Search Results - (Author, Cooperation:R. Walters)

2011-06-10

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

2018-06-22

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

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Genetics, Medicine, Diseases, Online Only

1089-7550

AIP Digital Archive

Physics

Results are presented of a deep level transient spectroscopy study of radiation-induced defects in p-type (Zn-doped) InP grown by metalorganic chemical vapor deposition. Three major hole traps (H3, H4, and H5) and two electron traps (EA and EB) were observed. The electron trap structure in particular is significantly different from that reported in the literature for p-type InP grown by other methods. Activation energies of 0.22 eV (EA) and 0.76 eV (EB) have been measured, and capture cross sections (σ∞) of 4.4×10−15 cm2 (EA), and 1.4×10−12 cm−2 (EB) have been determined. The H5 center has a thermally activated capture cross section with an energy barrier of 0.35 eV. The measured injection annealing rate of the primary hole trap (H4) was different than previously observed.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The reverse dark current-voltage (dark I-V) curves of InGaAs photodiodes have been measured as a function of temperature following irradiation with 1-MeV electrons. Prior to irradiation, the I-V curves are well described by a diffusion term alone indicating that the junctions are of good quality. Irradiation produces a large increase in the generation current which can be modelled as resulting from a single defect center with an energy Ec−0.29 eV. Such a defect center called E2 has been detected using deep level transient spectroscopy.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The annealing behavior of the reverse bias current-voltage curves of 1 MeV electron irradiated In0.53Ga0.47As photodiodes has been measured at 300 K. The observed decay is shown to be correlated with the reduction of the E2 peak height with time, as measured by deep level transient spectroscopy. The reverse current is found to decay with a logarithmic time dependence, which can be explained by a model in which the annealing of the E2 defects is controlled by a distribution of thermal energy barriers.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Deep-level transient spectroscopy has been used to monitor the effect of carrier concentration on the properties of radiation-induced defects in InP n+p mesa diodes grown by metalorganic chemical vapor deposition. The activation energy Ea for hole emission from H4 and H5 centers and the injection-enhanced annealing rate of H4 at 200 K have been measured as a function of carrier concentration NA over the range ∼1×1016– 4×1017 cm−3. The measured values of Ea decrease with increasing NA in a way that can be semi-quantitatively explained by a combination of the Frenkel–Poole effect and phonon-assisted tunneling produced by the electric field in the junction. The results suggest that hole emission from H4 and H5 centers takes place to maxima in different valence bands. The injection-enhanced annealing rate of H4 centers increases with increasing NA at low concentrations, but approaches a maximum value near NA ∼ 1017 cm−3, indicating a limiting dopant (Zn) concentration for impurity-enhanced defect annealing.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

A deep level transient spectroscopy study of proton irradiation induced defects in n+p InP mesa diodes grown by metalorganic chemical vapor deposition is reported. In contrast to results reported for InP grown by other methods, 3 MeV proton irradiation produced a DLTS spectrum similar to 1 MeV electron irradiation with the addition of two new peaks. Six majority carrier peaks: HP1(Ea=0.15 eV), H2(Ea=0.20 eV), H3(Ea=0.30 eV), H4(Ea=0.37 eV), H5(Ea=0.54 eV), and H7(Ea=0.61 eV) and three minority carrier peaks: EA(Ea=0.26 eV), EB(Ea=0.74 eV), and EC(Ea=0.16 eV) were detected. The H5 peak displayed a thermally activated capture cross section and a dependence of peak height on injection level. Isothermal annealing at 375 K was performed and thermal annealing rates are presented. Low temperature (200 K), minority carrier injection annealing rates are also presented. For most of the defects, a significant residual concentration remained after injection which could not be annealed further. An equation was developed for the annealing rate of the major defect, H4, as a function of injection level, carrier concentration, and temperature.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

