small amount of Li dopant could work as a carrier inhibitor. Several factors, such as electronegativity, standard electrode potential (SEP), and ion radius, need to be assessed for dopants to be used as carrier inhibitors [22,23]. The electronegativity of dopants and oxygen should significantly differ from each other. When the SEP of the dopant is lower than that of the host, the dopant-oxygen bond becomes stronger. The electronegativity and SEP of Sn hosts and Li dopants were 1.96 and −0.14 V and 0.98 and −3.04 V, respectively. Based on the aforementioned theory, the Li dopant could work as a carrier inhibitor successfully. Moreover, the radius of the dopant and host ions should not differ considerably to avoid the breakdown of the structure; the radii of Sn 4+ and Li + were 74 and 68 pm, respectively, making Li + suitable as the dopant. However, when 1.0-wt% Li was added, the relative percentage of O Vacancy increased from 21.2% to 23.3%, while the relative percentage of O Lattice decreased from 68.1% to 66.6%. from the (110) plane were 1.33 nm, 1.47 nm, and 1.47 nm for Li-doped SnO2 films with Li contents of 0.0 wt%, 0.5 wt%, and 1.0 wt%, respectively. The Li-doped SnO2 flims exhibited larger crystalline sizes than the undoped SnO2 films. The added Li could accelerate the growth process during the sol-gel process [20].  The O 1s peak was divided into three types of peaks [21]. One peak is for the O2 -combined with the adjacent ion in Lattice (OLattice), another for the Oxygen bonds related to the oxygen vacancy (OVacancy), and the other for the amount of OH. Figure 2d shows that the variation in the composition ratio of OVacancy, OLattice, and -OH groups as a function of Li (wt%). When a 0.5 wt% Li dopant was added, the relative percentage of OVacancy gradually decreased from 27.5% to 21.7%, whereas the relative percentage of OLattice gradually increased from 65.7% to 68.1%, which meant that a small amount of Li dopant could work as a carrier inhibitor. Several factors, such as electronegativity, standard electrode potential (SEP), and ion radius, need to be assessed for dopants to be used as carrier inhibitors [22,23]. The electronegativity of dopants and oxygen should significantly differ from each other. When the SEP of the dopant is lower than that of the host, the dopantoxygen bond becomes stronger. The electronegativity and SEP of Sn hosts and Li dopants were 1.96 and −0.14 V and 0.98 and −3.04 V, respectively. Based on the aforementioned theory, the Li dopant could work as a carrier inhibitor successfully. Moreover, the radius of the dopant and host ions should not differ considerably to avoid the breakdown of the structure; the radii of Sn 4+ and Li + were 74 and 68 pm, respectively, making Li + suitable as the dopant. However, when 1.0-wt% Li was added, the relative percentage of OVacancy increased from 21.2% to 23.3%, while the relative percentage of OLattice decreased from 68.1% to 66.6%. To elucidate the effect of the Li dopant amount on the optical bandgap of SnO2 films, the transmittance plots of the SnO2 films as a function of Li dopant amounts were obtained using a UV-vis spectrophotometer. Regardless of the Li dopant amount all the fabricated SnO2 films showed a transmittance of >97.5% in the visible range (380-900 nm) ( Figure  3a). However, at less than 350 nm, all the SnO2 films showed a drastic decrease in the bandgap absorption, corresponding to the optical bandgap of SnO2 films. The optical bandgap can be obtained by extrapolating the line segment in Figure 3b and using the following equation: where α is the absorption coefficient, A is a constant, and Eg is the optical bandgap of SnO2 films. For direct bandgap transition, the n value is ½. Using Equation (2), the estimated optical bandgaps of each film were 3.79 eV, 3.87 eV, and 3.93 eV. As a function of Li concentration, the optical bandgap of SnO2 films increased. Regardless of the Li dopant amount, the obtained optical bandgap of the fabricated SnO2 films was larger than that of bulk SnO2 (3.60 eV). If the film thickness was close to the Bohr radius owing to the quantum confinement effect, the optical bandgap could increase. The change in the optical bandgap can be calculated using the following equation [24,25]: where h is the Plank's constant, t is the thickness of SnO2 films, me * and mh * are the effective masses of electrons and holes, respectively (for SnO2, me * = 0.275m0, m0 = 9.1 × 10 −31 kg).
