Epitaxial growth of bismuth films and bismuth-antimony heterostructures

We describe an approach for fabricating high-quality Bi thin films and heterostructures on BaF₂ substrates by epitaxially growing them between layers of semiconducting Bi_1-x{Sb}ₓ$. We present results from reflection high-energy electron diffraction, scanning electron microscopy, and atomic force microscopy analysis and show that the films are single crystalline with typical rms roughness of 1 nm and a dislocation density of 2×{}10⁹ cm^-2. Low-temperature magnetoresistance measurements are discussed in detail for a 90-nm Bi0.95{Sb}0.05$/45-nm Bi/65-nm ${Bi}0.95Sb0.05 heterostructure. At liquid-helium temperatures, the electrical transport in the central, 45-nm-thick Bi layer is well described by a three-carrier model that takes into account high mobility electrons (μ₁=1.0×{}10⁵ cm²/V s) and holes ({ν}=3.1×{}10⁴ cm²/V s), as well as low mobility surface charges. The electron and hole densities are roughly equal and a factor of 6 higher than in the bulk. The epitaxial growth and clean interfaces result in a long electron elastic-scattering length, lₑₗ=0.38 {μ}m. From an analysis of the observed Shubnikov--de Haas oscillations we obtain values for the extremal cross section of the Fermi surface, the cyclotron mass, and the single-particle relaxation time. At 45 nm the film thickness is comparable to the Fermi wavelength and, due to quantum confinement, only a few two-dimensional subbands of the electron pocket are filled.