Correlation between spin transport signal and Heusler/semiconductor interface quality in lateral spin-valve devices

We show direct evidence for the impact of Heusler/semiconductor interfaces atomic structure on the spin transport signals in semiconductor-based lateral spin-valve (LSV) devices. Based on atomic scale Z-contrast scanning transmission electron microscopy and energy dispersive x-ray spectroscopy we show that atomic order/disorder of ${\mathrm{Co}}_2{\mathrm{FeAl}}_{0.5}{\mathrm{Si}}_{0.5}$ (CFAS)/$n$-Ge LSV devices is critical for the spin injection in Ge. By conducting a postannealing of the LSV devices, we find 90% decrease in the spin signal while there is no difference in the electrical properties of the $\mathrm{CFAS}\text{/}n$-Ge contacts and in the spin diffusion length of the $n$-Ge layer. We show that the reduction in the spin signals after annealing is attributed to the presence of intermixing phases at the Heusler/semiconductor interface. First-principles calculations show how that intermixed interface region has drastically reduced spin polarization at the Fermi level, which is the main cause for the significant decrease of the spin signal in the annealed devices above $300{}^{\circ }\mathrm{C}$.

Figure 1

(a) Schematic illustration of the fabricated $\mathrm{CFAS}\text{/}n$-Ge–based LSV. (b) Nonlocal magnetoresistance curve and (c) Hanle curves at 8 K for as-prepared and annealed LSVs. (d) $J\text{−}V$ characteristics for the $\mathrm{CFAS}\text{/}n$-Ge contacts in as-prepared and annealed LSVs.

Figure 2

(a) Low-magnification HAADF STEM image of the LSV device structure, showing an overall geometry of the fabricated device. The structural ordering of the $\mathrm{CFAS}\text{/}{n}^{+}$-Ge interface region of the as-prepared (b) and the annealed samples (c) viewed along the $\left[\overline{1}10\right]$ zone axis show drastic change after heat treatment. The annealed sample has multiple crystalline CFAS film domains.

Figure 3

(a) A HAADF STEM image of the $\mathrm{CFAS}\text{/}{n}^{+}$-Ge region, an EDS line scan of the as-prepared sample (b), is collected along the yellow dashed line. (c),(d) EDS line scan of the similar area of the annealed sample for comparison. Arrows are guides for the eye.