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[Hanjun Cho](https://orcid.org/0009-0009-2834-8846), [Masatake Tsuji](https://orcid.org/0000-0002-3404-6037), [Shigenori Ueda](https://orcid.org/0000-0001-9425-0614), [Junghwan Kim](https://orcid.org/0000-0001-9168-6260), [Hideo Hosono](https://orcid.org/0000-0001-9260-6728)

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[Oxygen Defects and Instability in Very Thin a‐IGZO TFTs](https://mdr.nims.go.jp/datasets/d2c95edf-a62b-4e7f-946d-96b8c3726902)

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Oxygen Defects and Instability in Very Thin a‐IGZO TFTsRESEARCH ARTICLEwww.advelectronicmat.deOxygen Defects and Instability in Very Thin a-IGZO TFTsHanjun Cho, Masatake Tsuji,* Shigenori Ueda, Junghwan Kim, and Hideo Hosono*Amorphous oxide semiconductor (AOS) thin-film transistors(TFT) have gained significant attention for their potential in capacitor-freenext-generation memory applications. However, improving thresholdvoltage (VTH) stability and precisely controlling carrier concentration inultra-thin channels remain critical challenges. In this study, an extraordinarilylarge positive-bias-stress (PBS) instability in hydrogen-free amorphousIGZO (a-IGZO)-TFTs that emerges as the channel thickness decreasesis reported. This instability can be attributed to acceptors interactingwith donors at shallow levels below the conduction band minimum (CBM).This model, based on temperature-dependent Hall effect measurements,reveals an unusual correlation between donor concentration and donor energylevels. Unlike in previously reported semiconductors, the energy differencebetween the CBM and donor energy level increases linearly in proportionto (donor concentration)1/3. The O 1s core-level hard X-ray photoemissionmeasurements suggest that the entity of the acceptors is oxygen vacancieswithout two electrons (VO2+) formed during deposition. These vacancies resultfrom strong donor−acceptor interactions arising from the formation of oxygenFrenkel defects in the thinner films. It is demonstrated that low-temperatureextra-annealing effectively suppresses PBS instability by inducingstructural relaxation of the Frenkel defects, thereby stabilizing the TFTs.1. IntroductionAmorphous oxide semiconductors (AOSs) offer signifi-cant advantages due to their ease of fabrication at lowH. Cho, M. Tsuji, J. Kim, H. HosonoMDX Research Center for Element Strategy, Institute of IntegratedResearchInstitute of Science TokyoYokohama 226–8501, JapanE-mail: tsuji.m.ac@m.titech.ac.jp; hosono.h.aa@m.titech.ac.jpS.Ueda,H.HosonoNational Institute forMaterials ScienceTsukuba, Ibaraki 305-0044, JapanJ. KimGraduate School of SemiconductorMaterials andDevices EngineeringUlsanNational Institute of Science andTechnologyUlsan 44919, Republic of KoreaThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/aelm.202500349© 2025 The Author(s). Advanced Electronic Materials published byWiley-VCH GmbH. This is an open access article under the terms of theCreative Commons Attribution License, which permits use, distributionand reproduction in any medium, provided the original work is properlycited.DOI: 10.1002/aelm.202500349temperatures, uniform thin-film forma-tion, and the ability to continuouslytailor their chemical composition andproperties.[1–4] However, the carrier gen-eration mechanism in AOS remains elu-sive. In amorphous IGZO (a-IGZO),which is widely used as a channel ma-terial in thin-film transistors (TFT) forthe backplane to drive flat-panel dis-play pixels, defects are present at mul-tiple levels: these include shallow levelsbelow the conduction band minimum(CBM) and deep levels above the valenceband maximum (VBM).[5] The precisecontrol of these defects is a significantchallenge in the development of amor-phous semiconductors. For instance, theperformance of TFTs is highly depen-dent on factors such as high-density hy-drogen impurities,[6,7] excess oxygen,[8]and carbon impurities,[9] all of whichoriginate from the fabrication process.Recently, AOS-TFTs have attractedattention for their potential in next-generation capacitor-free dynamicrandom access memory (DRAM)applications.[10–16] This interest isdriven by their ultra-low off-current of a-IGZO,[17] which isa pivotal for improving retention time in memory devices.[14]However, several obstacles must be overcome to achieve practicalimplementation. These challenges include the precise controlof carrier concentration, threshold voltage (VTH) stability inultra-thin channels,[18,19] and low contact resistance.[20,21] In thisarticle, we report that (1) the positive-bias-stress (PBS) instabilityof the VTH in a-IGZO-TFTs significantly increases as the channelbecomes very thin, (2) a model for two types of correlationsbetween donor concentration and activation energy, along withsupporting evidence, and (3) a proposed solution for improvingthe stability of very thin channel IGZO-TFTs based on thismodel.To investigate the mechanisms responsible for this instability,we utilized sputtering to deposit the TFT channels rather thanatomic layer deposition (ALD). Although ALD is often used forpractical fabrication due to its precise control over film thickness,it can introduce impurity carbon and hydrogen, potentiallymask-ing intrinsic phenomena . Addressing these impurities remainsa critical challenge in this field, requiring further systematic stud-ies. By employing sputtering under cryo-pumping conditions toreduce impurities of carbon and hydrogen, we aimed to fabricatecleaner TFT channels and reveal the intrinsic behavior of thesedevices. Understanding intrinsic defects in a-IGZO also providesreference data for addressing fabrication-induced variations inmemory devices that employing ALD and complex multi-layerAdv. Electron. Mater. 2025, 11, e00349 e00349 (1 of 12) © 2025 The Author(s). Advanced Electronic Materials published by Wiley-VCH GmbHhttp://www.advelectronicmat.demailto:tsuji.m.ac@m.titech.ac.jpmailto:hosono.h.aa@m.titech.ac.jphttps://doi.org/10.1002/aelm.202500349http://creativecommons.org/licenses/by/4.0/http://crossmark.crossref.org/dialog/?doi=10.1002%2Faelm.202500349&domain=pdf&date_stamp=2025-07-07www.advancedsciencenews.com www.advelectronicmat.dearchitectures. Such findings are pivotal to facilitate the oxidememory application. Our novel and unexpected findings suggestthat as the channel becomes thinner, the dominant interac-tion relevant to carrier generation switches from conventionaldonor−donor interaction to donor−acceptor interaction.We conductedHall effectmeasurements on a-IGZO thin-filmsas a function of temperature to evaluate the concentration andenergy levels of shallow donors. Understanding the relationshipbetween donor density and the energy level with respect to theCBM is crucial for elucidating the carrier generationmechanism.It is well known that the activation energy of donor Ea(ND) in n-type semiconductors with the donor concentration (ND) is pro-portional to −𝛼(ND)1/3, where 𝛼 is a coefficient.