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DOI: 10.1021/cg901380b

Epitaxial Growth of MgO and CoFe/MgO on Ge(001) Substrates by Molecular Beam Epitaxy

2010, Vol. 10 1346–1350

Kun-Rok Jeon, Chang-Yup Park, and Sung-Chul Shin* Department of Physics and Center for Nanospinics of Spintronic Materials, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea Received November 4, 2009; Revised Manuscript Received January 12, 2010

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ABSTRACT: We report the epitaxial growth of MgO and CoFe/MgO on Ge (001) substrates using molecular beam epitaxy. It was found that the epitaxial growth of a MgO film on Ge could be realized at a low growth temperature of 125 ( 5 °C and the MgO matches the Ge with a cell ratio of 21/2:1, which renders MgO rotated by 45° relative to Ge. In-situ and ex-situ structural characterizations reveal the epitaxial crystal growth of bcc CoFe/MgO on Ge with the in-plane crystallographic relationship of CoFe(001)[100] MgO(001)[110] Ge(001)[100], exhibiting sharp interfaces in the (001) matching planes. The saturation magnetization of the sample is 1430 ( 20 emu/cc, which is comparable to the value of bulk CoFe.

1. Introduction Semiconductor (SC) spintronics utilizing the spin degree of freedom as a new functionality in SC has received considerable interest as one of the possible candidates to solve the physical limit of complementary metal-oxide-semiconductor (CMOS) technologies in the near future.1-3 To realize SC spintronic devices, the electrical injection of a spin-polarized current from a ferromagnetic (FM) contact into a semiconductor (SC) is crucial factor.1,4-6 Electrical spin injection can be achieved by introducing spin-dependent interface resistance between the FM and SC, for instance, in the form of a tunnel barrier or a Schottky tunnel contact, to overcome the FM/SC conductivity mismatch.7-11 Also, the spin injection efficiency of this system strongly depends on its tunnel spin polarization (TSP).7,8 In this context, a bcc FM(001)/MgO(001) heterostructure on SC is considered to be promising, as this structure can solve the conductivity mismatch, provide a higher TSP, and avoid interdiffusion/intermixing between the FM and SC. Therefore, efficient spin injection can be achieved by using a crystalline FM/MgO/SC system, providing a high TSP as the theoretical prediction.12,13 For this purpose, FM/MgO/SC systems using GaAs have been studied due to a small lattice mismatch (-0.7%) between MgO and GaAs and on account of the feasibility of an optical approach.11,14-16 Recently, FM/ MgO/Si systems have attracted much attention because Si has a longer spin relaxation time than GaAs.17,18 However, fully epitaxial FM/MgO/Si has never been reported due to a large lattice mismatch (3.4%) between MgO and Si. Thus, another possible SC material which has a long spin relaxation time as well as a lattice that matches MgO should be explored. Thus, Ge is selected here as the SC of FM/MgO/SC system. Considering the fact that the lattice constant of Ge (ao = 5.658 A˚) is very close to that of GaAs (ao = 5.653 A˚), which matches MgO, epitaxial growth of FM/MgO on Ge can be expected. Moreover, as the primary type of spin relaxation in the n-type Ge is the Elliot-Yafet mechanism (which is one of the main spin relaxation mechanisms in SC), a long spin lifetime may be *Corresponding author. Phone: 82-42-350-2528; fax: 82-42-350-8100; e-mail: [email protected]. pubs.acs.org/crystal

