Spectroscopic ellipsometry and magneto-optical Kerr effect spectroscopy study of thermally treated Co60Fe20B20 thin films

We report the optical and magneto-optical properties of amorphous and crystalline Co60Fe20B20 films with thicknesses in the range of 10 nm to 20 nm characterized using spectroscopy ellipsometry (SE) and magneto-optical Kerr effect (MOKE) spectroscopy. We derived the spectral dependence of the dielectric tensor from experimental data for samples prior and after annealing in vacuum. The features of the dielectric function can be directly related to the transitions between electronic states and the observed changes upon annealing can be ascribed to an increase of the crystalline ordering of CoFeB.


Introduction
In recent years, CoFeB films have been extensively investigated due to the interest in understanding spin-dependent phenomena [1,2] and their suitability for applications such as magnetic recording media, sensors or microwave applications [3][4][5]. In the particular case of tunneling magnetoresistance (TMR) devices, large magnetoresistance (MR) yields were predicted [6] and observed [7] in magnetic tunnel junctions (MTJs) using (0 0 1)-textured MgO as a tunnel barrier. The amorphous nature of sputtered CoFeB allows a smooth growth of these films on the MgO barrier with minimal roughness, and a near-epitaxial relationship between MgO and CoFeB can be achieved upon a post-deposition thermal treatment [8]. In this process, the (0 0 1)-textured MgO serves as a template for the crystallization of the CoFeB layers, resulting in high TMR values.
As the current trends in spintronic devices demand new physical principles to provide scalability, there has been an increasing interest in the interaction between magnetism in CoFeB films and light. This includes studies of the optical driven magnetization dynamics on very short-time scales [9] or the development of all-optical switching mechanisms for spin-transfer-torque magnetic random-access memory [10]. As such, there is significant interest in studying the optical properties of CoFeB.
Spectroscopic ellipsometry (SE) in combination with magneto-optical Kerr effect (MOKE) spectroscopy can provide S Supplementary material for this article is available online (Some figures may appear in colour only in the online journal) Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. significant information on the electronic structure, as well as the crystallographic orientation and magnetization direction. There are a number of experimental and theoretical studies focusing on the optical properties of elemental Co and Fe [11][12][13][14][15] and also a few dedicated to Co-Fe alloys [16][17][18]. However, only limited attention has been paid to CoFeB, regarding solely the ellipsometry evaluation of its optical constants [19] or the comparison of optical and magneto-optical properties of granular nanostructures [20].
In this work, the optical and magneto-optical properties of Co 60 Fe 20 B 20 thin films, with thicknesses below and equal to 20 nm grown on Si covered with thermally grown SiO 2 , are investigated after annealing in vacuum by SE and MOKE spectroscopy. The changes observed in the dielectric function upon thermal annealing can be correlated to the crystallization of the films, while the increase in the MOKE signal indicates an increase in the magnetization driven by the annealing process.

Sample preparation and structural characterization
Co 60 Fe 20 B 20 thin films with nominal thicknesses (determined using a quartz crystal microbalance) ranging from 10 nm to 20 nm covered by a 3 nm gold passivation layer were deposited by dc magnetron sputtering on thermally oxidised silicon substrates. The depositions were performed at room temperature (RT) with a base pressure below 2 × 10 −4 Pa and a working pressure of 0.35 Pa. The samples were annealed for 30 min in vacuum at 350°C and subsequently again for 30 min at 400°C. The first annealing step was performed at 350°C as the transformation of the film from amorphous into a crystalline bcc Co-Fe phase is expected to occur above 325°C [21]. The characterization of the samples with the methods described below, was performed in the as-deposited state and repeated after each of the subsequent annealing steps.
X-ray reflectivity (XRR) and x-ray diffraction (XRD) measurements were performed on a 3000 PTS diffractometer from SEIFERT-FPM (today GE INSPECTION Technologies GmbH), allowing the film thickness and crystallinity to be assessed, independently of the ellipsometric model described below. The thickness of every single layer was determined through a simulation based on the experimental XRR data with the open-source software GenX [22]. In order to probe the crystallization of Co 60 Fe 20 B 20 as-deposited and postannealed layers x-ray diffractograms were recorded.
The surface of the samples was studied by atomic force microscopy (AFM) in AC mode with an Agilent 5500 Scanning Probe Microscope, using reflective Si AFM probes with a guaranteed tip radius below 10 nm, and scanning electron microscopy (SEM) on a ZEISS Supra 60 workstation.

