Highly Robust Atomic Layer Deposition‐Indium Gallium Zinc Oxide Thin‐Film Transistors with Hybrid Gate Insulator Fabricated via Two‐Step Atomic Layer Process for High‐Density Integrated All‐Oxide Vertical Complementary Metal‐Oxide‐Semiconductor Applications

Highly reliable atomic layer deposition (ALD)‐derived In‐Ga‐Zn‐O thin‐film transistors with high field‐effect mobility (μFE) and hydrogen (H) resistivity are crucial for the semiconductor industry. Herein, a hybrid Al2O3 gate insulator (GI) is proposed that is designed by controlling the plasma‐enhanced ALD and thermal ALD processes in situ to demonstrate robust characteristics. A hybrid GI is applied to the top‐gate geometry of an In0.71Ga0.08Zn0.21O active layer. The optimal device exhibits exceptional electrical characteristics, including a threshold voltage of 0.37 V, μFE of 150.7 cm2 V s−1, subthreshold swing of 64.0 mV decade−1, and hysteresis of 0.02 V. It demonstrates high resistance to H annealing and reliable positive bias temperature stress, as well as changes in VTH shifts of −0.43 and 0.00 V, respectively. The excellent electrical characteristics and high robustness of the device can be attributed to the precise control of H, oxygen, and carbon species within the upper region of the hybrid GI. The achievement of robust device characteristics enables the design of a novel vertical complementary metal‐oxide‐semiconductor inverter that exhibits a voltage gain of 44.7 V V−1 and a noise margin of 87.5% at a 10 V supply voltage.

Highly reliable atomic layer deposition (ALD)-derived In-Ga-Zn-O thin-film transistors with high field-effect mobility (μ FE ) and hydrogen (H) resistivity are crucial for the semiconductor industry.Herein, a hybrid Al 2 O 3 gate insulator (GI) is proposed that is designed by controlling the plasma-enhanced ALD and thermal ALD processes in situ to demonstrate robust characteristics.A hybrid GI is applied to the top-gate geometry of an In 0.71 Ga 0.08 Zn 0.21 O active layer.The optimal device exhibits exceptional electrical characteristics, including a threshold voltage of 0.37 V, μ FE of 150.7 cm 2 V s À1 , subthreshold swing of 64.0 mV decade À1 , and hysteresis of 0.02 V.It demonstrates high resistance to H annealing and reliable positive bias temperature stress, as well as changes in V TH shifts of À0.43 and 0.00 V, respectively.The excellent electrical characteristics and high robustness of the device can be attributed to the precise control of H, oxygen, and carbon species within the upper region of the hybrid GI.The achievement of robust device characteristics enables the design of a novel vertical complementary metal-oxide-semiconductor inverter that exhibits a voltage gain of 44.7 V V À1 and a noise margin of 87.5% at a 10 V supply voltage.
22][23] These O and H species can be incorporated adjacent to the a-IGZO layer during the fabrication process of a-IGZO TFTs because significant amounts of H and O are resolved in the buffer layer, gate insulator (GI), and passivation layers. [24,25]nother crucial requirement for channels in semiconductor devices is the ability to tolerate H incorporation. H treatment was employed to enhance the dry etching capabilities and regulate the defects of the insulation material during the fabrication process. [26,27]Consequently, the underlying a-IGZO TFTs were inevitably subjected to H incorporation, which eventually resulted in a negative threshold voltage (V TH ) shift.To enhance resistance to H diffusion, researchers have attempted to utilize Al 2 O 3 materials with inherent H diffusion barrier properties. [28]urthermore, C and O species present in Al 2 O 3 have been reported to bond chemically with H, thereby enhancing its H diffusion barrier properties. [29]Hence, an Al 2 O 3 material with precisely controlled amounts of C and O species that serve as an effective H-diffusion barrier adjacent to the a-IGZO layer must be employed.
In this study, we propose an approach for obtaining ALDderived a-IGZO TFTs that demonstrate both bias stability and H incorporation, in addition to high mobility.An ALD-Al 2 O 3 GI is implemented in a top gate-bottom contact (TG-BC) structure to obtain self-passivation properties for the a-IGZO layer.
Here, the ALD-Al 2 O 3 GI is deposited via plasma-enhanced ALD (PE-ALD, O 2 plasma) and thermal ALD (T-ALD, O 3 ) with distinct reactivity differences to precisely modulate the O, H, and C species within the Al 2 O 3 layer.By considering the effect of reactants for ALD-Al 2 O 3 on device characteristics, we design a hybrid Al 2 O 3 structure by controlling the reactants in situ.The achievement of robust device characteristics using T-ALD Al 2 O 3 with optimal thickness enables the development of a novel vertically stacked CMOS inverter designed for high-density devices.structures.The PE-ALD Al 2 O 3 demonstrated a slightly higher hard breakdown electrical field compared with the T-ALD Al 2 O 3 (PE-ALD:7.4MV cm À1 , T-ALD:6.6 MV cm À1 ).Furthermore, the dielectric constants of the PE-ALD Al 2 O 3 were higher than those of the T-ALD Al 2 O 3 (PE-ALD:6.4,T-ALD:5.9).Variations in the electrical characteristics of Al 2 O 3 have been reported to be strongly associated with the chemical bonding states and presence of C impurities. [30,31]To gain insight into the changes in the electrical characteristics based on the reactants, we conducted an X-Ray photoelectron spectroscopy (XPS) analysis.As shown in Figure 1c, the XPS O 1s peaks were deconvoluted using two Gaussian fits, with binding energy-entered peaks at 530.84 AE 0.07 eV (Al-O) and 532.26AE 0.02 eV (OH/CO). [32,33]Compared with the results of the PE-ALD Al  [34,35] This resulted in a decrease in film density in the T-ALD Al 2 O 3 (2.96g cm À3 ) compared with the case in the PE-ALD Al 2 O 3 (3.05g cm À3 ), as illustrated in Figure 1e.Because the reactivity of O 3 is lower than that of O 2 plasma, the observed C impurity in the T-ALD Al 2 O 3 was presumed to have originated from the residual methyl (CH 3 ) ligand of the TMA precursor. [36,37]Therefore, the variation in the electrical characteristics of Al 2 O 3 , depending on the reactants used, might have been caused by residual C impurities within the grown film.

