Speaker: Richard Hennig
Affiliation: Associate Professor, Materials Science and Engineering, University of Florida
Title: Materials Informatics for the Discover of Novel 2D Materials
Abstract: The rapid rise of novel single-layer materials presents the exciting opportunity for materials science to explore an entirely new class of materials. This comes at the time when mature computational methods provide the predictive capability to enable the computational discovery, characterization, and design of single-layer materials and provide the needed input and guidance to experimental studies. I will present our data-mining, chemical substitution, and evolutionary algorithm approaches to identify novel 2D materials with low formation energies and show how unexpected structures emerge when a material is reduced to sub-nanometers in thickness. To identify 2D materials that can be synthesized by exfoliation of bulk materials, we searched the Materials Project crystal structure database for materials possessing layered motifs in their crystal structures using a topology-scaling algorithm. The algorithm identifies and measures the sizes of bonded atomic clusters in a structure’s unit cell, and determines their scaling with cell size. The search yielded 680 monolayers with exfoliation energies below those of already-existent 2D materials. These materials guide future experimental synthesis efforts. Among the 2D materials, we find that for several 2D transition-metal chalcogenide compounds ferromagnetic order emerges at temperatures accessible to experiments. Calculations of the magnetic anisotropy show that many of the magnetic 2D materials exhibit an easy-plane for the magnetic moment and hence a Berezinsky-Kosterlitz-Thouless transition to a magnetically ordered low-temperature phase. A few 2D materials display an easy magnetization axis and thus an actual ferromagnetic ground state. Furthermore, we identify a family of three magnetic 2D materials with half-metallic band structures. Their purely spin-polarized currents and dispersive interlayer interactions should make these materials useful for 2D spin valves and other spintronic applications. These new 2D materials provide the opportunity to investigate the interplay of magnetic order and reduced dimensionality and may provide materials suitable for optoelectronic and spintronic applications. The structures and other calculated data for all 2D materials are available in the MaterialsWeb database at https://materialsweb.org.
Speaker: Darrin Pochan
More details will be provided.
More details will be provided.
Speaker: Theodosia Gougousi
Affiliation: Professor, University of Maryland, Baltimore County
Title: Interface Cleaning Mechanism during the Atomic Layer Deposition of Dielectrics on III-V Semiconductors
Abstract: Atomic layer deposition (ALD) is a thin film deposition technique which can be used to grow highly conformal thin films with sub-nanometer thickness control. Since its early development for the deposition of thin films for electroluminescent displays, the applications of ALD broadened into an expansive field ranging from microelectronics to biocompatible coatings. One of the most common applications is the formation of gate oxides in metal oxide semiconductor devices. During the ALD of Al2O3 from trimethyl aluminum (TMA) and H2O on GaAs, the thinning of the native oxide interfacial layer was observed.1,2 Since then, several systems exhibiting this native oxide “clean-up” behavior have been identified.3 One controversial observation was that for amine precursors the “clean-up” reaction seemed to proceed well after the native oxide surface had been covered with the formed film. To explain such observations a mechanism that will transport the surface oxides through the growing film is required.
In this talk I will present an overview of the literature data on the interface cleaning reaction for several dielectric/III-V systems and present evidence for native oxide migration in the growing film at typical processing conditions (150-250 °C). Subsequently I will present the experiments we designed to demonstrate the existence of this transport mechanism. In these experiments we coupled transmission Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) measurements to show both indirectly and directly that arsenic and gallium oxides can diffuse through 4 nm thick TiO2 films deposited on native oxide GaAs(100) surfaces at temperatures as low as 150°C. 4
1. Ye, P. D. et al. GaAs metal–oxide–semiconductor field-effect transistor with nanometer-thin dielectric grown by atomic layer deposition. Appl. Phys. Lett. 83, 180–182 (2003).
2. Frank, M. M. et al. HfO2 and Al2O3 gate dielectrics on GaAs grown by atomic layer deposition. Appl. Phys. Lett. 86, 152904 (2005).
3. Gougousi, T. Atomic layer deposition of high-k dielectrics on III–V semiconductor surfaces. Prog. Cryst. Growth Charact. Mater. 62, 1–21 (2016).
4. Henegar, A. J., Cook, A. J., Dang, P. & Gougousi, T. Native Oxide Transport and Removal During Atomic Layer Deposition of TiO2 Films on GaAs(100) Surfaces. ACS Appl. Mater. Interfaces 8, 1667–1675 (2016).
Bio: Theodosia Gougousi is a Professor in the Department of Physics at UMBC (University of Maryland, Baltimore County). She received her BS in Physics from the Aristotle University of Thessaloniki, Greece in 1990 and her MS and Ph.D. in Physics from the University of Pittsburgh in 1993 and 1996. Prior to joining the faculty at UMBC in 2004 she held postdoctoral appointments at the University of Maryland, College Park and North Carolina State University. Her research interests include the deposition mechanisms and properties of thin films, interfaces and other low dimensional materials. Her laboratory uses a combination of vacuum and high-pressure approaches to study the deposition of materials on planar and high aspect ratio topography. These techniques include Atomic Layer Deposition and Supercritical Fluid Deposition. She has authored or coauthored more than 45 journal articles, and has more than 50 contributions to conferences. She received an NSF CAREER award in 2009.