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MgO multilayer
Superlattices and Microstructures xxx (2015) 1e11
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Atomic diffusion processes in MgO/Fe/MgO multilayer Jitendra Pal Singh*, Weon Cheol Lim, Keun Hwa Chae** Advanced Analysis Center, Korea Institute of Science and Technology, Seoul, 02792, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 September 2015 Received in revised form 14 October 2015 Accepted 15 October 2015 Available online xxx
In present work, atomic diffusion processes in MgO/Fe/MgO structure is investigated. This structure is deposited by using e-beam evaporation method in ultra-high vacuum (107 Torr). X-ray diffraction pattern exhibits amorphous nature of MgO layer, however, Au and Fe layers are polycrystalline in nature. Structural quality investigated using transmission electron microscopy shows the presence of short-ranged crystalline ordering in MgO layer. Oxygen atoms of this layer exhibit its interaction with Mg to form MgO, however, Fe layer exists in metal form with onset of oxidation at interface. Rutherford backscattering study shows the diffusion of atoms from upper layers to the layers underneath and to the substrate. Au atoms exhibit exponential diffusion profile in the structure, however, the compositional variation of Fe atoms is described using the combination of exponential decay and exponential growth functions. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Microscopy Crystalline order Oxidation Diffusion
1. Introduction Superlattice structures find applications in spintronics [1,2], high mobility transistor [3], light emitting diodes [4]. For spintronic applications, devices based on Fe/MgO/Fe structures are considered to be most successful in recent years, however, characteristics of these structures are affected by interface quality and layer properties [5e7]. Perpendicular magnetic anisotropy, control of magnetic anisotropy by external effects are important phenomena that are mediated by interface in these structures [8e10]. Thus control of interfaces in these structures provides ample scope for researchers. In order to investigate the interface structure of Fe/MgO/Fe, Miynczak et al. has grown Fe/MgO and MgO/Fe epitaxial bilayers using molecular beam epitaxy. It is observed that MgO/Fe interface contains 63% of interfacial Fe atoms to have an oxidic character, however, almost 86% of Fe atoms are oxidized at Fe/MgO interface [11]. Very similar to this, interface structure of NiO/Fe (001) grown using same technique was studied by this group. Authors envisage presence of Fe3þ3.5Fe2þ1.75O7 layer at NiO/Fe interface [12]. CoO/Fe bilayer also exhibits onset of oxidized Fe at interface [13]. These several factors at interface affect exchange bias and magnetic anisotropy of these structures [11e13]. High resolution transmission electron microscopy (HRTEM) study shows interface oxidation, strain, and structural asymmetry at the interfaces of Fe/MgO/Fe structures. At interface Fe is bound with O to form oxides [14e16]. Surface X-ray diffraction (SXRD) study shows formation of substoichiometric FeOx(x ¼ 0.6 ± 0.1) layer at lower MgO/Fe(001) interface in Fe/MgO/Fe structures. Also the growth of Fe on the MgO spacer and the upper Fe/MgO interface structure strongly depends on the preparation method [17]. The formation of a bulk like six-fold-coordinated Mg at the MgO/Fe(001) interface is accounted for stabilizing the interface structure, in which
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J.P. Singh), [email protected] (K.H. Chae). http://dx.doi.org/10.1016/j.spmi.2015.10.022 0749-6036/© 2015 Elsevier Ltd. All rights reserved.
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several FeeO distances are strained up to ~10% with respect to their bulk analog as evidenced using SXRD [18]. X-ray absorption spectroscopy analysis envisages that Fe-layer forms a FeeO-rich interface region with MgO-layer in Fe/MgO/Fe multilayer stack [19], however, similar measurements shows oxidation free interfaces in Fe/MgO/Fe in another study [20]. These interfaces exhibit enhanced interface magnetic moments for buried interfaces as observed for MgO(3 nm)/Fe(12 nm)/ MgO(001) [21]. Beside the interface structure, inter-diffusion of atoms is commonly observed in bilayers, multilayers and superlattices [22e24]. Thus, present work is motivated to investigate the interface structure and atomic diffusion process in similar superlattices. In this context, we have deposited MgO/Fe/MgO structure which consists of MgO/Fe and Fe/MgO interface and are important for designing high-density magnetic random access memory due to presence of perpendicular magnetic anisotropy [25]. This structure is characterized by using HRTEM, X-ray absorption spectroscopy, X-ray reflectivity (XRR) and Rutherford backscattering study (RBS) in order to investigate interface structure and diffusion process.
2. Experimental details 2.1. Deposition of MgO/Fe/MgO structure MgO/Fe/MgO structures are deposited using same methodology which is used for deposition of Fe/MgO/Fe multilayer [6]. Prior to the deposition, Si-substrate was cleaned with trichloroethylene, acetone, isopropyl alcohol and de-ionized water in order to remove chemical impurity and surface contamination. The Si-substrate was dipped into hydrogen fluoride (HF) for 2e3 min to remove native SiO2. For deposition of MgO layers, MgO powder (purity 99.999%, Alfa- Aesar) was pressed into pellet form and evaporated. Fe and Au targets were used for metal layer deposition. All the layers were deposited at deposition rate of 0.1 nm/s with online monitoring of thickness using quartz crystal monitor. First, MgO buffer layer was deposited on Si(100) substrate in order to prevent silicide formation at Si/Fe. MgO layer also works as a good buffer layer to grow epitaxial Fe layer on high temperature annealing [26]. The substrate temperature was kept at 300 C during deposition. On top of MgO buffer layer, Fe thin film was deposited at 180 C. Subsequently, MgO barrier layer was deposited at the same growth temperature. As the capping layer plays an important role in determining characteristics of Fe/MgO/Fe like structures, hence Au layer was deposited at room temperature on the top of MgO layer [27,28]. Deposition of Au layer above MgO also avoids the formation of Mg(OH)2 [29]. 2.2. Characterization techniques X-ray diffraction (XRD) and XRR measurements were carried out at 1D XRS KIST-PAL beamline equipped with a bending magnet as the x-ray source. The main components of X-ray scattering unit of this beamline are scintillation detectors (Cyberstar2000, Oxford Danfysik, UK), beam path with slits and beam attenuators, a diffractometer with an analyzer, and an image plate (IP) detector system [30]. XRR curves were simulated using Parratt software [31]. Titan 80e300™ transmission electron microscope (TEM) was used for studying the structure of stacks and the crystallinity of layers. Near edge X-ray absorption fine structure (NEXAFS) measurements for deposited structure were performed at the soft X-ray beamline 10D XAS KIST (Korea Institute of Science and Technology) of the Pohang Accelerator Laboratory (PAL), operating at 3.0 GeV energy with a maximum storage current of 300 mA. All the spectra were collected in the total electron yield mode at room temperature in a vacuum ~1.5 108 Torr. Gratings with 1000 (G3) and 1800 grooves/mm (G4) and exit slit size of 100 mm 100 mm were used for collecting spectra in the energy range 300e800 eV and 800e1500 eV, respectively. The energy resolution was 0.6 and 1 eV for gratings G3 and G4 during measurement [32]. Obtained spectra were normalized with respect to the incident photon flux. Rutherford backscattering spectrometry (RBS) experiments were carried out using alpha particle beam from 1.7 MV Pelletron accelerator. RBS simulation was done using RUMP program [33].
