Anomalous electrical conductivity change in MoS2 during the transition from the amorphous to crystalline phase

Transition metal dichalcogenides exhibit unique properties, which make them interesting for fundamental studies and for applications in many devices. Here, we report on the anomalous electrical behavior during the amorphous-to-crystal phase transition of MoS2. While crystallization typically results in an increase in conductivity, the situation in MoS2 is opposite. Amorphous MoS2 shows a sheet resistance of 3.2 x 103 omega with the value remaining nearly constant until 200 degrees C, MoS2 samples annealed above 200 degrees C exhibit an unexpected two-step increase in sheet resistance. The first abrupt increase takes place after annealing at temperatures between 300 and 400 degrees C while the second increase occurs after heating to 700 degrees C with typical sheet resistance values of 1.2 x 105 and 8 x 106 omega after heating to 500 degrees C and 900 degrees C, respectively. Using a combination of X-ray photoelectron spectroscopy and X-ray diffraction studies and ab-initio modeling, we argue that the large electrical conductivity observed in amorphous MoS2 is associated with the existence of a large quantity of Mo-Mo homopolar bonds that exceed the percolation threshold. The dramatic increase in the sheet resistivity in the first step is accompanied by the formation of the Mo-S bonds upon the disassociation of homopolar Mo-Mo bonds for a further increase in the sheet resistance upon annealing temperatures of 700 degrees C that can be attributed to the grain boundary formation during crystallization of MoS2 into the 2H layered structure.