Evidence of Direct Exciton Polaron Formation upon Photoexcitation in Wurtzite ZnO

Abstract This paper extends the investigation of direct photoexcitation into polaronic states within transition metal oxide materials to ZnO, which has long been understood to form strongly bound excitons even at room temperature. The extent to which these excitons couple to the lattice to form exciton polarons and the nature of this coupling are less clear. Evidence for direct photoexcitation into polaronic states has been reported recently for several transition metal oxides; however, these investigations have been limited to electron or hole polarons. Here, we apply a combination of resonance Raman and temperature-dependent absorption and emission spectroscopies supported by phonon calculations to investigate exciton–phonon coupling in wurtzite ZnO thin films. We find that the A1 (LO) and E1 (LO) phonons exhibit strong coupling to excitonic transitions. These data are consistent with the direct excitation of intrinsic exciton polarons via coupling to thermally populated phonons within a pristine ZnO lattice.