3. Results and discussion 3.1. X-ray diffraction study Fig. 1 shows XRD pattern of MgO/Fe/MgO structure. In the XRD pattern, peaks are coming from body-centered Fe [JCPDS No.-06-0696], rocksalt-structured MgO [JCPDS-No.-78-0430], close-packed Au [JCPDS No.04e784]. Fe and Au layers are grown polycrystalline in nature. MgO layers appear to be amorphous in nature. XRD peaks corresponding to the phase of Fe and Au layer are broadened. The broadening of XRD peaks are caused by several factors such as presence of crystallite and self-induced strain. Considering, shape contributions to XRD peak due to crystallites and strain are Cauchy and Gaussian type, these factors were separated using pseudo-voigt fitting of the peak [34]. The size of these crystallites were estimated using Scherrer's formula from Cauchy (Lorentz) component of peak broadening [34e36],
D¼
0:94l ; uL cosq
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Fig. 1. X-ray diffraction pattern of MgO/Fe/MgO structure. Inset shows deposition sequence of structure.
where, l is wavelength of radiation used, q is diffraction angle for most intense peak and uL is Cauchy (Lorentz) components of peak broadening. Crystallite size of Au and Fe layers are 11 and 18 nm, respectively (Table 1). Further, we have estimated selfinduced strain (3i) from Gaussian component (uG) using the relation given below
εi ¼
uG : tanq
Values of strain for Au and Fe layers are 3.8 103 and 3.3 103, respectively (Table 1). It is clear that Au layer is more strained compared to Fe layer. This effect is ascribed to the presence of smaller crystallites as well as the growth of Au layer on thin MgO layer. 3.2. Cross-sectional transmission electron microscopy study Fig. 2(a) reflects the layered structure of MgO/Fe/MgO. Different layers in the structure are well resolved and clearly distinguishable [37]. Thicknesses of these layers from top to bottom are 14 nm (Au), 5 nm (MgO), 37 nm (Fe), and 50 nm (MgO), respectively (Table 2). Various layers in the structure seem to have uniform thickness except the Au layer. Below Au layer, MgO layer is very thin and structural quality of this layer is shown in zoomed region of Fig. 2(a). Fig. 2(b) shows structural quality of MgO layer deposited just above the substrate. In this figure, arrows indicate directions of different crystalline orientations. Encircle regions represent regions having no crystalline orientation. This shows the presence of amorphous region in MgO layer. Thus MgO layer is distinguished by the combination of crystalline and amorphous regions. The amorphous part is dominating in these layers, resulting absence/low intense peak of XRD pattern corresponding to rocksalt phase of MgO. Structural quality of Fe layer is shown in Fig. 2(c). In this layer, the different directions of crystalline orientation can be clearly seen. Fe crystallites in the layer grow in different directions resulting polycrystalline nature of Fe layer as envisaged by XRD pattern (Fig. 1). In this structure, Fe layer is surrounded by MgO layers, which causes Fe
Table 1 Position, Lorentz broadening (uL) and Gaussian broadening (uG) estimated from pseudo-voigt fitting of XRD peak. D and 3i are the cystallite size and strain of different layers. Layers
2q
uL
uG
Size (D) nm
Strain (3i) 103
Au Fe
38.15 44.79
0.79 0.50
0.30 0.31
11 18
3.8 3.3
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Fig. 2. (a) Sequence of MgO/Fe/MgO structure on Si substrate; crystalline quality of (b) MgO and (c) Fe layer. Zoomed area in (a) shows quality of MgO layer on the top of Fe layer.
atoms to form FeeO bonds at MgO/Fe and Fe/MgO. Due to this tendency, Fe atoms will try to move towards interface and strain will develop across Fe layer (Table 1) .
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Table 2 Results obtained from TEM measurements, XRR and RBS simulation for MgO/Fe/MgO structure. Here t and s represent thickness and roughness of the layers. Layers
TEM t (nm)
Au MgO Fe/MgO Fe MgO/Fe MgO
14 5 No clear interface 37 Mixed Interface (3~5 nm) 50
XRR
RBS
t (nm)
s (nm)
t (nm)
11.1 1.3 0.3 35.0 1.8 52.4
1.6 1.1 0.7 0.8 1.5 1.3
9.5 5 e 37 e 55
Fig. 3. (a) MgO/Fe and (b) Fe/MgO interface of MgO/Fe/MgO structure.
