NERSC Initiative for Scientific Exploration (NISE) 2011 Awards

First-Principles Calculation of Phonon-Assisted Optical Absorption in Indirect-Band-Gap Semiconductors.

Emmanouil Kioupakis, University of California - Santa Barbara

Associated NERSC Project: First-principles modeling of nitride and oxide materials for applications in electronic and optoelectronic devices. (m934)

Electrons in materials can absorb light by taking the energy of the incident radiation and jumping into a higher-energy excited state. In some materials however, such as silicon, this transition cannot happen in a single step and needs to be assisted by another mechanism that can provide some extra momentum. One mechanism that can give the extra push is the interaction of the electrons with the atoms of the crystal. Silicon solar cells, which dominate the photovoltaic industry, rely on this assisted process to absorb the incident sunlight and convert it to electricity. The goal of out research is to investigate this indirect light absorption process at the microscopic level and understand in detail how the silicon solar cells operate.

Optoelectronic materials are at the heart of CD/DVD/Bluray players, LED displays, solar cells, and fiber-optic communication systems and are poised to dominate in the areas of general illumination, projectors, and photovoltaic power generation. Understanding the way light interacts with charge carriers in materials is an important fundamental question that has paved the way to this array of technological applications. At present, the methodology to understand direct optical processes from first principles, including many-body effects and the electron-hole interaction, is well established. Another kind of optical processes are those that are mediated by an additional scattering mechanism, such as the coupling of the carrier motion to the lattice vibrations of the crystal. Such indirect optical processes are responsible, among other things, for the optical absorption of visible light in silicon and are hence at the core of the present silicon photovoltaic cells. These phonon-assisted optical absorption processes in indirect-band-gap semiconductors have not been investigated fully from first principles before because, due to the large number of initial and final states involved, the associated computational cost is prohibitively high.

In the context of our work on the efficiency issues in nitride lasers we investigated phonon-assisted optical absorption for the special case of absorption by free carriers near the band extrema of GaN. We found that these phonon-assisted processes are responsible for the internal reabsorption of the generated light in nitride lasers and proposed ways to mitigate the impact of this loss mechanism. As a supplemental research initiative related to our existing ERCAP proposal, we plan to generalize the methodology we developed and apply it to study the phonon-assisted absorption of light in technologically important indirect-band-gap semiconductors (such as Si, Ge, and GaP). The computational cost of the calculations can be substantially reduced by employing the interpolation formalism based on maximally-localized Wannier functions, which can yield the necessary band energies and electron-phonon coupling matrix elements for arbitrary pairs of states at a minimal cost. The resulting insights will shed light on the nature of phonon-assisted optical absorption in indirect-gap materials and the operation of silicon-based solar cells.