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Salmonella typhimurium 49a infection in a large dairy herd persisted for 3·5 years. Illness initially occurred in cows and calves but latterly although there were fewer clinical cases milk filters were culturally positive on 26 out of 73 samplings. Three associated human disease incidents occurred. Individual milk samples identified one cow as an excreter and the organism was recovered from the mammary gland of this animal at slaughter. Correlation between calving pattern, the times of calving and the occurrence of positive milk filters suggest that the cow may have been excreting the organism intermittently from the udder for 2·5 years.
To determine donor and acceptor concentrations affecting electrical properties in ZnO crystals, the relaxation time approximation (RTA) has been used to analyze mobility and carrier concentration data measured from 80 to 400 K. Five scattering mechanisms are included: polar-optical-phonon, piezoelectric potential, deformation potential, ionized impurity, and neutral impurity scattering. Temperature dependences of intrinsic mobilities and Hall r factors are determined. Neutral impurity (ni) scattering can play an important role in limiting the total mobility in ZnO single crystals. By including ni scattering, the experimental deformation potential E1 = 3.8 eV for ZnO can be used, rather than treating E1 as a fitting parameter. This approach yields “intrinsic” mobilities for n-type and p-type ZnO, and other II-VI materials. At 300 K, the intrinsic electron Hall mobility in ZnO is predicted to be 230 cm2/Vs. The results of fitting the temperature dependences of mobility and carrier concentrations from representative ZnO and CdSe bulk samples are shown. Intrinsic results for n-type CdS and ZnS will also be discussed. The 300-K intrinsic hole Hall mobility in ZnO is predicted to be about 50 cm2/Vs.
The ability of a poly(phenylene ethynylene) conjugated polyelectrolyte to detect a small molecule quencher in the solid state was assessed by coating the polymer onto a plastic-backed, non-fluorescent, silica-gel TLC plate. The efficiency of the emission quenching process was quantified by noting the resulting loss of emission with increasing quencher concentration, in accordance with the Stern-Volmer relation. The emission quenching induced by the addition of the analyte occurred with Stern-Volmer constants in the range of 103 M−1.
Differences in the optical activity of Be in GaN epilayers grown on different surface polarities by rf-plasma molecular beam epitaxy are investigated. Nitrogen-polar GaN doped with Be exhibits a significantly higher intensity of donor-acceptor pair (DAP) photoluminescence (PL) than similarly doped Ga-polar GaN, indicating the Be is incorporating at microscopically different sites, or possibly is forming different compensating complexes. Highly Be-doped Ga-polar GaN apparently forms isolated polarity-inverted regions which then incorporate Be via the N-polar mechanism resulting in the DAP PL. High temperature annealing of the Ga-polar layers both under nitrogen/hydrogen mixtures and under pure nitrogen atmospheres activates the DAP PL.
Photoluminescence (PL) experiments performed on bulk ZnO crystals are used to establish the ionization energy of the substitutional nitrogen acceptor. The temperature dependence of the nitrogen-related electron-acceptor (e,A°) emission band has been monitored in as-grown single crystals. A lineshape analysis of this band is used to determine the acceptor ionization energy. The temperature variation of the ZnO band gap was included in our analysis and the low-temperature acceptor ionization energy for substitutional nitrogen at an oxygen site in ZnO was found to be EA = 209 ± 3 meV. Electron paramagnetic resonance and Hall-effect measurements were also used to characterize these bulk ZnO samples.
Zinc oxide (ZnO) crystals grown by the seeded chemical vapor transport method have been studied using photoluminescence (PL), thermoluminescence (TL), and electron paramagnetic resonance (EPR) techniques. Lithium acceptors were diffused into the crystals during anneals in LiF powder at temperatures in the 750 to 850°C range. After a lithium diffusion, EPR was used to monitor neutral lithium acceptors and neutral shallow donors, as well as Ni3+, Fe3+, and Cu2+ impurities unintentionally present. Excitonic and deep-level PL emissions were also monitored before and after these diffusions. Two broad overlapping TL emission bands were observed at 117 and 145 K when a Li-diffused crystal was illuminated at 77 K with 325-nm light and then rapidly warmed to room temperature. The two TL bands have the same spectral dependence (the peak in wavelength is 540 nm when the intensity of the light reaches a maximum). These “glow” peaks occur when electrons are thermally released from Ni2+ and Fe2+ ions and recombine with holes at neutral lithium acceptors.
Bulk crystals of CdGeAs2 have been characterized using photoluminescence (PL), optical absorption, Hall effect, and electron paramagnetic resonance (EPR) techniques. An absorption band near 5.5 microns at room temperature is observed in all of the p-type samples we have studied. A correlation between the magnitude of this optical absorption and the excess hole concentration at room temperature is established. Also, an EPR signal is found to correlate with the strength of this absorption band. PL data are consistent with an increased concentration of shallow acceptors being present in high-absorption samples. From the EPR data, we suggest that a model for the paramagnetic defect associated with the absorption at 5.5 microns may be an acceptor on an anion site.
Bulk ZnGeP2 (ZGP) crystals grown for high-power tunable mid-infrared laser systems contain large concentrations of three native defects. Using EPR, these three defects have been identified as the Zn vacancy (an acceptor), the phosphorus vacancy (a donor), and the germanium-on-zinc antisite (also a donor). We have studied the wavelength dependence of the photoinduced changes in the EPR signal intensity of the three defect centers using 633 nm and 1064 nm light. We observe a significant increase in the EPR signal under 633 nm light. The 633-nm light also produces an increase in the defect-related 1-R.m absorption band, and we have used a spectrophotometer to measure the spectral shape of the photoinduced change in absorption in this near-IR region. The 633-nm wavelength produces paramagnetic forms of both donor centers, while 1064 nm light only produces the EPR-active center. Time decays of the photoinduced EPR signals have been measured for each of the donors.
Zinc germanium diphosphide (ZnGeP2) is a nonlinear optical material used in mid-infrared optical parametric oscillators. The near-infrared photoluminescence (PL) from single crystals of bulk ZnGeP2 has been studied as a function of excitation power, wavelength, temperature, and polarization. At 5 K, a broad PL band extending from 0.7 µm to beyond 1 µm is typically observed. Two distinct emissions with different polarization, power, and temperature behaviors have been resolved. These bands have peaks in intensity near 1.6 eV and 1.4 eV. The relative intensities of these two bands were found to correlate with the presence of phosphorus vacancies, as determined by electron paramagnetic resonance (EPR). A resonance in the intensity of the 1.6-eV band occurs when pumping into a level ∼90 meV below the minimum conduction band. This level is tentatively assigned to the shallow state.
Hydrogen incorporation in both undoped and nitrogen-doped CdTe and ZnSe is investigated. Evidence for a strong nitrogen-hydrogen interaction is presented. Preliminary data indicate that the growth of CdTe and ZnSe under an atomic hydrogen flux results in a significant concentration of paramagnetic defects possibly accompanied by enhanced auto-doping from residual impurities.