| In semiconductor alloys such as In1−xGaxAs, the energy band gap as well as the lattice parameter can be engineered by changing the concentration, x. Due to these properties, semiconductor alloys have found wide applications in optoelectronic devices. In these alloys, local structure information is of fundamental importance in understanding the physical properties such as band structure.; Using the high real-space resolution atomic Pair Distribution Function, we obtained more complete structural information such as bond length, bond length distributions, and far-neighbor distances and distributions. From such experimental information and the Kirkwood model we studied both local static displacements and correlations in the displacements of atoms. The 3-dimensional As and (In,Ga) atom iso-probability surfaces were obtained from the supercell relaxed using the Kirkwood potential. This shows that the As atom displacements are very directional and can be represented as a combination of ⟨100⟩ and ⟨111⟩ displacements. On the contrary, the (In,Ga) atom displacements are more or less isotropic. In addition, the single crystal diffuse scattering calculation of the relaxed supercell shows that the atomic displacements are correlated over longer range along [110] directions although the displacements of As atoms are along ⟨100⟩ and ⟨111⟩ directions.; Besides the local static displacements, we studied correlations in thermal atomic motions of atom pairs from the PDF peak width changes as a function of atom pair distance. In the PDF the near-neighbor peaks are sharper than those of far-neighbors due to the correlation in near-neighbor thermal motions. We also determined bond stretching and bond bending force constants of semiconductor compounds by fitting the nearest neighbor and far-neighbor peak widths to the lattice dynamic calculations using the Kirkwood model. |
