Observations and calculations of the exciton binding energy in (In,Ga)As/GaAs strained-quantum-well heterostructures

We present the first direct measurements of the heavy-hole exciton binding energy in a III-V-compound quantum-well system where the well widths are 25 Å or smaller. The 4-K photoluminescence excitation (PLE) spectrum from ${\mathrm{In}}_{0.15}$ ${\mathrm{Ga}}_{0.85}$As/GaAs multiple-well samples with 48-, 25-, and 14-Å wells displays a sharp heavy-hole exciton feature well resolved from the continuum edge; the separation between these features yields exciton binding energies of 9.2, 8.7, and 6.5 meV, respectively. The decrease in binding energy with decreasing well width confirms the predicted trend from our calculations. Quantitative agreement between the model calculations and the observations is good. In addition, we have seen fine structure in the PLE spectrum of the 14-Å sample in both the heavy- and light-hole excitonic peaks. The energy splitting in the fine structure equates with changes in In fraction of less than 0.5% in 15% or changes in average well width of