Effect of Thermal Annealing on Boron Diffusion, Micro-structural, Electrical and Magnetic properties of Laser Ablated CoFeB Thin Films

We report on Boron diffusion and subsequent crystallization of Co$_{40}$Fe$_{40}$B$_{20}$ (CoFeB) thin films on SiO$_2$/Si(001) substrate using pulsed laser deposition. Secondary ion mass spectroscopy reveals Boron diffusion at the interface in both amorphous and crystalline phase of CoFeB. High-resolution transmission electron microscopy reveals a small fraction of nano-crystallites embedded in the amorphous matrix of CoFeB. However, annealing at 400$^\circ$C results in crystallization of CoFe with \textit{bcc} structure along (110) orientation. As-deposited films are non-metallic in nature with the coercivity (H$_c$) of 5Oe while the films annealed at 400$^\circ$C are metallic with a H$_c$ of 135Oe.


I. INTRODUCTION
(Co x Fe 1−x ) 80 B 20 alloys have been under extensive research focus due to high tunneling magnetoresistance (TMR)and perpendicular magnetic anisotropy (PMA) observed in thin films of this material combined with ultrathin MgO layer [1][2][3][4]. A controlled transition of CoFeB from amorphous to crystalline phase is a necessary condition for the observation of giant TMR effect [1][2][3]. By post deposition thermal annealing in vacuum, TMR of the CoFeB based magnetic tunnel junctions (MTJs) increases abruptly [3]. Furthermore, recent results [4] have shown that although CoFeB/MgO system is widely used for in-planeanisotropy MTJs, it can also meet the requirements of high thermal stability and low induced current density magnetization switching for high performance perpendicular MTJs. Apart from this, CoFeB has lower coercivity (H c ) [5], high spin dependent scattering [6], stronger spin tunneling effect [1], and therefore, supports a higher output signal for particular giant magnetoresistance (GMR) and TMR ratios in CoFeB electrode based MTJs.
From the reported results, there is a sharp contradiction in opinions on TMR ratios due to Boron diffusion at the interface between ferromagnetic electrode and tunnel barrier. Earlier studies [7][8][9] have shown that improvement of TMR results from Boron diffusion at the interface of CoFeB and tunnel barrier during annealing because it forms an energetically favorable poly-crystalline Mg-B-O layer in case of CoFeB/MgO interface, whereas other groups [10][11][12][13] have claimed that Boron enrichment in the barrier is detrimental to TMR because it significantly suppresses the majority-channel conductance. Furthermore, crystallization of amorphous CoFeB during thermal annealing at the interface has been reported to be sensitive to its TMR effect due to enhance coherent tunneling [2,14,15]. However, crystallization of CoFeB electrode and change in the barrier properties due to Boron diffusion at the interface during annealing have not been fully characterized and understood till now. Therefore, it is important to study in detail of what happens to Boron at the interface of SiO 2 /CoFeB as a function of the degree of crystallization with respect to its original amorphous phase.
To date, CoFeB thin films have been grown by using different sputtering methods such as dc, rf and Ion beam sputtering [16,17]. Electron beam evaporation of CoFeB thin film has also been employed successfully [7]. However, there are no reports on pulsed laser deposited (PLD) thin films and junctions. The electron energy-loss spectroscopy and x-ray photo electron spectroscopy of CoFeB/MgO/CoFeB MTJs reveals that the existence of Boron as BO x in the barrier layer depends on the deposition method [18,19]. In the present study, we have explored PLD technique as it offers a unique advantage for the growth of multi-elemental films of desired stoichiometry of the elements with widely varying vapour pressures. CoFeB films can be either amorphous or nano crystalline depending on the thermal treatment, composition of the elements and film thickness [20]. We carried out Secondary ion mass spectroscopy (SIMS) depth profile measurements to investigate the diffusion of constituent elements especially the Boron. High-resolution transmission electron microscopy (HRTEM) observations reveal that CoFeB crystallized upon annealing at 400 • C.
The present work also focuses on to correlate annealing effects on micro-structural, electrical transport and magnetic properties of CoFeB thin films grown on amorphous layer of SiO 2 coated on (001) Si substrate.

II. EXPERIMENTAL
A KrF excimer laser that produces laser pulses of width ≈20 ns and wavelength of 248 nm operated at a repetition rate of 10 Hz was used to ablate a stoichiometric target of Co 40 Fe 40 B 20 (at%). The intensity of laser plume produced an aerial energy density of 4 Jcm −2 /pulse. The PLD deposition chamber was evacuated to the base pressure of 2x10 −7 mbar prior to deposition. The deposition pressure was kept at 2.5x10 −3 mbar. 6N purity Ar gas was used as a buffer gas. Under these conditions, a deposition rate of 0.4Ås −1 was realized. The films were annealed immediately after the deposition in the same chamber at 200, 300 and 400 • C for 1 hour under high vacuum. Our PLD grown films are highly consistent and reproducible. Crystallographic structure of the films was characterized using a PANalytical X'Pert PRO MRD X-ray diffractometer with CuKα 1 radiation. Elemental analysis was carried out on a Rigaku ZSX Primus II Wavelength Dispersive X-ray Fluores- The average crystallite size of the films is determined by the Scherrer formula is about 20 nm for annealed films at 400 • C. Strain induced in the film is calculated by Williamson-Hall method using the formula: β cos θ = κλ τ + η sin θ, where β is the full width at half-maximum in radians, θ is the Bragg diffraction angle of the peak, κ is the Scherrer constant, λ is the wavelength of the X-rays and τ is the crystallite size estimated from HRTEM results. The films have tensile strain of -2.44x10 −4 generated due to vacancy-type imperfections known as Schottky defects in the lattice created presumably by leaving Boron into the interface.

