Interface Regulation and Defect Passivation in Rare-Earth Doped Bilayer-Channel Thin Film Transistors for Enhanced Illumination Stability

Interface Regulation and Defect Passivation in Rare-Earth Doped Bilayer-Channel Thin Film Transistors for Enhanced Illumination Stability | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Interface Regulation and Defect Passivation in Rare-Earth Doped Bilayer-Channel Thin Film Transistors for Enhanced Illumination Stability Hongxu Cui, Meng Xu, Wentao Luo, Huajie Li, Longlong Chen, Cong Peng, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9586879/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The illumination-stress instability remains a critical bottleneck for amorphous oxide thin film transistors (TFTs). Here, we propose a rare-earth (RE) doped bilayer-channel structure to effectively enhance mobility while minimizing the deterioration of stability in solution-processed zinc-tin oxide (ZTO) TFTs. The microstructural analyses reveal that while RE dopants suppress oxygen vacancies, Nd retards sol-gel polycondensation, inducing residual hydroxyl traps and morphological degradation. Conversely, La facilitates thorough dehydroxylation, yielding an ultra-smooth defect-minimized network. Consequently, the La: ZTO TFT demonstrates enhanced NBIS stability compared to pristine ZTO. To overcome carrier suppression from RE doping, the bilayer-channel TFT is fabricated, in which the highly conductive ZTO underlayer ensures efficient transport, while the dense La: ZTO capping layer acts as a barrier to passivate interfacial defects and shield against ambient interactions. The ZTO/La: ZTO TFT exhibits excellent switching performance, while the mobility is more than 2 times that of single-layer devices, and small N/PBIS V th shifts (-3.55 /3.37 V). The RE-doped interface regulation strategy provides a highly viable pathway for fabricating stable, high-performance oxide semiconductor electronics. Thin Film Transistors Rare-Earth Doping Illumination-Stress Stability Interface Regulation Bilayer Channel Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. INTRODUCTION The distinctive electronic properties of oxide semiconductors originate from the (n-1)d¹⁰ns⁰ electron configuration of constituent metal cations, which leads to the formation of a conduction band characterized by spherical s-orbitals [1]. The significant overlap of s-orbitals establishes efficient percolation pathways for electron transport while minimizing carrier scattering, thereby endowing oxide thin film transistors (TFTs) with high mobility and low off-state current [2, 3]. Consequently, oxide TFTs have been extensively adopted as the backplane technology in various displays [4]. More importantly, TFTs are inevitably exposed to ambient light, backlight, or self-emission during practical operation, which imposes stringent demands on their photostability [5]. A critical reliability challenge in this context is the instability under negative bias illumination stress (NBIS), which manifests as threshold voltage shift and subthreshold degradation [6]. These instabilities are mainly attributed to photo-induced ionization of oxygen-vacancy (V O )-related states and the subsequent separation/trapping of photogenerated carriers under a negative gate bias via band bending. The transition from the neutral to the ionized V O 2+ is widely regarded as the fundamental cause of the persistent NBIS degradation [5, 6]. Therefore, developing effective strategies to mitigate such effects is of considerable significance, as it also provides deeper insights into material properties and potential applications of oxide semiconductor TFTs. Doping is a widely adopted material design strategy to enhance the stability of oxide TFTs, favored for its relatively simple, cost-effective fabrication and broad compatibility with diverse material systems [7, 8]. Among various dopants, rare-earth (RE) elements can suppress excess carriers through their ultra-low electronegativity and can also act as an efficient photoconversion medium due to their low charge-transfer energy [9, 10]. These effects have motivated the use of RE dopants to tailor the defect chemistry and illumination bias behavior of oxide semiconductors, although the resulting stability improvement remains material- and structure-dependent. So far, numerous studies have explored RE doping for TFT stabilization [9–14]. Wu et al. [15] demonstrated that Er doping in In₂O₃ effectively suppresses oxygen-vacancy–related defect states and reduces interface trap densities, thereby improving bias-stress stability with mitigated threshold-voltage shifts under PBS and NBS. This suppression is often attributed to the strong Er-O bonding, which stabilizes the local oxygen coordination and makes oxygen-vacancy formation energetically less favorable [16]. However, the mobility tends to decrease with increasing Er content due to carrier suppression. Therefore, further investigations are still required to improve device stability while preserving favorable carrier transport. Fundamentally, RE-doped semiconductors reshape the defect landscape and carrier transport properties from a perspective of materials engineering. This intrinsic feature allows them to be synergistically combined with other optimization strategies (e.g., interface band engineering) to further refine overall performance of devices [17–19]. Therefore, this work integrates RE doping with a bilayer channel design to comprehensively enhance the performance of zinc-tin oxide (ZTO) TFTs. While solution processing offers a promising route for low-cost, large-area fabrication of oxide TFTs, the thin films derived from this method often face inherent limitations. During the sol-gel conversion of the precursors, residual hydroxyl groups and incompletely condensed metal-oxygen networks may remain in the thin films [20, 21]. In addition, pores arising from solvent evaporation hinder full densification [22]. These structural imperfections collectively result in a high density of defect states, which exacerbates device instability. To this end, we first evaluate the electrical performance and stress stability of ZTO TFTs doped with different RE elements (La, Pr, and Nd). Following identification of the optimal dopant, a ZTO/RE: ZTO bilayer-channel TFT is fabricated. In this engineered structure, the bottom ZTO layer is designed to provide efficient carrier transport, whereas the top RE: ZTO layer functions as a passivation and defect-suppression capping layer [17–19]. 