Morphological control of GaAs quantum dots grown by droplet epitaxy using a thin AlGaAs capping layer

Abstract

We demonstrate the control of GaAs quantum dots morphology by using a thin AlGaAs capping layer. The AlGaAs layer uniformly covers the GaAs quantum dots and provides protections against thermally induced deformation up to 580 °C, which allows improved dot quality. In addition, annealing of AlGaAs-capped quantum dots at 640 °C flattens the top of the dots, leading to the formation of height-controlled quantum dots and their narrow inhomogeneous width of 28 meV.

Geometry of epitaxial GaAs/(Al,Ga)As quantum dots as seen by excitonic spectroscopy

“…1). Notwithstanding the shape, the QDs having calculated exciton energy in the range of the experimental measured exciton energy of 1.7-1.9 eV are seen to have a much smaller dot height of only 1-4 nm compared to the experimentally stated value (∼ 14 nm) by both AFM 15 and XSTM 8 approaches. This discrepancy, being well outside the measured size-distribution in the sample, indicates that the QDs measured by AFM or XSTM are not same as QDs seen by optical spectroscopy.…”

“…al. 8 of the dots grown by the same Sakoda's group 15,19 showed instead a rather different, Gaussian shape (schematic in right inset to Fig. 1) with an average base size of 40 nm, height of 14 nm and a size distribution of 10 − 20%.…”

“…1), with averaged [110]-elongated base size of 70 × 50 nm (spread ±10%) and dot height of 14 nm (spread ±19%) 15 . Subsequent, more refined XSTM measurement of capped dots by Keizer et.…”

“…Recent XSTM measurement 8 suggested that such QDs have Gaussian-shape instead of lens-shape as measured by Atomic force microscope (AFM) 2,15-17 and dot height is around 14 nm 8,15 . The measured exciton band gap by optical spectroscopy is about 1.7-1.9 eV 2,17-20 .…”

Optical characterization of individual GaAs quantum dots grown with height control technique

“…5 However, typical DE-QD emission spectra show quite large, inhomogeneously broadened, bands in the 60-100 meV range, which are the outcome of the initial size dispersion of the droplet, from which the QDs are formed, and of the complex crystallization dynamics. 11,12 Recently a morphological control of the QD heights has been proposed by using a thin AlGaAs capping layer and in situ annealing, 13 with the flattening of the top of the QDs, leading to the reduction of the inhomogeneous emission bandwidth from a large ensemble of high density QDs with superior uniformity; this property has been exploited for building laser diodes.…”