Investigation of interface properties of Ni/Cu multilayers by high kinetic energy photoelectron spectroscopy

High kinetic-energy photoelectron spectroscopy (HIKE) or hard x-ray photoelectron spectroscopy has been used to investigate the alloying of Ni/Cu (100) multilayers. Relative intensities of the corelevels and their chemical shifts derived from binding energy changes are shown to give precise information on physicochemical properties and quality of the buried layers. Interface roughening, including kinetic properties such as the rate of alloying, and temperature effects on the processes can be analyzed quantitatively. Using HIKE, we have been able to precisely follow the deterioration of the multilayer structure at the atomic scale and observe the diffusion of the capping layer into the multilayer structure which in turn is found to lead to a segregation in the ternary system. This is of great importance for future research on multilayered systems of this kind. Our experimental data are supplemented by first-principles theoretical calculations of the core-level shifts for a ternary alloy to allow for modeling of the influence of capping materials on the chemical shifts.

Figure 1

(Color online) The $\text{Cu}2p_{3/2}$ photoemission spectra of Pt capped ${\text{Ni}}_5{\text{Cu}}_5$ measured using 2010 eV photon energy. A reference spectrum of bulk Cu (100 ML Cu) is included. The inset presents the $\text{Cu}2p_{3/2}$ CLS as a function of temperature.

Figure 2

(Color online) The $\text{Cu}2p_{3/2}$ photoelectron spectra of ${\text{Ni}}_5{\text{Cu}}_5/\text{Pt}$ (a) and ${\text{Ni}}_5{\text{Cu}}_5$ (b) measured using 2010 eV photon energy. The black line corresponds the binding energy of bulk $\text{Cu}2p_{3/2}$.

Figure 3

(Color online) The theoretical core-level shifts of (a) Cu and (b) $\text{Ni}2p_{3/2}$ and (c) $\text{Pt}4f_{5/2}$ in fcc disordered ternary CuNiPt alloys are shown above. For clarity, the small circles mark incremental differences in $10\text{at}\text{.}\%$. Isolines are plotted in steps of 0.1 eV over the alloy compositions. Note that the sides of the triangles correspond to the respective binary alloys.

Figure 4

(Color online) The $\text{Ni}2p_{3/2}$ photoelectron spectra of ${\text{Ni}}_5{\text{Cu}}_5$ measured using 2010 eV photon energy.

Figure 5

(Color online) Intensity ratios between $\text{Cu}2p$, $\text{Ni}2p$, and $\text{Pt}4f$ corelevels of ${\text{Ni}}_5{\text{Cu}}_5/\text{Pt}$ (a) and ${\text{Ni}}_5{\text{Cu}}_2/\text{Pt}$ (b).

Figure 6

(Color online) The intensity ratio of $\text{Cu}2p$ and $\text{Ni}2p$ photoelectron spectra measured at different temperatures using 2010 eV photon energy. The spectra are normalized to a common background before the $\text{Ni}2p$ level. The increased intensity of $\text{Cu}2p$ relative to the intensity of $\text{Ni}2p$ as a function of heating temperature is due to segregation of Cu.

Figure 7

(Color online) The $\text{Cu}2p_{3/2}$ photoelectron spectra of ${\text{Ni}}_5{\text{Cu}}_5$ measured using 6030 eV photon energy.

Figure 8

(Color online) A two-peak-fit model of $\text{Cu}2p_{3/2}$ photoelectron spectra of ${\text{Ni}}_5{\text{Cu}}_5/\text{Pt}$ and ${\text{Ni}}_5{\text{Cu}}_5$ at photon energy 2010 eV (a and b, respectively) and ${\text{Ni}}_5{\text{Cu}}_5$ at photon energy 6030 eV (c). The insets show how the intensity ratio of the two components in the two-peak-fit model of ${\text{Ni}}_5{\text{Cu}}_5/\text{Pt}$, ${\text{Ni}}_5{\text{Cu}}_2/\text{Pt}$ and ${\text{Ni}}_5{\text{Cu}}_5$ changes as a function of temperature.

Figure 9

(Color online) Schematic illustration of intermixing and alloying of Ni/Cu multilayers and Pt cap as a function of temperature. The changes in the chemical environment are described by histograms that are based on the two-peak-fit models of ${\text{Ni}}_5{\text{Cu}}_5/\text{Pt}$ and ${\text{Ni}}_5{\text{Cu}}_5$.