Low-Temperature Atomic Layer Deposition of High-k SbOx for Thin Film Transistors

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY-NC-ND 4.0 https://creativecommons.org/licenses/by-nc-nd/4.0/ Low-Temperature Atomic Layer Deposition of High-k SbOx for Thin Film Transistors © 2022 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH. Published version Yang, Jun; Bahrami, Amin; Ding, Xingwei; Zhao, Panpan; He, Shiyang; Lehmann, Sebastian; Laitinen, Mikko; Julin, Jaakko; Kivekäs, Mikko; Sajavaara, Timo; Nielsch, Kornelius Yang, J., Bahrami, A., Ding, X., Zhao, P., He, S., Lehmann, S., Laitinen, M., Julin, J., Kivekäs, M., Sajavaara, T., & Nielsch, K. (2022). Low-Temperature Atomic Layer Deposition of High-k SbOx for Thin Film Transistors. Advanced Electronic Materials, 8(7), Article 2101334. https://doi.org/10.1002/aelm.202101334 2022 www.advelectronicmat.de 2101334 (1 of 10) © 2022 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH ReseaRch aRticle Low-Temperature Atomic Layer Deposition of High-k SbOx for Thin Film Transistors Jun Yang, Amin Bahrami,* Xingwei Ding, Panpan Zhao, Shiyang He, Sebastian Lehmann, Mikko Laitinen, Jaakko Julin, Mikko Kivekäs, Timo Sajavaara, and Kornelius Nielsch* DOI: 10.1002/aelm.202101334 1. Introduction There has been a substantial increase in the number of studies focused on metal oxide thin films due to their enormous potential for applications, either as insulators or semiconductors.[1,2] To date, thin film transistors (TFTs) based on oxide thin films have been studied because of their excellent electrical properties, high uniformity, and good transparency.[3] With the growing need for high-performance electronics, conventional low-k materials such as SiO2 are unable to fulfill demand due to direct tunneling, resulting in significant device reliability issues.[4] According to the generalizedequationfor thecapacitance, C= ε0εrS/d, where ε0, εr, S, and d are the dielectric constant of free space, relative dielectric constant, area, and thickness of the dielectric, respectively, a high-k dielectric material would be capable of providing strong capacitive coupling while also being scaled down to device size (below 100nm).[5] Furthermore, the higher capacitance of a high-k dielectric permits sufficient charge injection into the TFT semiconductor layer and boosts the device performance. Over the last decade, various dielectrics with high-k values (>9), such as zirconium dioxide (ZrO2), aluminum oxide (Al2O3), hafnium oxide (HfO2), neodymium oxide (Nd2O3), and yttrium oxide (Y2O3), have been investigated as prospective candidates to replace the conventional dielectric SiO2 gate in TFTs (Figure 1). Al2O3 was one of the first systems that has been studied as a substitute to SiO2 as a gate dielectric. The larger bandgap of Al2O3 makes it compatible with the complementary metal– oxide–semiconductor structure.[6] In addition, Hf-based dielectrics have been proposed as promising materials for application in large-scale integrations. In 2007, Intel announced that Hfbased high-k materials will be used in 45nm manufacturing, which is the first high-k material commercial production in the world.[7] Furthermore, Zeumault and Subramanian reported that the mobility of ZnO TFTs can be strongly improved by using ZrO2 as a booster gate dielectric via a thermally activated emissive process.[8] However, to date, the number of high-k dielectrics is still limited and new high-k material candidates still need to be further developed. Recently, antimony oxide was reported as a new dielectric material due to its good SbOx thin films are deposited by atomic layer deposition (ALD) using SbCl5 and Sb(NMe2)3 as antimony reactants and H2O and H2O2 as oxidizers at low temperatures. SbCl5 can react with both oxidizers, while no deposition is found to occur using Sb(NMe2)3 and H2O. For the first time, the reaction mechanism and dielectric properties of ALD-SbOx thin films are systematically studied, which exhibit a high breakdown field of ≈4 MV cm−1 and high areal capacitance ranging from 150 to 200 nF cm−2, corresponding to a dielectric constant ranging from 10 to 13. The ZnO semiconductor layer is integrated into a SbOx dielectric layer, and thin film transistors (TFTs) are successfully fabricated. A TFT with a SbOx dielectric layer deposited at 200°C from Sb(NMe2)3 and H2O2 presents excellent performance, such as a field effect mobility (µ) of 12.4 cm2 V−1 s−1, Ion/Ioff ratio of 4 × 108, subthreshold swing of 