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Kuan-Cheng Lu, Pen-Yuan Shih, Pin-Hsien Lin, Shih-Hao Wu, Kimitoshi Kono, [Wen-Bin Jian](https://orcid.org/0000-0002-1898-9641), Yu-Han Lin, Yo-Yao Ho, [Ching-Hwa Ho](https://orcid.org/0000-0002-7195-208X), Shin-Yuan Wang, Chao-Hsin Chien, [Ching-Yu Chiang](https://orcid.org/0000-0002-3640-0474), Shu-Jui Chang, Yu-Che Huang, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), [Kazuhito Tsukagoshi](https://orcid.org/0000-0001-9710-2692), [Chenming Hu](https://orcid.org/0000-0003-0836-6296)

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[Patternable laser-oxidized Ta2O5 dielectric and TaS2 contact for optimizing subthreshold swing of MoS2 field-effect transistors](https://mdr.nims.go.jp/datasets/13202d8e-f720-4746-9850-d505e0feb0f8)

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Patternable laser-oxidized Ta2O5 dielectric and TaS2 contact for optimizing subthreshold swing of MoS2 field-effect transistorsFull Length ArticlePatternable laser-oxidized Ta2O5 dielectric and TaS2 contact for optimizing subthreshold swing of MoS2 field-effect transistorsKuan-Cheng Lu a, Pen-Yuan Shih a, Pin-Hsien Lin a, Shih-Hao Wu a, Kimitoshi Kono a,  Wen-Bin Jian a,* , Yu-Han Lin b, Yo-Yao Ho b, Ching-Hwa Ho b , Shin-Yuan Wang c,  Chao-Hsin Chien c, Ching-Yu Chiang d , Shu-Jui Chang e, Yu-Che Huang f, Kenji Watanabe g ,  Takashi Taniguchi h, Kazuhito Tsukagoshi h, Chenming Hu f,ia Department of Electrophysics, National Yang Ming Chiao Tung University, Hsinchu 300093, Taiwanb Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei 106335, Taiwanc Institute of Electronics, National Yang Ming Chiao Tung University, Hsinchu 300093, Taiwand Scientific Research Division, National Synchrotron Radiation Research Center, Hsinchu 300092, Taiwane Corporate Research, Taiwan Semiconductor Manufacturing Company (TSMC), Hsinchu 308001, Taiwanf International College of Semiconductor Technology, National Yang Ming Chiao Tung University, Hsinchu 300093, Taiwang Research Center for Electronic and Optical Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japanh Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japani Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, Berkeley, CA 94720, USAA R T I C L E  I N F OKeywords:Two-dimensional semiconductorsMoS2Ultra-flat high-k dielectricTa2O5Ideal subthreshold swingvan der Waals contactA B S T R A C TRecent progress has achieved a high on-current density and a low contact resistivity for n-type field-effect transistors using two-dimensional (2D) semiconductors such as MoS2 and WSe2. However, other issues with device parameters remain unresolved, such as the use of complementary transistors with a single 2D semiconductor, environmental packaging, and dielectric layers. Integrating an optimal dielectric and ideal interfacial contact for mass production will play a crucial role in future electronics designs. Here, we demonstrate patternable laser oxidation to convert TaS2 into Ta2O5 as an ultra-flat dielectric layer, having been evidenced by energy-dispersive X-ray spectroscopy, nano-X-ray absorption near-edge structure, and atomic force microscopy. With this patternable conversion, we also demonstrate the implementation of TaS2 as contact electrodes on the MoS2 channel material. The Ta2O5 layer reveals a dielectric constant of ~15.98 and a breakdown field of ~5.5 MV/cm. A high on-current density of ~34.7 μA/μm is achievable in Bi-contacted devices at a channel length of 1.0 μm. Moreover, the TaS2-contacted MoS2 transistors on Ta2O5 present an extremely low subthreshold swing of ~59.8 mV/dec and a minimal hysteresis of ~0.15 V. This indicates the superior feature of the Ta2O5 dielectric through patternable laser oxidation while keeping an ultra-flat interfacial surface.1. IntroductionSince the successful demonstration of single-layer MoS2 transistors [1], layered structures of transitional metal dichalcogenides (TMDs) such as MoS2, WS2, and WSe2 have been implemented to build high-performance electronics [2–5]. The few-layer TMDs were alternatively applied to gas sensors [6], biosensors [7], optical sensors [8], and optoelectronics [9]. Among all layered TMDs, MoS2 was more popular and thoroughly researched. For example, the intrinsic scatterings in the FET channel of few-layer MoS2 were previously identified and investigated in detail [10], implying an important issue for making high-performance FETs. Most recently, the high performance of few-layer TMD field-effect transistors (FETs) was especially boosted by choosing better metal electrodes and contact processing [2,11]. It was demonstrated that the yttrium-doping lowered the contact resistance to 69 Ω μm, and that the on-current density reached 1.22 mA/μm for FETs with a gate length of 10 nm [11]. In addition to the contact issue, there are other environmental conditions, like gate dielectrics, device structures, and complementary FETs, leading to modulation of several key indicators of mobilities, subthreshold swings, and hysteresis for * Corresponding author.E-mail address: wbjian@nycu.edu.tw (W.