Spin dynamics in p-doped Germanium

By Jade Holt on June 7, 2016

Matthieu Jamet et al introduce their latest work, explaining the principles behind spintronics and how they located the spin current in Germanium films.

The field of semiconductor spintronics aims at introducing the electron spin as a new degree of freedom into existing microelectronic devices. To do this, one has to generate an out-of-equilibrium electron spin population in the semiconductor. In a sense, it makes this semiconductor magnetic until the local magnetization generated by the out-of-equilibrium spin population relaxes to zero. This happens for times longer than the spin lifetime in the semiconductor. Fortunately, due to crystal symmetry reasons, the spin lifetime is expected to be very long in silicon and germanium which are the materials used in today’s microelectronics.

Sketch of the three-terminal device used for (a) Hanle measurements and (b) spin pumping. © 2016 IOP Publishing Ltd

In our latest research, we create a spin population in p-doped germanium by injecting a spin polarized electrical current from a ferromagnetic electrode into a germanium film. This can be done by growing a magnetic tunnel junction Ta/CoFeB/MgO on top of the semiconductor. The way to evaluate the spin injection efficiency and the spin lifetime relies on the Hanle effect that we detect electrically after properly subtracting the magneto-resistance of the germanium film. The Hanle effect consists in applying a magnetic field perpendicular to the injected spins to trigger their Larmor precession and, by this, suppress the spin population in the germanium film. However, due to the unavoidable presence of electronic states confined at the MgO/Ge interface, part of the spin population or accumulation may be created into those interface states rather than in the germanium channel. It may lead to wrong values of the spin injection efficiency and spin lifetime in germanium. With germanium, interface states are located close to the top of the valence band. Therefore in p-doped germanium, interface states are close to the Fermi level limiting their confinement and favoring spin injection and accumulation into the germanium channel. However, due to strong spin-orbit coupling in the germanium valence band, the spin lifetime is expected to be very short, which limits Hanle effect measurements to low temperatures.

We indeed find that the electrical signal corresponding to the Hanle effect disappears above 80 K. The extracted spin lifetime is of the order of 1 ps. Working at low temperature offers the advantage of comparing this spin lifetime to the one extracted from weak anti-localization measurements. Weak anti-localization is a quantum correction to the electrical conductivity modulated by the spin-orbit interaction. By modelling the germanium magneto-conductance at low temperature, we are able to extract the spin diffusion length, which relates to the spin lifetime through the electron diffusion coefficient. We find a spin lifetime of the order of 1 ps in agreement with Hanle effect measurements in the 2-20 K temperature range. This observation confirms that a large part of the spin population is generated in the germanium valence band.

Finally, we use the spin pumping technique to achieve spin injection in germanium. In this case, a pure spin current (with no net charge current) is flowing into the germanium film at the ferromagnetic resonance of the CoFeB electrode grown on top. The spin current is then detected electrically by its conversion into a transverse charge current as a consequence of the inverse spin Hall effect (ISHE). We detect an ISHE signal down to very low temperatures confirming that a spin current is flowing into the germanium channel. Moreover, we found that the spin-to-charge conversion in germanium is determined by the spin-orbit coupling of ionized impurities (boron ions here).

You can read the full paper, F Rortais et al 2016 J. Phys.: Condens. Matter 28 165801, here.

About the authors

Fabien Rortais is a Ph.D candidate working at SPINTEC in Grenoble, France. He has a background in materials science and experimental physics. He is finishing his experimental thesis on spintronics in silicon and germanium.

Dr. Simon Oyarzun works at the University of Santiago in Chile. During his Ph.D and postdoc, he was involved in the study of spintronics in nanostructures, focusing in the last years in spin-orbit effects in 2 dimensional systems.

Dr. Matthieu Jamet is the head of the semiconductors and 2D spintronics group at SPINTEC in Grenoble, France. His main research activities are focusing on spintronics and magnetism in silicon, germanium and 2D materials like graphene and transition metal dichalcogenides.

This work is licensed under a Creative Commons Attribution 3.0 Unported License. Image taken from F Rortais et al 2016 J. Phys.: Condens. Matter 28 165801. Copyright IOP Publishing Ltd 2016.