Theoretical and experimental study of the optical-absorption spectrum of exciton resonance inIn0.53Ga0.47As/InP quantum wells

This paper presents a theoretical formulation of the integrated intensity and the profile of the optical-absorption spectrum of exciton resonance in quantum wells. Using our model, we analyzed the ground-state electron–heavy-hole exciton resonance in 20-period multiple quantum wells with 10-nm ${\mathrm{In}}_{0.53}$ ${\mathrm{Ga}}_{0.47}$As wells and 10-nm InP barriers. We found the intrinsic integrated intensity in this quantum well to be 1.1×${10}^{\mathrm{-}4}$ eV, which was about 70% of that in GaAs/${\mathrm{Al}}_{0.25}$ ${\mathrm{Ga}}_{0.75}$As quantum wells reported by Masselink. We demonstrate quantitatively that the smaller integrated intensity is due to the larger two-dimensional exciton radius caused by the larger static dielectric constant and the smaller in-plane reduced effective mass. We propose that the resonance spectrum profile should be formed by the convolution integral between the broadening function due to the spatial inhomogeneity of exciton energy and that due to the reduction of exciton lifetime by thermal phonon scattering. Inhomogeneous broadening was found to be a Gaussian distribution from the measured low-temperature spectrum. Assuming that the LO phonon is the major scattering source for excitons and that the distribution function of their lifetime is Lorentzian with a width of half the average, we could explain the measured profile of exciton resonance up to high temperatures. We found the average exciton lifetime to be 300 fs at room temperature. We argue that inhomogeneous broadening due to composition fluctuations in ${\mathrm{In}}_{0.53}$ ${\mathrm{Ga}}_{0.47}$As wells and the larger thermal broadening due to the larger exciton-phonon coupling make the exciton spectrum broader than GaAs/${\mathrm{Al}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ga}}_{\mathit{x}}$As quantum wells. We conclude that both the smaller integrated intensity and the broader spectrum make the exciton resonance in ${\mathrm{In}}_{0.53}$ ${\mathrm{Ga}}_{0.47}$As/InP quantum wells weaker. We present the condition of inhomogeneous broadening needed to observe exciton resonance at room temperature.