Disorder and Its Effects on Two-Dimensional Transition Metal Dichalcogenides

Semiconductor transition metal dichalcogenides (TMDs) have an inherent tunability where as the number of layers in the material increases, the band gap decreases and becomes indirect. This property gave rise to several band gap engineering studies for these 2D materials such as heterostructures, strain engineering, and twist angle studies. This work instead focuses on adjusting the band gap energy and other properties by tuning the degree of disorder. Disorder in a lattice refers to a state of a crystalline solid in which not all of the atoms occupy their predicted lattice sites. In other words, a completely ordered lattice would have all of the atoms occupy their equilibrium phase position and a completely disordered lattice would have the atoms randomly distributed over the lattice sites. A disordered state can occur even when the material maintains its predicted stoichiometry since antisite defects can preserve the correct composition. Although disorder in the lattice is typically unwanted, it has been shown to be useful in reliably tuning properties such as the band gap.<br/><br/>The degree of disorder has been quantified using the Bragg-Williams order parameter S to better understand the trends between disorder and significant material properties. This work extracts the order parameter of several of the TMD materials using Raman spectroscopy and SEM, with excellent agreement between these two techniques. For all of the materials in this work, there is a clear linear trend between the band gap values and S², which has previously been experimentally established for other materials like heterovalent ternary materials such as ZnSnN₂ and MgSnN₂, binary materials such as InN and GaN, and elementary materials such as Si.<br/><br/>This work focuses on WSe₂ films grown via metal-organic chemical vapor deposition (MOCVD) on sapphire substrates using a multi-step growth process which involved an initial nucleation step, a ripening step, and a final lateral growth step. This process was chosen as it has shown to achieve large-area, lateral growth of MOCVD grown TMDs in the past. Between each film growth, one process parameter was changed in order to determine how that parameter altered the degree of disorder. The process parameters tested were the substrate temperature, H₂Se flow rate, and length of the individual steps. Additionally, S² was also compared to the monolayer coverage and the thickness of the domains, which were extracted using atomic force microscopy to determine if these were the main drivers for the changes seen in the degree of ordering. Exploring the role of process parameters in the context of tuning the degree of disorder can not only allow for a dependable way of controlling of the band gap energy, but also could prove to be a reliable way of controlling other electrical properties such as carrier mobility.<br/><br/>The TMD materials analyzed for this study exhibited an expected continuous linear relationship between the band gap values and S². The amount of coverage and number of layers had the most discernable impact on the S² value, showing that as the number of layers or monolayer coverage increased, so too did the degree of ordering. However, process parameters such as the temperature during the nucleation step, H₂Se flow rate, and chamber pressure seemed to have an impact on the degree of ordering as well. Finally, an unexpected result emerged when adjusting the chamber pressure between film growths, which resulted in an ability to control domain alignment and domain directions.<br/><br/>Financial support for research at The Pennsylvania State University was provided by the National Science Foundation through the 2D Crystal Consortium – Materials Innovation Platform (2DCC-MIP) under NSF cooperative agreements DMR-1539916 and DMR-2039351.<br/><br/>Financial support for research at Western Michigan University was funded in part by the National Science Foundation (grant number DMR-2003581).