Fehr, M.; Schnegg, A.; Rech, B.; Lips K.; Astakhov O.; Finger, F.; Pfanner, G.; Freysoldt, C.; Neugebauer, J.; Bittl, R.; Teutloff, C.: Combined multifrequency EPR and DFT study of dangling bonds in a-Si:H. Physical Review B 84 (2011), p. 245203/1-10
Multifrequency pulsed electron paramagnetic resonance (EPR) spectroscopy using S-, X, Q, W-Band frequencies (3.6, 9.7, 34, and 94 GHz, respectively) was employed to study paramagnetic coordination defects in undoped hydrogenated amorphous silicon (a-Si:H). The improved spectral resolution at high magnetic field reveals a rhombic splitting of the g-tensor with the following principal values: gx = 2.0080, gy = 2.0060 and gz 21 = 2.0033 and shows pronounced g-strain, i.e., the principal values are widely distributed. The multifrequency approach furthermore yields precise 29Si hyperfine data. Density functional theory (DFT) calculations on 27 computer-generated a-Si:H dangling-bond models yielded g-values close to the experimental data but deviating hyperfine interaction values. We show that paramagnetic coordination defects in a-Si:H are more delocalized than computer-generated dangling-bond defects and discuss models to explain this discrepancy. In addition to coordination defects, we observed, for the first time, paramagnetic centers which show a strong 1H hyperfine interaction of 320 MHz. These centers are possibly due to paramagnetic configurations of Si-Si bond-centered hydrogen atoms, or spatially correlated hydrogen/dangling bond centers.