-B. Jian). Contents lists available at ScienceDirectApplied Surface Science Advancesjournal homepage: www.sciencedirect.com/journal/applied-surface-science-advanceshttps://doi.org/10.1016/j.apsadv.2026.100955Received 5 November 2025; Received in revised form 2 February 2026; Accepted 17 February 2026  Applied Surface Science Advances 32 (2026) 100955 Available online 24 February 2026 2666-5239/© 2026 The Authors. 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/ ). https://orcid.org/0000-0002-1898-9641https://orcid.org/0000-0002-1898-9641https://orcid.org/0000-0002-7195-208Xhttps://orcid.org/0000-0002-7195-208Xhttps://orcid.org/0000-0002-3640-0474https://orcid.org/0000-0002-3640-0474https://orcid.org/0000-0003-3701-8119https://orcid.org/0000-0003-3701-8119https://orcid.org/0000-0003-0836-6296https://orcid.org/0000-0003-0836-6296mailto:wbjian@nycu.edu.twwww.sciencedirect.com/science/journal/26665239https://www.sciencedirect.com/journal/applied-surface-science-advanceshttps://doi.org/10.1016/j.apsadv.2026.100955https://doi.org/10.1016/j.apsadv.2026.100955http://creativecommons.org/licenses/by-nc-nd/4.0/http://creativecommons.org/licenses/by-nc-nd/4.0/few-layer MoS2 FETs [12].In earlier studies, severe challenges existed when conventional silicon processing technologies were applied to TMDs. For example, atomic layer deposition (ALD) was a mature ultrathin-film preparation technique for high-κ dielectrics in semiconductor technology. However, growing uniform high-κ films on TMDs suffered physical difficulties [13]. In the earlier stage, hexagonal boron nitride (h-BN) was introduced as a single dielectric layer or a secondary layer above the substrate. Using h-BN significantly improved electron transport in the channel and boosted device performances of TMD FETs [10,14,15]. It implied necessary conditions of atomically flat and dangling-bond-free surfaces, thereby reducing electron scattering for carriers in TMD channels. However, the dielectric constant of the h-BN dielectric is ~4.0, which is much lower than other high-κ materials. Recent studies have turned to converting two-dimensional (2D) TMDs, such as TaS2 and HfS2, into ultra-flat oxide layers [16,17]. TaS2 was oxidized using a hot plate in the ambient atmosphere, and the dielectric constant of Ta2O5 was estimated to be ~15.5 [16]. HfS2 was selectively photo-oxidized by laser irradiation, and the dielectric constant of HfOx was evaluated to be ~15 [17]. Additionally, other oxidation approaches, such as oxygen plasma [18], ozone exposure [19], and oxidation scanning probe lithography [20], were implemented to oxidize TMDs.The van der Waals contact plays another important role for high- performance FETs. Recent literature has indicated that the thermal evaporation for metal electrode deposition leads to the generation of numerous defects and chemical bonding [21]. The defects and bonding resulted in the defect-induced gap states (DIGS) and metal-induced gap states (MIGS) that could further lead to Fermi-level pinning (FLP) [22–24]. To avoid the creation of DIGS and MIGS, it was proposed that the transfer electrode [21,25,26], van der Waals contact [27–29], and semimetal contact [2] techniques be applied to the preparation of metal electrodes. Graphene, several TMDs, like NbS2 and TaS2, and MXene were either inserted between the metal and TMD channel or used as contact electrodes [25,30–32]. Employing these 2D featured materials and van der Waals contact nature significantly avoids difficulties from DIGS, MIGS, and FLP, thus boosting device performances of TMD FETs.In this study, we integrate the ultra-flat dielectric layer of Ta2O5 and the van der Waals contact of TaS2 to boost the device performance of MoS2 FETs. Selective area oxidation using a patternable laser writing is performed after the van der Waals TaS2 contacting electrode is placed without any photolithography procedures. This approach ensures a clean interface without photoresist residues, giving an ultra-flat dielectric layer and a low hysteresis for FETs. The device structure and selective-area laser patterning exhibit a high potential for mass production and scalability for future electronics manufacturing.2. Method2.1. Device