Advith's Calendar

Seminar title: First-Principes Materials Science for Hydrogen Generation, Topological Semimetals and Microelectronics.

Speaker: Dr. Stephan Lany, National Laboratory of the Rockies, Golden, CO

This seminar surveys how Stephan Lany’s recent computational materials research establishes predictive control of interfaces, defects, and dopants, with direct impact on three fronts: hydrogen generation science, topological semimetals, and ultrawide-bandgap microelectronics.

Thermochemical hydrogen production utilizes an oxygen vacancy mediated redox mechanism to split water. However, the thermodynamic boundary conditions are very constrained [1], calling for research strategies that accommodate two antagonistic goals. 

First, broad materials screening and discovery efforts are enabled by a machine-learning defect Graph Neutral Network (dGNN) approach [2]. Second, a statistical mechanics based approach for the free energy of defect interaction enables quantitative predictions for the high defect concentrations encountered in the water slitting redox cycle.[3] While applied here to hydrogen production, both advances are broad utility in areas where materials properties and functionality depend on defect formation. 

For quantum materials, we address the problem of Fermi level control in the Dirac semimetal Cd3As2,[4,5] where an intrinsic stoichiometry imbalance causes a misalignment between EF and the Dirac point. Here, the system responds very sensitively and in a strongly temperature-dependent fashion to the charge balance between defects and dopants on one side and the population of the host density of states on the other side. 

Using SCAN-level DFT supercell calculations with spin-orbit coupling, in combination with quasi-particle GW band corrections, we identify both intrinsic and extrinsic defect control strategies to achieve the desired alignment of EF with the Dirac point. In addition, we formulate a pseudo-equilibrium theory that goes beyond the conventional “freeze-in” approximation after the growth step and captures short-range site exchanges, including vacancy–interstitial annihilation. This approach is broadly transferable to other materials where defect redistributions during cooling from growth or annealing are of interest. 

For ultrawide-gap nitrides, we turn to heterostructural interface engineering to facilitate lattice matched interfaces between semi-metallic substrates and ultrawide-bandgap (UWBG) semiconductors for power electronics.[6,7] Specifically, we model heterostructural rocksalt/wurtzite interfaces between TaC(111) substrates and Al0.5Ga0.5N(0001) UWBG semiconductors. High-throughput DFT calculations are used to enumerate the possible stacking sequences on the underlying hexagonal coincidence site lattice. Sampling octahedral, tetrahedral, and prismatic motifs, we determine a phase diagram for the favorable combinations of substrate termination, film nucleation, and polarity as function of synthesis conditions. Subsequent calculations of the electronic structure yield the GW-corrected band alignment and electrostatic-potential profiles, from which we obtain Schottky barriers heights and interface charges. Implementing these firstprinciples data into device modeling, we demonstrate the possibility of order-of-magnitude gains in power-handling over current GaN devices.

Together, these studies link ab initio predictions to device- and process-level metrics, offering synthesis-tunable “knobs” that accelerate progress across varied areas including energy conversion, quantum transport, and power electronics.

Bio: Bio: Stephan Lany is a theoretical/computational scientist with a background in first principle calculations for problems in materials physics and chemistry. He received his PhD in Physics in 2002 from the Universität des Saarlandes, in Saarbrücken, Germany. In 2003, he joined the National Laboratory of the Rockies in Golden, Colorado as a post-doctoral researcher, where he currently works as a senior scientist. His projects have covered a wide range of materials from transition metal oxides to topological semimetals to ultra-wide gap semiconductors. His research combines the power of density-functional and electronic structure calculations from first principles with approaches and methods in defect theory, alloys and disorder, structure prediction, and materials design and discovery.