Study of boron diffusion in MgO in CoFeB∣MgO film stacks using parallel electron energy loss spectroscopy Sankha S. Mukherjee, David MacMahon, Feiming Bai, Chih-Ling Lee, and Santosh K. Kurinec Citation: Applied Physics Letters 94, 082110 (2009); doi: 10.1063/1.3090035 View online: http://dx.doi.org/10.1063/1.3090035 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/94/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Influence of boron diffusion on the perpendicular magnetic anisotropy in Ta|CoFeB|MgO ultrathin films J. Appl. Phys. 117, 043913 (2015); 10.1063/1.4906096 A study of the effect of iron island morphology and interface oxidation on the magnetic hysteresis of Fe-MgO (001) thin film composites J. Appl. Phys. 112, 013905 (2012); 10.1063/1.4730630 Spatial control of magnetic anisotropy for current induced domain wall injection in perpendicularly magnetized CoFeB|MgO nanostructures Appl. Phys. Lett. 100, 192411 (2012); 10.1063/1.4711016 Fe diffusion, oxidation, and reduction at the CoFeB/MgO interface studied by soft x-ray absorption spectroscopy and magnetic circular dichroism Appl. Phys. Lett. 96, 092501 (2010); 10.1063/1.3332576 A nondestructive analysis of the B diffusion in Ta–CoFeB–MgO–CoFeB–Ta magnetic tunnel junctions by hard x-ray photoemission Appl. Phys. Lett. 96, 072105 (2010); 10.1063/1.3309702
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APPLIED PHYSICS LETTERS 94, 082110 共2009兲
Study of boron diffusion in MgO in CoFeB円MgO film stacks using parallel electron energy loss spectroscopy Sankha S. Mukherjee,1,a兲 David MacMahon,2 Feiming Bai,1 Chih-Ling Lee,3 and Santosh K. Kurinec1 1
Microelectronic Engineering, Rochester Institute of Technology, Rochester, New York 14623, USA Micron Technology, Manassas, Virginia 20110, USA 3 Veeco Instruments Inc., Fremont, California 94538, USA 2
共Received 2 January 2009; accepted 9 February 2009; published online 27 February 2009兲 Boron diffusion in MgO has been investigated in annealed film stacks of sputtered CoFeB兩MgO using transmission electron microscopy and parallel electron energy loss spectroscopy. The analyses show significant B movement when the films are annealed, with the formation of BOx complexes. Characteristic diffusion lengths have been estimated in films annealed at the commonly employed temperature range of 300– 400 ° C for the fabrication of magnetic tunnel junctions. An activation energy of 1.3 eV 共⫾0.4 eV兲 has been extracted from these data that represent B diffusion in MgO through vacancies and defect states mediated by the formation of BOx complexes. © 2009 American Institute of Physics. 关DOI: 10.1063/1.3090035兴 Magnetic tunnel junctions with a MgO tunneling layer have been proposed for the use in memory devices after theoretical predictions of significantly high tunneling magnetoresistance 共TMR兲 were reported.1,2 Steady improvements in the TMR have been achieved over the years. Recently, TMRs of 500% at room temperature and 1010% at 5 K for CoFeB兩MgO兩CoFeB structures have been achieved.3 The sputter-deposited CoFeB film is amorphous and subsequently crystallizes on annealing using MgO as a template for forming a continuous 共100兲 out-of-plane oriented crystal structure, which is critical to obtain high TMR.4 It is well known that the diffusion of B out of the CoFeB is necessary for proper crystallization of the CoFeB.5 B diffusion in MgO from CoFeB has been reported in the literature,6 however, its behavior at normally employed anneal temperatures has not been fully investigated. Typically employed MgO thicknesses of 1 – 3 nm in current technologies are generally insufficient to measure the diffusion characteristics of the diffusing species. In order to investigate B diffusion in MgO, special samples were prepared that consisted of several thin 共2 nm兲 MgO layers sandwiched between Co40Fe40B20 layers, and a 25 nm thick monitor MgO layer adjacent to the top CoFeB layer of the film stack. The samples were prepared by sputter deposition using a multitarget physical vapor deposition tool on oxidized silicon wafers. Samples were annealed in a N2 ambient in the temperature range of 300– 400 ° C for 1 h. Boron diffusion in MgO has been studied using transmission electron microscopy 共TEM兲 and parallel electron energy loss spectroscopy 共PEELS兲 analyses on the samples carefully prepared by focused ion beam technique. A Hitachi HD-2300A STEM fitted with a Gatan Enfina PEELS spectrometer without a monochrometer or an aberration corrector was used. A probe diameter of approximately 4 – 5 Å, a probe current of approximately 150 pA, a convergence semiangle of 10.6 mrad, and a PEELS acceptance semiangle of 21.8 mrad were used. The spectroscope dispersion was set to 0.5 eV per channel with a total of 1340 parallel channels, resulting in an energy resolution of approximately 1 eV. a兲
