Band filling in GaAs-GaAlAs quantum well graded-index injection lasers studied by photocurrent spectroscopy

GaAs-GaAlAs quantum well (QW) structures with a graded-index waveguide (GRIN) as used in high-power laser arrays have been studied by photocurrent spectroscopy in terms of dependence on temperature, polarization of light and additional unmodulated (DC) light in the photon energy range of optical transitions between bound QW states. In TM polarization, the spectrum is dominated by the first QW transition between the light hole and electron subbands lh1-e1. Higher transitions are not detected against the background of high absorption or are covered by the GRIN absorption. According to the observed temperature dependence, the escape mechanism of carriers from the QW is assigned to thermionic emission. The escape is determined from an Arrhenius plot and related to the band model. The photoresponse near the QW transitions is strongly suppressed by additional unmodulated light with photon energy of 1.96 eV. From the spectra recorded with different DC light intensities, the conclusion was drawn that this is only partially due to the decrease of the built-in voltage. The main effect is due to bleaching of QW transitions by subsequent filling of energy states in the QWs. The dependence on DC light intensity is strongly nonlinear due to the small density of states of QW systems.