Contact engineering of metallic Ni-integrated niobium sulfide via H₂S treatment for enhanced MoS₂ transistor performance and CMOS compatibility

Abstract

For scaled transistors utilizing transition metal dichalcogenide (TMDC) channels, we demonstrate that the Ni_0.19Nb_1.16S₂ is a promising contact material for n-type field-effect transistors (nFETs). This material is derived from the metallic TMDC NbS₂ through Ni incorporation and enables monolayer MoS₂-channel nFETs with Ni_0.19Nb_1.16S₂ source/drain contacts to achieve higher on-state current and superior thermal stability up to 600 °C compared to devices with NbS₂ or Ni contacts. Metallic NbS₂, owing to its high work function, is suitable as a contact material for p-type FETs (pFETs). However, the incorporation of Ni atoms into NbS₂ modifies both its crystal structure and work function, effectively converting its suitability from pFETs to nFETs applications. Therefore, Ni_0.19Nb_1.16S₂ represents a promising contact material for MoS₂-channel nFETs, while NbS₂ remains appropriate for pFETs, thereby advancing CMOS integration process technology for TMDC-based CMOS devices.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-026-41610-3.

Introduction

The performance of Silicon-based field-effect transistors (FETs) has been enhanced through continuous miniaturization, the introduction of new materials, and the adoption of advanced structures such as FinFET and gate-all-around (GAA) architectures. However, the carrier mobility of Si nanosheet decreases significantly when the thickness falls below ~ 5 nm^1–3, primarily due to potential fluctuations arising from atomic-scale variations in nanosheet thickness⁴. To address this issue, transition metal dichalcogenides (TMDCs) have attracted considerable attention as next-generation semiconductor channel materials to replace Si nanosheet, owing to their advantageous properties, particularly their high carrier mobility even at thickness below approximately 1 nm^5–8. One of the key challenges in realizing TMDC channel FETs is reducing the contact resistance at the interface between TMDC and metal electrode^9–13. To reduce contact resistance, van der Waals (vdW) contacts, formed by stacking two-dimensional layered metals onto TMDC semiconductors, have been reported^14,15. Examples include Au^16,17, In¹⁸, and Sb¹⁹ contacts for MoS₂-channel n-type FETs (nFETs) and WSe₂-channel p-type FETs (pFETs). Similarly, we have previously demonstrated that vdW contacts, in which Bi₂Te₃ is used for WSe₂-channel pFETs²⁰ and Sb₂Te₃ for MoS₂-channel nFETs²¹, lead to improved device performance. Furthermore, we recently demonstrated vdW contacts using the metallic TMDC NbS₂ for WSe₂-channel pFETs, achieving higher on-state currents compared to conventional metal contacts. This improvement is attributed to the high work function of NbS₂ (~ 6.2 eV)^22–24, which is synthesized from a Nb film through H₂S annealing at 600 °C, a temperature optimized to minimize sulfidation-induced damage to WSe₂²⁵. On the other hand, NbS₂ is not suitable as a contact material for nFETs due to its high work function. We therefore explore alternative contact materials with consideration for integration into the CMOS fabrication process²⁶.

In this study, we demonstrate the advantages of nickel-incorporated NbS₂ (Ni_xNb_yS_z) as a contact material for nFETs. Nickel is commonly used as a promoter for metal-induced growth and can readily diffuse within two-dimensional materials^27–29. These characteristics motivated us to investigate the possibility of tuning the structure and electronic properties of NbS₂ through Ni incorporation. Accordingly, we propose a TMDC-channel CMOS comprising WSe₂-channel pFETs with NbS₂ contacts²⁵ and MoS₂-channels nFETs with Ni_xNb_yS_z, as shown in Fig. 1(a). Both contact materials were synthesized using a common H₂S annealing process, enabling simplified integration into the CMOS fabrication process, as shown in Fig. 1b. We further elucidate the structure of Ni_xNb_yS_z on MoS₂ and evaluate the thermal stability of MoS₂-channels nFETs with Ni_xNb_yS_z contacts.

