Coexistence of photodarkening and photobleaching in Ge-Sb-Se thin films.

We have investigated photoinduced effects in as-deposited Ge 40−x Sb x Se 60 (x = 8, 15, and 20) films via in situ measurements of transmission spectra for wavelengths λ = 500 to 950 nm. The absorption coefficient increases initially, indicating the photodarkening (PD) effect, which rapidly saturates in tens of seconds. Then the absorption coefficient starts decreasing representing photobleaching (PB). The evolution of PD and PB can be described by stretched exponential function, yielding the time constants for the changes in absorption coefficients. The kinetics of PD is faster than that of PB. The coexistence of PB and PD suggests that the Sb and Ge based structural units of Ge-Sb-Se films respond to bandgap radiation independently. The structural origins of the observed opposing photoinduced effects are discussed. [ABSTRACT FROM AUTHOR]