Depth-resolved chemical speciation of a ${\text{W-B}}_4\mathrm{C}$ multilayer structure

We report results of simultaneous x-ray reflectivity and grazing incidence x-ray fluorescence measurements in combination with x-ray standing-wave-assisted depth-resolved near-edge x-ray absorption measurements to reveal new insights on chemical speciation of W in a ${\text{W-B}}_4\mathrm{C}$ multilayer structure. Interestingly, our results show the existence of various unusual electronic states for the W atoms especially those near to the surface and interface boundary of a thin film medium as compared to those of the bulk. These observations are found to be consistent with the results obtained using first-principles calculations. Unlike the conventional x-ray absorption measurements the present approach has an advantage that it permits the determination of the depth-resolved chemical nature of an element in the thin-layered materials at atomic length scale resolution.

Figure 1

A schematic representation showing the x-ray reflection from a ${\text{W-B}}_4\mathrm{C}$ multilayer structure consisting of $N=15$ bilayer repetitions. In this figure we have also depicted a buried interface layer of W inside the multilayer medium, on top of a Si substrate.

Figure 2

Measured and fitted XRR and $\text{W-L}\alpha$ fluorescence profiles for the ${\text{W-B}}_4\mathrm{C}$ multilayer structure comprising $N=15$ bilayer repetitions at an incident x-ray energy of 10230 eV. (a) XRR profile and (b) $\text{W-L}\alpha$ fluorescence profile.

Figure 3

X-ray field intensity (EFI) distribution inside a ${\text{W-B}}_4\mathrm{C}$ periodic multilayer structure computed using best-fitted parameters obtained from the XRR and $\text{W-L}\alpha$ fluorescence measurements at an incident x-ray energy of $E=10230\mathrm{eV}$. In this figure, we have also marked various dotted vertical lines showing different incident angles that have been chosen for depth selective XANES measurements of the ${\text{W-B}}_4\mathrm{C}$ multilayer structure.

Figure 4

Measured and fitted XRR profiles for a ${\text{W-B}}_4\mathrm{C}$ multilayer structure comprising $N=15$ bilayer repetitions at incident x-ray energies of 10192–10230 eV.

Figure 5

Determined optical constants (a) $\delta$ and (b) $\beta$, and (c) the ratio $\beta /\delta$ of W from the x-ray reflectivity measurements in a ${\text{W-B}}_4\mathrm{C}$ multilayer structure comprising $N=15$ bilayer repetitions in an x-ray energy region of 10192–10230 eV. The values of optical constants calculated from the Henke table are also given for comparison.

Figure 6

X-ray field intensity (EFI) distribution inside a ${\text{W-B}}_4\mathrm{C}$ periodic multilayer structure computed using best-fitted parameters obtained from the XRR and $\text{W-L}\alpha$ fluorescence measurements and determined optical constants ($\delta ,\beta$) at incident x-ray energies of (a)10205 eV, (b) 10207 eV, (c) 10212 eV, and (d) 10218 eV. One can clearly observe that the proving depth volume of the XSW wave field inside the ${\text{W-B}}_4\mathrm{C}$ multilayer structure across the ${\text{W-L}}_3$ absorption edge energy preserves a more or less constant volume, except for an angular shift of $\theta \sim 0.{01}^{\circ }$ at the high energy side ($E\sim 10230\mathrm{eV}$) of the ${\text{W-L}}_3$ edge.

Figure 7

X-ray standing-wave-induced XANES profiles of the ${\text{W-B}}_4\mathrm{C}$ multilayer structures measured at the ${\mathrm{L}}_3$ edge of W. (a) Measured XANES profile of a multilayer structure consisting of $N=15$ bilayer repetitions using the BL-16 beamline of the Indus-2 synchrotron facility. (b) ${\text{W-B}}_4\mathrm{C}$ B multilayer structure with $N=10$ bilayer repetitions measured at the IAEA GIXRF-XRR experimental facility (IAEAXspe) operated at the XRF beamline of Elettra Sincrotrone Trieste (BL-10.1L). On the right-hand side, (c) a schematic illustration depicts the real space distribution of W atoms at the surface-interface boundary as well as inside the bulk thin film medium.

Figure 8

X-ray standing-wave-induced XANES profiles of (a) a ${\text{W-B}}_4\mathrm{C}$ bilayer structure and (b) a W single thin film structure at the ${\mathrm{L}}_3$ edge of W. In the insets of panels (a) and (b) schematic model structures of a ${\text{W-B}}_4\mathrm{C}$ bilayer structure and a W single thin film structure are given. On the right hand side, computed x-ray field distribution near ${\text{W-L}}_3$ edge energy ($E\sim 10208\mathrm{eV}$) is shown inside the two mediums.

Figure 9

First-principles simulated (a) partial density of states of W ($5d$) and (b) core level (${\mathrm{L}}_3$) absorption spectra of pure bulk bcc W.

Figure 10

Measured GIXRD pattern of a ${\text{W-B}}_4\mathrm{C}$ multilayer consisting of $N=15$ bilayer repetitions at an incident x-ray energy of 15.5 keV. During the measurements the incidence angle of the primary x-ray beam was fixed at $1.0^{\circ }$.