Fig. 1.

(a) Transition metal dichalcogenides (TMDC)-channel CMOS integration technology utilizing NbS₂ for p-type FETs (pFETs) and Ni_xNb_yS_z contacts for n-type FETs (nFETs). Incorporation of Ni atoms into NbS₂ modifies both its crystal structure and electronic properties, effectively enabling its application in nFETs. (b) Contact formation process for both pFETs and nFETs, where Ni_xNb_yS_z and NbS₂ contacts are formed through a common H₂S annealing process.

Results and discussion

Synthesis and structural analysis of Ni_xNb_yS_z film on MoS₂

The Ni_xNb_yS_z film on single-crystal monolayer MoS₂ (ML-MoS₂) was synthesized by annealing a 1 nm-thick Nb film deposited on a 1 nm-thick Ni film atop MoS₂ (Nb/Ni/MoS₂) in an H₂S gas atmosphere, as shown in Fig. 2a. Notably, the structure of the underlying ML-MoS₂ was preserved after the formation of Ni_xNb_yS_z, and no diffusion or intermixing of Ni or Nb atoms into the ML-MoS₂ layer was observed. Compared to a Ni-only or Nb-only film on ML-MoS₂ subjected to the same annealing conditions used for Ni_xNb_yS_z formation, EDX analysis revealed diffusion of Ni or Nb atoms into the underlying MoS₂, as shown in Fig. 2b–c. The Ni/MoS₂ stack was likely converted into a Ni-Mo-S compound through sulfurization of Ni (Fig. 2b and Fig. S3), diffusion of Ni into MoS₂, and/or intermixing of Ni/MoS₂ during H₂S annealing. Accordingly, the three-layer laminated structure of Nb/Ni/MoS₂ is thermodynamically stabilized through the formation of Ni_xNb_yS_z on MoS₂, in contrast to the Ni/MoS₂ or Nb/MoS₂ stacks. It is also noteworthy that Nb diffusion into ML-MoS₂ was observed when the stacking order was changed from Nb/Ni/MoS₂ to Ni/Nb/MoS₂. This suggests that Nb atoms may have diffused into the ML-MoS₂ layer during the Nb deposition process³⁰. To elucidate the reaction mechanisms, various stacks were systematically investigated, as detailed in Supplementary Information. Notably, the Ni/Nb/MoS₂ stack did not result in the formation of Ni_xNb_yS_z on MoS₂, unlike the Ni/Nb/MoS₂ stack. This difference may be attributed to Nb diffusion into MoS₂, which likely suppresses the formation of Ni_xNb_yS_z.

Fig. 2.

Cross-sectional scanning transmission electron microscope (STEM) images and energy dispersive X-ray spectroscopy (EDX) mappings of (a) crystalline Ni_0.19Nb_1.16S₂ formed on monolayer MoS₂ (ML-MoS₂) after H₂S annealing at 600 °C, showing no diffusion or intermixing of Ni and Nb elements into the underlying ML-MoS₂. (b) Ni and (c) Nb on ML-MoS₂ after H₂S annealing at 600 °C, showing diffusion or intermixing of Ni or Nb elements into underlying ML-MoS₂.