2. EXPERIMENTAL SECTION The precursor solutions for pristine ZTO (Zn: Sn = 7: 3) and rare-earth-doped ZTO (RE: ZTO, where RE = La, Pr, or Nd; RE: Zn: Sn = 1: 70: 30) are synthesized using zinc acetate hydrate (Zn(CH 3 COO) 2 ·xH 2 O), tin chloride hydrate (SnCl 4 ·xH 2 O) and RE-nitrate hydrate (RE(NO 3 ) 3 ·xH 2 O) as the primary metal sources. These metal precursors are dissolved in 2-methoxyethanol to yield a total concentration of 0.3 M. Then the solutions are magnetically stirred at 70°C for 3 h and aged at room temperature for 24 h. The TFT devices are fabricated on 4-inch quartz glass substrates. First, 35-nm-thick ITO source/drain electrodes are deposited via PVD and patterned by heated oxalic acid. Then the ZTO-based solutions are spin-coated at 3000 rpm for 30 s to obtain a 20-nm-thick single-layer channel. In contrast, bilayer channels are spin-coated at 4000 rpm for 30 s to obtain a thickness of 10 nm for each layer. Next, the thin films are post-annealed at 500°C for 1 h. Subsequently, a 150-nm-thick SiO 2 gate-insulator layer is deposited via PECVD. Finally, a 35-nm-thick ITO thin film is deposited and patterned as the gate electrode to complete the top-gate ZTO-based TFTs. 3. RESULTS AND DISCUSSION To determine whether RE doping changes the crystallization state of ZTO thin films, the microstructure is examined through both grazing incidence X-ray diffraction (GIXRD) and Raman measurements. As shown in Fig. 1 (a), no distinct sharp diffraction peaks are observed over the measured 2θ range for the ZTO-based thin films. Instead, only a broad diffuse scattering peak located around 2θ = 30°-35° is visible, which is a classic hallmark of an amorphous state. It reveals that the incorporation of RE dopants (La, Pr, and Nd) into the ZTO does not induce any crystalline phase transformation or nanocluster precipitation [13]. The long-range structural disorder is highly advantageous for TFT applications because it prevents carrier scattering at grain boundaries and ensures excellent uniformity for the integration of large-area production. In order to further probe the short-range atomic arrangements and verify the absence of subtle phase separations, Raman spectroscopy is employed. It can be seen from Fig. 1 (b) that all thin films display strikingly similar Raman scattering profiles. The intense and sharp peaks centered at approximately 300 cm -1 and 520 cm -1 originate from the underlying silicon substrate [23]. Notably, the spectra of the La: ZTO, Pr: ZTO, and Nd: ZTO thin films exhibit a conspicuous absence of any distinct vibrational modes that are typically associated with crystalline rare-earth oxides or phase-separated ZnO/SnO 2 hexagonal crystal clusters [24]. The structural congruence between the Raman spectra of the RE-doped thin films and the pristine ZTO film corroborates the GIXRD findings. Collectively, these microstructural characterizations confirm that the RE elements are homogeneously incorporated into the amorphous zinc-tin-oxygen network. The atomic force microscopy (AFM) characterizations are conducted to further evaluate the influence of RE doping on the surface topography and interface quality for ZTO-based thin films. The pristine ZTO thin film exhibits an exceptionally smooth surface with a root-mean-square (RMS) roughness of merely 0.14 nm, with a scan area of 2 µm × 2 µm, as shown in Fig. 2 (a). Following the introduction of La and Pr, the RMS roughness values of the La: ZTO and Pr: ZTO films are maintained at ultralow levels of 0.18 nm and 0.16 nm, respectively. Such low surface roughness indicates that the appropriate incorporation of La and Pr does not disrupt the homogeneous polycondensation network during the sol-gel process, and it is beneficial for reducing trap state density at the GI/semiconductor interface, thus providing a favorable morphological basis for the electrical performance of the TFT devices [25]. Conversely, the RMS roughness of Nd: ZTO thin film abruptly increases to 0.73 nm, which is accompanied by pronounced local agglomerations and island-like topological fluctuations, as evidenced by the expanded height scale (3.0 nm) in Fig. 2 (d). As demonstrated by the preceding GIXRD and Raman analyses, the Nd: ZTO film is confirmed to be macroscopically amorphous. However, the slight microstructural degradation suggests that Nd doping may induce local lattice mismatch during the thermal annealing process [9]. Consequently, an exhaustive evaluation of the microstructural properties and surface topography has been demonstrated to reveal that La and Pr are comparatively superior dopant candidates, which can effectively protect the oxide amorphous network while maintaining high uniformity. Figure 3 presents the Fourier-transform infrared (FTIR) spectra, which can elucidate the chemical evolution and the efficiency of precursor decomposition within the sol-gel derived ZTO-based thin films. As illustrated in the absorbance spectra, the pristine ZTO, La: ZTO, and Pr: ZTO thin films exhibit relatively baseline-like profiles with minimal absorbance in the organic and hydroxyl regions, which indicate a high degree of conversion from precursors to metal-oxide-metal (M-O-M) networks. In contrast, the Nd: ZTO thin film displays anomalously intense and discrete absorption bands. Specifically, the prominent absorbance peaks at 3500–3800 cm -1 and approximately 2000 cm -1 represent the stretching and bending vibrations of residual hydroxyl (O-H) groups, respectively. The strong bands centered around 2900 cm -1 correspond to C–H stretching, while the complex peaks in the 1300–1700 cm⁻¹ region are attributed to the overlapping signals of C = O stretching and C–H bending from residual organic ligands [26, 27]. The trapped organic residues and hydroxyl groups are in agreement with the morphological degradation observed in the AFM analysis, which may originate from incomplete dehydroxylation process for Nd: ZTO thin film. To further investigate the elemental composition and chemical-state evolution of ZTO and RE: ZTO thin films, X-ray photoelectron spectroscopy (XPS) is employed. Figure 4 presents the high-resolution O 1s XPS spectra of the pristine ZTO and RE: ZTO thin films. All binding energies are calibrated using the adventitious carbon C 1s peak at 284.8 eV as a reference. The O 1s spectrum is deconvoluted into three peaks located at approximately 530.3 ± 0.1 eV, 531.3 ± 0.2 eV, and 532.2 ± 0.1 eV, respectively, which correspond to lattice oxygen bonded to metal cations (M–O), oxygen-vacancy-related species (Vo), and hydroxyl-related species (-OH), respectively [20, 21]. According to the fitting results, the relative area of the M–O component in La: ZTO, Pr: ZTO, and Nd: ZTO increased from 62.3% for pristine ZTO to 69.0%, 68.1%, and 64.0%, whereas the relative area of the oxygen-vacancy-related component decreased from 29.6% to 16.2%, 20.0%, and 23.8%, respectively. Among them, La doping can more effectively reduce oxygen-vacancy defects in ZTO thin films and enhance metal–oxygen bonding. Similar trends have also been reported previously for RE-doped oxide semiconductors [9–11, 13, 14]. In order to deeply investigate the influence of RE doping on the local chemical bonding states and defect configurations of the ZTO network, high-resolution Zn 2p₃/₂ and Sn 3d₅/₂ core-level XPS spectra are collected, as shown in Fig. 5 (a) and 5(b), respectively. For the pristine ZTO thin film, the binding energies of the Zn 2p 3/2 and Sn 3d 5/2 peaks are located at 1021.52 eV and 486.46 eV. Upon the incorporation of RE dopants, a distinct and consistent shift toward higher binding energies is observed across all doped samples. Fundamentally, RE elements (La, Pr, Nd) possess lower electronegativities compared to Zn and Sn. Consequently, the RE dopants act as strong Lewis acids that aggressively attract oxygen atoms to form highly stable RE-O bonds [16]. Specifically, the strongest La-O bonds can more easily extract electron density away from the adjacent Zn-O and Sn-O polyhedra, reducing the electron density around the Zn and Sn nuclei and thus increasing the core-level binding energies [16, 28]. Furthermore, in order to verify the energy shifts of Zn 2p and Sn 3d levels, it is necessary to consider the results are obtained from the corresponding Zn LMM and Sn MNN Auger spectra, which are more detailed. As shown in Fig. 