0.22 V dec−1, and a trapping state (Ntrap) of 1.1 × 1012 eV−1 cm−2. The amorphous structure and high areal capacitance of SbOx boosts the interface between the semiconductor and dielectric layer of TFT devices and provide a strong electric field for electrons to improve the device mobility. J. Yang, A. Bahrami, P. Zhao, S. He, S. Lehmann, K. Nielsch Institute for Metallic Materials Leibniz Institute of Solid State and Materials Science 01069 Dresden, Germany E-mail: [email protected]; [email protected] J. Yang, P. Zhao, S. He, K. Nielsch Institute of Materials Science Technische Universität Dresden 01062 Dresden, Germany X. Ding Key Laboratory of Advanced Display and System Application Ministry of Education, Shanghai University Shanghai 200072, China M. Laitinen, J. Julin, M. Kivekäs, T. Sajavaara Department of Physics University of Jyvaskyla Jyväskylä FI-40014, Finland The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aelm.202101334. © 2022 The Authors. Advanced Electronic Materials published by Wiley- VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. Adv. Electron. Mater. 2022, 2101334 www.advancedsciencenews.com www.advelectronicmat.de 2101334 (2 of 10) © 2022 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH dielectric property.[9] The breakdown field can reach a value of 2.5 MV cm−1 for a dielectric layer thickness of 100nm.[10] Liu etal. showed that monolayer molybdenum disulfide field-effect transistors supported by Sb2O3 dielectric substrate exhibit reduced transfer-curve hysteresis compared with when using SiO2 substrate.[11] However, it should be mentioned that Sb2O3 is classified as a carcinogen in large quantities. In addition to the composition of metal oxides, the thin film preparation method plays an important role in achieving high-performance devices. Thin films grown by magnetron sputtering and pulsed laser deposition techniques can lead to surface scattering and deterioration of the device stability.[12] Additionally, solution-based deposited (such as sol–gel and inkjet printing) thin films require very high postannealing temperatures to achieve a high-densification metal–oxygen–metal (MOM) structure. Furthermore, the reproducibility in these mentioned films is not always satisfactorily achievable.[13] The electrical properties of TFT devices are greatly influenced by the interface defects between the channel and insulator well as the roughness of the layers.[14] It is well-known that a dielectric material with a smooth surface is essential to enable proper electronic device function.[1] For instance, despite having a high dielectric constant, classical high-k thin films such as ZrO2 and HfO2 are susceptible to forming a crystal structure with a rough surface.[15] In addition, the dielectric property of high-k layer strongly depends on the preparation process, which dominates the impurity and residue groups in the material structure.[16] Therefore, it is crucial to look for an appropriate deposition technique with high reproducibility, excellent thickness controllability, accurate chemical composition control, and low growth temperatures. The atomic layer deposition (ALD) technique offers the highest conformality among all deposition techniques as well as atomic layer control over wide deposition temperature ranges, thanks to sequential and self-limiting surface reactions.[17] Due to the high conformability of ALD process, it is widely used in the coating of metal oxide thin films. To the best of our knowledge, the dielectric properties of ALD-processed SbOx have never been explored before. Herein, we develop SbOx dielectric thin films by thermal ALD using SbCl5 and Sb(NMe2)3 as antimony reactants and H2O and H2O2 as oxidizers at low temperatures (80–200°C). The electrical and structural properties of deposited SbOx films were investigated systematically. The amorphous structure of SbOx deposited in this study presents a smooth surface, resulting in a high-quality dielectric/semiconductor interface. Furthermore, the dielectric constant of SbOx is higher than 13, resulting in a satisfactory capacitance. Additionally, the elemental concentration in the films was studied by time-of-flight elastic recoil detection analysis (ToF-ERDA) to correlate the level of impurities with the device performance.