Chemical Vapor Deposition of MoS2 for Energy Harvesting: Evolution of the Interfacial Oxide Layer

The growth of two-dimensional (2D) materials directly on the substrates that are relevant to device fabrication is crucial for their large-area production and application. This is because their production via transfer processes not only increases the costs but, more importantly, induces contamination and mechanical defects in the transferred material. The presence of a dielectric interface layer and the control of its thickness in transistors and p-n heterojunctions are essential aspects in the semiconductor industry. In the present work, MoS2 flakes and films with thicknesses down to the monolayer limit were grown using chemical vapor deposition (CVD) on Si substrates covered with a native oxide layer. The high quality of the as-grown MoS2 resting on a flat SiO2 surface was documented by a combination of atomic force microscopy, optical spectroscopy, including tip-enhanced photoluminescence spectroscopy, and photoelectron microspectroscopy methods. The changes of the interfacial oxide were then interrogated using spectroscopic imaging ellipsometry and X-ray photoelectron spectroscopy, both with micrometer scale resolution, to show the increase of the oxide layer thickness by several nanometers during the heating and MoS2 growth processes. Our results evidence the possibility of growing high-quality MoS2 directly on thin dielectrics. However, at the same time, if this type of MoS2 deposition is to be used for device fabrication, the simultaneous increase of the SiO2 thickness makes it important to have proper knowledge and control of the growth process. For the applications in energy harvesting where only a thin (or none) insulating layer is required, alternative growth protocols, surface passivation, or a different dielectric material (e.g., Al2O3) are suggested.