[22–26] Oxide semi-conductors reported to date have 𝛼 > 0 to our best knowledge.This implies that an increase in the Ea originates from the elec-trostatic repulsive interaction between donors because (ND)−1/3 isproportional to the mean separation of defects (d), and Coulombpotential energy is inversely proportional to d. Nevertheless, inour preliminary study of thinner films, we observed the oppo-site correlation, that is, 𝛼 < 0. This new correlation would comefrom an attractive interaction between the donor and acceptor.This suggests that donor−acceptor interaction becomes domi-nant in the thinner film in place of donor−donor interaction. Weattribute the responsible acceptor to positively charged oxygen va-cancies (VO2+). A plausible model for VO2+ is the formation ofFrenkel defect pairs [VO2+ paired with negatively charged intersti-tial oxygen (Oi2−)] during the deposition process. It is well knownthat the concentration of Frenkel defect increases with tempera-ture. In the present case, the amorphous structure is frozen froma high-temperature state created by the sputtering process. Thus,the formation of a high concentration of oxygen Frenkel pair islikely to occur in the as-deposited thin a-IGZO thin films. Thisdonor–acceptor model is supported by our experimental resultson the O 1s core-level spectra obtained by hard X-ray photoelec-tron spectroscopy measurement (HAXPES).[27] We propose thatextra-annealing at low-temperature can effectively suppress se-vere PBS instability in very thin a-IGZOTFT by restoring the pro-posed acceptors, which is experimentally supported by the O 1score-level spectra obtained by X-ray photoelectron spectroscopy(XPS).2. TFT Characteristics and Channel ThicknessThe effect of channel thickness on VTH was investigated usinga bottom-gate and top-contact structure with channels rangingfrom 5 to 100 nm in thickness. Figure 1a shows the transfercurves of fabricated TFTs, indicating that VTH significantly shiftsto the positive side as the thickness decreases. Specifically, for athickness of 100 nm, VTH was − 7.1 V, whereas it was 1.5 V forthe 5-nm-thick channel. Since VTH tends to shift negatively withincreasing carrier concentration (ne) in the channel,[9,28] this re-sult is consistent with the Hall effect measurement results sum-marized in Table S1 (Supporting Information). These results in-dicate that ne decreases as the channel becomes thinner. E.g.,the ne was 6.6 × 1016 cm−3 for the 100-nm-thick film and 6.0× 1014 cm−3 for the 10-nm-thick film. Figure 1b,d demonstratesthat PBS instability becomes more severe as the channel thick-ness decreases, whereas the negative-bias-stress (NBS) instabil-ity shows no distinctive degradation. Figure 1e summarizes theVTH shift after 1 h when channel thickness is varied. The PBS sta-bility tends to degrade significantly for film thicknesses below 50nm, even when a Zn−Si−O[29] (ZSO, Zn: Si ratio of 70: 30 at.%)capping layer is employed. We used ZSOx (ZnO-SiO2) as a passi-vation layer suitable for a-IGZO channel because it contains Znwhich is common in the channel. The amorphous ZSO thin filmscan be easily deposited at room temperature (RT) by DC sput-tering. These features make ZSO favorable for passivation layerof a-IGZO channel. ZSO serves two key functions: (1) effectivelyblocks impurities from affecting the AOS channel,[9,20,30] and (2)enhances structural relaxation at the thin-film surface and theback-channel by the impact of high energy ions generated duringsputtering of ZSO. To investigate whether adsorbed moleculesforming surface dipoles on the exposed back-channel[31] con-tribute to PBS instability, we examined the effect of the ZSO cap-ping layer on the instability of TFTs in the ambient atmosphere.Initially, the device was placed in a vacuum environment (< 1× 10−4 Pa) to observe VTH stability without applying any bias.Measurements were taken at 2-h intervals over a 12-h period, asshown in Figure S2 (Supporting Information). The results indi-cate almost no shift in the VTH with ZSO capping, whereas sig-nificant instability was observed in the ZSO capping-free TFT.These results suggest that ZSO effectively suppresses the effectof adsorbed molecules, which are a potential cause of PBS insta-bility. The results of a subsequent PBS test with the ZSO cappinglayer are shown in Figure 1d. Although the large VTH shift (ΔVTH= 3.0 V) by PBS (see Figure 1e) was improved for a 5-nm-thickchannel, severe PBS VTH shifts (ΔVTH = 1.6 V) still remained.These findings suggest that PBS instability arises from two dis-tinct mechanisms: The first is an extrinsic factor, i.e., adsorbedmolecules on the surface, which is mitigated by ZSO capping.The second is an intrinsic factor observed in devices with thecapping layer, indicating that PBS instability in very thin chan-nels is related to acceptor-type trap states created in bulk. TheVTH dependence on channel thickness implies two possibilities:1) ne depends on the channel thickness, and 2) very large PBSVTH shifts arise not from surface adsorbed molecules, but ratherbulk defects.3. Correlation between ND and Ea in a-IGZOThin-FilmsFigure 2a shows the Arrhenius plot of carrier electron concen-tration (ne) in a-IGZO thin-films deposited in a different PO2,where thickness is fixed at 100 nm. Both ND and Ea were calcu-lated by assuming an Arrhenius (thermally activated)-type rela-tionship between temperature and electron carrier concentration(ne): ne = NDexp(−Ea/kBT), obtained from Hall effect measure-ments, where kB and T are Boltzmann constant and temperature,respectively. As indicated in Figure 2a, the ND increases and Eadecreases, when PO2 during deposition is decreased. Here, we fo-cus on the correlation betweenND and Ea. It is well known that inconventional single-crystalline semiconductors, the Ea decreasesas the dopant concentration increases, regardless of conductiontype (p or n), as in Si: B, diamond: P, In2O3, and GaN: Mg. It isa general consensus in science of amorphous solids that the firstneighboring structure such as the coordination number remainseven in the amorphous state. Carrier generation occurs from VOand the local structure around VO is almost the same as that inAdv. Electron. Mater. 