Interface structure of MgO/Fe and Fe/MgO are shown in Fig. 3. Fig. 3 shows mixing at MgO/Fe interface, however interface is not very well resolved Fe/MgO interface. Interfacial layer thickness is almost ~3e5 nm at MgO/Fe interface. As Fe/MgO layer is not well resolved, it is expected that some of Fe ions might have reacted with O ions of MgO to form Fe-Ox. The presence of interfacial region is general property of similar structures and corroborates from previous studies [14e16]. 3.3. Near edge X-ray absorption fine structure study 3.3.1. O K and Mg L-edge spectra Fig. 4(a) reflects O K-edge NEXAFS spectrum for MgO/Fe/MgO structure. Pre-edge spectral feature of O K-edge arises due to the excitation to the localized bound state [38]. The observed spectral features in this spectrum are A1 (534 eV), B1 (538 eV), C1(542 eV), and D1 (553 eV) [39,40]. These spectral features arise due to MgeO covalent bonding states, O(2p) nonbonding states, O(2p)-Mg(3s) antibonding states [41]. In Fig. 4(a), pre-edge spectral feature is sharper than usual [39]. In case of oxides containing Fe, pre-edge spectral feature represents Fe(3d)-O(2p) hybridized states [42,43]. Thus the presence of spectral features is ascribed to presence of FeeO like structures at interface. This effect is concomitant with TEM observation of Fe/ MgO interface (Fig. 3(b)). Mg K-edge NEXAFS spectrum shows only two spectral features (Fig. 4(b)). These spectral features are A2 (1318 eV) and B2 (1360 eV). Mg K-edge spectrum of MgO thin films of similar thickness exhibit five spectral features in this energy range (1300e1370 eV) that are associated with Mg (1s) to (3p) states [39]. Thus the presence of only two spectral features in Mg Kspectrum envisages weak hybridization among Mg and oxygen atoms [41,42]. Few atoms might have attached with Fe atoms. Intense pre-peak region in O K-edge also supports this effect as it might have contribution from O(2p)-Fe(3d) states [42,43]. Fig. 5 shows the Fe L-edge NEXAFS spectra for MgO/Fe/MgO structure. This spectrum exhibits two main spectral features A3 (710 eV) and B3 (723 eV). As mention earlier, Fe L-edge spectrum arises from the transition of Fe(2p) core level to the unoccupied 3d orbitals. Presence of spineorbit coupling at core level gives rise to 2p3/2 and 2p1/2 states and spectral features A3 and B3 appear in Fe L-edge spectrum [40,43,44]. Spectral features A3 and B3 exhibit shoulder like features. This effect can be Please cite this article in press as: J.P. Singh et al., Atomic diffusion processes in MgO/Fe/MgO multilayer, Superlattices and Microstructures (2015), http://dx.doi.org/10.1016/j.spmi.2015.10.022
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Fig. 4. (a) O K and (b) Mg K-edge spectra of MgO/Fe/MgO structure.
Fig. 5. Fe L-edge spectrum of MgO/Fe/MgO structure.
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Fig. 6. (a) Experimental and simulated XRR curves of MgO/Fe/MgO structure, and (b) Scattering length profile.
ascribed to the oxidation of Fe ions. Presence of octahedral crystal fields in Fe-Ox like system gives rise to degeneracy of 3d levels. As the result, two degenerate states t2g and eg are created and exhibits spectral features as shown by arrow in Fig. 5 [44]. Hence, it is contemplated that Fe/MgO interfaces in this structure is oxidized. This behavior is different from that are reported by Colonna et al., which envisages the absence of interface oxidation at Fe/MgO interface [45]. 3.4. X-ray reflectivity measurements Important information gathered from TEM and NEXAFS motivated us to simulate XRR curve for this structure. Fig. 6 shows experimental and simulated XRR curves along with scattering length profile for these structures. This curve was simulated using MgO layer, an interface layer (Fe-Ox) at MgO/Fe interface layer, Fe layer, interface layer (Fe-Ox) at Fe/MgO interface (Table 2). Table 2 shows values of thickness and interface roughness of various layers. Values of thickness of various layers in the structure are 11.1 nm (Au), 1.3 nm (MgO), 35 nm (Fe) and 50 nm (MgO). These values are slightly less than that are estimated from cross-sectional TEM (Table 1). This is due to the inclusion of interface region during thickness measurements for results obtained from TEM. XRR separately estimates thickness of this region. Values of thickness and interface roughness of Fe/MgO interface are 0.3 and 0.7 nm, respectively (Table 2). Due to this interfacial region, NEXAFS spectrum similar to oxidize Fe is observed. At MgO/Fe interface, Fe-Ox interface layer with thickness 1.8 nm and interface roughness 1.5 nm (Table 2) exists and can also be clearly seen in Fig. 5(a). Scattering length profile of MgO/Fe/MgO structure is shown in Fig. 6(b). d is value of real part of refractive index. Values of d are 4.7158 105, 2.247 105, and 1.15 105 for Au, Fe, and MgO, respectively. Simulated values of d are 4.27 105, 2.2 105, and 4.5 106 for these elements (Fig. 6b). Reduction in these values is due to the compositional change of different layers. 3.5. RBS study Fig. 7 shows simulated RBS spectra for this structure. Features corresponding to Au, Fe, Mg, and O atoms are well resolved (Fig. 7). In Fig. 7(a), simulation is done according to the results obtained from TEM and XRR (Table 2). In this case, thicknesses Please cite this article in press as: J.P. Singh et al., Atomic diffusion processes in MgO/Fe/MgO multilayer, Superlattices and Microstructures (2015), http://dx.doi.org/10.1016/j.spmi.2015.10.022
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Fig. 7. Rutherford Backscattering Spectra of MgO/Fe/MgO structure simulated (a) according to TEM and XRR, (b) considering diffusion of atoms.
of Au, Fe, and MgO layers are kept almost same to the thicknesses obtained from TEM and XRR. Here, tail region of Au and Fe peaks (shown by arrows) could not be simulated. Additionally, the intensity of feature corresponding to Fe is quite different from experimental data points. Since, diffusion is common in bilayer and trilayer structures [22e24], hence, the interdiffusion of atoms is considered for simulating RBS spectrum of this structure. The simulated spectrum considering diffusion is shown in Fig. 7(b). It is clear that experimental and simulated spectra are in good agreement with each other. Thickness Please cite this article in press as: J.P. Singh et al., Atomic diffusion processes in MgO/Fe/MgO multilayer, Superlattices and Microstructures (2015), http://dx.doi.org/10.1016/j.spmi.2015.10.022
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Table 3 Results obtained from RBS simulation (considering diffusion) for MgO/Fe/MgO structure. Layers
Thickness (nm)
Composition
Au MgO Fe/MgO Fe MgO/Fe MgO Si Si Si
6.4 3.4 6.1 17.50 8.00 51 70 50 100000
Au0.97 Au0.47Mg0.500 O0.37 Au0.18O0.37Fe0.05 Au0.008Fe0.97 Au0.025Fe0.53MgO.5O0.5 Au0.01Fe0.25MgO.5O0.5 Si1.00 Au0.03Fe0.08MgO.25O0.55 Si1.00 Au0.03Fe0.08MgO.25O0.55 Si1.0
Multilayer Region
Substrate Region
Fig. 8. Compositions of atoms in different layers of MgO/Fe/MgO structure.
