Co thin film with metastable bcc structure formed on GaAs ( 111 ) substrate

Co thin films are prepared on GaAs(111) substrates at temperatures ranging from room temperature to 600 oC by radio-frequency magnetron sputtering. The growth behavior and the detailed resulting film structure are investigated by in-situ reflection high-energy electron diffraction and X-ray diffraction. In early stages of film growth at temperatures lower than 200 oC, Co crystals with metastable A2 (bcc) structure are formed, where the crystal structure is stabilized through hetero-epitaxial growth. With increasing the film thickness beyond 2 nm, the metastable structure starts to transform into more stable A1 (fcc) structure through atomic displacements parallel to the A2{110} close-packed planes. The crystallographic orientation relationship between the A2 and the transformed A1 crystals is A1{111}<11 _ 0> || A2{110}<001>. When the substrate temperature is higher than 400 oC, Ga atoms of substrate diffuse into the Co films and a Co-Ga alloy with bcc-based ordered structure of B2 is formed.


Introduction
Cobalt (Co) is a 3d ferromagnetic transition metal which has two stable crystallographic phases, A3 (hcp) and A1 (fcc). A2 (bcc) phase is metastable and does not appear in the bulk phase diagram. Recently, application of Co thin film with A2 structure is providing a new possibility a new possibility in the development of magnetic devices. Tri-layer films consisting of A2-Co and oxide (MgO or SrTiO 3 ) layers are reported to show high tunnelling magnetoresistance ratios [1][2][3]. Understanding of the formation conditions is important in order to apply the A2-Co film for practical applications.
Formation of A2 phase has been recognized for Co films of a few nm thicknesses grown on GaAs substrates of (100) [4][5][6][7] and (110) [4,8,9] orientations by molecular beam epitaxy. With increasing the thickness, most of the A2-Co crystals tended to transform into more stable A3 or A1 crystals. The film growth behavior and the film structure vary depending on the substrate orientation. However, there are very few reports on preparation of Co films on GaAs(111) substrates. In the present study, Co films are deposited on GaAs(111) substrates by magnetron sputtering, which is suitable for practical applications. The growth process and the detailed resulting film structure are investigated.

Experimental procedure
A radio-frequency (RF) magnetron sputtering system equipped with a reflection high-energy electron diffraction (RHEED) facility was used. The base pressures were lower than 4×10 -7 Pa. Before film formation, GaAs(111) substrates were heated at 600 °C in the chamber to obtain clean surfaces. Figure 1(a-1) shows the RHEED pattern observed for a GaAs substrate after heating. A clear diffraction pattern from a clean B3(111) single-crystal surface shown in the schematic diagram of figure 1(a-2) is observed. A Co target of 3 in diameter was employed. The distance between target and substrate, the Ar gas pressure, and the RF power were respectively fixed at 150 mm, 0.67 Pa, and 54 W, where the deposition rate was 0.02 nm/s. Co films were deposited on the substrates at temperatures ranging between room temperature (RT) and 600 °C. The thickness was varied in a range from 1 to 40 nm.
The surface structure during sputter deposition process was studied by RHEED. The resulting film structure was investigated by 2T/Z-scan out-of-plane, 2TF/ĳ-scan in-plane, and pole-figure X-ray diffractions (XRDs) with Cu-KD radiation (O = 0.15418 nm). A Co single-crystal film with metastable A2 structure is obtained in an early stage of film growth on GaAs(111) substrate, similar to the cases of films deposited on GaAs(100) [4][5][6][7] and GaAs(110) [4,8,9] substrates. The    These relationships are similar to the Nishiyama-Wasserman orientation relationship [10,11]. In these configurations, the close-packed planes of A2(110), A2(101), and A2(011), which are 55° inclined from the in-plane, are parallel to the A2(111) closepacked plane, as shown for example in figures 3(a-1) and (b-1). The phase transformation is taking place through atomic displacements parallel to the closepacked planes. The stacking sequence of A1(111) is different between the types of A-1 and B-1, of A-2 and B-2, and of A-3 and B-3. Therefore, the A1 crystals with types A-1, A-2, and A-3 possess a (1 13 7) plane parallel to the substrate surface, whereas those with types B-1, B-2, and B-3 possess a (13 1 7) plane parallel to the substrate surface. Furthermore, there is a possibility that the transformation is also taking place through atomic displacements parallel to the A2(11  Figures 3(c-5) and (c-6) show the schematic diagrams of diffraction patterns simulated for A1 crystals transformed in the orientation relationships of types C and D, respectively. However, these diffraction patterns are not clearly recognized in the RHEED pattern observed for the 2-nm-thick Co film [ figure 1(c)]. However, the transformations of types C and D may occur mainly within the film, since the RHEED detects a crystallographic information around the film surface. In order to confirm the existence of A1 crystals with types C and D, polefigure XRD analysis, which is described later, was carried out. As the thickness increases beyond 5 nm Co films deposited at 200 °C. A diffraction pattern corresponding to A2(111) texture is observed for the 1nm-thick film [ figure 5(a-1)]. With increasing the thickness [figure 5(a-2)], a diffraction pattern from A1crystals transformed from A2 structure is observed. The nucleation and the transformation behaviours are similar to the case of film formation at RT. Figures 5(b) and (c) show the RHEED patterns observed for the Co films deposited at 400 and 600 °C, respectively. Clear diffraction patterns involving superlattice spots shown by the white circles in the schematic diagrams are observed. The RHEED patterns correspond to a formation of (111) crystal with bcc-based ordered structure of B2. It is considered that Ga atoms of substrate diffuse into the film and a Co-Ga alloy crystal with B2 structure is formed on the substrate. The crystallographic orient ion relationship is d termined as at e Co-Ga(111) [11 _ 0] B2 || GaAs(111) [11 _ 0] B3 . Figure 6 shows the out-of-plane and in-plane XRD patterns measured for the 10-nm-thick Co film deposited at 600 °C. The in-plane pattern is measured by making the scattering vector parallel to GaAs [11 _ 0]. Co-Ga(111) and Co-Ga(222) out-of-plane and Co-Ga(11 _ 0) and Co-Ga(22 _ 0) in-plane reflections are observed in addition to reflections from GaAs substrate. It is necessary to employ a low substrate temperature to prepare an A2-Co film on a GaAs substrate.

Conclusion
Co thin films are prepared on GaAs(111) substrates by varying the thickness from 1 to 40 nm and the substrate temperature from RT to 600 °C. The growth behavior and the film structure are studied by RHEED and XRD. Co crystals with A2 structure nucleate epitaxially on the substrates at temperatures lower than 200 °C. With increasing the thickness, the A2 structure transforms into more stable A1 structure through atomic displacements parallel to the A2{110} close-packed planes. When the substrate temperature increases beyond 400 °C, Ga atoms diffuse from the substrate into the Co film and a Co-Ga alloy film with B2 ordered structure is formed.