NSF Award
2101220
Non-Technical: The successful incorporation of high amount of nitrogen in silicon enables absorption of infra-red (IR) light, unabsorbed by pure silicon. The IR absorption of such modified silicon is tuned through the initial amount of incorporated N and subsequent heat treatments. The new material leads to a new generation of IR devices as well as enhancing the efficiency of widely used silicon solar cells. In addition, silicon absorbing IR enables easier integration of optical devices in microelectronics, high speed parallel microprocessors with optical interconnections, smart high resolution IR cameras, smart remote-control devices, and the detection of small amounts of IR light. These applications are important for the economy, national security, future appliances, and energy security and sustainability.<br/>This project offers research topics for graduate students in physics and materials science, as well as a strong research training for undergraduate students. The project will advance the understanding of the science underlying materials for next generation computers, consumer electronics, and many other applications, while it promotes teaching and training of underrepresented students. Furthermore, students will learn through this collaboration involving three major universities interested in improving silicon, as well as two national laboratories operating the most advanced materials characterization techniques that are appropriate for this research. Summer research activities will be offered to high school students and transfer students from neighboring two-year colleges. High school teachers and two-year college faculty will be included, to increase recruitment of underrepresented students in STEM programs.<br/><br/>Technical: This project will develop new understanding of the materials science of hyperdoping silicon with nitrogen, as well as the physics of IR optical processes. Preliminary data suggests that N complexes in hyperdoped silicon create an Intermediate Band (IB) that enhances absorption from near infrared (NIR) to Mid-IR through two photon absorption. Intertwined physics and materials science issues are addressed to understand phase transitions coupled to transformation of the electronic band structure and the IB generation. The emphasis is on the energy levels induced by V_x N_y O_z complexes (V represents a Si vacancy) in the bandgap of oxygen-rich Czochralski and Float Zone silicon, and their conversion to an optically efficient band. <br/>The challenging questions consist in unravelling the: i) atomic configurations of dominant V-N complexes that generate deep level donor centers, ii) their thermodynamic stability during heat treatments for hyperdoping, iii) the mechanism that drives local phase transformations in N-hyperdoped Si, whether by the Mott transition, electron pairing, or the energy level delocalization proposed in this project, iv) the requisites (e.g., band filling, sub-bands, band valleys, and other conditions) for the IB to work, and v) the IB efficiency versus the V-N type and concentration. Large N-related complexes are modeled as nanodot arrays; the effects of array symmetry breaking on energy level mixing in such coupled systems, level delocalization, level anti-crossing, and tunneling between levels are investigated to clarify conditions at which they contribute in IB generation.<br/>The experimental data is correlated with multiscale computer modeling of N-related complexes and disordered nanodot arrays. Both continuum and atomistic calculations are used. Strain at the atomic scale is considered in the calculations since it is a determinant factor for energy levels and the IB. Quantum group behavior of electrons in nanodot arrays will shed light on the energy delocalization that leads to IB formation. IR absorption and photoluminescence of N-hyperdoped Si will indicate the efficiency of the sequential IR optical transitions via the IB.<br/><br/>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.
