Electron paramagnetic resonance (EPR), photoluminescence, and infrared optical absorption have been used to investigate a ZnO crystal before and after a thermal anneal for 1 h in air at 900 °C. The sample was an undoped high quality crystal grown by the chemical vapor transport method. In addition to shallow donor impurities, the crystal contained trace amounts of copper ions. Prior to the thermal anneal, these ions were all in the Cu+ (3d10) state and the observed luminescence at 5 K, produced by 364 nm light, consisted of a broad structureless band peaking at 500 nm. After the high-temperature anneal, the Cu2+ (3d9) EPR spectrum was observed and the luminescence had changed significantly. The emission then peaked near 510 nm and showed structure identical to that reported by Dingle [Phys. Rev. Lett. 23, 579 (1969)]. Our data reaffirm that the structured green emission in ZnO is associated with Cu2+ ions. We suggest that the unstructured green emission (observed before the high-temperature anneal) is donor–acceptor pair recombination involving the Cu+ acceptors.

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies are used to track the temperature dependence of the A exciton energy (EXA) in undoped bulk ZnO crystals grown by the seeded-chemical-vapor-transport method. For T>150 K, the edge emission becomes broad as the A exciton recombination and its longitudinal-optical (LO) phonon replica become superimposed. We use PLE to determine the temperature dependence of EXA by monitoring the broad green emission commonly observed in as-grown ZnO crystals, and thus have established the energy difference between the EXA and PL emission peak energies. The PL emission at 3.26 eV at room temperature is shown to be offset by about 50 meV to lower energy than the actual EXA transition. The temperature dependence of the energy difference between the EXA and PL peaks is compared with predictions based on the lineshape function for the EXA– LO recombination. At 300 K, the PL is predominantly composed of EXA– LO recombination. Further, the temperature dependence of the EXA transition energy can be described using standard expressions and the Debye and Einstein temperatures are found to be 700±30 and 240±5 K, respectively. The slope of the EXA versus T curve for ZnO approaches a constant value of dEXA/dT=−0.35 meV/K near room temperature.

Zinc oxide crystals grown by the seeded chemical vapor transport method have been annealed in zinc vapor at 1100 °C for 30 min. These thermochemical reduction treatments produce a deep red coloration in the crystals and increase their n-type electrical conductivity. Electron paramagnetic resonance (EPR), optical absorption, and Hall measurements were used to monitor changes in the crystals. After an anneal, an intense optical absorption band is present that extends from the band edge out to approximately 550 nm, and the EPR signal near g=1.96 (due to shallow donors and/or conduction-band electrons), the free-carrier absorption, and the Hall electron concentration are all larger. Hydrogen was not present during these anneals, thus leaving oxygen vacancies and zinc interstitials as candidates for the added donors. Neutral oxygen vacancies are produced at high temperature by the additive-coloration mechanism, and are responsible for the broad near-edge absorption band. The observed increase in the number of free carriers is a result of either (1) the formation of zinc interstitials or (2) having the ground state of the neutral oxygen vacancy near the conduction band.

Nitrogen acceptors are formed when undoped single crystals of zinc oxide (ZnO) grown by the chemical-vapor transport method are annealed in air or nitrogen atmosphere at temperatures between 600 and 900 °C. After an anneal, an induced near-edge absorption band causes the crystals to appear yellow. Also, the concentration of neutral shallow donors, as monitored by electron paramagnetic resonance (EPR), is significantly reduced. A photoinduced EPR signal due to neutral nitrogen acceptors is observed when the annealed crystals are exposed to laser light (e.g., 364, 442, 458, or 514 nm) at low temperature. The nitrogens are initially in the nonparamagnetic singly ionized state (N−) in an annealed crystal, because of the large number of shallow donors, and the light converts a portion of them to the paramagnetic neutral acceptor state (N0).

Electron paramagnetic resonance (EPR) has been used to monitor oxygen vacancies and zinc vacancies in a ZnO crystal irradiated near room temperature with 1.5MeV electrons. Out-of-phase detection at 30K greatly enhances the EPR signals from these vacancies. Following the electron irradiation, but before illumination, Fe3+ ions and nonaxial singly ionized zinc vacancies are observed. Illumination with 325nm laser light at low temperature eliminates the Fe3+ signal while producing spectra from singly ionized oxygen vacancies, neutral zinc vacancies, and axial singly ionized zinc vacancies. This light also produces EPR spectra from zinc vacancies having a OH− ion at an adjacent oxygen site. The low-temperature response of the irradiated crystal to illumination wavelengths between 350 and 750nm is described. Wavelengths shorter than 600nm convert Fe3+ ions to Fe2+ ions and convert neutral oxygen vacancies to singly ionized oxygen vacancies. Neutral zinc vacancies are formed by wavelengths shorter than 500nm as electrons are removed from isolated singly ionized zinc vacancies. Warming above 120K in the dark reverses the effect of the illuminations. These wavelength-dependence results suggest that the ground state of the neutral oxygen vacancy is deep, approximately 1.3eV above the valence band, and that the ground state of the singly ionized zinc vacancy is also deep, about 0.9eV above the valence band.