Resolution of discrete excited states in Inx Ga1-x N multiple quantum wells using degenerate four-wave mixing

We report on two pulse, degenerate four-wave mixing (DFWM) measurements on shallow Inx Ga1-x N GaN multiquantum wells (MQWs) grown on sapphire substrates. These reveal pulse length limited signal decays. We have found a 10:1 resonant enhancement of the DFWM signal at the excitonic transition frequencies which thereby give a sharp discrimination of the discrete excitonic contributions within the featureless distribution seen in absorption spectra. The exciton resonances have peak positions, which yield good overall agreement with a full k•p model calculation for the quantum well (QW) energy levels and optical transition matrix elements. Inx Ga1-x N GaN MQWs generally exhibit strongly inhomogeneously broadened excitation spectra due to indium fluctuation effects; this approach therefore affords a practical method to extract information on the excited excitonic states not available previously.