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Fig. 9. Fitting to composition of (a) Au and (b) Fe metals in MgO/Fe/MgO structure.
and composition of various layers are shown in Table 3. It is clear that both composition and thickness of each layer are different than that estimated from TEM and XRR. Thickness of Au layer is 6.4 nm and its composition is Au0.97. Second deposited layer is MgO and RBS simulation shows a large amount of Au diffusion in this layer. Compositional variation of each atom with thickness is shown in Fig. 8. Fe and Au atoms diffuse dominantly in the layers underneath. Diffusion of Au and Fe is dominant due to evaporation of these atoms on the layers of lighter atoms (MgO and Si) [22e24]. Tendency of forming oxides by Fe is another reason for Fe-diffusion [16,17]. A small amount of Au and Fe also diffuses to the substrate. A large amount of Mg and O atoms is diffused to Si substrate as well as to other layers (Fig. 8). Au composition inside the layer can be fitted using exponential decay (Fig. 9). This indicates that Au atoms are diffused towards the layers underneath. Apart from this, the composition of Fe inside structure increases exponentially first, then decreases in same manner. This indicates the presence of some amount of Fe in upper MgO layer. This is due to diffusion of Fe atoms in upper MgO layer due to strong tendency of forming oxides. 4. Conclusion We have successfully deposited MgO/Fe/MgO structures using e-beam evaporation method. X-ray diffraction study shows amorphous and polycrystalline nature of MgO and Fe layers in the structures. Cross sectional high resolution transmission electron microscopy study shows partial crystallization of MgO layers. Fe and Au layers are crystallized and well separated from each other. At Fe/MgO interface, no clear boundary appears but clear boundary exists at MgO/Fe interface with the onset of interface mixing. Near edge X-ray absorption fine structure measurements shows slight oxidation of Fe at Fe/MgO interface. X-ray reflectivity measurement corroborates the results obtained from other measurements. RBS study shows atomic diffusion process. References [1] Zi-Zheng Guo, Spin-torque critical current of graphene-based lateral spin valves, Superlattices Microstruct. 86 (2015) 150e156. [2] F. Sattari, Spin dependent shot noise in magnetic graphene superlattice, Superlattices Microstruct. 86 (2015) 29e35. [3] M. Ravindran, P. Shankar, Superlattice structure modeling and simulation of high electron mobility transistor with novel device, Superlattices Microstruct. 86 (2015) 282e288. [4] W. Liu, D.G. Zhao, D.S. Jiang, P. Chen, Z.S. Liu, J.J. Zhu, X. Li, F. Liang, J.P. Liu, S.M. Zhang, H. Yang, Y.T. Zhang, G.T. Du, The difference in efficiency droop behaviors of two InGaN/GaN multiple-quantum-well green light-emitting diodes with modified structural parameters, Superlattice and Microstructure, 10.1016/j.spmi.2015.08.026.
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Armelles, Effect of a capping layer on the magnetic properties of island nanostructured Fe(110), J. Appl. Phys. 97 (2005) 104302. [28] Der-Sheng Lee, Hao-Ting Chang, Chih-Wei Cheng, G. Chern, Perpendicular magnetic anisotropy in MgO/CoFeB/Nb and a comparison of the cap layer effect, IEEE Trans. Magn. 50 (2014) 3201904. [29] S.G. Kim, K.H. Choi, J.H. Eun, H.J. Kim, C.S. Hwang, Effects of additives on properties of MgO thin films by electrostatic spray deposition, Thin Solid Films 377e378 (2000) 694e698. [30] Ik-Jae Lee, J. Yu, Y.D. Yun, C.S. Lee, in: D. Seo, H.-Y. Kim, W.-W. Lee, Keun Hwa Chae (Eds.), Construction of X-ray Scattering and X-ray Absorption Fine Structure Beamline at the Pohang Light Source, Rev. Sci. Inst., 81, 2010, p. 026103. [31] G. Dixit, J.P. Singh, R.C. Srivastava, H.M. Agrawal, R.J. Choudhary, A. Gupta, Annealing effect on the structural and magnetic properties of nickel ferrite thin films, Surf. Inter. Anal. 42 (2010) 151e157. [32] H.N. Hwang, H.S. Kim, B. Kim, C.C. Hwang, S.W. Moon, S.M. Chung, C. Jeon, C.Y. Park, K.H. Cha, W.K. Choi, Construction of a soft X-ray beamline at the PLS, Nucl. Inst. Methods Phys. Res. A 581 (2007) 850e855. [33] Q. Yang, A realistic PC-program for RBS simulation and data analysis, Nucl. Inst. Methods Phys. Res. B 90 (1994) 602e605. [34] T.H.H. de Keijser, J.I. Langford, E.J. Mittemeijer, A.B.P. Vogels, Use of the voigt function in a single-line method for the Analysis of X-ray diffraction line broadening, J. Appl. Cryst. 15 (1982) 308e314. [35] H.G. Jianga, M. Rühlea, E.J. Lavernia, On the applicability of the x-ray diffraction line profile analysis in extracting grain size and microstrain in nanocrystalline materials, J. Mater. Res. 14 (1999) 549e559. [36] G. Dixit, J.P. Singh, R.C. Srivastava, H.M. Agrawal, R.J. Choudhary, A. Gupta, Structural and magnetic behavior of NiFe2O4 thin film grown by pulsed laser deposition, Ind. J. Pure Appl. Phys. 48 (2010) 287e291. [37] M. Arita, Y. Okubo, K. Hamada, Y. Takahashi, Tunnel current measurement of MgO and MgO/Fe/MgO nanoregions during TEM observation, Superlattices Microstruct. 