Spectroscopic determination of the electron distribution in a quantum cascade structure

Complementary intersubband and interband optical measurements have been employed in order to study the bias dependence of the carrier distribution and energy level alignment within a GaAs-AlGaAs quantum cascade structure. Using these techniques, we have measured the redistribution of electrons throughout the bridging regions and upper states in the active regions with increasing bias. The high quality of the sample gives very narrow linewidths in the optical spectra, permitting the resolution of transitions involving closely spaced energy levels. This has allowed the direct observation of level alignment at the onset of current flow through the device. (C) 1999 American Institute of Physics. [S0003-6951(99)01840-9].