Optically induced metastable paramagnetic states in amorphous semiconductors

Mid-gap optical absorption and electron spin resonance (ESR) attributed to optically induced localized paramagnetic states have been studied in several chalcogenide glasses and amorphous arsenic for $T\le 80$ K. The observation of these optically induced metastable paramagnetic centers provides the most direct evidence yet obtained for the existence of localized gap states in these amorphous semiconductors. Irradiation with light whose energy corresponds to the Urbach tail of the absorption edge ($\alpha \simeq 100$ ${\mathrm{cm}}^{-1\mathrm{}}$) excites photoluminescence (PL) which fatigues (decays) with continuing excitation. The fatigue of the PL is accompanied by the appearance of a growing ESR signal which is not present before illumination. The occurrence of an associated induced optical absorption demonstrates that the paramagnetic states are located in the forbidden gap of these amorphous semiconductors. Subsequent prolonged irradiation of the sample with infrared light in the mid-gap induced absorption band reduces the strength of or "bleaches" both the optically induced ESR signal and the induced absorption and restores the fatigued PL to its cold dark (initial) efficiency. These facts suggest a mechanism whereby radiative recombination centers which become inactive during PL fatigue are closely associated with the production of the metastable paramagnetic centers. The optically induced ESR centers are apparently unique to the amorphous phase and their density ($N_s$) saturates at or below ${10}^{17}$ spins per ${\mathrm{cm}}^3$ in all materials measured to date regardless of light intensity. Analysis of the ESR spectra has led to the identification of a hole center which consists of an electron missing from a nonbonding lone pair chalcogen orbital and a center in As-containing glasses and amorphous arsenic which is localized in an As $p$ orbital. A detailed account of the ESR spectral analysis is presented. Changes in the local structural order as a function of composition appear as changes in the $g$ values and width of the optically induced ESR spectrum. The value of $N_s$ is also a function of composition with the highest values occurring in glasses containing substantial amounts of As such as ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$. The addition of Ge depresses $N_s$ to levels as low as ∼ 5 × ${10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$ in Ge${\mathrm{Se}}_2$, indicating that the structural differences which accompany the introduction of tetrahedrally coordinated Ge reduce the density of the specific structural anomaly which determines the value of $N_s$. Possible mechanisms for the generation of the optically induced states are presented, and the relationship between the experimental results and various models proposed to explain the properties of localized gap states is discussed.