Measurement and calculation of spontaneous recombination current and optical gain in GaAs-AlGaAs quantum-well structures

Experimental determinations have been made of the peak optical gain as a function of spontaneous recombination current density for GaAs quantum wells of width 25 and 58 Å bounded by AlGaAs barriers. These data were obtained from measurements of spontaneous emission spectra, observed through narrow windows in the 50-μm-wide contact stripes of oxide isolated lasers, using only a single reference value of the optical absorption coefficient above the band edge to calibrate the measurements in absolute units. These results are in good agreement with gain-current curves calculated using a model which includes unintentional monolayer well width fluctuations, band-gap narrowing and intraband carrier-carrier scattering. The characteristic intraband scattering time is calculated from first principles as a function of electron energy and carrier density on the basis of a 2-dimensional Auger-type process. This lifetime gives a much better representation of our observed spontaneous spectra than a lifetime which is simply dependent upon carrier density. The comparison between experiment and model calculation involves no adjustable parameters. For the 58-Å-wide wells there is a difference between the experimental and calculated gain-current curves at low values of gain. We show that this is a consequence of applying the Einstein relations to a broadened spectrum in the process of deriving the gain from the observed spontaneous emission spectrum. A direct comparison of the shapes of experimental and calculated spontaneous emission spectra at several injection levels provides a more rigorous, yet equally valid, verification of the computer model.