Fabrication of Epitaxial Fe3O4 Film on a Si(111) Substrate.

The application of magnetic oxides in spintronics has recently attracted much attention. The epitaxial growth of magnetic oxide on Si could be the first step of new functional spintronics devices with semiconductors. However, epitaxial spinel ferrite films are generally grown on oxide substrates, not on semiconductors. To combine oxide spintronics and semiconductor technology, we fabricated Fe3O4 films through epitaxial growth on a Si(111) substrate by inserting a γ-Al2O3 buffer layer. Both of γ-Al2O3 and Fe3O4 layer grew epitaxially on Si and the films exhibited the magnetic and electronic properties as same as bulk. Furthermore, we also found the buffer layer dependence of crystal structure of Fe3O4 by X-ray diffraction and high-resolution transmission electron microscope. The Fe3O4 films on an amorphous-Al2O3 buffer layer grown at room temperature grew uniaxially in the (111) orientation and had a textured structure in the plane. When Fe3O4 was deposited on Si(111) directly, the poly-crystal Fe3O4 films were obtained due to SiOx on Si substrate. The epitaxial Fe3O4 layer on Si substrates enable us the integration of highly functional spintoronic devices with Si technology.

. In the former method, it is difficult to optimize the oxidation of the Si layer and the thickness of Al film. In contrast, the latter method is simple if an ultra-high vacuum system is accessible.
In this study, the epitaxial γ-Al 2 O 3 buffer layers were prepared using an ultra-high vacuum system and the Fe 3 O 4 layer was fabricated by reactive molecular beam epitaxy. We investigated the crystal structure, magnetic and electric properties of the Fe 3 O 4 layer on Si(111) with an epitaxial γ-Al 2 O 3 buffer layer, an amorphous-Al 2 O 3 buffer layer, and without a buffer layer. We succeeded in the fabrication of high quality Fe 3 O 4 films on Si(111) substrates. The buffer layer had a significant effect on the crystal structure of the Fe 3 O 4 layers.  Fig. 1 (hereafter referred to as (a) EPI, (b) AMO and (c) W/O), respectively. After treatment of the Si substrate, we confirmed that the in-situ reflection high energy electron diffraction (RHEED) pattern of the Si substrate had a (7 × 7) streak pattern ( Supplementary Fig. S1). This means that the surface of Si was clean and flat. Figure 2(a) and (b) show the RHEED pattern of γ-Al 2 O 3 and Fe 3 O 4 in EPI. The direction of the incident electron beam was . The RHEED patterns of γ-Al 2 O 3 and Fe 3 O 4 were clear streak patterns indicating that γ-Al 2 O 3 and Fe 3 O 4 grew epitaxially. Therefore, the γ-Al 2 O 3 film was considered to play a role of a buffer layer for epitaxial growth of Fe 3 O 4 . The surface roughness of γ-Al 2 O 3 and Fe 3 O 4 were estimated to be very small in value by atomic force microscope (AFM) (shown in Supplementary Fig. S2).  Fig. 2(c), the Si (7 × 7) streak pattern turned into a halo pattern, which indicated that the Al 2 O 3 layer was amorphous. Figure 2 Fig. S3), which is smaller than the bulk lattice parameter. Therefore, the Fe 3 O 4 was considered to be compressed in-plane.

