IEEE TRANSACTIONS ON MAGNETICS, VOL. 54, NO. 1, JANUARY 2018 4400103 Interfacial Crystal Structures and Non-Local Spin Signals of Co 2 FeAl 0.5 Si 0.5 /n-GaAs Junctions Kohei Kataoka , Tatsuya Saito, Nobuki Tezuka, Masashi Matsuura, and Satoshi Sugimoto Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan We have investigated interfacial crystal structures and non-local spin signals 1 R of Co 2 FeAl 0.5 Si 0.5 (CFAS)/n-GaAs junctions. Cross-sectional transmission electron microscopy observations indicated that with the exception of Ga diffusion into CFAS of the sample deposited at 400 °C, the interfacial structure of the junctions and defect density at the interface were not very different for different CFAS fabrication temperatures of the substrate (T CFAS ). The obtained reflection high-energy electron diffraction patterns showed that all samples fabricated at T CFAS varying from room temperature to 400 °C exhibited the L2 1 ordered structure in the vicinity of CFAS/n-GaAs junctions. It is found that the junctions with larger rectifying characteristic as indicated by the conduction ratio G(0.5V)/ G(-0.5V) show larger spin signal 1 R. This may strongly affect the spin injection/detection efficiency. Index Terms— Co 2 FeAl 0.5 Si 0.5 (CFAS), full-Heusler alloy, Schottky junction, spin injection. I. I NTRODUCTION S EMICONDUCTOR (SC) spintronics devices are attracting considerable attention as candidates for avoiding the limi- tation of Moore’s law. Spin-based metal–oxide–semiconductor field-effect transistors (spin MOSFETs) are once such class of devices and consist of source and drain ferromagnets (FMs) electrodes [1], [2]. To realize spin MOSFETs, spin-polarized electrons from FM must be injected into SCs with high efficiency. However, it is difficult to obtain high efficiencies for spin injection from FM to SC comes due to conduc- tive mismatch [3]. The use of the FM with a high spin polarization or the insertion of a tunnel barrier between FM and SC are beneficial for improving the spin injection effi- ciency [4], allowing to detect the signal even at room temper- ature (RT) [5], [6]. Epitaxially grown Co 2 FeAl 0.5 Si 0.5 (CFAS) on GaAs is a good candidate for the FM/SC spin injection, because the CFAS Heusler alloy shows a high spin polarization as measured by the tunnel magnetoresistance effect [7], and its lattice constant is close to that of GaAs. Previously, our group has reported that the spin signals of CFAS/n-GaAs junctions were strongly affected by the substrate tempera- ture (T CFAS ) during the CFAS deposition. It was found that the junction deposited at T CFAS = 300 °C showed the largest spin signal compared to the previously reported non-local four terminal (4T) systems [8]. While it is known that this discrep- ancy may arise from the CFAS structure ordering, the inter- face structure of the junctions and/or electronic structure of the junction, it is still unclear what factor predominantly affects the spin signal in the junction. In our previous study, CFAS with the thickness of 30 nm showed the B2 structure with T CFAS below 200 °C and the L2 1 structure with T CFAS above 300 °C, and it was not possible to determine which Manuscript received March 8, 2017; revised April 12, 2017; accepted April 13, 2017. Date of publication November 16, 2017; date of current version December 20, 2017. Corresponding author: K. Kataoka (e-mail: kohei.kataoka.t1@dc.tohoku.ac.jp). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2017.2756986 CFAS ordering structure or GaAs surface were mainly influ- encing the spin signal. In this paper, we successfully fabricated the L2 1 structure in all specimens for the CFAS thickness of 5 nm, even for the samples deposited at T CFAS = RT, enabling the isolation of the GaAs contribution for the spin sig- nal without the contribution of the CFAS ordering structures. Therefore, the rectifying characteristics of the CFAS/n-GaAs junctions and the obtained spin signals 1 R, which are related to the electronic structure of the Schottky junction, have been investigated and compared to the results in our previous reports. II. EXPERIMENT All n-GaAs, n + -GaAs, and CFAS layers were prepared by molecular beam epitaxy under the base pressure of 1×10 -7 Pa. First, the GaAs (001) substrate was annealed at 700 °C to remove the native oxide, and then undoped GaAs was deposited at 680 °C. CFAS (5 or 30 nm)/n + -GaAs (20 nm)/ n-GaAs (500 nm) structures were deposited. The n-GaAs layer and the n + -GaAs layer were deposited at 680 °C with Si as the dopant deposited from a K-cell and these doped densities were 1 × 10 17 and 1 × 10 19 cm -3 , respectively. The deposition temperature (T CFAS ) of the CFAS layer varied from RT to 400 °C. The sample was then transferred to a sputtering system and capped with a 3 nm thick Ta layer. Details of the growth conditions have been already reported in our previous works. The crystal structure was analyzed by in situ reflection high-energy electron diffraction (RHEED), and transmission electron microscopy (TEM) with energy dispersive X-ray spectrometry (EDX). We fabricated 4T lateral spin transport devices using conventional electron beam lithography and Ar ion milling etching. The current–voltage measurements were performed by the dc four-probe method at 10 K. The spin transport measurements were carried out by applying the reverse bias current of 0.3 mA at 10 K. In these dc spin trans- port measurements, a magnetic field was applied along the in- plane GaAs [110] direction corresponding to the magnetization easy axis of the CFAS layer. 0018-9464 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.