Because me * << mh * , 1/mh * term can be ignored [24]. Regardless of Li dopant amount, the film thickness was approximately 5.5 nm, and the calculated optical bandgap was 3.90 eV. An additional change in the optical bandgap could be attributed to the free carrier concentration. Figure 3c presents the valence band (VB) spectra of Li-doped SnO2 films using XPS. The VB offset values between the maximum VB value and the Fermi level were extrapolated by linear fitting and the values were 3.65 eV, 3.46 eV, and 3.65 eV, respectively [26]. Using the obtained bandgap and VB offset values, the conduction band (CB) offset values between the minimum of CB value and Fermi level could be calculated. The calculated CB offset values were 0.14 eV, 0.41 eV, and 0.28 eV, respectively, and they were proportional to the carrier concentration. The CB offset value of the 0.5 wt% Li dopant was the lowest, implying the lowest carrier concentration among the three cases by suppressing OVacancy formation, determines the free carrier concentration in the MOS system [27]. To elucidate the effect of the Li dopant amount on the optical bandgap of SnO 2 films, the transmittance plots of the SnO 2 films as a function of Li dopant amounts were obtained using a UV-vis spectrophotometer. Regardless of the Li dopant amount all the fabricated SnO 2 films showed a transmittance of >97.5% in the visible range (380-900 nm) (Figure 3a). However, at less than 350 nm, all the SnO 2 films showed a drastic decrease in the bandgap absorption, corresponding to the optical bandgap of SnO 2 films. The optical bandgap can be obtained by extrapolating the line segment in Figure 3b and using the following equation: where α is the absorption coefficient, A is a constant, and E g is the optical bandgap of SnO 2 films. For direct bandgap transition, the n value is 1 2 . Using Equation (2), the estimated optical bandgaps of each film were 3.79 eV, 3.87 eV, and 3.93 eV. As a function of Li concentration, the optical bandgap of SnO 2 films increased. Regardless of the Li dopant amount, the obtained optical bandgap of the fabricated SnO 2 films was larger than that of bulk SnO 2 (3.60 eV). If the film thickness was close to the Bohr radius owing to the quantum confinement effect, the optical bandgap could increase. The change in the optical bandgap can be calculated using the following equation [24,25]: where h is the Plank's constant, t is the thickness of SnO 2 films, m e * and m h * are the effective masses of electrons and holes, respectively (for SnO 2 , m e * = 0.275m 0 , m 0 = 9.1 × 10 −31 kg). Because m e * << m h * , 1/m h * term can be ignored [24]. Regardless of Li dopant amount, the film thickness was approximately 5.5 nm, and the calculated optical bandgap was 3.90 eV. An additional change in the optical bandgap could be attributed to the free carrier concentration. Figure 3c presents the valence band (VB) spectra of Li-doped SnO 2 films using XPS. The VB offset values between the maximum VB value and the Fermi level were extrapolated by linear fitting and the values were 3.65 eV, 3.46 eV, and 3.65 eV, respectively [26]. Using the obtained bandgap and VB offset values, the conduction band (CB) offset values between the minimum of CB value and Fermi level could be calculated. The calculated CB offset values were 0.14 eV, 0.41 eV, and 0.28 eV, respectively, and they were proportional to the carrier concentration. The CB offset value of the 0.5 wt% Li dopant was the lowest, implying the lowest carrier concentration among the three cases by suppressing O Vacancy formation, determines the free carrier concentration in the MOS system [27]. Electronics 2021, 10, x FOR PEER REVIEW 5 of 9  TFTs showed the conventional n-type semiconductor properties and exhibited a high on/off current ratio (over 10 7 ). The relatively large work function of an Au source and a drain electrode on n-type semiconductor results in the formation of Schottky contacts. The nonlinear relationship between drain currents and voltages originates from the abovementioned energy barrier between electrodes and semiconductors [14]. The TFT fabricated using pure SnO2 showed the depletion-mode operation with a negative threshold voltage (Vth). This device, which is in the depletion mode, could easily form a channel at a gate voltage of 0.0 V owing to the presence of excessive