2025, 11, e00349 e00349 (2 of 12) © 2025 The Author(s). Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2025, 15, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/aelm.202500349 by National Institute For, Wiley Online Library on [22/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advelectronicmat.dewww.advancedsciencenews.com www.advelectronicmat.deFigure 1. Bias stability of the a-IGZO-TFTs with varying channel thicknesses. Transfer characteristics of a-In0.6Ga0.3Zn0.1Ox (a-IGZO631) TFTs under apartial oxygen pressure of 25% and threshold voltage (VTH) shift as a function of biasing duration a,b) without a capping layer and c,d) with a cappinglayer, respectively. e) VTH shifts of TFTs with and without a capping layer when channel thickness is varied. The TFT channels were patterned with a widthof 60 μm and a length of 30 μm. Transfer curves were acquired with a drain–source voltage of 0.1 V, and a bias of VTH ± 20 V was applied for 1 h in PBSand NBS tests.crystalline oxide semiconductors. Thus, the empirical relation ofEa – ND is valid even for an amorphous state. This decrease in Eawith increasing oxygen defect concentration is typically expressedby the empirical equation:Ea(ND)= Ea (0) − 𝛼(ND)1∕3(1)where Ea(ND) is the energy difference between shallow donorsand the CBM, i.e., the Ea of the donor, and Ea(0) is the Ea at adilute doping limit.[22–26] Note that in this equation, ND corre-sponds to the ionized donor concentration rather than the totaldonor concentration.[26] Again, (ND)−1/3 can relate to the meanseparation between the defects (d), where the Coulomb poten-tial energy is inversely proportional to d between charges. Asmentioned above, oxide semiconductors exhibit 𝛼 > 0, indicatingthat the decrease in Ea is due to enhanced interactions betweenionized donors. As the ND increases, the d decreases, leading tostronger repulsive interaction between the donors.[32] As a result,the energy level of the thermally active shallowest donor becomesshallower due to higher potential energy until it reaches a degen-erate state that corresponds to Ea = 0. Figure 2b shows the corre-lation between (ND)1/3 and Ea for a-IGZO631. A negative correla-tion between Ea and ND is evident; when partial oxygen pressure(PO2) is reduced, ND increases and Ea decreases, following therelationship described by Equation (1).[27] This result is straight-forward to understand that oxygen vacancies (VO) trapping elec-trons act as shallow donors in a-IGZO. The energy level of iso-lated VO without VO’s interactions, obtained by extrapolating NDto zero, is Ea = 0.06 eV (see also Figure S3, Supporting Infor-mation), which is nearly the same as that of a-IGZO111.[33] Thisvalue is close to that (0.09 eV) for single crystalline In2O3.[24] AsVO concentration increases by lowering PO2, as shown in Figure3a,b, the d decreases, causing the VO level to become shallower.The linear relationship between (ND)1/3 and Ea indicates that theinteractions between VO’s control the depth of the donor level.3.1. New Finding of 𝜶 < 0 in the Oxide SemiconductorFigure 2c shows the Arrhenius plot of ne in a-IGZO thin-filmswith different thicknesses (10–100 nm), where PO2 is fixed at25%. The ND increases and Ea increases, when the thin-film be-comes thinner. This trend between Ea and ND is opposite to thatobserved in Figure 2a when PO2 is varied. It is thus evident thatAdv. Electron. Mater. 2025, 11, e00349 e00349 (3 of 12) © 2025 The Author(s). Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2025, 15, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/aelm.202500349 by National Institute For, Wiley Online Library on [22/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advelectronicmat.dewww.advancedsciencenews.com www.advelectronicmat.dea bPO2c dlower PO2thinnerlower PO2Figure 2. Correlation between donor-level energy (Ea)measured fromCBMand donor concentration (ND) in a-IGZO631. a) The temperature dependenceof electron carrier concentration (ne) in the thin-films when partial O2 pressure (PO2) is varied during deposition. b) Correlation between (ND)1/3 andEa when PO2 is varied. A star symbol denotes the energy level of oxygen vacancy (VO) at dilution limit evaluated by extrapolation of ND to 0. c) Thetemperature dependence of ne with varying film thicknesses (10–100 nm). PO2 is 25%. d) The correlation between (ND)1/3 and Ea when PO2 or thicknessis varied. Two opposing types of correlation between (ND)1/3 and Ea are observed: a blue solid line (indicating a negative correlation, or 𝛼 > 0 inEquation 1) and a red solid line (indicating a positive correlation, or 𝛼 < 0 in Equation 1), corresponding to the dependence on PO2 and thickness,respectively. The results on PO2 = 0.25% are presented in Figure S4e (Supporting Information), where trends similar to PO2 = 25% are observed.there are two types of correlation between Ea and ND. As a-IGZObecomes thinner, a linear relationship (𝛼 < 0 in Equation 1) be-tween (ND)1/3 and Ea is observed as indicated by the red solidline in Figure 2d. This negative correlation (𝛼 > 0) switches tothe positive correlation (𝛼 < 0) as the film thickness decreases,even at PO2 = 0.25% (see Figure S4e, Supporting Information),which is a condition that favors the formation of VO. To explainthis positive correlation, an attractive factor for donors is nec-essary. To date, Equation (1) has been used to explain the de-crease in Ea with increasing ND (𝛼 > 0) and to analyze the dop-ing effects and electrical properties. Various ideas have been pro-posed for negative correlation (𝛼 > 0), including band edge tail-ing, the attraction between free electrons and positively chargedionized donors,[22] and donor−donor interactions.[32] However,each model explains only for negative correlation (𝛼 > 0), anddoes not fully provide a unified interpretation for our findingsof both 𝛼 > 0 and 𝛼 < 0. Here, we extend the donor−donor in-teraction model to defect−defect interactions to explain these ob-servations. We propose the presence of acceptors whose energylevels are located just below the CBM, and the acceptors inter-act with donors. Namely, as the d decreases, donor−acceptor de-fect pairs deepen the donor level through attractive interactions,in contrast to donor−donor interactions that induce repulsive ef-fects and lower the activation energy. As a result, donor activa-tion energy increases, as shown in Figure 3c,d. Hereafter, theelectronic states of the donor and acceptor are discussed in theground state at 0 K, which correspond to occupied and unoccu-pied states, respectively. Following this idea, Ea increases withAdv. Electron. Mater. 