Published on Web 02/05/2010

expected for lightly doped n-type Ge.3,19 As a result, Ge is considered to be a candidate SC for the FM/MgO/SC system. Despite the potential of the FM/MgO/Ge system for electrical spin injection purposes, there has been a lack of knowledge pertaining to the epitaxial growth of its system compared to FM/MgO/GaAs and FM/MgO/Si systems.14-18 In this article, we first report, by investigating the dependence crystallinity of MgO film on the growth temperature, the epitaxial growth of MgO and CoFe/MgO on Ge substrates without introducing any buffer layer. Also, we have investigated the magnetic properties of the present sample, which shows the cubic anisotropy with a negative value of Keff1 and the high saturation magnetization of 1430 ( 20 emu/cc. 2. Experimental Section The samples were grown on Ge(001) substrates (n-type, Sb-doped, F ≈ 16-20 Ω cm) in a molecular beam epitaxy (MBE) system with base pressure better than 5  10-10 torr. Before introducing the substrate to the load-lock chamber, it was degreased by acetone and methanol, immersed in an HCl/H2O [1:4] solution to remove the native oxide, and finally rinsed with deionized water. After the Ge substrate was transferred into the MBE chamber, the substrate was sputtered by 300-eV Arþ ions for 10 min at a (30° incidence angle at room temperature and was then annealed at 500 °C for 30 min. Through this cleaning procedure, a well-defined (2  1) reconstruction of the Ge (001) surface structure was obtained, as demonstrated by the streaky reflective high-energy electron diffractometer (RHEED) pattern in Figure 1a. This figure shows the remarkable half-order streaks, as indicated by the white arrow. All layers were deposited by e-beam evaporation with a working pressure better than 2  10-9 torr. A 7.5-nm thick MgO film was prepared by e-beam evaporation from a MgO single crystal source with a very slow deposition rate of 0.167 nm/min at growth temperatures varying from RT to 400 °C for an investigation of the temperature effect on the crystal quality of the MgO. The CoFe layer was prepared by e-beam evaporation at a deposition rate of 0.25 nm/min from a rodtype CoFe source with a composition of Co70Fe30 at RT. The sample was subsequently annealed in situ for 1 h at 300 °C in 2  10-9 torr. Finally, the sample was capped by a 1.5-nm thick Cr layer at RT to prevent oxidation of the sample. The crystal structures of the samples were characterized by in situ RHEED and ex situ Rigaku D/MAXRC MPA X-ray diffractometry (XRD) with Cu KR radiation. Crosssectional transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) measurements using a 200 kV analytic electron microscope (JEOL JEM2100F) with a r 2010 American Chemical Society

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Figure 1. (a) RHEED patterns of 7.5-nm MgO films grown on Ge(001) substrates at different temperatures along the azimuths of Ge[100] and Ge[110], and (b) the corresponding X-ray θ-2θ diffraction patterns. The white arrows mark the first half-order streaks indicating (2  1) surface reconstruction. (c) High-resolution TEM images of the 7.5-nm MgO film grown on Ge at 125 °C and (d) 225 °C. (e) Fourier-filtered image of the area enclosed by the dashed rectangle in (c). The red circle shows interface dislocation (typically, one dislocation was observed every 20 atomic planes).

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The formation of a single crystalline MgO(001) tunnel barrier on Ge is important in achieving spin injection with a high TSP via the symmetry-dependent spin filtering effect of MgO barrier in conjunction with bcc FM. Figure 1a shows in situ RHEED patterns of the 7.5-nm MgO films grown on Ge(001) substrates at different growth temperatures along the azimuths of [100]Ge and [110]Ge. The MgO film grown at RT is a (001)-oriented texture with no in-plane crystalline orientation, while the MgO films grown at 75, 125, and 175 °C show clear spotty patterns with a 4-fold in-plane crystalline symmetry. It is indicating that the epitaxial growth of a MgO film on Ge is realized even though some small crystalline islands might be present on the flat surface of the MgO. The contrast of the RHEED pattern and the 4-fold symmetry increases as the growth temperature increases from 75 to 125 °C. However, as the growth temperature surpasses 125 °C, the contrast of the pattern decreases and the 4-fold symmetry tends to elongate, as shown in Figure 1a, implying the degradation of the crystallinity of MgO film. When the growth temperature is higher than 225 °C, the diffraction pattern appears as complete ring patterns, indicating that the MgO film is polycrystalline without any preferential orientation. The θ2θ XRD results of the temperature effect on the crystallinity of the MgO film were found to be consistent with the RHEED studies, as shown in Figure 1b. These results are understandable because the lower growth temperature is not sufficient to give enough mobility to Mg and O atoms to overcome the

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3. Results and Discussion

lattice mismatch for epitaxial growth. This leads to a (001)textured MgO film corresponding to the lowest surface energy plane (1.16 J/m2).20 On the other hand, higher growth temperature gives rise to intermixing/interdiffusion, which leads to a polycrystalline MgO, due to activated mobility of Ge, Mg, and O atoms. This claim was supported by high-resolution TEM images of the MgO film grown on Ge at 125 and 225 °C in Figure 1c,d. It should be noted that (i) the growth temperature of 125 ( 5 °C for the epitaxial growth of the MgO film on Ge is relatively low compared to a GaAs or Si substrate (300400 °C).14-18 In addition, (ii) the crystallographic relationship of MgO(001)[110] Ge(001)[100] in Figure 1a contrasts with the epitaxial relationships of MgO(001)[100] GaAs or Si (001)[100].14-18 These results are possibly ascribed to the following reasons. (i) According to the strong dependence of diffusion and melting on the binding energy of atoms in solid, the activation energy for bulk diffusion has been known to be proportional to melting temperature (Tm).21 Assuming that this rule holds for interdiffusion/Intermixing during the growth, the activation energy for the interdiffusion/intermixing of Ge, Mg, and O during the growth is relatively low as compared to that of the GaAs or Si system due to the low melting temperature of Ge (1210 K).22 Therefore, it is reasonable that the growth temperature for the epitaxial growth of the MgO film on Ge is relatively low. (ii) Two possible in-plane epitaxial relationships for the MgO/Ge system are predicted in the view of lattice matching epitaxy (LME) and domain matching epitaxy (DME),23 respectively. In the LME, where films grow by one to one matching of lattice constants or pseudomorphically across the film and substrate for the lattice mismatch less the critical value of 7-8%,24 it is predicted the 45°-rotated configuration of MgO(001)[110] Ge(001)[100] with a unit cell matching ratio of 21/2:1 (5.3%). )

