Georgia Tech Team Explores Crystallization of Atomic Layer Deposited TiO2
In a work published in the ALD Journal [ALDJ], Dr. Jamie Wooding et al from Georgia Tech team have explored the fundamental crystallization kinetics of atomic layer deposited (ALD) TiO2 thin films. They discovered that especially when low temperatures were used during ALD, that a post-deposition anneal (PDA) created large anatase crystals. By creating these larger grains of anatase, the volume of the grain boundary is reduced, allowing more of the film to effectively contribute to its functional performance.
Titanium dioxide (TiO2) is a wide-bandgap semiconductor known for its high refractive index and high dielectric constant, which vary depending on its phase. The anatase phase of TiO2 is typically favored for photocatalysis and gas sensing, while the rutile phase is more suitable for high-k applications. In contrast, the amorphous form of TiO2 is often utilized as a diffusion barrier and for corrosion protection.
The creation of these thin films is frequently accomplished through ALD making the understanding of phase control in ALD films of TiO2 crucial. TiO2-ALD has been extensively explored with TiCl4 and H2O as the co-reactants. For TiCl4/H2O ALD chemistry, the films typically remain amorphous when deposited at temperatures below 150 °C. Crystallization of the anatase phase begins around 150 °C, whereas the rutile phase requires temperatures of 350 °C or higher.
While ALD TiO2 films derived from TDMAT and H2O have also been examined, they've been studied to a lesser extent compared to TiCl4. TDMAT has certain advantages as a precursor, including the absence of chlorine contamination in the resulting film and its attribute of generating "electrically leaky TiO2". However, films created with TDMAT/H2O are usually amorphous as-deposited, given the decomposition temperature of TDMAT (around 220 °C) restricts the ALD deposition temperature window.
Large-grained anatase can exhibit improved photoelectrochemical performance and photocatalytic activity because of its minimal grain boundary volume. The highly defected regions in the grain boundary don't contribute to the functional performance of the crystalline film, making it crucial to maximize grain size and minimize grain boundary volume to optimize material performance.
Figure 1 presents a review of literature on the ratio of crystal size to film thickness of TiO2 as it relates to deposition temperature in the ALD processes using TiCl4/H2O and TDMAT/H2O. The processes that yield large-grain TiO2 typically require deposition temperatures between 200 °C and 300 °C. These findings are predominantly from as-deposited films derived from TiCl4/H2O chemistry on both silicon and amorphous-Al2O3 substrates. The ratio of resulting TiO2 anatase grain size to film thickness for films on amorphous-Al2O3 is about ten times greater than for films deposited on silicon, with the maximum reported ratio values being 40 and 5, respectively. The growth of large anatase crystals is heavily influenced by the substrate and surface energy. This occurrence has not been documented for TDMAT/H2O ALD yet. Therefore, Figure 1 underscores the discovery how large grain anatase can be created using TDMAT/H2O chemistry through low temperature ALD combined with a post deposition anneal.
Figure 1. Plot of the crystal-size-to-film-thickness ratio as a function of the crystallization temperature compiled from various TiO2 ALD literature reports. All reports use thermal-ALD with water as the co-reactant. Varying titanium precursor and substrate chemistries are noted by data point shape as indicated in the legend. Conditions reported include: TiCl4/H2O on Si as deposited (black square), TiCl4/H2O on Al2O3 as deposited (green square), TiCl4/H2O on Al2O3 with PDA (circled green square), TDMAT/H2O on Si as deposited (red circle), and TDMAT/H2O on Si with PDA, which are the results from this study (circled red star). Films that undergo PDA are all circled in blue (from Wooding et al.)
TiO2 Crystal images:
Read the full article to see the SEM images of the TiO2 crystals, and learn how the team explained the remarkable crystal growth dynamics with a beautiful theoretical explanation!
Image: Losego's group photo.
About the Authors:
Shawn Gregory also did his Ph.D. at Georgia Institute of Technology (2017-2022), and his research was related to Charge Transport Physics, Semiconductors. Polymers, Thermoelectrics, Atomic Layer Deposition and Vapor Deposition.
Amalie Atassi is a Graduate Student Researcher at the Georgia Institute of Technology, based in the Greater Atlanta Area. She has been in this position since August 2019. Her research focuses on examining the charge and thermal transport properties of insulating and semiconducting organic materials. She has also attended the 2022 National School on Neutron and X-ray Scattering, a two-week program hosted by Argonne and Oak Ridge National Laboratories, where she gained hands-on experience with X-ray and neutron experiments and networked with scientists.
Dr. Kalaitzidou joined Georgia Tech as an Assistant Professor in the G.W. Woodruff School of Mechanical Engineering in November of 2007. She also holds an adjunct appointment in the School of Materials Science and Engineering. She obtained her Ph.D. in manufacturing and characterization of polymer nanocomposites (PNCs) from Michigan State University and worked as a post-doctoral researcher on mechanics of soft materials in the Polymer Science and Engineering Department at University of Massachusetts, Amherst.
Professor Mark D. Losego is an associate professor in the School of Materials Science and Engineering at Georgia Tech. The Losego research lab focuses on materials processing to develop novel organic-inorganic hybrid materials and interfaces for microelectronics, sustainable energy devices, national security technologies, and advanced textiles.
Professor Losego says that his group combines a unique set of solution and vapor phase processing methods to convert organic polymers into organic-inorganic hybrid materials, including developing the science to scale these processes for manufacturing. Prof. Losego’s work is primarily experimental, and researchers in his lab gain expertise in the vapor phase processing of materials (atomic layer deposition, physical vapor deposition, vapor phase infiltration, etc.), the design and construction of vacuum equipment, interfacial and surface science, and materials and surface characterization. Depending on the project, Losego Lab researchers explore a variety of properties ranging from electrical to electrochemical to optical to thermal to sorptive to catalytic and more.
Prof. Losego received his B.S. degree in materials science and engineering from Penn State University in 2003 (with a focus on electronic and photonic materials), earned an M.S. (2005) and Ph.D. (2008) in materials science and engineering from North Carolina State University (primarily in vapor phase deposition of functional oxide thin films), and completed postdoctoral studies at the University of Illinois (studying chemical surface modifications and nano-scale thermal transport across hybrid interfaces).
Prof. Losego is also the faculty founder and director of The Materials Innovation and Learning Laboratory (The MILL), an open-access make-and-measure facility operated by and for students with the goal of elevating experiential education and undergraduate student research.
Losego’s group webpage: http://losegolab.gatech.edu/Interfaces/Home.html
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