free carriers in the SnO2 TFT. However, 0.5-and 1.0-wt% Li-doped TFTs showed the enhancement-mode operation with a positive Vth. Compared with pure SnO2, the reduced OVacancy generated less free carriers, leading to a late turn on. Field effect mobility (FEM) in the saturation regime and Vth were obtained by linear fitting to the (ID) 0.5 -VG plot using the following equation: where Ci (insulator capacitance) = 3.45 10 / , W (channel width) = 1000 μm, L (channel length) = 100 μm. The FEM in the saturation regime of the SnO2 TFT was 5.91 cm 2 /Vs. The FEM values of the TFTs consisting of 0.5-and 1.0 wt% Li doped SnO2 TFTs were 2.0 and 4.42 cm 2 /Vs, respectively. Grain boundary scattering, related to the grain boundary number, inside the channel material was strongly affected the FEM of TFTs. For example, a transistor, consisting of channel material, having large crystallinity, showed increased FEM by suppressing the grain boundary scattering [28]. Although based on the GIXRD data, the calculated crystalline size of the pure SnO2 films was smaller than those  . The relatively large work function of an Au source and a drain electrode on n-type semiconductor results in the formation of Schottky contacts. The nonlinear relationship between drain currents and voltages originates from the abovementioned energy barrier between electrodes and semiconductors [14]. The TFT fabricated using pure SnO 2 showed the depletion-mode operation with a negative threshold voltage (V th ). This device, which is in the depletion mode, could easily form a channel at a gate voltage of 0.0 V owing to the presence of excessive free carriers in the SnO 2 TFT. However, 0.5-and 1.0-wt% Li-doped TFTs showed the enhancement-mode operation with a positive V th . Compared with pure SnO 2 , the reduced O Vacancy generated less free carriers, leading to a late turn on. Field effect mobility (FEM) in the saturation regime and V th were obtained by linear fitting to the (I D ) 0.5 -V G plot using the following equation: where C i (insulator capacitance) = 3.45 × 10 −8 F/cm 2 , W (channel width) = 1000 µm, L (channel length) = 100 µm. The FEM in the saturation regime of the SnO 2 TFT was 5.91 cm 2 /Vs. The FEM values of the TFTs consisting of 0.5-and 1.0 wt% Li doped SnO 2 TFTs were 2.0 and 4.42 cm 2 /Vs, respectively. Grain boundary scattering, related to the grain boundary number, inside the channel material was strongly affected the FEM of TFTs. For example, a transistor, consisting of channel material, having large crystallinity, showed increased FEM by suppressing the grain boundary scattering [28]. Although based on the GIXRD data, the calculated crystalline size of the pure SnO 2 films was smaller than those of the Li-doped SnO 2 films, the FEM in saturation regime of pure SnO 2 TFTs showed the highest values. For the MOS system, the main transportation mechanism was the percolation conduction mechanism. Based on this mechanism, the FEM could be improved by filling trap sites at high carrier concentrations [29]. The number of free carriers could be increased by forming O Vacancy inside MOSs. O Vacancy worked as donor levels and generated carriers using the following equation: The decreased FEM value of 0.5-wt% Li-doped SnO 2 -based TFT attributed to the decrease in the number of O Vacancy , which was confirmed using XPS data. This is a critical factor, compared with grain boundary scattering effects in this system. Additionally, The reduced number of O Vacancy was the reason for the low FEM value of the 0.5-wt% Li-doped SnO 2 -based TFT show compared with that of pure SnO 2 -based TFTs [16]. The 1.0 wt% Li-doped SnO 2 based TFTs again showed increased O Vacancy again, and which increased the FEM and induced a negative shift of V th compared with the 0.5 wt% Li-doped based SnO 2 TFTs. It was already reported that Li can be located at the interstitial site and substitutional site in the SnO 2 matrix. If Li is located at the interstitial site, this can increase carrier concentration by generating on electron. The increased carrier concentration results in the enhanced FEM, based on aforementioned the percolation conduction mechanism. However, if Li is located at a substitutional site in the SnO 2 matrix, it can neutralize the electrons, which are the system's main carriers; because of fewer electrons, FEM decreases.
