Maik Wagner-Reetz
Fraunhofer Institute for Photonic Microsystems, Group Spin-based Computing Königsbrücker Str. 178, 01099 Dresden, Germany
In semiconductor industry, the introduction and integration of unconventional materials, technologies and devices for new applications is a very challenging process with a lot of limitations and demands. Even if the expected benefit is tremendous, like it is foreseen for many spintronic applications, partially immense hurdles have to be mastered. First, the established contamination management of a Fab leads to a lot of restrictions and a limited availability with regard to approved elements from the periodic table. Furthermore, the Environmental, Health and Safety conditions from industry add various constraints, which lead to further limitations. Perhaps, the most important condition in semiconductor industry is the cost-of-ownership. The restrictions lead to a limited usability of known materials and difficulties to explore unconventional phases and compounds. Despite the restrictions, there is still plenty of room and a lot of new technologies were developed in recent years especially in the memory business.
Today, data is the life blood that is disrupting many industries. The vast majority of these data are stored in the form of non-volatile magnetic bits in hard disk drives, a technology developed more than half a century ago, that has reached fundamental scaling limits that impedes further increases in storage capacity. New approaches are needed. Based on very recent discoveries, spin-based implementations like e.g. Magnetic Random Access Memory (MRAM) or Racetrack Memory (RTM) are such approaches. The charge-to-spin-conversion and vice versa is a key element in spin-based computing systems and is addressed in recent research. Spin-Orbit-Coupling phenomena play a vital role in both, Spin-Orbit-Torque MRAM and RTM, where new materials with high Spin-Hall-Angles are needed. Therefore, several materials ranging from (heavy) metals to binary compounds are considered, like e.g. CoSi or TaP. A CoSi process sequence including wet chemical silicon oxide removal, Cobalt CVD deposition with annealing is available [1]. For TaP a thin film process is not yet available. Both CMOS-compatible compounds are considered to belong to the class of Weyl semimetals, which are theoretically proven to have high Spin Hall Angles [2,3]. Several open questions, like e.g. influences on the topological properties of interfaces or grain boundaries have to be addressed in order to pave the way for new unconventional approaches.
[2] Hu, J.; Liu, J. Y.; Graf, D.; Radmanesh, S. M. A.; Adams, D. J.; Chuang, A. et al. π Berry phase and Zeeman splitting of Weyl semimetal TaP. Scientific reports 6 (2016) 18674.
[3] D.A. Pshenay-Severin, Y.V. Ivanov, A.A. Burkov, A.T. Burkov. Band structure and unconventional electronic topology of CoSi. J. Phys.: Condens. Matter 30 (2018) 135501.