A detailed study of the effects of proton irradiation-induced defects in heteroepitaxially grown InP/Si solar cells has been made through a combination of cathodoluminescence (CL), electron beam induced current (EBIC), and electrochemical capacitance versus voltage (ECV) carrier profiling measurements. The CL data indicate the distribution of nonradiative recombination centers both before and after proton irradiation, and temperature dependent and spectroscopic analysis of the CL signal give an estimate of the energies of the dominant defect levels. The EBIC data yield an estimate of the magnitude and spatial variation of the minority carrier diffusion length (L) in the base region. Values of L determined from EBIC measurements made on solar cells irradiated by protons ranging in energy from 0.1 up to 4.5 MeV follow a single curve when plotted versus displacement dose, Dd, allowing a single proton damage coefficient to be determined. The ECV measurements show the evolution of the carrier concentration profile in the cell under irradiation, as carrier removal first depletes and eventually type converts the base region. From an in-depth analysis of the combined data, the physical defects that give rise the radiation-induced energy levels are suggested, and a detailed understanding of the physical mechanisms causing the radiation response of InP/Si solar cells is developed. © 1999 American Institute of Physics.

Electronic Resource

1475-4991

Blackwell Publishing Journal Backfiles 1879-2005

Economics

The Australian Bureau of Statistics (ABS) has recently published a set of experimental estimates of gross State product at factor cost and its income components for the States of Australia (1): The regular compilation and publication of the estimates is also commencing. In compiling the estimates, certain conceptual issues were confronted. Others were recognized but have not required final resolution as yet because they are beyond the scope of the estimates presently compiled.This paper considers the development of a framework for regional accounts for countries with regional characteristics broadly similar to those of the Australian States by starting with the SNA framework that applies at the national Ievel. The difficulties that arise are highlighted and alternative approaches for dealing with specific issues discussed. These alternative approaches can have a significant impact on measured State relativities and performance over time. In some instances the choice between approaches may be narrowed because of practical considerations in the same way that the SNA is a conceptual framework that nevertheless has to take account of practicalities. The issues largely involve consideration as to whether or not to regard certain activities as extra-territorial in nature. If an extra-territorial concept is not adopted choices must be made on how to allocate data items related to such activities by State.

