Hard x-ray photoemission study of near-Heusler Fe${}_x$Si${}_{1-x}$ alloys

The structural and electronic properties of epitaxial and amorphous Fe${}_x$Si${}_{1-x}$ alloys with $x$ $=$ 0.72 and 0.67 near the binary Heusler composition of $x$ $=$ 0.75 were determined using hard x-ray photoelectron spectroscopy (HXPS). By performing the measurements at a photon energy of 5950.3 eV, the bulk-sensitivity of the measurement is enhanced by a factor of 4–7 compared to conventional soft x-ray photoelectron spectroscopy at about 1000 keV. HXPS probes, on average, as far as 76 Å into the Fe${}_x$Si${}_{1-x}$ samples. Via core-level spectra, it is found in the amorphous alloy that, in spite of the disordered structure that could lead to a broad distribution of chemical environments, the Si environment is mostly unique. Valence-band spectra reveal a clear distinction between the contributions of the two inequivalent Fe sites of the most highly ordered ($x$ $=$ 0.72, $D$0${}_3$) epitaxial sample. The valence-band spectra are compared to results of fully relativistic coherent potential approximation calculations performed in the framework of the one-step model of photoemission, which reveal details of the atomic-orbital makeup of various features, and generally exhibit good agreement with experiment.

Figure 1

TEM image and electron diffraction pattern (inset) for the amorphous film $x$ $=$ 0.67, designated as $a$-Fe${}_{0.67}$Si${}_{0.33}$. The image displays an ∼5 nm diam nanocrystal (circled) in an amorphous matrix, which is estimated to represent ∼80% of the sample surface area. In addition to strong spots from the 〈110〉-oriented Si substrate, the electron diffraction pattern shows a broad diffuse ring originating from the amorphous film, as well as some random spots from nanocrystals.

Figure 2

X-ray diffraction patterns for epitaxial samples designated as epi-Fe${}_{0.72}$Si${}_{0.28}$ and epi-Fe${}_{0.67}$Si${}_{0.33}$. (a) θ-2θ scans for $x$ $=$ 0.72 (black) and $x$ $=$ 0.67 (red) epitaxial films. (b) ϕ scans on the in-plane (220) peak for $x$ $=$ 0.72 (black) and the (110) peak for $x$ $=$ 0.67 (red); fourfold symmetry is observed, confirming epitaxy.

Figure 3

(a) Si 1$s$ core peak spectra obtained for all three samples: epi-Fe${}_{0.72}$Si${}_{0.28}$, epi-Fe${}_{0.67}$Si${}_{0.33}$, and $a$-Fe${}_{0.67}$Si${}_{0.33}$. Two prominent features are the main Si 1$s$ peak and the Si oxide peak. (b)–(d) O 1$s$ peaks obtained for all three samples at the electron take-off angles (TOA) of 88° and 45°. The origins of the two components are identified to be the Si oxide layer and the solid C/O contaminant layer on the surface. (e) C 1$s$ peaks, originating from the solid C/O contaminant layer on the surface, obtained for all three samples.

Figure 4

(a) Experimental Fe 2$p$ spectra collected for all three samples, which do not exhibit any evidence of iron oxidation. (b) Fe 3$s$ and Si 2$p$ spectra obtained for all three samples. Fe 3$s$ also does not show evidence of oxidation.

Figure 5

(a) Extended valence-band spectra obtained for all three samples, including the valence-band features labeled $A$–$D$ and the subvalence feature, later decomposed into four components ($P$1–$P$4), and normalized to the high-binding-energy inelastic background tail. (b) Higher-binding-energy valence feature, background-subtracted and fitted, with a dominant peak ($P$2) corresponding to the photoemission from O 2$s$ levels in the SiO${}_2$ layer.

Figure 6

Total calculated densities of states for ordered $D$0${}_3$ Fe${}_3$Si (used as the model for epi-Fe${}_{0.72}$Si${}_{0.28}$), partially disordered FeFe${}_{0.3}$Si${}_{0.7}$ in the $B$2 structure (epi-Fe${}_{0.67}$Si${}_{0.33}$), and for chemically disordered bcc Fe${}_{0.67}$Si${}_{0.33}$, used for the amorphous $a$-Fe${}_{0.67}$Si${}_{0.33}$ sample.

Figure 7

Calculated element- and site-resolved angle-integrated XPS spectra for ordered $D$0${}_3$ Fe${}_3$Si (used to model epi-Fe${}_{0.72}$Si${}_{0.28}$) [cf. Fig. 6a], including 230 meV broadening due to the total experimental resolution.

Figure 8

Room-temperature XPS spectra for (a) ordered $D$0${}_3$ Fe${}_3$Si (used to model epi-Fe${}_{0.72}$Si${}_{0.28}$), (b) partially disordered FeFe${}_{0.3}$Si${}_{0.7}$ in the $B$2 structure (used to model epi-Fe${}_{0.67}$Si${}_{0.33}$), and (c) chemically disordered bcc $a$-Fe${}_{0.67}$Si${}_{0.33}$ at a photon energy of 5950.3 eV compared to calculated one-step theoretical spectra shown in gray. The black curves represent a broadening of the theoretical spectra with a 230 meV (FWHM) Gaussian to simulate the experimental resolution. Experimental data, normalized to the high-binding-energy inelastic background tail, are shown in red.