In a cross-departmental collaborative undertaking, scientists at the University of Minnesota have successfully grown a quantum material, bismuth selenide, by magnetron sputtering, a process compatible with large scale production, and currently in use commercially. The development bears significance for the semiconductor industry, holding the possibility of improved energy efficiency for computing and memory devices.
The details of the team’s research are reported in “Room-temperature high spin-orbit torque due to quantum confinement in sputtered BixSe(1-x) films,” published in Nature Materials.
TOPOLOGICAL MATERIALS AND SPIN-ORBIT TORQUES
The class of materials known as topological insulators (TI) has been of interest to scientists in the fields of physics and material science. The unique property they possess–insulating interior and conducting surface–can improve the energy efficiency of post-CMOS computing devices, and magnetoresistive random access memory (MRAM), a non-volatile memory. TIs can convert charge into non-equilibrium spin density efficiently because of the perpendicular locking of an electron’s spin with its momentum. Such efficient conversion is critical to magnetoresistive random access memory (MRAM) and post-CMOS computing devices for faster writing of data, reduced power consumption, and greater device reliability.
Typically spin-polarized current is generated using a ferromagnetic polarizer which transfers its spin angular momentum to another ferromagnetic layer. This is the mechanism for the creation of spin-transfer torque (STT). However, the inefficiency of the ferromagnetic polarizer entails higher power consumption, and lower reliability. On the other hand, spin-orbit torque (SOT), an outcome of spin-orbit interaction, essential to the atomic structure of specific materials such as heavy metals (for instance, tungsten, W) has advantages over STT. The absence of a separate polarizer gives SOT-based memory and logic devices an edge over STT-based devices in terms of energy efficiency and device reliability. Recently, topological materials have been reported to generate large spin-orbit torques.
GROWING BISMUTH SELENIDE
However these developments can be worthwhile only if they can be nudged into the mainstream, and commercially viable processes for growing TI are developed. Typically, topological insulators are created either using a single crystal growth process, or a process called molecular beam epitaxy. Both of these techniques cannot be easily scaled up for use in the semiconductor industry. In the present study, researchers started with bismuth selenide (Bi₂Se₃), a compound of bismuth and selenium. They then used a thin film deposition technique called “sputtering,” which is driven by momentum exchange between the ions and atoms in the target materials due to collisions. While the sputtering technique is common in the semiconductor industry, this is the first time it has been used to create a topological insulator material. Significantly, this could be scaled up for semiconductor and magnetic industry applications. It is also worth noting that on testing the new material, researchers found it to be 18 times more efficient in computing processing and memory compared to current materials.
According to lead scientist, and Distinguished McKnight University Professor and Robert F. Hartmann Chair in Electrical Engineering Jian-Ping Wang, “The unique feature of the sputtered bismuth selenide is the presence of nano-sized grains, which were missing in molecular beam epitaxy grown single crystalline topological insulator films.”
Prof. Tony Low, co-author of the paper and faculty in the Department of Electrical and Computer Engineering points out, “The nano-sized grains present in the sputtered bismuth selenide films yield higher spin-accumulation due to the spin momentum locking in the deeply located bands. As grains shrink, quantum confinement of the wave function further enhances spin-accumulation.”
Prof. Wang’s excitement is palpable, “Using the sputtering process to fabricate a quantum material like a bismuth-selenide-based topological insulator is against the intuitive instincts of all researchers in the field and actually is not supported by any existing theory. Four years ago, we started with a big idea to search for a practical pathway to grow and apply the topological insulator material for future computing and memory devices. Our surprising experimental discovery led to a new theory for topological insulator materials. Research is all about being patient and collaborating with team members. This time there was a big pay off,” Wang said.
THE ROAD AHEAD
With support from the Semiconductor Research Corporation, the researchers have filed for a materials-related patent, and experts in the semiconductor industry have requested samples of the material. The current paper represents the latest development in the research led by Prof. Wang, and collaborating scientists at the University of Minnesota and other institutions in the field of spintronics. His team has been at the forefront of the search for novel materials, and fabrication techniques that can enhance computation in semiconductor-based devices by improving storage and processing speeds, and the development of magnetic biomedical technologies and devices that can improve patient comfort and outcomes.
Recently, Prof. Wang as principal investigator received a $3.1 million grant from DARPA (Defence Advanced Research Projects Agency) under the Electronics Resurgence Initiative (ERI) for exploring novel materials and circuit structures that can take us beyond the limitations imposed by Moore’s Law, and meet looming engineering, economic, and defense challenges.
Another notable example of the pioneering work conducted by Prof. Wang’s research team is the development of Z-Lab, a portable diagnostic platform designed to perform on site testing of biological samples for various ailments. This is the first version of the prototype developed for point-of-care diagnostics. The details of the device and results of the test are reported in the paper “Portable GMR Handheld Platform for the Detection of Influenza A Virus” published recently in ACS Sensors.
Earlier in the year, a collaborative effort led by researchers at the University resulting in the experimental discovery of ferromagnetic properties in Ruthenium was published in Nature Communications under the title, “Demonstration of Ru as the 4th ferromagnetic element at room temperature.“ The discovery makes it the fourth single element with such properties in the periodic table. It opens an exciting new chapter in the fundamental studies of this element, and its application potential in the creation and scaling of magnetic memories, and breaking new ground in computing performance.
Late last year, Prof. Wang and his team in collaboration with researchers from Pennsylvania State University discovered the existence of magnetoresistance in topological insulator-ferromagnetic bilayers. This discovery has significant implications for the semiconductor industry, and opens up the door to enabling low power computing, brain-like computing, and chips for robots in the near future. The details of their research are published in Nature Communications under the title “Unidirectional spin-Hall and Rashba−Edelstein magnetoresistance in topological insulator-ferromagnet layer heterostructures.” The study confirms the existence of such unidirectional magnetoresistance and reveals that the adoption of TI, compared to heavy metals, improves the magnetoresistance performance by about twice at a temperature of 150 Kelvin (-123.15 Celsius).
Prof. Wang and his team of scientists are on an exciting path, unpacking and harnessing the potential of spinning electrons, translating it into applications disparate but critical to diverse aspects of our lives: low power computing, memory devices, biomedical devices. Translating these research outcomes into technologies that are adopted by industry, and impact our lives is only a matter of time. Our lives will certainly look different as these research outcomes make their way to industry and become commercially available.
The research for the paper, “Room-temperature high spin-orbit torque due to quantum confinement in sputtered BixSe(1-x) films,” was funded by the Center for Spintronic Materials, Interfaces and Novel Architectures (C-SPIN) at the University of Minnesota. C-SPIN is a Semiconductor Research Corporation program sponsored by the Microelectronics Advanced Research Corp. (MARCO) and the Defense Advanced Research Projects Agency (DARPA). C-SPIN is led by Prof. Wang.