44 (2008) 633e640. [38] T. Linder, H. Sauer, W. Engel, K. Kambe, Near-edge structure in electron-energy-loss spectra of MgO, Phys. Rev. B 33 (1986) 22e24. [39] J.P. Singh, S. Gautam, B.B. Singh, S. Chaudhary, D. Kabiraj, D. Kanjilal, K.H. Chae, R. Kotnala, J.M. Lee, Jin-Ming Chen, K. Asokan, Magnetic, electronic structure and interface study of Fe/MgO/Fe multilayer, Adv. Mater. Lett. 5 (2014) 372e377. [40] J.P. Singh, I. Sulania, J. Prakash, S. Gautam, K.H. Chae, D. Kanjilal, K. Asokan, Study of surface morphology and grain size of irradiated MgO thin films, Adv. Mat. Lett. 3 (2012) 112e117. [41] J. A. McLeod, R. G. Wilks, N. A. Skorikov, L. D. Finkelstein, Bandgaps and Electronic Structure of Alkaline Earth and Post-Transition Metal Oxides http:// arxiv.org/pdf/0908.1581. [42] J.P. Singh, C.L. Chen, C.L. Dong, J. Prakash, D. Kabiraj, D. Kanjilal, W.F. Pong, K. Asokan, Role of surface and subsurface defects in MgO thin film: XANES and magnetic investigations, Superlattices Microstruct. 77 (2015) 313e324. [43] J.P. Singh, C.L. Chen, C.L. Dong, R.C. Srivastava, H.M. Agrawal, W.F. Pong, K. Asokan, Effect of intermediate annealing on the structural, electrical and dielectric properties of zinc ferrite: an XANES investigation, Sci. Adv. Mater 5 (2013) 171e181. [44] Z.Y. Wu, S. Gota, F. Jollet, M. Pollak, M. Gautier-Soyer, C.R. Natoli, Characterization of iron oxides by x-ray absorption at the oxygen K edge using a full multiple-scattering approach, Phys. Rev. B 55 (1997) 2570. [45] S. Colonna, A. Cricenti, P. Luches, S. Valeri, F. Boscherini, J. Qi, Y. Xu, N. Tolk, Fe/NiO(100) and Fe/MgO(100) interfaces studied by X-ray absorption spectroscopy and non-linear Kerr effect, Superlattices Microstruct. 46 (2009) 107e113.
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Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Atomic diffusion processes in MgO/Fe/MgO multilayer Jitendra Pal Singh*, Weon Cheol Lim, Keun Hwa Chae** Advanced Analysis Center, Korea Institute of Science and Technology, Seoul, 02792, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 September 2015 Received in revised form 14 October 2015 Accepted 15 October 2015 Available online xxx
In present work, atomic diffusion processes in MgO/Fe/MgO structure is investigated. This structure is deposited by using e-beam evaporation method in ultra-high vacuum (107 Torr). X-ray diffraction pattern exhibits amorphous nature of MgO layer, however, Au and Fe layers are polycrystalline in nature. Structural quality investigated using transmission electron microscopy shows the presence of short-ranged crystalline ordering in MgO layer. Oxygen atoms of this layer exhibit its interaction with Mg to form MgO, however, Fe layer exists in metal form with onset of oxidation at interface. Rutherford backscattering study shows the diffusion of atoms from upper layers to the layers underneath and to the substrate. Au atoms exhibit exponential diffusion profile in the structure, however, the compositional variation of Fe atoms is described using the combination of exponential decay and exponential growth functions. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Microscopy Crystalline order Oxidation Diffusion
1. Introduction Superlattice structures find applications in spintronics [1,2], high mobility transistor [3], light emitting diodes [4]. For spintronic applications, devices based on Fe/MgO/Fe structures are considered to be most successful in recent years, however, characteristics of these structures are affected by interface quality and layer properties [5e7]. Perpendicular magnetic anisotropy, control of magnetic anisotropy by external effects are important phenomena that are mediated by interface in these structures [8e10]. Thus control of interfaces in these structures provides ample scope for researchers. In order to investigate the interface structure of Fe/MgO/Fe, Miynczak et al. has grown Fe/MgO and MgO/Fe epitaxial bilayers using molecular beam epitaxy. It is observed that MgO/Fe interface contains 63% of interfacial Fe atoms to have an oxidic character, however, almost 86% of Fe atoms are oxidized at Fe/MgO interface [11]. Very similar to this, interface structure of NiO/Fe (001) grown using same technique was studied by this group. Authors envisage presence of Fe3þ3.5Fe2þ1.75O7 layer at NiO/Fe interface [12]. CoO/Fe bilayer also exhibits onset of oxidized Fe at interface [13]. These several factors at interface affect exchange bias and magnetic anisotropy of these structures [11e13]. High resolution transmission electron microscopy (HRTEM) study shows interface oxidation, strain, and structural asymmetry at the interfaces of Fe/MgO/Fe structures. At interface Fe is bound with O to form oxides [14e16]. Surface X-ray diffraction (SXRD) study shows formation of substoichiometric FeOx(x ¼ 0.6 ± 0.1) layer at lower MgO/Fe(001) interface in Fe/MgO/Fe structures. Also the growth of Fe on the MgO spacer and the upper Fe/MgO interface structure strongly depends on the preparation method [17]. The formation of a bulk like six-fold-coordinated Mg at the MgO/Fe(001) interface is accounted for stabilizing the interface structure, in which
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J.P. Singh), [email protected] (K.H. Chae). http://dx.doi.org/10.1016/j.spmi.2015.10.022 0749-6036/© 2015 Elsevier Ltd. All rights reserved.