5 (c), the Zn LMM spectra exhibit two components located at approximately 991 and 987–988 eV. The higher-kinetic-energy component can be assigned to electron-rich Zn δ+ species (δ < 2), which are associated with oxygen-deficient local Zn environments, whereas the lower-kinetic-energy component is mainly attributed to oxidized Zn 2+ species in Zn-O coordination [29]. In Fig. 5 (d), the Sn MNN spectra show a characteristic feature near 432 eV, corresponding to oxidized Sn species in Sn 4+ /Sn–O coordination. These consistent variations in energy profiles further confirm that the pervasive reconstruction of the local electronic states induced by the RE atoms, with La doping showing the most pronounced modulation effect. Figure 6 shows the transfer characteristics curves of the ZTO and RE: ZTO TFTs measured at V DS =10 V, and the extracted electrical parameters are summarized in Table 1 . Compared to the pristine ZTO TFT, the introduction of RE dopants induces a systematic positive shift in the threshold voltage (V th ), alongside a concomitant reduction in the maximum on-state current (I on ). The electrical evolution is highly consistent with the preceding XPS analyses, that the strong oxygen-binding capabilities of the RE dopants effectively suppress the formation of oxygen vacancies, thereby reducing the carrier concentration [30]. The pristine ZTO and La: ZTO TFTs exhibit steep subthreshold swing (SS), indicating a high-quality channel interface with a low subgap trap state density. In contrast, the Nd: ZTO and Pr: ZTO TFT display severely degraded subthreshold characteristics, which correlates with our microstructural and chemical findings. These severe structural and chemical defects act as dense carrier scattering centers and deep trap states, which severely impede Fermi level movement and degrade both the SS and mobility [31]. Conversely, the La dopant successfully achieves the crucial balance, that it effectively modulates the oxygen vacancy concentration to induce a positive V th shift without introducing extra structural or chemical traps. Table 1 Electrical parameters extracted from the transfer characteristics of pristine ZTO and RE: ZTO TFTs Device µ (cm 2 V -1 s -1 ) V th (V) SS (V/dec) I on /I off ZTO 1.95 0.74 0.49 1.20×10 8 La: ZTO 1.64 1.52 0.50 9.93×10 7 Pr: ZTO 1.12 2.12 0.84 1.64×10 7 Nd: ZTO 1.26 2.58 1.01 1.78×10 7 To evaluate the operational stability of the TFT devices under practical conditions, positive and negative bias illumination stress (PBIS and NBIS) tests are conducted at an illumination intensity of 10,000 Lux. The electron trapping at the channel/gate-insulator interface typically dominates the degradation under PBIS. As shown in Fig. 7 , the pristine ZTO TFT exhibits a positive V th shift (∆V th ) of 4.39 V after 1800 s, whereas La: ZTO and Nd: ZTO TFTs show improved shifts of 2.96 and 2.95 V, respectively. However, the most critical and universally challenging instability for oxide TFTs occurs under NBIS, that photo-generated holes and the structural relaxation of ionized oxygen vacancies shift to the gate-insulator, causing severe negative ∆V th . Predictably, the pristine ZTO TFT suffers a substantial NBIS degradation with ∆V th of -4.70 V. Notably, La doping significantly suppresses the degradation, yielding a minimal ∆V th of -2.99 V. The enhancement is fundamentally attributed to the ultra-strong La-O bond energy, which stabilizes the local oxygen network and minimizes the generation of photosensitive Vo defects. Conversely, the Nd: ZTO TFT exhibits a severe NBIS degradation (∆V th =-9.00 V), which directly correlates with our FTIR and AFM findings, the massive density of residual -OH groups and trapped organic clusters act as deep hole-trapping centers, drastically exacerbating the illumination-stress instability. Although La dopant maximizes N/PBIS stability, rare-earth doping inevitably suppresses the intrinsic carrier concentration, which can compromise the overall mobility and on-state current. To further resolve the mobility-stability trade-off, the ZTO/La: ZTO bilayer-channel TFT is fabricated, as shown in the inset of Fig. 8 (a). In this engineered structure, the pristine ZTO serves as the highly conductive front channel to ensure superior carrier transport, while the optimized La: ZTO acts as a back-channel passivation layer. As depicted in Fig. 8 (a), the transfer curve of the bilayer-channel device inherits the excellent switching characteristics of single-layer pristine ZTO TFT, maintaining a steep subthreshold swing and high on-state current. Specifically, the mobility increased by more than 2 times to 4.22 cm 2 V −1 s − 1 , with V th of 0.48 V, SS of 0.26 V/dec, and I on /I off of 6.48×10 8 . Crucially, as shown in Figs. 8 (b) and 8(c), compared to the single-layer pristine ZTO TFT, the ZTO/La: ZTO TFT shows significant improvement under both PBIS and NBIS, with respective ∆V th values of 3.37 V and − 3.55 V. The La: ZTO top layer has been shown to function as an exceptional physical and chemical barrier so that it not only passivates surface defect states, but also physically shields the active ZTO channel from ambient moisture and oxygen, which are known to trigger surface charge exchange during illumination [32]. Consequently, the RE-doped bilayer-channel strategy successfully decouples the inherent mobility-stability trade-off, providing a highly viable pathway for the fabrication of high-performance and illumination-induced-stable oxide TFTs. 4. CONCLUSIONS In summary, this study presents a highly effective approach that integrates RE doping with bilayer-channel structure in modulating the electrical performance and illumination-stress stability of solution-processed ZTO TFTs. The comprehensive microstructural and chemical characterizations confirm that while all investigated RE dopants preserve the amorphous states, La incorporation specifically facilitates optimal polycondensation, yielding a dense, hydroxyl-minimized, and ultra-smooth surface. Furthermore, XPS analyses reveal that the RE dopants act as powerful oxygen binders, profoundly modifying the local metal–oxygen coordination environment and effectively suppressing oxygen-vacancy related defects. Consequently, while the suppression of donor states inevitably alters carrier transport, the single-layer La: ZTO TFT demonstrates an optimal balance, exhibiting significantly improved under NBIS due to the mitigation of photosensitive traps. To further overcome the inherent carrier suppression of RE doping, a functional ZTO/La: ZTO bilayer-channel TFT is engineered. The structure successfully leverages the highly conductive pristine ZTO underlayer for superior charge transport, while utilizing the dense La: ZTO capping layer for realizing defect passivation and environmental protection. The resulting bilayer-channel TFT achieves a mobility of 4.22 cm 2 V −1 s − 1 , a near-zero threshold voltage of 0.48 V, a steep subthreshold swing of 0.26 V/dec, and a high on/off current ratio of 6.48×10 8 , alongside markedly improved PBIS and NBIS stability. Ultimately, this work demonstrates that RE-doped defect regulation, when integrated with a bilayer-channel design, provides a highly viable and scalable strategy for developing high-performance, photo-stable oxide semiconductor electronics for next-generation displays. Declarations CONFLICTS OF INTEREST The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work is supported by the Ministry of Science and Technology of China (2023YFB3609004), the National Natural Science Foundation of China (U25A20481), and the Science and Technology Commission of Shanghai Municipality (25CL2901203). AUTHOR CONTRIBUTION Hongxu Cui: Writing-original draft, Methodology. Meng Xu: Writing-original draft, Methodology, Formal analysis, Funding acquisition. Wentao Luo: Investigation, Resources. Huajie Li: Investigation, Resources. Longlong Chen: Formal analysis, Resources. Cong Peng: Formal analysis, Resources. Xifeng Li: Conceptualization, Writing-review and editing, Supervision, Funding acquisition. Jianhua Zhang: Supervision. All authors have read and agreed to the published version of the manuscript. 