ZnO growth by MOCVD: numerical study

EnIII−V compound device fabrication is facing today challenging issues typically related to high volume manufacturing such as process reliability, process consistency, cost−reduction. Each step of the overall device manufacturing process must be carefully analysed and replicated to obtain reproducible device structures. Purifiers are commonly used in MOVPE processes and are becoming standard equipment in both research and production environments. In most cases implementation of gas purification strategies is enabling to achieve ultimate product purity and process reproducibility by defect and contamination control. In addition, an appropriate gas purification strategy is effective in high value component/chemicals protection (e.g. high purity MO sources), and as an assurance against line contamination due to human error or component failure. Purifier operating conditions can vary noticeably and a knowledge of which parameters can affect ultimate gas purity should be of interest to MOVPE operators to master gas distribution line contamination issues. Expertise on such parameters and their effect is essential to obtain a reliable product and sub−ppb contamination control throughout the purifier's lifetime and not only in spot demonstrations

Positron annihilation lifetime spectroscopy of ZnO bulk samples

In order to gain a further insight into the knowledge of point defects of ZnO, positron annihilation lifetime spectroscopy was performed on bulk samples annealed under different atmospheres. The samples were characterized at temperatures ranging from 10 to 500 K. Due to difficulties in the conventional fitting of the lifetime spectra caused by the low intensity of the defect signals, we have used an alternative method as a solution to overcome these difficulties and resolve all the lifetime components present in the spectra. Two different vacancy-type defects are identified in the samples: Zn vacancy complexes (VZn−X) and vacancy clusters consisting of up to five missing Zn-O pairs. In addition to the vacancies, we observe negative-ion-type defects, which are tentatively attributed to intrinsic defects in the Zn sublattice. The effect of the annealing on the observed defects is discussed. The concentrations of the VZn−X complexes and negative-ion-type defects are in the 0.2–2 ppm range, while the cluster concentrations are 1–2 orders of magnitude lower.Peer reviewe

Positron annihilation lifetime spectroscopy of ZnO bulk samples

In order to gain a further insight into the knowledge of point defects of ZnO, positron annihilation lifetime spectroscopy was performed on bulk samples annealed under different atmospheres. The samples were characterized at temperatures ranging from 10 to 500 K. Due to difficulties in the conventional fitting of the lifetime spectra caused by the low intensity of the defect signals, we have used an alternative method as a solution to overcome these difficulties and resolve all the lifetime components present in the spectra. Two different vacancy-type defects are identified in the samples: Zn vacancy complexes (VZn−X) and vacancy clusters consisting of up to five missing Zn-O pairs. In addition to the vacancies, we observe negative-ion-type defects, which are tentatively attributed to intrinsic defects in the Zn sublattice. The effect of the annealing on the observed defects is discussed. The concentrations of the VZn−X complexes and negative-ion-type defects are in the 0.2–2 ppm range, while the cluster concentrations are 1–2 orders of magnitude lower.Peer reviewe

Localized versus delocalized states: Photoluminescence from electrochemically synthesized ZnO nanowires

We analyze the near-band-edge photoluminescence of electrochemically deposited ZnO nanowires and directly correlate the photoluminescence properties with the carrier concentration in the nanowires as determined from electrochemical impedance spectroscopy. We find a donor density of 81019 cm−3 in the as-deposited nanowires and show that the near-band-edge emission results from band-to-band recombination processes delocalized states. A photoluminescence band centered at 3.328 eV scales with the diameter of the nanowires and is assigned to recombination processes involving surface states. We show that annealing at 500 °C in air reduces the donor density in the nanowires by more than one order of magnitude, leading to sharp excitonic transitions in the electrochemically deposited nanowire

Faceting and structural anisotropy of nanopatterned CdO(110) layers

CdO(110) layers with a self-organized surface structure have been grown on (10math0) sapphire (m plane) substrates by metal-organic vapor phase epitaxy. The epitaxial relationships between layer and substrate have been determined and a crystallographic model that accounts for the CdO in-plane orientation, which results in a reduced lattice mismatch when the CdO[001] direction is perpendicular to the sapphire c axis, has been proposed. Although the measured lattice parameters indicate that the layers are almost fully relaxed, an anisotropic mosaicity is detected with symmetrical rocking curves attaining minimum values when measured along the CdO[math10] direction. The layer morphology consists of a regular ridge-and-valley structure which defines, again, a preferential in-plane direction. The grooves run parallel to the CdO[001] axis and exhibit lateral surfaces sloped at 28° with respect to the (110) surface. The influence of growth temperature and VI∕II molar ratio on the anisotropic mosaicity and morphology has been [email protected] [email protected] [email protected]

Synthesis and Characterization of ZnO Nanowire–CdO Composite Nanostructures

ZnO nanowire–CdO composite nanostructures were fabricated by a simple two-step process involving ammonia solution method and thermal evaporation. First, ZnO nanowires (NWs) were grown on Si substrate by aqueous ammonia solution method and then CdO was deposited on these ZnO NWs by thermal evaporation of cadmium chloride powder. The surface morphology and structure of the synthesized composite structures were analyzed by scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The optical absorbance spectrum showed that ZnO NW–CdO composites can absorb light up to 550 nm. The photoluminescence spectrum of the composite structure does not show any CdO-related emission peak and also there was no band gap modification of ZnO due to CdO. The photocurrent measurements showed that ZnO NW–CdO composite structures have better photocurrent when compared with the bare ZnO NWs

Parametric Study on Dimensional Control of ZnO Nanowalls and Nanowires by Electrochemical Deposition

A simple electrochemical deposition technique is used to synthesize both two-dimensional (nanowall) and one-dimensional (nanowire) ZnO nanostructures on indium-tin-oxide-coated glass substrates at 70°C. By fine-tuning the deposition conditions, particularly the initial Zn(NO3)2·6H2O electrolyte concentration, the mean ledge thickness of the nanowalls (50–100 nm) and the average diameter of the nanowires (50–120 nm) can be easily varied. The KCl supporting electrolyte used in the electrodeposition also has a pronounced effect on the formation of the nanowalls, due to the adsorption of Cl− ions on the preferred (0001) growth plane of ZnO and thereby redirecting growth on the (100) and (20) planes. Furthermore, evolution from the formation of ZnO nanowalls to formation of nanowires is observed as the KCl concentration is reduced in the electrolyte. The crystalline properties and growth directions of the as-synthesized ZnO nanostructures are studied in details by glancing-incidence X-ray diffraction and transmission electron microscopy