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Christopher B Barrett - One of the best experts on this subject based on the ideXlab platform.
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improving food aid what reforms would yield the highest payoff
World Development, 2008Co-Authors: Erin C Lentz, Christopher B BarrettAbstract:Summary This paper develops an integrated model of the food aid distribution chain, from Donor appropriations through operational agency programming decisions to household consumption choices. We use this model to simulate alternative policies and to perform sensitivity analysis to establish how varying underlying conditions—for example, delivery costs, the political additionality of food, targeting efficacy—affect optimal food aid policy for improving the well-being of food insecure households. We find that improved targeting by operational agencies is crucial to advancing food security objectives. At the Donor Level, the key policy variable under most model parameterizations is ocean freight costs associated with cargo preference restrictions on the US food aid.
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improving food aid what reforms would yield the highest payoff
Social Science Research Network, 2007Co-Authors: Erin C Lentz, Christopher B BarrettAbstract:This paper develops an integrated model of the food aid distribution chain, from Donor appropriations through operational agency programming decisions to household consumption choices. We use this model to simulate alternative policies and to perform sensitivity analysis to establish how varying underlying conditions - e.g., delivery costs, the political additionality of food, targeting efficacy - affect optimal food aid policy for improving the well-being of food insecure households. We find that improved targeting by operational agencies is crucial to advancing food security objectives. At the Donor Level, the key policy variable under most model parameterizations is ocean freight costs associated with cargo preference restrictions on US food aid.
L Dobaczewski - One of the best experts on this subject based on the ideXlab platform.
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Donor Level of interstitial hydrogen in semiconductors deep Level transient spectroscopy
Physica B-condensed Matter, 2009Co-Authors: Vl Kolkovsky, L Dobaczewski, Bonde K Nielsen, Nylandsted A Larsen, Jacques WeberAbstract:Abstract The behaviour of hydrogen in crystalline semiconductors has attracted considerable interest during several decades. Due to its high diffusion rate and ability to react with a wide variety of lattice imperfections such as intrinsic point defects, impurities, interfaces and surfaces, hydrogen is an impurity of fundamental importance in semiconductor materials. It has been already evidenced in previous investigations that the most fundamental hydrogen-related defects in-group IV semiconductors are interstitial hydrogen atoms occupying the bond-centre site (BC) or the interstitial tetrahedral site (T). Using first-principles calculations Van de Walle predicted similar properties of isolated hydrogen in other II–VI and III–V semiconductors. Another interesting prediction shown in that work was the existence of a universal alignment for the hydrogen electronic (−/+) Level. Until now there is no direct experimental information regarding the individual isolated hydrogen states in compound semiconductors and most reported properties have been inferred indirectly. In the present work in-situ conventional deep Level transient spectroscopy (DLTS) and high-resolution Laplace DLTS techniques are used to analyse hydrogen-related Levels after low-temperature proton implantation in different II–VI and III–V semiconductors including GaAs, ZnO and CdTe. From these experimental observations the Donor Level of isolated hydrogen is found to keep almost a constant value in the absolute energy scale taking into account different band-offsets calculated for the whole group of semiconductors.
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e center in silicon has a Donor Level in the band gap
Physical Review Letters, 2006Co-Authors: Nylandsted A Larsen, R Jones, L Dobaczewski, Bonde K Nielsen, A Mesli, Kortegaard H Nielsen, J Adey, D W Palmer, P R Briddon, Sven ObergAbstract:It has been an accepted fact for more than 40 years that the E center in Si (the group-V impurity--vacancy pair)--one of the most studied defects in semiconductors--has only one energy Level in the band gap: namely, the acceptor Level at about 0.45 eV below the conduction band. We now demonstrate that it has a second Level, situated in the lower half of the band gap at 0.27 eV above the valence band. The existence of this Level, having a Donor character, is disclosed by a combination of different transient-capacitance techniques and electronic-structure calculations. The finding seriously questions some diffusion-modeling approaches performed in the past.
