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Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies are used to track the temperature dependence of the A exciton energy (E[SUBXA]) 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 E[SUBXA] by monitoring the broad green emission commonly observed in as-grown ZnO crystals, and thus have established the energy difference between the E[SUBXA] 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 E[SUBXA] transition. The temperature dependence of the energy difference between the E[SUBXA] and PL peaks is compared)with predictions based on the lineshape function for the E[[SUBXA]- LO recombination. At 300 K, the PL is predominantly composed of E[SUBXA]- LO recombination. Further, the temperature dependence of the E[SUBXA] 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 E[SUBXA] versus T curve for ZnO approaches a constant value of De[SUBXA]/Dt=-0.35 meV/K near room temperature. [ABSTRACT FROM AUTHOR]

Electron paramagnetic resonance (EPR) has been used to investigate molecular nitrogen and isolated nitrogen acceptors in single crystals of ZnO. These samples were grown by the seeded chemical vapor transport method with N[sub 2] added to the gas stream. A five-line EPR spectrum is observed at low temperature in the as-grown bulk crystals and is assigned to N[sup -, sub 2] molecules substituting for oxygen. This structure arises from nearly equal hyperfine interactions with two nitrogen nuclei ([sup 14]N, 99.63% abundant, I = 1). The spin Hamiltonian parameters for the N[sup -, sub 2] center are g[sub ] = 2.0036, g[sub ⊥] = 1.9935, A[sub ⊥] = 9.8 MHz, and A[sub ⊥] = 20.1 MHz, with the unique directions parallel to the c axis. Laser excitation at 9 K, with 325 or 442 nm light, eliminates the N[sup -, sub 2] spectrum (when the N[sup -, sub 2] convert to N[sup 0, sub 2]) and independently introduces the EPR spectrum due to isolated nitrogen acceptors (when N[sup -] acceptors convert to N[sup 0]). Removing the laser light and warming to approximately 100 K restores the crystal to its preilluminated state. In separate experiments, heating between 600 and 800 °C increases the number of N[sup -, sub 2] and N[sup 0] acceptors that can be observed. We suggest that the activation of these nitrogen acceptors occurs when complexes of hydrogen and nitrogen thermally dissociate. Further heating above 800 °C drives the two nitrogen acceptors to inactive forms. [ABSTRACT FROM AUTHOR]