fabricationFour-probe TaS2 and MoS2 FETs were fabricated on heavily p-doped Si substrates with a 300 nm SiO2 layer as the back-gate dielectric. Contact electrodes were patterned via electron beam lithography (EBL) and formed by thermal evaporation of 50 nm Au. The Ta2O5 parallel plate capacitor adopted a metal-insulator-metal (MIM) configuration. A 50 nm Au bottom electrode was first deposited on a SiO2 substrate. A few-layer TaS2 flake was exfoliated and transferred onto the electrode, then oxidized into Ta2O5 by 532 nm laser irradiation. The selective oxidation was carried out using a laser with a power density of 28 mW/ μm2, exposure time of 1 s per spot, and spot size of ~300 nm in diameter. The oxidation was performed by raster scanning across the surface with a step of ~500 nm, and the scanning procedure was repeated 10 times to ensure uniform oxidation. Finally, a 50 nm Au top electrode was deposited. Both bottom and top electrodes were defined by EBL and deposited by thermal evaporation. The MoS2 FETs incorporated a high-κ dielectric layer of Ta2O5. The fabrication began by defining a trench using EBL and etching it to a depth of 50 nm with a buffered oxide etch (BOE) solution. This trench was filled with a Ti/Au (10/40 nm) layer to make the embedded gate. Next, a TaS2 flake was transferred and patternable laser-oxidized to form Ta2O5 using the same oxidation parameters as the MIM capacitors. A few-layer MoS2 flake was transferred and served as the channel material. Source and drain electrodes were formed using thermally evaporated Au (100 nm), Bi/Au (30/50 nm), or unoxidized TaS2 flakes defined during the laser oxidation step. The bulk MoS2 crystal was purchased from SPI Co., while the TaS2 bulk crystal was provided by Prof. Ching-Hwa Ho (NTUST, Taiwan).2.2. Electrical properties measurementsThe transfer characteristics of the FETs were measured using high- precision electrometers (Keithley 6430 and 2400, Tektronix), a voltmeter (Keithley 2000, Tektronix), and a probe station (TTPX, Lake Shore Cryotronics Inc.) under a high-vacuum environment. The FET behavior was assessed using a two-terminal and four-terminal configuration. In the two-terminal configuration, the drain-source voltage (VDS) was fixed at 1 V, and the transfer current was recorded with the Keithley 6430 while the back-gate voltage was swept using the Keithley 2400. For the four-terminal configuration, a fixed voltage was applied to the outer electrodes, and the transport current was measured with the Keithley 6430, while the voltage drop between the inner electrodes was monitored using the Keithley 2000. The back-gate voltage was controlled by the Keithley 2400. The sheet Conductivities of the MoS2 and TaS2 flakes were extracted from the four-terminal configuration. The dielectric properties of the Ta2O5 capacitor were characterized by C–V measurements conducted under high-vacuum conditions at room temperature using a precision current–voltage analyzer (B1500A, Keysight). A voltage sweep ranging from +1 V to − 1 V was applied across the Ta2O5 capacitor, and measurements were performed at three distinct frequencies (1, 3, and 10 kHz).2.3. Material characterizationThe crystal phase and lattice constant of 1T-TaS2 were determined by X-ray diffraction (XRD) analysis (D2 PHASER, Bruker). The TaS2 crystal was finely ground into powder and analyzed using Copper Kα radiation (1.5418 Å) and the detector system (LYNXEYE XE-T, Bruker). Nano-X- ray absorption near-edge structure (nano-XANES) (BL21A, Taiwan Photon Source (TPS)) was employed to investigate the transformation of TaS2 into Ta2O5 by analyzing the Ta L₃-edge spectra. Further details for the nano-XANES are provided in the Supplementary material. The thicknesses of TaS2, Ta2O5, and MoS2 were measured using an atomic force microscope (AFM, SPA-300HV, Seiko Instruments Inc.) operated in tapping mode with AFM tips (PPP-RT-FMR, Nano sensors). The AFM tips had a radius of curvature of <10 nm, a resonance frequency of 75 kHz, and a force constant of 2.8 N/m. The cross-section specimen was prepared through dual beam focused ion beam (Dual Beam FIB, Helios 5 Dual Beam, Thermo Fisher Scientific Inc.). Platinum and amorphous carbon layers were first deposited for surface protection. Subsequently, 30 kV ion milling and lift-out with a micromanipulator were performed to obtain thin lamellae, which are affixed onto a TEM grid. The cross- sectional images and energy-dispersive spectrometer (EDS) analysis were obtained at 200 keV by high-resolution scanning transmission electron microscopy (HR-STEM, JEM-2800F, JEOL Ltd.)3. Experimental results and discussionsFig. 