[18] Furthermore, the ALD ZnO semiconductor material, owing to its nontoxicity, low cost, and high mobility characteristics, was integrated into a SbOx dielectric and fabricated TFT device. In this study, the ALD process provides an excellent interface state between the channel and insulator, boosting the performance of TFT devices. Therefore, as a new high-k material, ALD-pro- cessed SbOx is a great candidate that merits further research and development. Adv. Electron. Mater. 2022, 2101334 Figure 1. a) Schematic of the process from thin film to device. b) Summary of dielectric properties for high-k materials.[19,20] www.advancedsciencenews.com www.advelectronicmat.de 2101334 (3 of 10) © 2022 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH 2. Results and Discussion To understand the growth behavior and effect of using different precursors on the quality and performance of the as-grown SbOx thin films, H2O and H2O2 oxidants were selected to react with SbCl5 and Sb(NMe2)3. The postulated mechanism is shown in Figure 2. The hydroxyl saturated surface is exposed to the Sb precursor, which reacts with the hydroxyl group, leaving behind a Cl (or CH3) terminated surface. The Cl (or CH3) surface will be exposed to the oxygen source (H2O2 or H2O) and react on the surface to leave one layer of a SbOx thin film. The carrier gas removes excess precursors and byproducts in each half ALD cycle. Based on the reaction mechanism, the SbOx growth properties with respect to different precursors at different chamber temperatures are shown in Figure 3. The thickness and the X-ray reflectivity (XRR) patterns are shown in Figures S1 and S2 (Supporting Information), respectively. For all thin films, the growth per cycle (GPC) decreases as the deposition temperature (TD) increases. This is probably due to the desorption of reactive surface sites, such as OH, from the substrate at higher temperatures.[21] When sufficient Sb precursors are pulsed into the chamber and the surface reaction is not limited, the amount of OH groups on the surface is the crucial condition for determining the growth rate during the ALD process. Thus, the density of OH decreases with increasing TD, leading to a reduction in the GPC.[22] It should be mentioned that the SbOx deposited using SbCl5 and different oxygen sources shows a very different growth rate. For example, the GPC for SbOx (SbCl5+ H2O) is ≈1.3 Å, but it is only 0.9 Å for Adv. Electron. Mater. 2022, 2101334 Figure 2. Schematic diagram of the reaction mechanism for ALD SbOx using different precursors: SbCl5 and a) H2O2 or b) H2O, c) Sb(NMe2)3 and H2O2. Figure 3. a) The growth rate, b) density, and c) AFM images (scale bar: 1 µm) of the SbOx thin films. www.advancedsciencenews.com www.advelectronicmat.de 2101334 (4 of 10) © 2022 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH SbOx synthesized from the reaction between SbCl5 and H2O2. According to the Langmuir adsorption isotherm, the number of surface hydroxyl groups strongly depends on the vapor pressure of the oxidant precursor.[23] The vapor pressure of H2O2 is 18.4Torr, which is lower than that of H2O (23.8Torr) at room temperature. The relatively lower density of OH groups leads to a lower GPC for SbOx using SbCl5 and H2O2. The density values for the thin films obtained from the calculated fit to the XRR patterns are shown in Figure3b. As the TD increases, the density of the SbOx thin films increases from ≈4.9g cm−3 at 80°C to ≈5.5g cm−3 at 200°C. Impurity incorporation induced by a higher growth rate, as well as partial detachment of chemical legends during the purging process due to a lower deposition temperature, may affect thin film density in this case. The surface morphologies of SbOx were studied by atomic force microscopy (AFM) analyses with scanned areas of 5 × 5 µm2, as shown in Figure3c and Figure S3 (Supporting Information). The root mean squared value of the surface for SbOx films grown using SbCl5 is slightly higher than that of Sb(NMe2)3. The differences in the obtained roughness values can be attributed to several facts, i.e., i) films grown using metal chloride precursors always have an “incubation” time, imposing few ALD cycles to fulfill the requirement for uniform coverage of a hydrogen-terminated Si surface, which can lead to difficulty for the deposition at the initiating cycles and to a high degree of surface roughness,[24] ii) the formation of HCl as a byproduct that etches the thin films, leading to a nonuniform and rough surface.[25] The scanning electron microscopy (SEM) images are shown in Figures S4 and S5 (Supporting Information). There is a clear interface between the deposited thin films and Si substrate. As mentioned before, inhomogeneous SbOx deposition is observed when Sb(NMe2)3 and H2O are used as precursors, probably resulting from the weak oxidizing ability of H2O (Figure S5e, Supporting Information). Deposition of 900 cycles of SbOx using Sb(NMe2)3 and H2O2 results in a thickness of ≈106.2nm at a TD of 80°C, while it is reduced to ≈36.6nm when the TD is increased to 200°C, which is consistent with the XRR results. The transmission electron microscopy (TEM) analyses (Figure S5c, Supporting Information) and grazing incidence X-ray diffraction patterns (Figure S5d, Supporting Information) confirm the amorphous nature of all SbOx thin films deposited using different precursors at the studied deposition temperatures. The X-ray photoelectron spectroscopy (XPS) was used to identify the binding energies and chemical states of the elements in the SbOx thin films presented in Figure 4a–d and Figure S6 (Supporting Information). The C 1s state was used as a reference to calibrate the binding energy. The normalized O 1s area is shown in Figure 4b. The thin film deposited at 200 °C shows a smaller O 1s area compared to other films, which is consistent with the analysis of the change in the coordination number for Sb. Interestingly, the XPS peaks were found to shift toward lower binding energies for increasing TD. This phenomenon can be explained by two mechanisms: i) the progressive oxidization of Sb from SbOCl [or Sb(NMe2)2OH] to SbOx,[26] and ii) the decrease in the coordination number of Sb (from Sb2O5 to Sb2O3) can result in lower binding energy.[27] Although the O 1s spectra overlap with the Sb 3d spectra, the use of some strict rules makes it possible to separate out the O 1s spectra;[28] i) the full width at half maximum of 3d3/2 is equal to that of 3d5/2, ii) the center distance between 3d5/2 and 3d3/2 is 9.34 eV, and iii) the area of 3d5/2 is theoretically 1.5 times Adv. Electron. Mater. 2022, 2101334 Figure 4. a) XPS survey for Sb 3d for SbOx thin films (Sb(NMe2)3 and H2O2). b) Normalized O 1s area. c) Sb 3d spectra and d) fitting result. www.advancedsciencenews.com www.advelectronicmat.de 2101334 (5 of 10) © 2022 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH larger than that of 3d3/2. It is known that a strong MCl (M is the metal atom) bond can hardly be replaced by a MO bond during ALD. The first principle calculation shows that a high TD is necessary to overcome this high activation energy barrier.[29] In addition, a high TD can effectively reduce the incorporation of Cl impurities into the synthesized film[25] (see Figure S7 in the Supporting Information). ToF-ERDA analysis was used to detect different elements, such as Sb, O, H, C, and Cl, in the SbOx thin films, as presented in Figure 5a–d and Table 1. The content of some impurities, such as H, can strongly affect the performance of TFT devices.[20] Although an overabundance of hydrogen in the dielectric layer of TFTs is assumed to deteriorate device performance due to the generation of excess carriers, a controlled amount of hydrogen can efficiently passivate the electrode/SbOx interfacial states under a certain electric field and form an electric double layer, leading to a high areal capacitance.[30] Therefore, precise analysis of the H content is crucial for ALD films grown using hydrogen-containing precursors. ToF-ERDA outperforms X-ray photoelectron spectroscopy in terms of sensitivity and depth resolution, and it outperforms time-of-flight secondary ion mass spectrometry in terms of quantitative evaluation capabilities, which is suitable for detecting smaller trace of H, Cl, and C, and the Sb:O ratio in the SbOx film. From the ERDA measurement, it can be concluded that the Sb/O ratio increases with increasing deposition temperature. The same behavior was observed for hydrogen content for increasing deposition temperature, i.e., reduction of hydrogen content from ≈11.2% to ≈0.6% upon increase of the deposition temperature of SbOx using Sb(NMe2)3 and H2O2 from 80 to 200°C. It should also be mentioned that the materials deposited at lower temperatures (80°C) were not stable and were partially Adv. Electron. Mater. 2022, 2101334 Figure 5. ERDA spectrum of ALD SbOx deposited using Sb(NMe2)3 and H2O2 at a) 80°C and b) 200°C. In the 2D histogram of raw spectra, light elements are observed more toward the bottom left corner of the plot, where the energy and time of flight are at their minimum. A lower energy and a longer time-of-flight for each element indicate that the events originate deeper from the sample. c,d) ERDA depth profile of SbOx on a Si substrate, extracted by analyzing the spectra shown using the Potku software package. Table 1. Elemental compositions of ALD-grown films determined by ToF-ERDA. Sample TD [°C] H [at%] C [at%] Cl [at%] N [at%] Sb [at%] Sb:O SbCl5+ H2O280 22 ± 2 0.3 ± 0.06 1 ± 0.3 –25 ± 2 0.52 200 3.2 ± 1.5 0.3 ± 0.15 0.8 ± 0.3 –34.5 ± 1.5 0.56 SbCl5+ H2O80 19± 2 0.6 ± 0.2 1.5 ± 0.5 –25 ± 2 0.48 200 5.5 ± 1.5 0.1 ± 0.08 0.9 ± 0.3 –31 ± 3 0.51 Sb(NMe2)3+ H2O280 11.2 ± 2 4.0 ± 1.0 –2.4 ± 0.8 26.5 ± 1.5 0.47 200 0.6 ± 0.3 0.22 ± 0.07 –0.13 ± 0.06 33 ± 2 0.53 www.advancedsciencenews.com www.advelectronicmat.de 2101334 (6 of 10) © 2022 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH destroyed by the ion beam during the measurement, which may generate some ambiguity in the ERDA analysis results for such samples. A more detailed picture of the elemental composition for all deposited films is presented in Table1. To evaluate the optical properties of the deposited SbOx films, films were grown onto a quartz glass substrate, as shown in Figure 6a and Figure S8 (Supporting Information). In the visible range, the thin films are highly transparent at 80%, which can also be proven by the optical image shown in the inset figure, favoring potential application in transparent electronic devices. The obtained bandgap values are different for SbOx deposited using different precursors. It is noted that the bandgap values increase as the TD is increased (Figure S8, Supporting Information). Soon and co-workers calculated the band structure of SbOx by density functional theory.[31] They found that with the transition of SbOx phase from Sb2O5 to Sb2O3, the bandgap increases obviously. In our work, SbOx is a mixture of Sb2O5 (low bandgap) and Sb2O3 (high bandgap). The Sb:O ratio calculated by ToF-ERDA (Table 1) and XPS (Figure 4b) shows that the majority of the phase is Sb2O3 with a relative higher bandgap. By increasing the deposition temperature and reducing of coordination number of Sb, we will have more Sb2O3 and as a result, higher bandgap. The combination of ultraviolet photoelectron spectroscopy (UPS) and XPS provides knowledge for the electronic structure and energy band positions of various materials. UPS spectra were obtained to explore the band structure of the SbOx thin films. A clear shift of the secondary electron cutoff (Ecutoff) and the valence band edge to higher energies by increasing the deposition temperature can be observed in all three systems, as shown in Figure6b,c and Figure S9 (Supporting Information). From the secondary cutoff at high binding energy, the work function (ϕ) was determined by subtracting the cutoff energy from the photon energy (21.2 eV) and is presented in Figure 6d. The results confirm the reduction of ϕ due to increased deposition temperature and relocation of the Fermi level toward the conduction band edge. Kumar and co-workers reported the same phenomenon in their study of Ba0.5Sr0.5TiO3 system.[32] The induced vacancies disrupt the orbital character and cause Fermi level shift toward conduction band. Another explanation is that amorphous Sb2O3 behaves more as an n-type material than Sb2O5.[33] As mentioned before, the thin films underwent a phase transition from Sb2O5 to Sb2O3 when the deposition temperature was raised, as shown in ToF-ERDA (Table1) and XPS (Figure4b) results. Therefore, the presence of more Sb2O3 phase in the thin film at high temperatures might lead the Fermi level to be closer to the conduction band. The corresponding valence band maximum was calculated, and the energy band diagram is presented in Figure6e. Previous reports indicate that an offset for ϕ can result from the use of different processing conditions and composition variation.[34] Here, we postulate that the decrease in the coordination number of Sb (from Sb2O5 to Sb2O3) at higher temperature can reduce ϕ. Metal–insulator–metal structured capacitors were fabricated to evaluate the electrical and dielectric properties of ALD SbOx thin films deposited using different precursors and deposition temperatures. Figure 7a and Figure S10 (Supporting Information) show the leakage current behavior of the SbOx dielectrics. A significant leakage current density is observed for all SbOx thin films deposited at 80 °C (Figure S10, Supporting Information). At lower deposition temperatures, the thin films contain a high content of impurities originating from incomplete decomposition of precursors (see Table1), which act as leakage Adv. Electron. Mater. 