Photoluminescence excitation spectroscopy of InGaN epilayers

Photoluminescence (PL) has been reported from InGaN-based heterostructures, including thick epilayers on GaN, InGaN/GaN quantum wells and InGaN/GaN quantum boxes, with peak energies ranging from 3.44 to 1.31 eV at low temperature. The corresponding absorption spectra are not always easy to obtain, but photoluminescence excitation (PLE) spectroscopy provides an efficient means of obtaining comparable information. We describe here a comprehensive investigation of PLE spectra from a wide range of InGaN samples. Variation of the measured bandgap energy with the detection energy for individual samples suggests that the InGaN emission spectrum is inhomogeneously broadened. The PLE spectrum obtained at the peak emission energy of a particular sample is equivalent to the absorption spectrum of that sample. The data range of the band edge measurements is extended to lower energies by the PLE results. In general, the PLE data confirm the existence of a linear relationship between the optical bandgap and the emission energy.