Quantum Dot Mapping Points to Unthought of Applications

University of Michigan physicists have mapped quantum dots, crystals with wide-ranging applications in electronics and photovoltaics. The step may lead toward "designer dots" that can be tailored for specific applications, and demonstrates the usefulness of X-ray phasing techniques.

Alexander E. Braun, Senior Editor -- Semiconductor International, 11/5/2009

University of Michigan physicists have created the first atomic-scale maps of quantum dots, a major step toward the goal of producing "designer dots" that can be tailored for specific applications. The work provides the first atomic-scale mapping of the interface between epitaxial quantum dots and a substrate, and establishes the usefulness of X-ray phasing techniques for this and other similar systems.

Quantum dots — often called artificial atoms or nanoparticles — are tiny semiconductor crystals with wide-ranging potential applications in computing, photovoltaic cells, light-emitting devices and other technologies. Each dot is a well-ordered cluster of atoms, 10-50 atoms in diameter. Although the droplet epitaxy quantum dot fabrication method allows a wide range of material combinations to be used, practically nothing is really known about the actual growth mechanisms involved. Engineers are gaining the ability to manipulate the atoms in quantum dots to control their properties and behavior through a directed assembly process, but progress has been slowed, until now, by the lack of atomic-scale information about the structure and chemical makeup of quantum dots.

The researchers applied direct X-ray methods to derive sub-angstrom resolution maps of quantum dots crystallized from indium droplets exposed to antimony, as well as of their interface with a GaAs (100) substrate. They discovered that quantum dots form coherently and extend a few unit cells below the substrate surface. This enables a droplet/substrate exchange of atoms, resulting in core-shell structures with a surprisingly small amount of indium.

The new atomic-scale maps will fill knowledge gaps, leading to more rapid progress in the field of quantum-dot directed assembly, according to Roy Clarke, University of Michigan professor of physics and one of the leading researchers. Researchers have been able to chart the outline of quantum dots for quite some time, but this is the first time that anyone has mapped them at the atomic level to determine where the atoms are positioned, as well as their chemical composition.

To create the maps, Clarke's team illuminated the dots with a brilliant X-ray photon beam at Argonne National Laboratory's Advanced Photon Source. The X-beam acts like a microscope to reveal details about the quantum dot's structure and, because of X-rays' very short wavelengths, they can be used to create super high-resolution maps. This has enabled the measurement of the position and the chemical makeup of individual pieces of a quantum dot at a resolution of 1 pm (0.001 nm).

Atomic-scale map of the interface between an atomic dot and its substrate. Each peak represents a single atom. The map, made with high-intensity X-rays, is a slice through a vertical cross-section of the dot. (Source: University of Michigan)

The availability of this kind of high-resolution atomic-scale maps will quicken progress in the field of directed assembly, leading to new technologies based on quantum dots. According to Clarke, the dots have already been used to make highly efficient lasers and sensors, and they might help make quantum computers a reality.

For those applications to be optimized and become reality, a very good characterization of the structures of the quantum dots themselves is crucial — what their shape is, what the chemical composition is within the dot itself. These are objects that are on the order of 10 nm, so the means with which to characterize them were insufficient until this work, which enabled the interior mapping on an atomic scale. Having this kind of information — chemical, structural, strained shape — gives all the important parameters for tailoring the electronic and optical properties of quantum dots.

Composition of the quantum dots studied in the research. (Source: University of Michigan)

The importance of the development is the ability to be able to go in and map the quantum dot very carefully, under different growth deposition conditions, and use that information to control the parameters for specific applications such as infrared detection and infrared lasers, as well as solar cell applications resulting in highly absorbing, highly spectrally selective characteristics. Specialized semiconductor applications would also benefit by enabling, for example, single-electron transistors for quantum computing. This would require a very high degree of control over the uniformity of the quantum dots, as well as detailed knowledge of how the dot is constructed and what its shape is, how it is coupled to its semiconductor substrate. For example, the researchers made a GaAs and GaSb core, which is a mixture of two fairly widely separated bandgaps — GaAs being the larger bandgap and GaSb the smaller. Bandgap control has been referred to as bandgap engineering in planar structures like quantum wells; it now seems that this might be possible with quantum dots as well, opening an even greater range of possibilities.

Atomic-scale mapping provides information that is essential for the controlled fabrication of quantum dots. To make dots with a specific set of characteristics or a certain behavior, it is necessary to be able to place the atoms optimally and therefore know where everything is. Now the tools and methods exist to do just that.

The research was sponsored by a grant from the National Science Foundation. The U.S. Department of Energy supported work at Argonne National Laboratory's Advanced Photon Source.