A crystal structure of Ni_xNb_yS_z on ML-MoS₂ was clearly observed by high-resolution cross-sectional scanning transmission electron microscopy (STEM), as shown in Fig. 3a. Here, Ni_xNb_yS_z was synthesized by annealing a 1 nm-thick Nb film deposited on a 1 nm-thick Ni film atop MoS₂. Focused ion beam (FIB) processing was performed parallel to the sides of the triangular MoS₂ domains; consequently, the cross section was observed along the [210] direction of MoS₂, confirming the formation of a vdW junction with well-aligned Ni_xNb_yS_z on MoS₂. Also, we investigated the effect of the initial Ni film thickness on the formation of Ni_xNb_yS_z by preparing initial stacks of Nb (1 nm)/Ni (1 nm), Nb (1 nm)/Ni (0.2 nm), and Nb (1 nm)/Ni (0.1 nm) on ML-MoS₂ (Fig. 3a–c). After H₂S annealing, all Ni_xNb_yS_z films exhibited a uniform thickness of 2.9 nm, regardless of the initial Ni film thickness. This finding indicates that the Ni_xNb_yS_z film possesses a maximum Ni content, and its overall thickness is primarily determined by the initial Nb film thickness. STEM images further revealed the locations to which the excess Ni atoms had migrated, as shown in Fig. 3d–f. Ni aggregation was observed on the surface of the Nb (1 nm)/Ni (1 nm) and Nb (1 nm)/Ni (0.2 nm) samples after the H₂S annealing, with the amount of aggregation increasing as the initial Ni thickness increased. In contrast, surface Ni aggregation was scarcely observed in the Nb (1 nm)/Ni (0.1 nm) structure. Furthermore, we examined the Ni_xNb_yS_z film on MoS₂, as shown in Fig. 4a synthesized Nb (1 nm)/Ni (0.1 nm) on MoS₂ by H₂S annealing. This crystal structure closely resembles that of Nb_1.35S₂, which contains excess Nb atoms located between NbS₂ layers³¹. Figure 4b shows the positions of Ni atoms were analyzed using an electron energy loss spectroscopy (EELS) line scan in the STEM image. The Ni distribution (Fig. 4b) was clearly biased toward regions of higher contrast (Fig. 4a), which correspond to Nb substitution sites in NbS₂. This observation suggests that the displaced Nb atoms were subsequently ejected into the interlayer region as excess Nb, owing to the stability of the substituted Ni atoms, as shown in Fig. 4c and S4. Moreover, the strong structural similarity between 2H-Nb_1.35S₂³¹ and Ni_xNb_yS_z (Fig. 4a) suggests that similar bonding states may occur between excess Nb (or Ni) atoms and S atoms in Ni_0.19Nb_1.16S₂, as illustrated in Fig. 4c.

Fig. 3.

High resolution cross-sectional STEM images of Ni_0.19Nb_1.16S₂ formed by H₂S annealing of (a, d) Nb (1 nm) /Ni (1 nm), (b, e) Nb (1 nm)/Ni (0.2 nm) and (c, f) Nb (1 nm)/Ni (0.1 nm) stacks on ML-MoS₂. Red flames indicate Ni aggregation on the Ni_0.19Nb_1.16S₂ surface.

Fig. 4.

(a) High resolution cross-sectional STEM image of NixNbySz on ML-MoS₂. (b) Corresponding elemental profiles obtained from an electron energy loss spectroscopy (EELS) line scan, as indicated in Fig. 3c. (c) Schematic diagrams of Ni_0.19Nb_1.16S₂, illustrating partial substitution of Nb atoms in Nb_1.35S₂^31,32 by Ni atoms.

Assuming that all Ni atoms were incorporated into Nb_1.35S₂, the composition ratio x was calculated to be 0.19 in Ni_xNb_(1.35-x)S₂. It is noteworthy that the Ni_0.19Nb_1.16S₂ film likely possesses both a vdW interface and surface, leading to the formation of a vdW junction between Ni_0.19Nb_1.16S₂ and ML-MoS₂.

