  A wide variety of apparently contradictory piezoresistance (PZR) behaviors
have been reported in p-type silicon nanowires (SiNW), from the usual positive
bulk effect to anomalous (negative) PZR and giant PZR. The origin of such a
range of diverse phenomena is unclear, and consequently so too is the
importance of a number of parameters including SiNW type (top down or bottom
up), stress concentration, electrostatic field effects, or surface chemistry.
Here we observe all these PZR behaviors in a single set of nominally p-type,
$\langle 110 \rangle$ oriented, top-down SiNWs at uniaxial tensile stresses up
to 0.5 MPa. Longitudinal $\pi$-coefficients varying from $-800\times10^{-11}$
Pa$^{-1}$ to $3000\times10^{-11}$ Pa$^{-1}$ are measured. Micro-Raman
spectroscopy on chemically treated nanowires reveals that stress concentration
is the principal source of giant PZR. The sign and an excess PZR similar in
magnitude to the bulk effect are related to the chemical treatment of the SiNW.
