Electroabsorption spectroscopy on AlGaN heterostructure devices (invited talk)
Electroabsorption spectroscopy has proven to be a powerful and sensitive technique to accurately determine even small absorption changes in semiconductor materials. Surprisingly, only a few studies have been performed so far on group-III Nitride-based semiconductor films, although many optical properties in these films have yet to be accurately determined. For example, in wurtzite InGaN/GaN heterostructures, large internal fields exist due to strain-induced piezoelectric fields and differences in the spontaneous polarization. These internal fields have been discussed in many theoretical and experimental publications but reliable values for these fields are still lacking and their impact on light emitting devices is still debated. In this contribution we report on a method, which has allowed us to determine these fields with high accuracy. We have performed electroabsorption experiments on wurtzite InGaN/AlGaN quantum wells embedded in the intrinsic region of GaN heterostructure p-i-n diodes. The field induced absorption changes as well as the field dependent absorption spectra have been measured for various samples with a quantum well thickness ranging from 4nm to 20nm. In all cases the electroabsorption spectra are similar and reflect the absorption behavior expected for bulk semiconductors indicating that the carrier-confinement in the quantum well layers is relatively weak. No pronounced excitonic features are observable even if the strong internal polarization fields are compensated by an external bias. The absorption changes are clearly resolvable at room temperature even in diodes containing only a single narrow quantum well. The analysis of the electroabsorption data allows a precise determination of the strong internal fields, originating from strain-induced polarization and differences in the spontaneous polarization. The basic idea of this method is to determine the applied reverse voltage necessary to compensate the internal field within the InGaN layer. From this, and supplemental measurements of the space charge layer (CV-measurements), we are able to determine the magnitude of the strong piezoelectric field with high accuracy on functioning heterojunction diodes.
Kiesel, P. ; Schmidt, R. F. ; Kneissl, M. A. ; Johnson, N. M. Electroabsorption spectroscopy on AlGaN heterostructure devices (invited talk). ONR Workshop on Defect Characterization Techniques in Wide Gap Semiconductors; 2003 March 16-20; Wailea; HI; USA.