NEGATIVE CORRELATION-ENERGY AND VALENCE ALTERNATION IN AMORPHOUS SELENIUM - AN IN-SITU OPTICALLY INDUCED ESR STUDY

Two different kinds of electron spin resonance (ESR) signals, triclini c and isotropic, are observed in amorphous selenium under photoexcitat ion at 20 K with a concentration of up to 10(20) cm(-3). ESR annealing behavior shows that isotropic centers are not stable and an converted into triclinic defects. The defects are identified as singly and trip ly coordinated Se defects. The results present experimental evidence f or negative-U centers and valence alternation in amorphous selenium. T he relationship between the photoinduced ESR and reversible photodarke ning is discussed. [S0163-1829(98)07338-X].