C.-W. Lee's research website

As a computational materials scientist, my overarching goal is to develop and apply computational methods to speed up the development and deployment of new solutions for global challenges of energy, environment, and sustainability. Specifically, my research topics focus on discovery and engineering of wide-band-gap semiconductors, optically active point defects, and hetero-structures for the applications of power electronics, opto-electronics, and quantum communication/sensing. Close collaborations with experimental groups is also the integral part of my research program and the goal is to understand the underlying structure-property-processing relation in a synergistic manner.

Bridging the gap between new materials and functional devices

Semiconductor-based devices account for large portion of energy-related application, including photovoltaic, light-emitting diode, thermoelectrics, photocatalyst, power electronics, quantum information science, low-energy computing, etc. The breakthrough is typically achieved either by new device designs or by discoveries of new materials. Driven by Material Genome Initiative, which aims to significantly reduce the development time, high-throughput computational material discoveries and experimental combinatorial searches have led to fruitful identifications of new materials for diverse applications. However, such approach is generally based on intrinsic material properties and further works are still needed to fill the gap between a novel material and a functional device and to have translational impacts. For semiconductor-based devices, high quality substrate, low interfacial defect densities, dopability, proper band alignment, and suitable metal contacts are the common obstacles

Dopability in (Ultra)wide bandgap semiconductors

New ultrawide bandgap (UWBG) semiconductors are desirable for optoelectronics and power electronics due to larger electronic band gap and higher Baliga figure of merit (BFOM). BFOM is inversely related to on-state resistance of a conducting transistor under forward bias and is a metric critical for high-power applications. However, since UWBG materials are typically only n-type or p-type dopable and more likely the former, finding promising p-type UWBG semiconductors is critical to make heterojunction diodes.

Electronic structure of interfaces

Interfaces form when translation symmetry is broken either through different chemical species or through change in crystal structure or both. Such changes at atomic level haves significant impact on the electronic structure and thus on the thermal, electrical, magnetic, and optical properties. Therefore, understanding the underlying structure-property relation is critical for interface engineering (e.g. lower the electrical resistance at the interfaces for CMOS).

Thermochemical stability of interfaces

When two materials are in contact, chemical reactions can happen depending on thermodynamics. Gibbs free energy of the interface in principle should be included explicitly but atomic structure at the interface, which is needed for first-principles calculations, is non-trivial to construct. Instead, for bulk materials, the contribution of interface is sufficiently small, such that we can approximate the calculations with only Gibbs free energy of bulk phases. Therefore, once we can curate the formation Gibbs free energies (or enthalpies) of all the involving phases at a given pressure and temperature, we can predict the thermochemical stability of the interface.

Optically active point-defect qubits by design

The importance of QIS to funding agencies was embodied by the National Quantum Initiative Act and further detailed in the National Strategic Overview for QIS. Impurity/semiconductor-host qubits are among the promising qubit systems, as demonstrated by the successes in quantum sensing, quantum computing, and quantum communication. However, scalability and/or very low operating temperature remains the major challenges for semiconductor qubits.

Finding design principles for transition metal impurities in wide-band-gap semiconductors for quantum information technology

Transition metal (TM) impurities are common color centers in wide-band-gap (WBG) materials and some promising ones have been observed in diamond, SiC, and AlN for quantum communication and sensing. However, there is no quantitative design principle to achieve targeted optical property for a specific application like quantum communication or sensing. For example, key quantities are the relative position of defect levels with respect to band edges of the host materials, energy differences between defects levels, energetics of different spin configurations, zero phonon lines, etc. (see desired optical properties in the figure). The ultimate goal is discover the right combination of TM impurities and WBG semiconductors for a given application.

Synthesizability of point defects in WBG semiconductors

After a right combination of TM impurities and WBG semiconductors is identified, the next step is to fabricate a scalable device. Optically active defects are commonly generated via combination of ion irradiation and thermal annealing. The processing technique is great for creating an ensemble of optically active defects for proof of concept but the positions and types of created defects are stochastic in nature. However, to make scalable qubits, deterministic creation of single photon emitter is required. Approaching this with thermodynamic perspectives, I utilize first-principles calculations and machine-learning techniques to estimate the synthesizability of a targeted defect types and identify the external condition that maximize its probability to form as compared to competing defect types (for example, defect type of D1 has the highest probability to form under condition 1).

Material responses to non-thermal processing techniques

Non-thermal methods (e.g. laser, particle beams and electric field, etc.) are commonly used for materials characterization and processing. These stimuli pump energy into not only ionic systems but also electronic systems of materials. However, the effect of electronic excitation on how materials respond to non-thermal stimuli remains difficult to capture quantitatively. This is generally due to significantly shorter time scale of electron dynamics (10-100 fs) and the challenge to decouple electron and ion dynamics both experimentally and computationally. The goal is to utilize state-of-art first-principles method to quantify electronic excitation and incorporate the effect into larger length- and time-scale simulations such that we can predict the evolution of micro-structure under non-thermal processing.

Multiscale simulations of defect formations in ion-irradiated semiconductors

To capture the electronic excitation caused by non-thermal processing techniques, real-time time-dependent density functional theory is utilized. The excitation can be modeled by occupation numbers of initial Kohn-Sham orbitals and its effect on ion diffusion can be simulated using constrained DFT and NEB. To simulate radiation damage caused by ion-irradiation, two-temperature molecular dynamics can be used and coarse-graining key quantities from TDDFT and DFT calculations is the critical step for accurate predictions of defect formations.

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