Resonance phenomena and defects in amorphous selenium

Abstract

A three-level system, consisting of a basic/ground state B, a metastable excited state D* and a defect state D, with relative energies Δε_D* = 0·68 ± 0·01 eV and Δε_D* = 0·14±0·04 eV respectively, is constructed from experimental data for viscosity resonance in a magnetic field, crystallization resonance in an electric field and electron spin resonance in a-Se. For the first time, such low-energy defects in an amorphous substance have been revealed experimentally. The paramagnetic transition state D* is realized at the top of the potential barrier, between the basic state and that of the defect. When this barrier is transparent, one has a two-level system with tunnelling states, which is widely exploited for glasses, especially in the low-temperature region. In the general case of a three-level system all three states are successively produced in the form of cycles B → D* → D → D* → B, etc. for each atom. As a result, at sufficiently high temperatures the whole structure is intermittently populated by defects. The frequency of this intermittent population, which can be revealed by resonance experiments, is assumed to be essential for the following defect-assisted processes and corresponding properties of glasses. The concentration and lifetime of the defects and of the excited states in selenium have been determined as a function of temperature. On this quantitative basis the interrelationship of the totality of experimental data concerning both micro-properties (photoluminescence, photo-induced ESR, thermally-induced ESR) and macro-properties (crystallization, viscosity, photostructural changes) is examined.