2025, 11, e00349 e00349 (4 of 12) © 2025 The Author(s). Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2025, 15, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/aelm.202500349 by National Institute For, Wiley Online Library on [22/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advelectronicmat.dewww.advancedsciencenews.com www.advelectronicmat.deFigure 3. Two types of defect−defect interactions. a,b) Donor−donor repulsive interaction and c,d) donor−acceptor attractive interaction. The effect ofchange in separation between defects (d) on Ea is schematically illustrated. Note that d is relatively larger in (a,c) than in (b,d).increasing ND, in contrast to the case of donor−donor repulsiveinteraction.To further understand the correlation between thickness andND, we varied the substrate temperature (Tsub) during deposi-tion and observed its impact as a function of Tsub. The correla-tion between (ND)1/3 and Ea at Tsub during the deposition for a-IGZO111 at PO2 = 0.5% is shown in Figure S5 (Supporting Infor-mation). It is evident that ND decreases as Tsub increases, whichsuggests that structural relaxation during deposition plays a sig-nificant role in determining ND. Most of the defect pairs caus-ing positive correlations are formed during film deposition andare suppressed in part by the structural relaxation owing to sub-strate heating. Therefore, these defect pairs may be attributedto a metastable state created through a rapid quenching pro-cess during deposition. The positive slope shown in Figure 2dcan be explained as follows: sputtering deposition occurs byAr+ ion-bombardment to the target. Thus, the films grown atlow Tsub contain a quenched high-temperature state that occursby ion-bombardment and plasma exposure. Since the equilib-rium Frenkel defect concentration increases with temperature,the defect concentration in the as-sputtered thin-film at RT is ex-pected to be high. This rapidly quenched defect state undergoesrelaxation during sequential thin-film deposition. As a result,higher defect concentrations remain in the very thin depositedlayers.3.2. What Is the Entity of the Acceptor in a-IGZO?The next question to address is the entity of the acceptorin a-IGZO. To induce significant defect−defect interactions,small d is necessary for donor−donor interaction, whereas, fordonor−acceptor interaction, high concentration of acceptor de-fects is required. To gain insight into the entity of the acceptorinteracting with the donor, we measured the electronic states ofthin and thick a-IGZO thin-films by hard X-ray photoemissionspectroscopy (HAXPES) in combination with X-ray total reflec-tion (TR).[34,35] Figure 4a–c presents the HAXPES spectra in theO-1s region for two samples, one 100-nm-thick (Figure 4a,b) andthe other 10 nm (Figure 4c). Figure 4b,c shows the thickness de-pendence of the O 1s core spectra obtained by surface-sensitiveTR-HAXPES (see Note S2, Supporting Information). The mainpeak is located at around 530 eV and additional peaks appearat a higher binding energy side around ≈531 eV and ≈532 eV.The main peak comes from the oxygen ions bonded with metalcations (In−O, Ga−O, and Zn−O). The sub-peak around 532 eVis attributed to OH in deeper bulk regions.[36–38] The intensityof OH remains unchanged regardless of surface-sensitive (TR-HAXPES), bulk-sensitive (non-TR-HAXPES) measurements, orfilm thickness, indicating no significant difference is seen be-tween the thin and thick films. A notable difference between thethin (10 nm) and thick (100 nm) films is the intensity of the peakAdv. Electron. Mater. 2025, 11, e00349 e00349 (5 of 12) © 2025 The Author(s). Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2025, 15, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/aelm.202500349 by National Institute For, Wiley Online Library on [22/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advelectronicmat.dewww.advancedsciencenews.com www.advelectronicmat.deFigure 4. HAXPES spectra of a-IGZO111. a–c) O 1s core-level HAXPES spectra for a,b) 100-nm-thick and c) 10-nm-thick a-IGZO111 films. HAXPEScombined with X-ray total reflection (TR-HAXPES) was used to obtain the spectra in (b) and (c). The O 1s peaks are deconvoluted into three components.d) Valence band (VB) TR-HAXPES spectra of 10- and 100-nm-thick films and the difference spectrum between them. The photon energy was set to 5.95keV. e) Simplified schematic of the experimentally clarified density of states (DOS) of two types of oxygen vacancies in a-IGZO.at ≈531 eV relative to the main peak at ≈530 eV.[27] This obser-vation suggests the presence of oxygen species that are weaklybound than lattice O2−.[39] The origin of the O 1s peak com-ponents located at ≈531–532 eV in oxide semiconductors looksrather confusing. While not a few studies describe these peaksas an oxygen vacancy (so-called “VO”),[37,40] VO has no electronin the core-level region, in principle. This means that VO itselfshould not directly give any peak in the O 1s core-level region.Here, we propose a plausible model that the O 1s peak at ≈531eV is attributed to oxygen perturbed by the interstitial oxygen sug-gested in Refs.[41–43] This interstitial oxygen (Oi) leads to theformation of the acceptor, namely VO2+ (an oxygen vacancy with-out two electrons), which interacts with VO0 (an oxygen vacancywith two electrons). An origin for this donor−acceptor interac-tion would be the formation of Frenkel defect pairs. The conceptof Frenkel pairs in ionic solids is well-established and is observedeven in amorphous materials because the local structure in crys-tal remains even in the amorphous state. The clear formationof Frenkel defect was observed in a-SiO2.