Molecular Beam Epitaxy of van der Waals Materials for Applications in Novel Logic and Memory Devices

Abstract

The continued scaling toward high-density, low-power devices has pushed the current Si-based technology to the fundamental limit. To overcome these limitations, new types of materials are being researched and developed for implementation in the next-generation of devices. The ability to produce uniform crystals in nanoscale dimensions presents major challenges in device design and fabrication. However, van der Waals materials offer the advantage of crystallizing in a planar monolayer where strong in-plane covalent bonds along with saturated surface bonds allow for scaling beyond the current limit. To investigate the potential of certain van der Waals materials for novel device applications, this research utilizes molecular beam epitaxy (MBE) to study the formation of thin films with close control of the deposition conditions. This dissertation examines a number of van der Waals materials (HfSe₂, WSe₂, WTe₂, WSe(2-x)Tex, hBN, Bi₂Se₃) to determine the potential advantages and challenges of device integration. The results show that chalcogen-rich deposition conditions are essential in the crystallization of stoichiometric transition metal dichalcogenides (HfSe₂, WSe₂, WTe₂). However, in the pursuit of more complex alloys, a study of the MBE growth of WSe(2-x)Tex at 250°C reveals an energetic barrier of formation that results in phase-separated films from 14% to 79% Te concentrations. Furthermore, research investigating an insulating member of the van der Waals materials family, hexagonal boron nitride (hBN), revealed the necessity of high growth temperatures to form large-area crystals with monolayer thickness control. Finally, a number of studies surrounding the topological insulator Bi2Se3 showed the formation of stoichiometric, large-area crystals at low substrate temperatures (320°C) are very robust to physical damage. These studies shows that sputtering with He⁺ ions preferentially removed Se atoms from the crystal during the destructive technique, but failed to disrupt the topologically-protected surface states. Furthermore, inductively coupled plasma etching with chlorine and fluorine-based recipes revealed the necessity for strategic selection of etching constituents. Whereas fluorine-based recipes result in chemical reactions that produced insulating bismuth fluoride, chlorine-based recipes had little reactivity and resulted in films that had minimal changes to the surface states. Finally, a study showing the regeneration effects of Bi₂Se₃ demonstrated the ability to “heal” the damage induced during conventional processing. The totalities of these results reveal the potential of van der Waals materials for applications in novel logic and memory devices while highlighting the challenges associated with their synthesis. This dissertation serves as a guide for the design of next-generation devices where these van der Waals materials might be crucial components.