Electrical Properties of ALD-IGZO TFTs Depending on Reactants Used for Al 2 O 3 GI Deposition
To identify the effects of the reactants for ALD-Al 2 O 3 on the device characteristics, TG-BC a-IGZO TFTs were fabricated, as shown in Figure 2a.The a-IGZO active layer was identically deposited with a thickness of 10 nm via PE-ALD; the sole discrepancy between the two devices was the utilization of O 2 plasma and O 3 reactants for the deposition of the Al 2 O 3 GI.Figure 2b shows the transfer characteristics of the devices with an Al 2 O 3 GI grown via PE-ALD and T-ALD (hereinafter denoted as PE-ALD and T-ALD devices, respectively).The PE-ALD and T-ALD devices demonstrated V TH of 0.49 and À3.81 V, μ FE of 111.2 and 56.9 cm 2 Vs À1 , SS of 65.1 and 74.9 mV decade À1 , and hysteresis of 0.02 and 0.02 V, respectively.These results indicate that the PE-ALD device performed better than the T-ALD device.In previous research, a well-designed a-IGZO active layer has been identified as a crucial factor in achieving excellent electrical characteristics. [38,39]Nevertheless, since the only difference among these devices is the Al 2 O 3 GI deposition method, it is necessary to understand how the GI deposition method influences electrical characteristics.[42] To investigate whether the discrepancy in the electrical characteristics between the PE-ALD and T-ALD devices was associated with the O 2 plasma treatment applied to the a-IGZO layer, we performed O 2 plasma treatment prior to the T-ALD Al 2 O 3 GI deposition.Figure S1a   were the same, the result above cannot be explained by the difference in the adsorbed TMA precursors.Several researchers have reported that the TMA precursor exhibits high reactivity in ligand exchange reactions, which can result in a partially etched underlying IGZO layer during chemical reactions. [43,44]herefore, the difference in the Al signal between the stacked films with PE-ALD and T-ALD Al 2 O 3 likely originates from the additional energy provided by the O 2 plasma reactant, not the O 3 reactant, which facilitates enhanced chemical reactions.Al has a higher affinity for oxygen compared with In, Ga, and Zn.Consequently, doping IGZO with Al can potentially reduce the V O concentration, thus resulting in suppressed carriers and defects. [45,46]Additionally, the Al 2 O 3 /IGZO stacked films were subjected to SIMS analysis to investigate the variations in O and H in the IGZO active layer, as shown in Figure S2a, b, Supporting Information, respectively.The O signal in the a-IGZO layer exhibited negligible changes, whereas the intensity of H at the interface between Al 2 O 3 and a-IGZO for the PE-ALD Al 2 O 3 was lower than that for the T-ALD Al 2 O 3 .Generally, H species in IGZO serve as carriers because they appear in the forms of H i þ and H O þ .However, an excessive H concentration in oxide semiconductors can result in the formation of deep acceptor-like bistable centers (H i À , H O À , and M-H), leading to the degradation of device characteristics. [20,21]Therefore, the combination of resolved Al and accumulated H at the interface between Al 2 O 3 and a-IGZO might determine the discrepancy in electrical characteristics between the PE-ALD and T-ALD devices.

