Synthesis of large-area two-dimensional materials for vertical heterostructures
Campbell, Philip Michael
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Due to their intrinsic bandgap and thickness-dependent properties, transition metal dichalcogenides (TMDs) have attracted significant attention for applications in digital and analog electronics, flexible electronics, optical applications, and sensors. In particular, 2D vertical heterostructures composed of TMDs have a number of interesting applications, including digital logic, analog communications systems, and optical applications. However, the quality of currently available synthetic materials is not sufficient to realize many of these applications. Further, the impact of defects and layer-to-layer interactions on the electronic behavior of heterostructures is not well understood. Several synthesis methods for TMDs have been explored, ranging from chemical vapor deposition (CVD) to thin film alloying methods. A common drawback to both methods is the high synthesis temperature required, ranging from roughly 550 – 1050 °C. Through a combination of theory and experiment, this work provides insight into the relationship between material quality and performance in 2D vertical heterostructures. A theoretical model based on the Bardeen transfer Hamiltonian is used to explore the behavior of the heterostructures. In particular, TMD-based systems are identified for application in resonant tunneling and steep-slope devices. Further, the impact of scaling and defects on device performance is explored. From an experimental standpoint, this work demonstrates wafer-scale synthesis of TMDs using high temperature growth methods. In addition, plasma-enhanced synthesis processes are demonstrated which lower the required growth temperature. Temperature dependent conductivity measurements for the materials synthesized at low temperature demonstrated conduction through variable range hopping as a result of high defect densities. MoS2-Al2O3-MoS2 and MoS2-WS2 heterostructures are created using the low temperature, plasma-assisted growth processes. Extensive physical characterization of the films demonstrates good fidelity of the heterostructures, with no evidence of chemical bonding between the layers. Electrical characterization of two-terminal devices based on the MoS2-Al2O3-MoS2 heterostructure confirms tunneling between the MoS2 electrodes with a high degree of scattering. The MoS2-WS2 heterostructure, which relies only on the van der Waals gap as the tunnel barrier, exhibits a current-voltage characteristic dominated by tunneling through defects. Through a combination of simulation and experiment, the implications of defects and Fermi level pinning on device performance were explored. In particular, this work demonstrates the potential of 2D vertical heterostructure devices and provides a path toward realizing high performance devices through device design and optimization of synthesis.