[44–46] According to this“oxygens perturbed by O Frenkel defect”model, a comparison be-tween the 10- and 100-nm-thick a-IGZO thin-films in Figure 4b,creveals that the intensity of so-called “VO” relative to that of thelattice O2− is distinctly larger in the thinner (10 nm) film than inthe 100-nm-thick film.The observed difference in the chemical state of oxygen is ex-pected to affect the DOS of the VB. Figure 4d shows the VB TR-HAXPES spectra of the 10- and 100-nm-thick films, normalizedby the intensity of Zn 3d core-level located at ≈10.5 eV. The in-tensity difference spectrum shows the finite intensity, which isslightly enhanced in the 10-nm-thick film and is mainly com-posed of O 2p orbitals in the entire VB region. The enhancedintensity is attributed to the perturbed oxygen, which correlateswith the O 1s peak (so-called “Vo”) at ≈531 eV in Figure 4b,c.A possible explanation for the difference between the thick andthin a-IGZO is the enhancement of oxygen-related defects in thethinner films, such as interstitial oxygen (Oi),[8] O2−, or O22−.Among them, Oi and O22− are more likely than O2− because noEPR signal was observed (data not shown here), notwithstand-ing O2− is paramagnetic. Figure S7c (Supporting Information)presents thermal desorption spectra (TDS) ofm/Z= 32 (O2) froma-IGZO111 films. Desorption of O2 from the thin-films was notobserved; thus, the possibility of excess oxygen being incorpo-rated into the thin-films during deposition is unlikely. The ob-served results suggest the formation of interstitial oxygen speciesin stoichiometry by the Frenkel defect mechanism.Figure 4e illustrates that oxygen vacancies (VO) can be cate-gorized into two types: deep states and shallow states. The deepstates above the VB are observed by HAXPES, while the in-formation on the shallow states is obtained from temperature-dependent Hall effect measurements. We consider the distinc-tion between deep and shallow states lies in the size of the oxygenvacancy site based on molecular dynamics (MD) simulation:[47]Adv. Electron. Mater. 2025, 11, e00349 e00349 (6 of 12) © 2025 The Author(s). Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2025, 15, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/aelm.202500349 by National Institute For, Wiley Online Library on [22/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advelectronicmat.dewww.advancedsciencenews.com www.advelectronicmat.deFigure 5. A schematic model for two types of defect−defect interactions. a) A donor–donor interaction is dominant, and the responsible defect is Votrapping 2 electrons, originated from oxygen deficiency. b) Distortion of lattice oxygen in a-IGZO, leading to the formation of a Frenkel-type defect pair.c) Relaxed structure of a-IGZO with a VO2+–Oi2− pair, which may be regarded as Frenkel defect. In practice, these defects may be led by bond angledistortion in metal-oxygen octahedra (MO6). Namely, the second-neighboring oxygen, perturbed by VO2+–Oi2− pair (denoted as O*), is a plausiblecandidate responsible for the O 1s peak located at ≈531 eV, so-called “VO” detected by XPS.larger space forms an electron trap level deep in the band gap,otherwise, smaller space is necessary to form shallow states.There is a variety of sub-gap states in a-IGZO from multiplesizes around oxygen vacancy according to the MD simulation.[48]G.W. Mattson et al. reported that hydrogen incorporation createsa broad distribution of electronic states in mid-gap.[7] Here, weemphasize that our thin films are hydrogen-free a-IGZO, withhydrogen contamination below the TDS detection limit (≈1018cm−3) before and after annealing, respectively. For simplicity, wecategorize these states as shallow and deep states, with both statesdetected in the O 1s core-level HAXPES spectra through per-turbed oxygen. Notably, the deep states are quantitatively dom-inant, being 1−2 orders of magnitude larger than the shallowstates.3.3. Interstitial Oxygen and Oxygen Vacancy PairAccording to calculations[49–52] on defects in relevant oxides, in-terstitial oxygen (Oi) forms unoccupied states just below the CBMdepending on the chemical composition. In 𝛽-Ga2O3 and In2O3,the Oi and lattice oxygen are closer together to form energeticallystable molecular oxygen. Comparisons of the defect formationenergies assumed for In, Zn-rich, and Ga-rich IGZO are summa-rized in Figure S8 (Supporting Information). The preferentiallyformed interstitial oxygen-related defects depend on the chem-ical composition: in In-rich a-IGZO, Fermi-level (EF) is pinnedby VO and Oi, resulting in the formation of molecular oxygen. Incontrast, inGa-rich a-IGZO,Oi itself can be stabilized. It is impor-tant to note that these electronically active oxygen species wouldtypically be deactivated by competition with hydrogen-related im-purities. In this study, we suppose that the intrinsic defect in a-IGZO was observed because the concentration of hydrogen im-purities was below the detection limit of TDS (≈1018 cm−3) forthe as-deposited thin-films. The hydrogen impurity concentra-tion in this work is two to three orders of magnitude lower thanthat previously reported in conventional sputtering under turbo-molecular-pumping, which contains impurities of carbon andhydrogen.[53]Figure 5a–c illustrates two types of defect–defect interactionmodels. In the conventional a-IGZO case, as reported in previ-ous studies,[5] the formation of oxygen vacancies (VO0) trappingtwo electrons originated from oxygen deficiency is depicted inFigure 5a. For the donor–acceptor interaction in Figure 5b,c, VO2+is formed by distortion of lattice oxygen in a-IGZO (Figure 5b), es-pecially in the very thin films. Furthermore, the VO2+–Oi2− pair,which may be regarded as Frenkel defects (Figure 5c), leads tothe formation of large vacancy sites that may explain the originof deep states.The following is a rough estimate of the concentration of oxy-gen perturbed by the VO2+–Oi2− pair. On the assumption that theoxygen constituting a point-shared MO6 octahedron with a dis-torted bond angle is perturbed by the oxygen Frenkel defect pair,the number of second-neighboring oxygen (O*) can be expressedas:Number of O∗ = 2 ×(# of distorted MO6 octahedron)× (CN − 1) (2)Adv. Electron. Mater. 2025, 11, e00349 e00349 (7 of 12) © 2025 The Author(s). Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2025, 15, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/aelm.202500349 by National Institute For, Wiley Online Library on [22/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advelectronicmat.dewww.advancedsciencenews.com www.advelectronicmat.dewhere CN is the coordination number around the cation adja-cent to VO. In the case of a-IGZO, the estimated number of O*around the Frenkel defect is ≈40. Since the ND for a-IGZO111film is ≈1 × 1020 cm−3 for thinner film (Figure S4, SupportingInformation), the O* concentration in the thin-film is ≈4 × 1021cm−3. By comparison, the concentration calculated from the areaof the XPS O 1s peak at 531 eV is in the range of 5 × 1021 to1.5 × 1022 cm−3, corresponding to 10–30% of the total O2−. Thecomparable values of the XPS-derived concentration and the esti-mated concentration of perturbed oxygen (O*) support this ideathat the second-neighboring oxygen, perturbed by the VO2+–Oi2−defect pair, is responsible for the O 1s peak located at ≈531 eV.During TFT operation, the proposed acceptor (VO2+) may act as acarrier trap and deteriorate ON characteristics, resulting in a pos-itive VTH shift. In other words, even with the same ne, the filmthickness significantly affects the stability of the TFT operation.4. Improved PBS Stability in Very Thin a-IGZO TFTThese results indicate that both the fabrication conditions, suchas PO2 and thickness, and the chemical composition of a-IGZOmust be carefully considered to have improved TFT stability.It is known that increasing the Ga concentration reduces thene in a-IGZO.