Hybrid Al 2 O 3 GI Design Using a Two-Step in Situ ALD Process
Based on the electrical characteristics of the PE-ALD device, we designed hybrid Al 2 O 3 GI structures through the in situ control of O 2 plasma and O 3 reactants.This approach was employed to obtain insights into the effects of alterations in the bulk GI on the device characteristics.3b.The key electrical parameters are listed in Table 1 and their variations are compared in Figure 3c.As shown, the electrical characteristics exhibited significant variations as the top thickness of the T-ALD Al 2 O 3 layers increased.Subsequently, the V TH shifted negatively from devices A to D, with device D exhibiting a particularly high degree of V TH shift.Furthermore, device B exhibited a significantly higher increase in μ FE compared with device A, whereas μ FE decreased from devices B to D. Meanwhile, devices C and D, whose mobility reduced significantly, indicated degraded SS.To understand the primary factors affecting the changes in the key electrical parameters, we performed a technology computer-aided design (TCAD) simulation to extract the active-layer density of states (DOS).Figure S4, Supporting Information, presents the TCAD fitting results for the devices with hybrid Al 2 O 3 GI structures, and representative results for devices B and C are shown in Figure 3d.The obtained TCAD parameters are listed in Table S1, Supporting Information.Notable changes in the carrier concentration (n carrier ), acceptorlike tail state (N TA ), and Gaussian donor state (N GD ) are shown in Figure 3e.The variations in the TCAD parameters revealed a close relationship between the V TH , μ FE , and SS values and n carrier , N TA , and N GD , respectively.As shown in Figure 2, the resolved Al element at the interface between Al 2 O 3 and a-IGZO is likely to affect the changes in the TCAD parameters between devices A and D. However, we did not consider this to be a significant factor in devices A, B, and C because they comprised the same PE-ALD Al 2 O 3 component at the bottom of the hybrid Al 2 O 3 GI.We observed that the H intensity was different in both Al 2 O 3 and a-IGZO, depending on the ALD-Al 2 O 3 deposition method, as shown in Figure S2, Supporting Information,.This implies that the T-ALD Al 2 O 3 contained a higher amount of H than the PE-ALD Al 2 O 3 , and that a greater amount of H diffused from Al 2 O 3 to a-IGZO in the stacked film with T-ALD Al 2 O 3 during the annealing process.The correlation between the H content in Al 2 O 3 and a-IGZO is consistent with our results.Considering the observed negative V TH shift with an increase in n carrier from devices A to C, it is believed that the H content in a-IGZO would increase from device A to C owing to the higher proportion of H-resolved T-ALD Al 2 O 3 .In oxide semiconductors, H is primarily considered a shallow donor with the bonding status of H i þ , H O þ , and V O H. [20][21][22][23] Furthermore, these H states can passivate the localized trap and tail states adjacent to the conduction band minimum (CBM). [47,48]However, a few groups have claimed that H can appear as deep acceptor-like bistable centers (H i À , H O À , and M-H) when the Fermi energy level (E F ) is close to that of the CBM. [20,21,49]Hence, the decrease in N TA observed in device B compared with that in device A is attributable to the passivation effect on the tail states located near the CBM owing to its appropriate H content.However, the significant increases in N TA and N GD observed in device C, which is expected to contain an excess of H in a-IGZO, is likely due to the coexistence of acceptor-like H bistable centers caused by the elevation of E F arising from the excess H content.    [29] To confirm the changes in these species, the distributions of C and O species in the Al 2 O 3 deposited using PE-ALD and T-ALD were observed via SIMS analysis.Figure S5a,b,  4c) was due to the presence of C and O species caused by limited O 3 reactivity at 200 °C for oxidizing the TMA precursor. [36,37]In addition, because the channel region of the upper a-IGZO was in contact with the well-H-resolved T-ALD Al 2 O 3 , the slight increase in μ FE of device D after H 2 annealing is attributable to the improved carrier transport within the active channel layer.
Reliable bias stability must be ensured to guarantee the longterm use of semiconductor devices.Therefore, the devices were stressed at a gate field of AE2 MV cm À1 at a temperature of 95 °C for 10 000 s. Figure 6a shows the transfer characteristics of the devices as a function of positive bias temperature stress (PBTS) time.[16][17][18][19] More specifically, among the ) and 2þ-charged V O (V O 2þ ) were identified, and the transition from V O 2þ to V O 0þ under PBTS conditions induced charge trapping.][52] To understand the origins of the V TH shifts, we compared the amounts of H and O species obtained via SIMS analysis of Al 2 O 3 deposited using PE-ALD and T-ALD, as shown in Figure S2, Supporting Information.Figure 6b  and thermal energy in PBTS environments.Specifically, the accumulation of H þ species at the interface between the GI and active layer can result in the generation of electron image charges at the active layer. [41]Furthermore, the diffusion of H þ from the GI into the active layer, as facilitated by the donor-like states of H (H i þ and H O þ ) in oxide semiconductors, results in electron generation. [20,21]In addition to H þ migration, O 2À species accumulate at the interface between the gate and GI, thus potentially serving as a screen for positive gate biases. [41]herefore, it is believed that the transition from a positive to a negative V TH shift in the devices in PBTS environments occurs predominantly because of the increased proportion of H-and Orich T-ALD Al 2 O 3 .Figure S6, Supporting Information, shows the negative bias temperature stress (NBTS) results of the devices.As shown, the abnormal positive V TH shifts from þ0.10 V in device A to þ0.49V in device D increased gradually.In contrast to the case under the PBTS condition, H þ is likely to accumulate at the interface between the gate and GI, whereas O 2À accumulates at the interface between the GI and active layer, thus resulting in a positive V TH shift under NBTS conditions.Considering the increase in positive V TH shifts from devices A to D in NBTS environments, we can conclude that the abnormal V TH shift under the P/NBTS condition is related closely to the amounts of H and O species within the GI.