point-point resolution of 0.23 and 0.20 nm, respectively, were carried out for microstructural characterizations of the samples. The magnetic properties of the samples were measured by a vibrating sample magnetometer (VSM) (VT-800, Riken Denshi Co Ltd.) with an applied field of up to 25 kOe at RT.

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notice that the diffraction pattern of the CoFe layer grown at RT shows a spotty pattern, implying that the strain-induced roughness propagated through the CoFe layer. However, after in situ annealing at 300 °C for 1 h, the surface morphology and crystallinity were improved, as exhibited by the streaky pattern in Figure 2a, which is comparable to a CoFe layer grown on a MgO single-crystal substrate. The RHEED pattern along the [110] azimuth of the CoFe film annealed at 300 °C shows additional streak patterns in the middle of each pair of major neighboring streak patterns as indicated by the yellow arrows in Figure 2. This indicates the occurrence of c(2  2) surface reconstruction.25 Moreover, in Figure 2a, the in-plane orientation of the CoFe layer is rotated by 45° relative to the MgO layer, which is consistent with previously reported results.15 Hence, the in-plane crystallographic relationship of the CoFe/MgO/Ge sample determined by the diffraction periodicities in the RHEED patterns along the two different azimuths is expressed by CoFe(001)[100] MgO(001)[110] Ge(001)[100]. Figure 2b shows the three-dimensional atomic configuration of the structure. Further microstructural characterizations of the sample were performed using STEM-Z contrast image, crosssectional TEM images, diffractograms, and selected area electron diffraction (SAED) pattern. Figure 3a shows the STEM-Z contrast image of the sample, which shows the sharp interfaces of Ge/MgO and MgO/CoFe with almost no indication of interfacial reactions. Because the STEM-Z contrast is proportional to Z2, the MgO (12, 8) layer shows the darkest contrast and the CoFe (27, 26) and Cr (24) layers are indiscernible due to their similar Z values. The low-magnification and high-resolution TEM images of the sample in Figure 3b,c )

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On the other hand, in the DME, where integral multiples of lattice planes containing densely packed rows are matched across the interface for the lattice mismatch larger than the critical value, it is predicted the cubic on cubic configuration of MgO(001)[100] Ge(001)[100] with a 4:3 matching ratio (-0.7%) such as the MgO/GaAs (-0.7%) or Si (3.4%) system. Among the two possible configurations, only the 45°-rotated configuration in Figure 1c,e was observed. It seems that the 45°-rotated configuration facilitates extra release of energy in contrast to cubic on cubic under the low-temperature growth conditions. Although some interface dislocations appeared (typically, one dislocation was observed every 20 atomic planes) to reduce the strain energy induced by the large lattice mismatch between MgO and Ge (5.3%), our result reveals that highquality epitaxial growth of a MgO film on Ge at a low temperature (125 ( 5 °C) could be realized. Because of the low growth temperature, native oxide formation (GeOx) before the start of the actual MgO deposition and interdiffusion/intermixing during the deposition could be minimized, resulting in a sharp interface as shown in Figure 1c. A CoFe layer was grown at RT on a crystalline 3.3-nmthick MgO underlayer prepared at the optimal growth temperature of 125 °C and then, annealed at 300 °C. Figure 2a shows the RHEED patterns of the Ge (001) substrate after the ion bombardment and annealing (IBA) process, the deposition of MgO layer, the deposition of a 7.5-nm-thick CoFe layer at RT, and annealing the sample at 300 °C along the two different orientations of [100] and [110], together with a 7.5-nm CoFe layer grown on a MgO (001) single-crystal substrate under the same growth conditions for comparison. One might

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Figure 2. (a) Evolution of RHEED patterns during the growth processes of the CoFe(7.5 nm)/MgO(3.3 nm)/Ge(001) sample. (b) The RHEED pattern of a 7.5-nm CoFe grown on a MgO(001) substrate under the same growth conditions for comparison. The RHEED observations were carried out along the azimuths of Ge[100] and Ge[110]. The yellow arrows mark the first half-order streaks showing the occurrence of c(2  2) surface reconstruction. (c) 3D atomic configuration of the CoFe/MgO/Ge sample with the in-plane crystallographic relationship of CoFe(001)[100] MgO(001)[110] Ge(001)[100].