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Mattson, K. T. Vogt, J. F. Wager, M. W. Graham, Adv. Funct. Mater. 33, 2300742 (2023), https://doi.org/10.1002/adfm.202300742 L. Liang, H. Zhang, T. Li, W. Li, J. Gao, H. Zhang, M. Guo, S. Gao, Z. He, F. Liu, C. Ning, H. Cao, G. Yuan, C. Liu, Adv. Sci. 10, e2300373 (2023), https://doi.org/10.1002/advs.202300373 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9586879","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":637067357,"identity":"479c4a7b-2419-4ff8-9919-c3fcf5115e40","order_by":0,"name":"Hongxu Cui","email":"","orcid":"","institution":"Shanghai University","correspondingAuthor":false,"prefix":"","firstName":"Hongxu","middleName":"","lastName":"Cui","suffix":""},{"id":637067358,"identity":"a79f75ac-8754-4c68-9d94-1b4a0281197b","order_by":1,"name":"Meng Xu","email":"","orcid":"","institution":"Shanghai University","correspondingAuthor":false,"prefix":"","firstName":"Meng","middleName":"","lastName":"Xu","suffix":""},{"id":637067359,"identity":"edb644ba-e711-4661-a411-ded0f4aad3d0","order_by":2,"name":"Wentao Luo","email":"","orcid":"","institution":"Shanghai University","correspondingAuthor":false,"prefix":"","firstName":"Wentao","middleName":"","lastName":"Luo","suffix":""},{"id":637067360,"identity":"021141be-3a01-49ef-8793-6240b70ddcce","order_by":3,"name":"Huajie Li","email":"","orcid":"","institution":"Shanghai University","correspondingAuthor":false,"prefix":"","firstName":"Huajie","middleName":"","lastName":"Li","suffix":""},{"id":637067361,"identity":"f3f95bd4-8076-49e9-9351-b9ec6cb5e0b2","order_by":4,"name":"Longlong Chen","email":"","orcid":"","institution":"Shanghai University","correspondingAuthor":false,"prefix":"","firstName":"Longlong","middleName":"","lastName":"Chen","suffix":""},{"id":637067362,"identity":"a0bc02ab-6230-4d67-8a35-62bb352cf993","order_by":5,"name":"Cong Peng","email":"","orcid":"","institution":"Shanghai University","correspondingAuthor":false,"prefix":"","firstName":"Cong","middleName":"","lastName":"Peng","suffix":""},{"id":637067363,"identity":"e1479519-372c-4b4f-b9be-ee55e9c5c624","order_by":6,"name":"Xifeng Li","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA00lEQVRIie3RMQqDMBSA4SeCLqGu6dA7KIIUBL1KRKiLODs4ZHL1MIXOTwJOka4OnTvbC0gNtKtmLDT/8EggHyQEwGT6wQ42ANbrwgOi9vY+cRSR6+LItYkaivioTVwSINaP5HofBwp1nHF3xJ2LOQxRPvPbVF0oyCLjpGI7xEbxakUeTSSiVisyTom/QyyO/SLysJMrWbSIjdhzkfhQroRrEfWWQTA6leGZDUXYknKbeJ4MZ2xE6nUymOYmPnWu3CbfMq4mg89P6ZTqHjSZTKY/7A1fzUUA7P63dAAAAABJRU5ErkJggg==","orcid":"","institution":"Shanghai University","correspondingAuthor":true,"prefix":"","firstName":"Xifeng","middleName":"","lastName":"Li","suffix":""},{"id":637067364,"identity":"644785c4-b208-4dd8-848e-2a609de3bac7","order_by":7,"name":"Jianhua Zhang","email":"","orcid":"","institution":"Shanghai University","correspondingAuthor":false,"prefix":"","firstName":"Jianhua","middleName":"","lastName":"Zhang","suffix":""}],"badges":[],"createdAt":"2026-05-01 14:23:24","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9586879/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9586879/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":109205251,"identity":"775c54e3-12e9-4c4e-97b6-feaa7c550aec","added_by":"auto","created_at":"2026-05-13 15:03:54","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":227797,"visible":true,"origin":"","legend":"\u003cp\u003e(a) GIXRD patterns, and (b) Raman spectra of pristine and RE: ZTO thin films\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-9586879/v1/7a16d8f807739e5d1c0e31cc.png"},{"id":109206002,"identity":"b25d9010-4628-44aa-8114-700a9f93c071","added_by":"auto","created_at":"2026-05-13 15:10:26","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":533448,"visible":true,"origin":"","legend":"\u003cp\u003eHeight AFM images (2 μm×2 μm) of (a) ZTO, (b) La: ZTO, (c) Pr: ZTO, and (d) Nd: ZTO thin films\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-9586879/v1/ba3c1c9a1eb2520e54094d54.png"},{"id":109134364,"identity":"dd28ed9d-10e6-4ca8-b874-e1ba61642de4","added_by":"auto","created_at":"2026-05-12 23:18:43","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":272597,"visible":true,"origin":"","legend":"\u003cp\u003eFTIR spectra of pristine and RE: ZTO thin films\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-9586879/v1/2f8b9b4e7e9e137739c715c7.png"},{"id":109134366,"identity":"57b3454a-24bd-462f-9f7b-8316aa6e8743","added_by":"auto","created_at":"2026-05-12 23:18:43","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":280291,"visible":true,"origin":"","legend":"\u003cp\u003eO 1s XPS spectra of (a) ZTO, (b) La: ZTO, (c) Pr: ZTO, and (d) Nd: ZTO thin films\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-9586879/v1/ca2e43101b5cd5c49aefeed8.png"},{"id":109205094,"identity":"8c5ac769-0dc0-45cf-a252-0ed2a0705ed8","added_by":"auto","created_at":"2026-05-13 15:03:20","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":313030,"visible":true,"origin":"","legend":"\u003cp\u003eHigh-resolution XPS spectra of (a) Zn 2p₃/₂ and (b) Sn 3d₅/₂, and Auger spectra of (c) Zn LMM and (d) Sn MNN for pristine ZTO and RE: ZTO thin films\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-9586879/v1/94f649d0c9bf525e49512f8b.png"},{"id":109204978,"identity":"127e4b0e-ea0d-4c74-be54-4c58fe0243e9","added_by":"auto","created_at":"2026-05-13 15:03:04","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":142737,"visible":true,"origin":"","legend":"\u003cp\u003eThe transfer characteristics of single-layer pristine ZTO and RE: ZTO TFTs\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-9586879/v1/576e17807b9c910ffe87853e.png"},{"id":109204981,"identity":"9afed77d-3162-4792-9c51-b78045779fb2","added_by":"auto","created_at":"2026-05-13 15:03:04","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":471486,"visible":true,"origin":"","legend":"\u003cp\u003eThe PBIS and NBIS stability of (a) ZTO, (b) La: ZTO, (c) Pr: ZTO, and (d) Nd: ZTO TFTs\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-9586879/v1/58103ae5da8776f1f700f36d.png"},{"id":109134370,"identity":"dc3897dd-2ef4-4d3f-bf0e-f6e6307f60cb","added_by":"auto","created_at":"2026-05-12 23:18:43","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":253808,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The transfer characteristics, and (b) PBIS stability, (c) NBIS stability of the ZTO/La: ZTO bilayer-channel TFT\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-9586879/v1/f699d2fcb59812f2f8722c91.png"},{"id":109207322,"identity":"5740fbb4-4a83-4b5d-89b0-aa1231fe8f25","added_by":"auto","created_at":"2026-05-13 15:19:21","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2621394,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9586879/v1/d6dfa402-b27f-419c-a2a4-e3ddc99c8a2e.