  The effect of edge and corner diffusions on the morphology and on the density
of islands nucleated irreversibly on a flat substrate surface is studied.
Without edge and corner diffusion, islands are fractal. As an edge diffusion
constant $D_e$ increases, islands tend to take a cross shape with four needles
in the $< 10 >$ direction. Additional corner diffusion with a diffusion
constant $D_c$ yields square islands. When $D_e$ is small relative to the
surface diffusion constant $D_s$, the square corner shows the Berg instability
to produce hopper growth in the $<11>$ direction. The corner diffusion
influences the island number density $n$. At a deposition flux $F$ with a small
$D_c$, mainly monomers are mobile and $n \propto (F/D_s)^{1/3}$. At large
$D_c$, dimers and trimers are also mobile and $n \propto F^{3/7} D_s^{-5/21}
D_c^{-4/21}$. The $F$ dependence is in good agreement to the rate equation
analysis, but the dependence on $D_c$ cannot be explained by the theory.
