We investigated the effects of post-growth annealing in the temperature range of 873 to 1273 K on the spectral features of photoluminescence (PL) vis-à-vis the crystalline and compositional native defects of ZnO thin films grown at 773 K by pulsed laser deposition (PLD) on sapphire substrates. It is found in the PL spectra at 10 K that the deep level emission (DLE) shifted from red-orange spectral region of ∼1.8–2.4 eV to yellow–green region of ∼2.4–2.9 eV with the increasing temperature of annealing. We propose that the PL in red-orange region originating from the singly ionized oxygen vacancies diminished due to increased replenishment of oxygen with increasing annealing temperature and that in the yellow–green region originating from the oxygen interstitials and/or zinc vacancies increased due to enhanced concentration of these point defects. As the annealing temperature was increased, the overall intensity of PL in the DLE region increased slightly up to 973 K but beyond that it increased steeply and made a quantum leap at 1073 K. In contrast to that, intensity of PL due to the near band-edge emission (NBE) in UV region of ∼3.15 to 3.45 eV increased very steeply up to the annealing temperature of 973 K, which is found to be due to improvement in the crystalline and compositional qualities of the films and beyond that it dropped drastically due to deteriorations of these qualities. The high resolution PL spectra at 10 K in the NBE region mainly consisted of peaks due to the recombinations of neutral donor bound excitons’ complexes (D0X) at ∼3.36 eV, free excitons (FXA) at ∼3.38 eV with their conspicuous LO phonon replicas and some other features such as exciton complexes bound to surface states or transitions of conduction band electrons to acceptor levels located in stacking faults and recombination of neutral acceptor bound excitons. The relative intensities of these individual features were strongly dependent on the annealing temperature of the films and the ensuing crystalline and compositional qualities. The 10 K PL spectra from the interfacial region of the annealed ZnO films and the sapphire substrates observed from the backside of the samples showed that the annealing temperature affected the crystalline and compositional qualities at the interface in a complex manner. Particularly, the features corresponding to the interface deteriorations resulting from the diffusion of Al into the ZnO films and the crystalline defects at the interface caused by the sputtering due to the PLD plume were prominently present in these PL spectra. These studies provide deeper insight into the fundamental PL processes in ZnO thin films annealed at different temperatures.

We show that the size of nanorods grown by a vapour phase transport method on different substrates can be easily changed by an order of magnitude. This allows producing customized nanorod arrays that can improve the efficiency of Gratzel-type solar cells as well as polymer hybrid solar cells. Luminescence measurements show that doping is possible simply by the choice of the substrate. The crystalline quality is examined by high resolution TEM measurements. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The research on ZnO has a long history but experiences an extremely vivid revival during the last 10 years. We critically discuss in this didactical review old and new results concentrating on optical properties but presenting shortly also a few aspects of other fields like transport or magnetic properties. We start generally with the properties of bulk samples, proceed then to epitaxial layers and nanorods, which have in many respects properties identical to bulk samples and end in several cases with data on quantum wells or nano crystallites. Since it is a didactical review, we present explicitly misconceptions found frequently in submitted or published papers, with the aim to help young scientists entering this field to improve the quality of their submitted manuscripts. We finish with an appendix on quasi two‐ and one‐dimensional exciton cavity polaritons.

Hexagonal and arrow-headed ZnO nanorod structures have been grown by low pressure chemical vapour deposition (CVD) and atmospheric pressure metal-organic CVD. The technology ensures a high optical quality of the produced nanostructures to act as gain medium for stimulated emission in the ultraviolet spectral region in combination with high quality factor laser resonators. Multiple sharp lasing peaks related to the guided modes were realized from single hexagonal nanorods and arrays of hexagonal ZnO nanorods. A comparative analysis of the variations of lasing spectra from shot to shot of pumping, and the dependence of lasing threshold on the area of pump beam spot on the sample surface in disordered agglomerations of hexagonal nanorods and in layers consisting of arrow-headed nanorods, demonstrate that lasing is determined by the superposition of guided modes in the first case, while random lasing occurs in the second case.

Nanocrystalline ZnO powders can act as gain and scattering medium in a random laser where the light emission can be strongly amplified. In this work, we compare the luminescence properties of samples with different particle sizes in the regime of linear and nonlinear optics. In the high-excitation regime random lasing is observed in all samples. Here, the lasing threshold depends strongly on the size distribution in the ensemble. Additional characterization of the samples has been done by determining the absolute quantum efficiency of the radiative processes in the powder. The values are in the 10% range and the near-edge luminescence is strongly influenced by the particle sizes. We show that by annealing the nanocrystals coalesce to larger polycrystalline grains, which results in a new emission band at 3.333eV due to the grain boundaries. Furthermore, it is found that in the annealed samples the threshold for random lasing could be considerably decreased.

