Direct determination of the built-in polarization field in InGaN/GaN quantum wells

Results of absorption and electroabsorption measurements on wurtzite (In)GaN light emitting diodes are reported. Despite the relatively high excitonic binding energy of 25 meV in GaN no pronounced excitonic features could be observed in the absorption spectra. Electroabsorption measurements were performed to study the field induced absorption changes both in the InGaN quantum wells and the GaN barriers. This technique allows precise determination of the strong internal field changes at the GaN/InGaN-hetero-interface that originate from strain-induced polarization and differences in spontaneous polarization. We find field-induced absorption changes as large as 7000 cm^-1 below and almost 20000 cm^-1 above the band edge. The deduced polarization fields vary between 1.1 and 1.4 MV/cm for indium concentrations in InGaN quantum wells ranging from 7% to 9%.