Although spintronic devices can process an order of magnitude more information than conventional electronics, current efforts to realize commercial spintronic devices are limited, due to the inefficiencies associated with spin injection across semiconductor interfaces. The primary obstacle to realizing commercial spintronics is the need to set and maintain the electron spin orientation. Most devices tune spin orientation using ferromagnets and magnetic fields. This approach makes it difficult for carriers to retain their spin orientation when they move from high- to low-conductivity materials — for example, from metallic ferromagnets to the undoped silicon and conjugated polymer materials that comprise most semiconductors. In a study led by the University of Utah and the National Renewable Energy Laboratory (NREL), researchers demonstrated, for the first time, that traditional optoelectronic devices can be adapted to control electron spin at room temperature, without a ferromagnet or magnetic field. Their work builds on previous research in which the team developed an active spin filter made of two layers of chiral hybrid organic-inorganic halide perovskites. The left-handed chiral layer of the filter allows electrons with “up” spins to pass through it and blocks electrons with “down” spins. The right-handed chiral layer performs the same process in reverse. The researchers found that chiral-induced spin selectivity — the spin-dependent transmission of charge carriers through an oriented chiral potential — allows for spin control at room temperature in a semiconductor platform when it is applied to chiral semiconductors based on halide perovskites. Stack of the spin-LED emitting circularly polarized electroluminescence. The (R-MBA2Pbl) acts as a spin filter, allowing only polarized carriers (blue circles) to flow through the LED and recombine in the multiple quantum wells emitting circularly polarized light (yellow helix). Courtesy of M. Hautzinger et al. Nature (2024) DOI: 10.1038/s41586-024-07560-4. They combined chiral semiconductors based on halide perovskites with a standard III-V LED through a direct semiconductor-to-semiconductor interface, replacing the electrodes in the LEDs with the spin filter made from chiral hybrid organic-inorganic halide perovskites. Spin injection across the chiral halide perovskite/III-V interface led to spin accumulation in a standard semiconductor III-V, multiple quantum well LED. The researchers detected the spin accumulation in the multiple quantum well through the emission of circularly polarized light with a degree of polarization of up to 15 ± 4%. “We took an LED from the shelf. We removed one electrode and put the spin filter material and another regular electrode. And voila! The light was highly circularly polarized,” said professor Valy Vardeny of the University of Utah. The spin LED comprises a stack of several layers. Each layer has different properties. The first layer is a common transparent metallic electrode. The second layer is a chirality-induced spin filter that blocks the electrons that spin in the wrong direction. The third layer is a standard semiconductor that is also used as an active layer in conventional LEDs. The spin-aligned electrons recombine in the third layer. The injected spin-aligned electrons cause this layer to produce photons that move in unison along a spiral path, rather than in a conventional wave pattern, to produce circularly polarized electroluminescence in the LED. “This work demonstrates the unique and powerful ability for these emerging ‘hybrid’ semiconductors to combine and take advantage of the interplay of the distinct properties of organic and inorganic systems,” said NREL researcher Matthew Beard. “Here the chirality is borrowed from the organic molecules and provides control over spin, while the inorganic component both orients the organic component and provides conductivity or control over charge.” The team characterized the chiral halide perovskite/III-V interface with X-ray photoelectron spectroscopy, cross-sectional scanning Kelvin probe force microscopy, and cross-sectional transmission electron microscopy imaging. The characterizations showed a clean semiconductor-to-semiconductor interface. The work shows that direct contact between chiral halide perovskite and traditional semiconductors is possible, and that the chiral halide perovskite semiconductor behaves as another semiconductor would within the device stack. The integration of chiral semiconductors based on halide perovskites can transform existing commercial III-V LEDs from a conventional LED semiconductor structure that controls the interconversion of light and charge to one that also controls spin-to-light. The spin-based semiconductor structure developed by the team operates at room temperature with no external magnetic fields. The team said that its approach can be used with other types of chiral materials, such as DNA, which would allow spin-based optoelectronics to be used in various contexts. “For decades, we’ve been unable to efficiently inject spin-aligned electrons into semiconductors because of the mismatch of metallic ferromagnets and nonmagnetic semiconductors,” Vardeny said. “All kinds of devices that use spin and optoelectronics, like spin-LEDs or magnetic memory, will be thrilled by this discovery.” The chiral halide perovskite semiconductor/traditional semiconductor interface could provide the foundation for a new class of spin injectors to achieve spin accumulation for a variety of spin functionalities. Spin accumulation in semiconductor structures at room temperature and without magnetic fields is key to broadening the functionality of optoelectronics. Although the team has confirmed that the spin-LED device works as intended, more research is needed to understand exactly what mechanisms are at work to create the polarized spins. “That’s the $64,000 question for a theorist to answer,” Vardeny said. The research was published in Nature (www.doi.org/10.1038/s41586-024-07560-4).
