Collaborative Research: DMREF: Quasi-Direct Semiconductors

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Nontechnical DescriptionThe rapid identification of materials and structures with properties tailored to specific applications is a fundamental aspect of the Materials Genome Initiative. A basic prerequisite for the success of the Initiative is the ability to predict the targeted properties starting from basic information about the atomic composition and configurations in the material. For devices such as solar cells, detectors, light-emitting diodes, and lasers, the key design consideration is the fraction of the incident light energy absorbed at any particular wavelength. This DMREF project focuses on a particular class of materials, dubbed “quasi-direct” semiconductors, for which a satisfactory theory of light absorption does not exist. The project will develop the theoretical tools needed for the calculation of optical absorption in quasi-direct semiconductors and validate the new theoretical methods by carrying out optical experiments in structures optimized for the accurate measurement of the absorption coefficient. More than 200 quasi-direct semiconductors have already been identified in the Materials Project database, and this project will make it possible to incorporate such materials as optical components of future devices. All codes released will be open source to maximize societal impact, and the semiconductor industry will also benefit from the highly trained STEM workforce delivered by the project. A strong educational focus will be placed on undergraduate students by partnering with the Arizona State University (ASU) Sundial Project, which recruits and mentors students who traditionally have limited access to STEM careers. At the University of Texas at Austin (UT Austin), undergraduates will be directly involved in the development of the new codes. Technical DescriptionIn quasi-direct semiconductors, the energy threshold for vibration-assisted light absorption (the so-called indirect gap) is only slightly below the energy threshold for direct light absorption (the direct gap). Because of this proximity, the quantum-mechanical second-order perturbation theory expressions for vibrational-assisted absorption diverge as the photon energy approaches the direct gap, leading to unphysical predictions. The proposed solutions include the development of many-body quasi-degenerate perturbation theory and the use of a non-perturbative special displacement method by the team at UT Austin. The experimental validation requires special samples and materials, since the absorption coefficient must be determined with high accuracy over a spectral range where it changes by orders of magnitude. The needed samples will be fabricated at ASU using custom Chemical Vapor Deposition methods, and the optical measurements will also be performed by the ASU team.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.