Excitonic effects associated with saddle points and subband dispersion were studied in a series of GaAs/${\mathrm{Al}}_{0.18}$${\mathrm{Ga}}_{0.82}$As superlattices at 5 K using photoluminescence excitation spectroscopy. The well width was fixed at 75 Å and the barrier widths ($L_b$’s) were 105, 60, and 35 Å. Distinct new peaks and structures were detected in addition to the major n=1 excitonic peaks (1HH, 1LH). The origin of these peaks in superlattices is attributed to the excitonic resonance occurring near the lower band edge and the upper saddle points of the subband. This excitonic resonance is brought about by the redistribution of the oscillator strengths of the continuum states due to the electron-hole Coulomb interaction. Excellent agreement was found between the data and theoretically generated curves which incorporate saddle-point excitonic effects and valence-band mixing. We also observed a decrease in exciton binding energies, approaching the bulk GaAs limit, with a decrease in $L_b$.

• Received 2 December 1988

DOI:https://doi.org/10.1103/PhysRevB.39.5562