Impurity Bands and Electroluminescence in SiC p-n Junctions

Abstract

A study of the electroluminescence of certain SiC p-n junctions, between 77°K and 830°K, and over a range of 104 in current density, has been used to verify and to extend a 3-part model (P−N*−N) of the junctions derived from electrical measurements. The electroluminescence, due to recombination in N*, consists of two parts, which may be called impurity luminescence and intrinsic recombination radiation. At low temperatures (≥200°K) only the former is present. Several effects of impurity banding on the electroluminescence can be predicted, and some of these have now been observed. The most striking of these effects is the ``Fermi-level emission edge'' in the low-temperature spectra, an edge which moves to higher energies with increasing current density because of the impurity band injection of electrons. The predominance of impurity band injection at low temperatures excludes the possibility of intrinsic recombination radiation. At higher temperatures, however, electron injection is by way of the conduction band. The intrinsic recombination radiation observed at higher temperatures has been compared with that calculated from our earlier absorption data, using the theory of van Roosbroeck and Shockley. From the comparison we obtain the density of electrons in the conduction band, and the electron lifetime (10−8 sec) at room temperature. This lifetime is very short compared with the measured hole lifetime, and is possibly a result of the merging of impurity band and conduction band levels. Our junction model is now detailed enough to estimate impurity level densities and the hole and electron densities. From these, we are able to calculate cross sections for the various recombination processes involved. All luminescent processes have been found to have a recombination cross section of approximately 10−23 cm2.