Growth and Ferroelectric Properties of Sputtered Aluminum Nitride-Based Thin Films
Open Access
- Author:
- Hayden, John
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 15, 2024
- Committee Members:
- John Mauro, Program Head/Chair
Susan Trolier-McKinstry, Major Field Member
Tom Jackson, Outside Unit & Field Member
Jon-Paul Maria, Chair & Dissertation Advisor
Ismaila Dabo, Major Field Member - Keywords:
- Ferroelectrics
Wurtzite
Nitride
Thin Film
Sputtering - Abstract:
- Ferroelectrics are materials with crystallographically defined spontaneous polarizations that can be reoriented by application of an electric field. The polarization remains after removal of the electric field, allowing one to design devices that make use of the polarization state for non-volatile information storage. Random access memory devices using ferroelectrics in place of the linear dielectric were proposed in the early 1950s, but synthesis challenges and poor compatibility of existing materials with semiconductor processing prevented commercialization of such technologies until the 1990s. Even then, the ferroelectric materials used in these devices were limited by the extremes to which they could be scaled and were easily outperformed by their volatile counterparts. As Moore’s law demanded increasingly smaller microprocessor components, ferroelectric memories were relegated to specialty use cases. Decades of focused research on ferroelectric memories pointed to the conclusion that the limitations were material based, and not just a matter of solving engineering problems associated with integrating the materials. The community was understandably excited with the emergence of ferroelectricity in the HfO2 material system in 2011, introducing a new class of ferroelectrics with a simple chemistry, excellent compatibility with mainstream silicon processing, and preexisting 3D-friendly deposition processes. A little over a decade has since passed and the first HfO2-based nonvolatile memory technologies are beginning to surface, though challenges related to data retention, cycling endurance, and reliable ferroelectric phase stabilization need to be addressed before the technology can be widely adapted. In 2019, ferroelectricity was discovered in the AlN material system, addressing some of the challenges facing HfO2, but also introducing some of its own. This work investigates the growth and properties of sputter-deposited ferroelectric AlN-based thin films. An introduction to ferroelectricity in wurtzite structured materials is provided, followed by an overview of the experimental techniques employed throughout this project. The main body of this work focuses on B-substituted AlN thin films, starting with epitaxial growth on W coated Al2O3 substrates, followed by process migration to Si wafers using a plasma treatment and templated electrode growth, and finally scale up to 300 mm Si wafers using industrial deposition tools. The electronic, structural, and microstructural properties of the films are thoroughly characterized, demonstrating that the films are highly textured with smooth surfaces and robust ferroelectric properties. With the overarching goal of reducing switching voltages for use in ferroelectric memories, films as thin as 10 nm are demonstrated to exhibit robust switching at voltages less than 8 V. Co-substitution with B and Sc is explored as a means to engineer lattice parameters, coercive field values, and leakage currents. Multiple quaternary Al-B-Sc-N compositions are shown to lattice match to GaN, enabling growth of heterostructures with exceptional crystal quality, matching that of films grown by molecular beam epitaxy. Finally, ancillary works focusing on understanding and addressing leakage currents in wurtzite ferroelectrics are provided as a starting point for future studies.