Half space theorem for the Allen-Cahn equation and related problems

In this paper we obtain rigidity results for a bounded non-constant entire solution $u$ of the Allen-Cahn equation in ${\mathbb{R}}^n$, whose level set $\{u=0\}$ is contained in a half-space. If $n\le 3$ we prove that the solution must be one-dimensional. In dimension $n\ge 4$, we prove that either the solution is one-dimensional or stays below a one-dimensional solution and converges to it after suitable translations. Some generalizations to one phase free boundary problems are also obtained.