TRAPPING EFFECTS IN ALPHA-SIH INVESTIGATED BY SMALL-SIGNAL TRANSIENT PHOTOCONDUCTIVITY AND THE STEADY-STATE PHOTOCARRIER-GRATING TECHNIQUE

We report on trapping effects observed in small-signal photoconductivity and steady-state photocarrier-grating (SSPG) experiments. In the former case trapping effects appear since the small-signal photocurrent decays exponentially with time after the excitation is turned off, but with a decay time which depends on the ratio of free to trapped carriers and is determined by the intensity of the cw background illumination. In the case of (steady-state) SSPG measurements, carried out as a function of electric field, an electron drift mobility is obtained which is proportional to the ratio of free to trapped carriers and is therefore again determined by the background illumination. Both experiments show that around 0.4 eV below the electron mobility edge the density of states falls off much more slowly with energy towards midgap than near the mobility edge and may even exhibit a minimum.