Thermal Stability of Titanium Contacts to MoS2

Abstract

Thermal annealing of Ti contacts is commonly implemented in the fabrication of MoS₂ devices; however, its effects on interface chemistry have not been previously reported in the literature. In this work, the thermal stability of titanium contacts deposited on geological bulk single crystals of MoS₂ in ultrahigh vacuum (UHV) is investigated with X-ray photoelectron spectroscopy and scanning transmission electron microscopy (STEM). In the as-deposited condition, the reaction of Ti with MoS₂ is observed resulting in a diffuse interface between the two materials that comprises metallic molybdenum and titanium sulfide compounds. Annealing Ti/MoS₂ sequentially at 100, 300, and 600 °C for 30 min in UHV results in a gradual increase in the reaction products as measured by XPS. Accordingly, STEM reveals the formation of a new ordered phase and a Mo-rich layer at the interface following heating. Due to the high degree of reactivity, the Ti/MoS₂ interface is not thermally stable even at a transistor operating temperature of 100 °C, while post-deposition annealing further enhances the interfacial reactions. These findings have important consequences for electrical transport properties, highlighting the importance of interface chemistry in the metal contact design and fabrication.

Keywords: X-ray photoelectron spectroscopy; contacts; interface reactions; thermal annealing; transmission electron microscopy.

Figure 1.

Mo 3d and S 2p core levels before and after Ti deposition.

Figure 2.

(a) Cross-sectional ADF-STEM image of as deposited Ti/MoS₂ showing disordered Ti-rich and Mo/S-rich layers, (b) 90° rotated ADF-STEM image showing the dotted white line where an EDS and EELS line-scan was simultaneously acquired, and (c) the corresponding EDS line profiles of the Ti-K, Mo-K (enlarged five times), and the overlapped Mo-L and S-K edges, and (d) EELS line profiles of S-L and Ti-L edges. False colors are added in the ADF-STEM images to aid the eye.

Figure 3.

(a) XPS spectra acquired following 30 min anneals at each temperature. These were performed sequentially on the same sample. (b) Intensity ratios based on the data in panel (a), whereas panel (c) highlights the changes that occur at 100 °C.

Figure 4.

(a) Cross-sectional ADF-STEM image of Ti/MoS₂ after 30 min anneal at 400 °C showing a Mo-rich layer and a partially recrystallized layer grown out from the disordered Mo/S-rich layer, (b,c) FFT images of the white dotted-line framed regions in panel (a), (d) 90° rotated ADF-STEM image showing the dotted white line where an EELS line-scan was acquired, and (e) the corresponding EELS line profiles of the S-L and Ti-L edges, the black dash-dot lines showing the fast dip of S and Ti in the Mo-rich layer. (f) ADF-STEM image showing the white dotted-line framed region where EDS mapping was acquired and (g) the corresponding Mo-K, (h) Ti-K, and (i) the overlapped Mo-L and S-K edge maps. False colors are added to aid the eye.