Paper EM-TuM3
Application of Thermodynamics to Processing Transition Metal Dichalcogenides

Tuesday, October 20, 2015, 8:40 am, Room 210E

We have applied thermodynamics to guide us in processing transition metal dichalcogenides for ohmic contact formation, oxidation, and etching. Annealing has been reported by a number of researchers to reduce the resistance of electrical contacts to transition metal dichalcogenides. To better understand the effect of annealing and guide our ongoing experiments, we have surveyed the condensed phase equilibria in the transition metal-Mo-S systems. The phase diagrams we have calculated or found in the literature fall into three categories: the metal is in thermodynamic equilibrium with MoS2, there is a driving force for the metal to reduce MoS2, or there is a stable solid solution or ternary phase that dominates the phase diagram. We have performed a similar analysis of the metal-W-Se systems, although there is less thermodynamic data available for the transition metal selenides than the transition metal sulfides. In this presentation, we will first compare materials characterization of annealed contacts to MoS₂ and WSe₂ to our predictions. Then, we will turn our attention to oxidation and etching. Introducing transition metal dichalcogenides to an oxidizing environment can have different effects on the material, depending on the temperature and partial pressure of the oxidizing agent. Using O₂, we find from our thermodynamic calculations that a solid product of oxidation forms on MoS₂ and WSe₂ at mildly elevated temperatures, whereas at higher temperatures we can use O₂ as a vapor phase etchant due to the volatility of the oxygen-bearing reaction products. We have found good agreement between our predictions and characterization of processed samples using light microscopy, atomic force microscopy, scanning electron microscopy, and scanning Auger microscopy.