Evaluations of All-Oxide ALD-Vertical CMOS Inverter Implemented with Hybrid Al 2 O 3 GI
Owing to the reliable H incorporation and bias stability exhibited by device B, we applied the latter to a novel vertically stacked CMOS structure integrated with a p-type ALD-SnO TFT to achieve high-density devices.Figure 7a shows a schematic illustration and the fabrication process of the ALD-vertical CMOS using device B. To confirm the changes in the electrical characteristics of the underlying n-type IGZO TFT, device B was measured through a step-by-step process that involved pristine measurements, followed by the fabrication and subsequent postannealing of a p-type SnO TFT. Figure 7b shows the transfer characteristics of the underlying n-type IGZO TFT based on the p-type SnO TFT fabrication process.The variations in the key electrical parameters are summarized in Figure 7c.The variations in V TH , μ FE , and SS were calculated to be þ0.59V, þ5.6 cm 2 Vs À1 , and À0.6 mV decade À1 , respectively, after the fabrication of the p-type SnO TFT, which might have been affected by the incorporation of O radicals into the active layer during the deposition of p-type GI via PE-ALD.After the p-type SnO TFT was fabricated, RTA was performed at 300 °C for 5 min under an N 2 ambient to activate the p-type SnO TFT. Figure S7a, Supporting Information, shows that the SnO TFT exhibits conventional p-type characteristics with reasonable V TH , μ FE , and SS values of À1.74 V, 0.86 cm 2 Vs À1 , and 141 mV decade À1 , respectively.Because of the deposition of the p-type SnO active layer and Al 2 O 3 in situ passivation layer using T-ALD with an H 2 O reactant, the underlying n-type IGZO TFT was exposed to a significant amount of H during the postannealing processes.However, no significant changes in the electrical characteristics were observed in the p-type SnO TFT after postannealing processes were performed.The V TH , μ FE , and SS varied by À0.16 V, þ11.1 cm 2 Vs À1 , and 3.9 mV decade À1 , respectively, which is attributable to the excellent tolerance of the H incorporation of device B with the hybrid Al 2 O 3 GI, as shown in Figure 4a. Figure 7d,e shows the voltage transfer characteristics (VTCs) and voltage gain of the all-oxide ALD vertically stacked CMOS inverter as a function of the input voltage (V IN ), respectively.Based on the VTCs of the CMOS with a supply voltage (V DD ) ranging from 2 to 10 V, the CMOS exhibited clear rail-to-rail inverter output curves.The voltage gain (ÀdV out /dV in ) increased with V DD and exhibited a maximum value of 44.7 V V À1 at a V DD of 10 V (Figure S7b, Supporting Information.Figure 7f shows the noise margin and transition voltage extracted from the output voltage (V out ) as a function of V in .The noise margin was calculated as follows Noise margin ð%Þ ¼ where NM H and NM L are the high and low noise margins, respectively.The NM H and NM L were calculated using the logical high-output voltage (V OH = V DD )-input voltage (V IH ) and logical low-input voltage (V IL )-output voltage (V OL = 0 V), respectively.The V IH and V IL values were determined from the high and low input voltage values at the À1 V V À1 gain value, respectively.NM H and NM L as a function of V DD are presented in Figure S7c, Supporting Information,.In addition, the transition voltage was extracted from V in , where V in = V out .An increase in the noise margin and a positive shift in the transition voltage were observed as V DD increased.The noise margin and transition voltage exhibited reasonable values of 87.5% and 1.25 V, respectively, at a V DD = 10 V.

