'Racetrack' for fast electrons in semiconductor structures

In order to realize the electrical units of voltage, resistance and current with highest accuracy quantum effects in nano-circuits are nowadays used. Important prerequisites are extremely pure semiconductor layers where high-mobile electrons move through the crystal without collision with residual impurities. To be prepared for future developments in quantum metrology PTB has set up a new molecular beam epitaxy system optimized for the fabrication of highest purity semiconductor structures. Already the first structures have shown ex-treme high electron mobility with a value five times better compared to the ones from the conventional epitaxy system. With this, PTB extends its leading position in electrical quantum metrology with semiconductor structures. Semiconductors are the basic material for computer and home electronics. In electrical quantum metrology semiconductor devices are used in two areas. First, electrical resis-tance values are reproduced by the quantum-Hall effect with uncertainties below one part in a billion. Second, high mobile electron devices are used to fabricate single-electron pumps allowing controlled transport of electrons passing the device one by one. These single-electron pumps are a hot topic in metrology, today since they might allow the redefinition of the electrical current as a product of frequency and electron charge. Such definition would connect the current unit Ampere to a spatial and time invariant fundamental constant, the elementary charge.

For both applications semiconductor heterostructures made of GaAs and AlGaAs are em-ployed. In those crystals a few nanometer thin electron layer forms at the interface be-tween both materials which is called two-dimensional electron gas. The mobility of these electrons increases with decreasing impurities collisions and it is therefore a measure for the quality of the semiconductor structure.

The fabrication of pure semiconductors is achieved by molecular-beam epitaxy. In an ultrahigh vacuum chamber the source materials, e.g. gallium and arsenic, are evaporated at a background pressure 15 orders of magnitude lower than atmospheric pressure. The growth is highly controlled allowing deposition of atomic layers of GaAs and AlGaAs one by one.

The purity of the semiconductor crystal is determined by the quality of the vacuum in the molecular beam epitaxy system. PTB's new facility has an extremely powerful pumping system to produce ultra-low pressure in the growth chamber. Moreover, special cooling panels, adsorb residual impurities before they can be incorporated in the semiconductor crystal.

The result of these efforts are two-dimensional electron gases with electron mobilties up to 7.5 million cm2/Vs at low temperatures which is a five times higher compared to sam-ples from the standard epitaxy machine. The new structures thus provide a formula-1 race track for electrons to achieve highest mobilties.

By means of the new epitaxial technology PTB will promote the development of semicon-ductor single-electron pumps. Furthermore, the universality of electrical quantum Hall resistance metrology will be investigated focussing on measurements of the fractional quantum Hall effect.

Source: Physikalisch-Technische Bundesanstalt (PTB)