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Interface Regulation and Defect Passivation in Rare-Earth Doped Bilayer-Channel Thin Film Transistors for Enhanced Illumination Stability","fulltext":[{"header":"1. INTRODUCTION","content":"\u003cp\u003eThe distinctive electronic properties of oxide semiconductors originate from the (n-1)d\u0026sup1;⁰ns⁰ electron configuration of constituent metal cations, which leads to the formation of a conduction band characterized by spherical s-orbitals [1]. The significant overlap of s-orbitals establishes efficient percolation pathways for electron transport while minimizing carrier scattering, thereby endowing oxide thin film transistors (TFTs) with high mobility and low off-state current [2, 3]. Consequently, oxide TFTs have been extensively adopted as the backplane technology in various displays [4]. More importantly, TFTs are inevitably exposed to ambient light, backlight, or self-emission during practical operation, which imposes stringent demands on their photostability [5]. A critical reliability challenge in this context is the instability under negative bias illumination stress (NBIS), which manifests as threshold voltage shift and subthreshold degradation [6]. These instabilities are mainly attributed to photo-induced ionization of oxygen-vacancy (V\u003csub\u003eO\u003c/sub\u003e)-related states and the subsequent separation/trapping of photogenerated carriers under a negative gate bias via band bending. The transition from the neutral to the ionized V\u003csub\u003eO\u003c/sub\u003e\u003csup\u003e2+\u003c/sup\u003e is widely regarded as the fundamental cause of the persistent NBIS degradation [5, 6]. Therefore, developing effective strategies to mitigate such effects is of considerable significance, as it also provides deeper insights into material properties and potential applications of oxide semiconductor TFTs.\u003c/p\u003e \u003cp\u003eDoping is a widely adopted material design strategy to enhance the stability of oxide TFTs, favored for its relatively simple, cost-effective fabrication and broad compatibility with diverse material systems [7, 8]. Among various dopants, rare-earth (RE) elements can suppress excess carriers through their ultra-low electronegativity and can also act as an efficient photoconversion medium due to their low charge-transfer energy [9, 10]. These effects have motivated the use of RE dopants to tailor the defect chemistry and illumination bias behavior of oxide semiconductors, although the resulting stability improvement remains material- and structure-dependent. So far, numerous studies have explored RE doping for TFT stabilization [9\u0026ndash;14]. Wu et al. [15] demonstrated that Er doping in In₂O₃ effectively suppresses oxygen-vacancy\u0026ndash;related defect states and reduces interface trap densities, thereby improving bias-stress stability with mitigated threshold-voltage shifts under PBS and NBS. This suppression is often attributed to the strong Er-O bonding, which stabilizes the local oxygen coordination and makes oxygen-vacancy formation energetically less favorable [16]. However, the mobility tends to decrease with increasing Er content due to carrier suppression. Therefore, further investigations are still required to improve device stability while preserving favorable carrier transport. Fundamentally, RE-doped semiconductors reshape the defect landscape and carrier transport properties from a perspective of materials engineering. This intrinsic feature allows them to be synergistically combined with other optimization strategies (e.g., interface band engineering) to further refine overall performance of devices [17\u0026ndash;19]. Therefore, this work integrates RE doping with a bilayer channel design to comprehensively enhance the performance of zinc-tin oxide (ZTO) TFTs. While solution processing offers a promising route for low-cost, large-area fabrication of oxide TFTs, the thin films derived from this method often face inherent limitations. During the sol-gel conversion of the precursors, residual hydroxyl groups and incompletely condensed metal-oxygen networks may remain in the thin films [20, 21]. In addition, pores arising from solvent evaporation hinder full densification [22]. These structural imperfections collectively result in a high density of defect states, which exacerbates device instability. To this end, we first evaluate the electrical performance and stress stability of ZTO TFTs doped with different RE elements (La, Pr, and Nd). Following identification of the optimal dopant, a ZTO/RE: ZTO bilayer-channel TFT is fabricated. In this engineered structure, the bottom ZTO layer is designed to provide efficient carrier transport, whereas the top RE: ZTO layer functions as a passivation and defect-suppression capping layer [17\u0026ndash;19].\u003c/p\u003e"},{"header":"2. EXPERIMENTAL SECTION","content":"\u003cp\u003eThe precursor solutions for pristine ZTO (Zn: Sn\u0026thinsp;=\u0026thinsp;7: 3) and rare-earth-doped ZTO (RE: ZTO, where RE\u0026thinsp;=\u0026thinsp;La, Pr, or Nd; RE: Zn: Sn\u0026thinsp;=\u0026thinsp;1: 70: 30) are synthesized using zinc acetate hydrate (Zn(CH\u003csub\u003e3\u003c/sub\u003eCOO)\u003csub\u003e2\u003c/sub\u003e\u0026middot;xH\u003csub\u003e2\u003c/sub\u003eO), tin chloride hydrate (SnCl\u003csub\u003e4\u003c/sub\u003e\u0026middot;xH\u003csub\u003e2\u003c/sub\u003eO) and RE-nitrate hydrate (RE(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e\u0026middot;xH\u003csub\u003e2\u003c/sub\u003eO) as the primary metal sources. These metal precursors are dissolved in 2-methoxyethanol to yield a total concentration of 0.3 M. Then the solutions are magnetically stirred at 70\u0026deg;C for 3 h and aged at room temperature for 24 h. The TFT devices are fabricated on 4-inch quartz glass substrates. First, 35-nm-thick ITO source/drain electrodes are deposited via PVD and patterned by heated oxalic acid. Then the ZTO-based solutions are spin-coated at 3000 rpm for 30 s to obtain a 20-nm-thick single-layer channel. In contrast, bilayer channels are spin-coated at 4000 rpm for 30 s to obtain a thickness of 10 nm for each layer. Next, the thin films are post-annealed at 500\u0026deg;C for 1 h. Subsequently, a 150-nm-thick SiO\u003csub\u003e2\u003c/sub\u003e gate-insulator layer is deposited via PECVD. Finally, a 35-nm-thick ITO thin film is deposited and patterned as the gate electrode to complete the top-gate ZTO-based TFTs.\u003c/p\u003e"},{"header":"3. RESULTS AND DISCUSSION","content":"\u003cp\u003eTo determine whether RE doping changes the crystallization state of ZTO thin films, the microstructure is examined through both grazing incidence X-ray diffraction (GIXRD) and Raman measurements. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a), no distinct sharp diffraction peaks are observed over the measured 2θ range for the ZTO-based thin films. Instead, only a broad diffuse scattering peak located around 2θ\u0026thinsp;=\u0026thinsp;30\u0026deg;-35\u0026deg; is visible, which is a classic hallmark of an amorphous state. It reveals that the incorporation of RE dopants (La, Pr, and Nd) into the ZTO does not induce any crystalline phase transformation or nanocluster precipitation [13]. The long-range structural disorder is highly advantageous for TFT applications because it prevents carrier scattering at grain boundaries and ensures excellent uniformity for the integration of large-area production.