Dependence of Positive Bias Stress Instability on Threshold Voltage and Its Origin in Solution-Processed Aluminum-Doped Indium Oxide Thin-Film Transistors

The initial electrical characteristics and bias stabilities of thin-film transistors (TFTs) are vital factors regarding the practical use of electronic devices. In this study, the dependence of positive bias stress (PBS) instability on an initial threshold voltage (VTH) and its origin were analyzed by understanding the roles of slow and fast traps in solution-processed oxide TFTs. To control the initial VTH of oxide TFTs, the indium oxide (InOx) semiconductor was doped with aluminum (Al), which functioned as a carrier suppressor. The concentration of oxygen vacancies decreased as the Al doping concentration increased, causing a positive VTH shift in the InOx TFTs. The VTH shift (∆VTH) caused by PBS increased exponentially when VTH was increased, and a distinct tendency was observed as the gate bias stress increased due to a high vertical electric field in the oxide dielectric. In addition, the recovery behavior was analyzed to reveal the influence of fast and slow traps on ∆VTH by PBS. Results revealed that the effect of the slow trap increased as the VTH moved in the positive direction; this occured because the main electron trap location moved away from the interface as the Fermi level approached the conduction band minimum. Understanding the correlation between VTH and PBS instability can contribute to optimizing the fabrication of oxide TFT-based circuits for electronic applications.


Introduction
Oxide thin-film transistors (TFTs) have been considered to be the building blocks for various next-generation electronic devices owing to their advantages, such as low processing temperature, high mobility, device-to-device uniformity, optical transparency, and high mechanical flexibility [1][2][3].Large-area organic light-emitting diode (OLED) displays with indium-gallium-zinc-oxide TFTs have been commercialized [4,5].Recently, the use of oxide TFTs in displays and monolithic three-dimensional integration has been actively studied [6].The commercial feasibility of TFTs depends on their initial electrical characteristics.Among the various electrical parameters, such as mobility, subthreshold swing (SS), on/off current ratio (I on/off ), and threshold voltage (V TH ), V TH is vital because it determines the depletion or enhancement mode of TFTs.Depletion-and enhancementmode TFTs should be appropriately selected based on the product's characteristics [7,8].In addition, V TH should be effectively controlled to greater or less than 0. For example, when configuring a compensation circuit within a pixel in a display, the compensation error may increase if V TH is too high or low [9,10].In addition, a positive V TH is required to reduce circuit design complexity, as well as power consumption, when it is used as a gate driver circuit [11,12].Thus, V TH is an important initial parameter and should be effectively controlled during fabrication.
Based on their commercial usage, the robustness of TFT devices and their temperature, light, humidity, and bias stabilities must be ensured.Various external stresses, such as light or humidity, can be partially blocked by appropriately designing the device's structure, as well as introducing additional layers [13,14].However, blocking the bias stress in TFTs is difficult as the electrical bias applied to drive the transistors is an internal stress.Therefore, the origin of bias stability must be studied to determine the robustness of the device.Extreme V TH shifts and SS degradation occurs when negative gate bias and illumination stress are simultaneously applied [15][16][17].When the oxide TFTs act as a switch rather than the current supplier to the OLEDs in displays, negative bias illumination stress (NBIS) is critical because the switching TFTs are typically turned off by negative gate bias [18].However, the NBIS can be improved through the reduction of oxygen vacancies or peroxides, or introducing light-blocking metals [19].Recently, oxide TFTs have been used in OLED displays [20], and positive bias stress (PBS) is becoming increasingly important because it is applied to the driving transistor that supplies a current to the OLED and gate driver circuit [18].Although initial V TH and PBS must be simultaneously considered, their correlation is not entirely clear.The PBS stability can be further improved based upon the initial V TH .Moreover, solution-processed oxide TFTs have been actively studied owing to their advantages of high scalability and throughput [21][22][23].However, they have poorer electrical stabilities than those fabricated by conventional sputtering because of the high density of physical and chemical defects that are generated during fabrication [24,25].Therefore, the relation between V TH and PBS stability must be further deduced.Additionally, VTH must be decreased in solution-processed oxide TFTs with relatively poor PBS stability than the sputtered oxide TFTs.
In this study, the correlation between an initial V TH and PBS instability in solutionprocessed oxide TFTs was investigated.The degradation mechanism of PBS based on V TH was demonstrated through the observation of the recovery behavior of oxide TFTs after PBS.Various molarities of Al were doped in the InO x semiconductor to examine the dependency of V TH on PBS instability; V TH was successfully controlled in a wide range.The V TH shift of Al-doped oxide TFTs was determined via the chemical and optical analysis of InO x and indium-aluminum-oxide (InAlO x ) thin-film characteristics.Through measuring the PBS and the recovery of oxide TFTs, it was found that the initial V TH and ∆V TH induced by PBS have an exponential relation, regardless of the gate bias stress (V G,stress ).Moreover, analysis of the recovery behavior revealed that the role of slow traps in ∆V TH became dominant in oxide TFTs with positive V TH .

