Negative correlation energy and valence alternation in amorphous selenium: Anin situoptically induced ESR study

Two different kinds of electron spin resonance (ESR) signals, triclinic and isotropic, are observed in amorphous selenium under photoexcitation at 20 K with a concentration of up to 10²⁰cm^-3. ESR annealing behavior shows that isotropic centers are not stable and are converted into triclinic defects. The defects are identified as singly and triply coordinated Se defects. The results present experimental evidence for negative-U centers and valence alternation in amorphous selenium. The relationship between the photoinduced ESR and reversible photodarkening is discussed.