Band gap engineering and transport properties of BaInO: effect of fluorine doping and hydration.

Two types of fluorine doped barium indate solid solutions were prepared by a solid state method: BaInOF (x = 0, 0.1, 0.2) and BaInOF (y = 0, 0.1, 0.25). Good agreement between the theoretical and experimental values of the pycnometric densities for the studied solid solutions confirms these two distinctly different models of solid solution formation. According to analysis of the XRD data introduction of fluorine ions into the oxygen sublattice of barium indate leads to a decrease in the a lattice parameter and unit cell volume for both types of solid solutions. The effect of fluorine doping and hydration on the band gap of barium indate was studied by means of diffuse reflectance spectroscopy. The estimated value of the band gap width E for undoped BaInO is 2.94 eV and is in good agreement with literature data. Introduction of fluorine results in a slight increase of E and emergence of an additional absorption band in the region of 2.56-2.65 eV, near the fundamental absorption edge, which can be attributed to the -defects appearing upon fluorine doping. The increase of E with fluorine introduction correlates well with the decrease of the electronic transport numbers and can be explained by two competing effects: (1) lowering of the top of the valence band due to replacement of O ions by F; and (2) changes in the local structure, i.e., lattice contraction and tilting of the InO(F) framework. Hydration of the barium indate also leads to an increase of E, which was attributed to structural transformation from orthorhombic symmetry to tetragonal. However, with an increase in the fluorine concentration such changes of E become less pronounced because of the decrease of the hydration degree due to formation of -defects.