  A theory based on localized-orbital approaches is developed to describe the
valley splitting observed in silicon quantum wells. The theory is appropriate
in the limit of low electron density and relevant for proposed quantum
computing architectures. The valley splitting is computed for realistic devices
using the quantitative nanoelectronic modeling tool NEMO. A simple,
analytically solvable tight-binding model is developed, it yields much physical
insight, and it reproduces the behavior of the splitting in the NEMO results.
The splitting is in general nonzero even in the absence of electric field in
contrast to previous works. The splitting in a square well oscillates as a
function of S, the number of layers in the quantum well, with a period that is
determined by the location of the valley minimum in the Brillouin zone. The
envelope of the splitting decays as $S^3$. Finally the feasibility of observing
such oscillations experimentally in modern Si/SiGe heterostructures is
discussed.
