Optically induced electron-spin resonance inAsxS1−

Abstract

Four different metastable paramagnetic centers have been identified in the low-temperature, light-induced electron-spin resonance (ESR) spectrum of glassy As&S3. Two of the centers anneal at significantly lower temperatures than the other two, allowing the line shapes to be partially separated with isochronal annealing experiments. The two centers which anneal at lower temperatures, labeled type-I centers, constitute approximately 15% of the induced spins after long-time irradiation at high intensities ( 100 mW/crn~). These centers consist of a hole on a nonbonding 3p orbital of a sulfur atom (Si) and an electron on an s-p hybridized orbital of an arsenic atom (As&). Similar kinetic behavior suggests that the origin of these two centers is a single event, which may be the breaking of an arsenic-sulfur bond. The type-II centers, which are thermally more stable, represent -85%%uo of the induced spins and are concluded to be due to an electron in a nonbonding 4p wave function on a twofold-coordinated arsenic atom (As») and a hole on a nonbonding 3p orbital of a sulfur atom (Si&).The origin of these centers is suggested to be the breaking of As -As and S -S bonds. The densities of the different spins vary rapidly with the stoichiometry. Interpretation of the kinetic behavior of the type-I and type-II ESR signals suggests the existence of a third intermediate metastable state in addition to the ground state and the excited paramagnetic state. High-intensity () 100 mWcm ) light with energy above the band gap (A,~514.5 nm) creates new structural defects in the glass at densities which exceed 10' cm '. At high temperatures (T)250 K) the shift of the opticalabsorption edge to lower energies, which is known as the photodarkening effect, exhibits different kinetics from the electron-spin resonance. This fact suggests that there exists no one-to-one correlation between these two effects. There is, however, a close parallel at all temperatures between absorption well below the gap (midgap absorption) and the type-I ESR centers.

Electronic structure and the glass transition in pnictide and chalcogenide semiconductor alloys. I. The formation of the ppσ-network

“…At the same time, the two substances exhibit opposite trends in terms of intrinsic and light induced ESR response. 21,22,30,31 Such simultaneous trends are particularly vivid in the Ge x Se 1−x series 32 for 1/3 < x < 1/2, which exhibit coordination ranging from tetrahedral (smaller x) to distorted octahedral (larger x). In this series, the octahedral ordering seems to correlate with the separation between s and p bands 33,34 and light induced ESR, and anti-correlate with the presence of unpaired spins 31 and the glassforming ability.…”

“…On the other hand, these amorphous arsenic chalcogenides and similar materials do indeed exhibit several electronic and optical anomalies that could be accounted for by the interface-based states, in a unified fashion. 14 These anomalies include light-induced electron spin resonance (ESR) and midgap absorption, 21,22 two types of photoluminescence, 23 and field-induced ultrasonic attenuation. 24 Thus general arguments, on the one hand, and observation, on the other, seem to converge on the uniqueness of chalcogenide and pnictide glasses with regard to their potential ability to host topological midgap states.…”

Nanoscale mechanism of photoinduced metastability and reversible photodarkening in chalcogenide vitreous semiconductors

“…Later, however, it was argued [39] that temperature behaviours of the photo-induced ESR and photodarkening were quite different: ESR disappeared at temperatures quite a bit lower than the photodarkening. This result was the grounds for a conclusion that photo-induced ESR and photodarkening were not related.…”

Photoluminescence and optical absorption in glassy AsxSe1−x

“…In the As x S 1Àx system, the presence of S-S and As-As bonds (so-called homopolar or like-atom bonds) has been documented by Raman spectroscopy [10], X-ray absorption fine structure (EXAFS) measurements [11], and nuclear quadrupole resonance (NQR) spectroscopy [12]. Also in this system, homopolar bonds have been invoked to explain the compositional dependence of optically induced, paramagnetic defects [13,14]. The results in the As x Se 1Àx system are less complete, but the general features should be the same.…”