  We consider a reaction-diffusion model incorporating the reactions A -> 0, A
-> 2A and 2A -> 3A. Depending on the relative rates for sexual and asexual
reproduction of the quantity A, the model exhibits either a continuous or
first-order absorbing phase transition to an extinct state. A tricritical point
separates the two phase lines. As well as briefly examining this critical
behavior in 2+1 dimensions, we pay particular attention to the cluster
geometry. We observe the different cluster structures that form at criticality
for the three different types of critical behavior and show that there exists a
linear relationship for the probability of survival against initial cluster
size at the tricritical point only.
