Monday, March 28, 2022

Let quantum dots grow regularly

With this experimental setup, the researchers check the quality of the quantum dots. Green laser light is used to stimulate the quantum dots that then emit infrared light.
© İsmail Bölükbaşı

With the previous manufacturing process, the density of the structures was difficult to control. Now researchers can create a kind of checkerboard pattern. A step towards application, for example in a quantum computer.

Quantum points could one day form the basic information units of quantum computers. Researchers at the Ruhr University Bochum (RUB) and the Technical University of Munich (TUM) have significantly improved the manufacturing process for these tiny semiconductor structures, together with colleagues from Copenhagen and Basel. The quantum dots are generated on a wafer, a thin semiconductor crystal disc. So far, the density of the structures on it has been difficult to control. Now scientists can create specific arrangements - an important step towards an applicable component that should have a large number of quantum dots.

The team published the results on 28. March 2022 in the journal Nature Communications. A group led by Nikolai Bart, Prof. Dr. Andreas Wieck and Dr. Arne Ludwig from the RUB Chair for Applied Solid State Physics with the team around Christian Dangel and Prof. Dr. Jonathan Finley from the TUM working group semiconductor nanostructures and quantum systems as well as with colleagues from the universities of Copenhagen and Basel.

Like the mushrooms in the forest

Quantum points are narrowly defined areas in a semiconductor, in which, for example, a single electron can be locked. From the outside, this can be manipulated with light, for example, so that information can be stored in the quantum dot. The researchers from Bochum are experts in the production of quantum dots. They create the structures on a wafer from a semiconductor material that is about the size of a beer mat. The quantum dots have a diameter of only about 30 nanometers.

"Our quantum dots used to grow like mushrooms in the forest," Andreas Wieck describes the initial situation. “We knew that they would be created somewhere on the wafer, but not exactly where. "The researchers then chose a suitable mushroom in the forest for their experiments with the quantum dots.

First breeding attempts

In various preparatory work, the team had already tried to influence the growth of the quantum dots on the wafer. The physicists had irradiated the wafer with focused ions at individual points, thus creating defects in the semiconductor crystal lattice. These acted like condensation germs and provoked the growth of quantum dots. "But just like cultivated mushrooms taste a bit bland and forest mushrooms, on the other hand, were great, the quantum dots produced in this way were not as high quality as the naturally grown quantum dots," illustrates Andreas Wieck. They didn't radiate light so perfectly.

Therefore, the team continued to work with the naturally grown quantum dots. For the work, the beer mat-sized wafer was cut into millimeter-small rectangles. They could not examine the whole wafer at once because the vacuum chamber of the equipment at the RUB was simply not large enough for it. However, the researchers observed that some wafer rectangles contained many quantum dots, others few. "At first we didn't notice a system behind it," recalls Andreas Wieck - because the researchers never saw the whole picture.

Quantum points of high quality

The Moiré pattern: Here a green screen was photographed with a digital camera. Both the monitor and the semiconductor chip in the digital camera have a regular pixel grid. The overlay of the two grids and minimal distortions in image generation by the optical lens system lead to strong image artifacts.
© Arne Ludwig

To get to the bottom of this, the Bochum team cooperated with the colleagues from TUM, who had a measuring device with a larger sample chamber at an early stage. In these analyses, the group found that there was a strange distribution of areas with high and low quantum dot densities on the wafer. “The structures were strongly reminiscent of a Moiré pattern that often occurs in digital images. I quickly had the idea that it should actually be a concentric pattern, i.e. rings, and that these can be seen in correlation to our crystal growth,” reports Arne Ludwig. Indeed, higher resolution measurements showed that the density of the quantum dots was concentrically distributed. In the following, the researchers confirmed that this arrangement was due to the manufacturing process.

The wafer is first coated with additional atomic layers. The geometry of the coating system creates ring-shaped structures that have a complete atomic layer, i.e. where an atom is not missing at any point on the layer. Similar wide areas form between the rings, which are not provided with a complete atomic layer and therefore have a rougher surface because individual atoms are missing. This has consequences for the growth of the quantum dots. "To stay in the picture: The mushrooms prefer to grow on the loose forest floor, i.e. in the rough areas, then on a concrete surface," says Andreas Wieck.

The researchers optimized the coating process in such a way that the rough areas were created on the wafer at regular intervals - less than a millimeter - and that the rings crossed. This resulted in an almost checkerboard-like pattern with quantum dots of high quality, as the colleagues from Basel and Copenhagen showed.

Measurement of a wafer (red circle): The color scale shows how much light the quantum dots on the wafer emit at wavelengths between 1,000 and 1,300 nanometers - the higher the emission, the higher the density of the quantum dots. The dashed lines show the checkerboard-like course of high quantum dot densities.
© Nikolai Bart / Marcel Schmidt


The work was financially supported by: Federal Ministry of Education and Research (Funding code 13N14846; QR.X projects 16KISQ009 and 16KISQ027) German research community (EXC-2111/1 - 390814868; MCQST EXC-2089/1 - 390776260; FI947-5; FI947-6; INST 95-164; TRR 160/2 - project B04; DFG 383065199; DFH / UFA CDFA-05-06) European Commission as part of Horizon 2020 (QLUSTER - Project 862035; HiFig - 840453) Danish National Research Foundation (Center of Excellence "Hy-Q", Grant number DNRF139) Swiss National Science Foundation (200020_204069; NCCR QSIT).

Source/Credit: Ruhr University Bochum