1(a) and 1(b) exhibit optical microscope images of four-probe FETs with TaS2 and MoS2 flakes, respectively. The inset to Fig. 1(a) presents the AFM measurement of the line profile indicated as a dashed line on the SEM image. It reveals the thickness of ~8 nm (9 layers) for the TaS2 flake. On the other hand, the 3-layer thickness of the MoS2 flake shown in Fig. 1(b) is estimated using the optical contrast measurement. K.-C. Lu et al.                                                                                                                                                                                                                                   Applied Surface Science Advances 32 (2026) 100955 2 The crystalline structure of the layered TaS2 bulk is further characterized by XRD. The XRD spectrum shown in Fig. 1(c) with marked lattice planes corresponds to the JCPDS card No. 02-1317, indicating the crystalline phase of 1T-TaS2 with lattice constants calculated to be a =3.341 and b = 5.990Å. Fig. 1(d) presents the TaS2 and MoS2 FET device transfer curves, as shown in Fig. 1(a) and 1(b) using two-probe measurements at a source-drain voltage (VDS) of 0.1 V and back-gating voltages (Vg) ramping from − 60 to 60 V. The TaS2 FET exhibits a high channel current of 82.38 μA/μm that exposes negligible variation of current densities with regard to the Vg. In contrast, the MoS2 FET shows an n-type behavior with an on-off ratio of ∼ 107, an on-current density of 3.98 μA/μm, and a mobility of 82.2 cm2V− 1s− 1. The inset in Fig. 1(d) shows linear current-voltage behaviors to confirm an Ohmic contact for TaS2 and MoS2 FETs. Fig. 1(e) illustrates temperature behaviors of the sheet conductivities of TaS2 and MoS2 flakes at the Vg of 60 V using the four-probe measurement. In contrast to the semiconducting feature of the MoS2 flake, the TaS2 flake offers a metallic character with a high sheet conductivity of 1.62 mS/ϒ at a temperature of 300 K. The high conductivity of TaS2 flakes manifests itself as ideal contact electrodes for MoS2 FETs. The concept is checked and confirmed in Figure S1 in the Supplementary material.The few-layered TaS2 flake is area-selectively converted to TaOx using programmable laser irradiation in the ambient atmosphere. It is attributed to laser-induced photothermal heating, which activates the oxidation of TaS2 with oxygen and moisture in air, thereby forming amorphous TaOx in the irradiated region while keeping other regions of TaS2 intact. Fig. 2(a) shows an optical image of a TaS2 flake on a Si substrate before laser-induced oxidation. A specified area of 2 μm × 2 μm is patterned by laser writing, and the result is presented in Fig. 2(b). The oxidation area, marked by an orange frame, reveals a faded color, indicating material transformation. The transformation is inspected using nano-XANES spectra for the determination of the valence state of Ta atoms. The Ta L3-edge nano-XANES spectra are displayed in Fig. S2 in the Supplementary material. After laser oxidation, a blue shift of the spectrum is clearly observed, which is in line with previous studies of XANES spectra of TaS2 and Ta2O5 and offering clear evidence of the valence state transformation from Ta4+ to Ta5+ [33,34]. In addition to material conversion, Fig. 2(c) reveals a thickness variation before and after laser-induced oxidation, where the thicknesses are measured by AFM. The flake thickness decreases to ~68 % of its original thickness (see the slope in Fig. 2(c)). The thickness of Ta2O5 ranges from 4.8 to 9.8 nm and can be controlled according to the initial thickness of the TaS2 flake. AFM topography further reveals that the laser-oxidized Ta2O5 possesses an ultra-flat surface with a root-mean-square (RMS) roughness of ~0.812 nm, compared to ~0.700 nm for the pristine TaS2 flake. Notably, in comparison with Ta2O5 films deposited using conventional techniques, the laser-oxidized Ta2O5 achieves superior flatness. The comparisons are presented in detail in Fig. S3 and Table S1 in the Supplementary material.The electrical properties of TaS2 flakes are measured before and after oxidation. Fig. 2(d) shows current-voltage (I-V) curves of TaS2 and Ta2O5, corresponding to the optical images of the devices displayed in Figs. S4(a) and S4(b) in the Supplementary material. After laser- induced oxidation on a selective area, the sheet resistivity largely changes from 770 Ω/□ to 370 MΩ/□, indicating a conversion from the metallic TaS2 to insulating Ta2O5. Further, the capacitive properties and breakdown electric fields are investigated. The inset to Fig. 2(e) shows the scheme of the metal-insulator-metal device structure and