Miniature Molybdenum Disulfide and Iron Magnets Enable Spin + Charge Transistor Technology

one month ago by Biljana Ognenova

The two-dimensional molybdenum disulfide material combines ferromagnetism and semiconductivity to help set the stage for future logic and memory devices based on spintronics.

Miniaturisation has been a long-prevailing trend in semiconductor manufacturing technology. However, you can only go so small with electronic devices. At some point, it becomes impossible to produce invisible-to-the-naked-eye transistors.

Even if Moore’s law is not entirely dead, for at least a few more years, moving beyond it has been a priority for the electronics industry for some time now. 

Spintronics is a fertile new ground for exploration, and variations of electron-spin transistors are being researched and tested.


Molecular diagram ferromagnetic.

A molecular diagram representing the structure of a ferromagnetic, two-atom-thick semiconductor: it has green, blue, and red spheres, which are respectively sulfur, molybdenum, and iron (Fe) aroma. Image credited to Stevens Institute of Technology.


A Chance to Abandon Physical Scaling of Electronic Devices

Spintronic devices are not only necessary in terms of spintronic circuits and more energy-efficient transistors, but also because electron spin+charge technology enables the creation of memory-packed storage devices of the future, which are expected to be omnipresent in upcoming IoT networks.

Furthermore, spintronics is crucial for developing a new generation of quantum devices, particularly given its potential to overcome the significant limits of Moore’s law by either advancing spin polarisation with new materials or improving spin filtering at current devices. (The latter is achieved by finding new ways to generate spin-polarised currents.)

The recent progress made by Stevens Institute of Technology (SIOT) researchers, in collaboration with several other research teams, is a step towards a new methodology for producing electronic devices that can be faster and more efficient, without needing to be smaller.


Ambient Temperature is Necessary for Operating Safety

Instead of focusing on the physical scaling of electronic components, the SIOT-led researchers developed a ferromagnetic semiconductor that can harness the energy from both electron charge and spin at ambient temperature.

The fact that spintronics is largely limited to functioning in cryogenic conditions has been a critical problem in such a field of research (and this has called for a lot of R&D into how the technology can work at room temperature). This new, atomically-thin ferromagnet enables faster processing speed, less energy consumption, and increased storage capacity.

As it appears to have solved the safe operating temperature problem, it has been deemed ready to be tested for immediate integration with the current semiconductor production technologies.


Old-generation transistors.

A group of various older generation transistors. Consider the improvements that can be made to transistors with the use of spintronics—particularly when utilising molybdenum disulfide and ferromagnetic. Image credited to Wikimedia Commons.


A Two-dimensional Ferromagnetic Semiconductor 

MoS2 has good chemical and thermal stability. The two-dimensional (two-atom-thick) material was made by doping MoS2 with isolated iron atoms that take the crystal’s place during such a doping process. This technique enables the design of transistors that are lightweight and flexible, and yet are able to control electron spin on the up-and-down axis.

The thin ferromagnetic material created in this way remains magnetised at room temperature and can be used as a semiconductor component. Although such magnets are weak and have a magnetic field strength of only 0.5 mT (millitesla)—which is incapable of moving even a tiny nail—the researchers found that it is still powerful enough to both alter electron spin and play a part in quantum bits applications.


In Situ Substitutional Doping

The method of directly replacing MoS2 crystals with iron atoms is called ‘in situ substitutional doping’. The most crucial requirement for materials that are produced with this method is the simultaneous existence of ferromagnetism and semiconductivity. This hybrid property enables the combining of spin degrees of freedom and charge degrees of freedom for next-gen quantum heterostructures.

It took the combined efforts of Stevens Institute of Technology and various research institutes with specialities in physics and mechanical engineering to develop a solution that produces permanent magnetisation with Mo-Fe atom replacement. This development is yet further proof that the electronic components industry does much better when taking the interdisciplinary approach to research.