Preparation of Oxide Precursor Solutions
InO x and InAlO x precursor solutions were prepared for fabricating oxide semiconductor thin films.A 0.1 M InO x precursor solution was prepared by dissolving In(NO 3 ) 3 •xH 2 O (Sigma-Aldrich, St. Louis, MO, USA) in 2-methoxyethanol (2-ME), which is generally used as a solvent for oxide precursor solutions [26,27].To control the initial V TH , InAlO x precursor solutions were prepared by adding 0.005, 0.010, and 0.015 M Al(NO 3 ) 3 •9H 2 O (Sigma-Aldrich, St. Louis, MO, USA) to 0.1 M InO x precursor solution.The molar ratio of Al/In of the three prepared solutions are 0.05, 0.10, and 0.15.All solutions were stirred at 50 • C for 12 h.

Fabrication of Devices
Si/SiO 2 wafer substrates were prepared for gates and gate dielectrics.Wafers were cleaned via sonication for 10 min each in acetone, isopropyl alcohol, and deionized water.Then, the substrates were dried in an N 2 atmosphere, and annealed at 300 • C for 5 min to remove any residual moisture.The water etchant-based photo patterning method was used to deposit the patterned oxide semiconductor for the fabrication of high-performance oxide TFTs [28].Figure 1a schematizes the fabrication process of InO x and InAlO x thin films.The precursor solutions were spin-coated at 3000 rpm for 20 s onto a cleaned Si/SiO 2 wafer (Step 1).The precursor solution-deposited wafer was soft baked at 100 • C for 30 s (Step 2).Then, a fine metal mask was placed on the wafer to irradiate ultraviolet (UV) light in a selective area.The UV was exposed to InO x for 120s and InAlO x for 150 s under an N 2 atmosphere (Step 3).UV irradiation (25 mW cm −2 ) was conducted via a low-pressure mercury lamp with two main wavelengths, namely 253.7 (90%) and 184.9 nm (10%).UV photons induced the decomposition of nitrate ligand via photochemical cleavage.Then, the wafer was etched in a deionized water etchant for 1 min (Step 4).Patterned InO x and InAlO x were annealed at 100 • C for 10 min and 270 • C for 2 h, respectively, on a hot plate in the air (Step 5).In this series of processes, hydrolysis and condensation reactions were promoted, which yielded low-defect high-quality oxide thin films.Lastly, a 50-nm-thick Al film was deposited for source and drain electrodes using thermal evaporation (Step 6).The channel length (L) and width (W) were 100 and 1000 µm, respectively.The patterned thin films or TFTs fabricated using InO x , InAlO x with 0.005 M Al doping, InAlO x with 0.010 M Al doping, and InAlO x with 0.015 M Al doping were named InO x , InAlO x -1, InAlO x -2, and InAlO x -3, respectively, as shown in Figure 1b.Then, the substrates were dried in an N2 atmosphere, and annealed at 300 °C for 5 min to remove any residual moisture.The water etchant-based photo patterning method was used to deposit the patterned oxide semiconductor for the fabrication of high-performance oxide TFTs [28].Figure 1a schematizes the fabrication process of InOx and InAlOx thin films.The precursor solutions were spin-coated at 3000 rpm for 20 s onto a cleaned Si/SiO2 wafer (Step 1).The precursor solution-deposited wafer was soft baked at 100 °C for 30 s (Step 2).Then, a fine metal mask was placed on the wafer to irradiate ultraviolet (UV) light in a selective area.The UV was exposed to InOx for 120s and InAlOx for 150 s under an N2 atmosphere (Step 3).UV irradiation (25 mW cm −2 ) was conducted via a lowpressure mercury lamp with two main wavelengths, namely 253.7 (90%) and 184.9 nm (10%).UV photons induced the decomposition of nitrate ligand via photochemical cleavage.Then, the wafer was etched in a deionized water etchant for 1 min (Step 4).Patterned InOx and InAlOx were annealed at 100 °C for 10 min and 270 °C for 2 h, respectively, on a hot plate in the air (Step 5).In this series of processes, hydrolysis and condensation reactions were promoted, which yielded low-defect high-quality oxide thin films.Lastly, a 50nm-thick Al film was deposited for source and drain electrodes using thermal evaporation (Step 6).The channel length (L) and width (W) were 100 and 1000 μm, respectively.The patterned thin films or TFTs fabricated using InOx, InAlOx with 0.005 M Al doping, InAlOx with 0.010 M Al doping, and InAlOx with 0.015 M Al doping were named InOx, InAlOx-1, InAlOx-2, and InAlOx-3, respectively, as shown in Figure 1b.