[2,54] The previous analysis focused solely on newithout considering Ea or ND. Here, we employed temperature-dependent Hall measurements to determine ND and Ea, asshown in Figure 6a. Notably, the Ea values strongly depend onthe chemical composition, and decreasing the In/Ga ratio doesnot necessarily reduce the oxygen defect concentration (ND). In-stead, higher Ga content to In leads to deepens the donor lev-els, which explains the observed decrease in ne. The trends inEa as a function of (ND)1/3 for the different In/Ga ratios in a-IGZO are summarized in Figure S4 (Supporting Information).Figure 6b illustrates the relationship between (ND)1/3 and Ea un-der different fabrication conditions, such as PO2, film thickness,and the chemical composition of a-IGZO. A negative correla-tion (blue dashed arrow in Figure 6b) is observed with increas-ing film thickness, decreasing PO2, or increasing the In/Ga ra-tio, indicating enhanced donor−donor interaction. Conversely,a positive correlation (red dashed arrow in Figure 6b) is ob-served as the thickness decreases (see also Figure S4e,f), sug-gesting that donor−acceptor interaction becomes dominant. Ad-ditionally, as the PO2 increases or the In/Ga ratio decreases (reddashed arrow in Figure 6b), the concentration of Vo0 (oxygen va-cancies with two electrons) decreases, leading to a reduced con-tribution from donor (Vo0)−donor (Vo0) interactions. As a re-sult, the donor (Vo0)−acceptor (Vo2+) interaction becomes appar-ent. Based on these observations, we can identify two origins forthe formation of “oxygen defects”: i) the oxygen-deficient type,which is commonly known, Vo0 (with 2 electrons), formed in off-stoichiometric oxide semiconductors for transparent conductiveoxides (TCOs), and ii) intrinsic VO0, which forms as the Frenkeldefect during deposition, even in stoichiometric and hydrogen-free a-IGZO thin-films.Figure 6c summarizes the dominant defect−defect interac-tions and their impact on TFT instability in the Ea−(ND)1/3 di-agram. Three types of oxygen-related defects influence electrontransport properties: (i) VO0, caused by oxygen deficiency, whichcan be categorized into deep and shallow states, with the shallowstates affecting electron transport properties; (ii) Oi2−; and (iii)VO2+, located just below theCBM, acting as a gate-bias-dependentacceptor. A reduction in Frenkel defects (VO2+–Oi2−) leads to thedominance of donor (VO0), resulting in deteriorated NBS insta-bility. Conversely, an increase in Frenkel defects causes acceptor(VO2+) to become dominant, leading to deteriorated PBS insta-bility. Viewing the diagram with the above-consideration helpsexplain the extraordinarily large PBS instability observed in verythin a-IGZO TFTs and highlights the existence of a “sweet spot”for AOS-TFTs, where the device is least affected by NBS and/orPBS instability. This optimal condition can be achieved by in-creasing the AOS thickness, adjusting the In/Ga ratio, or reduc-ing PO2 during deposition. Conversely, for TFTs prone to NBSinstability due to dominant donor−donor interactions, reducingthe AOS thickness, decreasing the In/Ga ratio, or increasing thePO2 during depositionmay be necessary to reach the sweet spotby suppression of donor formation. Understanding these dom-inant defect interactions and fine-tuning fabrication conditionsare crucial for successfully producing high-performance TFTs.By identifying these factors, the performance and reliability ofdevices can be enhanced, ensuring that the devices are tailoredto withstand specific stress conditions. Furthermore, the nega-tive and positive correlations between (ND)1/3 and Ea in Figure 6cprovide insight into the fundamental differences between trans-parent oxide semiconductors and TCOs. TCOs such as Sn-dopedIn2O3, F-doped SnO2, and Al-doped ZnO require high electricalconductivity like metals.[55] Therefore, increasing ND with de-creasing Ea (i.e., negative correlation) is typically desirable forTCOs.Historically, AOS-TFTs with thick channels have been suscep-tible to NBS instability due to the dominance of donor−donorinteractions. However, as the channel thickness decreases to 10nm or less, oxygen-related acceptors, such as Frenkel defects, be-come traps, making PBS instability more prominent. To addressthe serious PBS instability caused by Frenkel defects in very thina-IGZO, it was necessary to reduce the acceptor concentration.We modified the conventional thin-film process by introducingan additional low-temperature annealing step after deposition.The process began with a 400 °C post-deposition thermal anneal-ing in air for 1 h. After cooling to RT, the samples underwent anadditional low-temperature thermal annealing at 200 °C in airfor 1 h. As a result, we observed a noticeable decrease in ND andEa, indicating a reduction in the concentration of donor−acceptorpairs, as illustrated in Figure 6d. It is important to note thatthe initial thermal annealing and the extra-annealing serve dif-ferent roles and operate via different mechanisms. The first an-nealing cycle, involving heating and cooling (known as the post-annealing process) above 300 °C, is well known for improvingTFT performance, stability, and uniformity.[5,56,57] It is reportedthat the densification is observed to occur ≈1% around 400 °C asa result of macro-relaxation.[58] This process was suitable for theelimination of defects and voids so far. However, temperaturesabove 300 °C are too high to restore Frenkel defects to regularM−O−M bonds. On the other hand, low-temperature annealingbelow 300 °C has been reported to reduce tail-state optical ab-sorption while preserving the amorphous structure without de-tectable densification.[58] Therefore, additional low-temperaturethermal annealing at around 150−250 °C is necessary to stabi-lize oxygen species.Adv. Electron. Mater. 