Low density InAs/(In)GaAs quantum dots emitting at long wavelengths

We present research carried out on molecular beam epitaxy grown InAs/(In)GaAs quantum dot structures for single- photon operation at long wavelengths. The optical and morphological properties of the structures are studied as functions of quantum dot growth parameters and of the InGaAs upper confining layer thickness and composition. We show that low growth rate, high growth temperature and reduced quantum dot coverage are very effective in reducing the quantum dot density but, owing to In desorption effects and quantum dot size reduction, this result is not always concomitant with the achievement of long wavelength emission. To this aim, we show that the use of InGaAs upper confining layers allows the redshift of quantum dot emission energy without affecting their density. Both the thickness and composition of the InGaAs layer have to be carefully chosen to provide a complete coverage of quantum dots and not to exceed the critical thickness for plastic relaxation. Our results led to the preparation of quantum dot structures with densities in the low 10(9) cm(-2) range, 1.33 mu m emission at 10 K and full widths at half maximum of 22 meV.