Kelvin G. Lynn

Infrared spectroscopy of hydrogen in ZnO

Zinc oxide (ZnO) is a wide-band gap semiconductor that has attracted tremendous interest for optical, electronic, and mechanical applications. First-principles calculations by [C. G. Van de Walle, Phys. Rev. Lett. 85, 1012 (2000)] have predicted that hydrogen impurities in ZnO are shallow donors. In order to determine the microscopic structure of hydrogen donors, we have used IR spectroscopy to measure local vibrational modes in ZnO annealed in hydrogen gas. An oxygen–hydrogen stretch mode is observed at 3326.3 cm−1 at a temperature of 8 K, in good agreement with the theoretical predictions for hydrogen in an antibonding configuration. The results of this study suggest that hydrogen annealing may be a practical method for controlled n-type doping of ZnO.

Open volume defects (measured by positron annihilation spectroscopy) in thin film hydrogen-silsesquioxane spin-on-glass; correlation with dielectric constant

We used Doppler broadening positron annihilation spectroscopy as a nanovoid characterization tool in the study of low dielectric constant (low-k) hydrogen-silsesquioxane (HSSQ) thin films. The high void sensitivity of this method, combined with depth-resolving capability, enables one to observe changes in the local electronic environment in a thin film. We established a correlation between the annihilation parameters and the dielectric properties for a series of samples subjected to various annealing conditions in nitrogen. Qualitative differences are found between the pore structures of isochronally and isothermally annealed films, suggesting a complex relationship between the film dielectric constant and pore size and pore density. Results showing changes in the chemical environment caused by exposure to various processing environments are also presented. First, the changes in the HSSQ films were determined after a year of exposure to ambient air in which the film properties changed with time due to wat...

Identification of defects in Y3Al5O12 crystals by positron annihilation spectroscopy

Positron annihilation, thermoluminescence, and optical absorption measurements were applied with the aid of several annealing and diffusion procedures to investigate the nature of point defects in Y3Al5O12 (YAG) single crystals. By annealing at 1500°C in air or O, and diffusing Al into a Ce doped YAG single crystal, a reduction of nearly two orders of magnitude in vacancy concentration was observed. Scintillation measurements showed a significant improvement in energy resolution after Al diffusion. This study revealed the presence of vacancy-defect complexes, most likely associated with cation antisites in YAG crystals.

Porosity characterization by beam-based three-photon positron annihilation spectroscopy

We present a straightforward and fast positron annihilation spectroscopy (PAS) technique for measuring the 2 to 3 photon annihilation ratio of Ps (electron-positron) atoms (3γ PAS), utilized here for the nondestructive characterization of mesoporous (pore size >1 nm) dielectric films. Examples are given for ∼1-μm-thick foamed methyl-silsesquioxane (MSSQ) films, produced by mixing MSSQ (0–90 wt % fraction) with a sacrificial foaming agent (porogen). Probing these films as a function of depth allows one to monitor Ps escape from interconnected pores and to determine the threshold for pore interconnectivity to the film surface. A classical treatment of Ps diffusion is used to calculate the open and closed porosity fractions as a function of the initial porogen load.

Probing capped and uncapped mesoporous low-dielectric constant films using positron annihilation lifetime spectroscopy

We have measured uncapped mesoporous low-dielectric-constant films using positron annihilation lifetime spectroscopy (PALS), a nondestructive technique, which yields both pore-size distributions and the threshold for pore interconnectivity. Pairs of fully cured capped and uncapped identical films, initially containing 5%–50% porogen additions, exhibited similar signatures in PALS, establishing a technique in which film capping is unnecessary. We also found that it was possible to distinguish between closed and percolated pores, without film capping, by comparing lifetime spectra of a film taken using different detector configurations. Interconnected pores were observed in cured samples, which had greater than 20% porogen additions.

