Artificial 3-D Quantum Dot Crystal Created

Michael A. Greenwood

Although quantum dots are being looked at for a growing number of applications, the scale of the nanosize semiconductors continues to make it difficult to arrange them into the precision patterns necessary for applications such as optoelectronics, spintronics and, potentially, quantum computing.

A quantum dot crystal created by researchers consisted of 11 germanium dot layers separated by 10 silicon spacer layers. Extreme ultraviolet interference lithography was used to prepare the prepatterned substrates, and molecular beam epitaxy was used for overgrowth. Image courtesy of Christian Dais, Paul Scherrer Institut.

Researchers in Europe report that they have created a three-dimensional array of quantum dots with symmetry comparable to the atomic structure of a natural crystal. The investigators were led by Detlev Grützmacher of Forschungszentrum Jülich in Germany. Researchers from the Paul Scherrer Institut in Villigen, Switzerland, from Johannes Kepler University in Linz, Austria, from Technical University Munich, in Germany, and from Masaryk University in Brno, Czech Republic, also participated.

Although a number of methods exist to build quantum dot arrays, they have some important limitations, including a broad size distribution and the number of dimensions that can be fabricated.

To overcome these problems, the researchers turned to templated self-organization to achieve highly accurate positioning. They used extreme ultraviolet interference lithography at 13.5 nm to produce a prepattern for the growth of quantum dot arrays. This technique allows a relatively large area to be exposed in a short time with high accuracy.

The prepattern was transferred by reactive ion etching into the silicon substrate with a depth ranging from 8 to 10 nm. Contaminants were removed, and a silicon buffer layer was deposited with molecular beam epitaxy. At this point, the scientists deposited seven monolayers of pure germanium at 450 °C, which led to the formation of faceted islands in the pits on the silicon surface.

The result was an extremely ordered array of quantum dots that were uniform in size. Analysis of the quantum dots revealed that those in the first layer had a diameter of 45 ±3.2 nm and a height of 4.2 ±0.37 nm, less than a 9 percent variation.

Subsequent layers of germanium quantum dots were added, creating a 3-D quantum dot crystal with high lateral and vertical accuracy. The array had a lateral periodicity of 90 × 100 nm and a vertical periodicity of ∼11 nm.

Modeling of the electronic states in the 3-D quantum dot states indicates the onset of electronic coupling. The investigators’ final goal is to create a 3-D quantum dot crystal with strongly coupled states. This will require an even shorter lateral period of the quantum dots.

Nano Letters, October 2007, pp. 3150-3156.