Number 257 (Story #1), February 6, 1996 by Phillip F. Schewe and Ben Stein|
SELF-ASSEMBLED QUANTUM DOTS . Reducing the dimensionality of semiconductor structures not only saves space, but also brings about beneficial quantum effects. Quantizing allowable electron energies, for example, can make an electronic device more efficient and allow it to operate at lower voltages and higher speeds. Quantum wells, semiconductor sandwiches in which electrons are confined to a plane, are already at work in some devices. Quantum wires (electrons restricted to one dimension) and quantum dots (electrons restricted to a point volume) might prove even more useful. Dotlike structures can be made lithographically (with electron beams and etching) but scientists have tried to achieve greater control and economy by "growing" dots. A group of scientists at the University of Wisconsin (Max Lagally, 608-263-2078), AT&T Bell Labs, and IBM have deposited layers of SiGe on a Si substrate. The strain which arises from the atomic mismatch between the two materials is accommodated by the development of SiGe islands. After several cycles of SiGe and Si layers, the strain built into the material results in the nucleation of a regular array of 100-nm- wide, 3-10-nm-tall SiGe pyramids. A simple theoretical explanation of the phenomenon has now been achieved (J. Tersoff et al., Physical Review Letters, 4 March 1996). These self- organized nanostructures are not exactly point-like (they contain about 10**16 atoms each), but the researchers hope to make them smaller and more isolated by controlling the deposition conditions during growth. Meanwhile, a group at the University of Southern California has made InAs islands on top of GaAs (Qianghua Xie et al., Phys. Rev. Lett., 25 Sept. 1995). What can one do with an array of nanodots? Possibly read and write data into them, probably not through wires, but possibly with tiny bursts of laser light.