In this letter we investigate the changes in the surface morphology and emission wavelength of InAs quantum dots (QDs) during initial GaAs encapsulation by atomic force microscopy and photoluminescence. The density (2.9×1010 cm−2) and height (7.9±0.4 nm) of the uncapped QDs decrease and saturate at 0.6×1010 cm−2 and 4 nm, respectively, after the deposition of 4 monolayers (MLs) of GaAs. A model for the evolution of surface morphology is proposed. Photoluminescence spectra of the surface dots show a wavelength shift from 1.58 to 1.22 μm when the GaAs capping layer thickness increases from 0 to 8 MLs.

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By capping InAs quantum dots (QDs) with a thin intermediate layer of InAlAs instead of GaAs, the radiative transition wavelengths are redshifted. Surface morphology studies confirm that the redshift is due to a better preserved QD height as compared with capping by GaAs only. In contrast, the energy levels are blueshifted when using AlGaAs instead of GaAs as the barrier material. In both cases, the energy separation between the ground and the first-excited state increases significantly. Combining these approaches, we demonstrate InAs QDs with a record transition energy separation of 108 meV and ground-state emission at 1.3 μm.

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We have performed experimental and theoretical studies of the effects of inhomogeneous broadening on the luminescence properties of a self-assembled InAs quantum dot (QD) assembly. From atomic force microscopic (AFM) images the InAs QD assembly is found to have an average lateral size of 20–22 nm and a height of 10–12 nm, and the dot density is in the range of 1–2×1010 cm−2. Using the statistical distribution of the QD size from AFM measurements and the results from the theoretical analysis of the photoluminescene (PL) spectrum, it is found that the distance between QDs is larger than 30 nm (the average distance is about 100 nm), the penetration of the ground-state wave function into the GaAs barrier is negligible, and the calculated PL spectrum agrees well with that measured when the carriers in each QD are assumed to be at a local thermal equilibrium state, resulting in the conclusion that the QDs are physically independent. The width of the PL peak is determined by the inhomogeneous QD size.