  The effects of mobility of small islands on island growth in molecular beam
epitaxy are studied. It is shown that small island mobility affects both the
scaling and morphology of islands during growth. Three microscopic models are
considered, in which the critical island sizes are $i^*=1,2$ and 3 (such that
islands of size $s \le i^*$ are mobile while islands of size $s \ge i^{\ast}+1$
are immobile). As i^* increases, islands become more compact, while the
exponent $\gamma$ which relates the island density to deposition rate
increases. The morphological changes are quantified by using fractal analysis.
It is shown that the fractal dimensions are rather insensitive to changes in
i^*. However, the prefactors provide a quantitative measure of the changing
morphologies.
