Luminescence at 1.3 /spl mu/m and 1.5 /spl mu/m with GaAs-capped InAs quantum dots

In recent years MBE-grown In(Ga)As/GaAs quantum dots (QDs) have been explored as a source of light in a range not accessible to GaAs-based quantum wells. In fact, InGaAs-capped InAs QD lasers emitting at 1.3 /spl mu/m with low threshold current densities (< 100 A/cm/sup 2/) have been demonstrated. There is, however, great interest in extending the emission of GaAs-based structures towards even longer wavelengths (/spl sim/1.55 /spl mu/m) in order to fit the useful window for optical fiber communication. One of the approaches used to obtain luminescence at 1.3 /spl mu/m is the growth of In(Ga)As islands at low rates (< 0.01 ML/s). The long wavelength emission is provided by a distribution of large QDs peaked at a height around 10 nm. In this work we demonstrate that besides the /spl sim/10 nm-high QDs, the MBE growth at low rates can also create a distribution of even larger islands (/spl sim/15-20 nm-high) that can be optically active in the range of 1.40-1.50 /spl mu/m (at 300 K). We studied the structural and optical properties of islands formed on GaAs after the deposition of 2.4 ML, 3.2 ML and 4.0 ML of InAs, respectively, at a rate of 0.003 ML/s. The TEM study suggests that the additional structures are relaxed by the introduction of dislocation and are not expected to be optically active.