\u003c/p\u003e \u003cp\u003eIn order to further probe the short-range atomic arrangements and verify the absence of subtle phase separations, Raman spectroscopy is employed. It can be seen from Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b) that all thin films display strikingly similar Raman scattering profiles. The intense and sharp peaks centered at approximately 300 cm\u003csup\u003e-1\u003c/sup\u003e and 520 cm\u003csup\u003e-1\u003c/sup\u003e originate from the underlying silicon substrate [23]. Notably, the spectra of the La: ZTO, Pr: ZTO, and Nd: ZTO thin films exhibit a conspicuous absence of any distinct vibrational modes that are typically associated with crystalline rare-earth oxides or phase-separated ZnO/SnO\u003csub\u003e2\u003c/sub\u003e hexagonal crystal clusters [24]. The structural congruence between the Raman spectra of the RE-doped thin films and the pristine ZTO film corroborates the GIXRD findings. Collectively, these microstructural characterizations confirm that the RE elements are homogeneously incorporated into the amorphous zinc-tin-oxygen network.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe atomic force microscopy (AFM) characterizations are conducted to further evaluate the influence of RE doping on the surface topography and interface quality for ZTO-based thin films. The pristine ZTO thin film exhibits an exceptionally smooth surface with a root-mean-square (RMS) roughness of merely 0.14 nm, with a scan area of 2 \u0026micro;m \u0026times; 2 \u0026micro;m, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a). Following the introduction of La and Pr, the RMS roughness values of the La: ZTO and Pr: ZTO films are maintained at ultralow levels of 0.18 nm and 0.16 nm, respectively. Such low surface roughness indicates that the appropriate incorporation of La and Pr does not disrupt the homogeneous polycondensation network during the sol-gel process, and it is beneficial for reducing trap state density at the GI/semiconductor interface, thus providing a favorable morphological basis for the electrical performance of the TFT devices [25]. Conversely, the RMS roughness of Nd: ZTO thin film abruptly increases to 0.73 nm, which is accompanied by pronounced local agglomerations and island-like topological fluctuations, as evidenced by the expanded height scale (3.0 nm) in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(d). As demonstrated by the preceding GIXRD and Raman analyses, the Nd: ZTO film is confirmed to be macroscopically amorphous. However, the slight microstructural degradation suggests that Nd doping may induce local lattice mismatch during the thermal annealing process [9]. Consequently, an exhaustive evaluation of the microstructural properties and surface topography has been demonstrated to reveal that La and Pr are comparatively superior dopant candidates, which can effectively protect the oxide amorphous network while maintaining high uniformity.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e presents the Fourier-transform infrared (FTIR) spectra, which can elucidate the chemical evolution and the efficiency of precursor decomposition within the sol-gel derived ZTO-based thin films. As illustrated in the absorbance spectra, the pristine ZTO, La: ZTO, and Pr: ZTO thin films exhibit relatively baseline-like profiles with minimal absorbance in the organic and hydroxyl regions, which indicate a high degree of conversion from precursors to metal-oxide-metal (M-O-M) networks. In contrast, the Nd: ZTO thin film displays anomalously intense and discrete absorption bands. Specifically, the prominent absorbance peaks at 3500\u0026ndash;3800 cm\u003csup\u003e-1\u003c/sup\u003e and approximately 2000 cm\u003csup\u003e-1\u003c/sup\u003e represent the stretching and bending vibrations of residual hydroxyl (O-H) groups, respectively. The strong bands centered around 2900 cm\u003csup\u003e-1\u003c/sup\u003e correspond to C\u0026ndash;H stretching, while the complex peaks in the 1300\u0026ndash;1700 cm⁻\u0026sup1; region are attributed to the overlapping signals of C\u0026thinsp;=\u0026thinsp;O stretching and C\u0026ndash;H bending from residual organic ligands [26, 27]. The trapped organic residues and hydroxyl groups are in agreement with the morphological degradation observed in the AFM analysis, which may originate from incomplete dehydroxylation process for Nd: ZTO thin film.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo further investigate the elemental composition and chemical-state evolution of ZTO and RE: ZTO thin films, X-ray photoelectron spectroscopy (XPS) is employed. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e presents the high-resolution O 1s XPS spectra of the pristine ZTO and RE: ZTO thin films. All binding energies are calibrated using the adventitious carbon C 1s peak at 284.8 eV as a reference. The O 1s spectrum is deconvoluted into three peaks located at approximately 530.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 eV, 531.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2 eV, and 532.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 eV, respectively, which correspond to lattice oxygen bonded to metal cations (M\u0026ndash;O), oxygen-vacancy-related species (Vo), and hydroxyl-related species (-OH), respectively [20, 21]. According to the fitting results, the relative area of the M\u0026ndash;O component in La: ZTO, Pr: ZTO, and Nd: ZTO increased from 62.3% for pristine ZTO to 69.0%, 68.1%, and 64.0%, whereas the relative area of the oxygen-vacancy-related component decreased from 29.6% to 16.2%, 20.0%, and 23.8%, respectively. Among them, La doping can more effectively reduce oxygen-vacancy defects in ZTO thin films and enhance metal\u0026ndash;oxygen bonding. Similar trends have also been reported previously for RE-doped oxide semiconductors [9\u0026ndash;11, 13, 14].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn order to deeply investigate the influence of RE doping on the local chemical bonding states and defect configurations of the ZTO network, high-resolution Zn 2p₃/₂ and Sn 3d₅/₂ core-level XPS spectra are collected, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a) and 5(b), respectively. For the pristine ZTO thin film, the binding energies of the Zn 2p\u003csub\u003e3/2\u003c/sub\u003e and Sn 3d\u003csub\u003e5/2\u003c/sub\u003e peaks are located at 1021.52 eV and 486.46 eV. Upon the incorporation of RE dopants, a distinct and consistent shift toward higher binding energies is observed across all doped samples. Fundamentally, RE elements (La, Pr, Nd) possess lower electronegativities compared to Zn and Sn. Consequently, the RE dopants act as strong Lewis acids that aggressively attract oxygen atoms to form highly stable RE-O bonds [16]. Specifically, the strongest La-O bonds can more easily extract electron density away from the adjacent Zn-O and Sn-O polyhedra, reducing the electron density around the Zn and Sn nuclei and thus increasing the core-level binding energies [16, 28]. Furthermore, in order to verify the energy shifts of Zn 2p and Sn 3d levels, it is necessary to consider the results are obtained from the corresponding Zn LMM and Sn MNN Auger spectra, which are more detailed. