Type of presentation: Oral

MS-8-O-1683 Structure and magnetism in strained Ge1−x−ySnxMny films grown on Ge(001) by low temperature molecular beam epitaxy

Prestat E.1,2,3, Barski A.1, Bellet-Amalric E.1, Jacquot J. F.1, Morel R.1, Tainoff D.1, Jain A.1, Porret C.1, Bayle-Guillemaud P.1, Jamet M.1
1INAC, CEA and Université Joseph Fourier, 17 rue des Martyrs, 38054 Grenoble, France. , 2Karlsruher Institut für Technologie (KIT), Laboratorium für Elektronenmikroskopie, D-76128 Karlsruhe, Germany, 3School of Materials, University of Manchester, Manchester M13 9PL, UK
eric.prestat@manchester.ac.uk

For spintronics, ferromagnetic semiconductors are of great interest because they allow the injection of spin polarized current into a non-magnetic semiconductor. However, diluted magnetic semiconductors with a Curie temperature (Tc) below room-temperature, are not usable in current spintronic devices. One alternative route to fabricate room temperature ferromagnetic semiconductors is to use the spinodal decomposition to form high Tc nanostructures, as demonstrated in the GeMn system [1]. This GeMn system appears as a promising candidate to be used in spintronic devices, especially because of its perfect compatibility with silicon technologies.

In this presentation, we report on the structural and magnetic properties of GeSnMn films grown on Ge(001) by low temperature molecular beam epitaxy using high resolution X-ray diffraction (HRXRD), high resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS) and superconducting quantum interference device (SQUID). Chemical mechanical wedge polishing was used to obtain high quality samples with flat and large surfaces free of damages and almost free of amorphous layers. HRTEM, STEM-EELS observations were performed on a FEI Titan3 Ultimate 80-300 microscope operating at 200 kV and equipped with probe-side and image-side aberration-correctors.

Similar to Mn-doped Ge films, GeMn nanocolumns of a few nanometers in diameter are formed during the growth, as revealed by the HRTEM images in figure 1a-b and the EELS elemental map (figure 3a-c). Sn map (figure 2c) shows that the matrix exhibits a GeSn solid solution while there is a Sn-rich GeSn shell around GeMn nanocolumns. The interface Ge/GeSnMn is perfectly coherent as demonstrated by HRTEM (figure 1a) and HRXRD (figure2b), leading to a pseudomorphic growth of the GeSnMn layers. The out-of-plane lattice distance can be monitored by the Sn concentration, as shown by HRXRD in figure 2a.

Electron diffraction patterns exhibit the presence of the forbidden (200)- and (020)-Bragg reflections in the GeSnMn layer -figure 2f. These diffracted peaks have a peculiar cross-like feature oriented along the [110] and [110] directions. We show how the GeSn shells provide these reflections.

The magnetization in GeSnMn layers is higher than in GeMn films (figure 1e). This magnetic moment enhancement is independent of the Sn concentration and thus the strain state around the Mn-rich nanocolumns. However, the Sn-rich shell, which is formed around the nanocolumns, could change the electronic structure of Mn atoms in the nanocolumns, which could explain the magnetization enhancement [2].

[1] M Jamet et al, Nature Materials 5 (2006), p. 653
[2] E Prestat et al, Applied Physic Letters 103 (2023), 012403

Fig. 1: HRTEM images of a GeSnMn layers on Ge substrate:(a) in cross-section and (b) in plane view.

Fig. 2: (a) θ–2θ X-ray diffraction spectra: the position of the peaks between 65° and 66° of the θ-2θ X-ray diffraction spectra corresponding to the GeSnMn epitaxial depends on the Sn content (b) X-ray map around the (-1-15) Bragg peak. (c) Magnetization versus temperature of GeMn and GeSnMn layers.

Fig. 3: (a-c) Ge, Mn and Sn elemental map obtained by EELS. (d) Mn versus Sn composite map showing the Sn shell around the Mn-rich nanocolumns. (e) Electron diffraction pattern in the [001]-zone axis.(f) Magnified image of the forbidden (200)-Bragg reflection showing the peculiar cross-like feature of the diffracted peak.