Ultralow-Resistance Contacts to Heavily Doped p-Type Nb_xW_1-xS_y Thin Films Grown by Atomic Layer Deposition

Transition metal dichalcogenides (TMDs) are an important class of materials for future microelectronics. Of particular interest are TMDs deposited by atomic layer deposition (ALD) since this technique allows both back-end-of-line (BEOL) compatible deposition and the ability to create heavily doped regions for contact formation. In this work, we characterize ∼3 nm-thick heavily doped Nb_xW_1-xS_y thin films grown by plasma-enhanced ALD using gated transfer-length measurement (TLM) structures. An analysis of films with different Nb concentrations, x, found that films with x = 0.22 had the lowest sheet resistivity of 86 kΩ/sq along with an ultrahigh carrier concentration of 4.2 × 10²⁰ cm^-3. The contact resistance, R_C, of different metals to Nb_xW_1-xS_y thin films was also analyzed. Among Pd, Ni, and Ti contacts, Pd was found to have the lowest R_C, whereas Ni (Ti) had an average R_C that was 6× (20×) higher than Pd. Physical analysis of the films using Raman spectroscopy and transmission electron microscopy shows that the crystal quality degrades going from x = 0.08 to 0.33, while Kelvin probe force microscopy, complemented by density functional theory, is used to explain the Nb concentration of the extracted work function. The best TLM structures have an R_C value as low as 0.30 ± 0.26 kΩ-μm and a mean specific contact resistivity, ρ_C, of 11 ± 27 nΩ-cm². Even after accounting for experimental error, this value is lower than the other values reported for p-type TMD contacts in the literature. These results suggest that Nb_xW_1-xS_y can be a promising intermediate layer between metal contacts and monolayer WSe₂ in future scaled-down TMD MOSFETs.