Extrinsic and intrinsic p-type doping of pulsed laser deposited few-layer WS2 films

Abstract

X-ray photoelectron spectroscopy confirmed (XPS) that Nb substitutes on W lattice sites yielding substitutional extrinsic p-type doping in pulsed laser deposited (PLD) few-layer WS2:Nb films. A Fermi level separation of 0.31 eV from the valence band edge with 0.5 atomic % of Nb doping was measured by ultraviolet photoelectron spectroscopy (UPS), which decreased to 0.18 eV when doping was increased to 1.1 atomic %. The corresponding hole sheet concentrations increased from 3.9 x 10 12 to 8.6 x 10 13 cm -2 respectively while the mobility exhibited the opposite trend, presumably due to increased ionized impurity scattering. Separately, undoped PLD few-layer WS2 films exhibited a conductivity switch from n to p-type when the composition changed from sulfur-deficient to sulfur-rich. UPS revealed a workfunction increase from 3.36 to 4.52 eV and a corresponding change in the Fermi level separation from the valence band edge. The intrinsic p-type conductivity is assigned to excess sulfur in the form of interstitials. Despite the relatively large change in workfunction in these films, Ohmic contacts were obtained to both the n and p-type materials with indium, albeit with different contact resistances. This suggests that in addition to the interfacial energy barrier, tunneling from gap states associated with point defects and surface contamination are likely contributors to charge injection. The approach demonstrates the potential of PLD for controlled doping and the creation of p-n junctions from transition metal dichalcogenides.