UC Santa Barbara

In the past several decades, nitride-based semiconductors have impacted everyday life in sectors such as energy efficient lighting and high-resolution display technology. While the (Al, In, Ga)N alloys are the most heavily utilized nitride-based semiconductors, hexagonal boron nitride (hBN) is essential to the two-dimensional (2D) material system, mainly due to its ability to passivate 2D materials better than conventional dielectric materials like SiOx.1 hBN plays an instrumental role in 2D material and device research, making the maturation of scalable, thin-film hBN growth techniques critical to the field.

This thesis presents plasma-enhanced growth techniques for next-generation nitride thin-films. Nucleation and growth processes were examined in-vacuo and ex-situ for a greater understanding of these synthesis techniques. A high-temperature 1450-1500°C, plasma-enhanced chemical beam epitaxy (PE-CBE) process was utilized to grow hBN on silicon carbide (SiC) substrates. Film morphology and epitaxial alignment were examined via in-situ reflection high-energy electron diffraction (RHEED), as well as ex-situ atomic force microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). Spectroscopic techniques, such as in-vacuo X-ray photoelectron spectroscopy (XPS) and ex-situ energy-dispersive X-ray spectroscopy (EDS), provided information on stoichiometry and interface bonding. Characterization of hBN nuclei showed that a 30° metastable rotational alignment between hBN and a graphinated SiC surface could be stabilized by tuning substrate morphology and growth parameters, providing a route to grow 2D heterostructures with incommensurate alignments without relying on manual rotation of the layers. These ''twisted” hBN/graphene structures could be utilized in future twistronic devices for their exotic physical properties.2

Low-temperature (<400°C) atomic layer epitaxy (ALE) was explored in collaboration with the U.S. Naval Research Lab as a means to produce novel heterostructures. These depositions utilized in-vacuo RHEED and XPS to greater understand the ALE mechanism and the required properties of the substrate's starting surface. To facilitate these depositions, low-temperature cleaning processes for bulk GaN substrates were developed and optimized for subsequent ALE regrowth. A unique, crystalline superconductor-semiconductor-superconductor Josephson junction (JJ) for future applications in quantum computation was chosen as a test structure for low-temperature ALE. The ALE AlN barriers were deposited on Al epitaxially grown at cryogenic temperatures on GaAs(001) substrates. Cross-sectional TEM analysis showed the low temperature ALE allowed for the Al to remain epitaxial, resulting in improved junction crystallinity. These crystalline JJs may provide enhanced tunneling performance over junctions with amorphous barrier dielectrics.

1 C.R. Dean, et al., Nat. Nanotechnol. 5, 722 (2010).

2 Y. Cao, et al., Nature. 556, 43-50 (2018).