Field Dependent Trap Parameters

The computer simulation of charge transport in electron-beam irradiated SiO2 layers has shown that electric fields up to some 106 V.cm-1 are possible within the negative space charge region. An experimental variation of the parameters strongly influencing the maximum field strength FM results in systematic shifts of the glow peaks of a-Al2O3 powder by up to ± 5 K only. Furthermore the peaks often agree with those of TSEE, TSL and TSC given in the literature for different forms of Al2O3 after quite different excitation. The assumption of the resolution limit of T-shifts of 3 K due to complex trap spectra gives FM " 103 V.cm-1 for single positively charge coulomb centres. For screened coulomb potentials, FM depends on temperature T as well as on trap concentration H. With H = 5 x 1017 cm-3, FM _ (1.3 to 3.5) 103 V.cm-1 (T = 273 to 773 K) and d (log FM) /d (log H) " 0.5. FM for neutral traps increases up to 105 V.cm-1 depending on their nature (potential) as well as on potential calibration. Similar behaviour should hold for carrier release due to annealing of defects or relaxation, for example, of strained bonds. A further increase in FM may proceed from field-dependent frequency factors owing to their connection with the capture cross section of traps which in general decrease with increasing fields. So, indeed, fields up to 106 V.cm-1 could not influence peak positions of TSEE. More insight into these problems is expected from direct parameter determination for simpler trap spectra, for example, in MIM or MOS structures as well as from investigation of the nature and concentration of traps in connection with theoretical studies of thermally stimulated relaxation phenomena.