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.

On the Contact Optimization of ALD-Based MoS2 FETs: Correlation of Processing Conditions and Interface Chemistry with Device Electrical Performance

“…As a result, a disordered and an intermixed region would be formed at the Ti-to-MoS 2 interface, as has previously been verified for the annealed Ti contacts to the exfoliated MoS 2 films. 59 Furthermore, the annealing process in this study is performed in a HV chamber. The presence of background water and/or oxygen and its possible reaction with the MoS 2 surface at the channel regions, as well as the Ti-to-MoS 2 interfaces (from the Ti edge sites), may lead to the formation of oxide-containing compounds, all of which could impede the enhancements in the device on-state.…”

Electron Irradiation of Metal Contacts in Monolayer MoS2 Field-Effect Transistors

“… 48 A disordered, compositionally graded layer, composed of Mo and Ti x S y species, forms on the surface of the MoS 2 crystal following the deposition of Ti, and thermal annealing in the 100–600 °C temperature range can cause Ti diffusion inducing further chemical and structural changes at the Ti–MoS 2 interface. 49 , 50 It is also possible that diffusion of Au atoms to the interface with MoS 2 occurs under the energetic electron beam irradiation. Au does not react with MoS 2 but reduces the contact resistance and therefore the Schottky barrier height.…”

Molybdenum Disulfide Field Effect Transistors under Electron Beam Irradiation and External Electric Fields

“…We note that the reduction of the contact resistance by chemical reactions between the metal contacts and MoS2 channel has been reported for metal deposited under ultrahigh vacuum [42] and contact laser annealing [43]. A disordered, compositionally graded layer, composed of Mo and Ti x S y species, forms at the surface of the MoS2 crystal following the deposition of Ti, and thermal annealing in the 100-600 °C temperature range can cause Ti diffusion inducing further chemical and structural changes at the Ti/MoS2 interface [44,45]. It is also possible that diffusion of Au atoms to the interface with MoS2 occurs under the energetic electron beam irradiation.…”