The Experts below are selected from a list of 15675 Experts worldwide ranked by ideXlab platform
S D Mahanti - One of the best experts on this subject based on the ideXlab platform.
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electronic structure of ga in and tl doped pbte a supercell study of the impurity bands
Physical Review B, 2008Co-Authors: Khang Hoang, S D MahantiAbstract:The physics of Deep Defect states associated with group-III (Ga, In, Tl) impurities in PbTe has been of great interest over the last several decades. Three different models have been proposed to understand interesting and unusual properties exhibited by these impurities. These are ``impurity level'' model, ``mixed-valence'' model, and ``autocompensation'' model. Recent studies that were carried out using ab initio density-functional theory and supercell models give a detailed microscopic picture of the Deep Defect states associated with these impurities. The computed single-particle electronic density of states does not support the mixed-valence model where the impurities are supposed to be in ${3}^{+}$ and ${1}^{+}$ charge states, rather it is in favor of the impurity level model. In this paper, we present a detailed analysis of the impurity bands, impurity levels in the dilute limit, and the hybridization between the impurity level and the host PbTe bands. We find that the impurity level overlaps the conduction band (In), is in the gap (Ga), and overlaps the valence band (Tl). The possibility of the autocompensation model where impurities in charge states ${1}^{+}$ and ${1}^{\ensuremath{-}}$ coexist (a different type of mixed valency) is briefly discussed.
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ab initio study of Deep Defect states in narrow band gap semiconductors group iii impurities in pbte
Physical Review Letters, 2006Co-Authors: Salameh Ahmad, Khang Hoang, S D MahantiAbstract:The nature of Deep Defect states, in general, and those associated with group III elements (Ga, In, Tl) in narrow band-gap IV-VI semiconductors (PbTe and PbSe), in particular, have been of great interest over the past three decades. We present ab initio electronic structure calculations that give a new picture of these states compared to the currently accepted model in terms of a negative-U Hubbard model. The Fermi surface pinning and why In-doped PbTe and related compounds show excellent high temperature thermoelectric behavior can be understood within the new picture.
David J Cohen - One of the best experts on this subject based on the ideXlab platform.
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light induced Defects in hydrogenated amorphous silicon germanium alloys
Solar Energy Materials and Solar Cells, 2003Co-Authors: David J CohenAbstract:Abstract A selected survey of the phenomenon of light-induced Deep Defect creation in the hydrogenated amorphous silicon–germanium is presented. First a general review of the early studies that established the key salient features of light-induced degradation in a-Si,Ge:H is given. This is followed by a discussion of a couple of complicating issues that have more recently come to light; namely, the possibility that charged Defects play a more central role in the alloys, and that both Si and Ge metastable dangling bonds may be playing a significant role in the alloys with germanium fractions below 20 at%. Following this, the results of some recent studies are summarized that have been focusing on the details of degradation in the low Ge fraction alloys to gain insight into the fundamentals of degradation of amorphous silicon materials in general. This review concludes with an overall assessment of the level of our understanding of degradation in the a-Si,Ge:H alloys and where some key issues are still remaining to be resolved.
J P Conde - One of the best experts on this subject based on the ideXlab platform.
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amorphous and microcrystalline silicon films grown at low temperatures by radio frequency and hot wire chemical vapor deposition
Journal of Applied Physics, 1999Co-Authors: P Alpuim, V Chu, J P CondeAbstract:The effect of hydrogen dilution on the optical, transport, and structural properties of amorphous and microcrystalline silicon thin films deposited by hot-wire (HW) chemical vapor deposition and radio-frequency (rf) plasma-enhanced chemical vapor deposition using substrate temperatures (Tsub) of 100 and 25 °C is reported. Microcrystalline silicon (μc-Si:H) is obtained using HW with a large crystalline fraction and a crystallite size of ∼30 nm for hydrogen dilutions above 85% independently of Tsub. The deposition of μc-Si:H by rf, with a crystallite size of ∼8 nm, requires increasing the hydrogen dilution and shows decreasing crystalline fraction as Tsub is decreased. The photoconductivity, Defect density, and structure factor of the amorphous silicon films (a-Si:H) are strongly improved by the use of hydrogen dilution in the Tsub range studied. a-Si:H films with a photoconductivity-to-dark conductivity ratio above 105, a Deep Defect density below 1017 cm−3, an Urbach energy below 60 meV and a structure fa...
A Shah - One of the best experts on this subject based on the ideXlab platform.
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mobility lifetime product a tool for correlating a si h film properties and solar cell performances
Journal of Applied Physics, 1996Co-Authors: N Beck, N Wyrsch, Ch Hof, A ShahAbstract:The missing correlation between film characteristics and a‐Si:H‐based p‐i‐n solar cells is still a controversial subject. The authors present a new parameter μ0τ0, evaluated from steady‐state transport measurements on a‐Si:H layers, which can indeed relate film quality and cell performance as far as the latter is limited by the quality of the intrinsic 〈i〉 layer. Thereby, two specific features of the evaluated μ0τ0 product can explain its successful role as a quality parameter for a‐Si:H: First, the computation of μ0τ0 takes into account the effects of the prevailing dangling bond occupation, which is very different in uniform films as compared to the occupation profile prevailing through the i layer of a p‐i‐n solar cell; second, the evaluated μ0τ0 product combines information about band mobility and Defect density; furthermore it avoids some of the well‐known pitfalls of usual Deep Defect density measurements such as constant photocurrent method and photothermal deflection spectroscopy. Experimental dat...
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subbandgap absorption spectra of slightly doped a si h measured with constant photocurrent method cpm and photothermal deflection spectroscopy pds
Solid State Communications, 1993Co-Authors: E Sauvain, N Wyrsch, Andreas Mettler, A ShahAbstract:Abstract The authors report on systematic measurements of the absorption spectra in the visible near-infrared range performed on a series of slightly phosphorus- and boron-doped amorphous hydrogenated silicon (a-Si:H) samples. Optical transmission spectra, Constant Photocurrent Method (CPM) and Photothermal Deflection Spectroscopy (PDS) have been used to measure the absorption coefficient (α) in differently doped samples. Variation of the gas phase doping levels in the ppm range is shown to lead to relevant differences between CPM and PDS spectra. The latter demonstrate that additional care has to be taken when evaluating Deep Defect density from CPM when the Fermi level is not constant, and especially so, if the Fermi level is below midgap.
O Saadane - One of the best experts on this subject based on the ideXlab platform.
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role of interstitial hydrogen and voids in light induced metastable Defect formation in hydrogenated amorphous silicon a model
Physical Review B, 2002Co-Authors: Christophe Longeaud, D Roy, O SaadaneAbstract:Densities of states of a large set of samples have been measured. Samples were deposited by radio-frequency powered glow discharge under various conditions of temperature, power, gas pressure, and dilution of silane. Density of states was studied in the as-deposited, light-soaked, and annealed states. For all the samples light soaking resulted in an increase of both the Deep Defect density and of the conduction-band tail states. For samples deposited on the edge of crystallinity and polymorphous materials irreversible modifications of the density of states were observed after light soaking followed by annealing. Since none of the existing models of the metastability can account for this behavior we propose a model. In this model, light induced creation of dangling bonds is mediated by interstitial hydrogen. Hydrogen coming from the breaking of Si-H bonds is trapped into voids or platelets during light soaking and released in the lattice during annealing. This model fully explains our experimental results and also many other experimental observations found in the literature.
