Oxidation and oxidative vapor-phase etching of few-layer MoS2

Understanding oxidation of layered chalcogenide semiconductors is important for device processing, as oxidation can be both an intentional and unintentional result of processing steps. Here, the authors investigate chemical and morphological changes in mechanically exfoliated few-layer MoS2 in oxidizing and inert environments using different microscopies (optical, scanning electron, and atomic force) and spectroscopy (Raman, x-ray photoelectron, and Auger electron) techniques. The environments studied were oxygen, oxygen and water vapor, argon, argon and water vapor, and ultraviolet-generated ozone at temperatures from 25 to 550 °C. Oxidation at low temperatures resulted in the formation of a condensed molybdenum oxide phase and sulfur trioxide gas. At sufficiently elevated temperatures, all the products of oxidation volatilize, resulting in a vapor-phase etch. The kinetics of oxidation and etching depended upon the annealing gas, temperature, time, and the number of layers of MoS2. Conditions can be selected to create isolated etch pits, smooth oxide layers, oxide islands, or flakes of reduced lateral dimensions (etching from the flakes' edges). These results can provide useful guidance for MoS2 device processing.