Results and Discussion
To investigate the in-plane epitaxial relationship, we conducted φ-scan measurements of Si(311) and Fe 3 O 4 (4-40), as shown in Fig. 3  Si(111), as exhibited in Fig. 3(c). In addition, the peaks of the Fe 3 O 4 film were broader than that of the Si substrate. There was a lattice mismatch of 5.7% at γ- The θ-2θ XRD diffraction pattern of Fe 3 O 4 in AMO (blue line) exhibited four peaks, which was identical with the diffraction pattern of Fe 3 O 4 in EPI. Therefore, the Fe 3 O 4 in AMO was also (111)-oriented. However, the RHEED pattern in Fig. 1(d) implied the presence of a polycrystalline structure. Furthermore, the Fe 3 O 4 (4-40) diffraction peak was not observed in the φ-scan measurement. Therefore, we concluded that the Fe 3 O 4 had a textured structure and the growth direction was (111).
The θ-2θ XRD diffraction pattern of  To investigate the crystallinity of the Fe 3 O 4 layer in detail, we carried out X-ray reciprocal space mapping around the symmetric (222) diffraction for Fe 3 O 4 in EPI and AMO (Fig. 3(d)). The symmetrical scan showed that the  Transmission electron microscope observation. We conducted cross-sectional transition electron microscopy (TEM) analysis to confirm the crystallinity and compositions of the materials. Figure 4 shows the cross-section TEM images in which the electron beams were incident along the Si [1-10] zone axis. In Fig. 4(a), the TEM image shows that the Fe atoms of Fe 3 O 4 were orderly aligned; thus, the Fe 3 O 4 film was epitaxial. The electron diffraction (ED) of Fe 3 O 4 in EPI shown in Fig. 4(b) was in good agreement with the simulation of spinel structure. The left side in Fig. 4(a) shows the epitaxial relationship on   Fig. S4(c)), which were almost the same as the out-of-plane lattice constant (4.84 Å) determined by XRD in Fig. 3(a) and that of bulk Fe 3 O 4 (4.85 Å). In contrast, the TEM image of Fe 3 O 4 in AMO shown in Fig. 4(c) demonstrated that the structure was polycrystalline and grain boundaries were clearly observed. The ED image in Fig. 4(d) consisted of the diffraction from the grains with some crystal orientations. In the low magnification TEM image (supplementary Fig. S4(b)), some grains with a size of 15-30 nm appeared.
With respect to the buffer layer, the thickness of γ-Al 2 O 3 was estimated from the HRTEM image ( Fig. 4(a)) to be approximately 1 nm, which was thinner than the nominal value measured by the crystal oscillator in the chamber. The reason for this difference in thickness was unclear; however, it could be due to the fluctuation of the crystal oscillator or re-evaporation of Al 2 O 3 because the γ-Al 2 O 3 was grown at a high temperature (900 °C). We could see the amorphous layer under the γ-Al 2 O 3 layer, which was determined to be a SiO x layer by HAADF and Energy dispersive X-ray spectroscopy (EDS) mapping images (Fig. 5). The SiO x layer was considered to form during the growth of Fe 3 O 4 because the Fe 3 O 4 was grown in 4 × 10 −4 Pa O 2 gas. It was reported that Si was oxidized through the γ-Al 2 O 3 layer of less than 2.0 nm by introducing oxygen (>10 −3 Pa) 32 . To confirm that, we fabricated a γ-Al 2 O 3 (7.5 nm) film on Si(111), and carried out XRD and TEM observations (supplementary Fig. S5(a) and (b)). The γ-Al 2 O 3 grew epitaxially on Si and we found no amorphous layer at the Si(111)/γ-Al 2 O 3 (7.5 nm) interface.  Fig. 6(a). The directions of the magnetic field were in-plane , in-plane [1][2][3][4][5][6][7][8][9][10] and out-of-plane [111]. The hysteresis curve along  was the same as that along [1][2][3][4][5][6][7][8][9][10] and the Fe 3 O 4 film had in-plane magnetization. The saturation magnetization (M s ) was 480 emu/cm 3 for all magnetic field directions. The remanent magnetization (M r ), the coercive field (H c ), and the remanent ratio (M r /M s ) in the in-plane field were 280 emu/cm 3 500 Oe, and 0.48, respectively, and those for the out-of-plane field were 47 emu/cm 3 , 225 Oe, and 0.08, respectively. The hysteresis loops for Fe 3 O 4 in EPI, AMO, and W/O are illustrated in Fig. 6(b). Fe 3 O 4 in EPI had the largest H c and M s among the three samples. The M s of Fe 3 O 4 in EPI was the same as the value of bulk Fe 3 O 4 . Although the reason for small magnetization for AMO and W/O has not been clear so far, the antiphase boundary or disordered structure at grain boundary could be responsible for it 33, 34 . Transport characteristics. Figure 7 shows that the dependence of the resistance on temperature for the Fe 3 O 4 film in EPI. As is well-known, Fe 3 O 4 is an electric conductor at room temperature and the resistivity increases exponentially with decreasing temperature. The resistivity of the film at 300 K was 2.5 × 10 −4 Ωcm, which is lower than the bulk value (5 × 10 −3 Ωcm) 35 . The dlogR/dT plots (inset) show a valley at approximately 120 K. This anomaly corresponds to a Verwey transition 36 , which is a famous phase transition in Fe 3 O 4 . The Verwey transition has been reported to sharply change the resistivity by approximately one digit 37 ; however, the transition is easily disappeared by impurities or structure defects 34,38 . As the Fe 3 O 4 in EPI possessed magnetic and electric characteristics that were comparable to bulk Fe 3 O 4 , the Fe 3 O 4 on γ-Al 2 O 3 buffer layer was very good quality.  Measurements. The epitaxial growth and crystal structure were confirmed by RHEED, XRD (Rigaku SmartLab (9 kW)), and TEM (FEI Titan3 G2 60-300). Cross-sectional samples for TEM were prepared by using conventional mechanical polishing and dimpling techniques 42 . The magnetic properties of Fe 3 O 4 were measured by vibrating sample magnetometer (VSM) and the electrical properties were measured by direct current (DC) measurements.