X-ray crystallographic study of the optically active semiconductor(GaxIn1−x)2Se3

Abstract

Crystal structures of two kinds of optically active semiconductors ${(\mathrm{G}\mathrm{a}}_{0.60}{\mathrm{In}}_{0.40}{)}_{2}{\mathrm{Se}}_{3}$ and ${(\mathrm{G}\mathrm{a}}_{0.40}{\mathrm{In}}_{0.60}{)}_{2}{\mathrm{Se}}_{3},$ both of which are defect wurtzite structures but show different reduced ${c}_{w}{/a}_{w}$ ratios, have been studied using powder X-ray diffraction and Rietveld analysis. The final reliability factors are as follows: ${R}_{\mathrm{wp}}=10.00%,$ $S=2.4234,$ ${R}_{F}=4.19%$ for ${(\mathrm{G}\mathrm{a}}_{0.60}{\mathrm{In}}_{0.40}{)}_{2}{\mathrm{Se}}_{3}$ and ${R}_{\mathrm{wp}}=10.48%,$ $S=2.4324,$ ${R}_{F}=2.78%$ for ${(\mathrm{G}\mathrm{a}}_{0.40}{\mathrm{In}}_{0.60}{)}_{2}{\mathrm{Se}}_{3}.$ The structure of ${(\mathrm{G}\mathrm{a}}_{0.60}{\mathrm{In}}_{0.40}{)}_{2}{\mathrm{Se}}_{3}$ with ${c}_{w}{/a}_{w}=1.633$ is called the ideal defect wurtzite structure; the bond lengths are nearly the same and the bond angles are almost to be the ideal bond angle of tetrahedral bonding 109.47\ifmmode^\circ\else\textdegree\fi{}. Meanwhile, the structure of ${(\mathrm{G}\mathrm{a}}_{0.40}{\mathrm{In}}_{0.60}{)}_{2}{\mathrm{Se}}_{3}$ with ${c}_{w}{/a}_{w}=1.565$ is called the layered defect wurtzite structure; the nearest ${(001)}_{\mathrm{group}\mathrm{}\mathrm{III}}$ and ${(001)}_{\mathrm{Se}}$ layers are waved and very close to each other as if they merged into one atomic layer. The average bond length along the c axis is 5.89% longer than that between the atoms in the merged ${(001)}_{\mathrm{group}\mathrm{}\mathrm{III}}$ and ${(001)}_{\mathrm{Se}}$ layers; the average bond angle whose one side is the bond along the c axis is 13.11% smaller than 109.47\ifmmode^\circ\else\textdegree\fi{}. Based on these structural results, the anomalous dependence of the optical band gap on composition is discussed from the view of bonding nature. The additional ionic bonding between the merged (001) layers in the layered defect wurtzite structure makes the c parameter smaller but the band gap larger than those of the ideal defect wurtzite structure although the ionic bonding nature is dominant for the both defect wurtzite structures.