Analysis of Thin Films and Devices
Chemical compositions of InOx and InAlOx thin films were investigated using X-ray photoelectron spectroscopy (XPS; ThermoFisher Scientific, NEXSA, Waltham, MA, USA) with an Al Kα (1486.6 eV) light source.To determine the optical properties, such as the bandgap, of the deposited oxide semiconductor films, a UV-visible (UV-Vis) spectrophotometer (Perkin Elmer, Waltham, MA, USA, LAMBDA 265) was used.The InOx and In-AlOx films deposited on cleaned glass were measured by a transmittance mode.A probe station (MS Tech, Oak Ridge, TN, USA, MST-6VC) was used to measure the transfer curves of InOx TFTs under temperature variations.The electrical characteristics of the devices were obtained using a semiconductor parameter analyzer (4200-SCS, Keithley, Cleveland, OH, USA) combined with an ultrafast I-V module (4225-PMU, Keithley).All devices were measured at room temperature in a dark environment.

Analysis of Thin Films and Devices
Chemical compositions of InO x and InAlO x thin films were investigated using X-ray photoelectron spectroscopy (XPS; ThermoFisher Scientific, NEXSA, Waltham, MA, USA) with an Al Kα (1486.6 eV) light source.To determine the optical properties, such as the bandgap, of the deposited oxide semiconductor films, a UV-visible (UV-Vis) spectrophotometer (Perkin Elmer, Waltham, MA, USA, LAMBDA 265) was used.The InO x and InAlO x films deposited on cleaned glass were measured by a transmittance mode.A probe station (MS Tech, Oak Ridge, TN, USA, MST-6VC) was used to measure the transfer curves of InO x TFTs under temperature variations.The electrical characteristics of the devices were obtained using a semiconductor parameter analyzer (4200-SCS, Keithley, Cleveland, OH, USA) combined with an ultrafast I-V module (4225-PMU, Keithley).All devices were measured at room temperature in a dark environment.

Results
InO x and InAlO x thin films with various molarities were fabricated to demonstrate their electrical characteristics, as shown in Figure 1a.The changes in the atomic structure with increasing Al doping concentration were evaluated via XPS analysis.Figure 2a shows the O1s spectra of InO x , InAlO x -1, InAlO x -2, and InAlO x -3.The oxygen bonding states were analyzed via deconvolution into three Gaussian Lorentzian peaks corresponding to the oxygen species of the metal-oxide (M-O), oxygen vacancy (V o ), and metal-hydroxide (M-OH) centered at ~529.6, ~531.0, and ~531.8 eV, respectively [29][30][31].The oxygen-binding energy of Al was higher than that of In; therefore, Al suppressed the oxygen defects in the active layer [32].As shown in Figure 2b, as the Al doping concentration increases, M-O bonding increases and V o decreases, thereby increasing the carrier concentration [33].The M-OH bonding increases with the Al doping concentration, which is explained via the M-OH condensation reaction.At a certain temperature, two M-OH bonds form an M-O-M bond via condensation; however, the reaction does not progress when Al is doped.Figure 2c shows the binding energy of the element extracted from XPS.As the Al doping concentration increases, the Al2p peak can be clearly observed.In step 2 (Figure 1a), a large amount of nitrate ligands in the oxide film disappears; in step 3 (Figure 1a), the nitrate ligands inside the film are decomposed by UV irradiation, hydroxyl radical species, and M−OH bond formation [28].Therefore, the N1s peak was not clear for all the TFT devices.The C-O/C-OH bond with a binding energy of ~287.0 eV was decomposed via UV irradiation, and only the C-H/C-C bond with a binding energy of ~285.0 eV remained; all TFT devices showed the same c1s peak [34].

