NERSC Initiative for Scientific Exploration (NISE) 2010 Awards

First-principles Modeling of Charged-Defect-Assisted Loss Mechanisms in Nitride Light Emitters

Emmanouil Kioupakis, University of California, Santa Barbara

Associated NERSC Project: First principles modeling of group-III-nitride compounds and alloys for applications in electronic and optoelectronic devices (m934), Principal Investigator: Patrick Rinke, University of California, Santa Barbara

Nitride light-emitting devices already have a wide range of applications, such as the lasers in Blu-Ray players and the white LEDs of bicycle lights. In the future, they may also be used as general white light sources, replacing the existing incandescent and fluorescent light bulbs, or in tiny laser projectors that can fit inside a cell phone. At present, however, these devices are not as efficient at the high intensities required for these applications. We will investigate why nitride light-emitting devices lose their efficiency at high intensities and suggest ways to fix this problem.

Nitride-based light emitters in the UV to green part of the optical spectrum have revolutionized the field of solid-state lighting and hold great promise for applications in general illumination and laser projectors. The performance of these devices at high drive currents, however, is limited by a prominent efficiency loss, the origin of which is not fully understood. One mechanism that has been blamed for the efficiency droop of nitride light-emitting diodes is Auger recombination, a 3-particle non-radiative recombination process that becomes dominant at high injected-carrier densities. The Auger process may occur either in a direct way, or mediated by a scattering mechanism, such as the electron-phonon coupling and alloy-scattering. An additional loss mechanism in nitride lasers is the absorption of the generated light by free and impurity-bound carriers in the active region and surrounding material. Our preliminary results indicate that phonon-mediated absorption due to non-ionized acceptor atoms in the p-type material is substantial and a source of concern for nitride lasers.

An additional carrier scattering mechanism in materials is the scattering by charged defects. Lattice imperfections and impurity atoms are omnipresent in materials and in certain cases they acquire an overall charge, rendering them efficient scattering centers via their Coulomb interaction with the charge carriers. This is particularly true for nitride devices, whose n- and p-type layers are intentionally doped. Charged-defect scattering has profound effects for e.g. the carrier mobility in semiconductors and may also provide the momentum required for indirect transitions. As a supplemental research initiative related to our existing ERCAP proposal, we plan to investigate charged-defect-mediated indirect Auger recombination and free-carrier absorption processes in nitride materials and evaluate their significance for nitride optoelectronic devices.