Electron paramagnetic resonance and Hall measurements show consistently the presence of two donors ( D1 and D2) in state-of-the-art, nominally undoped ZnO single crystals. Using electron nuclear double resonance it is found that D1 shows hyperfine interaction with more than 50 shells of surrounding 67Zn nuclei, proving that it is a shallow, effective-mass-like donor. In addition D1 exhibits a single interaction with a H nucleus ( a(H) = 1.4 MHz), thus H is the defining element. It is in agreement with the prediction of Van de Walle [Phys. Rev. Lett. 85, 1012 (2000)] that H acts as a donor in ZnO. The concentration of D1 is 6x10(16) cm(-3) emphasizing its relevance for carrier statistics and applications.

The recently reported ability to dope ZnO p-type opens novel possibilities for opto-electronic emitters in the blue spectral range [1]. However, in ZnO emission in the green spectral range is commonly reported, but the responsible defects are not identified; extrinsic (copper) as well as intrinsic defects (O-or Zn-vacancies) are discussed [2,3].We have investigated undoped ZnO single crystals, which are commercially available from Eagle-Picher, by photoluminescence (PL) and optically detected magnetic resonance (ODMR) spectroscopy. The electrical properties of this material are very similar to the samples investigated in Ref. [4]. The total residual shallow donor concentration is about 1 Â 10 17 cm À3 . The low temperature emission is dominated by the donor bound exciton (D 0 X) at 3.366 eV. At 2.45 eV the broad, unstructured "green" emission is located, its full width at half maximum is 320 meV (Fig. 1). The temperature dependence of the PL reveals that this green band maintains its peak energy up to 450 K, which is a feature typical of a recombination within a localised defect, while the D 0 X emission follows the shrinkage of the bandgap with increasing temperature.Performing the ODMR experiment on the green band, i.e. applying an external magnetic field (B 0 ) and exposing the sample to microwaves (24 GHz, 200 mW, TE 011 -cavity) we find two types of ODMR signals (Fig. 2). Two resonance signals enhance the luminescence intensity in the order of 0.5%, and one signal decreases the emission intensity. Using amplitude modulation of the microwave power which serves as reference for the lock-in detection, we find that the two intense signals are detected in phase (spectrum I in Fig. 2a). The low intensity signal (spectrum II in Fig. 2a) is detected with a "90 phase". These observations indicate that the two types of signals are of different origin. This is also evident from the orientation dependence of the resonances with respect to B 0 (Fig. 2b). The small signal is isotropic and the corresponding g-value is g ¼ 1.956, i.e. the g-value of shallow donors in ZnO [5]. Its small anisotropy, originating from the wurtzite crystal structure (g || ¼ 1.957 and g ? ¼ 1.956, || and ? to the crystallographic c-axis) is not resolvable in our experiment. The two intense resonances show the characteristic angular dependence of a spintriplet system (S ¼ 1). The spectra are explained by solutions of the spin Hamiltonian:

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The 1.8 eV luminescence band in GaN grown by molecular beam epitaxy is studied by photo-luminescence (PL) and magnetic resonance. Optically detected magnetic resonance (ODMR) shows that deep centres with g = 1.98 and g = 2.01 are involved in the recombination. The results of the PL experiments indicate that the 1.8 eV recombination can be further fed by shallow centres like shallow donors or excitons.