Conclusion
We

Experimental Section
ALD-IGZO Active Layer Deposition: To obtain the IGZO active layer, we utilized a lateral flow-type PE-ALD system with a radio frequency (RF) (13.56 MHz) direct capacitance-coupled plasma (CCP) source (ISAC Research Inc.) in a 6 00 Â 6 00 chamber.During deposition, a constant flow of argon (Ar) (purity: 99.999%) was maintained in the chamber to regulate the operating pressure at 1.2 Torr using a throttle valve.The In, Ga, and Zn precursors used were (3-dimethylaminopropyl)dimethylindium (DADI), trimethylgallium (TMGa), and diethylzinc (DEZ).Considering the vapor pressures of the precursors, the canister temperatures for DADI and TMGa were adjusted to 55 and 15 °C, respectively.The DADI precursor was transported to the reaction chamber via Ar as a carrier gas.The chemical reaction was realized using oxygen (O 2 ) (purity: 99.999%) plasma with a power of 100 W at 200 °C.Supercycling was employed to deposit the active IGZO layer.The cation composition of IGZO, as determined via X-Ray fluorescence spectrometry (XRF), was confirmed to be In 0.71 Ga 0.08 Zn 0.21 O.
ALD-Al 2 O 3 GI Deposition: A vertical flow-type ALD chamber (ISAC Research Inc.) measuring 6 00 Â 6 00 was used to deposit the Al 2 O 3 film.The ALD chamber was equipped with a RF (13.56 MHz) direct CCP source.Ar (purity: 99.999%) was used as the carrier gas during the deposition process.The operating pressure was set to 1.2 Torr for the PE-ALD process and 350 mTorr for the T-ALD process using a throttle valve.TMA was used as the Al precursor.To oxidize the TMA precursor, either O 2 (purity: 99.999%) plasma with a power of 100 W or ozone (O 3 ) with a concentration of 200 g m À3 , generated via an O 3 generator (CN-1, Ozonetech Co.), was employed at a temperature of 200 °C.The durations of O 2 plasma ignition and O 3 dosing were set to 1 and 5 s, respectively.To evaluate the electrical properties of Al 2 O 3 deposited via the PE-ALD and T-ALD processes, we fabricated MIM structures.These structures were fabricated on a highly boron-doped Si wafer with an Al 2 O 3 layer deposited on it.Additionally, a 100-nm-thick indium tin oxide (ITO) electrode was applied using RF sputtering.The ITO electrode was patterned to cover a 100 μm Â 100 μm area using conventional photography methods.A schematic illustration of the ALD-Al 2 O 3 process is shown in Figure 1a.
TG-BC ALD-IGZO TFTs Fabrication: The device fabrication process began with the ultrasonic cleaning of a thermally-grown 100 nm-thick SiO 2 buffered substrate using acetone, ethanol, and deionized water, in that particular sequence.To define the source and drain regions, a 100 nm-thick ITO electrode was deposited using RF sputtering and then patterned via photolithography.Subsequently, a 10 nm-thick IGZO active layer was deposited via PE-ALD.The active layer was then defined using conventional photolithography followed by wet etching.For the GI, a 15 nm-thick Al 2 O 3 was deposited using PE-ALD, T-ALD, as well as in situ PE-ALD and T-ALD processes, and contact holes for the source/drain were created via wet etching.To deposit the gate electrode, a 100 nm-thick ITO electrode was deposited and patterned via photolithography.The channel width and length of the fabricated device were 40 and 20 μm, respectively.Finally, the devices were postannealed at 350 °C for 2 h under dry air 2 21%/N 2 79%, purity: 99.999%).The fabrication process of the TG-BC ALD-IGZO TFTs is shown in Figure 2a.
Fabrication of Vertically Stacked All-Oxide ALD CMOS Inverter: A vertically stacked CMOS inverter integrated with a p-type ALD-SnO TFT was fabricated to achieve high-density devices.For the bottom gate-bottom contact p-type SnO TFT, a 15 nm-thick layer of Al 2 O 3 was deposited using PE-ALD to serve as the GI on the n-type device.The n-type device was an IGZO TFT with a hybrid GI composed of Al 2 O 3 layers deposited using T-ALD and PE-ALD with thicknesses of 6 and 9 nm, respectively.Next, the contact hole for the n-type drain electrode was created via wet etching, and a 100 nm-thick ITO electrode was deposited via RF sputtering.The in situ ALD-stacked 10 and 7 nm-thick Al 2 O 3 /SnO layers were deposited as capping and p-type channel layers, respectively.The detailed fabrication process and deposition conditions of the p-type in situ Al 2 O 3 /SnO device are available in our previous report. [53]The contact holes, electrodes, and active layers were defined using photolithography.A schematic illustration of the fabricated vertical CMOS structure is shown in Figure 7a.The inverter characteristics were obtained with channel width and length of 40/60 and 40/20 μm for the n-type IGZO and p-type SnO TFTs, respectively.
Characterization: The cation atomic composition of IGZO was determined via energy-dispersive XRF (ARL QUANT'X, Thermo Scientific).The chemical bonds of O and C in the Al 2 O 3 bulk region were analyzed using XPS (K-alpha þ , Thermo Fisher Scientific) with a monochromatic aluminum (Al) Kα (1486.6 eV) source.The density of the deposited Al 2 O 3 films, which were fabricated via PE-ALD and T-ALD, was obtained via grazing incidence-XRR (SmartLab, Rigaku) using a Cu-Kα (1.5405 Å) target.To investigate the variations in the Al, O, H, and C signals in the Al 2 O 3 /IGZO stacked films, dynamic SIMS (IMS-7 F_Auto, CAMECA, UK) was conducted using a cesium (Cs þ ) ion source at 6 keV.Cross-sectional information regarding the Al 2 O 3 /IGZO stacked films were obtained via TEM and EDS line scans using high-resolution TEM (Talos F200X, Thermo Fisher Scientific).TCAD was used to identify the relationship between the electrical characteristics and DOS.The gas permeation properties were evaluated using a constant-volume/variable pressure system (time lag, Airrane).The film was subjected to a 100% H 2 gas with a pressure of 6500 Torr at a temperature of 35 °C.The measurements were terminated when the downstream pressure reached 2 Torr.The electrical characteristics of the devices were evaluated in a dark environment using a TOP Engineering S3000 electrical parameter analyzer.The dielectric constant of Al 2 O 3 was measured using an Agilent 4284 A precision LCR meter.
Statical Analysis: The V TH was determined using the constant-current method, characterized by V TH ¼ V GS @I DS ¼ W L À Á Â 100pA, where V GS is the gate voltage, I DS is the drain current, W is the channel width, and L is the channel length.The μ FE of the transistors was estimated by the linear transfer characteristics as where g m(max) is the maximum transconductance, C ox is the oxide capacitance of the GI, and V DS is the drain voltage.The SS was calculated using the following equation: SS ¼ dðV GS Þ dlogðI DS Þ .The hysteresis was extracted by the difference of V TH between the forward and reverse sweep of the transfer curve.These key electrical parameters are expressed in the format of "average AE standard deviation".The average and standard deviation values of electrical properties were extracted by measuring ten devices with a size of 1 Â1 in.

