New method produces quantum dots that offer advantages for light-based technologies

The manufacturing strategy allows for the production of single and entangled photon emitters with low density, high symmetry, and wavelengths that are more suitable for integrated photonics.

By José Tadeu Arantes  |  Agência FAPESP – Light-based quantum technologies, such as quantum communication and photonic quantum computing, require reliable sources of individual photons and, ideally, pairs of entangled photons. Semiconductor quantum dots are promising candidates for this purpose. These nanostructures have electrical conductivity between that of insulators and conductors and are capable of confining electrons and holes. This property causes them to emit light at well-defined frequencies when excited by a laser.

However, traditional manufacturing methods have significant limitations, including very high dot density, which makes it difficult to isolate a single emitter; structural asymmetries that impair entanglement; relatively long emission times; and electronic “noise” that reduces light quality. The challenge is to manufacture quantum dots that are more symmetrical, faster, and more predictable and that emit light at wavelengths more suitable for integrated photonic devices.

A study conducted through international cooperation demonstrated a new strategy for manufacturing semiconductor quantum dots capable of emitting light at longer wavelengths while maintaining optical properties ideal for quantum technology applications, such as simulation, secure communication, and photonic computing. The lead author of the study was Saimon Filipe Covre da Silva, a researcher at the Gleb Wataghin Physics Institute at the State University of Campinas (IFGW-UNICAMP) in Brazil. The study was published in the journal Nano Letters.

“Our work has shown that it’s possible to produce indium gallium arsenide [InGaAs] quantum dots in aluminum gallium arsenide [AlGaAs] with low surface density, fast photon emission, and high structural symmetry, which are essential characteristics for single-photon and entangled photon sources on demand,” says Silva.

Many pioneering experiments in quantum optics have used InGaAs quantum dots grown using the Stranski-Krastanov (SK) method. This epitaxial growth regime involves one crystal growing on top of another crystal, following the crystallographic orientation of the substrate – that is, with alignment defined by the crystal lattice of the base. In the 1930s, Bulgarian physicists Ivan Stranski (1897-1979) and Lyubomir Krastanov (1904-1971) formulated a theoretical model describing the transition from layer growth to the formation of three-dimensional islands. In the aforementioned experiments, these islands constitute the quantum dots.

Although efficient, the method produces quantum dots with the following drawbacks: high surface density, high structural variability, and relatively long radiative lifetimes of around 1 nanosecond. The method also results in the undesirable preservation of the “wetting layer,” which is the thin, continuous, two-dimensional initial layer of deposited material that “wets” the substrate. Growth ceases to occur in a flat manner once the layer reaches a critical thickness, resulting in three-dimensional islands. The grown materials have different sizes and grow under tension. This tension causes the islands to form but also presents disadvantages for light sources.

These characteristics make it difficult to optically address individual dots and can introduce decoherence, which impairs applications that require single photons or pairs of entangled photons. For this reason, an alternative approach known as local droplet etching (LDE) has gained prominence in recent years. In this method, small metal droplets (usually gallium or aluminum) form during epitaxial growth, creating nearly identical nanocavities on the surface of the material. These cavities can then be filled in a controlled manner to create highly symmetrical quantum dots with adjustable density.

Until now, this technique had mainly been used to produce gallium arsenide (GaAs) quantum dots in aluminum gallium arsenide (AlGaAs). The optical emission of these quantum dots is limited to wavelengths close to 815 nanometers, a value imposed by the GaAs bandgap at low temperatures. The new study broadens this horizon. “We’ve shown that by filling the nanocavities excavated in aluminum gallium arsenide with a thin layer, about 1 nanometer thick, of indium gallium arsenide, it’s possible to obtain quantum dots that are almost free of mechanical deformation and have excellent optical properties,” the researcher points out.

The nominal indium (In) fraction varied from 0.1 to 0.4, which allowed adjustment of the emission wavelength. Microphotoluminescence measurements revealed an extremely low surface density of 0.2 to 0.3 quantum dots per square micrometer (μm⁻²). Meanwhile, the quantum dots exhibited extremely short radiative lifetimes close to 300 picoseconds (ps) – about three times shorter than those observed in indium gallium arsenide quantum dots grown using the Stranski-Krastanov method within the same spectral range.

One of the main results of the study is the extension of the emission wavelength. The researchers observed a progressive shift in the emission by increasing the indium concentration, which could be adjusted between 780 and approximately 900 nanometers at cryogenic temperatures around 10 kelvin (K). “This range is particularly relevant for integrated photonics because optical losses due to scattering and absorption in AlGaAs structures decrease with increasing wavelength. Also, this spectral range is compatible with optical technologies already developed for conventional InGaAs quantum dots,” Silva comments.

Another crucial parameter analyzed was the fine structure splitting (FSS). This quantity is crucial for generating polarization-entangled photon pairs. The study presented values comparable to the best results obtained with GaAs quantum dots by droplet excavation. In practical terms, such small values indicate high potential for applications in quantum cryptography and quantum networks, where entanglement is a central feature. The experiment showed that the light source almost never emits two photons simultaneously, but rather one photon at a time – exactly the behavior expected from a reliable source of single photons.

“The combination of low density, high symmetry, fast emission, and extended wavelength makes these new quantum dots particularly promising for integrated quantum photonics. Additionally, the greater energy separation between the s and p electronic levels – up to twice that of GaAs quantum dots – may enable quantum devices to operate at significantly higher temperatures, exceeding 40 K,” Silva points out.

Together, these results suggest a new generation of solid-state quantum light sources that are more resistant to decoherence effects and more compatible with scalable quantum device architectures.

The study was supported by FAPESP through the project “Solid-State Single-Photon Sources for Telecommunications Frequencies” (projects 24/08527-2 and 24/21615-8). These projects are linked to the FAPESP QuTIa (Quantum Technologies Initiative) Program in Quantum Technologies.

The article “Low-density InGaAs/AlGaAs quantum dots in droplet-etched nanoholes” can be accessed at pubs.acs.org/doi/full/10.1021/acs.nanolett.5c04426.