Growth conditions influence on thermally stimulated luminescence of sapphire single crystals

The results of investigation of thermally stimulated and photoluminescence as well as absorption spectra of a-sapphire grown by a technique of horizontally directed crystallization in a protective gas medium of varying composition and pressure are presented. This technique has an advantage of minimization of W and Mo erosion, thus providing a considerably reduced prime cost. At the same time, there are some problems connected with emergence of considerable number of anion vacancies, whose in-axis concentration gradient is directed oppositely to that of dopants (mainly Ti). The correlations are found between the conditions of -sapphire growing and the data of absorption and luminescent spectroscopy. An interaction between the anion vacancies and Ti dopants, which induces a transition from electric-charge state Ti⁴+ to Ti³+, is discussed

Temperature dependence of the charge carrier mobility in disordered organic semiconductors at large carrier concentrations

Temperature-activated charge transport in disordered organic semiconductors at large carrier concentrations, especially relevant in organic field-effect transistors (OFETs), has been thoroughly considered using a recently developed analytical formalism assuming a Gaussian density-of-states (DOS) distribution and Miller-Abrahams jump rates. We demonstrate that the apparent Meyer-Neldel compensation rule (MNR) is recovered regarding the temperature dependences of the charge carrier mobility upon varying the carrier concentration but not regarding varying the width of the DOS. We show that establishment of the MNR is a characteristic signature of hopping transport in a random system with variable carrier concentration. The polaron formation was not involved to rationalize this phenomenon. The MNR effect has been studied in a OFET based on C60 films, a material with negligible electron-phonon coupling, and successfully described by the present model. We show that this phenomenon is entirely due to the evolution of the occupational DOS profile upon increasing carrier concentration and this mechanism is specific to materials with Gaussian-shaped DOS. The suggested model provides compact analytical relations which can be readily used for the evaluation of important material parameters from experimentally accessible data on temperature dependence of the mobility in organic electronic devices. Experimental results on temperature-dependent charge mobility reported before for organic semiconductors by other authors can be well interpreted by using the model presented in this paper. In addition, the presented analytical formalism predicts a transition to a Mott-type charge carrier hopping regime at very low temperatures, which also manifests a MNR effect