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several FeeO distances are strained up to ~10% with respect to their bulk analog as evidenced using SXRD [18]. X-ray absorption spectroscopy analysis envisages that Fe-layer forms a FeeO-rich interface region with MgO-layer in Fe/MgO/Fe multilayer stack [19], however, similar measurements shows oxidation free interfaces in Fe/MgO/Fe in another study [20]. These interfaces exhibit enhanced interface magnetic moments for buried interfaces as observed for MgO(3 nm)/Fe(12 nm)/ MgO(001) [21]. Beside the interface structure, inter-diffusion of atoms is commonly observed in bilayers, multilayers and superlattices [22e24]. Thus, present work is motivated to investigate the interface structure and atomic diffusion process in similar superlattices. In this context, we have deposited MgO/Fe/MgO structure which consists of MgO/Fe and Fe/MgO interface and are important for designing high-density magnetic random access memory due to presence of perpendicular magnetic anisotropy [25]. This structure is characterized by using HRTEM, X-ray absorption spectroscopy, X-ray reflectivity (XRR) and Rutherford backscattering study (RBS) in order to investigate interface structure and diffusion process.
2. Experimental details 2.1. Deposition of MgO/Fe/MgO structure MgO/Fe/MgO structures are deposited using same methodology which is used for deposition of Fe/MgO/Fe multilayer [6]. Prior to the deposition, Si-substrate was cleaned with trichloroethylene, acetone, isopropyl alcohol and de-ionized water in order to remove chemical impurity and surface contamination. The Si-substrate was dipped into hydrogen fluoride (HF) for 2e3 min to remove native SiO2. For deposition of MgO layers, MgO powder (purity 99.999%, Alfa- Aesar) was pressed into pellet form and evaporated. Fe and Au targets were used for metal layer deposition. All the layers were deposited at deposition rate of 0.1 nm/s with online monitoring of thickness using quartz crystal monitor. First, MgO buffer layer was deposited on Si(100) substrate in order to prevent silicide formation at Si/Fe. MgO layer also works as a good buffer layer to grow epitaxial Fe layer on high temperature annealing [26]. The substrate temperature was kept at 300 C during deposition. On top of MgO buffer layer, Fe thin film was deposited at 180 C. Subsequently, MgO barrier layer was deposited at the same growth temperature. As the capping layer plays an important role in determining characteristics of Fe/MgO/Fe like structures, hence Au layer was deposited at room temperature on the top of MgO layer [27,28]. Deposition of Au layer above MgO also avoids the formation of Mg(OH)2 [29]. 2.2. Characterization techniques X-ray diffraction (XRD) and XRR measurements were carried out at 1D XRS KIST-PAL beamline equipped with a bending magnet as the x-ray source. The main components of X-ray scattering unit of this beamline are scintillation detectors (Cyberstar2000, Oxford Danfysik, UK), beam path with slits and beam attenuators, a diffractometer with an analyzer, and an image plate (IP) detector system [30]. XRR curves were simulated using Parratt software [31]. Titan 80e300™ transmission electron microscope (TEM) was used for studying the structure of stacks and the crystallinity of layers. Near edge X-ray absorption fine structure (NEXAFS) measurements for deposited structure were performed at the soft X-ray beamline 10D XAS KIST (Korea Institute of Science and Technology) of the Pohang Accelerator Laboratory (PAL), operating at 3.0 GeV energy with a maximum storage current of 300 mA. All the spectra were collected in the total electron yield mode at room temperature in a vacuum ~1.5 108 Torr. Gratings with 1000 (G3) and 1800 grooves/mm (G4) and exit slit size of 100 mm 100 mm were used for collecting spectra in the energy range 300e800 eV and 800e1500 eV, respectively. The energy resolution was 0.6 and 1 eV for gratings G3 and G4 during measurement [32]. Obtained spectra were normalized with respect to the incident photon flux. Rutherford backscattering spectrometry (RBS) experiments were carried out using alpha particle beam from 1.7 MV Pelletron accelerator. RBS simulation was done using RUMP program [33].
3. Results and discussion 3.1. X-ray diffraction study Fig. 1 shows XRD pattern of MgO/Fe/MgO structure. In the XRD pattern, peaks are coming from body-centered Fe [JCPDS No.-06-0696], rocksalt-structured MgO [JCPDS-No.-78-0430], close-packed Au [JCPDS No.04e784]. Fe and Au layers are grown polycrystalline in nature. MgO layers appear to be amorphous in nature. XRD peaks corresponding to the phase of Fe and Au layer are broadened. The broadening of XRD peaks are caused by several factors such as presence of crystallite and self-induced strain. Considering, shape contributions to XRD peak due to crystallites and strain are Cauchy and Gaussian type, these factors were separated using pseudo-voigt fitting of the peak [34]. The size of these crystallites were estimated using Scherrer's formula from Cauchy (Lorentz) component of peak broadening [34e36],
D¼
0:94l ; uL cosq
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Fig. 1. X-ray diffraction pattern of MgO/Fe/MgO structure. Inset shows deposition sequence of structure.
where, l is wavelength of radiation used, q is diffraction angle for most intense peak and uL is Cauchy (Lorentz) components of peak broadening. Crystallite size of Au and Fe layers are 11 and 18 nm, respectively (Table 1). Further, we have estimated selfinduced strain (3i) from Gaussian component (uG) using the relation given below
εi ¼
uG : tanq
Values of strain for Au and Fe layers are 3.8 103 and 3.3 103, respectively (Table 1). It is clear that Au layer is more strained compared to Fe layer. This effect is ascribed to the presence of smaller crystallites as well as the growth of Au layer on thin MgO layer. 3.2. Cross-sectional transmission electron microscopy study Fig. 2(a) reflects the layered structure of MgO/Fe/MgO. Different layers in the structure are well resolved and clearly distinguishable [37]. Thicknesses of these layers from top to bottom are 14 nm (Au), 5 nm (MgO), 37 nm (Fe), and 50 nm (MgO), respectively (Table 2). Various layers in the structure seem to have uniform thickness except the Au layer. Below Au layer, MgO layer is very thin and structural quality of this layer is shown in zoomed region of Fig. 2(a). Fig. 2(b) shows structural quality of MgO layer deposited just above the substrate. In this figure, arrows indicate directions of different crystalline orientations. Encircle regions represent regions having no crystalline orientation. This shows the presence of amorphous region in MgO layer. Thus MgO layer is distinguished by the combination of crystalline and amorphous regions. The amorphous part is dominating in these layers, resulting absence/low intense peak of XRD pattern corresponding to rocksalt phase of MgO. Structural quality of Fe layer is shown in Fig. 2(c). In this layer, the different directions of crystalline orientation can be clearly seen. Fe crystallites in the layer grow in different directions resulting polycrystalline nature of Fe layer as envisaged by XRD pattern (Fig. 1). In this structure, Fe layer is surrounded by MgO layers, which causes Fe
Table 1 Position, Lorentz broadening (uL) and Gaussian broadening (uG) estimated from pseudo-voigt fitting of XRD peak. D and 3i are the cystallite size and strain of different layers. Layers
2q
uL
uG
Size (D) nm
Strain (3i) 103
Au Fe
38.15 44.79
0.79 0.50
0.30 0.31
11 18
3.8 3.3
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Fig. 2. (a) Sequence of MgO/Fe/MgO structure on Si substrate; crystalline quality of (b) MgO and (c) Fe layer. Zoomed area in (a) shows quality of MgO layer on the top of Fe layer.
