Due to the dramatic reduction in device size down to 0.1
micrometers, electrons start to see a few dozens of discrete
dopant atoms randomly scattered in the channel. The
characteristics may be different among the transistors that are
designed to be the same, and this places a serious limitation for
integration. A fundamental solution is to create electronics with
atomically precise elements, which could be fabricated with atom
manipulation technology. Atomic chain electronics belongs to this
category. Foreign atoms are placed to form chains on an
atomically regulated substrate surface. Using the tight-binding
theory with universal parameters, it has been predicted that Si
chains are metallic and Mg chains are insulating, regardless of
the lattice constant .
For electronic applications, it is essential to establish a
method to dope a semiconducting chain, which is to control the
Fermi energy position without altering the original band
structure. If we replace some of the chain atoms with dopant
atoms randomly, the electrons will see random potential along the
chain and will be localized strongly in space (Anderson
localization). However, if we replace periodically, although the
electrons can spread over the chain, there will generally appear
new bands and band gaps reflecting the new periodicity of dopant
atoms. This will change the original band structure
significantly. In order to overcome this dilemma, we may place a
dopant atom beside the chain at every N lattice periods (N
>> 1). Because of the periodic arrangement of dopant atoms,
we can avoid the unwanted Anderson localization. Moreover, since
the dopant atoms do not constitute the chain, the overlap
interaction between them is minimized, and the band structure
modification can be made smallest. Some tight-binding results
will be discussed to demonstrate the present idea.
 T. Yamada, Y. Yamamoto, and W. A. Harrison, J. Vac. Sci.
Technol. B 14, 1243 (1996); T. Yamada, ibid B 15, 1019 (1997).