Strain and optical transitions in InAs quantum dots on (001) GaAs

To investigate the strain characteristics of InAs quantum dots grown on (001) GaAs by solid source molecular beam epitaxy we have compared calculated transition energies with those obtained from photoluminescence measurements. Atomic force microscopy shows the typical lateral size of the quantum dots as 20-22 nm with a height of 10-12 nm. and photoluminescence spectra show strong emission at 1.26 mum when the sample is capped with a GaAs layer. The luminescence peak wavelength is red-shifted to 1.33 mum when the dots are capped by an In0.4Ga0.6As layer. Excluding the strain it is shown that the theoretical expectation of the ground-state optical transition energy is only 0.566 eV (2.19 mum), whereas a model with three-dimensionally-distributed strain results in a transition energy of 0.989 eV (1.25 mum). It has thus been concluded that the InAs quantum dot is spatially strained. The InGaAs capping layer reduces the effective barrier height so that the transition energy becomes red-shifted. (C) 2001 Academic Press.