A rigidity result for overdetermined elliptic problems in the plane

Let $f:[0,+\infty) \to \mathbb{R}$ be a (locally) Lipschitz function and $\Omega \subset \mathbb{R}^2$ a $C^{1,\alpha}$ domain whose boundary is unbounded and connected. If there exists a positive bounded solution to the overdetermined elliptic problem $$\left\{\begin{array} {ll} \Delta u + f(u) = 0 & \mbox{in }\; \Omega \\ u= 0\, \, \, , \, \, \, \frac{\partial u}{\partial \vec{\nu}}=1 &\mbox{on }\; \partial \Omega \end{array}\right.$$ we prove that $\Omega$ is a half-plane. In particular, we obtain a partial answer to a question raised by H. Berestycki, L. Caffarelli and L. Nirenberg in 1997.