Photoluminescence and EPR of donors and acceptors in ZnO

Photoluminescence (PL) and electron paramagnetic resonance (EPR) are high-resolution techniques used to study donors and acceptors in optoelectronic materials. Zinc oxide (ZnO), with a room-temperature band gap of 3.37 eV, has significant potential for applications ranging from light emission to sensors and detectors. The low-temperature near-edge PL of ZnO is rich in detail, with sharp-line emissions from bound excitons related to various donors and acceptors. Strong phonon couplings in this material produce a series of LO and TO phonon sidebands at slightly lower energies. Donor-acceptor pair and electron-acceptor recombinations related to nitrogen (E A = 209 meV) and lithium (E A ~ 0.6 eV) are detected. Copper and iron impurities show characteristic luminescence spectra in the visible. Thermal anneals in air induce significant changes in the PL spectra. Complementary information can be obtained from EPR and photoinduced EPR experiments performed at low temperature. In ZnO, EPR spectra have been observed from neutral nitrogen acceptors, neutral copper acceptors, neutral lithium acceptors, hydrogenic shallow donors, as well as deeper donors such as nickel and iron. In previous work, EPR spectra have been assigned to singly ionized oxygen vacancies and singly ionized zinc vacancies in electron-irradiated crystals.