the electrical circuit connection for the capacitive property measurements. Fig. 2(e) depicts areal capacitance (Cox) as a function of back gate voltage at three different frequencies from 1 kHz to 10 kHz. The Fig. 1. (a) Optical micrograph with a scale bar for the 9-layer thick 1T-TaS2 device on the 300-nm thick SiO2 substrate. The inset displays the line profile of the white dashed segment marked in Fig. 1a. (b) Optical microscope of the 3-layer thick MoS2 FET device on the SiO2 substrate. (c) XRD spectrum of the 1T-TaS2 bulk with the standard pattern of red peaks from the JPCDS card No. 02-0137. (d) Current densities as a function of the back-gating voltage, Vg , for the 1T-TaS2 and the MoS2 FET devices at a bias voltage of 0.1 V using the source and drain for the two-probe measurement. The inset shows current-voltage (I-V) curves of the 1T-TaS2 and the MoS2 FET devices at the Vg of 60 V using the two-probe measurement. The current densities of the MoS2 FET device are magnified to be 10 times larger. (e) Sheet conductivities as a function of temperature for the 1T-TaS2 and the MoS2 FET devices at Vg of 60 V using four-probe measurement.K.-C. Lu et al.                                                                                                                                                                                                                                   Applied Surface Science Advances 32 (2026) 100955 3 dielectric constant (κ) of Ta2O5 is evaluated from the areal capacitance according to the equation Cox = κε0/t, where ε0 is the vacuum permittivity of electric fields. The evaluated dielectric constant is in the range from 15.98 to 15.68, revealing a very small change regarding gating voltages and frequencies. Fig. 2(f) presents the areal current as a function of the electric field and gives the breakdown electric field of ~5.5 MV/cm at a current density of ~0.001 A/cm² for Ta2O5. The inset to Fig. 2(f) provides a view of the electric field behavior on a logarithmic scale.To demonstrate patternable laser oxidation, we make MoS2 FETs with an embedded gate electrode and a Ta2O5 layer. The Ta2O5 layer is converted from a specific thickness of a TaS2 flake through patternable laser-induced oxidation. Fig. 3(a) shows a schematic diagram of the embedded-gate MoS2 FET. The source/drain contact electrode is directly deposited on the surface of the MoS2 flake. Fig. 3(b) presents a typical optical image of a MoS2 FET (Device MS-TaO-Au3) with a Ta2O5 dielectric layer and directly contacted Au electrodes. All represented devices with physical parameters and device performances are listed in Table S2 in the Supplementary material. The MoS2 FET in Fig. 3(b) consists of a 7-layer-thick MoS2 flake and a 6.0-nm-thick Ta2O5 layer. The transfer curves, current density (J) as a function of Vg, in linear and logarithmic scales for the Device MS-TaO-Au3 after annealing at 170 ◦C in a high vacuum (<10− 6 Torr) for 30 min are shown in Fig. 3(c). The measurements are carried out at the source-drain voltage (VDS) of 1 V with Vg ranging from − 2 to 1 V at room temperature. The on-current density is ~3.6 μA/μm at Vg of 1 V, where the device channel length (L) is ~4.0 μm On the logarithmic scale, the minimum subthreshold swing (SS) is calculated to be ~62.2 mV/dec at 300 K from the slope of the green linear fitting line in the subthreshold region, according to 1 /SS = d(logJ)/ dVg [35]. A more detailed analysis of the annealing temperature and the SS as a function of current density is provided in Figs. S5 and S6(a) in the Supporting Information. The field-effect mobility (μ) of ~1.43 cm2/V s is obtained from the current density in linear scale using the equation of μ =(dJ /dVg)(LCox /VDS), where dJ/dVg is obtained from the slope of the blue linear fitting line in the linear scale. The Cox of 15.98 is used in the calculation (see description of Fig. 2(e)). The linear scale in Fig. 3(c) further gives the evaluation of the threshold voltage (Vth) of about − 0.6 V, which is the intercept of the linear fitting (the blue dotted line) and the zero current density. In addition, the corresponding gate leakage current (Ig) is below 1.4 ×10− 14 A/μm, which is the minimum detection limit of the current meter and shown as the gray dotted line in Fig. 3(c). To assess the interface quality of the Ta2O5 layer, we perform the hysteresis loop measurements of the transfer curve. The hysteresis in the transfer curve for Device MS-TaO-Au3 is about 0.15 V (see Fig. S7 and corresponding descriptions in the Supplementary material).The importance of the van der Waals contact and the capability of the patternable