Quasi-two-dimensional arrays of nearly parallel hexagonal ZnO nanorods and a three-dimensional cylindrical microstructure consisting of ZnO nanorods have been grown by low pressure chemical vapor deposition (CVD) and carbothermal evaporation technologies, respectively. The technology ensures high optical quality of the produced nanostructures so as to act as a gain medium for stimulated emission in the ultraviolet spectral region in combination with high quality factor laser resonators. Multiple sharp lasing peaks were realized from the produced structures under nanosecond pulse optical excitation. The lasing peaks display successive onset and saturation with increasing excitation power density in accordance with the lasing behavior of guided modes in ZnO nanorods. The produced structures are expected to find applications in integrated nanoscale optoelectronics, photonics, and sensor technologies.

High optical quality, well end leg faceted ZnO microtetrapods with leg length between 1 and 12 μm have been grown by carbothermal chemical vapor deposition. Lasing with mode quality factors of 2500–3000 is demonstrated. The origin of laser resonator cavity is discussed as a function of the tetrapod size. It is shown that in big tetrapods with legs of 12 μm in length the laser emission lines are well explained by longitudinal Fabry–Pérot modes generated in cavities formed by individual tetrapod legs. The dispersion of the ZnO refractive index is experimentally determined from the position of lasing modes in the temperature interval from 10 to 300 K. It is shown that the lasing mode structure is seriously affected by the decrease of the tetrapod size. For a small tetrapod with a leg length of 1 μm, the lasing modes cannot be explained anymore by the formation of longitudinal Fabry–Pérot modes in separate tetrapod legs, and the generation of guided modes by multiple total internal reflections in single tetrapod legs or in pairs of legs should be taken into account. The correlations between the lasing threshold and the tetrapod size are discussed.

In this work we examine the lasing properties of single ZnO nanorods under ns- and fs-pulsed excitation. By measuring the photoluminescence of individual nanorods excited by a single ns-pulse we demonstrate that several stable lasing modes are emitted by the nanorod. In this quasi-stationary excitation regime lasing up to room temperature can be achieved. Measurements of the temperature dependent laser threshold and of the spectral lasing region are presented. The excitation with fs-pulsed excitation combined with a time resolved photoluminescence measurement gives furthermore an insight in the lasing dynamics.[GRAPHICS]Scanning electron microscope picture of a sample with an array of ZnO nanorods grown perpendicular aligned on the substrate. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The electrical properties of field-effect transistors fabricated on the basis of single ZnO nanorods were analyzed under ambient conditions and in the chamber of a scanning electron microscope under high-vacuum conditions. Under ambient conditions, the threshold voltage and conductivity may depend strongly on the details of the measurement procedure as the chosen gate voltage range and gate voltage sweep direction. Electron irradiation in a scanning electron microscope under high-vacuum conditions at ∼10−5 mbar leads to desorption of oxygen and other electronegative molecules, which can increase the conductivity by more than two orders of magnitude.

In linear optics, we report on measurements of the absolute external quantum efficiency of bulk ZnO and powders using an integrating sphere. At low temperature the near band edge emission efficiency can reach 0.15 in the best samples. For deep center luminescence this value may be even higher. When going to room temperature (RT) the quantum efficiency drops by about one order of magnitude. From time resolved luminescence measurements we deduce the lifetime of the free and bound excitons to be in the sub ns regime and find for the latter a systematic increase with increasing binding energy.

We report on the investigation of CdSe/ZnSe heterostructures by transmission electron microscopy (TEM) and photoluminescence spectroscopy (PL). CdSe layers with nominal thicknesses t(CdS), between 0.5 and 8 monolayers (ML) were embedded in a ZnSe matrix and grown on a GaAs(001) substrate by motecular-beam epitaxy at 280 degrees C. The Cd-distribution was obtained from high-resolution TEM lattice fringe images using composition evaluation by lattice fringe analysis technique. The measured minimum, average and maximum Cd-concentrations and the overall CdSe contents in the layers increase with the nominal CdSe layer thickness and reach a constant value at t(CdSe) = 4 ML. The measured CdSe content in the regions with the maximal Cd-concentration continues to increase for t(CdSe) >= 5 ML. The increasing of measured Cd-concentrations/CdSe contents is correlated with a red shift of PL spectra. In the CdSe layers with t(CdSe) >= 5 ML, formation of defects is observed. The increasing density of defects and decreasing intensity of PL spectra with tCdS, suggest that the critical thickness for defect formation during CdSe growth on the ZnSe(001) is between 4 ML and 5 ML. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ZnO(0001) substrates were ion implanted with 100 keV of Co and 300 keV of Gd at different fluences ranging from 5×1013–1×1015/cm2. The resulting Co:ZnO and Gd:ZnO samples were analyzed with respect to their structural, magnetic, and optical properties. The effect of annealing at 350 °C on the structure and the resulting magnetic and optical properties were investigated as well. For Co:ZnO hardly any changes were observable, neither in the structural nor in the magnetic properties, even though the existence of substitutional Co2+ in the ZnO lattice could be shown by means of low temperature photoluminescence especially for Zn-annealed samples. For the much larger Gd ion the implantation leads to a changed crystal structure, which leads to a ferromagneticlike behavior for higher implantation doses, which could even be enhanced by annealing in vacuum. Ferromagnetic behavior in annealed Gd:ZnO is corroborated by ferromagnetic resonance measurements at low temperatures. The distinct behavior of Gd- and Co-implanted ZnO highlights the importance of defects in the understanding of the magnetic properties in dilute magnetic semiconductors.

