Experimental evidence for negative correlation energy and valence alternation in amorphous selenium

Abstract

Two different kinds of electron spin resonance signals, triclinic and isotropic, are observed in amorphous selenium under photoexcitation at 20 K, their concentration being around ${10}^{20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}.$ At higher temperatures, isotropic centers are converted into triclinic ones. These centers are identified as singly and triply coordinated Se defects. The results present experimental evidence for negative$\ensuremath{-}U$ centers and valence alternation in amorphous selenium.