2. 1 .
Figure 1.a) Schematic illustration of PE-ALD and T-ALD processes to achieve Al 2 O 3 films.b) Breakdown field and dielectric constant of the Al 2 O 3 films extracted from MIM structure.XPS c) O 1s and d) C 1s spectra of the Al 2 O 3 films.e) X-Ray reflectometry (XRR) spectra of the Al 2 O 3 films and comparison of the critical angle (inset).
2 O 3 , the Al-O peak decreased from 84.3% to 77.3%, whereas the OH/CO peak increased from 15.7% to 22.7% in the T-ALD Al 2 O 3 .These results are supported by the XPS C 1s spectra of the Al 2 O 3 bulk region obtained via Ar þ etching, as shown in Figure 1d.The C content was not analyzed in the PE-ALD Al 2 O 3 samples; meanwhile, the T-ALD Al 2 O 3 samples clearly contained approximately 1 at% C with C-O, C-C, and C-H bonding states.
, Supporting Information, shows a schematic illustration of the fabrication process of the O 2 plasma-treated T-ALD Al 2 O 3 device.Considering the cumulative time of the O 2 plasma used for PE-ALD Al 2 O 3 deposition, the a-IGZO was subjected to the same O 2 plasma conditions for 140 s prior to the deposition of T-ALD Al 2 O 3 .Figure S1b, Supporting Information, shows a comparison of the transfer characteristics with and without O 2 plasma treatment before T-ALD Al 2 O 3 deposition.As shown, the O 2 plasma treatment before T-ALD Al 2 O 3 deposition resulted in a slight positive shift of 0.38 V in V TH .However, the electrical characteristics of the O 2 plasma-treated device differed significantly from those of the PE-ALD device.This result indicates that whereas O 2 plasma treatment during PE-ALD Al 2 O 3 GI deposition can assist in adjusting the V TH value, other factors significantly affect the device performance.
Figure 2c shows the secondary ion mass spectrometry (SIMS) depth profiles of Al in the Al 2 O 3 /IGZO stacked films.Notably, the Al signal at the interface between Al 2 O 3 and a-IGZO in the stacked film with PE-ALD Al 2 O 3 was higher than that of the T-ALD Al 2 O 3 .To observe this more clearly, we performed a transmission electron microscopy (TEM)-energy dispersive spectroscopy (EDS) analysis of the stacked film.
Figure 2d shows the cross-sectional TEM images and EDS line scans of the PE-ALD and T-ALD Al 2 O 3 stacked films.Both the TEM images and fast Fourier transform patterns of the a-IGZO (inset) revealed similar results for the stacked films, where a smooth interface was indicated between Al 2 O 3 and a-IGZO, as well as a diffuse ring.However, in the case of the stacked film with PE-ALD Al 2 O 3 , the Al element was presumed to have resolved at the a-IGZO surface area as well as appeared predominantly at the interface between Al 2 O 3 and a-IGZO, as compared with the case involving the T-ALD Al 2 O 3 .In our experiment, because the deposition temperatures of both PE-ALD and T-ALD Al 2 O 3 Figure 3a shows a schematic illustration of the hybrid Al 2 O 3 GI structure deposited on the a-IGZO layer.The overall thickness of the Al 2 O 3 GI remained at 15 nm.In this case, as the bottom thickness of the PE-ALD Al 2 O 3 film decreased, the top thickness of the T-ALD Al 2 O 3 film increased.Devices A, B, C, and D denote devices with T-ALD/PE-ALD Al 2 O 3 thicknesses of 0/15, 6/9, 12/3, and 15/0 nm, respectively.Figure S3a,b, Supporting Information.shows the transfer and output characteristics of devices with hybrid Al 2 O 3 GI structures, respectively.The representative properties of devices B and C are shown in Figure