Electronic Resource

1365-2494

Blackwell Publishing Journal Backfiles 1879-2005

Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition

Electronic Resource

1365-2494

Blackwell Publishing Journal Backfiles 1879-2005

Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The annealing behavior of electron, proton, and alpha particle irradiated, epitaxial n+p InP solar cells has been characterized using several techniques. Current–voltage measurements were made under simulated 1 sun, AM0 solar illumination and in the dark. The radiation-induced defect spectra were monitored using deep level transient spectroscopy and the base carrier concentration profiles were determined through capacitance–voltage measurements. The irradiated cells were annealed at temperatures ranging from 300 up to 500 K. Some cells were annealed while under illumination at short circuit while others were annealed in the dark. These experiments produced essentially the same results independent of illumination and independent of the irradiating particle. An annealing stage was observed between 400 and 500 K, in which the radiation-induced defects labeled H3 and H4 were removed and the carrier concentration recovered slightly. Concurrently there was a small reduction in the junction recombination current and a slight increase in the photovoltaic (PV) output of the cell; however, most of the radiation-induced defects did not anneal, and the overall PV recovery was very small. A full analysis of the annealing data is given, and a model for the radiation response and annealing behavior of the cells is presented. The results are compared to those reported previously for irradiated, diffused junction InP solar cells. Although the radiation-induced degradation mechanisms appear to be essentially the same in the two cell types, the recovery of the PV output is found to be quite different. This difference in cell recovery is explained in terms of the defect annealing characteristics in the individual cell types. © 1996 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The degradation and annealing properties of 1 MeV electron-irradiated n+p diffused junction InP solar cells are reported in detail. The solar cells were characterized through current–voltage measurements under simulated solar illumination at 1 sun, AM0. The radiation-induced defect spectra were characterized through deep level transient spectroscopy. At fluences up to 1015 cm−2, cell degradation was primarily due to a decrease in the short-circuit current Isc which occurred during the introduction of the hole trap, H4. Most of this degradation could be removed by minority-carrier injection annealing of the H4 defect at temperatures as low as 225 K. At higher irradiation fluences, up to 1016 cm−2, cell degradation was dominated by a decrease in both the open-circuit voltage Voc and the fill factor. This degradation was caused by a large radiation-induced recombination current and by carrier removal which was associated with the introduction of the hole trap H5 and the electron traps EA, EC, and ED. Most of the effects of the recombination current and some of the carrier removal were removed by concurrent injection and thermal annealing between 373 and 400 K where the residual H4 defect concentration and the H5 defect were removed. Essentially full cell recovery was achieved after subsequent annealing between 450 and 500 K where the electron traps also showed a partial annealing stage. Thermal annealing without illumination in the range of 350–500 K showed the same defect annealing stages suggesting that the cell recovery in this temperature range is due solely to thermal annealing. The data are summarized to give a model for the radiation-induced degradation and annealing of these InP solar cells. © 1995 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Results are presented for 1 MeV electron-irradiated, two terminal, monolithic InP/Ga0.47In0.53As tandem solar cells. These highly efficient prototype cells show radiation resistance that is comparable to single junction InP cells. A current mismatch between the subcells does not occur until high fluence levels, that is, near 3×1015 e−/cm2. This value for the onset of current mismatch and the measured remaining absolute efficiency of 9.4% at 1×1016 e−/cm2 are excellent results reported for a tandem cell designed for space applications. © 1995 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The effect of 1 MeV electron and 3 MeV proton irradiation on the performance of n+p InP solar cells grown heteroepitaxially on Si (InP/Si) substrates is presented. The radiation response of the cells was characterized by a comprehensive series of measurements of current versus voltage (I–V), capacitance versus voltage (C–V), quantum efficiency (QE), and deep level transient spectroscopy (DLTS). The degradation of the photovoltaic response of the cells, measured under simulated 1 sun, AM0 solar illumination, is analyzed in terms of displacement damage dose (Dd) which enables a characteristic degradation curve to be determined. This curve is used to accurately predict measured cell degradation under proton irradiation with energies from 4.5 down to 1 MeV. From the QE measurements, the base minority carrier diffusion length is determined as a function of particle fluence, and a diffusion length damage coefficient is calculated. From the C–V measurements, the radiation-induced carrier removal rate in the base region of the cells is determined. The DLTS data show the electron and proton irradiations to produce essentially the same defect spectra, and the spectra are essentially the same as observed in irradiated homoepitaxial n+p InP. From the DLTS data, the introduction rate of each defect level is determined. From the dark I–V curves, the effect of irradiation on the various contributions to the dark current are determined. The data are analyzed, and a detailed description of the physical mechanisms for the radiation response of these cells is given. The results enable a model to be developed for the radiation response of the cells. © 1997 American Institute of Physics.

Electronic Resource

1365-2494

Blackwell Publishing Journal Backfiles 1879-2005

Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition

A sampling procedure for estimating herbage intake by the ‘Difference’ method over short grazing periods using an electrically powered sheep-shearing head to cut long narrow sample strips, was compared with several indirect animal techniques on single-variety grass swards. The method provided intake estimates in good general agreement with those obtained by animal methods but discrepancies arose when used on a semi-prostrate variety due, probably, to incomplete recovery of sampled herbage cut from post-grazing residues.It was concluded that the method might be usefully adopted for the preliminary assessment of the intake characteristics of herbage varieties in routine evaluation programmes, provided that care is taken to ensure the efficient collection of herbage sampled post-grazing and that a reduction in the land requirement can be achieved without undue loss of accuracy or precision. Confirmation of results should be sought using animal methods when possible.

Electronic Resource

1365-2494

Blackwell Publishing Journal Backfiles 1879-2005

Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition

Stage of growth, leafiness and the accumulation of dead material were evaluated as factors causing differences in digestibility between varieties of perennial ryegrass, cocksfoot and timothy. In first growth these factors only partly accounted for the variation in digestibility between species but almost entirely accounted for the variation between varieties within each species. In the regrowths, heading behavior and leafiness were not closely associated with digestibility, which indicated that the variation between grasses following first growth may be a result of inherent differences.

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] Because deer liver is a choice food item of man, it was of interest to assay samples for fall-out radionuclides. Livers and rumen contents were obtained from wild mule deer (Odocoileus hemionus hemionus) which were collected approximately weekly during the period May 1963-August 1964 from a 580 ...

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] The ability or inability of a cell recovering from radiation damage to support synthesis of functional proteins can be established by introducing into the intact, irradiated cell an alien messenger species and determining whether or not the cellular synthetic machinery (that is ribosomes, transfer ...

Electronic Resource

0004-8402

Philosophy