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Figure 1 shows a STEM cross section of the film stack investigated in this study showing the monitor layer and two thin MgO layers of the as-deposited stack. Labels a, b, and c are three locations within the sample whose B-K and O-K energy loss spectra are shown in Fig. 2 for the sample annealed at 395 ° C. It can be observed in Fig. 2共a兲 that the B-K edge deep within the MgO monitor layer shows very low 共if any兲 B signal, indicating that the B has not been able to diffuse deeply into the MgO layer. In the PEELS signal closer to the interface, there is a strong peak at 193 eV indicating the oxidation of the B atoms,6 which is true even for the as-deposited samples. In the CoFeB regions, however, no distinct B-K peaks are observed, indicating metallic B in these regions.6 The PEELS data of the O-K edge are shown in Fig. 2共b兲. It can be seen that the O-K edge is distinctly different from that of a standard MgO sample, with the presence of a large peak at 530 eV, especially farther away from the CoFeB兩MgO interface. This peak has been observed by Cha et al.6 and has been attributed to gap states resulting from vacancies and other structural defects. All samples, including the unannealed sample, have B-K and O-K edge distributions similar to that shown in Fig. 2. PEELS line scans were performed at different annealing temperatures, starting deep within the MgO monitor layer and ending in the second CoFeB layer. Assuming that the intensities are proportional to the concentration of the atoms within a particular region, a study of the concentration as a function of distance was carried out. Normalized B intensi-
FIG. 1. TEM cross-section image of an as-deposited sample, showing the capping MgO layer in the top right of the figure, and several layers of CoFeB and MgO toward the bottom left of the figure.
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FIG. 4. Plot of boron doffusion coefficient in MgO as a function of temperature.
FIG. 2. B K edge 共a兲 and O K edge 共b兲 at a location deep inside the MgO capping layer 共open squares兲, near the interface 共solid line兲 and inside the CoFeB layer 共open circles兲 for the sample annealed at 395 ° C.
ties at 193 and 230 eV and the O intensity around 536 eV energy loss regions, as well as the 530 eV gap-state intensities, are plotted as a function of distance in Fig. 3. Two separate energies have been used for the B concentration so that they may serve to corroborate one another. A strong 530 eV O-K edge appears to be present in regions deficient
in B, which becomes significantly smaller in regions in which B is present, similar to previous observations.6 This suggests a strong interaction between B and O, and further indicates that the gap states are significantly more pervasive than had been previously believed, since previous measurements have only been performed in MgO layers containing copious concentrations of B. This is also strongly indicative of B diffusion mediated mainly through vacancies and structural defects via BOx complex formation. The effect of these gap states as well as the effect of BOx complexes on the electronic configuration of MgO needs to be theoretically investigated. Using the O-K edge intensity as a reference, the B diffusion into the monitor MgO layer is estimated for different annealing temperatures. On a logarithmic scale of intensities 共not shown兲, the concentration profiles appear to follow a complementary error-function characteristic, similar to that found in constant-source diffusion profiles. Assuming that the normalized B concentration reaches a tenth of its maximum value a unit distance 共normalized to half the diffusion length兲 away from the CoFeB兩MgO edge,7 diffusion lengths have been obtained as 2.8, 5.1, and 6.3 nm for annealing temperatures of 350, 385, and 395 ° C, respectively, for 60 min each. These data show an Arrhenius relationship for the diffusion coefficient as shown in Fig. 4 providing an activation energy of 1.3⫾ 0.4 eV with a pre-exponential factor of 9.5⫻ 10−8 cm2 / s. The spread in the activation value takes into account the PEELS measurement resolution. The activation energy of 1.3 eV may represent B diffusion in MgO through vacancies and structural defects at these temperatures. Although this value of diffusivity is empirically obtained in an effort to understand nanoscale diffusion phenomena, it provides a model for accounting for B diffusion in these and similar structures. The authors acknowledge the support provided by the National Science Foundation, through the Grant No. EEC530575.
FIG. 3. The variation in the normalized intensity of the oxygen peak around 1 W. H. Butler, X.-G. Zhang, T. C. Schulthess, and J. M. MacLaren, Phys. 536 eV 共solid line兲, the gap state energy loss at 530 eV 共dashed lines兲, and Rev. B 63, 054416 共2001兲. for boron around 193 eV 共open squares兲 and 230 eV 共open circles兲, as a 2 J.to Mathon and A. Phys. Rev. B 63, 220403 共2001兲.Downloaded to IP: function of PEELS scan distance. in the article. Reuse of AIP content is subject This article is copyrighted as indicated the terms at: Umerski, http://scitation.aip.org/termsconditions.
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