This question is answered (affirmatively and somewhat more generally) in the following paper: Chat Yin Ho, Projective planes with a regular collineation group and a question about powers of a primeProjective planes with a regular collineation group and a question about powers of a prime, J. Algebra 154 (1993), no. 1, 141–151. The proof there uses the ring of Eisenstein integers. (The issue is available online via Open Access).