Heteroepitaxial growth of the intrinsic vacancy semiconductor ${\text{Al}}_2{\text{Se}}_3$ on Si(111): Initial structure and morphology

The evolution of nanostructure morphology and local chemical environment during heteroepitaxial growth of aluminum selenide on Si(111) was investigated with scanning tunneling microscopy and high-resolution photoemission spectroscopy. Despite the strong similarity to GaSe in atomic and electronic structure during deposition of the first AlSe bilayer, subsequent growth is quite different—resulting in an alternating Al-Se-Al-Se stacking sequence consistent with defected-wurtzite-structure ${\text{Al}}_2{\text{Se}}_3$. The first bilayer is completed on a given terrace before the second layer nucleates, but subsequent layers nucleate before completion of the second layer. The surfaces of well-formed ${\text{Al}}_x{\text{Se}}_y$ islands are smooth and terminated by Se atoms; Al then sticks before additional Se, resulting in rougher incomplete islands with Al-rich disordered surfaces. Growth with extra Al in the incident flux does not result in layered AlSe and induces only subtle differences in film morphology.

Figure 1

(Color online) Photoemission of ${\text{Al}}_2{\text{Se}}_3$ deposition on Si(111). (a)–(c) Spectra from wedge film 0 to 4 BL thick (see inset). ${\text{Al}}_2{\text{Se}}_3$ $\text{flux}=6.5\text{Å}/\text{min}$; $T_{\text{sub}}=550°\text{C}$. Photon energy $h\nu =185\text{eV}$. (a) Si $2p$, (b) Al $2p$, and (c) Se $3d$. Vertical (blue) lines mark peak ($2p_{3/2}$ or $3d_{5/2}$) for single BL. Fitted Se $3d$ components are shown for the thickest film. (d) Intensity of fitted components vs position. Component energy was held constant during fitting. Points between 0 and 1 are from a 0–1 BL wedge (${\text{Al}}_2{\text{Se}}_3$ $\text{flux}=1.6\text{Å}/\text{min}$, $625°\text{C}$, and $h\nu =240\text{eV}$); points between 1 and 4 are from the same 0–4 BL wedge as (a)–(c). Position 1 corresponds to a single BL for each sample; spectra from both samples are normalized to 1 at this position. Varying sticking coefficients with coverage results in a monotonic, but not necessarily linear, dependence of thickness on position.

Figure 2

(Color online) Bilayer formation. STM images of $\lesssim 1\text{ML}$ Al plus Se deposited from combined ${\text{Al}}_2{\text{Se}}_3$ and Al sources at $T_{\text{sub}}=550°\text{C}$. (a) $39\times 39{\text{nm}}^2$ ($-2.6\text{V}$, 0.12 nA, empty state). (b) $88\times 44{\text{nm}}^2$ ($-2.32\text{V}$, 0.17 nA) in higher coverage region. (c) $14\times 4.6{\text{nm}}^2$, enlargement of upper rectangle in (a), showing atomic resolution of AlSe BL; circle highlights a defect similar to the most common defect on GaSe/Si(111). (d) $14\times 7{\text{nm}}^2$, enlargement of lower rectangle in (a). (e) Tip height vs position along lines parallel to $⟨1\overline{1}0⟩$ drawn on images (c) (solid blue line) and (d) (dotted red line). Rhombuses in (a), (c), and (d) denote $7\times 7$ substrate unit cell for reference.

Figure 3

(Color online) Overlayer nucleation and growth of ${\text{Al}}_2{\text{Se}}_3$. (a)–(c) $\sim 1.6\text{BL}$ ${\text{Al}}_x{\text{Se}}_y$ deposited at $T_{\text{sub}}=585°\text{C}$ with both ${\text{Al}}_2{\text{Se}}_3$ and Al sources. (a) $180\times 180{\text{nm}}^2$, $z$ scale of 2.65 nm. (b) $39\times 39{\text{nm}}^2$ ($-2.7\text{V}$, 0.19 nA). (c) Horizontal gradient of (b). (d)–(h) $\sim 1.8\text{BL}$, single source. (d) $180\times 180{\text{nm}}^2$, $z$ scale of 2.65 nm. (e) $37\times 37{\text{nm}}^2$ occupied state image ($-2.4\text{V}$, 0.16 nA). (f) Same region (and color scale) as (e); empty-state image ($+2.4\text{V}$, 0.16 nA). (g) Height profiles along lines in (e) and (f). (h) Horizontal gradient of (f).

Figure 4

Development of valence-band structure with thickness. Normal emission at photon energy $h\nu =120\text{eV}$. Wedge sample, $\sim 2–6\text{BL}$, $T_{\text{sub}}=550°\text{C}$, single source ${\text{Al}}_2{\text{Se}}_3$, $\text{flux}=6.5\text{Å}/\text{min}$.