laser-induced oxidation are mentioned previously in the introduction. The concept has materialized as presented in Fig. 4(a). The insert provides atomic structures of the scheme for the TaS2 contacts on both the MoS2 channel and the Au electrode. Only the specified region in dark brown of the TaS2 flake is selectively oxidized and converted to the Ta2O5 dielectric layer. Fig. 4(b) shows the top-view optical micrograph of the Device MS-TaO-TaS1, and Fig. 4(c) displays the corresponding transfer curves measured after annealing at 170 ◦C for 30 min. With the TaS2 used as a van der Waals contact, the on-current density increased to ~6.3 μA/μm, and the mobility is ~4.83 cm2/V s, where the device channel length is ~4.0 μm In addition, the subthreshold swing is a little bit lowered to be ~59.8 mV/dec with a hysteresis of ~0.15 V at 300 K, and the detailed analysis of the SS as a function of current level is provided in Fig. S6(b) in the Supporting Information. Notably, this value is close to the thermodynamic limit in theory, which can be expressed as SSlimit = (kBT /q)ln (10) ≈ 59.526 mV /dec, where kB is the Boltzmann constant, T = 300 K is the temperature and q is the charge. The Fig. 2. Optical micrographs with scale bars of a few-layered TaS2 flake (a) and after (b) laser-patterning oxidation. The orange dashed rectangle indicates the area illuminated by the 532-nm laser. (c) Thickness of the oxidized TaS2 flake (Ta2O5 flake) as a function of its original TaS2 thickness. The red solid line presents the best linear least-squares fitting to the data, giving a slope of ~0.68. (d) Current as a function of voltage using the two-probe measurement for the same TaS2 flake before and after laser-induced oxidation. The sheet resistivities are evaluated as about 770 Ω and 370 MΩ for the original TaS2 and the oxidized Ta2O5 flake, respectively. (e) Areal capacitance as a function of gating voltage for a 9.8-nm thick Ta2O5 flake measured at frequencies of 1, 3, and 10 kHz. The inset illustrates the device structure and connecting circuit. (f) Areal current density as a function of electric field for the Ta2O5 flake. The breakdown field of ~5.5 MV/cm at the pre-breakdown current density of ~0.001 A/cm2 is marked. The inset offers the same graph with the current density on a logarithmic scale.K.-C. Lu et al.                                                                                                                                                                                                                                   Applied Surface Science Advances 32 (2026) 100955 4 performance metrics and parameters of other devices are summarized inTable S2 of the Supplementary Material. On average, the TaS2 contacted MoS2 FETs show higher current densities as compared with Au contacted MoS2 FETs. To further explore the contact issue in the MoS2 FET devices, we further make devices with a direct metal contact, while the Bi is selected to replace the Au metal contact. Device MS-TaO- Bi2 is the Bi-contacted MoS2 FET, and it reveals an on-current density of ~34.7 μA/μm at VDS of 1 V with channel length of 1.0 μm It also reveals an SS value of 68.7 mV/dec at a low VDS of 0.1 V. The tendency of lower SS value is observed at a low driving VDS. The corresponding gate leakage current is below 1.3 × 10− 14 A/μm, indicated by the gray dotted line in Fig. 4(c). Table 1 summarizes the SS values for most of TMD FETs with several different dielectric materials. It is noted that the dielectric of Ta2O5 achieves the lowest SS value. This work also presents the benefits of controllable thickness and patternable oxidation for converting TaS2 to an ultra-flat Ta2O5 layer. On the other hand, we also make MoS2 FETs with other dielectric materials, such as SiO2, Si3N4, and h-BN, for comparison. Transfer curves of MoS2 FETs on dielectrics of Ta2O5, h-BN, SiO2, and Si3N4 are displayed in Figs. S8(a)–S8(d), and the device performances are summarized in Fig. S9 and Table S3 in the Supplementary material. In comparison to other dielectrics, the Ta2O5 dielectric for FETs reveals the lowest interface trap states of 3.23×10− 11 eV− 1cm− 2. The MoS2 FETs on the Ta2O5 dielectric exhibit the lowest SS value and the smallest hysteresis, all confirming the superior feature of the Ta2O5 dielectric layer.The cross-sectional scanning transmission electron microscope (STEM) with energy-dispersive X-ray spectroscopy (EDS) mapping for elemental analyses is implemented to check the dielectric layer of Ta2O5. Fig. 5(a) shows the optical micrograph of Device MS-TaO-TaS1. The red line indicates where the section specimen of the cross-sectional