Structural Transformation of Bi₂Se₃(001) Surface during Sn Monolayer Annealing

Using in situ X-ray photoelectron spectroscopy under ultra-high vacuum conditions, the adsorption of a thin tin layer (~1 monolayer <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\approx 0.2$</tex> nm) on the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{Bi}_{2}\mathbf{Se}_{3}(0001)$</tex> surface at room temperature followed by annealing at 21 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0\ {}^{\circ} \mathbf{C}$</tex> was investigated. Changes in the X-ray photoelectron spectra of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{Bi}_{2}\mathbf{Se}_{3}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{Sn}^{0}$</tex> revealed the formation of crystalline bismuth and Sn-Se compounds indicating the substitution of bismuth atoms by tin, subsequent exit to the surface, and the formation of Bi(111) bilayers during annealing. Scanning electron microscopy in backscattered and secondary electron detection modes has visualized a labyrinth-like structure where regions with an enhanced tin concentration occupy approximately 50% of the sample area, which suggests the formation of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{SnSe}_{2}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{SnSe}$</tex> compounds in the near-surface region. Atomic force microscopy images of the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{Bi}_{2} \mathbf{Se}_{3}(0001)$</tex> surface morphology after the deposition and annealing of 1 monolayer of tin show height variations corresponding to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{SnSe}_{2}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{SnSe}$</tex> van der Waals molecular layers and the Bi(111) bilayers.