Fe nanoparticles on ZnSe: Reversible temperature dependence of the surface barrier potential

The Fe growth on ZnSe(001) takes place via the initial formation of superparamagnetic nano-islands that subsequently coalesce, giving rise to a continuous film for a nominal thickness of 8 Fe monolayers. For a very low Fe coverage (2 Fe monolayers), we show that the surface barrier potential (i.e. the barrier potential seen by electrons incident on the surface), measured by absorbed current spectroscopy, attains very large values (6.9 eV at room temperature) and dramatically changes as a function of temperature, with an increase of $\sim$1.5 eV from room temperature down to 130 K, largely exceeding similar changes observed in both thin films and nanoparticles. This phenomenon disappears as the thickness increases and is fully reversible with temperature. Nonequilibrium phenomena due to the experimental conditions are present, but are not able to explain the observed data. Inverse photoemission, core level photoemission, x-ray photoemission diffraction, and scanning tunneling microscopy are employed in order to find temperature-dependent properties of the Fe islands: while only minor changes as a function of temperature are present in the electronic band structure, the Fe crystal structure, and the morphology of the islands, a noticeable temperature dependence of the Se segregation through the Fe islands is found.

Figure 1

Absorbed current spectroscopy spectra, taken at room temperature, of (i) Fe/ZnSe(001) films with Fe nominal thicknesses of 2, 4, 6, and 8 ML, (ii) a clean ZnSe(001) substrate, and (iii) a bulk Fe(001) single-crystal sample. The ACS signal is defined as the ratio between the current absorbed by the sample and the total current at the entrance of the electron gun. In the inset, the energy scheme for the determination of the surface barrier potential ($\phi$) from the ACS spectra is reported.

Figure 2

Absorbed current spectroscopy spectra of Fe/ZnSe(001) films with nominal thicknesses of (a) 2 and (b) 6 ML, measured at different temperatures. In the inset of (b), the surface barrier potentials ($\phi$) are reported as a function of the temperature (the red dashed lines are only guides for the eye).

Figure 3

Mechanisms in play for explaining the surface barrier potential data: (a) nonequilibrium phenomena, giving rise to a misalignment of the Fe and ZnSe Fermi levels, and (b) surface barrier potential modification. Among the nonequilibrium phenomena, two mechanisms are considered: (c) reverse biased Schottky diode behavior and (d) surface electron-in voltage in ZnSe.

Figure 4

Inverse photoemission spectroscopy spectra of an Fe/ZnSe(001) film with a nominal thickness of 2 ML, measured at 130 K (full dots) and 290 K (empty dots).

Figure 5

X-ray photoelectron diffraction polar scans measured on Fe/ZnSe(001) films with nominal thicknesses of 2 (top), 4 (center), and 6 ML (bottom) at two different temperatures, 150 K (empty dots) and 300 K (full dots). The intensities of the Fe 2$p_{3/2}$ peak, excited by photons with $h\nu$ $=$ 1100 eV, were recorded at different polar angles of emission from the surface, with the sample oriented along the [110] azimuth. In the inset, the main forward scattering directions are indicated.

Figure 6

Scanning tunneling microscopy images (50 $\times$ 50 nm${}^2$) of an Fe/ZnSe(001) film with a nominal thickness of 2 ML, measured in constant current mode at (a) 300 K ($I$ $=$ 1 nA; $V$ $=$ 1 V) and (b) 140 K ($I$ $=$ 1 nA; $V$ $=$ 0.05 V).

Figure 7

X-ray photoemission spectroscopy spectra, measured with a photon energy $h\nu$ $=$ 170 eV, of a Fe/ZnSe(001) film with a nominal thickness of 2 ML: (a) Se3$d$ and Zn3$d$ at RT (300 K) and LT (150 K); Se3$d$ peak at (b) RT and (c) LT fitted by a substrate component, a residual component, and the Fe3$p$ contribution. An integral background due to the secondary electrons has been previously subtracted. The form of each of the Se contributions has been taken from the Se3$d$ spectrum measured on a clean ZnSe(001) substrate [see the inset of (c)].