Figure 3 .
Figure 3. a) Schematic illustration of hybrid Al 2 O 3 GI deposited by in situ controlling PE-ALD and T-ALD processes.b) Transfer characteristics and μ FE of the devices with hybrid Al 2 O 3 GI and c) variations in key electrical parameters.The average and standard deviation values of electrical properties were extracted by measuring ten devices with a size of 1 Â1 in.d) Simulation results obtained using TCAD for transfer characteristics of devices with hybrid Al 2 O 3 GI, and e) representative changes in DOS extracted from TCAD result.

2. 4 .
Hydrogen Resistivity and Bias Stability of ALD-IGZO TFTs with Hybrid Al 2 O 3 GIConsidering the compatibility with the semiconductor device fabrication process, the devices with hybrid Al 2 O 3 GI were annealed at 300 °C for 2 h under a pressure of 10 Torr using 100% H 2 concentration gas.Figure4ashows the transfer characteristics of the devices before and after H 2 annealing.To clearly observe the effects of the H-annealing process, the variations in the key electrical parameters before and after H 2 annealing were compared, as shown in Figure4b.Device A exhibited significant changes in the V TH , μ FE , and SS values following H 2 annealing.However, the variations in these values decreased from devices A to C; eventually, device D showed minimal changes, except for its μ FE .To investigate the changes in the electrical parameters, we performed a SIMS depth profile analysis of H in the Al 2 O 3 /IGZO stacked films fabricated via different Al 2 O 3 deposition methods, i.e., PE-ALD and T-ALD, as shown in Figure 4c.The H intensity in the a-IGZO region increased after the H 2 annealing process in the stacked film with Al 2 O 3 deposited via PE-ALD as compared with the case for the Al 2 O 3 deposited by T-ALD.Therefore, the degraded electrical characteristics observed in device A are believed to be caused by the increased n carrier , N TA , and N GD values resulting from the higher concentration of H species in the a-IGZO layer, as shown in Figure 3e.Because of the increased top thickness of the T-ALD Al 2 O 3 in the hybrid GI, the amount of H incorporated into a-IGZO after H 2 annealing decreased gradually from device A to D. However, one must understand the reason contributing to the reduced incorporation of H into the a-IGZO in the T-ALD Al 2 O 3 compared with that in the PE-ALD Al 2 O 3 , in addition to the slight increase in μ FE of device D after H 2 annealing.The barrier properties to H 2 gas of the Al 2 O 3 films obtained via PE-ALD and T-ALD were evaluated using a constant-volume/ variable pressure system (time lag), as shown in Figure5a.The evaluation involved monitoring the changing vacuum level over time while supplying high-pressure H 2 gas at 100% concentration from the top through the thin film to the bottom.To obtain more accurate information, 15-nm-thick Al 2 O 3 films were deposited on a polyimide (PI) substrate, which is known to exhibit high permeation properties for H 2 gas.During the Al 2 O 3 deposition via PE-ALD and T-ALD, the barrier properties of H 2 gas may be affected by variations in the PI surface properties.Hence, a 1 nm common layer of T-ALD Al 2 O 3 was applied to both samples.Figure5bshows a comparison of the H 2 barrier properties of the Al 2 O 3 films deposited using PE-ALD and T-ALD.The T-ALD Al 2 O 3 showed a lower H 2 permeability than the PE-ALD Al 2 O 3 , which is attributed to the high H 2 solubility and low H 2 diffusivity of the T-ALD Al 2 O 3 film.This indicates that externally supplied H can readily dissolve within the T-ALD Al 2 O 3 film, effectively inhibiting its diffusion toward the underlying layer.The presence of C and O species in the Al 2 O 3 has been reported to enhance the barrier properties of H.

