Photocurrent spectroscopy of QW GRIN laser structures

GaAs/GaAlAs quantum well (QW) structures with a graded-index waveguide (GRIN) designed for high-power laser arrays have been studied by photocurrent spectroscopy in relation to temperature and polarization. Optical transitions between confined QW states at photon energies between 1.4 and 1.8 eV were observed: with TM polarization the transitions lh1-e1 and lh2-e2. The lh1-e1 subband edge is best resolved and can be followed up between 170 and 375 K. The height of the lh1-e1 spectral peak depends nonmonotonously on temperature: At temperatures below 325 K the peak height increases with temperature, the photocurrent is attributed to thermionic emission. At temperatures above 325 K the peak height decreases with temperature, this is attributed to pair recombination in the 2D QW. The accuracy of extracting energies from the spectra is discussed in terms of the 2D Elliott formula and broadening. The experimental technique of light bias is used to increase the sensitivity. An example is given to use the high accuracy reached for studying mounting induced strain in centimeter laser array bars.