Exciton spectra of ZnO epitaxial layers on lattice-matched substrates grown with laser-molecular-beam epitaxy

Optical properties in undoped-ZnO epilayers grown by the laser-molecular-beam epitaxy method on lattice-matched ${\mathrm{ScAlMgO}}_{\mathrm{4}}$ substrates were investigated. The absorption spectrum at 5 K has two sharp peaks, both of which are attributed to resonances of $A$ and $B$ excitons, which reflect a small nonradiative damping constant of excitons as well as high film crystallinity accomplished by the virtue of lattice matching. The coupling strengths of exciton-acoustic phonon and of exciton–longitudinal-optical phonon were directly determined from the temperature dependence of exciton absorption spectra independently for $A$ and $B$ excitons, which are close in energy and obey the same selection rule for each other.