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 angstrom bounded by AlGaAs barriers. These data were obtained from measurements of spontaneous emission spectra, observed through narrow windows in the 50-mu-m-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-angstrom-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.