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(a) Scheme of the three process steps for the fabrication of GaAs QDs by utilizing LDE: Al droplets deposition in Volmer-Weber mode, hole etching during post-growth annealing, and QD generation by hole filling with GaAs and AlGaAs capping. The walls surrounding the hole openings consist of optically inactive AlAs21 and, thus, are not shown in (b)-(d). (b)-(d) Scheme of different types of GaAs QDs realized by applying the LDE technique. (b) Non optimized LDE on AlGaAs yields non-uniform nanoholes and a broad QD size distribution with mostly completely filled holes.22 (c) Homogeneous, partially filled nanoholes are generated by integrating an AlAs layer before the etching step.21 (d) The present ULD QDs are fabricated by optimizing the As flux during droplet deposition.
(a)-(c) Atomic force microscopy images of LDE samples with varied arsenic flux during droplet deposition as indicated. On all samples, LDE was performed at T = 620 °C substrate temperature, with 3.2 ML Al, and an annealing time of 5 min. A typical shallow hole is marked by S and a deep hole by D in the AFM graphs. (d) Density of shallow (blue squares) and deep holes (red triangles) in dependence of . The lines are guides to the eye. The ULD regime, where exclusively deep holes are formed, is shaded.
(a)-(c) AFM pictures and corresponding line scans along  and [1−10] crystal directions of ULD nanoholes. The samples are fabricated at and T = 630 °C. After etching, the nanoholes were covered with an AlGaAs layer of different thicknesses as indicated. (c) The buried holes are still visible after covering with 200 nm of AlGaAs.
(a) Density N of the nanoholes as function of the process temperature T measured and calculated as is described in the text. (b) Hole depth d measured and calculated.
(a) Power dependant single-dot PL spectra from a 8.1 nm high QD. The exciton X and biexciton XX peaks are marked and their splitting of 2.28 meV is indicated. The spectra are vertically shifted for clarity. (b) Excitonic emission energies , and (c) exciton-biexciton splitting in dependence of the QD height given by the number of GaAs filling pulses. Every data point represents at least five individual QDs on the sample.
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