The optical properties of excitonic recombinations in bulk, n-type ZnO are investigated by photoluminescence (PL) and spatially resolved cathodoluminescence (CL) measurements. At liquid helium temperature in undoped crystals the neutral donor bound excitons dominate in the PL spectrum. Two electron satellite transitions (TES) of the donor bound excitons allow to determine the donor binding energies ranging from 46 to 73 meV. These results are in line with the temperature dependent Hall effect measurements. In the as-grown crystals a shallow donor with an activation energy of 30 meV controls the conductivity. Annealing annihilates this shallow donor which has a bound exciton recombination at 3.3628 eV. Correlated by magnetic resonance experiments we attribute this particular donor to hydrogen. The Al, Ga and In donor bound exciton recombinations are identified based on doping and diffusion experiments and using secondary ion mass spectroscopy. We give a special focus on the recombination around 3.333 eV, i.e. about 50 meV below the free exciton transition. From temperature dependent measurements one obtains a small thermal activation energy for the quenching of the luminescence of 10 ± 2 meV despite the large localization energy of 50 meV. Spatially resolved CL measurements show that the 3.333 eV lines are particularly strong at crystal irregularities and occur only at certain spots hence are not homogeneously distributed within the crystal contrary to the bound exciton recombinations. We attribute them to excitons bound to structural defects (Y-line defect) very common in II-VI semiconductors. For the bound exciton lines which seem to be correlated with Li and Na doping we offer a different interpretation. Li and Na do not introduce any shallow acceptor level in ZnO which otherwise should show up in donor -acceptor pair recombinations. Nitrogen creates a shallow acceptor level in ZnO. Donor -acceptor pair recombination with the 165 meV deep N-acceptor is found in nitrogen doped and implanted ZnO samples, respectively. In the best undoped samples excited rotational states of the donor bound excitons can be seen in low temperature PL measurements. At higher temperatures we also see the appearance of the excitons bound to the B-valence band, which are approximately 4.7 meV higher in energy.

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.

We study the influence of nitrogen, a potential acceptor in ZnO, on the lattice dynamics of ZnO. A series of samples grown by chemical vapor deposition ͑CVD͒ containing different nitrogen concentrations, as determined by secondary ion mass spectroscopy ͑SIMS͒, was investigated. The Raman spectra revealed vibrational modes at 275, 510, 582, 643, and 856 cm Ϫ1 in addition to the host phonons of ZnO. The intensity of these additional modes correlates linearly with the nitrogen concentration and can be used as a quantitative measure of nitrogen in ZnO. These modes are interpreted as local vibrational modes. Furthermore, SIMS showed a correlation between the concentration of incorporated nitrogen and unintentional hydrogen, similar to the incorporation of the p-dopant magnesium and hydrogen in GaN during metalorganic CVD.There is increasing interest in investigating the properties of ZnO epitaxial films with a direct gap of 3.37 eV at room temperature. 1 The material is a potential competitor for GaN-based light-emitting devices in the ultraviolet and blue spectral range. There are reports of superior ZnO properties such as a high exciton binding energy combined with a low lasing threshold density 2 and a good resistance to bombardment with high-energy particles. 3,4 For other wide-band-gap semiconductors as GaN ͑Ref. 5͒ and ZnSe ͑Ref. 6͒ controlled p-type doping is problematic. As-grown ZnO typically has n-type conductivity with background concentrations between 10 16 and 10 17 cm Ϫ3 . However, there have been reports on the synthesis of p-conducting ZnO doped with As ͑Ref. 7͒ and a Ga/N codoping 8 as well as the fabrication of a p-n-junction by excimer-laser doping. 9 In this letter, we report on doping experiments with nitrogen as a potential acceptor and its influence on the lattice dynamics of ZnO.The ZnO thin films under investigation were grown by chemical vapor deposition ͑CVD͒ using a home built epitaxy system which consists of a horizontal quartz reactor and a resistance heating with different temperature zones. Metallic zinc was kept in one zone at a temperature of 470°C the growth temperature was 650°C. We used NO 2 as oxygen precursor and NH 3 as nitrogen source for the doping experiments. The epitaxial films were deposited on GaN/sapphire templates which offers the advantage of a lattice parameter similar to ZnO. We investigated samples containing different nitrogen concentrations. Secondary ion mass spectroscopy ͑SIMS͒ was applied to determine the concentration of nitrogen and unintentional dopants such as hydrogen. The primary ion species was cesium. Nitrogen was detected as 14 N 16 O Ϫ and hydrogen as 64 Zn 1 H Ϫ clusters. The given abso-lute concentrations are accurate to within half an order of magnitude. Despite this accuracy the relative error is less than 10%. The Raman-scattering experiments were carried out in backscattering geometry with a triple-grating spectrometer equipped with a cooled charge-coupled device detector. The lines at 488 and 514.5 nm of an Ar ϩ /Kr ϩ mixedgas laser were used...

No abstract

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: