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[Yong-Lie Sun](https://orcid.org/0000-0003-1113-1658), Toshihide Nabatame, [Jong Won Chung](https://orcid.org/0000-0002-9799-7438), Tomomi Sawada, Hiromi Miura, Manami Miyamoto, [Kazuhito Tsukagoshi](https://orcid.org/0000-0001-9710-2692)

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[Compositional changes between metastable SnO and stable SnO2 in a sputtered film for p-type thin-film transistors](https://mdr.nims.go.jp/datasets/9f552f01-0852-4f4a-bf74-f6e672c04fe3)

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Compositional changes between metastable SnO and stable SnO2 in a sputtered film for p-type thin-film transistorsCompositional changes between metastable SnO and stable SnO2 in a sputtered film for p-type thin-film transistorsYong-Lie Sun a, Toshihide Nabatame a,*, Jong Won Chung b, Tomomi Sawada a, Hiromi Miura a,  Manami Miyamoto a, Kazuhito Tsukagoshi a,*a Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba 305-0044, Japanb Samsung Advanced Institute of Technology (SAIT), Samsung Electronics, Suwon 16678, South KoreaA R T I C L E  I N F OKeywords:p-Type SnOTin oxideThin-film transistorsSputteringHigh vacuum annealingA B S T R A C Tp-Type tin(II) oxide (SnO (Sn2+)) formation using radiofrequency (RF) reactive magnetron sputtering and post- deposition annealing (PDA) processes was investigated. The as-grown SnOx film deposited from an SnOx (SnO:Sn = 60:40) target by RF sputtering at an oxygen partial pressure (PO2) of 0 Pa consisted of 2 % Sn (Sn0), 42 % Sn2+, and 56 % SnO2 (Sn4+). However, compared with the Sn2+ fraction observed after PDA under N2 and low-vacuum (~1 Pa) conditions, that after PDA at 300 ◦C under high vacuum (< 5 × 10− 4 Pa) (HVPDA) increased substantially to greater than 62 %. This result was attributed to the transformation from SnO2 to SnO during HVPDA. A staggered bottom-gate thin-film transistor with an SnO channel (10 nm), which was fabricated by HVPDA at 300 ◦C, exhibited p-type properties, including a relatively high on-current/off-current (Ion/Ioff) ratio of 5.1 × 104 and a hole field-effect mobility (µFE) of 1.8 cm2/(V⋅s).1. Introductionn-Type oxide semiconductors stand out in cutting-edge electronic applications because of their high mobility, low off-current, and good compatibility with low-temperature fabrication methods. These materials have therefore been identified as highly promising semiconductors for use in advanced displays, sensors, and ferroelectric transistor channels [1-5]. Numerous In2O3-based oxide semiconductors, including In2O3, C-doped In2O3, In–Si–O, In–W–O, and In–Ga–Zn–O, have been widely investigated as n-type oxide semiconductors [6-10].The development of p-type oxide semiconductors, such as tin(II) oxide (SnO), nickel(II) oxide (NiO), and copper(II) oxide (CuO), continues to encounter problems related to low hole mobility and poor reliability [11,12]. Among these materials, SnO has emerged as a notable p-type oxide semiconductor; however, achieving both high field-effect mobility (μFE) and a large Ion/Ioff ratio in SnO-based thin-film transistors (TFTs) is challenging because SnO (Sn2+) is a thermodynamically metastable phase [13-21]. Its metastability often leads to the formation of n-type SnO2 (Sn4+) or other intermediate phases such as Sn2O3 or Sn3O4, and sometimes phase-pure residual metallic Sn (Sn0), resulting in the coexistence of both p-type SnO and n-type phases and further degradation of device efficiency [18-20,22,23]. Thus, the fabrication of stable p-type SnO by controlling the deposition and post-deposition annealing (PDA) conditions is important. p-Type SnO films have typically been fabricated using two reaction methods. SnO has been predominantly fabricated by oxidizing a deposited film composed of Sn-rich SnOx and Sn [24,25]. As a minority phase, SnO was formed via decomposition of SnO2 [26].Metallic Sn [19,27], ceramic SnO [20,27–30] and SnO2 [31,32], and Sn+SnO2 [33,34] and Sn+SnO [35] mixtures have been used as targets in sputtering methods. As-grown SnO films are expected to have a stoichiometric O/Sn ratio of 1/1 and a large SnO fraction. Metallic Sn targets are limited by their low melting point and the narrow processing window for oxygen partial pressure and sputtering power during sputtering of pure SnO films [19,35]. A pure SnO2 target predominantly forms n-type SnO2 because O–Sn–O bonds involving Sn4+ are more stable than those involving Sn2+ particularly at high-temperature. Meanwhile, sputtering targets with mixtures of metals and ceramics offer both reliable film production and an enlarged processing window, making their use a promising approach for attaining stable and manageable p-type SnO thin films.The effect of the annealing atmosphere and temperature during post- deposition annealing (PDA) of SnO films has also been studied. Lin et al. explored how SnO films react to annealing under various atmospheres * Corresponding authors.E-mail addresses: nabatame.toshihide@nims.go.jp (T. Nabatame), tsukagoshi.kazuhito@nims.go.jp (K. Tsukagoshi). Contents lists available at ScienceDirectThin Solid Filmsjournal homepage: www.elsevier.com/locate/tsfhttps://doi.org/10.1016/j.tsf.2024.140548Received 5 May 2024; Received in revised form 30 September 2024; Accepted 6 October 2024  Thin Solid Films 807 (2024) 140548 Available online 9 October 2024 0040-6090/© 2024 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by- nc-nd/4.0/ ). mailto:nabatame.toshihide@nims.go.jpmailto:tsukagoshi.kazuhito@nims.go.jpwww.sciencedirect.com/science/journal/00406090https://www.elsevier.com/locate/tsfhttps://doi.org/10.1016/j.tsf.2024.140548https://doi.org/10.1016/j.tsf.2024.140548http://creativecommons.org/licenses/by-nc-nd/4.0/http://creativecommons.org/licenses/by-nc-nd/4.0/(including vacuum, air, O2, and N2) over the temperature range 100–500 ◦C and found that films annealed at 500 ◦C contained both SnO and SnO2 phases under all of the tested atmospheres [32]. Contrarily, other researchers have reported attaining single-phase p-type SnO thin films after annealing at 500 ◦C [36] or even at 600 ◦C [13] for 30 min in vacuum. Although the atmosphere during PDA is an important factor for forming p-type SnO, the effect of vacuum pressure on SnO is unclear. In addition, the literature contains no reports on the electrical properties of TFTs with an SnO channel formed by PDA under high vacuum.In the present study, we investigate the effects of sputtering with an SnOx target (SnO:Sn = 60:40) and PDA under three different atmospheres (N2 gas, low vacuum (~1 Pa) (LVPDA), and high vacuum (<5 ×10− 4 Pa) (HVPDA)) on the formation of p-type SnO. We also demonstrate the transistor performance of SnO-channel staggered bottom-gate TFTs fabricated on p-doped Si substrates with an SiO2 insulator layer.2. Experimental methods2.1. SnOx film fabricationThe SnOx films were deposited on p-doped Si substrates (15 × 15 mm) with an SiO2 layer at room temperature under a deposition power of 125 W via radiofrequency (RF) reactive magnetron sputtering using an SnOx target (SnO-to-Sn volume ratio of 60:40) (Fig. 1(a)). The target, purchased from Kojundo Chemical Laboratory Co.,Ltd., has a density of 86 % and a size of 3 inches. Unlike conventional ceramic targets, which often contain a binder metal such as Zn, the SnOx target does not contain a binder additive. Then, the SnOx target is composed entirely of Sn and oxygen. The target-to-substrate distance is set to 82 mm. The SiO2 thickness was 1.5 nm for surface analysis, 200 nm for Hall measurements, and 250 nm for TFT characterization. Here, substrates with 1.5 nm thick SiO2 is bare Si substrates with a native oxide, substrates with 200 nm and 250 nm thick SiO2 are thermal oxide Si. The oxygen partial pressure (PO2) was varied from 0 to 0.055 Pa by changing the O2/Ar gas flow. As-grown SnOx films deposited at PO2 = 0 Pa were ~10 nm thick, as determined from a cross-sectional transmission electron image (Fig. 1(b)). The SnOx films were subsequently annealed by PDA under three different atmospheres: N2, LVPDA at ~1 Pa, and HVPDA at <5 × 10− 4 Pa. PDA was carried out at 300 ◦C for 60 min. For Hall effect measurements, 10-nm-thick SnOx films were deposited on p-doped Si/SiO2 substrates. HVPDA was carried out for 60 nm at a temperature of 200 to 400 ◦C. Finally, Ni electrodes (100 nm) were formed on the SnOx films at four diagonal locations through a metal stencil mask. Other SnOx films were also prepared via direct-current sputtering at room temperature with a metallic Sn target under a sputtering power of 25–50 W and a PO2 of 0.025–0.26 Pa.2.2. p-Type SnO TFT fabricationFig. 1(c) shows a back-gate-type TFT with an SnO channel. A 10-nm- thick SnO channel was first deposited on a heavily p-doped Si/SiO2 substrate through a metal mask under the same RF sputtering conditions using an SnOx target. HVPDA was then carried out in the temperature range 200–400 ◦C for 60 min. For ohmic contacts, Ni (100 nm) films were formed as source–drain electrodes through a metal mask by thermal evaporation at room temperature. The channel length (L) was varied from 50 to 350 μm, and the channel width (W) was 1000 μm (Fig. 1(d)).2.3. CharacterizationThe crystal structures of the SnOx films were analyzed using X-ray diffraction (XRD) (Bragg-Brentano geometry) with a Cu Kα radiation Fig. 1. (a) Photograph of SnOx target. (b) Cross-sectional TEM image of as-grown SnOx film deposited at PO2 = 0 Pa using SnOx target. (c) Schematic of bottom-gate TFT with SnO channel (Length = 250 μm, Width = 1000 μm) and (d) optical microphotograph of TFT with SnO channel.Y.-L. Sun et al.                                                                                                                                                                                                                                  Thin Solid Films 807 (2024) 140548 2 source (Cu Kα1 1.541Å and Kα2 1.544Å) operating at a voltage of 40 kV and current of 30 mA (SmartLab 3 kW, Rigaku, Japan). The XRD measurement was performed in a low angle X-ray incident configuration with a fixed angle of 1◦ The chemical binding states were characterized using X-ray photoelectron spectroscopy (XPS) using a monochromatized Al Kα source (1486.6 eV) with a detection area of 100 µmϕ, a detection Fig. 2. XRD patterns for as-grown SnOx films prepared using (a) SnOx and (b) Sn targets. PO2 was varied for each sputtering process.Fig. 3. (a) Wide-scan XPS spectra and (b) Sn 4d XPS spectra of as-grown SnOx films prepared using SnOx target. (c) Sn0, Sn2+, and Sn4+ fractions of as-grown SnOx films as function of PO2 during sputtering.Y.-L. Sun et al.                                                                                                                                                                                                                                  Thin Solid Films 807 (2024) 140548 3 depth of 4–5 nm, a take-off angle of 45◦, and a base pressure of ≦5 ×10− 6 Pa (QuanteraSXM, ULVAC-PHI, Japan). The calibration of XPS binding energy scale was carried out using Au and Cu reference samples and surface sputtering was not performed in this study. XPS peak fitting was performed by the Foundation for Promotion of Material Science and Technology of Japan (MST) with an in-house software with the Shirley background subtraction, and the deconvolution error is claimed to be of ± 3 %. The Raman spectra were collected using a micro-Raman scattering system (Nanofinder FLEX, Tokyo Instruments, Japan) with a 100× objective lens; a green laser (532 nm) was used as the excitation source. Hall mobility, carrier density, and conductivity data were extracted from the results of room-temperature Hall effect measurements conducted using a Hall effect measurement system (HMS-5000, Ecopia, South Korea). Electrical measurements of the SnO TFTs were conducted using a precision semiconductor parameter analyzer (4156C, Keysight Technologies, US) in the dark on a probe station under vacuum. Performance metrics of linear-region μFE, on-current/off-current (Ion/ Ioff) ratio, subthreshold swing (SS), and threshold voltage (VTH) were evaluated following the standard model for metal-oxide semiconductor field-effect transistors.3. Results and discussion3.1. Characterization of SnOx filmsXRD patterns for the as-grown SnOx films fabricated using SnOx and Sn targets are shown in Fig. 2. The thickness of the SnOx films was 50 nm. In the pattern for the film deposited using an SnOx target at PO2 =0 Pa, small broad peaks associated with diffraction from (200), (101), (220), and (211) planes in the tetragonal Sn phase (PDF code: 00–004–0673) are observed. No peaks could be assigned to other phases such as SnO, Sn2O3, Sn3O4, and SnO2. These peaks for the Sn phase disappear as PO2 increases above 0.017 Pa, indicating that the SnOx films have an amorphous structure. However, strong sharp peaks due to the Sn phase are observed even at a PO2 of 0.025 Pa and are maintained in the patterns for the SnOx films prepared with PO2 as high as 0.05 Pa using the Sn target. An amorphous structure is formed at PO2 = 0.26 Pa. The difference in the amounts of Sn formed in the SnOx films is attributed to differences in the oxygen concentration in the target, suggesting that the SnOx target has an advantage in terms of suppressing the formation of Sn metal.XPS wide-range spectra (Fig. 3(a)) and Sn 4d core-line spectra of as- grown SnOx films fabricated using the SnOx target at PO2 = 0, 0.017, and 0.055 Pa (Fig. 3(b)) are shown in Fig. 3. The thickness of the SnOx films was 10 nm. The Sn 4d signals are deconvoluted into three components. In the deconvolution, the 4d3/2 and 4d5/2 spin–orbit components are considered to be doublets of Gaussian peaks. The Sn0, Sn2+, and Sn4+components are observed at 24, 25.5, and 26.2 eV, respectively; chemical shifts of ~1.5 and ~0.7 eV toward lower binding energy from Sn2+to Sn0 and from Sn4+ to Sn2+, respectively, are also observed. In the spectrum of the film prepared at PO2 = 0 Pa, two large peaks due to Sn2+and Sn4+ are observed. A small peak associated with Sn0 is also seen, consistent with the small, broad Sn peak observed in the XRD pattern (Fig. 2(a)). The Sn0 peak disappears and the Sn4+ peak intensity decreases as PO2 increases. Fig. 3(c) shows the Sn0, Sn2+, and Sn4+ fractions of the as-grown SnOx films (10 nm) fabricated using the SnOx and Sn targets. Each fraction was estimated from the XPS data. SnOx films with an Sn2+ fraction >40 % were obtained at low Po2 levels. However, to attain an Sn2+ fraction >40 % using the Sn target, a high Po2 of 0.05 Pa was required. This difference is attributed to the oxidation of SnO (Sn2+) into the thermodynamically stable form by external oxygen during sputtering and to the SnO2 (Sn4+) component increasing with increasing PO2 [35]. Therefore, in subsequent experiments, we used an SnOx target and PO2 = 0 Pa to eliminate the effect of oxidation during sputtering.The PDA process plays a critical role in the crystallization of the as- deposited SnO films, with the PDA atmosphere being particularly important. Raman spectra of as-grown and annealed SnOx films (10 nm) prepared using the SnOx target are shown in Fig. 4(a). PDA was carried out at 300 ◦C for 60 min under different conditions: N2, LVPDA, and HVPDA. No peaks are observed in the Raman spectrum of the as-grown film. By contrast, the spectra of the films prepared under N2, LVPDA, and HVPDA show a strong peak at ~220 cm− 1 and a weak peak at ~120 cm− 1; however, no peaks due to Sn3O4 or SnO2 are observed [37]. The peaks at ~120 and ~220 cm− 1 correspond to the B1g and A1g vibrational modes of SnO, respectively, indicating its crystallization [38].Fig. 4. (a) Raman spectra, (b) wide-scan XPS spectra, (c) XPS 4d spectra, and (d) Sn0, Sn2+, and Sn4+ fractions of as-grown and annealed SnOx films.Y.-L. Sun et al.                                                                                                                                                                                                                                  Thin Solid Films 807 (2024) 140548 4 We carried out XPS analyses (Fig. 4(b) and 4(c)) to evaluate the Sn0, Sn2+, and Sn4+ fractions under different atmospheres. Fig. 4(c) shows the Sn 4d XPS spectra of the as-grown film and the films annealed under N2, LVPDA, and HVPDA conditions. The Sn2+ fraction for the HVPDA film is substantially higher than those for the films annealed under N2 and LVPDA conditions. The Sn0, Sn2+, and Sn4+ fractions are summarized in Fig. 4(d). The films annealed under N2 and LVPDA conditions maintained the same Sn2+ and Sn4+ fractions as the as-grown film. The HVPDA film showed a maximum Sn2+ fraction greater than 62 % and a Sn2+/Sn4+ ratio greater than 1.6. Here, we express this unintelligible result: although the Sn4+ fraction was clearly detected in XPS analysis, SnO2 binding signal has not been obtained in Raman spectroscopy (Fig. 4(a)). Then, it is difficult to find unified interpretation of this issue at moment.The presence of excess oxygen has been reported to lead to the transformation of metastable SnO to stable SnO2 as a result of a local disproportionate redistribution of internal oxygen [39]. The as-grown SnOx film (10 nm) had an amorphous structure (results not shown). This amorphous film is speculated to be composed of a network of Sn0, Sn2+, and Sn4+. The film was partially crystallized with the SnO (Sn2+) structure during PDA, as determined from the Raman results in Fig. 4(a). In addition, the Sn2+ fraction in the N2 film slightly increased compared with that in the as-grown film because Sn metal transforms to SnO by acquiring oxygen from the nearest SnO2. Under an O2 atmosphere, the oxidation of Sn to SnO2 via reaction (1) occurs rather than the oxidation of Sn to SnO via reaction (2) [40]: Sn (s) + O2 (g) → SnO2 (s) − 577 kJ mol− 1                                     (1)Sn (s) + 0.5O2 (g) → SnO (s) − 282 kJ mol− 1                                  (2)It was reported that an oxygen-free atmosphere during PDA was found to be a key factor for the formation of SnO from Sn. With increasing vacuum level, the Sn2+ fraction increased, possibly because of accelerated decomposition of the Sn4+ network. Chetri et al. reported that surface oxygen vacancies are easily formed in as-prepared SnO2 under vacuum annealing and that SnO2 compensates for these oxygen vacancies by transforming itself into SnO [41]. This hypothesis is supported by the experimental observations that the phase transition from SnO2 to SnO occurs through removal of bridging oxygen atoms from a stoichiometric SnO2 surface [26]. Therefore, the increase in the SnO fraction after HVPDA was found to involve decomposition of the SnO2 network. Thus, HVPDA is a suitable annealing process for SnO formation.The influence of the annealing temperature during HVPDA on SnO crystallization was examined. Fig. 5(a) shows Raman spectra of the SnOx films (10 nm) after HVPDA at 200–400 ◦C. Raman spectroscopic analysis confirmed that SnO crystallized at all of the investigated temperatures, and other phases were not detected. We attempted to probe the XRD pattern of the 10 nm thick SnOx films, but the films were too thin to detect any peaks with our facility. Thus, 50 nm thick films were used to obtain the XRD patterns, which are shown in Fig. 5(b), and also confirmed that no other phases were present. By the way, although the Raman peak intensity of the SnO film annealed at 200 ◦C is high, it is broadened towards lower wavenumbers with a large full width at half maximum (FWHM) of 29 cm⁻1, indicating small grain size. In contrast, the peak of the film annealed at 300 ◦C shows a smaller FWHM of 19 cm⁻1, indicating better crystal quality. The polycrystalline structure formed at 300 ◦C was also confirmed by cross-sectional transmission electron microscopy observations (Fig. 5(c)).3.2. Electrical properties of SnOx filmsThe electrical properties of 10-nm-thick SnOx films annealed under N2, LVPDA, and HVPDA conditions were first examined using Hall effect measurements. The HVPDA film exhibited p-type conductivity with moderate Hall mobility, whereas the N2 and LVPDA films showed n-type conductivity (results not shown). This n-type behavior is primarily attributed to a high SnO2 content, consistent with the XPS results in Fig. 4(c). Corresponding to the XPS data, we found that the Sn2+/Sn4+ratio determines the conduction type, where a ratio greater than 1 is the Fig. 5. (a) Raman spectra of SnOx films after HVPDA at 200–400 ◦C. (b) XRD patterns of SnO films with a thickness of 50 nm. (c) High-resolution cross-sectional TEM image of SnOx films after HVPDA at 300 ◦C.Y.-L. Sun et al.                                                                                                                                                                                                                                  Thin Solid Films 807 (2024) 140548 5 criterion for p-type SnO formation.Fig. 6 shows the conductivity, carrier density, and mobility of the SnOx films (10 nm) after HVPDA, as evaluated using Hall effect measurements. No conductivity was observed in the SnOx film subjected to HVPDA at 200 ◦C. In contrast, p-type behavior was observed in films annealed at a wide range of temperatures from 250 to 400 ◦C. The conductivity, carrier density, and Hall mobility were 0.5–1.8 S/cm, (0.3–1.6) × 1019 cm− 3, and 0.8–1.4 cm2/(V⋅s), respectively. Increases in the conductivity and carrier density and a reduction of the Hall mobility were observed after HVPDA at 400 ◦C, suggesting that n-type components (e.g., intermediate oxides and Sn metal) might be incorporated in the p-type SnO channel. This result potentially means that high- temperature HVPDA at 400 ◦C may lead to a transformation from SnO to other uncontrollable oxides of SnOx and Sn metal, or self- compensating defects, such as oxygen vacancies at high temperatures, may form in the film [42]. Intermediate oxides have been reported to be formed via thermal decomposition of SnO [43].We fabricated staggered bottom-gate TFTs using SnO films subjected to HVPDA and measured their electrical properties in the dark under vacuum conditions. Fig. 7(a) shows the transfer (ID–VG) characteristics of the TFTs fabricated with SnO channels subjected to various PDA temperatures. The VDS was maintained at − 0.1 V. p-Type behavior was observed for all of the TFTs, as indicated by the increase in IDS with increasingly negative VG. The output (ID–VD) characteristics of the TFT fabricated at 300 ◦C show linear and saturation regions (Fig. 7(b)). The linear region indicates that the Ni contacts form an ohmic contact with the SnO channels. The contact resistance, extracted using the transmission line method (TLM; channel length of 50–350 μm), was estimated to be in the range from 102 to 103 Ω⋅cm. Parameters such as the linear-region field effect mobility (μFE), subthreshold sing (SS), on-off current change (Ion/Ioff) ratio, and threshold voltage (VTH) were extracted from the transfer characteristics; the extracted values are plotted in Fig. 7(c). Relatively high μFE values greater than 1.8 cm2/ (V⋅s), small SS values of 10 V/dec, and high Ion/Ioff ratios greater than 3.5 × 104 were maintained at 250–300 ◦C. These properties were degraded when the HVPDA temperature was increased to 350 or 400 ◦C. The SS value is known to depend on the trap density in the interface between an SiO2 dielectric and an SnO channel. A trade-off relationship Fig. 6. Conductivity, carrier density, and Hall mobility of SnOx films after HVPDA at 200–400 ◦C, as characterized using Hall effect measurements.Fig. 7. (a) Id–Vg at VDS of − 0.1 V, (b) Id–Vd at 300 ◦C, and (c) mobility, SS, Ion/Ioff, and VTH properties of bottom-gate TFTs with SnO channel (channel length = 250 μm, width = 1000 μm).Y.-L. Sun et al.                                                                                                                                                                                                                                  Thin Solid Films 807 (2024) 140548 6 between SS and μFE with respect to the HVPDA temperature was clearly observed, indicating that the trap density at the SiO2/SnO interface increased as a function of HVPDA temperature. The VTH value shifted in the positive direction as the HVPDA temperature increased, indicating that electron traps were formed in the SiO2/SnO structure of the TFT. One of the origins of these phenomena is that n-type components were incorporated into the p-type SnO channel. The Ioff current, which corresponds to electron transfer in the accumulation region, was substantially smaller in the films annealed at 300 ◦C than in those annealed at 250 ◦C (Fig. 7(a)). The large Ioff current in the film annealed at 250 ◦C is attributed to insufficient transformation from SnO2 to SnO as a result of low-temperature HVPDA. We found that the optimal HVPDA temperature to obtain stable p-type SnO is 300 ◦C. The relatively high μFE of 1.8 cm2/(V⋅s) and high Ion/Ioff ratio of 5.1 × 104 are comparable to previously reported values [19,21,44]. Therefore, HVPDA at the optimal temperature is clearly a suitable method to increase the p-type SnO fraction.4. ConclusionThe effects of the PDA atmosphere and temperature on the SnO fraction in SnOx films were investigated. As-grown SnOx films deposited by RF sputtering using an SnOx (SnO:Sn = 60:40) target had an SnO (Sn2+) fraction of 42 % even when deposited at PO2 = 0 Pa. HVPDA (<5 × 10− 4 Pa) was found to substantially increase the Sn2+component to 62 % and induce p-type behavior, unlike PDA conducted under N2 and low- vacuum (~1 Pa) conditions, which resulted in n-type behavior. This p- type behavior was attributed to the transformation from SnO2 to SnO during HVPDA. For staggered bottom-gate TFTs with an SnO channel, HVPDA at temperatures ranging from 200 to 400 ◦C consistently led to p-type characteristics, with the best performance observed for the device corresponding to HVPDA at 300 ◦C, where the resultant film exhibited an Ion/Ioff ratio of 5.1 × 104 and a μFE of 1.8 cm2/(V⋅s). Our investigation into the critical parameters for achieving high-performance p-type TFTs provides valuable insights for optimizing SnO sputtering and PDA processes.CRediT authorship contribution statementYong-Lie Sun: Writing – original draft, Investigation, Data curation. Toshihide Nabatame: Writing – review & editing, Writing – original draft, Supervision, Investigation, Conceptualization. Jong Won Chung: Writing – review & editing, Investigation, Conceptualization. Tomomi Sawada: Data curation. Hiromi Miura: Data curation. Manami Miyamoto: Data curation. Kazuhito Tsukagoshi: Writing – review & editing, Writing – original draft, Supervision, Investigation, Conceptualization.Declaration of competing interestThe authors declare the following financial interests/personal relationships which may be considered as potential competing interests:Kazuhito Tsukagoshi reports financial support was provided by SAIT. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.AcknowledgmentsPart of this work was performed under the Cooperative Research Program of Institute for Joining and Welding Research Institute, Osaka University. This research was supported by the Samsung Advanced Institute of Technology (SAIT).Data availabilityNo data was used for the research described in the article. References[1] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, H. 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