Title: Materials Genome Approach to Defects in Amorphous Oxide Semiconductors
Speaker: Julia Medvedeva, professor of physics and Materials Research Center senior investigator at the Missouri University of Science and Technology
Date: Friday, March 14, 2025
Time: 9:30 a.m.
Location: Pettit Microelectronics Building, Room 102A

Defects in semiconductors are a cornerstone of the material’s performance. While oxygen vacancies in crystalline oxides are well understood from extensive theoretical and experimental investigations, allowing for unprecedented levels of control over the carrier generation and charge/ion transport, the concept of "vacancy" becomes ambiguous in disordered structures where periodicity and symmetry-defined lattice sites are absent. For amorphous oxide semiconductors (AOS) with weak metal-oxygen bonding, such as In- or Sn-based oxides, disorder invades the short- and medium-range structure, making every metal-oxygen polyhedron unique in itself and/or its environment. The intricate coordination morphology combined with an increased number of degrees of freedom supports coexistence of extended, weakly localized, and deep trap defects in sub-stoichiometric amorphous oxides and promotes switching between shallow and deeply bound states, invisible to conventional x-ray or electron beam probes and to static measurements of carrier concentration and mobility. Lack of understanding of the origins of oxygen defects with various degree of electron localization, different binding energies, and unique dynamical properties, make the electron transport and optical transmission hard to control experimentally even in commercialized AOS.

Elucidating the microscopic behavior of hydrogen in AOS is another formidable challenge. H may passivate either under-coordinated oxygen or metal atoms, forming covalent (M-OH) or ionic (M-H-M) bonds, respectively, yielding strikingly different electronic behavior and, hence, introducing competing mechanisms for carrier generation and scattering. The wide coordination distributions and strong distortions in the M-O polyhedra dramatically increase the number of possible (meta-)stable sites for H and the diversity of its immediate neighbor environment – in contrast to unambiguous H passivation of dangling bonds in covalent semiconductors. The increased number of degrees of freedom leads to pronounced structural dynamics associated with bond-rearrangements, enhanced by oxygen non-stoichiometry. All the above strongly influence the formation, activation, and stability of the competing H defects as well as H mobility through the disordered structure. Accurate and statistically-significant calculations are essential to elucidate the complex H behavior in AOS and its role in the resulting macroscopic properties.

In this work, ab-initio non-stoichiometric liquid-quench molecular dynamics simulations, advanced time- and temperature-dependent structural analysis, and accurate hybrid-functional electronic structure calculations are employed to identify, classify, and quantify various oxygen and hydrogen defects in prototype AOS. To derive the materials genome of the defect formation, multiple descriptors of the defect’s local structure are considered in concert with extended bond reconfiguration that occurs to accommodate the defect in the disordered lattice. The time-resolved behavior sheds light on the defect stability, defect transformations, and defect diffusion. Matching the results with the defect’s ability to induce, compensate, or trap electronic charges, helps detangle the role of specific defects in competing mechanisms for carrier generation, charge scattering, instabilities, and optical absorption.

Bio: Julia E. Medvedeva is a professor of physics and a senior investigator at Materials Research Center of Missouri University of Science and Technology. She received PhD in physics and mathematics in 2002 and worked as a pre- and post-doctoral fellow at Northwestern University in Arthur Freeman’s group. She joined Missouri S&T in 2005. Julia’s expertise is in first-principles density functional calculations of the structural, electronic, optical, and mechanical properties of a wide range of materials, including metal oxides, nitrides, chalcogenides, alloys, and strongly-correlated materials. She is a leading expert in the area of transparent conducting oxides and amorphous oxide semiconductors. Julia’s career-long publication impact puts her among the top 2% of researchers in the world in the fields of Applied Physics and Materials. Her research work has been funded by federal and private agencies as well as industry. She has served as a Lead-PI on a materials genome grant funded by the NSF-DMREF (Designing Materials to Revolutionize and Engineer our Future) program, a co-PI on NSF-MRSEC (Materials Research Science and Engineering Center) at Northwestern University, and a co-PI on NSF-MRI (Major Research Instrumentation) grant to scale-up high-performance computational resources at Missouri S&T. Currently, Julia is a co-PI on two collaborative grants funded by the U.S. Department of Energy Solar Energy Technologies Office and the U.S. Army Research Laboratory.