Hall and drift mobilities in molecular beam epitaxial grown GaAs

A series of nominally undoped and Si-doped GaAs samples have been grown by molecular beam epitaxy (MBE) with Hall concentrations ranging from 10¹⁵ to 10¹⁹ cm⁻³ and mobilities measured at 77 and 300K by Hall-van der Pauw methods. Drift mobilities were calculated using the variational principle method and Hall scattering factors obtained from a relaxation-time approximation to permit cross-correlation of experimental data with drift or Hall mobilities and actual or Hall electron concentrations. At 77K, both high purity and heavily doped samples are well represented by either drift or Hall values since piezoelectric acoustic phonon scattering and strongly screened ionized impurity scattering hold the Hall factor close to unity in the respective regimes. Between n≊10¹⁵ and 10¹⁷ cm⁻³, where lightly screened ionized impority scattering predominates, Hall mobility overestimates drift mobility by up to 50 percent and Hall concentration similarly underestimates n. At 300K, polar optical phonons limit mobility and a Hall factor up to 1.4 is found in the lowest doped material, falling close to unity above about 10¹⁶ cm⁻³. Our calculation also agrees remarkably well with the Hall mobility of the highest purity MBE grown sample reported to date.     

Listeria monocytogenes: Its importance in the dairy industry

Recent outbreaks of listeriosis associated with consumption of milk products contaminated with Listeria monocytogenes have focused attention on the importance of food-borne transmission of this disease. This review outlines current knowledge of the incidence of L monocytogenes in raw milk and its incidence in, and ability to survive, the manufacturing process for other dairy products—notably soft cheeses. Discrepancies in results obtained by researchers concerning growth of the organism at refrigeration temperatures and the bacterium's ability to survive pasteurisation are discussed.     

Evidence for transient composition variations at GaAs/Ga1−xAlxAs heterostructure interfaces prepared by metal-organic chemical vapour deposition

The composition profiles of GaAs/Ga_1?xAl_xAs heterostructures prepared in two different Metal-Organic Chemical Vapour Deposition (MOCVD) reactors have been studied. Transmission electron microscopy (TEM). and Auger and secondary ion mass spectrometry (SIMS) sputter profiling results are in good agreement and interface widths below 20 å have been achieved. Significant new results on transient phenomena have been obtained. showing that large and sometimes very sharp excursions in composition can be associated with valve switching actions. Direct evidence is presented that gas pressure or flow transients can occur during valve operations. and it is suggested that such instabilities are the cause of the effects observed.     

Highly Efficient,Solution‐Processed,Single‐Layer,Electrophosphorescent Diodes and the Effect of Molecular Dipole Moment

A new family of highly soluble electrophosphorescent dopants based on a series of tris‐cyclometalated iridium(III) complexes (1–4) of 2‐(carbazol‐3‐yl)‐4/5‐R‐pyridine ligands with varying molecular dipole strengths have been synthesized. Highly efficient, solution‐processed, single‐layer, electrophosphorescent diodes utilizing these complexes have been prepared and characterized. The high triplet energy poly(9‐vinylcarbazole) PVK is used as a host polymer doped with 2‐(4‐biphenylyl)‐5‐(4‐tert‐butyl‐phenyl)‐1,3,4‐oxadiazole (PBD) for electron transport. Devices with a current efficiency of 40 cd A^?1 corresponding to an EQE of 12% can thus be achieved. The effect of the type and position of the substituent (electron‐withdrawing group (CF₃) and electron‐donating group (OMe)) on the molecular dipole moment of the complexes has been investigated. A correlation between the absorption strength of the singlet metal‐to‐ligand charge‐transfer (¹MLCT) transition and the luminance spectral red shift as a function of solvent polarity is observed. The strength of the transition dipole moments for complexes 1–4 has also been obtained from TD‐DFT computations, and is found to be consistent with the observed molecular dipole moments of these complexes. The relatively long lifetime of the excitons of the phosphorescence (microseconds) compared to the charge‐carrier scattering time (less than nanoseconds), allows the transition dipole moment to be considered as a “quasi permanent dipole”. Therefore, the carrier mobility is sufficiently affected by the long‐lived transition dipole moments of the phosphorescent molecules, which are randomly oriented in the medium. The dopant dipoles cause positional and energetic disorder because of the locally modified polarization energy. Furthermore, the electron‐withdrawing group CF₃ induces strong carrier dispersion that enhances the electron mobility. Therefore, the strong transition dipole moment in complexes 3 and 4 perturbs both electron and hole mobilities, yielding a reduction in exciton formation and an increase in the device dark current, thereby decreasing the device efficiency.     

