ReaxFF Studies of Sapphire Surface Properties for Applications in Epitaxy of Thin Films and Semiconductor Manufacturing
Open Access
- Author:
- Zhang, Yuwei
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 13, 2025
- Committee Members:
- John Mauro, Program Head/Chair
Lukas Muechler, Outside Unit & Field Member
Susan Sinnott, Major Field Member
Joan Redwing, Major Field Member
Adri van Duin, Chair & Dissertation Advisor - Keywords:
- ReaxFF
Molecular dynamics simulations
Sapphire surface
Thin film epitaxy
Atomic layer etching (ALE)
Hydroxylation
Fluorination
Surface chemistry
MoS2/α-Al2O3 interactions
H2O/α-Al2O3 interactions - Abstract:
- The surface of sapphire plays a pivotal role in a wide spectrum of scientific and technological applications. Investigating its interactions with gas-phase molecules, such as water and hydrogen, provides fundamental insights into processes involved in thin film epitaxy and atomic layer etching—techniques commonly applied in semiconductor manufacturing. This dissertation presents three ReaxFF studies on sapphire surface reactions to H2/H2O, H2S, and HF gas-phase molecules, discussed in Chapters 4, 5, and 6, respectively, with the exploration of MoS2 epitaxial behavior on sapphire presented in Chapter 5. Chapter 1 provides an overview of the significance of surface science and the materials central to this research. Chapter 2 describes the concepts of molecular dynamics, which is the main computational technique used in this work. Chapter 3 introduces the ReaxFF reactive force field method. The Al/O/H ReaxFF force field discussed in Chapter 4 enables simulations of sapphire surface interactions with H2/H2O gas-phase molecules. This force field reproduces the surface energy trend for stoichiometric surfaces along the C-, R-, A-, and M-planes (C < R < A < M), as well as the energy profiles of H2O hydrolysis on sapphire via the 1-2 and 1-4 pathways, in good agreement with density functional theory (DFT) predictions. Simulations of H2O/α-Al2O3 (0001) interactions reveal the atomic-scale mechanisms of Al-terminated surface hydroxylation across a broad temperature range and H₂O desorption at high temperatures. Water autocatalysis and surface poisoning are observed at moderate and high H2O concentrations, respectively. H2/α-Al2O3 interactions are simulated to investigate H2 pre-annealing of sapphire, which removes surface oxygen and creates an Al-terminated surface before thin-film epitaxy. High surface O density and low thermodynamic stability of the crystallographic plane leads to increased surface reactivity to H2. The simulation of H2 pre-annealing of O-terminated C-plane sapphire was accelerated by reducing the H-H sigma bond strength and completed within 1.5 ns. This simulation reveals incomplete O removal, which leaves residual hydroxyl groups that may affect subsequent epitaxy. In Chapter 5, H2S/α-Al2O3 (0001) interactions are simulated using the Al/O/H/S ReaxFF force field. The results show a reversed surface reactivity trend for the 100% Al-terminated, 50% Al-terminated, and 100% O-terminated surfaces compared to the H2O/α-Al2O3 (0001) system. The 100% Al-terminated surface remains nonreactive to H2S across a wide temperature range, suggesting that no -S surface groups are introduced on perfectly Al-terminated surfaces under H2S exposure. However, Al-termination achieved by H2 pre-annealing of O-terminated sapphire retains residual hydroxyls, which can react with H2S to form H2O, leading to further O removal and irreversible Al-S bonding. The Al/O/H/S force field also includes Mo-related parameters, though limited to describing MoS2/Al2O3 interactions. Simulations of MoS2 adhesion on flat and stepped α-Al2O3 (0001) with 100% Al-termination show that S-passivation disrupts the 0o alignment on both surfaces, triggering a 30o orientation change. Rarefied H2O exposure severely impairs MoS2 adhesion on flat sapphire while markedly improving it on stepped sapphire. This difference arises because MoS2 alignment on flat sapphire is lattice-guided and highly sensitive to surface impurities and gas-phase environments, while MoS2 alignment is guided by step edges on stepped sapphire, and H2O molecules compete with S species for Al-sites at the step edge, providing a protective effect that prevents irreversible sulfur-induced etching. In Chapter 6, the self-limiting HF-induced fluorination of sapphire is simulated using the Al/O/H/F ReaxFF force field. The extent of fluorination at equilibrium is influenced by surface termination, with 100% Al-terminated α-Al2O3 (0001) exhibiting the highest degree of fluorination, while the O-rich surface undergoes negligible conversion. This occurs because HF dissociation relies on Al-F bond formation. Continued fluorination of the deeper oxide layer in the presence of a solid AlFx layer relies on hydrogen diffusion from adsorbed HF on the AlFx layer to residual hydroxyls at the AlFx/Al2O3 interface. As the AlFx layer thickens, the efficiency of hydrogen diffusion declines, eventually terminating further transformation. HF concentration also influences fluorination behavior, with simulations identifying an optimal concentration that maximizes the extent of fluorination. Expanding the force field to include carbon-related parameters would enable the simulation of ligand-exchange reactions for removing solid AlFx during thermal atomic layer etching (ALE), contributing to comprehensive atomic-level studies of thermal ALE on sapphire.