Results
InOx and InAlOx thin films with various molarities were fabricated to demonstrate their electrical characteristics, as shown in Figure 1a.The changes in the atomic structure with increasing Al doping concentration were evaluated via XPS analysis.Figure 2a shows the O1s spectra of InOx, InAlOx-1, InAlOx-2, and InAlOx-3.The oxygen bonding states were analyzed via deconvolution into three Gaussian Lorentzian peaks corresponding to the oxygen species of the metal-oxide (M-O), oxygen vacancy (Vo), and metal-hydroxide (M-OH) centered at ~529.6, ~531.0, and ~531.8 eV, respectively [29][30][31].The oxygen-binding energy of Al was higher than that of In; therefore, Al suppressed the oxygen defects in the active layer [32].As shown in Figure 2b, as the Al doping concentration increases, M-O bonding increases and Vo decreases, thereby increasing the carrier concentration [33].The M-OH bonding increases with the Al doping concentration, which is explained via the M-OH condensation reaction.At a certain temperature, two M-OH bonds form an M-O-M bond via condensation; however, the reaction does not progress when Al is doped.Figure 2c shows the binding energy of the element extracted from XPS.As the Al doping concentration increases, the Al2p peak can be clearly observed.In step 2 (Figure 1a), a large amount of nitrate ligands in the oxide film disappears; in step 3 (Figure 1a), the nitrate ligands inside the film are decomposed by UV irradiation, hydroxyl radical species, and M−OH bond formation [28].Therefore, the N1s peak was not clear for all the TFT devices.The C-O/C-OH bond with a binding energy of ~287.0 eV was decomposed via UV irradiation, and only the C-H/C-C bond with a binding energy of ~285.0 eV remained; all TFT devices showed the same c1s peak [34].To understand the charge transport mechanism, the interfacial energy-level alignments between the active layer and source/drain electrode must be elucidated.The energy band alignment of InOx and InAlOx thin films was investigated via UV-Vis To understand the charge transport mechanism, the interfacial energy-level alignments between the active layer and source/drain electrode must be elucidated.The energy band alignment of InO x and InAlO x thin films was investigated via UV-Vis spectrophotometry and XPS analysis.The optical bandgap (E g ) was obtained from the extrapolated linear fit of (αhν) 2 versus the photon energy, as shown in Figure 3a.The extracted E g values of InO x , InAlO x -1, InAlO x -2, and InAlO x -3 were 3.72, 3.80, 3.81, and 3.86 eV, respectively.The atomic radius of In was 156 pm, larger than that of Al.Thus, the substitution of a smaller atom (Al) in the composition with a larger atom (In) decreases the E g ; these results coincide with the theoretical expectations [35].As shown in Figure 3b, as the Al doping concentration increases, the valence band offset decreases from 2.58 to 2.11.Thus, with increasing Al doping concentration, E g increases and valence band offset decreases, thereby increasing the conduction band offset (Figure 3c).This result suggests that as the Al doping concentration increases, V o and carrier concentration decrease [36,37].
Nanomaterials 2024, 14, x FOR PEER REVIEW 5 of 12 spectrophotometry and XPS analysis.The optical bandgap (Eg) was obtained from the extrapolated linear fit of (αhν) 2 versus the photon energy, as shown in Figure 3a.The extracted Eg values of InOx, InAlOx-1, InAlOx-2, and InAlOx-3 were 3.72, 3.80, 3.81, and 3.86 eV, respectively.The atomic radius of In was 156 pm, larger than that of Al.Thus, the substitution of a smaller atom (Al) in the composition with a larger atom (In) decreases the Eg; these results coincide with the theoretical expectations [35].As shown in Figure 3b, as the Al doping concentration increases, the valence band offset decreases from 2.58 to 2.11.Thus, with increasing Al doping concentration, Eg increases and valence band offset decreases, thereby increasing the conduction band offset (Figure 3c).This result suggests that as the Al doping concentration increases, Vo and carrier concentration decrease [36,37].To demonstrate the electrical characteristics based on the carrier concentration of the oxide semiconductor, TFTs containing InOx, InAlOx-1, InAlOx-2, and InAlOx-3 with a bottom-gate, top-contact structure were fabricated.Figure 4a shows the transfer characteristics of InOx, InAlOx-1, InAlOx-2, and InAlOx-3.Gate voltage (VG) swept from −20 to 30 V with a 0.5 V step, and drain voltage (VD) was fixed at 30 V. The gray line shows the gate leakage current (IG).As shown in the transfer curves, all TFTs had acceptable switching characteristics with low off current levels of 10 −12 -10 −11 A, regardless of the doping To demonstrate the electrical characteristics based on the carrier concentration of the oxide semiconductor, TFTs containing InO x , InAlO x -1, InAlO x -2, and InAlO x -3 with a bottom-gate, top-contact structure were fabricated.Figure 4a shows the transfer characteristics of InO x , InAlO x -1, InAlO x -2, and InAlO x -3.Gate voltage (V G ) swept from −20 to 30 V with a 0.5 V step, and drain voltage (V D ) was fixed at 30 V. The gray line shows the gate leakage current (I G ).As shown in the transfer curves, all TFTs had acceptable switching characteristics with low off current levels of 10 −12 -10 −11 A, regardless of the doping concentration.In contrast, the curves positively shifted, and on current gradually decreased as the Al doping concentration increased, which originated from the decrease in the carrier concentration of semiconductors as the Al doping concentration increased.For a more detailed comparison, the summarized results of the electrical parameters of InO x and InAlO x TFTs are shown in Figure 4b.Here, V TH was defined as the value of V G that induces drain current (I D ) = 10 nA × W/L at V D = 30 V. A positive shift of V TH , increase in SS, and decrease in field-effect mobility in saturation region (µ FE ) was observed.A positive shift of V TH and a decrease in µ FE are reasonable because the Al doping suppresses the formation of V o , thereby decreasing the carrier concentration, as confirmed by the XPS results.In addition, the maximum trap density (N T,max ) increased with Al doping concentration due to an increase in the SS based on the following equation: where k, q, T, and C i are the Boltzmann constant, elementary electron charge, and absolute temperature, respectively.N T and D it are the number of fast bulk traps and semiconductorinsulator interfacial traps.N T,max of InO x , InAlO x -1, InAlO x -2, and InAlO x -3 are 4.66, 6.11, 6.96, 7.96 × 10 18 cm −3 eV −1 , respectively.