atoms to form FeeO bonds at MgO/Fe and Fe/MgO. Due to this tendency, Fe atoms will try to move towards interface and strain will develop across Fe layer (Table 1) .
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Table 2 Results obtained from TEM measurements, XRR and RBS simulation for MgO/Fe/MgO structure. Here t and s represent thickness and roughness of the layers. Layers
TEM t (nm)
Au MgO Fe/MgO Fe MgO/Fe MgO
14 5 No clear interface 37 Mixed Interface (3~5 nm) 50
XRR
RBS
t (nm)
s (nm)
t (nm)
11.1 1.3 0.3 35.0 1.8 52.4
1.6 1.1 0.7 0.8 1.5 1.3
9.5 5 e 37 e 55
Fig. 3. (a) MgO/Fe and (b) Fe/MgO interface of MgO/Fe/MgO structure.
Interface structure of MgO/Fe and Fe/MgO are shown in Fig. 3. Fig. 3 shows mixing at MgO/Fe interface, however interface is not very well resolved Fe/MgO interface. Interfacial layer thickness is almost ~3e5 nm at MgO/Fe interface. As Fe/MgO layer is not well resolved, it is expected that some of Fe ions might have reacted with O ions of MgO to form Fe-Ox. The presence of interfacial region is general property of similar structures and corroborates from previous studies [14e16]. 3.3. Near edge X-ray absorption fine structure study 3.3.1. O K and Mg L-edge spectra Fig. 4(a) reflects O K-edge NEXAFS spectrum for MgO/Fe/MgO structure. Pre-edge spectral feature of O K-edge arises due to the excitation to the localized bound state [38]. The observed spectral features in this spectrum are A1 (534 eV), B1 (538 eV), C1(542 eV), and D1 (553 eV) [39,40]. These spectral features arise due to MgeO covalent bonding states, O(2p) nonbonding states, O(2p)-Mg(3s) antibonding states [41]. In Fig. 4(a), pre-edge spectral feature is sharper than usual [39]. In case of oxides containing Fe, pre-edge spectral feature represents Fe(3d)-O(2p) hybridized states [42,43]. Thus the presence of spectral features is ascribed to presence of FeeO like structures at interface. This effect is concomitant with TEM observation of Fe/ MgO interface (Fig. 3(b)). Mg K-edge NEXAFS spectrum shows only two spectral features (Fig. 4(b)). These spectral features are A2 (1318 eV) and B2 (1360 eV). Mg K-edge spectrum of MgO thin films of similar thickness exhibit five spectral features in this energy range (1300e1370 eV) that are associated with Mg (1s) to (3p) states [39]. Thus the presence of only two spectral features in Mg Kspectrum envisages weak hybridization among Mg and oxygen atoms [41,42]. Few atoms might have attached with Fe atoms. Intense pre-peak region in O K-edge also supports this effect as it might have contribution from O(2p)-Fe(3d) states [42,43]. Fig. 5 shows the Fe L-edge NEXAFS spectra for MgO/Fe/MgO structure. This spectrum exhibits two main spectral features A3 (710 eV) and B3 (723 eV). As mention earlier, Fe L-edge spectrum arises from the transition of Fe(2p) core level to the unoccupied 3d orbitals. Presence of spineorbit coupling at core level gives rise to 2p3/2 and 2p1/2 states and spectral features A3 and B3 appear in Fe L-edge spectrum [40,43,44]. Spectral features A3 and B3 exhibit shoulder like features. This effect can be Please cite this article in press as: J.P. Singh et al., Atomic diffusion processes in MgO/Fe/MgO multilayer, Superlattices and Microstructures (2015), http://dx.doi.org/10.1016/j.spmi.2015.10.022
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Fig. 4. (a) O K and (b) Mg K-edge spectra of MgO/Fe/MgO structure.
Fig. 5. Fe L-edge spectrum of MgO/Fe/MgO structure.
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Fig. 6. (a) Experimental and simulated XRR curves of MgO/Fe/MgO structure, and (b) Scattering length profile.