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Figure 3. (a) STEM-Z contrast, (b) low-magnification, and (c) high-resolution TEM images of the sample. (d) Corresponding diffractograms for the three regions of CoFe, MgO, and Ge of the sample and SAED pattern covering the whole region. The zone axis is parallel to the [110] direction of Ge substrate. (e) Simulated diffraction pattern of CoFe(001)[100] MgO(001)[110] Ge(001)[100] along the [110] direction of Ge.

Figure 4. (a) M-H hysteresis loop of 300 °C-annealed 7.5-nm CoFe film grown on 3.3-nm MgO/Ge(001) and (b) MgO(001) single-crystal substrate under same growth conditions along the [110] and [100] directions of a Ge substrate (corresponding to [110] and [100] of CoFe, respectively).

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with adjacent layers even at 300 °C annealing temperature. Figure 3d shows the corresponding diffractograms for the three regions of Ge, MgO, and Ge of the sample and the SAED pattern covering the whole region, with the zone axis along the [110] direction of Ge. Comparing the simulated diffraction pattern of CoFe(001)[100] MgO(001)[110] Ge(001)[100] in Figure 3e, we can confirm the single crystal nature of the sample and the in-plane crystallographic relationship expected by the in situ RHEED observation. Consequently, these measurements clearly demonstrate that the epitaxial growth of bcc CoFe(001)/MgO(001) on Ge is realized. This crystalline structure with a 4-fold in-plane )

along the [110] direction of the Ge substrate again clarify the high quality of the interfaces with the abruptness, within variation of two or three monolayers. It should be noticed here that the interdiffusion/intermixing of Ge, Mg, and O near the MgO/Ge interface did not occur even after annealing at 300 °C for 1 h. This thermal stability of the MgO/Ge interface can be related to the large binding energy between Mg and O in solid state, which leads to higher activation energy for the interdiffusion/intermixing of Ge, Mg, and O in the solid state than the activation energy during the formation of the film. So, one could expect that the large binding energy of MgO layer in solid state prevents interdiffusion/intermixing of MgO

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crystalline symmetry can provide the conservation of the symmetry of the Bloch states (Δ1, Δ5, Δ2(20 )) at the Fermi level from the CoFe electrode through the MgO barrier (k = 0) and the symmetry-related different decay rate of the evanescent states in the MgO barrier (Δ2(20 ) > Δ5 . Δ1) for transient electrons. Thus, the Δ1 Bloch states in the CoFe electrode which only exist at EF in the majority-spin band have the smallest decay rate across the MgO, leading to the high TSP of the injection current in Ge. Therefore, the present sample is considered promising for efficient spin injection. The magnetic properties of the present sample are as well crucial for successful application to the electrical spin injection. Figure 4a shows the M-H hysteresis loops obtained by VSM at RT for the sample annealed at 300 °C along the [110] and [100] directions of CoFe. It clearly exhibits the cubic anisotropy imposed by the crystal symmetry of CoFe with the easy axis of [110] and the hard axis of [100] directions; that is, the cubic anisotropy constant Keff1 is negative, which differs from the bulk bcc-CoFe with its positive value of K1. The negative value of Keff1 is possibly ascribed to the straininduced magnetoelastic contribution and the interface structure of CoFe/MgO.15,26,27 We have witnessed that the M-H loops of the sample were similar to those of CoFe grown on a MgO(001) single-crystal substrate under the same growth conditions as shown in Figure 4b, implying that the MgO interfacial layer has a highly crystalline structure. It should be also mentioned that the saturation magnetization of the sample is 1430 ( 20 emu/cc, comparable to the value of bulk CoFe. 4. Conclusions

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We have demonstrated the epitaxial growth of MgO and bcc CoFe/MgO on Ge with the in-plane crystallographic relationship of CoFe(001)[100] MgO(001)[110] Ge(001)[100] with high-quality interfaces. The magnetic properties of the sample are similar to those of CoFe grown on a MgO(001) substrate and its saturation magnetization is 1430 ( 20 emu/cc, comparable to the value of bulk CoFe. Acknowledgment. This work was supported by the National Research Laboratory Program (Contract No. R0A-2007-000-20026-0), the WCU (World Class University) Program (Contract No. R33-2008-000-10078-0), and the GPP (Contract No. K2070200001408E020001410) through the National Research Foundation of Korea funded by the

Ministry of Education, Science and Technology. The authors would like to thank Byoung-Chul Min and Hyung-Jun Kim at KIST for their valuable discussions and advices, and also Stuart Parkin at IBM for helpful discussions.

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