MoS₂-channel n-type FETs with Ni_0.19Nb_1.16S₂ contacts

We evaluated ML-MoS₂-channel nFETs with the Ni_0.19Nb_1.16S₂ contacts formed from the Nb (1 nm)/Ni (0.1 nm) stack, compared to the NbS₂ contacts formed from Nb (2 nm) using the same H₂S annealing at 600 °C, as shown in Fig. 5a,b. The Ni_0.19Nb_1.16S₂ contacts exhibited a higher I_on than the NbS₂ contacts, primarily because the contact resistivity of the NbS₂ contacts is approximately three orders of magnitude lager, as shown in Fig. S6. This high resistivity originates from the intrinsically high work function of NbS₂. In contrast, Ni incorporation effectively reduced the work function of NbS₂ by forming Ni_0.19Nb_1.16S₂. The work function of NbS₂ is known to depend strongly on its crystal phase, decreasing by roughly 1 eV when transitioning from the trigonal prismatic (H phase) to the octahedral (T phase) structure, due to differences in the splitting of metal d-orbitals²². The incorporation of Ni may thus promote modifications in the crystal phase and electronic states, resulting in a reduced work function. To substantiate this hypothesis, the electron Schottky barrier height at Ni_0.19Nb_1.16S₂/MoS₂ interface was experimentally determined using temperature-dependent transport measurements^9,33. The extracted barrier height was relatively low, approximately 0.12 eV, as detailed in the Supplementary Information. This result supports the conclusion that the electron Schottky barrier height of Ni_0.19Nb_1.16S₂/MoS₂ contact is lower than that of NbS₂ and its close to that of Ni, as evidenced by the transfer characteristics shown in Fig. 5a, b, and d.

Fig. 5.

Drain current (I_D)- gate voltage (V_G) characteristics of ML-MoS₂ channel nFETs with (a) Ni_0.19Nb_1.16S₂ and (b) NbS₂ contacts. (c) On-state current (I_on) of ML-MoS₂ nFETs with Ni_1.35−xNb_xS₂ contacts synthesized from Nb (1 nm)/Ni (1 nm) and (1 nm)/Ni (0.1 nm) stacks, and NbS₂ contacts synthesized from Nb (1.5 nm)/Ni (0.1 nm), Nb (2 nm)/Ni (0.1 nm), and Nb (2 nm). I_D-V_G characteristics of ML-MoS₂ nFETs with Ni contacts (d) before and (e) after Ar annealing at 600 °C, and (f) after H₂S annealing at 600 °C. (g) Electron mobility derived from ML-MoS₂ nFETs with Ni_0.19Nb_1.16S₂, NbS₂, and Ni contacts.

To clarify the influence of the initial Nb/Ni film thickness ratio on contact characteristics, ML-MoS₂-channel nFETs were fabricated using the following Nb/Ni configurations: Nb (1 nm)/Ni (1 nm), Nb (1 nm)/Ni (0.1 nm), Nb (1.5 nm)/Ni (0.1 nm), Nb (2 nm)/Ni (0.1 nm) and Nb (2 nm). The on-state current (I_on) of 30 devices, defined as the drain current (I_D) at V_G-V_th=100 V, is statistically distributed as shown in Fig. 5c. Compared with Ni_0.19Nb_1.16S₂ contacts formed from different initial stacks, those derived from Nb (1 nm)/Ni (0.1 nm) exhibited a broader I_on distribution, attributed to a discontinuous Ni layer with ultrathin thickness (0.1 nm). In contrast, contacts formed with initial films of Nb (1.5 nm)/Ni (0.1 nm) or Nb (2 nm)/Ni (0.1 nm) showed significantly lower I_on, approaching that of NbS₂ contacts formed form Nb (2 nm), likely due to NbS₂ formation resulting from the higher Nb ratio.

To assess the thermal stability of ML-MoS₂-channel nFETs with Ni_0.19Nb_1.16S₂ contacts, devices with Ni contacts were annealed under identical conditions for comparison. Prior to annealing, Ni contacts exhibited an I_on comparable to that of Ni_0.19Nb_1.16S₂ contacts. However, after Ar or H₂S annealing at 600 °C, Ni contacts showed significant variation and reduced I_on, as shown in Fig. 5e and f. This degradation is attributed to the intermixing between Ni and MoS₂, as previously discussed, which leads to an increase in contact resistance. These findings demonstrate that Ni_0.19Nb_1.16S₂ contacts provide superior thermal stability when interfaced with ML-MoS₂. Figure. 5 g shows the extracted electron mobility of the fabricated transistors, calculated using following equation:

where is the electron mobility, is the maximum transconductance, L_G= 2 μm and W = 5 μm are the channel length and width, respectively. is the gate-oxide capacitance, calculated form dielectric constant (3.9) and oxide thickness (280 nm). The drain voltage was fixed at V_D = 2 V. The Ni_0.19Nb_1.16S₂ contacts exhibited higher mobility compared to NbS₂ contacts, and their performance was comparable to that of the Ni contacts.

