  We discuss the mechanism responsible for the observed improvement in the
structural properties of In doped GaSe, a layered material of great current
interest. Formation energy calculations show that by tuning the Fermi energy,
In can substitute for Ga or can go as an interstitial charged
defect$(\text{In}_{\text{i}}^{\text{3+}})$. We find that
$\text{In}_{\text{i}}^{\text{3+}}$ dramatically increases the shear stiffness
of GaSe, explaining the observed enhancement in the rigidity of In doped
p-GaSe. The mechanism responsible for rigidity enhancement discussed here is
quite general and applicable to a large class of layered solids with weak
interlayer bonding.
