Fabrication of ultralow-density quantum dots by droplet etching epitaxy

Abstract

Isolated single quantum dots enable the investigation of quantum-optics phenomena for the application of quantum information technologies. In this work, ultralow density InAs quantum dots are grown by combining Droplet Etching Epitaxy and the conventional epitaxy growth mode. The extremely low density of quantum dots (~10 6 cm -2 ) are realized by creating low density self-assembled nanoholes with the high temperature Droplet Etching Epitaxy technique and then nanohole-filling. The preferred nucleation of quantum dots in nanoholes has been explained by a theoretical model. Atomic force microscopy and photoluminescence technique are used to investigate the morphological and optical properties the quantum dot samples. By varying In coverages, the size of InAs quantum dots can be controlled. Moreover, with a thin GaAs cap layer, the position of quantum dots remains visible on the sample surface. Such a low density and surface signature of quantum dots make this growth method promising for single quantum dot investigation and single dot device fabrication.

Mechanism of nucleation and critical layer formation during In/GaAs droplet epitaxy

“…A temperature increase leads to a significant increase in the droplet size which is attributed to an increase of the adatom activity, intensification of the decay of unstable clusters, including dimers, and Ostwald ripening being significant at higher temperatures [35,48]. The high-temperature growth also yields a low density of islands which is useful for some specific applications, such as quantum computing and integrated photonics [11][12][13]. Furthermore, it is commonly considered that high-temperature droplet epitaxy allows obtaining quantum structures of better quality [49].…”

“…It has been successfully used in the fabrication of semiconductor lasers, photodetectors, memory cells etc, but further improvement in the device parameters is associated with the possibilities that cannot be achieved by the Stranski-Krastanov method, namely the formation of nanostructures in lattice-matched material systems, wide control over the shape of nanostructures, independent control over the surface density and size of quantum dots. Meanwhile, the latter possibility would allow the formation of tiny nanostructures with a large distance between them [7][8][9][10][11][12][13], which is necessary to create efficient single photon sources for integrated photonics, quantum computing and cryptography [1,2,[14][15][16][17]. A good alternative to the Stranski-Krastanov mechanism is a method of droplet epitaxy that has a peculiarity of the presence of two growth stages which are formation of metallic droplets from group III atoms and subsequent crystallization in a flux of group V molecules.…”

“…This is especially important from a technological point of view, since one of the main advantages of droplet epitaxy is the ability to control the size of quantum dots at a constant density. Thus, using this technique, it is possible to obtain an array of nanostructures with ultra-low density (∼10 6 cm −2 ) and different sizes [8,9,13], which is unattainable by the Stranski-Krastanov growth mechanism. It is well-known that the average droplet size decreases with decreasing substrate temperature and/or growth rate, but in both cases this decrease is associated with an increase in the surface density [35,39,42].…”

Study of the effect of ultra-low arsenic flux on the formation of In(As)/GaAs nanostructures by droplet epitaxy

“…A special part in the technology of self-organizing semiconductor nanostructures is played by control of their surface density: depending on device purpose, structures both with high-density and low-density arrays of quantum dots are required [6][7][8][9]. In particular, high-efficiency sources of single and entangled photons of the near infrared band should be created using structures with single InAs/GaAs quantum dots, which presupposes the formation of quantum dot arrays having a low surface density (not higher than 10 8 −10 9 cm −2 ) to enable their subsequent isolation from each other [10,11]. However, the traditional method for the making of InAs/GaAs quantum dots using the Stranski-Krastanov mechanism has several shortcomings chiefly related to the fundamental limitation (due to the mechanism of their formation) of possibilities for independent control of nanostructures' surface density, shape and size [12,13].…”

“…Thanks to the multi-stage nature of droplet epitaxy, this method can be used to form quantum dots [7,11], quantum-dot molecules [18], quantum rings [19], nanosized recesses [20] and other types of nanostructures [21]. As a rule, the arsenic flux is used in the droplet epitaxy method to change the nanostructure shape and/or transform metal droplets into semiconductor nanostructures.…”