Growth and characterization of epitaxial bismuth films.

Abstract

The present work describes the growth of the first thin (0.1--2 \ensuremath{\mu}m) epitaxial films of pure bismuth using molecular-beam-epitaxy techniques. These structures were grown at elevated temperatures on single-crystal barium fluoride substrates of 〈111〉 orientation. Electron-microscope observations show the films to be featureless and defect free on the scale of 0.1 \ensuremath{\mu}m. The films grow with their trigonal axis parallel to the 〈111〉 axis of the substrate, and Laue-backscattering pictures show they are epitaxial. Mobilities at room temperature are on the order of 2 ${\mathrm{m}}^{2}$ ${\mathit{V}}^{\mathrm{\ensuremath{-}}1}$ ${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$, and increase to over 10 at 20 K and 100 at liquid-helium temperatures. These values are far superior to those of other bismuth films grown to date, and approach mobilities observed in single-crystal bismuth. Further evidence of their single-crystal nature is given by the temperature-dependent resistivity below 6K, which is more akin to that of a bulk single crystal, rather than polycrystal, bismuth, and by the thickness dependence of the film mobilities, which are limited by scattering on film boundaries. The carrier density, as deduced from Hall measurements, is in the range (4--8)\ifmmode\times\else\texttimes\fi{}${10}^{24}$ ${\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$ at room temperature and decreases as the temperature is lowered, becoming constant below about 50 K at approximately 5\ifmmode\times\else\texttimes\fi{}${10}^{23}$ ${\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$. We also observe Shubnikov--de Haas oscillations in the resistivity and Hall coefficient at 4.2 and 0.4 K. The carrier density calculated from the period of these oscillations correlates well with that found from Hall measurements.