Nanomaterials 2024, 14, x FOR PEER REVIEW 6 of 12 concentration.In contrast, the curves positively shifted, and on current gradually decreased as the Al doping concentration increased, which originated from the decrease in the carrier concentration of semiconductors as the Al doping concentration increased.For a more detailed comparison, the summarized results of the electrical parameters of InOx and InAlOx TFTs are shown in Figure 4b.Here, VTH was defined as the value of VG that induces drain current (ID) = 10 nA × W/L at VD = 30 V. A positive shift of VTH, increase in SS, and decrease in field-effect mobility in saturation region (μFE) was observed.A positive shift of VTH and a decrease in μFE are reasonable because the Al doping suppresses the formation of Vo, thereby decreasing the carrier concentration, as confirmed by the XPS results.In addition, the maximum trap density (NT,max) increased with Al doping concentration due to an increase in the SS based on the following equation: where k, q, T, and Ci are the Boltzmann constant, elementary electron charge, and absolute temperature, respectively.NT and Dit are the number of fast bulk traps and semiconductor-insulator interfacial traps.NT,max of InOx, InAlOx-1, InAlOx-2, and InAlOx-3 are 4.66, 6.11, 6.96, 7.96 10 18 cm −3 eV −1 , respectively.Unlike single-crystal Si-based transistors that follow the band transport theory, oxide semiconductor-based transistors have a different charge transport mechanism due to their atomic structural disorder.In general, the current-voltage behavior of oxide TFTs is modeled based on trap-limited conduction (TLC) and percolation [38,39].In particular, the charge transport mechanism varies based on the electron concentration in the semiconductor, which is identified through the power-law dependence of mobility [40].Here, a power-law equation was employed in the (V G -V TH )-µ FE curve to investigate the influence of Al doping on carrier trapping and thermal release at the tail states and percolation in the above-threshold regime in oxide TFTs, as shown in Figure 4c.
where V P is the percolation voltage, and K and γ are related to the nature of the electron transport.In a typical power-law model, TLC affects the constant K, and percolation conduction determines the exponent γ [41].To observe the transition of the electron conduction mechanism by V G , each curve was fitted in the low V G and high V G regions by the power-law equation.In low V G region, the K values of InO x , InAlO x -1, InAlO x -2, and InAlO x -3 are 0.46, 0.24, 0.04, and 0.01, respectively.This suggests that the TLC prevails as the initial V TH positively shifts in the oxide TFTs.As V TH shifts positively in the low V G region, γ also tends to increase, suggesting that electron conduction is greatly restricted by the potential barrier.Meanwhile, in high V G region, K and γ values according to Initial V TH show the same tendency as those in the low V G region.Interestingly, K and γ overall show higher and lower values, respectively, compared to the low V G region.This implies that charge transport by percolation is dominant rather than the tail states as the V G increase.
The PBS stability, along with the initial characteristics of the device, should be considered for the practical use of the device.Then, to verify the influence of Al doping concentration on the PBS-induced instability, V TH characteristics were determined based on the PBS and recovery.Figure 5 shows the time evolution of V TH shift (∆V TH ) during PBS and subsequent recovery.For PBS, a V G,stress of 10 or 30 V was applied for 1000 s.To avoid TFT degradation and asymmetric charge trapping due to current stress, a V D of 0 V was applied during stress.Immediately after 1000 s of PBS, recovery was followed for 2000 s at V G = 0 V and V D = 0 V.As shown in Figure 5a, V TH shifted positively during PBS with a V G,stress of 10 V, and the degree of recovery tended to increase with Al doping concentration.V TH recovers rapidly in the early stages of recovery after stress and then tends to saturate [42,43].InO x TFTs recovered at the fastest rates, and after recovery for 2000 s, V TH returned close to its initial state.InAlO x -3 with a high Al doping concentration showed the slowest recovery speed and a high ∆V TH , even after recovery at 2000 s.This characteristic means that some of the electrons trapped in the interface and gate dielectric by PBS are easily detrapped by the recovery phase, but the rest remain without being detrapped.Moreover, the results show that these trap/detrap characteristics differ depending on the Al doping concentration.
The stretched-exponential time dependence of ∆V TH suggests that ∆V TH originated from electron trapping at the trap sites [43,44].The stretched-exponential equation for ∆V TH (t) is as follows: where ∆V TH0 is the ∆V TH at infinite time, β is the stretched-exponential exponent, and τ is the characteristic trapping time of carriers.∆V TH by PBS with a V G,stress of 10 V for InO x and InAlO x TFTs were fitted to the stretched-exponential equation, and β and τ for each device were extracted, as shown in Figure 5b.τ decreased as the Al doping concentration increased, implying that electrons were easily trapped in the trap site.Figure 5c,d show the time evolution of ∆V TH by PBS with a V G,stress of 30 V and subsequent recovery, and the corresponding β and τ during PBS, respectively.∆V TH at a V G,stress of 30 V shows a similar trend to the PBS with V G,stress = 10 V, as shown in Figure 5a,b.Notably, as V G,stress increases, ∆V TH becomes larger in the same device and the correlation between Al doping concentration and ∆V TH becomes clearer.Through a quantitative comparison of τ, it is confirmed that charge trapping occurs more effectively with strong vertical electric field as V G,stress increases.The stretched-exponential time dependence of ∆VTH suggests that ∆VTH originated from electron trapping at the trap sites [43,44].The stretched-exponential equation for ∆VTH(t) is as follows: where ∆VTH0 is the ∆VTH at infinite time, β is the stretched-exponential exponent, and τ is the characteristic trapping time of carriers.