Abstract
Strain-free epitaxial quantum dots (QDs) are fabricated by a combination of Al local droplet etching (LDE) of nanoholes in AlGaAs surfaces and subsequent hole filling with GaAs. The whole process is performed in a conventional molecular beam epitaxy (MBE) chamber. Autocorrelation measurements establish single-photon emission from LDE QDs with a very small correlation function g (2)(0)≃ 0.01 of the exciton emission. Here, we focus on the influence of the initial hole depth on the QD optical properties with the goal to create deep holes suited for filling with more complex nanostructures like quantum dot molecules (QDM). The depth of droplet etched nanoholes is controlled by the droplet material coverage and the process temperature, where a higher coverage or temperature yields deeper holes. The requirements of high quantum dot uniformity and narrow luminescence linewidth, which are often found in applications, set limits to the process temperature. At high temperatures, the hole depths become inhomogeneous and the linewidth rapidly increases beyond 640 °C. With the present process technique, we identify an upper limit of 40-nm hole depth if the linewidth has to remain below 100 μeV. Furthermore, we study the exciton fine-structure splitting which is increased from 4.6 μeV in 15-nm-deep to 7.9 μeV in 35-nm-deep holes. As an example for the functionalization of deep nanoholes, self-aligned vertically stacked GaAs QD pairs are fabricated by filling of holes with 35 nm depth. Exciton peaks from stacked dots show linewidths below 100 μeV which is close to that from single QDs.
Highlights
Local droplet etching (LDE) [1] has been established as a powerful extension of conventional molecular beam epitaxy (MBE) that allows the self-assembled fabrication of strain-free nanostructures
Nanohole uniformity Process parameters influencing the hole depth are the As background pressure [24], the surface coverage θ with deposited Al droplet material [10], and the process temperature T during LDE [10]
With our results, we expand the possibilities of the local droplet etching method
Summary
Local droplet etching (LDE) [1] has been established as a powerful extension of conventional molecular beam epitaxy (MBE) that allows the self-assembled fabrication of strain-free nanostructures. LDE was first demonstrated for drilling of nanoholes into GaAs or AlGaAs surfaces with Ga droplets as etchant [1,2,3,4,5,6]. We have established etching of AlGaAs and AlAs surfaces using LDE with Al droplets [7, 8] (Fig. 1). The usage of Al droplets is advantageous for quantum dot (QD) fabrication, since here, the walls around the nanohole openings (Fig. 1e) are composed of optically inactive AlAs [7]. We fill the nanoholes with GaAs to fabricate strain-free quantum dots (QDs) [7]. These QDs exhibit controlled and uniform size [7], clear excitonic features [8], and small excitonic fine-structure splitting [9]. Shallow nanoholes with depth of 10 to 15 nm are used as template for fabrication of QDs with narrow optical linewidth
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