  Using atomic-force microscopy (AFM) and wide-band (0.02-8.5 eV) spectroscopic
ellipsometry techniques we investigated morphology and optical properties of
Cd3As2 films grown by non-reactive rf magnetron sputtering on two types of
oriented crystalline substrates (100)p-Si and (001) alpha-Al2O3. The AFM study
revealed grainy morphology of the films due to island incorporation during the
film growth. The complex dielectric function spectra of the annealed
Cd3As2/Al2O3 films manifest pronounced interband optical transitions at 1.2 and
3.0 eV, in excellent agreement with the theoretical calculations for the body
centered tetragonal Cd3As2 crystal structure. We discovered that due to
electronic excitations to the Cd(s) conical bands the low-energy absorption
edge of the annealed Cd3As2 films reveals linear dependence. We found that for
the annealed Cd3As2 films the Cd(s) conical node may be shifted in energy by
about 0.08-0.18 eV above the heavy-flat As(p) valence band, determining the
optical gap value. The as-grown Cd3As2 films exhibit the pronounced changes of
the electronic band structure due to doping effect associated with Cd
non-stoichiometry, where fine-tuning of Cd concentration may result in the
gapless electronic band structure of Dirac semimetals.
