Assessing site selectivity of Si-Ge in GaAs by isotopic dependent vibrational modes

Abstract

In doubly doped GaAs with silicon (Si) and germanium (Ge) defects, the observed localized vibrational modes (LVMs) by high resolution infrared absorption spectroscopy are carefully assessed in terms of a comprehensive Green’s function (GF) theory. Accurate values of phonons generated by a realistic lattice-dynamical model are meticulously integrated to simulate the GFs and perturbation matrix elements for attaining the site selectivity of Si and Ge impurities in GaAs. By considering two nearest-neighbor (NN) donor–acceptor prototypes: (A)SiGa+-GeAs- and (B) GeGa+-SiAs- of C3v-symmetry with strict sets of perturbation parameters, we have cautiously appraised the two models by calculating their impurity vibrational modes. Careful analysis of the experimental vibrational traits has convincingly established the observed split modes at 403 and 373 cm−1 as radial (a1+) and transverse (e+) modes, respectively of a NN SiGa+-GeAs-pair where Ge acts as an acceptor. For this pair structure, the high frequency broad band at 403 cm−1 with several phonon features is justified by relating it to the five naturally occurring Ge isotopes. Although, the sharp line at 373 cm−1 overlaps with the 30SiGa isotopic LVM we, assigned it, however, to the transverse vibration of the pair-defect. In the two NN Ge-Si defect structures, the accuracy of calculated impurity modes is checked by using a simple model which correctly predicted isotopic shifts of the triply degenerate (F2) phonons of isolated SiGa+ and SiAs- impurities involved in the (SiGa+-GeAs- and GeGa+-SiAs-) donor–acceptor pairs.____________________________________________________________________________