Carbon nanotubes and ZnO based functional coaxial heterostructured nanotubes have been fabricated by using atomic layer deposition. An irregular structured shell composed of ZnO nanocrystals was deposited on pristine nanotubes, while a highly defined ZnO shell was deposited on the tubes after its functionalization with Al2O3. Photoluminescence measurements of the ZnO shell on Al2O3/nanotube show a broad green band emission, whereas the shell grown on the bare nanotube shows a band shifted to the orange spectral range.

In this contribution we present a critical review of experimental data and theoretical considerations concerning the stimulated emission in ZnO. The main processes discussed in literature leading to stimulated emission at RT are inelastic exciton-exciton scattering (P-band) and recombination of an inverted band-to-band transition in an electron-hole plasma (EHP). While the latter process becomes dominant at RT for densities close to 10(19) cm(-3) theory predicts a lower threshold for inelastic exciton-free carrier scattering compared to the P-band [1]. In addition to that the exciton-nLO phonon process might also have a lower threshold than the P-band. A detailed analysis of the temperature dependence of the band gap and of the homogenous broadening of the exciton resonance allows us to distinguish between the various processes but also casts some doubt on the frequently given claim of excitonic RT lasing in ZnO. Therefore we argue that inelastic scattering processes with carriers, phonons or plasmons in a still nondegenerate and strongly coulomb correlated carrier gas are relevant processes for stimulated emission at RT.

We investigate the dynamics of electrons injected into InAs/GaAs quantum dots by initializing and further observing the spin state of the electrons. For this purpose, we use spin polarized light-emitting diodes where the electron spin is set in a semimagnetic ZnMnSe layer. We find that the degree of optical polarization depends strongly on the ground state energy of the quantum dot. A dependence of polarization on dopant concentration in the spin aligner suggests an influence of residual electrons in the quantum dots.

Single-crystal phosphorus-doped ZnO nanowires were synthesized by using a single-source precursor-based vapor transport method. The photoluminescence spectra of phosphorus-doped ZnO nanowires and undoped nanowires are compared. While both show several shallow bound exciton complexes, the phosphorus-doped nanowires reveal an additional distinct emission feature at 3.316 eV. Additionally, the time-resolved PL measurements were conducted to characterize the recombination dynamics.

Electrical spin injection from an n-type ZWnSe spin aligner into III-V p-i-n diode structures with InGaAs quantum dots (QDs) in the active layer is investigated. Analysis of the circular polarization degree (CPD) of the device emission indicates the spin polarization of the injected electrons. Values > 70% are obtained for the electroluminescence (EL) from the wetting layer and QDs with high ground-state energy. Towards the low-energy end of the emission spectrum, the CPD drops strongly. Temperature-dependent measurements suggest, that this is due to spin relaxation taking place at a stage, when the electrons are not yet finally captured in the dots, i.e. in the GaAs spacer or the wetting layer. Furthermore, we demonstrate electrical spin injection into single InGaAs QDs, a prerequisite for future single spin manipulation experiments within the context of quantum information processing.

We report on the injection of electron spins into InGaAs quantum dots with an efficiency of up to 60%. This injection is observed in p-i-n light-emitting diode structures using the diluted magnetic semiconductor ZnMnSe as spin aligner (spin-LED). The degree of spin polarization is deduced from the circular polarization degree of the photons emitted when the injected electrons recombine in the quantum dots with unpolarized holes. We observe a strong energy dependence of the polarization degree with a strong increase starting from zero to a high value on the high energy side of the emission spectrum. To study the origin of this dependence, we compare results of two quantum-dot samples with emission peaks at 1.2 eV and 1.33 eV, respectively. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The influence of the growth conditions during molecular-beam epitaxy on the properties of InAs/GaAs quantum dot structures were systematically investigated by low temperature photoluminescence spectroscopy and transmission electron microscopy. The circular polarization degree (CPD) of the electroluminescence was compared for two quantum-dot spin-injection light-emitting diodes. The CPD depends on the position of the emission energy in the luminescence band. This correlation is similar for both samples despite the strongly different quantum dot morphologies. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We report on efficient injection of electron spins into InGaAs-based nanostructures. The spin light-emitting diodes incorporate an InGaAs quantum well or quantum dots, respectively, as well as a semimagnetic ZnMnSe spin-aligner layer. We show a circular polarization degree of up to 35% for the electroluminescence from InGaAs quantum wells and up to 21% for InGaAs quantum dots. We can clearly attribute the polarization of the emitted photons to the spin alignment in the semimagnetic layer by comparison to results from reference devices (where the ZnMnSe is replaced by ZnSe) and from all-optical measurements.