Conclusions

The Ni_0.19Nb_1.16S₂ contact provides enhanced performance for monolayer MoS₂-channel nFETs. The thickness of Ni_0.19Nb_1.16S₂ was determined by the initial Nb film thickness, with excess Ni expelled to the surface. Its crystal structure closely resembles 2H-Nb_1.35S₂, which consists of Nb interstitials between hexagonal NbS₂ layers. Ni_0.19Nb_1.16S₂ is formed by the substitution of some Nb atoms in Nb_1.35S₂ with Ni atoms, originating from the initial Nb/Ni stack via H₂S annealing on monolayer MoS₂. Importantly, neither Ni nor Nb atoms diffuse into the underlaying MoS₂, in contrast to Ni- or Nb-only films on MoS₂. These structural characteristics are consistent with the observed transistor performance, demonstrating excellent on-state current and thermal stability of Ni_0.19Nb_1.16S₂ contacts up to 600 °C. In contrast, NbS₂ contacts are unsuitable for MoS₂-channle nFETs due to their high work function. Incorporation of Ni atoms into NbS₂ effectively reduces the work function, converting its suitability to nFET applications. Overall, Ni_0.19Nb_1.16S₂ represents a promising contact material for nFETs, while NbS₂ remains appropriate for pFETs, enabling the integration of 2D materials into CMOS transistors.

Synthesis of Ni_xNb_yS_z on MoS₂

Monolayer MoS₂ (ML-MoS₂) on SiO₂ (280 nm)/Si substrate used in this work is a commercially available product (2D semiconductors). To synthesize Ni_xNb_yS_z, a Ni film was deposited on ML-MoS₂ via electron-beam (EB) evaporation at -5 °C, followed by continuous deposition of an Nb film on the Ni layer using the same method. The thickness uniformity of the Ni and Nb layers was approximately 5–10% across the 2-inch substrate. In the EB evaporation system, the thickness was controlled using a quartz crystal oscillator, which is a widely adopted standard method. Therefore, the reported thickness for very thin film represents an estimated value and may exhibit slightly reduced accuracy. Subsequently, H₂S annealing was carried out at 600–700 °C for 30 min under a pressure of 50 kPa.

Fabrication of ML-MoS₂-channel nFETs with Ni_0.19Nb_1.16S₂ contacts

After cleaning the ML-MoS₂ on a SiO₂/Si substrate with acetone and isopropanol (IPA), the source and drain contact regions were patterned by photolithography. Nb and Ni films were then deposited onto the ML-MoS₂ by EB evaporation. The contact sections were formed using a lift-off process in acetone and IPA, followed by continuous H₂S annealing to synthesize Ni_0.19Nb_1.16S₂. Subsequently, 30 nm-thick Au electrodes were fabricated on the Ni_0.19Nb_1.16S₂ layer by photolithography and lift-off processing. The backside SiO₂ of the devices was then removed using a hydrogen fluoride (HF) solution, and Au was deposited to form the back-gate electrode. Finally, the channel areas were isolated by photolithography and reactive ion etching (RIE) using SF₆ gas.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

K. H. fabricated the devices and evaluated their electrical characteristics. N. O. conceived and supervised the experiment. The manuscript was written by K. H., and N. O., with contributions and discussions from all authors.

Funding

This research was supported by JPS KAKENHI (JP23H01574) and JST MIRAI (JPMJMI22708192).

Data availability

The data set used in this study is available on reasonable request to the corresponding author.

Declarations

Competing interests

The authors declare no competing interests.