2022, 2101334 Figure 6. a) Transmittance for SbOx grown at 200°C. UPS spectra obtained for b) the secondary electron onset and c) valence band edge for a SbOx thin film grown using Sb(NMe2)3 and H2O2. d) The work function and e) the band structure diagram for all SbOx thin films. www.advancedsciencenews.com www.advelectronicmat.de 2101334 (7 of 10) © 2022 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH current paths and lead to a high current density. At higher deposition temperatures, a decreased leakage current can be an indication of full decomposition of precursors, and, as a result, less impurities in the final deposited film, which agrees with the density values obtained using XRR analyses and the impurity content indicated by the ToF-ERDA method. Furthermore, at a deposition temperature of 200°C, a lower current density and higher breakdown field are measured for films grown using Sb(NMe2)3 compared to that for films grown using SbCl5 (Figure 7a), which indicates a higher density of traps in the metal–oxide framework of films derived from chlorine-based precursors. Kukli etal. observed that the trap density in films grown from chlorine-based precursors is higher than that for films grown from different precursors,[35] resulting in a higher current density and lower breakdown field. Figure7b shows the capacitance–frequency properties of the SbOx dielectric grown at 200°C with different precursors. The SbOx thin film deposited by Sb(NMe2)3 shows a high dielectric constant of about 13.2 (Figure S10d, Supporting Information). To achieve low-leakage current TFTs, the offset of the dielectric conduction band minimum should be at least 1eV relative to the semiconductor material in an n-type transistor.[36] A band offset of higher than 1eV effectively confines the charges within the ZnOchannel during the device measurement (Figure S11, Supporting Information). To confirm the possible application of SbOx thin films as a dielectric layer in TFTs, bottom-gated TFTs integrated with an ALD ZnO channel layer were fabricated. The typical transfer curves are shown in Figure7c. The key electrical parameters are summarized in Table 2. Here, a high field-effect mobility of 12.4 cm2 V−1 s−1 and a minimum subthreshold swing (SS) value of 0.22V dec−1 for TFTs based on the Sb(NMe2)3 precursor were calculated via the derivative of the transfer curve determining the slope. However, the TFT based on the SbOx dielectric grown from SbCl5 shows a relatively unsatisfactory performance, i.e., a low mobility of 6.3 cm2 V−1 s−1 and a high SS value of 0.88 V dec−1. Moreover, the maximum areal density of states (Ntrap) was further obtained from SS values using the following formula[37] ln 10 1 B 2 ox trap SS kT q q CN=+      (1) Adv. Electron. Mater. 2022, 2101334 Figure 7. a) Leakage current density characteristics and b) capacitance frequencies for SbOx dielectrics deposited at 200°C. c) Transfer curves for ZnO/SbOx thin film transistors. d) Output curve with a fixed gate voltage of 20 V. e) Positive bias stress test results. f) Illustration of the metal– insulator–semiconductor mechanism for electron emission. Table 2. Performance parameters for ZnO/SbOx TFTs with ZnO as the channel and SbOx as the dielectric layer (µ, Vth, Ion/Ioff, SS, and Ntrap are the field-effect mobility, threshold voltage, ratio of on and off currents, subthreshold swing, and trapping state). Precursors Vth [V] µ [cm2 V−1 s−1]Ion/Ioff SS [V dec−1]Ntrap [eV−1 cm−2] SbCl5 and H2O22.57 6.3 5 × 1070.88 7.5 × 1012 SbCl5 and H2O 4.72 8.7 2 × 1070.65 3.7 × 1012 Sb(NMe2)3 and H2O25.03 12.4 4 × 1080.22 1.1 × 1012 www.advancedsciencenews.com www.advelectronicmat.de 2101334 (8 of 10) © 2022 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH Adv. Electron. Mater. 2022, 2101334 where kB is the Boltzmann’s constant, T is the temperature in Kelvin, Cox is the gate oxide capacitance, and q is the electron charge. As shown in Table2, a low Ntrap was obtained for SbOx grown from Sb(NMe2)3- and H2O2-based TFTs, indicating an excellent interface between the channel layer (ZnO) and dielectric layer (SbOx).