2025, 11, e00349 e00349 (8 of 12) © 2025 The Author(s). Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2025, 15, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/aelm.202500349 by National Institute For, Wiley Online Library on [22/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advelectronicmat.dewww.advancedsciencenews.com www.advelectronicmat.deFigure 6. Improved TFTs stability through low-temperature extra-annealing. a) Temperature dependence of the ne in a-In0.33Ga0.33Zn0.33Ox (a-IGZO111),a-In0.6Ga0.3Zn0.1Ox (a-IGZO631), and a-In0.6Ga0.1Zn0.3Ox (a-IGZO613). b) Correlation between (ND)1/3 and Ea with deposition conditions; PO2, filmthickness, and chemical composition. Depending on the fabrication conditions, correlations switch from negative to positive (red dashed arrow) orfrom positive to negative (blue dashed arrow). c) Schematic relationship between two types of correlation and PBS/NBS instabilities in an Ea−(ND)1/3diagram. The purple area indicates the optimal ND and Ea region for NBS/PBS-stable TFTs. d–f) Extra-annealing effect on transfer TFT characteristics.To achieve a high-mobility, ultra-thin TFT with a-IGZO631 channel was employed. d) ND and Ea after extra-annealing at various temperatures. Resultsof bias stability tests of 200 °C extra-annealed a-IGZO631 TFTs with e) 10-nm-thick and f) 5-nm-thick active layers. For PBS and NBS tests, a positive ornegative bias of VTH ± 20 V was applied with a VDS of 0.1 V. g, h) Extra-annealing effect on the O 1s core-level XPS spectra in 10-nm-thick a-IGZO111films. g) 400 °C-1 h annealed thin-film; h) thin-film after 200 °C-1 h extra-annealing.Figure 6e,f demonstrates the improved characteristics of the10-nm-thick and 5-nm-thick TFT channels after undergoing low-temperature extra-annealing at 200 °C. The concentration ofoxygen-related acceptors decreased as the oxygen became morestable due to the annealing process. This stabilization led to thethermal restoration of the oxygen-related acceptors, resulting inan increase in ne and a negative shift in VTH from 0.2 V to −4.5V for the 10-nm-thick TFT channel and 0.4 V to −2.5 V for the5-nm-thick TFT channel (see Figure S11, Supporting Informa-tion). Additionally, the extraordinarily large PBS VTH shift wasAdv. Electron. Mater. 2025, 11, e00349 e00349 (9 of 12) © 2025 The Author(s). Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2025, 15, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/aelm.202500349 by National Institute For, Wiley Online Library on [22/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advelectronicmat.dewww.advancedsciencenews.com www.advelectronicmat.designificantly reduced from 2.5 V to 0.06 V for 10-nm-thick and3.2 V to 0.5 V for 5-nm-thick, as shown in Figure 6e,f. The re-maining PBS VTH shift (0.5 V) in the 5 nm-thick TFT may beattributed to surface effects such as adsorption of molecules. ForTFTs with a channel thickness of less than 10 nm, further inves-tigation is required to optimize the chemical composition andfabrication conditions. However, there was a minor degradationin NBS performance, which can be attributed to the shift in thedominant interaction from donor−acceptor to donor−donor af-ter extra-annealing, making the donor−donor interaction moreprominent when the gate bias is applied. To further understandhow low-temperature extra-annealing restores Frenkel defects,we measured the electronic states of very thin a-IGZO111 filmsusing conventional XPS using Al K𝛼 light source (1486.7 eV).Figure 6g,h shows the O 1s core-level XPS spectra for two films:one annealed at 400 °C (Figure 6g) and the other subjected to anadditional low-temperature annealing at 200 °C (Figure 6h). No-tably, there is a clear reduction in O 1s peak intensity at ≈531 eV.These results suggest that the concentration of oxygen-related ac-ceptors occupying interstitial sites decreases by extra-annealing,leading to a reduction in donor−acceptor pairs.5. ConclusionsThe primary points of the present paper are summarized as fol-lows:I. An extraordinarily large PBS instability was observed inhydrogen-free a-IGZO thin films (≈5 nm) deposited by sput-tering, and its mechanism was elucidated. The instabilitywas caused by insufficient ne due to donor level deepeningfrom donor−acceptor interactions at VG = 0 V and carriertrapping by acceptors under positive VG.II. Two types of correlations between ND and Ea in a-IGZOwere identified, and the opposite Ea shifts with ND wereexplained by interactions between defects. In the very thinfilms, high-concentration donor−acceptor pairs increase Eawith ND, suppressing ne. The dominant correlation is de-termined by fabrication conditions, such as film thickness,deposition PO2, and the In/Ga ratio, which control dom-inant defect−defect interactions and indicate whether theVTH shift is more prone to NBS or PBS instability.III. The presence of oxygen-related acceptors is suggested byHAXPES measurements on the VB and O 1s. These TFTand HAXPES measurements revealed that high concentra-tions of VO0 (as the donor) and VO2+ (as the acceptor) wereformed in thinner a-IGZO films. The VO2+–Oi2− pair may beregarded as a Frenkel defect and VO2+ can act as a gate-bias-dependent acceptor and was identified as a plausible originof the PBS instability.IV. Low-temperature extra-annealingwas demonstrated to be ef-fective to restore oxygen-related acceptor defects, allowingthe fabrication of very thin a-IGZO TFTs with high stability.V. A plausible structural model for so-called “VO” componentin O 1s XPS spectra was proposed.This study provides a pathway for developing high-bias-stability a-IGZO TFT devices with very thin channels by combin-ing precise control of defect interactions through fabrication tun-ing and the implementation of low-temperature extra-annealing.6. Experimental SectionThin-Film Deposition and Hall Effect Measurement: To compare donor-level and concentration characteristics across varying carrier concen-trations and chemical compositions, three different indium/gallium(In/Ga) ratios were prepared: a-In0.33Ga0.33Zn0.33Ox (a-IGZO111), a-In0.6Ga0.3Zn0.1Ox (a-IGZO631), and a-In0.6Ga0.1Zn0.3Ox (a-IGZO613).These films, ranging from 10 to 100 nm in thickness, were deposited us-ing RF magnetron sputtering without intentional heating. The sputteringconditions were as follows: RF power of 150 W, 3-inch diameter targets,total pressure of 0.4 Pa, and back-pressure of the deposition chamber of≈2 × 10−7 Pa, achieved using cryo-pumping. The partial oxygen pressure(PO2) was varied