Gallium-isotope fine structure of impurity modes due to defect complexes in GaAs

A comprehensive Green's-function calculation was performed in order to understand the recent high-resolution Fourier-transform infrared absorption (FTIR) data of localized vibrational modes (LVM's) due to isolated (SiGa donor, BGa isoelectronic, SiAs, LiAs, BAs acceptor) and pair (SiGa-SiAs, SiGa-As-LiGa) defects in GaAs. For the defect centers with Td, C₃ᵥ, Cₛ, and C₂ᵥ symmetries, we have considered simple perturbation models using mass changes at the impurity sites and nearest-neighbor (NN) force-constant changes between impurity-host atoms.For the host GaAs, all the involved Green's-function matrix elements are numerically evaluated by incorporating ``phonons'' generated by an eleven-parameter rigid-ion model (RIM11) fitted to the neutron data of phonon dispersions. After analyzing nearly eighty cases of LVM's in fifteen elemental and compound semiconductors, we strongly argue that there exists a close relationship between the force perturbation and the variation of the covalency of the bond. Based on simple physical reasonings we have proposed empirical relationships which provide correction to the force constants for closest mass isoelectronic (i), donor (d⁺), and acceptor (a^-) impurities carrying static charges. Such trends of force variation are found to be important not only in studying the pair defect vibrations (SiGa-SiAs, LiGa-SiGa) but also in understanding the possibility of the occurrence of boron antisite (B_As^-) defects in GaAs.Using the force perturbation parameter obtained from the Td case, and considering two Ga (⁶⁹Ga, ⁷¹Ga) isotopic masses in different NN configurations with C₃ᵥ and C₂ᵥ symmetries, we have qualitatively explained the recent Ga-isotopic fine structure of LVM's due to B_As^-. To understand the origin of the 367- and 369-cm^-1 bands observed in Si-doped GaAs samples, two perturbation models each involving one ²⁸Si and a self-antisite defect are proposed and evaluated in terms of the Green's-function theory. The results are compared and discussed with the existing theoretical and experimental data.