Positron Annihilation Spectroscopy Study of Interfacial Defects Formed by Dissolution of Aluminum in Aqueous Sodium Hydroxide

High-purity aluminum foils were examined using positron annihilation spectroscopy (PAS) after dissolution for various times in 1 M NaOH at room temperature. Measurements of the S and W shape parameters of the annihilation photopeak at 511 keV show the presence of voids of at least nanometer dimension located at the metal-oxide film interface. The large S parameter suggests that the metallic surface of the void is free of oxide. Voids are found in as-received foils and are also produced by dissolution in NaOH, evidently by a solid-state interfacial process. Atomic force microscopy (AFM) images of NaOH-dissolved foils, after stripping the surface oxide film in chromic-phosphoric acid bath, reveal cavities on the order of 100 nm size The average cavity depth is in quantitative agreement with the PAS-derived thickness of the interfacial void-containing layer, and the dissolution time dependence of the defect layer S parameter closely parallels that of the fractional coverage of the foil surface by cavities; thus, the cavities are believed to he interfacial voids created along with those detected by PAS. The cavity distribution on the surface closely resembles that of corrosion pits formed by anodic etching in 1 M HCl, thereby suggesting that the interfacial voids revealed by AFM serve as sites for pit initiation.

Doppler broadening positron annihilation spectroscopy: A technique for measuring open-volume defects in silsesquioxane spin-on glass films

Doppler broadening positron annihilation spectroscopy is used to measure the concentration, spatial distribution, and size of open-volume defects in low dielectric constant (low-k) hydrogen- and methyl-silsesquioxane thin films. A simple correlation between the number of open-volume defects and the dielectric constant is obtained. In addition, the depth-resolving capability enables profiling of the local electronic environment of open-volume defects as a function of depth. The potential for using this technique for measuring k as a function of film depth is also discussed.

Nanometer-scale pores in low-k dielectric films probed by positron annihilation lifetime spectroscopy

We measured positron annihilation lifetime spectra in mesoporous low dielectric constant (low-k) methyl-silsesquioxane (MSSQ) films versus porogen load Φ from Φ=0% to 50%. The ortho-positronium lifetime parameters were obtained using both the maximum entropy and discrete lifetime analyses. Open and closed porosity distributions and the average radius of closed pores were obtained. The total porosity and the fraction of open/closed porosities were evaluated. The total porosity increases linearly with porogen load, consistent with the porosity obtained from density measurements. Open porosity occurs from 20% porogen load upwards.

A simple positron lifetime spectrometer for a magnetically guided low-energy beam

We present a new, simple, and inexpensive positron lifetime spectrometer intended for the depth-resolved characterization of thin films and buried interfaces. The spectrometer operates on a conventional magnetically guided positron beam with energies ranging from 1 to ∼50 keV. Given is a detailed description of the performance of the apparatus, built on thorough experimental investigations and computer simulations. A timing resolution of 350±13 ps at full-width-at-half-maximum was obtained. The count rate for thin films (low positron energies) was of the order of 1000 s−1. A maximum peak-to-background ratio>105, aiding the measurements of long-living (10–100 ns) positronium in voids, was achieved by constant beam rate reduction and by beam chopping. Examples are presented for measured lifetimes in well-characterized systems.

STUDY OF DX CENTER IN CD0.8ZN0.2TE:CL BY POSITRON ANNIHILATION

Variable energy positron beam and positron annihilation lifetime experiments have been carried out to study the DX center in Cd0.8Zn0.2Te:Cl at 50 K. A short positron effective diffusion length of 275±25 A and a large intensity of 79.0%±0.3% for the long lifetime component indicate a strong trapping effect at DX centers. A trapping rate of κ=1.53±0.05×109 s−1 and a positron lifetime of 335±2 ps at the DX center were obtained. The concentration of DX centers is found to be 5.9±0.7×1016 cm−3, which is in good agreement with the results obtained using Hall effect and thermo-electric effect measurements.