A photoelectron study of annealing induced changes to workfunction and majority carrier type in pulsed laser deposited few layer WS 2 films

Annealing few layer pulsed laser deposited WS 2 films in sulfur increased the S/W ratio from 1.3 to 2.2, improved the mobility from 0.5 to 28 cm 2 V −1 s −1 , and switched the conductivity from n to p-type. The annealing induced n to p-type switch was confirmed by ultraviolet photoelectron spectroscopy which revealed a workfunction increase from 3.36 to 4.52 eV, a corresponding change in the Fermi level separation from the valence band edge, and a shift of the tungsten X-ray photoelectron spectrum to lower binding energy by ~ 1 eV. Current-voltage measurements indicated that charge injection from ohmic contacts was independent of the interfacial energy barrier, and was governed instead by tunneling from gap states associated with intrinsic point defects and surface contamination. The increased mobility with annealing is ascribed to improved stoichiometry and reduced incoherent scattering from point defects. The p-type conductivity is possibly due to excess sulfur in the form of interstitial ions.