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Donor Level of bond center hydrogen in germanium
Physical Review B, 2004Co-Authors: L Dobaczewski, Bonde K Nielsen, N Zangenberg, Bech B Nielsen, A R Peaker, V P MarkevichAbstract:We apply Laplace deep-Level transient spectroscopy (LDLTS) in situ after low-temperature proton implantation into crystalline $n$-type germanium and identify a deep metastable Donor center. The activation energy of the Donor emission is $\ensuremath{\sim}110\phantom{\rule{0.3em}{0ex}}\text{meV}$ when extrapolated to zero electric field. We obtain the split patterns of the emission signal for uniaxial stress applied along three major crystal directions $⟨100⟩$, $⟨110⟩$, and $⟨111⟩$, and conclude that the symmetry of the center is trigonal. We compare the annealing characteristics with those of bond-center hydrogen in silicon and with those of a trigonal center in germanium previously identified as bond-center hydrogen by in situ local-mode infrared absorption spectroscopy. From this comparison it is concluded that the observed Donor emission originates from bond-center hydrogen. Infrared absorption also revealed another trigonal center tentatively ascribed to hydrogen occupying an antibonding configuration. A search for a corresponding deep Level (as a hole or electron trap) failed, indicating that such Level must be near midgap or resonant with (close to) the valence band.
Bonde K Nielsen - One of the best experts on this subject based on the ideXlab platform.
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Donor Level of interstitial hydrogen in semiconductors deep Level transient spectroscopy
Physica B-condensed Matter, 2009Co-Authors: Vl Kolkovsky, L Dobaczewski, Bonde K Nielsen, Nylandsted A Larsen, Jacques WeberAbstract:Abstract The behaviour of hydrogen in crystalline semiconductors has attracted considerable interest during several decades. Due to its high diffusion rate and ability to react with a wide variety of lattice imperfections such as intrinsic point defects, impurities, interfaces and surfaces, hydrogen is an impurity of fundamental importance in semiconductor materials. It has been already evidenced in previous investigations that the most fundamental hydrogen-related defects in-group IV semiconductors are interstitial hydrogen atoms occupying the bond-centre site (BC) or the interstitial tetrahedral site (T). Using first-principles calculations Van de Walle predicted similar properties of isolated hydrogen in other II–VI and III–V semiconductors. Another interesting prediction shown in that work was the existence of a universal alignment for the hydrogen electronic (−/+) Level. Until now there is no direct experimental information regarding the individual isolated hydrogen states in compound semiconductors and most reported properties have been inferred indirectly. In the present work in-situ conventional deep Level transient spectroscopy (DLTS) and high-resolution Laplace DLTS techniques are used to analyse hydrogen-related Levels after low-temperature proton implantation in different II–VI and III–V semiconductors including GaAs, ZnO and CdTe. From these experimental observations the Donor Level of isolated hydrogen is found to keep almost a constant value in the absolute energy scale taking into account different band-offsets calculated for the whole group of semiconductors.
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e center in silicon has a Donor Level in the band gap
Physical Review Letters, 2006Co-Authors: Nylandsted A Larsen, R Jones, L Dobaczewski, Bonde K Nielsen, A Mesli, Kortegaard H Nielsen, J Adey, D W Palmer, P R Briddon, Sven ObergAbstract:It has been an accepted fact for more than 40 years that the E center in Si (the group-V impurity--vacancy pair)--one of the most studied defects in semiconductors--has only one energy Level in the band gap: namely, the acceptor Level at about 0.45 eV below the conduction band. We now demonstrate that it has a second Level, situated in the lower half of the band gap at 0.27 eV above the valence band. The existence of this Level, having a Donor character, is disclosed by a combination of different transient-capacitance techniques and electronic-structure calculations. The finding seriously questions some diffusion-modeling approaches performed in the past.
