High-quality ultra-flat BiSbTe₃films grown by MBE

We report on the growth of high-quality topological insulator BiSbTe₃films (thickness of 30 nm) on sapphire (0001) substrates through the molecular beam epitaxy (MBE) technique and discuss the possibility to improve the film quality and surface flatness through annealing. Instead of using elemental Sb, the choice of Sb₂Te₃as well as the use of solid sources of Bi and Te assures excellent stability during thermal evaporation enabling layer-by-layer epitaxial growth of high-quality films of BiSbTe₃. The crystallinity and the terrace size of the BiSbTe₃films are found to be improved through increasing the deposition temperature and/or after annealing at 540 K-620 K for 1-4 hours. The films grown at 485 K, 500 K, and 515 K exhibit root-mean-square (RMS) roughness of 2.9 nm, 2.3 nm, and 2.3 nm, respectively, whereas the RMS roughness is reduced to 0.6 nm or less when the films are annealed at 580 K for 2-4 hours. Annealing the film at too high temperature, such as 620 K, introduces a rougher surface due to the loss of material during annealing. A relatively low electron density of~2.2×10¹⁸cm^-3(at 2 K) is achieved for the as-grown films deposited at 485 K and 500 K. Significantly enhanced electron density is found in the case of either increasing growth temperature or increasing annealing temperature. An exception is the film annealed at 620 K, which became a p-type conductor. In addition, weak antilocalization effects are evident for the n-type films, but they nearly vanish for p-type conducting films. The significant influence of temperature on the crystallinity, the surface roughness, and the electronic transport in BiSbTe₃films will be instructive for further investigations of the transport behavior of surface states in BiSbTe₃films.