Achieving highly-enhanced UV photoluminescence and its origin in ZnO nanocrystalline films

AbstractZnO is an efficient luminescent material in the UV-range ∼3.4 eV with a wide range of applications in optical technologies. Sputtering is a cost-effective and relatively straightforward growth technique for ZnO films; however, most as-grown films are observed to contain intrinsic defects which can significantly diminish the desirable UV-emission. In this research the defect dynamics and optical properties of ZnO sputtered films were studied via post-growth annealing in Ar or O2 ambient, with X-ray diffraction (XRD), imaging, transmission and Urbach analysis, Raman scattering, and photoluminescence (PL). The imaging, XRD, Raman and Urbach analyses indicate significant improvement in crystal morphology and band-edge characteristics upon annealing, which is nearly independent of the annealing environment. The native defects specific to the as-grown films, which were analyzed via PL, are assigned to Zni related centers that luminesce at 2.8 eV. Their presence is attributed to the nature of the sputtering growth technique, which supports Zn-rich growth conditions. After annealing, in either environment the 2.8 eV center diminished accompanied by morphology improvement, and the desirable UV-PL significantly increased. The O2 ambient was found to introduce nominal Oi centers while the Ar ambient was found to be the ideal environment for the enhancement of the UV-light emission: an enhancement of ∼40 times was achieved. The increase in the UV-PL is attributed to the reduction of Zni-related defects, the presence of which in ZnO provides a competing route to the UV emission. Also, the effect of the annealing was to decrease the compressive stress in the films. Finally, the dominant UV-PL at the cold temperature regime is attributed to luminescent centers not associated with the usual excitons of ZnO, but rather to structural defects

Understanding the Affects of Increased Special Boundary Fraction on Dynamic Recrystallization

Deformations introduced into low to medium stacking fault energy materials at or above the recrystallization temperature cause simultaneous formation and annihilation of defects. Annihilation of defects by nucleating strain free grains is called dynamic recrystallization (DRX). Nucleation during DRX has been shown to occur more extensively on general (high angle) boundaries then on special (low angle) boundaries. Grain boundary engineering (GBE) is a process that improves the structural and functional properties of polycrystalline materials by manipulating the occurrence of low sigma or so called special grain boundaries. An iterative GBE cold rolling based process has been created to increase the occurrence of the Σ3 low angle special boundaries within stainless steel 316L (SS316L). The affect of increased Σ3 content on DRX has been studied on the MTS 810 Material Test System at elevated temperatures and strains specific to the creep regime. Analysis of a material’s microstructure, including grain boundary type and energy, has been performed with a crystallographic based microstructural characterization technique known as electron backscatter diffraction (EBSD). Through EBSD analysis, the affect of GBE SS316L, specifically to increase the Σ3 content, on DRX has been explored

The Role of Grain Boundary Character on Dynamic Recrystallization

This study investigates the role of grain boundary character on dynamic recrystallization. During hot working, the induced stress can be relieved through nucleation of new grains; this process is known as dynamic recrystallization (DRX). This study will use experimental analysis and computer modeling to determine the effects of grain boundary character on DRX in stainless steel and nickel alloys. In the experimental analysis, stainless steel 316L and nickel will undergo grain boundary engineering (GBE). This technique uses a series of cold-working and annealing steps to obtain predetermined levels of special boundaries. The GBE material will be deformed at high temperature, and the effects of the varying levels of special boundaries on the dynamic recrystallization behavior will be analyzed. The computer modeling will be based on the results of the experimental work and will offer insight to the role that the fraction of low energy boundaries has on the deformation behavior

P-type conductivity in annealed strontium titanate

Hall-effect measurements indicate p-type conductivity in bulk, single-crystal strontium titanate (SrTiO3, or STO) samples that were annealed at 1200°C. Room-temperature mobilities above 100 cm2/V s were measured, an order of magnitude higher than those for electrons (5-10 cm2/V s). Average hole densities were in the 109-1010 cm−3 range, consistent with a deep acceptor

A New Frontier for Smart Phone Gorilla Glass: Alternative Strengthening Method for Increased Reliability and Efficiency

More than 1 billion people utilize smartphones and touchscreens worldwide. The touchscreen glass, a key component of these devices, needs to be durable and scratch resistant to survive everyday use. The defining weakness of glass products is their poor ability to withstand tensile stresses. Sandia National Laboratories is exploring sodium-for-potassium ion exchange techniques, which induce surface compression to increase glass strength. Conventional ion exchange methods require lengthy processes (6-48 hours) for effective strengthening. An alternative strengthening method, field assisted ion exchange (FAIE), may reduce processing times (\u3c 2 hours), while providing high surface and near-surface compressive stresses. Two apparatuses were designed for the investigation of FAIE-based glass strengthening. Both apparatuses are electrochemical cells bisected with square (30 x 30 x 2 mm) Corning 2317 alumino-silicate glass. An electric field is applied across the glass, which drives potassium ions from the molten potassium nitrate into the glass. Potassium ion exchange depth was determined using energy dispersive spectroscopy. The effect of applied electric field and time on the resulting exchange depth will be evaluated using a design of experiments approach. Determining the feasibility and strengthening effect of FAIE may lead to producing more efficient and reliable glass for industrial applications. Such glass could be used not only for smart phones, but also for strengthening architectural structures and transportation vehicles

Potassium acceptor doping of ZnO crystals

ZnO bulk single crystals were doped with potassium by diffusion at 950°C. Positron annihilation spectroscopy confirms the filling of zinc vacancies and a different trapping center for positrons. Secondary ion mass spectroscopy measurements show the diffusion of potassium up to 10 μm with concentration ∼1 × 1016 cm−3. IR measurements show a local vibrational mode (LVM) at 3226 cm−1, at a temperature of 9 K, in a potassium doped sample that was subsequently hydrogenated. The LVM is attributed to an O–H bond-stretching mode adjacent to a potassium acceptor. When deuterium substitutes for hydrogen, a peak is observed at 2378 cm−1. The O-H peak is much broader than the O-D peak, perhaps due to an unusually low vibrational lifetime. The isotopic frequency ratio is similar to values found in other hydrogen complexes. Potassium doping increases the resistivity up to 3 orders of magnitude at room temperature. The doped sample has a donor level at 0.30 eV

Understanding the Effect of Grain Boundary Character on Dynamic Recrystallization in Stainless Steel 316L

Dynamic recrystallization (DRX) occurs during high-temperature deformation in metals and alloys with low to medium stacking fault energies. Previous simulations and experimental research have shown the effect of temperature and grain size on DRX behavior, but not the effect of the grain boundary character distribution. To investigate the effects of the distribution of grain boundary types, experimental testing was performed on stainless steel 316L specimens with different initial special boundary fractions (SBF). This work was completed in conjunction with computer simulations that used a modified Monte Carlo method which allowed for the addition of anisotropic grain boundary energies using orientation data from electron backscatter diffraction (EBSD). The correlation of the experimental and simulation work allows for a better understanding of how the input parameters in the simulations correspond to what occurs experimentally. Results from both simulations and experiments showed that a higher fraction of so-called ‘‘special’’ boundaries (e.g., Σ3 twin boundaries) delayed the onset of recrystallization to larger strains and that it is energetically favorable for nuclei to form on triple junctions without these so-called ‘‘special’’ boundaries