$7\mathrm{.}3$ For Ge on $(100)$ Si$,$ calculate the critical cluster size $x_c$ if ${}_{Ge}=1\mathrm{.}9eV/$atom and ${}_{Si}=2\mathrm{.}0eV/$atom$.$ Remember if we have Stransi$-$Krastanov $($S$-$K$)$ growth, an epitaxial wetting layer is formed of $Ge$ on $Si.$ The problem requires a derivation of $x_c$ in the $S-K$ situation.