Correlation Between the Crystalline Phase of Molybdenum Oxide and Horizontal Alignment in Thin MoS2 Films

Abstract

The crystallographic alignment of an ultrathin MoS2 film strongly influences its properties and is, therefore, substantial for various applications. Developing methods for controlled growth is of primary importance for reproducible film fabrication. Among them, sulfurization of predeposited molybdenum or molybdenum oxide films is a method capable of growing large-area MoS2 layers with reasonable control over the layer alignment. However, the question of which growth parameters define the layer alignment still remains unresolved. We prepared 10 nm thick films of Mo and Mo oxides with different degrees of crystallinity and sulfurized them all under the same conditions. We demonstrate that the presence of the crystalline phase of MoO2 or MoO3 in the parent films promotes the growth of the horizontally aligned (HA) MoS2. This observation provides a simple criterion for controlling the layer alignment and for the growth of horizontal MoS2 layers of thicknesses at which only the vertical layers have hitherto been assumed to grow.