  The steady-state, space-charge-limited piezoresistance (PZR) of
defect-engineered, silicon-on-insulator device layers containing silicon
divacancy defects changes sign as a function of applied bias. Above a
punch-through voltage ($V_t$) corresponding to the onset of a
space-charge-limited hole current, the longitudinal $\langle 110 \rangle$ PZR
$\pi$-coefficient is $\pi \approx 65 \times 10^{-11}$~Pa$^{-1}$, similar to the
value obtained in charge-neutral, p-type silicon. Below $V_t$, the mechanical
stress dependence of the Shockley-Read-Hall (SRH) recombination parameters,
specifically the divacancy trap energy $E_T$ which is estimated to vary by
$\approx 30$~$\mu$V/MPa, yields $\pi \approx -25 \times 10^{-11}$~Pa$^{-1}$.
The combination of space-charge-limited transport and defect engineering which
significantly reduces SRH recombination lifetimes makes this work directly
relevant to discussions of giant or anomalous PZR at small strains in
nano-silicon whose characteristic dimension is larger than a few nanometers. In
this limit the reduced electrostatic dimensionality lowers $V_t$ and amplifies
space-charge-limited currents and efficient SRH recombination occurs via
surface defects. The results reinforce the growing evidence that in steady
state, electro-mechanically active defects can result in anomalous, but not
giant, PZR.