area is prepared by a focused ion beam (FIB). The corresponding STEM image of the section specimen, consisting of the source, drain electrodes, and the embedded gate electrode, is presented in Fig. 5(b). The area marked in the red rectangle is closely inspected and shown as a high- resolution STEM image in Fig. 5(c). Regions i and ii represent the contact and embedded back electrodes, respectively. Region iii displays the MoS2 channel, TaS2 flake, and Ta2O5 between SiO2 and Pt. High- resolution STEM images of regions close to the contact and gate electrodes are exhibited in Figs. S10(a) and S10 (b), respectively. The STEM image of the area close to the gate region shows the MoS2 on Ta2O5 (Fig. S10(a)), while that of the area close to the contact region reveals the unoxidized TaS2 used as a van der Waals contact electrode (Fig. S10 (b)). The remaining Regions v and vi in Fig. 5(c) mark the protection layers of Pt and amorphous carbon for the FIB cross-sectioning. Figs. 5(d) and 5(e) display EDS elemental mapping for Regions i and ii, respectively. Fig. 5(d) shows Ta and S elements distributed in the same depth, which indicates the TaS2 contact electrode. In contrast, Fig. 5(e) shows clear oxygen elements near the gate electrode, where the TaS2 flake is oxidized to Ta2O5 layer. The detailed depth distributions of individual elements are provided in Figs. 5(f) and 5(g) for the contact and gate regions, respectively. The depth distributions of Ta and O elements offer curves for the calculation of areas under the curves, which confirms again the ratio of approximately 2 to 5 for Ta and O elements in Ta2O5.4. ConclusionsWe have demonstrated patternable oxidation to convert a 1T-TaS2 flake to an ultra-flat Ta2O5 dielectric layer with a thickness of 5–6 nm. The patternable capability leads to the fabrication of either direct metal- or 2D TaS2 van der Waals-contacted MoS2 FETs. The dielectric properties of Ta2O5 are characterized to reveal its dielectric constant κ of ~15.98 and its breakdown electric field of ~5.5 MV/cm. The nano- Fig. 3. (a) Schematic diagram of the MoS2 FET device with Au as a metal contact and Ta2O5 as a dielectric. (b) Optical micrograph of the Device MS-TaO-Au3 of Au- contacted MoS2 FET on the Ta2O5 dielectric. The dark brown dashed frame indicates the Ta2O5 dielectric, which is below the MoS2 flake (marked by the blue dashed frame). The source, drain, and gating electrodes are labeled by S, D, and G characters, respectively. (c) Current density as a function of embedded gating voltage for the Device MS-TaO-Au3. The corresponding gate leakage current (Ig) is shown as the gray dotted line on the logarithmic scale. The threshold voltage (Vth) is denoted, and the subthreshold swing is estimated to be ~62.2 mV/dec.K.-C. Lu et al.                                                                                                                                                                                                                                   Applied Surface Science Advances 32 (2026) 100955 5 XANES spectra and the STEM equipped with depth distribution of Ta and O elements are used to determine the ratio of 2 to 5 in the Ta2O5 dielectric layer. Further, the STEM images present the ultra-flat Ta2O5 layer as well as the perfect van der Waals contact of TaS2 on the MoS2 channel. The Bi-contacted MoS2 FET on the Ta2O5 dielectric shows a high on-current density of ~34.7 μA/μm at a channel length of 1.0 μm In addition, the TaS2-contacted MoS2 FET on the Ta2O5 dielectric presents the lowest SS value of 59.8 mV/dec and the smallest hysteresis of 0.15 V. The van der Waals contact features a damage-free 2D channel surface and the prevention of the Fermi level pinning. Comparing the Ta2O5 to SiO2, Si3N4, and h-BN dielectrics, the MoS2 FET on the ultra-flat and thin Ta2O5 dielectric possesses the lowest SS value, the lowest interface trap states, and the smallest hysteresis, indicating the superior feature of its dielectric properties for making 2D FETs.CRediT authorship contribution statementKuan-Cheng Lu: Writing – original draft, Validation, Methodology, Investigation, Conceptualization. Pen-Yuan Shih: Investigation, Data curation. Pin-Hsien Lin: Investigation, Data curation. Shih-Hao Wu: Investigation, Data curation. Kimitoshi Kono: Resources. Wen-Bin Jian: Writing – review & editing, Validation, Supervision, Resources, Project administration, Funding acquisition, Conceptualization. Yu-Han Lin: Data curation. Yo-Yao Ho: Data curation. Ching-Hwa Ho: Resources. Shin-Yuan Wang: Data curation. Chao-Hsin Chien: Resources. Ching-Yu Chiang: Resources. Shu-Jui Chang: Resources. Yu- Che