T. Su

Structural and photostructural properties of chalcogenide glasses: recent results from magnetic resonance measurements

Abstract We review nuclear quadrupolar resonance (NQR) and high-field (up to 30 T) nuclear magnetic resonance (NMR) measurements of 75 As in crystalline and glassy samples of arsenic chalcogenides. The results, which yield asymmetries in the electric field gradient at the arsenic sites, provide detailed information on the local structural order and the photoinduced structural changes in these glasses and crystals – order that cannot easily be obtained from other techniques such as X-ray or neutron scattering. In crystalline and glassy As 2 Se 3 and As 2 S 3 these results disagree with those inferred from previous low field Zeeman-perturbed NQR studies, and they are inconsistent with the predictions of a common model (Raft Model) for the structure of glassy As 2 Se 3 and other chalcogenide glasses. Results on optically irradiated and annealed, glassy As 2 S 3 place severe restrictions on the possible changes in local and intermediate range order that can occur on photodarkening. The high-field, NMR technique should be useful for many other solids that contain one of the 20 elements whose nuclei have spin 3/2 and relatively large quadrupole moments.

Photodarkening in glassy As2S3

Abstract Nuclear magnetic resonance (NMR) of 75As at 17 T has been employed to study photodarkening (the shift of the optical absorption edge to lower energies after excitation with light of energy near the optical band edge) in glassy As2S3. After irradiation at 514.5 nm for 230 h with 170 mW/cm2 the average asymmetry parameter of the electric field gradient (EFG) tensor increases from about 0.09±0.01 to about 0.12±0.01. This change is reversible on annealing at 200°C for 1.75 h. An increase in the asymmetry parameter implies an increase in the departure from cylindrical symmetry in the bonding at the arsenic pyramidal sites.

NMR probe of band tail states in a-Si:H

Abstract We measured the dipolar spin–lattice relaxation time (T1d) of the bonded hydrogen in hydrogenated amorphous silicon at low temperature. At 15 K, after the sample is irradiated by λ=6500 A light for 2 s, T1d is reduced by about one order of magnitude. The reduction of T1d is attributed to the cross-relaxation between the hydrogen and localized paramagnetic band tail states that are induced by the light. Within experimental error, T1d is proportional to the inverse of the square of the density of the paramagnetic states, consistent with a previously proposed model.