∆VTH by PBS with a VG,stress of 10 V for InOx and InAlOx TFTs were fitted to the stretched-exponential equation, and β and τ for each device were extracted, as shown in Figure 5b.τ decreased as the Al doping concentration increased, implying that electrons were easily trapped in the trap site.Figure 5c,d show the time evolution of ∆VTH by PBS with a VG,stress of 30 V and subsequent recovery, and the corresponding β and τ during PBS, respectively.∆VTH at a VG,stress of 30 V shows a similar trend to the PBS with VG,stress = 10 V, as shown in Figure 5a,b.Notably, as VG,stress increases, ΔVTH becomes larger in the same device and the correlation between Al doping concentration and ΔVTH becomes clearer.Through a quantitative comparison of τ, it is confirmed that charge trapping occurs more effectively with strong vertical electric field as VG,stress increases.
Based on the initial electrical characteristics according to the Al doping concentration in InOx TFTs, the influence of initial VTH on ∆VTH is further revealed.Figure 6a depicts the time evolution of ∆VTH.Al-doped InOx TFTs show higher ∆VTH as VTH shifts more positively, or as VG,stress increases (Figure 5).Here, electrons that are not easily detrapped during the recovery phase are considered as slow traps, whereas those that are detrapped Based on the initial electrical characteristics according to the Al doping concentration in InO x TFTs, the influence of initial V TH on ∆V TH is further revealed.Figure 6a depicts the time evolution of ∆V TH .Al-doped InO x TFTs show higher ∆V TH as V TH shifts more positively, or as V G,stress increases (Figure 5).Here, electrons that are not easily detrapped during the recovery phase are considered as slow traps, whereas those that are detrapped during the recovery phase are treated as fast traps [42].Variations in ∆V TH based on V TH were plotted by increasing the number of measured samples to clearly identify the correlation between V TH and ∆V TH , as shown in Figure 6b.Regardless of V G,stress , ∆V TH tended to increase exponentially as V TH increased.The ∆V TH by PBS with 10 and 30 V for devices with a V TH in the range of −2-4 V was fitted with an exponential function, and showed high R 2 values of 0.88 and 0.93, respectively.As V G,stress increased from 10 to 30 V, the trend became clear.∆V TH,slow was extracted from the recovery behavior, and the ratio of slow trap according to V TH is shown in Figure 6c.In enhancement-mode TFTs with a V TH of 0 V or higher, ∆V TH,slow /∆V TH tended to increase exponentially as V TH increased, whereas in depletion-mode TFTs, an opposite trend was confirmed.This trend could be fitted with a parabola function and showed high R 2 values of 0.66 and 0.81 when V G,stress was 10 and 30 V, respectively.PBS with a V G,stress of 30 V had a higher slow trap rate than when that with a V G,stress of 10 V.
trap according to VTH is shown in Figure 6c.In enhancement-mode TFTs with a VTH of 0 V or higher, ∆VTH,slow/∆VTH tended to increase exponentially as VTH increased, whereas in depletion-mode TFTs, an opposite trend was confirmed.This trend could be fitted with a parabola function and showed high R 2 values of 0.66 and 0.81 when VG,stress was 10 and 30 V, respectively.PBS with a VG,stress of 30 V had a higher slow trap rate than when that with a VG,stress of 10 V. To estimate the charge transport mechanism, we divided the plot of ∆VTH against VTH into three regions, with VTH of −2-0 V, 0-2 V, and 2-4 V named as regions 1, 2, and 3, respectively (Figure 6a,b).Figure 6d illustrates the transfer characteristics at the initial state, after stress, and after recovery by region.From Region 1 to 3, the initial VTH shifts positively and shows a large ∆VTH after stress.This ∆VTH dependence on VTH can be explained based on the band diagram, where x0 is the intersection of the Fermi level (EF) and trap level (ET), and most electrons are trapped between x0 and x1, as shown in Figure 6e [45,46].As shown in Figure 3c, as the Al doping concentration increases, EF approaches the conduction band minimum.Thus, x1, where the injected charge is mainly located, moves away from the interface based on the function of the electron transfer distance [45].In other words, electrons trapped deep cannot be easily detrapped through recovery at To estimate the charge transport mechanism, we divided the plot of ∆V TH against V TH into three regions, with V TH of −2-0 V, 0-2 V, and 2-4 V named as regions 1, 2, and 3, respectively (Figure 6a,b).Figure 6d illustrates the transfer characteristics at the initial state, after stress, and after recovery by region.From Region 1 to 3, the initial V TH shifts positively and shows a large ∆V TH after stress.This ∆V TH dependence on V TH can be explained based on the band diagram, where x 0 is the intersection of the Fermi level (E F ) and trap level (E T ), and most electrons are trapped between x 0 and x 1 , as shown in Figure 6e [45,46].As shown in Figure 3c, as the Al doping concentration increases, E F approaches the conduction band minimum.Thus, x 1 , where the injected charge is mainly located, moves away from the interface based on the function of the electron transfer distance [45].In other words, electrons trapped deep cannot be easily detrapped through recovery at room temperature, and as V TH shifts positively, the rate of slow trap increases.Meanwhile, ∆V TH increases with V G and stress caused by x 1 moving away from the semiconductor-dielectric interface as the electric field applied to the oxide dielectric increases, implying that the electrons are trapped deeper [46].Notably, compared to ∆V TH , the degree of recovery in region 1 was less than that in region 2.This is because depletionmode TFTs do not form a flat band at V G = 0 V, but still form charge accumulation by band bending, forming x 0 .In addition, region 2 showed smaller ∆V TH and higher recovery characteristics compared to region 3.These results suggest that as the initial V TH increases or the carrier concentration of the semiconductor decreases, the influence of the slow trap on PBS degradation increases.Thus, an inappropriate positive V TH may considerably deteriorate the short-and long-term stabilities.