Spectroscopic ellipsometry
SE data were recorded with a J A Wollam Co., Inc. M-2000 ellipsometer between 0.74 eV and 5.03 eV at incidence angles of 50°, 55°, 60°, 65°, and 70°. The experimentally measured SE data ψ and Δ, are related to the ratio of amplitudes of p-and s-polarized components of the light reflected on the sample and their phase difference, respectively: The optical constants for each sample were determined through a Kramers-Kronig consistent regression modelling of the ellipsometric parameters ψ and Δ [23]. In the optical layer stack model, the layer thicknesses obtained from the XRR measurements were used as input parameters for the fitting and kept fixed throughout the study. On top of the Au layer, a surface roughness layer (where the dielectric function of the cap layer is mixed with 50% void) based on the RMS roughness values obtained from the AFM measurements was used. For the dielectric function, a multi-sample analysis based on a parametric model consisting of one Drude free electron model and a series of five Gauss-Lorentz oscillators was employed in order to represent the fine structure of the optical dispersion in the Co 60 Fe 20 B 20 in the entire investigated spectral range. In the multi-sample analysis, the dielectric function of the samples with thicknesses from 10 nm to 20 nm was considered to be identical. This model was then further adjusted to respond to the structural changes induced by the annealing steps. The data evaluation for all ellipsometry experiments was performed using the software WVASE32 ™ .

MOKE spectroscopy
The polar Kerr rotation θ K and ellipticity η K spectra were measured with an indigenous developed MOKE spectrometer. The spectrometer measures simultaneously both rotation and ellipticity of the light reflected on the sample at near normal incidence by means of a polarization modulation method using a piezo-birefringent modulator also known as photoelastic modulator (PEM). A detailed discussion of this measurement technique can be found elsewhere [24]. All the spectra were measured at RT in a photon energy range of 1.5 eV-5 eV with a magnetic field of ~1 T applied perpend icularly to the plane of the layers.

Structural characterization
The thickness of the Co 60 Fe 20 B 20 /Au bilayers was initially verified using XRR measurements on the as-deposited samples, yielding differences below 1 nm with respect to the nominal values. The best fit simulation results for the Co 60 Fe 20 B 20 (20 nm)/Au (3 nm) sample are shown in figure 1.
The layers were further characterized using XRD before and after annealing at 350°C, and 400°C and the x-ray diffractograms are shown in figure 2. For the as-deposited state of the sample, only the Au (1 1 1) and Si (substrate) peaks at 38.2° and 69.1°, respectively, were observed, indicating that the Co 60 Fe 20 B 20 film is amorphous. Even after annealing at 350°C, no peak corresponding to the CoFe crystalline phase was detected, suggesting that the Co 60 Fe 20 B 20 layer is still in an amorphous phase or that the eventually formed crystallites are not large enough to be detected by our XRD equipment. A further annealing step at 400°C resulted in the occurrence of a well distinguishable peak around 45°, characteristic for the CoFe(1 1 0) crystalline phase. For this peak, a crystallites size of (8.9 ± 2.0) nm was calculated using the Scherrer formula [25].
The values of the root-mean-square (RMS) roughness were obtained from the AFM data, before and after annealing, as any changes in this parameter need to be accounted for in the ellipsometric modelling. The initial roughness was measured to be around 0.5 nm, and a substantial increase was registered upon annealing to 1.5 nm (350°C) and 2 nm (400°C). These changes in the roughness and grain structure upon annealing were further confirmed by SEM images.

