Quasi-Dynamic Approach in Structural Disorder Analysis: an Ion Beam Irradiated Silica

Optical properties of glassy silicon dioxide irradiated with 30 keV rhenium ions were investigated. The blurring and low-energy shift of the sample absorption edge was established in 5 x 10(15) -5 x 10(17) cm(-2) range of ion fluences. Dose dependences of characteristic Urbach energy E-U and optical gap width were obtained for direct interband transitions. The ratio of deformation potential constants D/K was determined on the basis of the linear correlation between E-U and parameters. The constancy of D/K ratio at different ion fluences indicates proportionality between optical gap values and band tails caused by local displacements of atoms in SiO2 host matrix. An interpretation of established regularities was given in the framework of thermodynamic approach. The structural damages induced by ion irradiation as a set of various system microstates and relationship with Boltzmann entropy value were postulated. Thermodynamic temperature and ion fluence were considered as equivalent factors that determine a certain degree of atomic disorder in the system within the framework of the above concepts. A special parameter a is introduced as an effective disordering cross section to analyze the dynamics of radiation-induced defect formation. Functional dependence of 6-parameter on the ion fluence was experimentally established. The analytical expressions for describing dose dependences of optical gap and characteristic energy E-U were obtained. It is assumed that these expressions can be used to predict the radiation-induced modification of some electron-optical properties in energy gap materials.