Acquisition of Instrument for the Study of Artificial Atoms

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9700818 Kastner The power of computers has increased exponentially with time in large part because the size of devices has decreased exponentially with time, resulting in a higher density of transistors on a chip. Unfortunately, the cost of facilities for producing these devices has also increased exponentially, and although the market for devices has grown rapidly, it has not grown exponentially. Thus we are rapidly approaching the time when facilities for new generations of conventional devices may be too expensive to build. In the past it seemed that novel devices could not compete with conventional ones. However, the economic limitation to future fabrication facilities provides a new motivation for research into classes of devices that may have cost advantages. Single- electron devices may be one of these classes. Whereas single- electron devices have, in the past, operated only at very low temperatures, recent reports of operation near room temperature bring useful applications much closer to reality. This project builds upon past work at MIT in which it was discovered that when a transistor is made very small it behaves in an unusual way. Whereas a conventional transistor turns on only once as electrons are added to it, and whereas such a transistor requires the addition of about 103 electrons to change from the off-state to the on-state, the new transistor turns on and off again every time an electron is added to it. The single electron transistor is made by isolating a very small droplet of electrons in such a way that electrons can move into and out of the droplet only by quantum mechanical tunneling. The MIT work has shown that such a transistor behaves like an artificial atom, in that the number of electrons in the droplet and the energy are both quantized. The transistors made so far have used the semiconductor GaAs with metal electrodes on the surface, patterned using electron-beam lithography, to confine the droplet of electrons. Beca use the electrons in such GaAs structures are typically 100nm below the surface, the droplet of electrons has typically been ~100nm in radius r. The energy for adding an extra electron to the droplet scales like r-1, and this limits single electron operation to temperatures below ~1K for r=100nm. The goal of this project is to explore ways of increasing the operating temperature of single electron transistors and to better understand their physics. In particular, the MIT group has recently discovered a series of phase transitions in the droplet of electrons confined in the artificial atom at high magnetic fields. The acquisition of a higher-field magnet and dedicated refrigerator are proposed here to study these phase transitions further. %%% New artificial atoms will be fabricated using Si instead of GaAs. Since the electrons in these devices can be as close as 5nm from the surface, much smaller droplets and, consequently, much higher energy and temperature scales are expected. The devices will be fabricated in collaboration with MIT Lincoln Laboratory using the outstanding electron- beam and optical lithography capability developed there under Air Force sponsorship. In this way, the effect of the NSF funds will be strongly amplified. In addition to higher operating temperatures, Si offers the advantage that single electron devices can be easily integrated with conventional ones. It is expected that single electron devices in Si can be made to operate well above liquid nitrogen temperature. Higher magnetic fields are required for the study of the state of electrons in these smaller structures. It is also proposed to fabricate devices in GaAs in which the droplets of electrons will be smaller. This will be accomplished by growing structures in which the electrons are closer to the surface. A collaboration has been established with the Weizmann Institute in Israel where the expertise in making such structures has already been developed. Like the col laboration with Lincoln Laboratory, this amplifies the impact of NSF funding. Single electron devices have the advantage that they can be capacitively coupled. It has been proposed that, because of this, an array of such devices may function as an associative memory. That is, any set of input voltages close to, but not exactly equal to, a special set will give the same output. It is proposed that such an array be fabricated and studied. Although the first such memories will be read-only, understanding their physics may lead to ways of writing as well. ***