Absorption and photoluminescence of ultrathin pseudomorphic InAs/GaAs quantum wells

We present absorption data for 2–4-monolayer InAs/GaAs single quantum wells obtained at 77 K using a polarization-based measurement technique. The spectral contribution of the optical loss features arising from bulk GaAs was minimized using the polarization selectivity of absorption in single quantum wells. Based on polarization properties and the results of an envelope-function calculation, the double structure observed in the spectra is attributed to transitions involving confined heavy holes and both confined and unconfined electron states. No evidence of transitions with the participation of confined light holes was found. Well-width fluctuations deduced from the absorption data are a fraction of one monolayer in our samples. The absorption lines were broadened, indicating interface roughness on a scale comparable to the exciton diameter. A single photoluminescence peak overlapping with the lower-energy portion of the fundamental absorption feature was observed in all the samples at 77 K. Although the absorption line of the undoped two-monolayer sample displayed the least broadening, the photoluminescence was red shifted by approximately 70 meV relative to the absorption feature. Presumably, this behavior indicates long-range diffusion of the excitations. Measurements of the temperature dependence and pump-power dependence of the red shift in this sample support this interpretation.