Recombination-enhanced defect reactions strong new evidence for an old concept in semiconductors

Abstract

Significant evidence linking changes in the macroscopic properties of semiconductors with recombination processes associated with injection current components has accumulated during the past 15 years or so. These effects play a critical role in performance degradation for many semiconductor devices. In this paper we review the developments in understanding of this effect. We concentrate on some very recent detailed studies which provide convincing evidence that carrier capture or recombination processes at deep traps can induce or enhance dissociation and/or migration of the defect or impurity centres which produce them. Thus, the ‘phonon-kick’ mechanism invoked to explain the early macroscopic property changes has now received strong support. We show that the new findings provide a hierarchy of increasingly detailed information on the properties of traps which may show this effect, now termed a recombination enhanced (or induced) defect reaction (REDR). Carrier capture or recombination at these traps must be predominantly non-radiative, to provide the rapid, large evolution of localized vibrational energy necessary for the effect. Deep level transient capacitance spectroscopy has located such energy levels, particularly in irradiated GaAs and GaP, and has clearly demonstrated the sensitivity of the concentration of the relevant centres to electron-hole recombination processes. The resultant defect motility can improve the minority carrier lifetime, particularly adjacent to dislocations which act as sinks for these defects. The defect capture can also induce the type of dislocation climb which is a key feature in the degradation of semiconductor lamps. Direct evidence for these effects has been obtained very recently by electron microscopy, which has also shown the coalescence into small loops and precipitates of defects left behind climbing dislocations in degraded material without prior radiation damage. We show that luminescence spectroscopy has also recently provided evidence for the dissociation of complex radiative centres under REDR, for example the familiar Zn-O activator in red GaP LEDs and an H-related centre which is luminescent in several polytypes of H-implanted SiC. The dissociation and hence the luminescence can be largely restored by annealing in both examples. We discuss the properties of the H-related spectra which provide uniquely specific evidence for the vulnerability of this centre to dissociation under REDR, even at the lowest temperature. We also give recent evidence that a complex Zn-related luminescent centre can be formed by REDR in irradiated GaP. Finally, we review a recent theoretical description of REDR which assumes quasi-equilibrium for the energy liberated locally at the centre. We note that an adiabatic model may be more appropriate for those centres where the reactions are not enhanced thermal processes but rather are induced athermally.