ascribed to the oxidation of Fe ions. Presence of octahedral crystal fields in Fe-Ox like system gives rise to degeneracy of 3d levels. As the result, two degenerate states t2g and eg are created and exhibits spectral features as shown by arrow in Fig. 5 [44]. Hence, it is contemplated that Fe/MgO interfaces in this structure is oxidized. This behavior is different from that are reported by Colonna et al., which envisages the absence of interface oxidation at Fe/MgO interface [45]. 3.4. X-ray reflectivity measurements Important information gathered from TEM and NEXAFS motivated us to simulate XRR curve for this structure. Fig. 6 shows experimental and simulated XRR curves along with scattering length profile for these structures. This curve was simulated using MgO layer, an interface layer (Fe-Ox) at MgO/Fe interface layer, Fe layer, interface layer (Fe-Ox) at Fe/MgO interface (Table 2). Table 2 shows values of thickness and interface roughness of various layers. Values of thickness of various layers in the structure are 11.1 nm (Au), 1.3 nm (MgO), 35 nm (Fe) and 50 nm (MgO). These values are slightly less than that are estimated from cross-sectional TEM (Table 1). This is due to the inclusion of interface region during thickness measurements for results obtained from TEM. XRR separately estimates thickness of this region. Values of thickness and interface roughness of Fe/MgO interface are 0.3 and 0.7 nm, respectively (Table 2). Due to this interfacial region, NEXAFS spectrum similar to oxidize Fe is observed. At MgO/Fe interface, Fe-Ox interface layer with thickness 1.8 nm and interface roughness 1.5 nm (Table 2) exists and can also be clearly seen in Fig. 5(a). Scattering length profile of MgO/Fe/MgO structure is shown in Fig. 6(b). d is value of real part of refractive index. Values of d are 4.7158 105, 2.247 105, and 1.15 105 for Au, Fe, and MgO, respectively. Simulated values of d are 4.27 105, 2.2 105, and 4.5 106 for these elements (Fig. 6b). Reduction in these values is due to the compositional change of different layers. 3.5. RBS study Fig. 7 shows simulated RBS spectra for this structure. Features corresponding to Au, Fe, Mg, and O atoms are well resolved (Fig. 7). In Fig. 7(a), simulation is done according to the results obtained from TEM and XRR (Table 2). In this case, thicknesses Please cite this article in press as: J.P. Singh et al., Atomic diffusion processes in MgO/Fe/MgO multilayer, Superlattices and Microstructures (2015), http://dx.doi.org/10.1016/j.spmi.2015.10.022
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Fig. 7. Rutherford Backscattering Spectra of MgO/Fe/MgO structure simulated (a) according to TEM and XRR, (b) considering diffusion of atoms.
of Au, Fe, and MgO layers are kept almost same to the thicknesses obtained from TEM and XRR. Here, tail region of Au and Fe peaks (shown by arrows) could not be simulated. Additionally, the intensity of feature corresponding to Fe is quite different from experimental data points. Since, diffusion is common in bilayer and trilayer structures [22e24], hence, the interdiffusion of atoms is considered for simulating RBS spectrum of this structure. The simulated spectrum considering diffusion is shown in Fig. 7(b). It is clear that experimental and simulated spectra are in good agreement with each other. Thickness Please cite this article in press as: J.P. Singh et al., Atomic diffusion processes in MgO/Fe/MgO multilayer, Superlattices and Microstructures (2015), http://dx.doi.org/10.1016/j.spmi.2015.10.022
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Table 3 Results obtained from RBS simulation (considering diffusion) for MgO/Fe/MgO structure. Layers
Thickness (nm)
Composition
Au MgO Fe/MgO Fe MgO/Fe MgO Si Si Si
6.4 3.4 6.1 17.50 8.00 51 70 50 100000
Au0.97 Au0.47Mg0.500 O0.37 Au0.18O0.37Fe0.05 Au0.008Fe0.97 Au0.025Fe0.53MgO.5O0.5 Au0.01Fe0.25MgO.5O0.5 Si1.00 Au0.03Fe0.08MgO.25O0.55 Si1.00 Au0.03Fe0.08MgO.25O0.55 Si1.0
Multilayer Region
Substrate Region
Fig. 8. Compositions of atoms in different layers of MgO/Fe/MgO structure.
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Fig. 9. Fitting to composition of (a) Au and (b) Fe metals in MgO/Fe/MgO structure.
and composition of various layers are shown in Table 3. It is clear that both composition and thickness of each layer are different than that estimated from TEM and XRR. Thickness of Au layer is 6.4 nm and its composition is Au0.97. Second deposited layer is MgO and RBS simulation shows a large amount of Au diffusion in this layer. Compositional variation of each atom with thickness is shown in Fig. 8. Fe and Au atoms diffuse dominantly in the layers underneath. Diffusion of Au and Fe is dominant due to evaporation of these atoms on the layers of lighter atoms (MgO and Si) [22e24]. Tendency of forming oxides by Fe is another reason for Fe-diffusion [16,17]. A small amount of Au and Fe also diffuses to the substrate. A large amount of Mg and O atoms is diffused to Si substrate as well as to other layers (Fig. 8). Au composition inside the layer can be fitted using exponential decay (Fig. 9). This indicates that Au atoms are diffused towards the layers underneath. Apart from this, the composition of Fe inside structure increases exponentially first, then decreases in same manner. This indicates the presence of some amount of Fe in upper MgO layer. This is due to diffusion of Fe atoms in upper MgO layer due to strong tendency of forming oxides. 4. Conclusion We have successfully deposited MgO/Fe/MgO structures using e-beam evaporation method. X-ray diffraction study shows amorphous and polycrystalline nature of MgO and Fe layers in the structures. Cross sectional high resolution transmission electron microscopy study shows partial crystallization of MgO layers. Fe and Au layers are crystallized and well separated from each other. At Fe/MgO interface, no clear boundary appears but clear boundary exists at MgO/Fe interface with the onset of interface mixing. Near edge X-ray absorption fine structure measurements shows slight oxidation of Fe at Fe/MgO interface. X-ray reflectivity measurement corroborates the results obtained from other measurements. RBS study shows atomic diffusion process. References [1] Zi-Zheng Guo, Spin-torque critical current of graphene-based lateral spin valves, Superlattices Microstruct. 86 (2015) 150e156. [2] F. Sattari, Spin dependent shot noise in magnetic graphene superlattice, Superlattices Microstruct. 86 (2015) 29e35. [3] M. Ravindran, P. Shankar, Superlattice structure modeling and simulation of high electron mobility transistor with novel device, Superlattices Microstruct. 86 (2015) 282e288. [4] W. Liu, D.G. Zhao, D.S. Jiang, P. Chen, Z.S. Liu, J.J. Zhu, X. Li, F. Liang, J.P. Liu, S.M. Zhang, H. Yang, Y.T. Zhang, G.T. Du, The difference in efficiency droop behaviors of two InGaN/GaN multiple-quantum-well green light-emitting diodes with modified structural parameters, Superlattice and Microstructure, 10.1016/j.spmi.2015.08.026.
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Please cite this article in press as: J.P. Singh et al., Atomic diffusion processes in MgO/Fe/MgO multilayer, Superlattices and Microstructures (2015), http://dx.doi.org/10.1016/j.spmi.2015.10.022