ENG-SEMICON: Ferrimagnetic / Transition Metal Dichalcogenide Heterostructures for Efficient Opto-Spintronic Devices

Non-technical: The light emitting diode (LED) is a crucial opto-electronic device which has transformed modern lighting technology as well as high- speed communications based on optical-fibers. In standard LEDs, electrons and the absence of electrons, termed holes, are injected from metallic contacts into opposite ends of a semiconductor junction, where they recombine to generate a photon. The recombination process, and hence the light intensity, can be quickly modulated for information encoding and communications. This project aims to also control the spin of the injected electrons, adding an additional degree of freedom to the information encoding through efficient modulation of the light circular polarization. Spin is a fundamental property of an electron, as is the electron’s charge and mass. Spin injection into semiconductors is challenging because of the large mismatch of electrical resistance between common magnetic metal electrical contacts and the semiconductor. The project will explore magnetic electrical contacts comprised of a class of materials termed ferrimagnetic rare earth / transition metal (RE/TM) alloys that can reduce this electrical resistance mismatch, and thereby greatly improve spin injection efficiency. Different combinations of two-dimensional semiconductors, almost atomically thin, with spin-selective light polarization emission, will be tested. Changing the magnetic state of the RE/TM contacts, even within very short time scales, reverses the injected electron’s spin and hence the circular polarization (left or right) of the emitted light. The results of this project can lead to innovative new technical capabilities including ultrahigh speed optical communications, three-dimensional displays, quantum encryption and other quantum information applications, and secure long-range communication. The research will support two graduate students and will expose numerous undergraduates in advanced research methods. Technical: This project will combine electronics, spintronics, valleytronics and photonics into an integrated device that can serve as an efficient, electrically controlled emitter or sensor for circularly polarized photons. At the heart of the proposed device structure are thin layers of two-dimensional (2D) semiconductor transition metal dichalcogenides (TMDs), such as WSe2 or MoS2. Single layer TMDs have a crystal structure that lacks in-plane inversion symmetry, leading to an electronic band structure characterized by spin-inequivalent valleys at distinct points in momentum space (K and K’ points). This close connection of spins and electronic band structures is an example of spin-momentum locking. For light emission applications, spin polarized carriers will be injected from metallic magnetic electrodes, thereby disproportionately populating the K or K’ points. The project focuses on two important advances: (1) use of tunable and low work function ferrimagnetic electrodes, namely Gd based rare earth / transition metal (RE/TM) alloys, to greatly improve the spin injection into the TMDs; and (2) growth of lateral TMD-1 / TMD-2 heterojunctions (e.g. MoSe2 / WSe2) with dissimilar p-doping and n-doping, which then naturally form nearly-perfect, one dimensional p-n junctions. The spins injected at the ferrimagnetic electrode will diffuse to the p-n junction, recombine with holes across the junction, and emit chiral (circularly polarized) photons. Reversing the magnetization of the ferrimagnet will reverse the sign of the injected spins and hence reverse the photon chirality. Several materials optimization steps will be needed to achieve this result, and the project will also develop precision optical magnetic metrology methods based on the spin Hall effect of light (SHEL) to better understand the diffusion length and lifetime of the injected spins. The results will establish a new class of 3D / 2D heterostructures for opto-spintronic applications. 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.