Conclusions
The linearity and resolution of the delay line has a great effect on transmitter performance.In order to overcome the bottleneck of low linearity and low resolution, an improved delay line structure is proposed with a calibration algorithm to conquer PVT variations for this all-digital design.Measurement results show that the proposed structure with the calibration algorithm can evidently improve the linearity and resolution of the delay line.In summary, we demonstrated the correlation between V TH and ∆V TH in solution-processed oxide TFTs.The oxygen vacancy and bandgap were controlled by tuning the Al doping concentration in the Al-doped InO x , which changed the initial V TH of the oxide TFTs.As V TH increased, ∆V TH due to PBS increased exponentially, and the deterioration became more severe as the V G,stress increased from 10 to 30 V. Using the stretched-exponential function, it was revealed that the positive V TH and high V G,stress decreased the characteristic trapping time of carriers, implying that the charge trapping occurs effectively.Based on the recovery characteristics after PBS, the slow trap acts dominantly to ∆V TH , and trapped electrons are not easily detrapped under a high V G,stress or in TFTs with a positive V TH .In case of PBS with a V G,stress of 30 V, ∆V TH,slow /∆V TH showed a significant difference of 10.07% and 53.12% for TFTs with V TH of 0.49 and 3.93 V, respectively.The results of this study can be used to determine the initial V TH considering PBS and improving PBS in solution-processed oxide TFTs.

