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.

PACS: 61.72.Vv; 78.47.+p; 78.55.Et Similar to other wide-band gap semiconductors such as GaN [1] and ZnSe [2] p-type doping in ZnO has been a challenge for many years. As-grown ZnO typically shows n-type conductivity with background concentrations between 10 16 and 10 17 cm --3 . These residual donor concentrations have to be overcome by a suitable acceptor impurity. Suitable means absence of site competition, e.g. substitutional Li (acceptor) versus interstitial Li (donor) [3], a high enough solubility, small size mismatch and hence no lattice relaxation, and avoiding if possible the formation of compensating/passivating centres. Nitrogen on an oxygen site can be considered the best candidate as demonstrated by its success in p-type doping ZnSe [2] and ZnS [4]. In this communication we present the optical properties of the shallow nitrogen acceptor in ZnO and give an estimate of its binding energy.The epitaxial films were deposited in a home-built epitaxy system which consists of a horizontal quartz reactor, and a resistance heating with different temperature zones. Metallic Zn with 6N purity was kept in one zone at a temperature of 470 C. The growth temperature was between 600 and 650 C. As oxygen precursor we used NO 2 and for nitrogen doping ammonia. We grew on GaN-templates which were grown by MOCVD on (0001) sapphire substrates. The films were investigated by time-integrated and time-resolved photoluminescence (PL). The 325 nm line of a HeCd laser and a pulsed dye laser (pulse width l ¼ 292 nm) were used as excitation source.In order to get information on the energy level of the nitrogen acceptor in the band gap of ZnO we performed low temperature photoluminescence experiments. In Fig. 1 we compare undoped and N-doped ZnO layers. The undoped films show predominantly bound exciton emission (Fig. 1a), in the inset the energy range around 3.36 eV is shown on an expanded scale. The two recombinations occur at 3.362 and 3.361 eV (note that the line positions may depend on strain in the films). They are commonly attributed to neutral donor bound excitons having localisation energies between 14 and 15 meV. From the observation of two-electron satellites (TES) we could determine the donor binding energies to between 52 and 54 meV. Characteristic changes occured in the nitrogen doped film (see Fig. 1b). The excitonic recombinations can no longer be distinguished due to the increase in linewidth. The recombination at 3.33 eV was related by spatially resolved cathodoluminescence experiments to structural defects. At lower energies pronounced donor-acceptor pair transitions with an intensity comparable to the excitonic recombination are observed. Its zero phonon line (ZPL) peaks at 3.235 eV and it is repeated by longitudinal optical phonon replicas with an energy of 73 meV. The intensity of the ZPL with respect to 1LO, 2LO, . . . replicas is described by a Poisson distribution (weak-coupling regime)

We present results of magneto-optical measurements and theoretical analysis of shallow bound exciton complexes in bulk ZnO. Polarization and angular dependencies of magneto-photoluminescence spectra at 5 T suggest that the upper valence band has Γ7 symmetry. Nitrogen doping leads to the formation of an acceptor center that compensates shallow donors. This is confirmed by the observation of excitons bound to ionized donors in nitrogen doped ZnO. The strongest transition in the ZnO:N (I9 transition) is associated with a donor bound exciton. This conclusion is based on its thermalization behavior in temperature-dependent magneto-transmission measurements and is supported by comparison of the thermalization properties of the I9 and I4 emission lines in temperature-dependent magneto-photoluminescence investigations.

We measured the near-infrared photoluminescence decay time in several different-chirality single-walled carbon nanotubes by time-resolved picosecond luminescence spectroscopy. Together with the results of resonant pump-and-probe spectroscopy this leads to a carrier lifetime in the first excited state of semiconducting nanotubes exceeding 30 ps, which is one order of magnitude larger than the carrier dynamics observed in nanotube bundles. Our findings show that the absence of photoluminescence in nanotube bundles is due to a tunneling of the free carriers from semiconducting into metallic nanotubes.Optical techniques are powerful methods used to study the electronic band structure of materials away from the Fermi level. Photoluminescence and excitation spectroscopy yield information on the electronic energies and the symmetries of the states. The dynamics of the excited carriers and their recombination is studied by time-resolved measurements. One-dimensional systems like single-walled carbon nanotubes are again special in that they are expected to have particularly intense and strongly structured emission and absorption spectra, because of their square-root-like singularities in the electronic density of states. In contrast to this expectation, however, as grown samples of nanotubeswhich occur in bundles of 50-100 individual tubes-did not show photoluminescence. Moreover, their absorption spectra are broad and unstructured. 1 Large progress in the spectroscopy of carbon nanotubes was made when O'Connell et al. 2 reported that photoluminescence and narrow optical absorption peaks could be observed in carbon nanotubes if the originally bundled tubes were isolated in micelles. [3][4][5] While the broadening of the optical absorption spectra in bundled nanotubes is due to the intertube electronic dispersion perpendicular to the tube axis, 6 the absence of photoluminescence in bundles remained unclear. It was suggested that the photoluminescence is quenched by the presence of metallic tubes; although the physical mechanism for the quenching remained unelucidated. 2 For the understanding of these processes, it thus became essential to determine the radiative lifetime in isolated single-walled carbon nanotubes.Here we report the relaxation times of photoluminescence of isolated semiconducting nanotubes tuned into several distinct chiral indices. The observed decay times ͑Ϸ30 ps͒ are more than ten times longer than in nanotube bundles. The much more rapid quenching in bundled tubes is explained by tunneling of the free carriers into metallic tubes. Resonant pump-and-probe experiments confirmed that our decay times correspond to a minimum of Ϸ30 ps for the lifetime of carriers in the excited state. 7 Combined with the rapid depopulation of the higher excited states our findings suggest the application of individual carbon nanotubes in optoelectronics such as light-emitting nanodevices or even four-level lasers.The dynamics of photocarriers in single-walled carbon nanotubes bundles was studied by Hertel et al. 8,9 with tim...

Indium-rich fluctuations in ultrathin InGaN layers act at low temperatures as a dense ensemble of quantum dots (QD). This leads to a complex potential landscape with localization sites of widely varying depth for excitons. We report on investigations of the recombination mechanisms of excitons localized in InGaN∕GaN QD structures by time-resolved and spatially resolved photoluminescence (PL) measurements. The structures were grown by metal-organic chemical-vapor deposition on Si (111) substrates. Sharp lines originating from single QDs could be observed. Their PL decays show monoexponential behavior. Similar transition energies have different time constants. Thus, the well-known nonexponential PL decay of the QD ensemble is assigned to the summation of monoexponential decays originating from individual QDs with different exciton lifetimes.