Ultrahigh Efficiency Fluorescent Single and Bi‐Layer Organic Light Emitting Diodes: The Key Role of Triplet Fusion

A new family of anthracene core, highly fluorescent emitters is synthesized which include diphenylamine hole transport end groups. Using a very simple one or two layer organic light emitting diode (OLED) structure, devices without outcoupling achieve an external quantum efficiency of 6% and photonic efficiencies of 20 cd/A. The theoretical maximum efficiency of such devices should not exceed 3.55%. Detailed photophysical characterization shows that for these anthracene based emitters 2T₁≤T_n and so in this special case, triplet fusion can achieve a singlet production yield of 0.5. Indeed, delayed electroluminescence measurements show that triplet fusion contributes 59% of all singlets produced in these devices. This demonstrates that when triplet fusion becomes very efficient, fluorescent OLEDs even with very simple structures can approach an internal singlet production yield close to the theoretical absolute maximum of 62.5% and rival phosphorescent‐based OLEDs with the added advantage of much improved stability.     

Effect of high amounts of dietary zinc and age upon tissue zinc in young chicks

Weight gains of younger broiler chicks were not reduced by up to 2400 p.p.m. added zinc fed to four weeks of age. Tissue zinc was not changed significantly by 600 or 1200 p.p.m. supplemental dietary zinc, but at 2400 p.p.m. added zinc, blood, kidney, and liver zinc were significantly elevated (P less than 0.05). In the heart, zinc was not affected by the added dietary zinc but increased over time with all diets. Unlike the calf and weanling pig, the metabolism of zinc in the young chick does not appear to change appreciably with increasing maturity. Thus, the homeostatic control mechanisms of the young chick are effective for up to 1200 p.p.m. dietary zinc, but are partially overcome by 2400 p.p.m. added dietary zinc.     

A comparative study of passive haemagglutination methods for the detection of hepatitis B surface antigen in routine hospital practice

Two passive haemagglutination methods for detecting HBsAg were compared. In general, the method using turkey erythrocytes was found preferable to the method employing sheep cells since it is more rapid and more sensitive, and less frequently gave rise to false positive reactions with sera from staff, blood donors, and patients not receiving haemodialysis. The turkey cell test gives rise to more false positive screen tests than the sheep cell test when monitoring renal dialysis patients since approximately 10% of the sera of these patients were found to contain turkey cell agglutinins, but this presents no particular difficulty if the recommended absorption procedures are used.     

Contamination from 131I, 103Ru, and 239Np in the eluate of 99Mo-99mTc generators loaded with (n, gamma)-produced 99Mo

Iodine-131, ruthenium-103, and neptunium-239 are present as contaminants in the eluate of 99Mo-99mTc generators loaded with 99Mo prepared by thermal-neutron irradiation of enriched 98Mo. The elution pattern of each of these contaminants is determined, together with the amounts found in the eluate of all generators tested over a 7-month period.     

The successful induction of molting in the adult male lobster (Homarus americanus) with a slow-release form of ecdysterone

In an initial, and then a confirmatory experiment, adult, male lobsters were injected with solvent, ecdysterone (E, 2.0 mug/g live weight) or ecdysterone acetate (EAc, 2.5 or 5.0 mug/g live weight) emulsions in Freund's incomplete adjuvant (FIA). Control lobsters underwent no molts and only one death in the two cases. The E treated animals all died (average: exp. 1, 19.2 +/-2.1 days; exp. 2, 25.3 +/- 8 days). After the lobsters were treated twice with 2.5 mug EAc/g live weight, in the first experiment, four out of five molted and one died; in the second experiment six out of eight molted, one died and one remained refractory. The high EAc dose resulted in five deaths, one molt and two pseudomolts after one treatment. It is concluded that the use of the oil emulsion and EAc sufficiently slowed the release of free ecdysterone to allow complete premolt development in the lobster.