PHOTOACOUSTIC SPECTROSCOPIC ESTIMATION OF OPTICAL-ABSORPTION AND THERMAL-DIFFUSIVITY OF AMORPHOUS GE15ASXSE85-X

Photoacoustic spectroscopy (PAS) has been applied to measure the compo sition dependent optical energy gap (E(o)), optical absorption coeffic ient (alpha), and thermal diffusivity (sigma(s)) of glassy Ge15AsxSe85 -x (0 less than or equal to x less than or equal to 25) alloys. The en ergy gap is found to decrease with increase of As concentration and sh ows a threshold behavior around x similar to 15. which corresponds to the average coordination number [r]similar to 2.45. This behavior is f ound to be consistent with the Phillips-Thorpe theory. The variation o f the optical gap (E(o)) with composition (x) is analyzed on the basis of the Kastner's model of bond energies. The observed exponential edg e (Urbach edge) can be considered as an internal Franz-Keldysh effect arising due to the charged impurity generated, as well as ''frozen-in' ' optical phonon-generated, electric microfields. It could be describe d in the framework of Tauc and Dow-Redfield model which ascribes the U rbach rule to the ionization of the exciton as an extension of the sta rk shift. The concentration (x) dependent thermal diffusivity (sigma(s )) estimated from the PAS studies also showed a similar critical behav ior at the same concentration x (approximate to 15) which arises due t o the threshold percolation of rigidity of the system. The measurement s of glass transition temperatures (T-g) and the magnetic susceptibili ties (chi) of the samples also support this critical behavior around x =15. (C) 1996 American Institute of Physics.