Electronics 2021, 10, x FOR PEER REVIEW 6 of 9 of the Li-doped SnO2 films, the FEM in saturation regime of pure SnO2 TFTs showed the highest values. For the MOS system, the main transportation mechanism was the percolation conduction mechanism. Based on this mechanism, the FEM could be improved by filling trap sites at high carrier concentrations [29]. The number of free carriers could be increased by forming OVacancy inside MOSs. OVacancy worked as donor levels and generated carriers using the following equation: The decreased FEM value of 0.5-wt% Li-doped SnO2-based TFT attributed to the decrease in the number of OVacancy, which was confirmed using XPS data. This is a critical factor, compared with grain boundary scattering effects in this system. Additionally, The reduced number of OVacancy was the reason for the low FEM value of the 0.5-wt% Li-doped SnO2-based TFT show compared with that of pure SnO2-based TFTs [16]. The 1.0 wt% Lidoped SnO2 based TFTs again showed increased OVacancy again, and which increased the FEM and induced a negative shift of Vth compared with the 0.5 wt% Li-doped based SnO2 TFTs. It was already reported that Li can be located at the interstitial site and substitutional site in the SnO2 matrix. If Li is located at the interstitial site, this can increase carrier concentration by generating on electron. The increased carrier concentration results in the enhanced FEM, based on aforementioned the percolation conduction mechanism. However, if Li is located at a substitutional site in the SnO2 matrix, it can neutralize the electrons, which are the system's main carriers; because of fewer electrons, FEM decreases.     [18,19,30]. In addition, compared with sol-gel-processed Mg-and Ti-doped based SnO 2 , Li-doped SnO 2 based TFTs show more stable NBS properties [19,30]. Comparing to the previous results, Li-doped SnO 2 TFTs show similar or higher field effect mobility and comparable negative bias stability. The reported Si-and Ga-doped SnO 2 based TFTs show higher FEM and the Si-doped SnO 2 based TFTs show the better NBS properties [31]. Corresponding to deposition method, the crystallinity, chemical composition, and carrier concentration were changed. In this paper, the sol-gel-processed SnO 2 -based thin film transistors show lower field effect mobility, compared to reported SnO 2 semiconductors deposited by the vacuum-based deposition technique. However, sol-gel-processed Li doping can be applied to other types of the solution-processed oxide TFTs.
Electronics 2021, 10, x FOR PEER REVIEW 7 of 9 the lowest Vth shift among the three cases. Based on XPS data, 0.5 wt% Li-doped SnO2 based TFT showed the lowest OVacancy concentration. Based on the percolation conduction mechanism, the lowest OVacancy leads to the lowest FEM and improved NBS properties. This negatively shifted Vth originated from the tapping of positive charges at the interface between semiconductors and insulators. OVacancy could form VO 2+ and 2e − . VO 2+ could move to the interface between semiconductors and insulators during the NBS test. By adding the optimal amount Li dopant, the OVacancy could be decreased by the formation of Li-OVacancy pairs, leading to improved NBS properties [18,19,30]. In addition, compared with sol-gel-processed Mg-and Ti-doped based SnO2, Li-doped SnO2 based TFTs show more stable NBS properties [19,30]. Comparing to the previous results, Li-doped SnO2 TFTs show similar or higher field effect mobility and comparable negative bias stability. The reported Si-and Ga-doped SnO2 based TFTs show higher FEM and the Si-doped SnO2 based TFTs show the better NBS properties [31]. Corresponding to deposition method, the crystallinity, chemical composition, and carrier concentration were changed. In this paper, the sol-gel-processed SnO2-based thin film transistors show lower field effect mobility, compared to reported SnO2 semiconductors deposited by the vacuum-based deposition technique. However, sol-gel-processed Li doping can be applied to other types of the solution-processed oxide TFTs.

Conclusions
In this study, sol-gel-processed Li-doped SnO2-based TFTs were fabricated on SiO2/p+ Si substrates. The influence of the Li dopant amount on the structural, chemical, optical, and electrical characteristics of the fabricated Li-doped SnO2-based TFTs was investigated. By adding 0.5-wt% Li, the oxygen vacancy formation process was successfully suppressed. The TFTs fabricated using 0.5-wt% Li-doped SnO2 semiconductor films exhibited FEM in the saturation regime of 2.0 cm 2 /Vs and showed the enhancement-mode operation. The decreased OVacancy inside the SnO2-based TFTs fabricated using 0.5-wt% Li improved the negative bias stability of TFTs.

Conclusions
In this study, sol-gel-processed Li-doped SnO 2 -based TFTs were fabricated on SiO 2 /p + Si substrates. The influence of the Li dopant amount on the structural, chemical, optical, and electrical characteristics of the fabricated Li-doped SnO 2 -based TFTs was investigated. By adding 0.5-wt% Li, the oxygen vacancy formation process was successfully suppressed. The TFTs fabricated using 0.5-wt% Li-doped SnO 2 semiconductor films exhibited FEM in the saturation regime of 2.0 cm 2 /Vs and showed the enhancement-mode operation. The decreased O Vacancy inside the SnO 2 -based TFTs fabricated using 0.5-wt% Li improved the negative bias stability of TFTs.