Crystal Structure, Optical Properties, and Characteristics of the Band Gap of Ca2Si Semiconductor Films on an Al2O3(0001) Substrate

A nanocrystalline Ca2Si film on Al2O3(0001) is formed for the first time by converting a sacrificial 2D Mg2Si template into a Ca2Si seed layer with the preliminary formation of an amorphous 2D silicon layer. A Ca2Si seed layer on a sapphire substrate facilitates the growth of oriented Ca2Si films by molecular-beam epitaxy (MBE) at a temperature of 250 degrees C, for which one epitaxial ratio is observed, Ca2Si(211)/Al2O3(0001). Studies of the optical properties and parameters of the band structure of Ca2Si on sapphire reveal the nature of the fundamental direct transition with an energy of 0.88 +/- 0.01 eV. Direct interband transitions are observed in the band structure of Ca2Si: at 0.88, 1.16, 1.49, and 1.61 eV with increasing oscillator strength. At photon energies from 0.78 to 0.88 eV, the Urbach tail on defects in Ca2Si nanocrystals makes the main contribution to absorption. Absorption at grain boundaries is observed at photon energies from 0.6 to 0.78 eV, and at free carriers, at energies below 0.6 eV. The results obtained are important for optoelectronics in the infrared (IR) region of the spectrum.