Determining the Composition of Quantum Dots from Top to Bottom

Hank Hogan

Quantum dots may shine, but shedding light on their make-up has proved to be difficult because they are small, with sizes in the tens of nanometers. Several analysis techniques average over many quantum dots, smoothing out possible variations. Techniques that target individual dots probe only the surface or just a cross section. Thus, researchers have had to make assumptions about structure to explain the electronic and optical properties of quantum dots.

Now investigators are employing scanning probe microscopy to determine the composition of quantum dots as they scan through the particles using selective wet chemical etching. Their concept — performed on SiGe islands — could prove useful when determining the composition of other types of quantum dots, noted team member Armando Rastelli of Leibniz Institut für Festkörper- und Werkstoffforschung (IFW) Dresden in Germany.

“Based on the availability of suitable etchants, our approach could be used also to characterize optically active quantum dots and to model their properties,” he said.

Besides Rastelli and colleagues from IFW Dresden, the team comprised members from Max Planck Institut für Festkörperforschung in Stuttgart, Germany, and from the European Synchrotron Radiation Facility in Grenoble, France.

Rastelli developed the software needed for reconstruction of the composition from the atomic force microscopy data. This, he noted, was a crucial part of the overall effort.

Atomic force microscopy combined with wet chemical etching reveals a complete picture of a quantum dot. Upper left is a 3-D view of a SiGe quantum dot, demonstrating how the germanium content is shown by the etch rate of an etchant that is sensitive to germanium concentration. Repeating the measurement and etching steps peels the quantum dot down to the substrate. Upper right is a map of various quantum dots; below is the germanium fraction at various heights through the quantum dots. Courtesy of Armando Rastelli, IFW Dresden.

The group worked with SiGe islands grown atop a silicon substrate, depositing 15 monolayers of germanium using solid-source molecular beam epitaxy. For characterization, they used a multimode atomic force microscope from Veeco Instruments Inc. of Plainview, N.Y., combining this with a standard wet etch done at room temperature using an ammonium hydroxide and hydrogen peroxide mix.

They selected this particular etch because of its characteristics: It is strongly sensitive to composition, and the etch rate increases with the germanium content. It also has no preferential etch direction and is almost totally unaffected by strain in the SiGe.

After creating the islands, the researchers collected an atomic force microscope image of them, dipped them in the etchant and captured another image. They repeated this process at varying intervals, reconstructing the local etch rate and, hence, the local composition.

When they looked at the data, they found that the germanium fraction decreased as they etched from the top of the island down to the substrate. The fraction depended on the height but not on the size or shape of the island. The one exception was a class of islands known as small plastically relaxed superdomes. In those, trenches formed at the foot of each island, causing silicon to migrate upward, which affected the germanium ratio.

As a check of their work, the investigators used anomalous x-ray scattering to determine the average material characteristics over a large sample area. These readings supported those determined by their method, which they likened tonanotomography.

The researchers will utilize the technique to characterize the shape, size and composition of SiGe islands used in devices, Rastelli said. “The structural parameters can then be used to model the expected electronic properties.”

Nano Letters, May 2008, pp. 1404-1409.