On the optical band gap of zinc oxide

Three different values (3.1, 3.2, and 3.3 eV) have been reported for the optical band gap of zinc oxide single crystals at room temperature. By comparing the optical properties of ZnO crystals using a variety of optical techniques it is concluded that the room temperature band gap is 3.3 eV and that the other values are attributable to a valence band-donor transition at $\text{∼3.15\hspace{0.17em}}\mathrm{eV}$ that can dominate the optical absorption when the bulk of a single crystal is probed.