Researchers have cultivated ultralow-density isolated indium arsenide (InAs) quantum dots (QDs) with distinct labelling in self-assembled nanoholes by combining high-temperature droplet etching epitaxy with the conventional epitaxial growth mode. The large-scale and low density of the QDs as well as the very good control over their position holds promise for technological applications in quantum computing and quantum cryptography. The results are published in a recent issue of the Journal of Materials Research.

Single QDs help researchers investigate quantum-optics phenomena and gain control of the photon-generation process. These isolated nanostructures can act as sources of single photons, a feature that is ultimately important in the terrain of quantum information processing. In quantum cryptography, for example, the state of a single quantum dot system is modified the moment an eavesdropper interferes with the transmitted message. Quantum computers can also benefit from low-density QDs as researchers gain insights on how to control and implement basic quantum-computational elements in a physical system.

The technique that the team of researchers, formed by Jiang Wu, from the University College London (UCL), Zhiming M. Wang, from the University of Electronic Science and Technology of China, and their colleagues at the University of Arkansas, developed, is based on a scheme that starts with high temperature droplet etching epitaxy to create a low-density self-assembly of nanoholes, followed by the filling of the nanoholes with QDs using molecular beam epitaxy.

Gallium arsenide (GaAs) with face-centred cubic (fcc) structure was used as the substrate material for the growth of the samples. The formation of the nanoholes started with the deposition of gallium droplets at 580°C on the top of the GaAs surface, and was followed by high temperature annealing at 600°C for 5 minutes. This process is known to cause gallium droplets to act like “electrochemical nano-drills,” etching away the GaAs substrate beneath, to give rise to nanoholes. The substrate temperature of 580°C, which is much higher than previously reported, resulted in a density of nanoholes of ~4 × 10⁶ cm^-2.

In the next phase, the substrate temperature was reduced to 500°C and deposition of InAs at a very low growth rate (0.03 ML/s) followed. Thus, the ideal conditions for the diffusion of indium adatoms (individual atoms on the GaAs surface), were created and the final nucleation of InAs QDs solely in the nanoholes took place. To acquire control over the size of the InAs QDs, the researchers used different indium coverages (0.9 ML, 1.2 ML, and 1.35 ML). The samples were then quenched to room temperature and examined with atomic force microscopy.

According to Wu, planting a single quantum dot in each hole, while at the same time preventing any quantum dot nucleation on the surfaces between the holes, was the most challenging part in this work. He says that the difficulty lay not only in the very low density of nanoholes, but also in the relatively large distance between them. “This gives a good chance for quantum dot nucleation on the surface areas between nanoholes instead of in the holes,” Wu says. Furthermore, quantum dots may also nucleate around the nanoholes, areas in which QD nucleation is favored by chemical potential minima. These problems meant that the team needed to obtain very good control of the experimental conditions along with a good understanding of the growth mechanism.

Interestingly enough, when a second set of samples was grown (by the same procedure), in which the QDs were covered with a 10-nm thick layer of GaAs for photoluminescence (PL) studies, surface marks appeared on the QDs. These distinct surface structures, which can assist in locating the position of buried QDs, were not expected since they are not observed after capping quantum dots with the conventional growth method (Stranski–Krastanov growth mode). “Self-labelling is useful and important to fabricate microcavities with good alignment with the quantum dots,” Wu says.

Huiyun Liu, professor of semiconductor photonics at UCL, believes that this is an interesting technique for low-density quantum dots fabrication. ‘’In comparison to the conventional method, it seems to have better control in dot density, especially at such a low level of ~10⁶ cm^-2,” he says. According to Liu the finding may open new opportunities for quantum information devices. The researchers are now planning to optimize the symmetry and material quality of the quantum dots and then study their optical properties. For Wu and Wang, the ultimate goal is to fabricate efficient, indistinguishable, single photon emitters.

Read the article in the Journal of Materials Research.