Heterointerface formation of aluminum selenide with silicon: Electronic and atomic structure of Si(111):AlSe

In this paper we present a new, stable, unreconstructed surface termination of silicon, Si(111):AlSe. The structure forms the interface layer when aluminum sesquiselenide $\left({\mathrm{Al}}_2{\mathrm{Se}}_3\right)$ is deposited on Si(111) by molecular beam epitaxy. The atomic structure of the interface layer was investigated using angle-resolved valence and core-level photoelectron spectroscopy and diffraction. The ${\mathrm{Al}}_2{\mathrm{Se}}_3∕\mathrm{Si}\left(111\right)$ interface forms an unreconstructed bilayer structure similar to GaSe-terminated Si, with Al directly above the top Si atom and Se over the hollow site, although the temperatures for bilayer formation and for Se re-evaporation from the film are higher for AlSe than for GaSe. In addition, the valence band structure shows that the AlSe bilayer electronically passivates the bulk Si, with all interface states lying within the bulk Si bands.

Figure 1

Integrated photoemission intensity versus polar angle along the $\left[11\overline{2}\right]$ direction $(\theta >0)$ and the $\left[\overline{1}\overline{1}2\right]$ direction $(\theta <0)$ of the Si substrate for Mg $\mathrm{K}\alpha (h\nu =1253.6\mathrm{eV})$ excited (a) Se $3d$, $\mathrm{KE}=1198\mathrm{eV}$, and (b) Al $2p$, $\mathrm{KE}=1179\mathrm{eV}$. Solid lines represent experimental data and dotted lines the calculated pattern using a selenium-on-aluminum bilayer model (see the text). Vertical dashed lines mark the Al-Se bond angle position along $11\overline{2}(+63.0\mathrm{deg})$ and the Se along $\overline{1}\overline{1}2(-75\mathrm{deg})$ determined from fitting the calculated pattern to the data.

Figure 2

Possible high-symmetry sites for Al on the Si(111) surface: Top, directly above a top layer Si atom; $T_4$, above a second-layer Si atom; and $H_3$, above a fourth-layer Si atom.

Figure 3

Al $2p\chi$ functions versus electron wave vector, $k$, taken normal to the sample surface. Calculated diffraction scans for aluminum occupying (a) an $H_3$ site, (b) a $T_4$ site, and (c) a Top site relative to the silicon surface. (d) Experimental energy-dependent photoelectron diffraction. The photon energy range for this data is 250 eV to 500 eV with 5 eV steps.

Figure 4

Experimentally determined structural parameters for the AlSe bilayer on the Si(111) surface.

Figure 5

Low kinetic energy holograms for Se $3d$ (a) experiment and (b) theory, and Al $2p$ (c) experiment and (d) theory. White indicates the greatest intensity and black the lowest intensity.

Figure 6

Valence band of an AlSe bilayer on Si(111) in the $\left[1\overline{1}0\right]$ and $\left[\overline{1}10\right]$ directions, $\overline{\Gamma }-\overline{K}-\overline{M}$. Solid white lines represent the boundaries of the calculated Si projected bulk bands, with vertical black stripes marking forbidden regions below the valence band maximum. Dots represent the calculated bulk Si valence band emission along the scan direction. Photon energy $h\nu =130\mathrm{eV}$.

Figure 7

Valence band of an AlSe bilayer on Si(111) in the $\left[1\overline{1}0\right]$ and $\left[\overline{1}10\right]$ directions, $\overline{\Gamma }-\overline{K}-\overline{M}$. Open circles are the plotted peak positions from the image in Fig. 6. Shaded regions are forbidden regions in the calculated Si projected bulk bands. Solid lines on the left half of the graph are the calculated Si bulk emission along the scan direction at $h\nu =130\mathrm{eV}$ (shown as dots in Fig. 6). Solid lines on the right half of the graph are the measured band structure of the bilayer on the silicon taken with He I, $h\nu =21.2\mathrm{eV}$. Bold open circles are those that do not overlap a silicon bulk state on the left, but note that they do correspond with a state observed at the same $k_{\Vert }$ and different $k_{\perp }$ on the right—indicative of their two-dimensional nature.