Optical spectroscopy of quasimonolayer InAs at the onset of quantum-dot nucleation

We have performed low-temperature photoluminescence (PL), resonant PL (RPL), and PL excitation (PLE) measurements on different series of self-organized InAs/GaAs quantum dots (QD’s) in samples with an InAs nominal coverage (L) varying from 1.2 to 3 monolayers (ML). Drastic changes in the PL spectra have been observed for L values spanning across the so-called critical thickness $L_c\hspace{0.5em}(\sim 1.7$ ML). RPL has shown that both QD’s and quasi-three-dimensional QD precursors contribute to the spectra of samples with $L\lesssim L_c.$ PLE measurements have allowed us to introduce an accurate determination of $L_c$ and to verify its dependence on sample growth conditions. Finally, the analysis of PL spectra in all investigated samples and its comparison with spatially resolved PL measurements has suggested a different interpretation of doublet and triplet bands usually found for $L\sim L_c$ and previously ascribed to excited states or multimodal distributions of QD families.