Figure 1 .
Figure 1.(a) Schematic of the device fabrication process.(b) Schematic of the InAlOx thin films with various Al doping concentration.

Figure 1 .
Figure 1.(a) Schematic of the device fabrication process.(b) Schematic of the InAlO x thin films with various Al doping concentration.

Figure 2 .
Figure 2. (a) XPS O 1s spectra for InOx and InAlOx semiconductors with various Al doping concentrations.(b) The comparison of the atomic percentages of M-O, Vo + C-O, and M-OH of InAlOx semiconductors with various Al doping concentrations.(c) XPS Al 2p, C 1s, and N 1s spectra for InOx and InAlOx semiconductors with various Al doping concentrations.

Figure 2 .
Figure 2. (a) XPS O 1s spectra for InO x and InAlO x semiconductors with various Al doping concentrations.(b) The comparison of the atomic percentages of M-O, Vo + C-O, and M-OH of InAlO x semiconductors with various Al doping concentrations.(c) XPS Al 2p, C 1s, and N 1s spectra for InO x and InAlO x semiconductors with various Al doping concentrations.

Figure 3 .
Figure 3. (a) Optical bandgap and (b) valence band offset spectra for InOx and InAlOx semiconductors with various Al doping concentrations.(c) Energy band alignment of InOx and InAlOx semiconductors with various Al doping concentrations.

Figure 3 .
Figure 3. (a) Optical bandgap and (b) valence band offset spectra for InO x and InAlO x semiconductors with various Al doping concentrations.(c) Energy band alignment of InO x and InAlO x semiconductors with various Al doping concentrations.

Figure 4 .
Figure 4. (a) Transfer characteristics of InAlOx TFTs with various Al doping concentrations.(b).Electrical parameters including threshold voltage, subthreshold swing, and field-effect mobility of InAlOx TFTs with various Al doping concentrations.(c) Field-effect mobility versus overdrive voltage of InAlOx TFTs with various Al doping concentrations.

Figure 4 .
Figure 4. (a) Transfer characteristics of InAlO x TFTs with various Al doping concentrations.(b) Electrical parameters including threshold voltage, subthreshold swing, and field-effect mobility of InAlO x TFTs with various Al doping concentrations.(c) Field-effect mobility versus overdrive voltage of InAlO x TFTs with various Al doping concentrations.

Figure 5 .
Figure 5. (a) Time evolution of ∆V TH during the low V G stress (V G = 10 V, V D = 0 V, t = 1000 s) and subsequent recovery (V G = 0 V, V D = 0 V, t = 2000 s) phases, and (b) corresponding stretched exponential function parameters of InAlOx TFTs with various Al doping concentrations.(c) Time evolution of ∆V TH during the high V G stress (V GS = 30 V, V DS = 0 V, t = 1000 s) and subsequent recovery (V GS = 0 V, V DS = 0 V, t = 2000 s) phases and (d) corresponding stretched-exponential function parameters of InAlO x TFTs with various Al doping concentrations.

Figure 6 .
Figure 6.Dependence of (a) time evolution of ΔVTH and corresponding slow trapping (ΔVTH,slow) and fast trapping (ΔVTH,fast) components.(b) ΔVTH and (c) ΔVTH,slow/ΔVTH on the initial VTH of oxide TFTs.Two devices each from InOx, InAlOx-1, InAlOx-2, and InAlOx-3 were measured.(d) Schematic of transfer characteristics at initial state, after stress, and after recovery of oxide TFTs in regions 1, 2, and 3. (e) The energy band of oxide TFTs and extracted trapped electron distribution in the fast and deep gate dielectric traps.

Figure 6 .
Figure 6.Dependence of (a) time evolution of ∆V TH and corresponding slow trapping (∆V TH,slow ) and fast trapping (∆V TH,fast ) components.(b) ∆V TH and (c) ∆V TH,slow /∆V TH on the initial V TH of oxide TFTs.Two devices each from InO x , InAlO x -1, InAlO x -2, and InAlO x -3 were measured.(d) Schematic of transfer characteristics at initial state, after stress, and after recovery of oxide TFTs in regions 1, 2, and 3. (e) The energy band of oxide TFTs and extracted trapped electron distribution in the fast and deep gate dielectric traps.