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(c), the Zn LMM spectra exhibit two components located at approximately 991 and 987\u0026ndash;988 eV. The higher-kinetic-energy component can be assigned to electron-rich Zn\u003csup\u003eδ+\u003c/sup\u003e species (δ\u0026thinsp;\u0026lt;\u0026thinsp;2), which are associated with oxygen-deficient local Zn environments, whereas the lower-kinetic-energy component is mainly attributed to oxidized Zn\u003csup\u003e2+\u003c/sup\u003e species in Zn-O coordination [29]. In Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(d), the Sn MNN spectra show a characteristic feature near 432 eV, corresponding to oxidized Sn species in Sn\u003csup\u003e4+\u003c/sup\u003e/Sn\u0026ndash;O coordination. These consistent variations in energy profiles further confirm that the pervasive reconstruction of the local electronic states induced by the RE atoms, with La doping showing the most pronounced modulation effect.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the transfer characteristics curves of the ZTO and RE: ZTO TFTs measured at V\u003csub\u003eDS\u003c/sub\u003e=10 V, and the extracted electrical parameters are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Compared to the pristine ZTO TFT, the introduction of RE dopants induces a systematic positive shift in the threshold voltage (V\u003csub\u003eth\u003c/sub\u003e), alongside a concomitant reduction in the maximum on-state current (I\u003csub\u003eon\u003c/sub\u003e). The electrical evolution is highly consistent with the preceding XPS analyses, that the strong oxygen-binding capabilities of the RE dopants effectively suppress the formation of oxygen vacancies, thereby reducing the carrier concentration [30]. The pristine ZTO and La: ZTO TFTs exhibit steep subthreshold swing (SS), indicating a high-quality channel interface with a low subgap trap state density. In contrast, the Nd: ZTO and Pr: ZTO TFT display severely degraded subthreshold characteristics, which correlates with our microstructural and chemical findings. These severe structural and chemical defects act as dense carrier scattering centers and deep trap states, which severely impede Fermi level movement and degrade both the SS and mobility [31]. Conversely, the La dopant successfully achieves the crucial balance, that it effectively modulates the oxygen vacancy concentration to induce a positive V\u003csub\u003eth\u003c/sub\u003e shift without introducing extra structural or chemical traps.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eElectrical parameters extracted from the transfer characteristics of pristine ZTO and RE: ZTO TFTs\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026times;\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDevice\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026micro; (cm\u003csup\u003e2\u003c/sup\u003eV\u003csup\u003e-1\u003c/sup\u003es\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eV\u003csub\u003eth\u003c/sub\u003e (V)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSS (V/dec)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eI\u003csub\u003eon\u003c/sub\u003e/I\u003csub\u003eoff\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eZTO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.95\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.74\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.49\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c5\"\u003e \u003cp\u003e1.20\u0026times;10\u003csup\u003e8\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLa: ZTO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.64\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c5\"\u003e \u003cp\u003e9.93\u0026times;10\u003csup\u003e7\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePr: ZTO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.84\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c5\"\u003e \u003cp\u003e1.64\u0026times;10\u003csup\u003e7\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNd: ZTO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.58\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c5\"\u003e \u003cp\u003e1.78\u0026times;10\u003csup\u003e7\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eTo evaluate the operational stability of the TFT devices under practical conditions, positive and negative bias illumination stress (PBIS and NBIS) tests are conducted at an illumination intensity of 10,000 Lux. The electron trapping at the channel/gate-insulator interface typically dominates the degradation under PBIS. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, the pristine ZTO TFT exhibits a positive V\u003csub\u003eth\u003c/sub\u003e shift (∆V\u003csub\u003eth\u003c/sub\u003e) of 4.39 V after 1800 s, whereas La: ZTO and Nd: ZTO TFTs show improved shifts of 2.96 and 2.95 V, respectively. However, the most critical and universally challenging instability for oxide TFTs occurs under NBIS, that photo-generated holes and the structural relaxation of ionized oxygen vacancies shift to the gate-insulator, causing severe negative ∆V\u003csub\u003eth\u003c/sub\u003e. Predictably, the pristine ZTO TFT suffers a substantial NBIS degradation with ∆V\u003csub\u003eth\u003c/sub\u003e of -4.70 V. Notably, La doping significantly suppresses the degradation, yielding a minimal ∆V\u003csub\u003eth\u003c/sub\u003e of -2.99 V. The enhancement is fundamentally attributed to the ultra-strong La-O bond energy, which stabilizes the local oxygen network and minimizes the generation of photosensitive Vo defects. Conversely, the Nd: ZTO TFT exhibits a severe NBIS degradation (∆V\u003csub\u003eth\u003c/sub\u003e=-9.00 V), which directly correlates with our FTIR and AFM findings, the massive density of residual -OH groups and trapped organic clusters act as deep hole-trapping centers, drastically exacerbating the illumination-stress instability.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAlthough La dopant maximizes N/PBIS stability, rare-earth doping inevitably suppresses the intrinsic carrier concentration, which can compromise the overall mobility and on-state current. To further resolve the mobility-stability trade-off, the ZTO/La: ZTO bilayer-channel TFT is fabricated, as shown in the inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(a). In this engineered structure, the pristine ZTO serves as the highly conductive front channel to ensure superior carrier transport, while the optimized La: ZTO acts as a back-channel passivation layer. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(a), the transfer curve of the bilayer-channel device inherits the excellent switching characteristics of single-layer pristine ZTO TFT, maintaining a steep subthreshold swing and high on-state current. Specifically, the mobility increased by more than 2 times to 4.22 cm\u003csup\u003e2\u003c/sup\u003eV\u003csup\u003e\u0026minus;1\u003c/sup\u003es\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, with V\u003csub\u003eth\u003c/sub\u003e of 0.48 V, SS of 0.26 V/dec, and I\u003csub\u003eon\u003c/sub\u003e/I\u003csub\u003eoff\u003c/sub\u003e of 6.48\u0026times;10\u003csup\u003e8\u003c/sup\u003e. Crucially, as shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(b) and 8(c), compared to the single-layer pristine ZTO TFT, the ZTO/La: ZTO TFT shows significant improvement under both PBIS and NBIS, with respective ∆V\u003csub\u003eth\u003c/sub\u003e values of 3.37 V and \u0026minus;\u0026thinsp;3.55 V. The La: ZTO top layer has been shown to function as an exceptional physical and chemical barrier so that it not only passivates surface defect states, but also physically shields the active ZTO channel from ambient moisture and oxygen, which are known to trigger surface charge exchange during illumination [32]. Consequently, the RE-doped bilayer-channel strategy successfully decouples the inherent mobility-stability trade-off, providing a highly viable pathway for the fabrication of high-performance and illumination-induced-stable oxide TFTs.