Figure 4 .
Figure 4. a) Transfer characteristics and b) changes in key electrical parameters of devices with hybrid Al 2 O 3 GI before and after H 2 annealing.The average and standard deviation values of electrical properties were extracted by measuring ten devices with a size of 1 Â1 in.c) SIMS depth profile analysis of H in Al 2 O 3 /IGZO stacked films before and after H 2 annealing.

Figure 5 .
Figure 5. a) Photographs showing the time-lag system for evaluating the H 2 barrier properties of the Al 2 O 3 films.b) H 2 barrier properties (permeability, solubility, and diffusivity) of Al 2 O 3 films deposited via PE-ALD and T-ALD.

Figure 6 .
Figure 6.a) PBTS stability of devices with hybrid Al 2 O 3 GI.b) Changes in V TH shift after PBTS stability of devices with hybrid Al 2 O 3 GI and expected values of H and O species in hybrid Al 2 O 3 GI.c) Schematic illustration of migration of H and O species within T-ALD Al 2 O 3 GI in PBTS environments.
shows the V TH shift under the PBTS conditions, in addition to the corresponding expected values of the H and O species within the GI of the devices.Considering the higher SIMS intensity of the H and O species in the T-ALD Al 2 O 3 compared with that in the PE-ALD Al 2 O 3 , the amounts of H and O species are expected to increase progressively from devices A to D. Generally, these H and O species can exist as O 2À and H þ within Al 2 O 3 .As shown in Figure 6c, in the case of a device with H-and O-rich T-ALD Al 2 O 3 , charged H and O species are likely to actively migrate within Al 2 O 3 because of the positive gate bias

Figure 7 .
Figure 7. a) Schematic illustration of vertically stacked CMOS inverter and fabrication process.b) Comparison of transfer characteristics and c) variations in key electrical parameters of underlying n-type IGZO TFT based on the fabrication process of p-type SnO TFT.The average and standard deviation values of electrical properties were extracted by measuring ten devices with a size of 1 Â 1 in.d) VTCs and e) voltage gain of vertical CMOS inverter as a function of V in with increasing V DD .f ) Noise margin and transition voltage versus V DD .
successfully fabricated TG-BC ALD-IGZO TFTs that demonstrated bias stability, H incorporation, and extremely high mobility through hybrid GI deposition via in situ PE-ALD and T-ALD processes.To precisely modulate H, C, and O species within the Al 2 O 3 layer, we designed a hybrid Al 2 O 3 GI structure by adopting in situ PE-ALD and T-ALD processes.The device with a hybrid Al 2 O 3 GI exhibited exceptional electrical characteristics, including a V TH of 0.37 V, μ FE of 150.7 cm 2 Vs À1 , SS of 64.0 mV decade À1 , and hysteresis of 0.02 V at a specific T-ALD Al 2 O 3 thickness of 6 nm.The passivation effect on the tail states located near the CBM is attributed to the diffusion of the appropriate H content from the GI into the active layer.We annealed the fabricated devices at 300 °C for 2 h under a pressure of 10 Torr using 100% H 2 concentration gas.The device with a hybrid Al 2 O 3 GI exhibited a reduced degree of negative V TH shifts from À2.10 to À0.08 V as the T-ALD Al 2 O 3 thickness increased.In addition, the devices were tested for PBTS stability under a gate field of þ2 MV cm À1 at a temperature of 95 °C for 10 000 s.The devices exhibited a transition from positive to negative V TH shifts under PBTS stability as the thickness of the T-ALD Al 2 O 3 increased.These results were primarily attributed to the increase in H, C, and O species in the hybrid GI with the T-ALD Al 2 O 3 thickness.Device B with excellent electrical characteristics indicates variations in the V TH shift of À0.43 and 0.00 V for the H 2 annealing process and PBTS stability, respectively.Finally, device B was utilized in a novel vertically stacked CMOS inverter to achieve high-density devices.The all-oxide vertical CMOS exhibited clear inverter characteristics, such as a voltage gain of 44.7 V V À1 and a noise margin of 87.5% at a V DD of 10 V.

Table 1 .
Comparison of device key parameters of ALD-IGZO TFTs with hybrid Al 2 O 3 GI.The average and standard deviation values of electrical properties were extracted by measuring 10 devices with a size of 1Â1 in.