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 (similar to 1.7 ML). RPL has shown that both QD's and quasi-three-dimensional QD precursors contribute to the spectra of samples with L less than or equal to 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 similar to L-c and previously ascribed to excited states or multimodal distributions of QD families. [S0163-1829(99)05627-1].