High elemental selectivity to Sn submonolayers embedded in Al using positron annihilation spectroscopy

In the present work, we demonstrate that metal layers in the submonolayer range embedded in a matrix are revealed with unprecedented sensitivity by coincident Doppler-broadening spectroscopy of the positron annihilation using a monoenergetic positron beam. The measured electron momentum distribution specific for Sn is clearly observable in $\mathrm{Al}∕\mathrm{Sn}∕\mathrm{Al}$-layered samples even at a Sn area density of as low as $7.3\times {10}^{-2}\mu \mathrm{g}∕{\mathrm{cm}}^2$ below $200\mathrm{nm}$ Al. An explanation for the high elemental selectivity for the thin Sn layers is set forward in terms of efficient positron trapping due to the changing positron affinity at the $\mathrm{Al}∕\mathrm{Sn}$-interface and quantum-dot-like positron states in Sn nanoparticles.

Figure 1

(Color online) CDBS of the $511\mathrm{keV}$ annihilation line profiles for pure Al, Sn, and the $\mathrm{Al}∕\mathrm{Sn}∕\mathrm{Al}$ layered samples with various Sn thicknesses (normalized to the same intensity). The inset shows a zoom into the high-momentum region and highlights the differences between the samples.

Figure 2

(Color online) Ratio curves of the Doppler broadened $511\mathrm{keV}$ line depicted in Fig. 1. All spectra are divided by the reference spectrum obtained for pure annealed Al. The uppermost curve shows the elemental signature of Sn. Even at an amount of $0.1\mathrm{nm}$, the signature of Sn emerges clearly. The data of the layered samples were fit by a linear combination of the photon intensities recorded for Sn and Al (solid lines).

Figure 3

(a) Implantation profile calculated for $6\mathrm{keV}$ positrons in the $\mathrm{Al}∕{\mathrm{Sn}}_{25}∕\mathrm{Al}$ sample with the positron fraction $f$ implanted in Sn (gray region). The high fraction of positrons stopped in the Sn layer reflects the high stopping power of Sn. (b) Distribution of accumulated annihilation events during diffusion and trapping in Sn. The fractions of positrons annihilating at Sn atoms $\eta$ are determined from the CDB spectra.