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"4. CONCLUSIONS","content":"\u003cp\u003eIn summary, this study presents a highly effective approach that integrates RE doping with bilayer-channel structure in modulating the electrical performance and illumination-stress stability of solution-processed ZTO TFTs. The comprehensive microstructural and chemical characterizations confirm that while all investigated RE dopants preserve the amorphous states, La incorporation specifically facilitates optimal polycondensation, yielding a dense, hydroxyl-minimized, and ultra-smooth surface. Furthermore, XPS analyses reveal that the RE dopants act as powerful oxygen binders, profoundly modifying the local metal\u0026ndash;oxygen coordination environment and effectively suppressing oxygen-vacancy related defects. Consequently, while the suppression of donor states inevitably alters carrier transport, the single-layer La: ZTO TFT demonstrates an optimal balance, exhibiting significantly improved under NBIS due to the mitigation of photosensitive traps. To further overcome the inherent carrier suppression of RE doping, a functional ZTO/La: ZTO bilayer-channel TFT is engineered. The structure successfully leverages the highly conductive pristine ZTO underlayer for superior charge transport, while utilizing the dense La: ZTO capping layer for realizing defect passivation and environmental protection. The resulting bilayer-channel TFT achieves a mobility of 4.22 cm\u003csup\u003e2\u003c/sup\u003eV\u003csup\u003e\u0026minus;1\u003c/sup\u003es\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, a near-zero threshold voltage of 0.48 V, a steep subthreshold swing of 0.26 V/dec, and a high on/off current ratio of 6.48\u0026times;10\u003csup\u003e8\u003c/sup\u003e, alongside markedly improved PBIS and NBIS stability. Ultimately, this work demonstrates that RE-doped defect regulation, when integrated with a bilayer-channel design, provides a highly viable and scalable strategy for developing high-performance, photo-stable oxide semiconductor electronics for next-generation displays.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eCONFLICTS OF INTEREST\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing financial interest.\u003c/p\u003e\n\u003cp\u003eACKNOWLEDGEMENTS\u003c/p\u003e\n\u003cp\u003eThis work is supported by the Ministry of Science and Technology of China (2023YFB3609004), the National Natural Science Foundation of China (U25A20481), and the Science and Technology Commission of Shanghai Municipality (25CL2901203).\u003c/p\u003e\n\u003cp\u003eAUTHOR CONTRIBUTION\u003c/p\u003e\n\u003cp\u003eHongxu Cui: Writing-original draft, Methodology. Meng Xu: Writing-original draft, Methodology, Formal analysis, Funding acquisition. Wentao Luo: Investigation, Resources. Huajie Li: Investigation, Resources. Longlong Chen: Formal analysis, Resources. Cong Peng: Formal analysis, Resources. Xifeng Li: Conceptualization, Writing-review and editing, Supervision, Funding acquisition. Jianhua Zhang: Supervision. All authors have read and agreed to the published version of the manuscript.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;FUNDING\u003c/p\u003e\n\u003cp\u003eThis work is supported by the Ministry of Science and Technology of China (2023YFB3609004), the National Natural Science Foundation of China (U25A20481), and the Science and Technology Commission of Shanghai Municipality (25CL2901203).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eA. Takagi, K. Nomura, H. Ohta, H. Yanagi, T. Kamiya, M. Hirano, H. Hosono, Thin Solid Films 486, 38-41 (2005), https://doi.org/10.1016/j.tsf.2004.11.223\u003c/li\u003e\n\u003cli\u003eK. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, H. 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Sci. 10, e2300373 (2023), https://doi.org/10.1002/advs.202300373\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Thin Film Transistors, Rare-Earth Doping, Illumination-Stress Stability, Interface Regulation, Bilayer Channel","lastPublishedDoi":"10.21203/rs.3.rs-9586879/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9586879/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe illumination-stress instability remains a critical bottleneck for amorphous oxide thin film transistors (TFTs). Here, we propose a rare-earth (RE) doped bilayer-channel structure to effectively enhance mobility while minimizing the deterioration of stability in solution-processed zinc-tin oxide (ZTO) TFTs. The microstructural analyses reveal that while RE dopants suppress oxygen vacancies, Nd retards sol-gel polycondensation, inducing residual hydroxyl traps and morphological degradation. Conversely, La facilitates thorough dehydroxylation, yielding an ultra-smooth defect-minimized network. Consequently, the La: ZTO TFT demonstrates enhanced NBIS stability compared to pristine ZTO. To overcome carrier suppression from RE doping, the bilayer-channel TFT is fabricated, in which the highly conductive ZTO underlayer ensures efficient transport, while the dense La: ZTO capping layer acts as a barrier to passivate interfacial defects and shield against ambient interactions. The ZTO/La: ZTO TFT exhibits excellent switching performance, while the mobility is more than 2 times that of single-layer devices, and small N/PBIS V\u003csub\u003eth\u003c/sub\u003e shifts (-3.55 /3.37 V). The RE-doped interface regulation strategy provides a highly viable pathway for fabricating stable, high-performance oxide semiconductor electronics.\u003c/p\u003e","manuscriptTitle":"Interface Regulation and Defect Passivation in Rare-Earth Doped Bilayer-Channel Thin Film Transistors for Enhanced Illumination Stability","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-05-12 23:18:38","doi":"10.21203/rs.3.rs-9586879/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"4688c728-871c-49b4-84e3-253dbda89493","owner":[],"postedDate":"May 12th, 2026","published":true,"recentEditorialEvents":[{"type":"reviewerAgreed","content":"71684742026656846029418417358641667091","date":"2026-05-08T17:56:46+00:00","index":16,"fulltext":""},{"type":"reviewerAgreed","content":"7460920984126428412282712590190715121","date":"2026-05-06T12:35:46+00:00","index":15,"fulltext":""},{"type":"reviewerAgreed","content":"73353254672486409114699479109204855320","date":"2026-05-05T00:49:52+00:00","index":7,"fulltext":""},{"type":"reviewersInvited","content":"5","date":"2026-05-05T00:48:40+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-05-04T15:06:01+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-05-04T15:05:52+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Materials Science: Materials in Electronics","date":"2026-05-01T14:17:10+00:00","index":"","fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-05-12T23:18:39+00:00","versionOfRecord":[],"versionCreatedAt":"2026-05-12 23:18:38","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9586879","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9586879","identity":"rs-9586879","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}