Homogeneity improvement of InAs/GaAs self-assembled quantum dots grown by molecular beam epitaxy

Abstract

Different methods to improve the size homogeneity of InAs/GaAs self-assembled quantum dots (QDs) have been investigated in this work. The QD fabrication process relies on self-assembled growth in the Stranski-Krastanow mode by molecular beam epitaxy. The homogeneity of the QD ensembles was measured by ex situ atomic force microscopy (AFM) and room-temperature photoluminescence (PL).The investigated QD arrays consist of large, low-density QDs (average height of about 10.7 nm, average diameter of about 37 nm and density of about 3-6x10x10x10x10x10x10x10x10x10 cm-2) and small, high-density QDs (average height of about 4.5 nm, average diameter of about 33 nm and density of about 4x10x10x10x10x10x10x10x10x10x10 cm-2). The two different QD types were grown by using InAs growth rates of 0.01 and 0.2 monolayer/s, respectively. The QD growth temperature was fixed at 500 ํC and the amount of deposited InAs was 1.8 monolayer.We found that for the large QDs a 30s growth interruption prior to capping the QDs with GaAs improves the size homogeneity of the QD ensemble. The homogeneity improvement was measured in terms of PL linewidths, which narrows from 38 meV to 32 meV in case the growth interruption was introduced. This phenomenon can be explained by strain-dependent adatom diffusion processes on surface incorporated with QDs.In case of small QDs, we improve the size homogeneity by a newly developed repetitive desorption-regrowth. By this repetitive desorption-regrowth, the QDs improve their size (height) distribution from 67% to 21%.Finally, we improve the PL linewidth of both large and small QDs by capping at low temperature (470 ํC). The PL results reveal that low temperature capping of large QDs redshifts the emission wavelength to 1.3 mm with a narrow peak linewidth of 23 meV. For the small QDs low temperature capping also narrows the linewidth from 51 meV (capped at QD growth temperature) to 26 meV.