Key researcher: Dr. S. Zhou (s(dot)zhou(at)hzdr.de)
Dr. M. Khalid (from November 2012), Mr. Ye Yuan (y(dot)yuan(at)hzdr.de)
The fashion word "spintronics" (short for "spin electronics") refers to devices that manipulate the spin degree of freedom. A new generation of devices based on the manipulation of spins may have completely new functionalities, which, therefore, could drastically improve the computational speed and reduce power consumption. Datta and Das proposed a spin-FET (field effect transistor), where the source and the drain are ferromagnets acting as the injector and detector of the electron spin. By modifying the gate voltage, the charge-carrier spin can be controlled. The spin injector can be a ferromagnetic metal or a ferromagnetic semiconductor. The crucial problem is the efficiency of the spin injection, i.e., the amount of carriers that can keep their spin state while moving sufficient distances. While the degree of spin polarization for metallic spin injection is limited due to the conductivity mismatch, a ferromagnetic semiconductor could allow a robust spin injection into a nonmagnetic semiconductor.
To fabricate the diluted ferromagnetic semiconductors, we have to overcome the low solid solubility limit of transition metals in semiconductors. The goal is not only to introduce a large number of dopant atoms, but also to ensure that these atoms are electrically active. However, at high concentrations, dopant atoms, especially transition metal atoms, usually tend to cluster in semiconductors. Therefore, one needs highly nonequilibrium methods to introduce enough dopants and a short-time annealing to activate them. Both ion implantation and pulsed-laser (or flash-lamp) annealing occur far enough from thermodynamic equilibrium conditions. Ion implantation introduces enough dopants. The subsequent short-time annealing deposits energy in the near-surface region, leading to a rapid liquid-phase epitaxial growth. Such a nonequilibrium process maintains the supersaturation induced by ion implantation. We focus on the preparation of magnetic semiconductors. We try to understand the new artificial advanced materials better by ion beam analysis, optical characterization as well as X-ray spectroscopy. The investigated materials include Mn doped Ge/Si and III-V compound semiconductors.
1. Ferromagnetic InMnAs on InAs with perpendicular anisotropy
Fig. 1 (Left) Field dependence of magnetization measured by SQUID magnetometry. The inset shows the temperature dependence of magnetization measured under 0.005 T but after saturation at 1 T. The field is applied either perpendicular or parallel to the film. The magnetic anisotropy can be clearly observed and the surface normal is the easy axis. Figure is from Appl. Phys. Express 5, 093007 (2012).
Fig. 2 (Right) Mn L3, 2 total electron yield (TEY) (a) XAS for magnetization and helicity parallel (µ+) and antiparallel (µ-) and (b) XMCD (µ+ - µ-) for InMnAs measured at around 10 K under an external field of 5 T applied perpendicular to the surface. Figure is from Appl. Phys. Express 5, 093007 (2012).
(1) Shengqiang Zhou, Yutian Wang, Zenan Jiang, Eugen Weschke, and Manfred Helm, Ferromagnetic InMnAs on InAs Prepared by Ion Implantation and Pulsed Laser Annealing, Appl. Phys. Express 5, 093007 (2012), Full version DOI: 10.1143/APEX.5.093007.
(2) M. Khalid, E. Weschke, W. Skorupa, M. Helm, S. Zhou, Ferromagnetism and impurity band in a magnetic semiconductor: InMnP, Phys. Rev. B 89, 121301(R) (2014).
(3) S. Prucnal, K. Gao, I. Skorupa, L. Rebohle, L. Vines, H. Schmidt, M. Khalid, Y. Wang, E. Weschke, W. Skorupa, J. Grenzer, R. Huebner, M. Helm, S. Zhou, Bandgap narrowing in Mn doped GaAs probed by room-temperature photoluminescence, Phys. Rev. B 92, 224407 (2015).
(4) S. Zhou, L. Li, Y. Yuan, A. W. Rushforth, L. Chen, Y. Wang, R. Boettger, R. Heller, J. Zhao, K. W. Edmonds, R. P. Campion, B. L. Gallagher, C. Timm, M. Helm, Precise tuning of the Curie temperature of (Ga,Mn)As-based magnetic semiconductors by hole compensation: Support for valence-band ferromagnetism, Phys. Rev. B 95, 075205 (2016).
(5) Y. Yuan, R. Hübner, F. Liu, M. Sawicki, O. Gordan, G. Salvan, D. R. T. Zahn, D. Banerjee, C. Baehtz, M. Helm, S. Zhou, Ferromagnetic Mn-Implanted GaP: Microstructures vs Magnetic Properties, ACS Applied Materials and Interfaces 8, 3912-3918 (2016).
(6) Y. Yuan, C. Xu, R. Hübner, R. Jakiela, R. Böttger, M. Helm, M. Sawicki, T. Dietl, S. Zhou, Interplay between localization and magnetism in (Ga,Mn)As and (In,Mn)As, Phys. Rev. Mater. 1, 054401 (2017).