Effect of Substrate Pre-treatment on Growth of Ultra-thin Bi₂Se₃ Films.

Bi₂Se₃ is a widely-studied topological insulator due to its potential applications in optics and spintronics. When the thickness of Bi₂Se₃ is reduced to below 6 nm, the wavefunctions of the electrons on the top and bottom surfaces couple, and a small non-trivial gap opens at the Dirac point. The film then acts as a quantum spin Hall insulator. To explore this state, growing coalesced ultra-thin Bi₂Se₃ films at a wafer scale with defined thickness is critical. We explored the growth of ultra-thin Bi₂Se₃ films on sapphire substrates using molecular beam epitaxy. The films were characterized by room temperature Hall measurements, atomic force microscopy, and Raman mapping. We observed that depositing films directly on sapphire without any pretreatment resulted in fragmented non-coalesced films. Pre-treatment of the substrate by growing a few nanometers of Bi₂Se₃ and decomposing it completely resulted in coalesced films. In addition, the two-step growth method resulted in typical triangular domain morphology. Furthermore, higher growth rates and lower growth temperatures resulted in reduced surface roughness contrary to what is observed for conventional epitaxy. Overall, the growth of well-coalesced Bi₂Se₃ ultra-thin films with lower surface roughness makes them reliable for optical or electronic studies.