B. SIMS depth profiles
A uniform distribution of Co and Fe is clearly seen across the thickness of as-deposited and annealed film (Fig.2). The peak of Boron intensity profile (black in color) indicates strong Boron segregation at the apex of the interface of SiO 2 /CoFeB. It is interesting to note that while sputtered CoFeB films suggests the presence of Boron in amorphous phase [21,22], our PLD grown films in as-deposited state confirms that Boron segregates and resides at the interface leaving CoFe in amorphous phase. The Si signal starts rising at around 30 nm and becomes constant at 50 nm which indicates an out-diffusion of Si into the film structure.
The Cobalt, Iron and Boron signals tends to reduce after 40 nm which is typically the film thickness but they are found to diffuse into the substrate as well. Si diffusion into the film is comparatively less and it becomes constant at 44 nm in the annealed film. The diffusion of Cobalt, Iron and Boron into the substrate is observed to be more in the annealed sample as it is clearly visible in the depth profile. The reported results on first principle calculations [10] stated that rather than inside the CoFe matrix, segregation of Boron at the interface is energetically favorable which is consistent for our PLD grown amorphous films.
The HRTEM micrograph of the as-deposited samples in general shows a feature-less contrast of amorphous phase in Fig.3(a). In some selective regions dispersed in the microstructure, little crystalline zones have been noted [ Figs.3(b) and (c)]. Further, the microstructure observed in these regions reveals some interesting ultra-fine details. In Fig.3(c), it is evident that the crystalline regions are in selectively embedded state in the amorphous matrix. A faint contrast of atomic planes in Fig.3(c) further authenticates the significant fraction of amorphous phase surrounding the crystalline phase, which is negligible compared to large fraction of amorphous structure in the matrix. A region surrounded by white dotted line displays a set of moiré fringes evolved due to overlapping of ultra-thin tiny crystals (Fig.3c).
The careful measurement of inter planar spacings (d hkl ) between the planes in Fig.3 to that of (200) planes (Fig.3c).  Fig.4(d)] further elucidates a set of atomic planes in reciprocal space with inter-planar spacings of 0.29 and 0.14 nm.

C. Electrical transport studies
Electrical transport properties of CoFeB films in their as-deposited state and annealed at 400 • C are shown in Fig.5. Resistivity data has been normalized to its value at 273K.
The room temperature resistivity of as-deposited films are about ≈70µΩcm and for samples annealed at 400 • C is observed to be ≈8µΩcm respectively. The abrupt increase in resistivity values for amorphous films and observed non-metallic behavior could be due to intragrain tunneling and the SiO 2 inclusions into the film was reported previously in granular CoFeB-SiO 2 amorphous thin film system [23]. Our HRTEM results indicate the presence of nano crystalline regions in the amorphous matrix of CoFeB, these crystalline regions are well separated and leading to the absence of metallic channels support the observed transport behavior. The resistivity is found to reduce gradually on decreasing temperature from 300K followed by a clear increase in its value below 22K for the film annealed at 400 • C. A fit to the part of our experimental data below the resistivity minimum for annealed sample using the following empirical relation ρ(T ) = β 0 + βlnT, 5K ≤ T ≤ 22K is shown as a solid line in the figure. Clear agreement with experimental data suggests finite magnetic contribution as has been observed in several amorphous ferromagnetic alloys at low temperature [24]. The detailed analysis of our transport measurements is under progress.

Magnetization [M(T)] measurement performed on as-deposited and annealed CoFeB films
at different temperatures are shown in Fig.6. The ferromagnetic hysteresis loops after substrate correction for in-plane and out-of-plane orientations confirmed that films exhibit inplane easy axis of magnetization. In the annealed film, the easy axis loop is squared shape, while the hard axis of the film displays a much slanted curve indicating the presence of magnetic anisotropy [25]. The as-deposited samples and samples annealed up to 300 • C shows H c of ∼10Oe. The films annealed at 400 • C are found to show increase in H c from 5Oe to 135Oe. A sharp increase in H c at annealing temperature of 400 • C is a clear evidence of crystallization of amorphous CoFeB films.

IV. CONCLUSIONS
In summary, we have grown thin film of CoFeB using pulsed laser ablation of an alloy target. HRTEM together with resistivity measurements reveal that annealing at 400 • C increases the metallicity of the film by forming nano-crystallites. The sudden increase in H c from 5Oe to 135Oe after annealing at 400 • C also confirms the crystallization of CoFeB.
Magnetic hysteresis loops of in-plane and out-of-plane measurements reveal that our films have in-plane easy axis of magnetization. We believe Boron segregation at the interface  Inset shows the room temperature magnetic hysteresis loops for 400 • C annealed film measured along parallel and perpendicular field direction with respect to film plane.