Search

An introduction to the operation of quantum-dot cellular automata (QCA) is presented, along with recent experimental results. QCA is a transistorless computation paradigm that addresses the issues of device density and interconnection. The basic building blocks of the QCA architecture, such as AND, OR, and NOT are presented. The experimental device is a four-dot QCA cell with two electrometers. The dots are metal islands, which are coupled by capacitors and tunnel junctions. An improved design of the cell is presented in which all four dots of the cell are coupled by tunnel junctions. A noninvasive electrometer is presented which improves the sensitivity and linearity of dot potential measurements. The operation of this basic cell is confirmed by an externally controlled polarization change of the cell.

Measurable disorder exists on GaAs(001) surfaces during MBE even though these surfaces subsequently incorporate into exceedingly well‐ordered bulk GaAs lattices. We have used reflection high energy electron diffraction (RHEED) to characterize this disorder. On the As stabilized surface obtained at 850 K, some of the fractional order beams are broadened indicating that there is antiphase disorder present on the surface. Models of this disorder are presented. In addition, if the growth is stopped by shuttering the Ga flux both integral and quarter order beams sharpen, indicating that a second type of disorder exists. From the lengths of the RHEED streaks we determine that during growth the long range order in the twofold direction is about 500 A. About 10 min after growth is stopped, the extent of order in this direction increases to 3000 A. At a given As flux the ordering process is slightly temperature dependent over the narrow range that this disordered 2×4 exists. At lower temperatures, the disordered s...

Reflection high‐energy electron diffraction (RHEED) is so sensitive to surface morphology that it is difficult to separate the roles of instrument response and surface imperfection in the diffraction. To address this difficulty we have used MBE grown GaAs(001) as a test surface to study the angular dimensions of the diffracted beams. This is important if RHEED is to be useful as a quantitive probe of surface structure for in situ studies of crystal growth. The limitations placed by instrumental uncertainties on the maximum resolvable distance are estimated. Measurements of the angular length of the RHEED steaks versus angle of incidence are presented which show the changes expected from a combination of a uniformly broadened reciprocal lattice rod and an instrument limit due to angular uncertainties. Resolvable distances are obtained with RHEED that are much larger than those typically obtained with low‐energy electron diffraction.

Reflection high‐energy electron diffraction (RHEED) is so well suited to studies of epitaxial growth that it has become the standard in situ technique for monitoring the growth of III–V semiconductors during molecular beam epitaxy. The characteristic diffraction pattern for a smooth surface is a family of parallel streaks. The intensity along the length of these streaks is shown to be due primarily to the distribution of atomic steps over the surface. These steps are readily observable because RHEED is able to resolve order over long distances. During epitaxial growth synchronous oscillations in the intensity and width of the diffracted beams are observed. These are due to the competition between cluster nucleation and step propagation. The period of the oscillations is the time required to deposit a monolayer.