[20] Figure7d shows the output characteristics of TFTs, clearly indicating pinch-off behavior and drain current saturation at high VDS. The electrical stability is also carried out under long-term positive bias stress (PBS), i.e., applying constant positive bias stress (+20V) between the drain and source for 1 h followed by measurement of the transfer curve (Figure S12, Supporting Information). When a positive voltage is applied under atmospheric conditions, electrons will accumulate in the ZnO semiconductor. The surrounding oxygen molecules with large electronegativity can capture electrons from the conduction band to form O2− species. Therefore, the carriers in the ZnO layer can be removed, leading to a shift in the positive threshold voltage (∆Vth).[20] The energy band diagram for TFTs under PBS conditions and the variations in the threshold voltage shift (∆Vth) as a function of stress time are shown in Figure7e and Figure S13 (Supporting Information). It can be observed that the TFT based on SbCl5 and H2O2 shows a higher voltage shift of 5.1V, while for the TFT-based Sb(NMe2)3, the voltage shift is only 1.2V, which reveals that there are only a small number of defects at the interface between the semiconductor and dielectric layer. The superior performance of TFTs based on the Sb(NMe2)3 precursor can be explained from different aspects. First, chlorine is a strongly electronegative impurity and strongly degrades the dielectric property of SbOx thin films. However, the SbOx thin film grown from the Sb(NMe2)3 precursor possesses a low content of impurities, revealing a lower density of states. Second, the amorphous structure presents smoother surfaces than the crystalline surfaces, resulting in a good quality dielectric/semiconductor interface. In addition, the smooth interface can decrease the surface scattering and is beneficial to the performance of TFT devices.[38] Third, it is postulated that low-temperature- processed SbOx dielectrics generate intrinsic donor-like electron traps. For increasing transverse VGS, the traps inject electrons into the ZnO semiconductor by thermally activated emission, increasing the electron concentration in bulk ZnO and resulting in a high field-effect mobility in TFT devices (Figure7f).[8,39] Furthermore, the right amount of hydrogen can efficiently passivate the interfacial states and enhance the TFT device performance.[20] Finally, in oxide semiconductors (ZnO), electrical conduction is mainly dominated by electron-trapping sites (Figure 8a). Based on the multiple-trapping-and-release model, trapping sites can be introduced by electron transport. Most of the electrons fill the localized state before participating in conduction transport (Figure8b), and only a small amount of electrons can move into the transport bands. The SbOx dielectric provides a high capacitance, which exhibits a stronger electric field and provides more energy to electrons. Therefore, a large number of electrons easily fill the lower-lying localized state and are rapidly trapped in the upper-lying localized states, which can remarkably enhance the TFT mobility.[20,40] 3. Conclusion In summary, SbOx thin films were successfully synthesized using a new combination of antimony reactants and oxidizers, namely, SbCl5, Sb(NMe2)3, H2O, and H2O2, in the temperature range of 80–200 °C. Furthermore, the dielectric properties of ALD-processed SbOx thin films were fully studied. Thin film structural and compositional characterization using XRD, TEM, AFM, and XRR confirms the uniformity, high density, and amorphous nature of all the films studied. Additionally, increasing the deposition temperature results in the deposition of denser and purer SbOx phases, as proved by ToF-ERDA and XPS. The UPS spectra confirm that the work function decreases and the Fermi level shifts toward the conduction band edge with increasing deposition temperature. The TFT performance of devices is affected by the type of precursor and deposition temperature. For SbCl5-based SbOx, it is easy to form SbO bonds during the ALD process at high deposition temperatures due to the high bonding energy of MCl. The Cl impurity can degrade the dielectric performance of SbOx thin films and result in a high leakage current and a low breakdown electric field. In comparison, the Sb(NMe2)3-based SbOx films demonstrate good dielectric properties, i.e., a breakdown field of ≈4 MV cm−1 and a high areal capacitance of ≈200nF cm−2 at 200 °C. To evaluate the performance of deposited films in Figure 8. a) Schematic of the electron-transporting mode. b) Energy band diagram of oxide semiconductor thin films. A very high electric field is induced by SbOx, which makes it easier to quickly trap electrons in the upper-lying localized states.