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Semiconductor quantum dots with single-atom precision

Another step towards a new generation of atomic and molecular electronic integrated circuits
Scientists at NTT Basic Research Laboratories in Japan, the Paul-Drude-Institute in Germany, and the Naval Research Laboratory in America have developed quantum dot and combined artificial molecules with single-atom precision in terms of position and configuration. They achieved this on a clean surface of semiconductor single crystal thin film manufactured by MBE using a low-temperature Scanning Tunneling Microscope (STM) to integrate atoms one-by-one.

According to the researchers, this technology has made it possible to implement quantum dots with identical properties, like natural atoms, and do this flexibly at the semiconductor substrate for the first time at this level of precision. As a result, they say, it should be possible to manufacture quantum devices with atomic-level reproducibility e.g. a single photon source with a uniform wavelength, or an array of quantum bits with uniform functions, which has not been possible before. These results will be published in the Nature Nanotechnology on 29th, June 2014.

If the fabrication and characterization of quantum structures with atomic precision is possible at the semiconductor substrate surface, this will be a major leap toward being able to make a new types of integrated circuits that combine wafer-level semiconductor technology and atomic and molecular electronics.

For the base of the atom manipulation, the team used an (111)A-oriented surface of indium arsenide crystal. The (111)A surface has periodic hollow sites caused by a specific atomic structure of compound semiconductors. The structure formation can be exactly controlled by placing each atom at each hollow site. The high quality InAs thin film has been grown at NTT-BRL on the (111)A-oriented substrate with atomically controlled thickness. After the grown InAs surface was covered by a protection film (amorphous As), the sample was transferred from NTT to PDI.

Atom-by-atom quantum structure fabrication
When the sample was loaded into STM instruments at PDI, the protection film was removed in an ultra-high-vacuum to recover the clean (111)A surface, on which it is feasible to perform atom manipulation. The indium atom is self-ionized at the InAs surface to be +1 charged ion with releasing an electron. By using the low-temperature STM, we can not only observe surface atomic arrangement but also form nanostructures by atom manipulation of these ions as building blocks. Artificial atoms (6 ≤ the number of atoms ≤ 25) have been manufactured by arranging each In atom one-by-one in a line at the (111)A surface. The row of such ions behaves as a 'core' of an artificial atom and electronic states at the semiconductor surface are confined to the induced local potential well.

Future Plans
The team expect that the present achievements will open the door to developing new electronic technology by combining atomic and molecular electronics with semiconductor thin film technology. By exploring novel properties of many integrated atoms and the interaction with semiconductor heterostructures, they plan to develop architectures for quantum computers and high-performance semiconductor devices composed of well-defined semiconductor nanostructures with robust fidelity. Further study will bring many benefits to a broad range of science and technology fields.

Publication information:  Quantum dots with single-atom precision by S. Fölsch etc al, Nature Nanotechnology (2014).DOI: 10.1038/NNANO.2014.129 

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