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Donor Level of bond center hydrogen in germanium
Physical Review B, 2004Co-Authors: L Dobaczewski, Bonde K Nielsen, N Zangenberg, Bech B Nielsen, A R Peaker, V P MarkevichAbstract:We apply Laplace deep-Level transient spectroscopy (LDLTS) in situ after low-temperature proton implantation into crystalline $n$-type germanium and identify a deep metastable Donor center. The activation energy of the Donor emission is $\ensuremath{\sim}110\phantom{\rule{0.3em}{0ex}}\text{meV}$ when extrapolated to zero electric field. We obtain the split patterns of the emission signal for uniaxial stress applied along three major crystal directions $⟨100⟩$, $⟨110⟩$, and $⟨111⟩$, and conclude that the symmetry of the center is trigonal. We compare the annealing characteristics with those of bond-center hydrogen in silicon and with those of a trigonal center in germanium previously identified as bond-center hydrogen by in situ local-mode infrared absorption spectroscopy. From this comparison it is concluded that the observed Donor emission originates from bond-center hydrogen. Infrared absorption also revealed another trigonal center tentatively ascribed to hydrogen occupying an antibonding configuration. A search for a corresponding deep Level (as a hole or electron trap) failed, indicating that such Level must be near midgap or resonant with (close to) the valence band.
Erin C Lentz - One of the best experts on this subject based on the ideXlab platform.
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improving food aid what reforms would yield the highest payoff
World Development, 2008Co-Authors: Erin C Lentz, Christopher B BarrettAbstract:Summary This paper develops an integrated model of the food aid distribution chain, from Donor appropriations through operational agency programming decisions to household consumption choices. We use this model to simulate alternative policies and to perform sensitivity analysis to establish how varying underlying conditions—for example, delivery costs, the political additionality of food, targeting efficacy—affect optimal food aid policy for improving the well-being of food insecure households. We find that improved targeting by operational agencies is crucial to advancing food security objectives. At the Donor Level, the key policy variable under most model parameterizations is ocean freight costs associated with cargo preference restrictions on the US food aid.
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improving food aid what reforms would yield the highest payoff
Social Science Research Network, 2007Co-Authors: Erin C Lentz, Christopher B BarrettAbstract:This paper develops an integrated model of the food aid distribution chain, from Donor appropriations through operational agency programming decisions to household consumption choices. We use this model to simulate alternative policies and to perform sensitivity analysis to establish how varying underlying conditions - e.g., delivery costs, the political additionality of food, targeting efficacy - affect optimal food aid policy for improving the well-being of food insecure households. We find that improved targeting by operational agencies is crucial to advancing food security objectives. At the Donor Level, the key policy variable under most model parameterizations is ocean freight costs associated with cargo preference restrictions on US food aid.
S B Zhang - One of the best experts on this subject based on the ideXlab platform.
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Donor Donor binding in semiconductors engineering shallow Donor Levels for znte
Applied Physics Letters, 2003Co-Authors: Anderson Janotti, Suhuai Wei, S B ZhangAbstract:In the past, codoping by mixing Donors with acceptors has been proposed to lower the dopant ionization energy. However, the Level repulsion between Donor and acceptor states is weak due to symmetry considerations. Here, we propose an innovative approach to lower the Donor ionization energy by combining Donor with Donor. Using first-principles band structure method, we demonstrated this concept with n-type doping in ZnTe. For example, we find that the BrTe–SnZn pair has a binding energy of 0.9 eV and a shallow e(+/0) Donor Level at 70 meV below the conduction-band minimum, compared to 240 meV for the isolated BrTe impurity.
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effects of ga addition to cuinse2 on its electronic structural and defect properties
Applied Physics Letters, 1998Co-Authors: Suhuai Wei, S B Zhang, Alex ZungerAbstract:Using a first-principles band structure method we have theoretically studied the effects of Ga additions on the electronic and structural properties of CuInSe2. We find that (i) with increasing xGa, the valence band maximum of CuIn1−xGaxSe2 (CIGS) decreases slightly, while the conduction band minimum (and the band gap) of CIGS increases significantly, (ii) the acceptor formation energies are similar in both CuInSe2 (CIS) and CuGaSe2 (CGS), but the Donor formation energy is larger in CGS than in CIS, (iii) the acceptor transition Levels are shallower in CGS than in CIS, but the GaCu Donor Level in CGS is much deeper than the InCu Donor Level in CIS, and (iv) the stability domain of the chalcopyrite phase increases with respect to ordered defect compounds. Our results are compared with available experimental observations.