SnSe fi lms thicker than 20 nm go back to the GeS structure. We checked the surface morphology of a 20 nm fi lm with STM to see how the transition from rock-salt to GeS structures takes place. As shown in Figure 4 d, there are islands distributed on the surface with the height of 11.3 Å, obviously in GeS structure. RHEED and ARPES results show that most part of the fi lm is still of rock-salt SnSe. With more SnSe deposited, the fi lm surface is gradually covered by the GeS-type islands. Therefore, above 20 nm, GeS-type SnSe forms on the top of the rock-salt SnSe fi lm instead of changing the fi lm into the GeS structure. The epitaxial rock-salt SnSe fi lms possess some unique properties of great importance. Since SnSe is electron-doped and SnTe is usually hole-doped, by mixing them into SnTe x Se 1− x , we can control the carrier density and type by Te/Se ratio. Considering that both rock-salt-type SnSe and SnTe are TCIs with similar bulk gap, [ 2,21 ] the topological property and bulk gap will be little changed by the alloying, a remarkable advantage over Pb 1− x Sn x Te(Se). The relatively weak spinorbit coupling may bring rock-salt SnSe different physical properties from other TI and TCI materials [ 1,19 ] for example longer spin relaxation time, which is important for spintronic applications. As a metastable phase, the lattice structure of rock-salt SnSe should be more sensitive to perturbations, a superior property for TCI-based fi eld-effect devices. It is also very helpful and interesting to study the thermoelectric properties of rock-salt-type Adv. Mater. 2015, 27, 4150-4154 www.advmat.de www.MaterialsViews.com Figure 4. The structural change of rock-salt SnSe fi lms above 20 nm. a) RHEED pattern of a nominal 70 nm SnSe fi lm along the [ 112 ] direction. b) ARPES spectra of the 70 nm SnSe fi lm around Γ point. c) STM image of the 70 nm SnSe fi lms (130 nm × 130 nm). The inset shows the line profi le along the black line. d) STM image of a nominal 20 nm SnSe fi lm (150 nm × 150 nm). The inset shows the line profi le along the black line.Figure 3. The energy band structure of a 16 nm SnSe(111) fi lm. The bandmap around Γ point obtained by a) ARPES and b) fi rst-principles calculations. The bandmap around M point obtained by c) ARPES and d) fi rst-principles calculations. The bandmap close to E F around Γ point obtained by e) ARPES and f) fi rst-principles calculations. The bandmap close to E F around M point obtained by g) ARPES and h) fi rst-principles calculations. The red dashed lines in (a-h) indicate E F . i) Real space partial charge distributions of the states of Dirac points at Γ point and M point. 4154 wileyonlinelibrary.com

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