between 0.25–25%, corresponding to the limits of stableplasma generation under oxygen-poor and oxygen-rich conditions. Afterthe deposition, the samples were annealed at 400 °C for 1 h in ambientair. For ohmic contacts, Mo (80 nm) electrodes were sputtered througha shadow mask for a-IGZO111, and Au (80 nm) electrodes were used fora-IGZO631 and a-IGZO613. It should be noted that the measurement ofcarrier concentration for thin-films thinner than 10 nm was not feasibledue to the detection limit.TFT Device Fabrication: To achieve a high-mobility, ultra-thin TFT, a-IGZO631 was employed in the fabrication of bottom-gate and top-contactTFT structures. P++-silicon substrates were used, featuring a 150 nm ther-mally grown SiO2 gate insulator. The a-IGZO631 films, ranging from 5 to100 nm in thickness, were deposited via RF magnetron sputtering with-out intentional heating. The sputtering parameters were as follows: RFpower of 150 W, 3-inch diameter target, total pressure of 0.4 Pa, and back-pressure of ≈2 × 10−7 Pa, with a PO2 of 25%. Post-deposition heat treat-ment was conducted under the same conditions as described above. TheTFT channels were patterned with a width of 60 μm and a length of 30μm by photolithography (MA-10, Mikasa). Following this, conductive Mo(10 nm) and Au (70 nm) were deposited as the source/drain electrodes.Additionally, a 5-nm-thick amorphous Zn−Si−O[9,20,29,30] (ZSO) cappinglayer, with a Zn: Si ratio of 70:30 at.%, was deposited under a total pressureof 0.4 Pa, back-pressure of ≈2 × 10−7 Pa and PO2 of 25%.Samples for Hard X-Ray Photoelectron Spectroscopy (HAXPES) Measure-ment: To facilitate HAXPES measurements, 10-nm- and 100-nm-thick a-IGZO111 films were prepared, using the standard a-IGZO chemical com-position. Due to the limitations of Hall measurements at 5-nm thickness,data consistency could not be verified for such thin-films. For these sam-ples, Conductive Ti (10 nm) and Pt (50 nm) layers were deposited ontosilicon substrates. The substrates underwent thermal annealing at 800 °Cfor 1 h in a vacuum to remove adsorbed molecules. Subsequently, a-IGZO111 films were deposited via RF magnetron sputtering without in-tentional heating, using the following sputtering parameters: RF power of150W, 3-inch diameter target, total pressure of 0.4 Pa, back-pressure of≈2× 10−7 Pa, and PO2 of 0.25%. After deposition, the samples were annealedunder the same conditions as previously described.Characterization of Devices and Thin-Films: The film thickness wasmeasured using grazing-incidence X-ray reflectivity spectroscopy (Smart-Lab, Rigaku). The presence of oxygen molecules in a-IGZO thin-films wasevaluated by thermal desorption spectroscopy (TDS, ESCO). Carrier trans-port properties of the a-IGZO thin-films were investigated through ACmagnetic field Hall effect measurement (ResiTest8400, TOYO) with thevan der Pauw configuration, under an AC magnetic field of 0.4 T at 300–150 K in a helium atmosphere (Note S1, Supporting Information). Depth-dependent electronic structures were measured using hard X-ray photoe-mission spectroscopy (HAXPES) in combination with X-ray TR[34,35] forthe 100- and 10-nm thick a-IGZO films at RT at the undulator beamlineBL09XU[59] of SPring-8 (Note S2, Supporting Information). The electronicstructures for the 10-nm-thick a-IGZO films were also analyzed by conven-tional X-ray photoemission spectroscopy (XPS) using Al K𝛼 radiation (h𝜈= 1486.6 eV) (Note S2, Supporting Information). Ar+ ion sputtering wasAdv. Electron. Mater. 2025, 11, e00349 e00349 (10 of 12) © 2025 The Author(s). Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2025, 15, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/aelm.202500349 by National Institute For, Wiley Online Library on [22/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advelectronicmat.dewww.advancedsciencenews.com www.advelectronicmat.deperformed to remove surface contamination from carbon and hydroxylgroups (Figure S9, Supporting Information).Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsH.C. and M.T. contributed equally to this work. H.H. conceived and super-vised the research. H.C., M.T., and J.K. conducted the experiments. H.C.,M.T., and S.U. performed theHAXPESmeasurements. All the authors con-tributed to the discussion on the results and mechanism. H.C., M.T., andH.H. wrote the manuscript mainly. All the authors have given their ap-proval to the final version of the manuscript. The authors thank MinoruTazoe and Yukiko Sato for their experimental assistance, andMichiko Satofor XPSmeasurements. The HAXPESmeasurements were performed withthe approval of JASRI/SPring-8 (Proposals 2023A1572, 2023A1720, and2023A1917). The authors thank Akira Yasui for technical assistance in theHAXPES experiments. This work was partly supported by the MEXT Pro-gram: Data Creation and Utilization Type Material Research and Devel-opment Project (Grant No. JPMXP1122683430). M.T. was supported byKAKENHI (Grant Numbers JP23K19266 and JP24K17753). This work waspartly supported by Samsung Electronics Co., Ltd (Grant No. IO240217-09010-01).Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available from thecorresponding author upon reasonable request.Keywordsamorphous oxide semiconductors (AOS), defects, instabilities, thin-filmtransistors (TFT)Received: May 24, 2025Published online: July 7, 2025[1] H. Hosono, H. Kumomi,Amorphous Oxide Semiconductors: IGZO andRelatedMaterials for Display andMemory, Wiley, Hoboken,New Jersey2022.[2] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, H. Hosono,Nature 2004, 432, 488.[3] A. Nathan, S. Lee, S. Jeon, J. Robertson, J. Display Technol. 2014, 10,917.[4] X. Yu, T. J. Marks, A. Facchetti, Nat. Mater. 2016, 15, 383.[5] K. Ide, K. Nomura, H. Hosono, T. Kamiya, Phys. Status Solidi (a) 2019,216, 1800372.[6] T. Kamiya, H. Hosono, ECS Trans. 2013, 54, 103.[7] G. W. Mattson, K. T. Vogt, J. F. Wager, M. 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See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advelectronicmat.de Oxygen Defects and Instability in Very Thin a-IGZO TFTs 1. Introduction 2. TFT Characteristics and Channel Thickness 3. Correlation between ND and Ea in a-IGZO Thin-Films 3.1. New Finding of 83± 80< 0 in the Oxide Semiconductor 3.2. What Is the Entity of the Acceptor in a-IGZO? 3.3. Interstitial Oxygen and Oxygen Vacancy Pair 4. Improved PBS Stability in Very Thin a-IGZO TFT 5. Conclusions 6. Experimental Section Supporting Information Acknowledgements Conflict of Interest Data Availability Statement Keywords