Optical and magneto-optical properties
The measured ψ and Δ spectra and the fitted spectra at an angle of incidence (AOI) of 60° for all the samples before annealing are shown in figure 3. For the fitting, the layer thicknesses of CoFeB and Au were kept constant at the values determined by XRR (see table 1) and the thickness of SiO 2     [27] and Si [28] were used, leaving only the optical constants of Co 60 Fe 20 B 20 as the fit parameters. The quality of the fit was evaluated based on the mean square error (MSE) values, calculated from N, C and S parameters (Mueller matrix components for isotropic material) [29], along with the agreement between experimental values and simulated curves. MSE values of less than ten were achieved for all results presented in this work.
The changes found in the real (ε 1 ) and imaginary (ε 2 ) parts of the dielectric function of Co 60 Fe 20 B 20 obtained from the SE measurements, before and after annealing, are shown in figure 4. As mentioned previously, all thicknesses as well as the optical constants of the substrate (Si/SiO 2 ) and capping layer (Au) were kept constant throughout the modelling, considering only a change in the surface roughness upon annealing, as measured by AFM. Hence, only the optical constants of CoFeB were fitted.
For the as-deposited films, the dielectric function relates well to the typical line shape of the optical function reported for amorphous Co 20 Fe 60 B 20 films with thicknesses between 10 nm and 40 nm grown on Si/ SiO 2 (1000 nm), i.e. with very broad features [19]. It should be noted that thicker films of Co 20 Fe 60 B 20 (100 nm) investigated here exhibit even lower spectral features (see supplementary material, figure S1 (stacks.iop.org/JPhysCM/32/055702/mmedia)). While for the modelling of thicker films one Drude oscillator and two Gauss oscillators located at 0.5 eV and 2.2 eV were employed, in the case of the multi-sample analysis performed for the films thicknesses in the range from 10 nm to 20 nm additional oscillators located at 1.4 eV, 3.3 eV, 3.7 eV and 4.6 eV had to be employed in order to simulate the experimental data. This apparent discrepancy between the thick and thin film optical properties might have several reasons: a possible oxidation of the top layer of Co 60 Fe 20 B 20 would have a larger contrib ution to the optical response of the thin compared to the thicker films. In addition, a granular structure of the film which is more pronounced in the case of the thinner films or the related plasmonic effects in the thin Au overlayer [30] or in the Co 60 Fe 20 B 20 might affect the thin film dielectric function.
Upon annealing of our films, two effects become visible: (i) a change in the low energy slope of ε 2 and (ii) a sharpening of the spectral features.
The low energy slope of ε 2 initially increases from the as-deposited state to the sample annealed at 350°C and then it decreases again in the sampled annealed at 400°C. The decrease in slope of ε 2 with annealing temperature corresponds to the characteristic behavior of metals upon crystallization and the corresponding reduction in the dielectric losses [31]. Liang et al [19] reported merely no changes of the Co 20 Fe 60 B 20 optical constants of a 40 nm film upon annealing at 300°C for 2 h, whereby 300°C is below the crystallization temperature of CoFeB.
The most significant changes upon annealing of our films occur at 2.6 eV and 3.8 eV in ε 1 . The lineshape of the spectra obtained after annealing resembles the reported one for Co 20 Fe 60 B 20 films (2 nm) [19]. Liang et al explained such a lineshape by the optical anomaly occurring when isolated metal islands percolate into a continuous metallic layer. Extrapolating to the case of our system, such a structurerich lineshape of both ε 1 and ε 2 might be consistent with the increasing roughness of the film after annealing. A slight oxidation of the CoFeB and/or the diffusion of the B towards the film surface (as recently reported in [32]) or on the grain boundaries of CoFe upon annealing are factors that might influence the dielectric function and can hardly be taken into account by modelling. In order to account for possible oxidation of the CoFeB layer and/or a new layer of B on top of the CoFeB, we included a layer defined by the Bruggeman effective medium approximation on top of the CoFeB layer. The resulting dielectric function exhibits a similar lineshape (see supplementary material, figure S2). Nevertheless, the occurrence of a clear CoFe(1 1 0) peak in the x-ray diffractogram of the 20 nm film annealed at 400°C (see figure 2) indicates that the observed changes in the dielectric function (figure 4) are more likely to be related to the crystallisation of CoFe(B).
The electronic origin of the observed spectral features is discussed in the following. At low photon energies (NIR and visible), the optical spectra are expected to relate mostly to intraband transitions. At higher photon energies (UV) the features in the spectra relate to interband transitions, namely transitions from the occupied d-bands to unoccupied ones. A broad pronounced feature at ~2.6 eV was previously observed for Fe-Co alloys in the bcc phase [17,18] and ascribed to direct interband transitions in the minority-spin bands between occupied d-and unoccupied p-states [13]. It is worth mentioning that the response may also be influenced by interband transitions from gold typically found around 2.5 eV [33] or by plasmonic effects in Au or even in CoFeB. While the interband transitions are taken into account by the optical constants for gold [26] used in our model, the plasmonic effects were not taken into account [34]. Since in our case we have an additional degree of complexity relating to the B content of our alloy, it is not straightforward to compare to simple Co-Fe alloys, and since the complete removal of the Au signature at  In the context of pure bcc-Fe, the peaks at 2.8 eV and 4 eV were ascribed to transitions along high symmetry points of the crystal lattice [35]. It can be assumed that this is also the case for a CoFeB alloy, suggesting that the observed transitions are likely to be consistent with the presence of a bcc crystalline phase, in agreement with the presence of the CoFe(1 1 0) peak in the diffractogram of the 400°C annealed sample. In any case, because the major plasmonic effects due to gold are expected at energies below the plasma edge (2.5 eV) [34], the observed changes in the spectra are more likely to be due to the crystallization of CoFeB.
Next, we compare the change in the Kerr spectra before and after annealing in figure 5. A broad feature centred at about 2.5 eV is in this case also very pronounced, becoming clearly narrower upon annealing.
The spectra compare well with previous studies on Co-Fe alloys [18], suggesting that the B content in our alloy may have a minimal influence on the magneto-optical response transitions in the studied energy range. The amplitude of the signal improves with the annealing, similarly to the previously reported case of FePt alloys [36]. Following the interpretation in [36], the enhancement of the MOKE signal can be regarded as a sign of improvement in the magnetic ordering of the films.

Conclusions
In this work, the optical and magneto-optical properties of Co 60 Fe 20 B 20 were investigated by means of ellipsometry and MOKE spectroscopy, before and after annealing in vacuum. The dielectric function of the Co 60 Fe 20 B 20 layers (diagonal elements) was determined from a multi-sample analysis of layers with thickness from 10 nm to 20 nm. The dielectric function of the layers after annealing is consistent with previous experimental and theoretical studies on Co-Fe alloys with a bcc crystalline structure, indicating the formation of this crystalline phase in the present alloy already after an annealing step of 30 min at 350°C. The crystallization of the films was also confirmed by XRD for films annealed at 400°C. This study shows that the optical and magneto-optical spectra are sensitive to the crystallization of the Co 60 Fe 20 B 20 layers already at initial stages that are not yet detectable by XRD. Figure 5. Comparison of Kerr rotation θ K (black) and ellipticity η K (red) for Si/SiO 2 (100 nm)/Co 60 Fe 20 B 20 (20 nm)/Au(3 nm) before and after annealing.