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Semiconductor islands: From the artificial atom to the molecule

Semiconductor structures with two stacked islands of a few nanometres in size have been produced for the first time. The island structures were grown by a self-organizing process and the electrical transport properties were characterized. These structures are so tiny that they display atom-like (quantum) properties. Their realization represents a step forward in the direction of a new quantum standard for the unit ampere.

Schematic section of two stacked semiconductor quantum dots (left) and transmission electron micrograph of a resonant tunnel diode (right).

In the foreseeable future, progress in modern electronics which involves ever smaller circuits must overcome one boundary: as the structures are reduced to only a few nanometres in size, the laws of classical physics become invalid and quantum mechanics must be applied. Such tiny circuit components are also of great importance for precision metrology. In these components, single electrons can be manipulated to a greater extent than before. This is a prerequisite for a new standard for the unit of current, the ampere, based on counting electrons.

Growing semiconductor crystals utilizes a self-organizing effect which produces tiny “islands” of a few nanometres in size. By exploiting this effect skilfully one can even stack the islands vertically. In experiments they then show new interesting properties. For this purpose, the self-organized semiconductor islands are embedded in the barrier of a resonant tunnel diode. Resonant tunnel diodes with two stacked nano-islands of indium-arsenide have been created for the first time. Their properties were observed during electrical transport experiments.

In the structures, the two islands in a way represent the emitter and the collector of a transistor. Sequential quantum-mechanical tunnelling processes determine the dependence of an electrical current flow as a function of the applied voltage. Just like atoms the quantum islands have typical discrete energy states for 0-dimensional systems and sharp current peaks are observed instead of steps as the voltage is changed. The sharp peaks hardly show any broadening with an increase in temperature, as they do not depend on the distribution of the most energetic electrons in the highly doped three-dimensional regions.

A new degree of freedom occurs by coupling the two nano-islands. By decreasing the thickness of the separating interlayer the two nano-islands become increasingly coupled and in the extreme case the electron wave functions of the two islands superimpose. Virtually speaking, a nano-island molecule is created. This was demonstrated by the influence of a modified spin splitting of the electrons in a magnetic field in cooperation with the “Nanostructures” Division of the University of Hannover.

The investigations also open up prospects for structures with more than two vertically coupled quantum dots. A practical advantage, viz. the reduction of an undesired background current through the barrier which is now thicker, could already be achieved.

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