Chapter 5
Spin Diffusion in Silicon
”I keep six honest serving men, They taught me all I knew: Their names are
What and Why and When and How and Where and Who.”
Rudyard Kipling
In order to design and fabricate high-performance silicon-based spintronic
devices, a comprehensive understanding of the spin transport properties of a
semiconducting channel (~ μm) is needed. In this chapter, the spin injection in
silicon from a ferromagnetic semiconductor (FMS) by electrical means is
investigated, when spin is injected from a charge neutral source. Even
though a sufficient analysis of the spin transport under charge neutrality is
available in literature [173, 174, 175], the attention there has been on the
magnetoresistance. Thus, it becomes mandatory to also investigate the
spin signal when the charge neutrality condition is violated. In such a
case, one has to solve the spin drift-diffusion equations coupled with the
Poisson equation. This is done because from the Poisson equation one can
derive the electric potential (and hence the electric field) in the conducting
channel, which prominently influences the spin diffusion [173]. In fact,
experiments have shown that electric fields can severely affect spin diffusion in
semiconductors [176, 177].
At first the fundamental charge transport equations [178] will be discussed.
Afterwards it will be shown how to extend those equations for spin transport.