Huang: Resources. Kenji Watanabe: Resources. Takashi Taniguchi: Resources. Kazuhito Tsukagoshi: Resources. Chenming Hu: Resources.Fig. 4. Schematic diagram of the MoS2 FET device with TaS2 as a metal contact and Ta2O5 as a dielectric. (b) Optical micrograph of the Device MS-TaO-TaS1 of TaS2- contacted MoS2 FET on the Ta2O5 dielectric. The dark brown, blue, and orange dashed frames point to the Ta2O5 dielectric, MoS2 flake, and TaS2 flake. (c) Current density as a function of embedded gating voltage for the Device MS-TaO-TaS1. The corresponding gate leakage current (Ig) is shown as gray dotted line on the logarithmic scale. The subthreshold swing is evaluated as ~59.8 mV/dec.Table 1 SS values of TMD FETs from our work in comparison with others’ results. TG, BG, SG, and DG are the abbreviations of top gate, back gate, single gate, and double gate. *Vertical-channel FET.TMD Dielectric Tox (nm) L (μm) W (μm) Gate SS (mV/dec) Refs.3L-MoS2 Ta2O5 5.5 4 0.5 SG 59.8 Our work9L-MoS2 Ta2O5 31 5 5 SG 61 [16]MoS2 h-BN 4 0.009 2 SG 93 [36]2L-MoS2 ZrO2 5.8 0.5 2.5 SG 65 [37]3L-WS2 HfO2 2.8 0.04 - SG 67.1 [38]4L-MoS2* HfO2 10 0.0087 - SG 73 [39]MoS2 Gd2O5 8.5 3 4 SG 61.5 [40]Bi2O2Se Bi2SeO5 5 10.4 34.3 SG 75 [41]1L-MoS2 PTCDA/HfO2 (TG) Al2O3 (BG)3 302.4 5.2 DG 60 [42]7L-MoS2 h-BN (TG) h-BN (BG)8.9 8.48 8 DG 65.5 [43]K.-C. Lu et al.                                                                                                                                                                                                                                   Applied Surface Science Advances 32 (2026) 100955 6 Declaration of competing interestThe authors declare the following financial interests/personal relationships which may be considered as potential competing interests:Wen-Bin Jian reports financial support was provided by National Science and Technology Council Taiwan. 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.AcknowledgmentThis work was supported by the National Science and Technology Council, Taiwan, under Grant No. NSTC-114-2112-M-A49-008. This work was also financially supported by NSTC T-Star Center Project: Future Semiconductor Technology Research Center under NSTC 114- 2634-F-A49-001. Kenji Watanabe and Takashi Taniguchi acknowledge support from the JSPS KAKENHI (21H05233 and 23H02052), the CREST (JPMJCR24A5), JST and World Premier International Research Center Initiative (WPI), MEXT, Japan.Supplementary materialsSupplementary material associated with this article can be found, in the online version, at doi:10.1016/j.apsadv.2026.100955.Data availabilityData will be made available on request.References[1] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Single-layer MoS2 transistors, Nat. Nanotechnol. 6 (2011) 147–150.[2] W. Li, X. Gong, Z. Yu, L. Ma, W. Sun, S. Gao, Ç. Köroğlu, W. Wang, L. Liu, T. Li, Approaching the quantum limit in two-dimensional semiconductor contacts, Nature 613 (2023) 274–279.[3] L. Zhan, X. Pei, J. Tang, S. Li, S. Li, Y. Li, L. Li, C. Wan, Y. Deng, Y. Shi, Highly oriented WS2 monolayers for high-performance electronics, Adv. Mater. 37 (2025) 2414100.[4] X. Li, P. Zhou, X. Hu, E. Rivers, K. Watanabe, T. Taniguchi, D. 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(c) High-resolution STEM image of the area denoted by the red frame in Fig. 5(b). The symbols for the regions are the contact electrode (i), the gate electrode (ii), the channel of MoS2/TaS2(Ta2O5) (iii), the substrate of SiO2 (iv), the Pt metal (v), and the amorphous carbon (vi). (d) EDS mapping of the contact electrode (Region i in Fig. 5(c)). The elements Au, Mo, S, Ta, and Si are colored in orange, light blue, green, purple, and yellow, respectively. (e) EDS mapping of the gate electrode (Region ii in Fig. 5(c)). The elements Mo, S, Ta, O, and Au are colored in light blue, green, purple, deep blue, and orange. (f) Line profile of the EDS mapping for Region i marked by the orange frame. The sections of MoS2 and TaS2 are colored in green and pink. (g) Line profile of the EDS mapping for Region ii marked by the purple frame. The sections of MoS2 and Ta2O5 are colored in green and blue, respectively.K.-C. 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Patternable laser-oxidized Ta2O5 dielectric and TaS2 contact for optimizing subthreshold swing of MoS2 field-effect transistors 1 Introduction 2 Method 2.1 Device fabrication 2.2 Electrical properties measurements 2.3 Material characterization 3 Experimental results and discussions 4 Conclusions CRediT authorship contribution statement Declaration of competing interest Acknowledgment Supplementary materials Data availability References