(Press release material)
June 27, 2014
Nippon Telegraph and Telephone Corp. (NTT; Head Office: Chiyoda-ku, Tokyo; President: Hiroo Unoura), the Paul-Drude-Institute (PDI; Germany), and the Naval Research Laboratory (NRL: USA) have cooperatively developed a novel quantum dot*1 (artificial atom*2) and combined artificial molecules*2 with single-atom precision of the position and configuration. This has been realized at a clean surface of semiconductor single crystal thin film manufactured by Molecular Beam Epitaxy (MBE) *3 by using low-temperature Scanning Tunneling Microscope (STM) *4 to integrate atoms one-by-one with an atom manipulation*5 technique.
This technology enables us to implement quantum dots with identical properties like natural atoms flexibly at the semiconductor substrate. Usage of these quantum dots enables us to manufacture ultimate quantum devices with atomic-level reproducibility like a single photon source*6 with a uniform wavelength and an array of quantum bits*7 with uniform functions, which have not been available due to statistical errors of structural fluctuations. Furthermore, integration and control of these precise nanostructures*8 will help us to develop a quantum computer and a next-generation technology called ‘Beyond CMOS’*9 to overcome the limits of conventional silicon technology.
These results will be published in the UK science journal “Nature Nanotechnology” on 29th, June 2014.
Through the design and processing of materials at the nanometer-level (1 nm = 1/1000000000 m), nanotechnology allows us to create new properties and functionalities not currently available due to the nature of original materials. A quantum dot (QD) is a nanostructure that confines electrons in a nanometer-level narrow space to make quantum mechanical effects evident. Therefore, QD is called an ‘artificial atom’ and is expected to be applied to a wide range of applications in various fields like optical and electronic devices, display panels, biotechnology, solar cells, and quantum information processing. However, the influence of microfabrication error becomes more serious as the manufactured device becomes smaller. Conventional lithography and self-assembling methods are problematic in the processing precision. Position and configuration of the fabricated nanostructures have included unavoidable variations (Figure 1 ). Thus, how to improve the device processing accuracy has been a problem to be solved.
If the fabrication and characterization of quantum structures with atomic precision become available at the semiconductor substrate surface, this will be a major leap toward the realization of a new integrated circuit technology expected about 10 years from now by the fusion of ‘wafer-level semiconductor technology’ and ‘atomic and molecular electronics’*10. NTT Basic Research Laboratories (NTT-BRL) has studied quantized states of semiconductor nanocrystals and adatoms by using high quality semiconductor thin film growth technology and low-temperature STM observation. Furthermore, NTT-BRL and PDI have cooperatively cultivated the atom manipulation technology at semiconductor surfaces using the low-temperature STM as a new approach to surpass conventional nanostructure fabrication technologies.
A team of physicists from NTT-BRL, PDI, and NRL has succeeded in building QDs (artificial atoms) and combined nanostructures (artificial molecules) with ultimate atomic-level accuracy of positions and sizes for the first time, which is extremely more precise than any other conventional structure processing methods.
We applied an atom manipulation method (Figure 2 ) using low-temperature STM at a clean surface of a high-quality semiconductor thin film grown by MBE (Figure 3 ). By the synergistic effect of coupling between extremely high processing accuracy at no difference in single atom and high crystallinity of the semiconductor thin film, we have succeeded in fabricating QDs with identical properties (Figure 4 ). Moreover, we have succeeded in integrating three QDs of a few nm in a 10 nm square area, meaning that we have achieved ultimate-high-density integration of functional quantum structures. This degree of integration is local but is about 1000 times as high as that of LSIs used in the present computers.
For the base of the atom manipulation, we used an (111)A-oriented surface of indium arsenide (InAs)*11 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.
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 (In) 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.
Density Functional Theory (DFT)*12 at NRL has played a vital role in the analysis and design of artificial atoms and molecules. By the calculation, the states of the artificial atom are found to originate in the quantized electronic states at semiconductor surface, but not atomic orbitals of In adatoms. Moreover, comparison of quantized states among artificial molecules and theoretical results has confirmed that the properties of these nanostructures have atomic-level fidelity without error of structural fluctuation.
We 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, we 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.
S. Fölsch, J. Martínez-Blanco, J. Yang, K. Kanisawa and S. C. Erwin
“Quantum dots with single-atom precision”
Nature Nanotechnology (2014).
Nippon Telegraph and Telephone Corporation
Science and Core Technology Laboratory Group, Public Relations
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