  Under the inhomogenous stress field set up by nanoindentation, the smaller
the extent of the stress field the greater the yield pressure. When the
specimen contains a thin layer of softer material, the yield pressure is
reduced if plasticity initiates in the soft material. A series of specimens
with the soft layer at different depths enables the depth at which plasticity
initiates to be mapped. InGaAs lattice-matched to InP is used, and the soft
layer is 320nm of strained-layer InGaAs superlattice. Under nanoindentation, we
find that plastic yield initiates throughout a region ranging from 300nm to
over 1um depending on the indenter tip radius. The region matches the depth
range over which the stress exceeds the yield stress of the InGaAs. Thus a
requirement for yield is an overload, or excess stress, throughout a finite
volume.
