Author

# Christophe Longeaud

Other affiliations: University of Paris, École Normale Supérieure, Université Paris-Saclay ...read more

Bio: Christophe Longeaud is an academic researcher from Pierre-and-Marie-Curie University. The author has contributed to research in topics: Amorphous silicon & Density of states. The author has an hindex of 19, co-authored 99 publications receiving 1300 citations. Previous affiliations of Christophe Longeaud include University of Paris & École Normale Supérieure.

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##### Papers

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TL;DR: An analysis of the MPC experiment applied to samples of amorphous semiconductors built in coplanar geometry shows that, if one type of carrier is predominant, the modulated photocurrent gives the density as well as the capture cross section of the localized states interacting with these carriers.

Abstract: In this paper we concentrate on an analysis of the modulated photocurrent (MPC) experiment applied to samples of amorphous semiconductors built in coplanar geometry. Taking into account both types of photocarriers, the basic equations describing the modulation of the occupation of the localized states are derived according to the statistics of Simmons and Taylor. Generalized expressions for the phase shift and the modulus of the modulated photocurrent are obtained without any restrictive assumptions and discussed. It is shown that, if one type of carrier is predominant, the modulated photocurrent gives the density as well as the capture cross section of the localized states interacting with these carriers. The precise conditions under which the two-carrier system is reduced to a single-carrier system are given. The main features of the method are illustrated by means of a simulation, where we study the influence of several parameters. We show that the dominant contribution to the modulated photocurrent comes from the carrier type which presents the higher value of \ensuremath{\mu}/(N\ensuremath{\sigma}), where \ensuremath{\mu} is the free-carrier mobility; \ensuremath{\sigma}, the capture cross section; and N, the density of trapping states for which the emission rate is equal to the angular frequency \ensuremath{\omega} at which the experiment is performed. Consequently, only the trapping states corresponding to this type of carrier can be probed. Our study underlines some possible experimental misuses of the MPC technique which could lead to erroneous results regarding the inferred density of states.

100 citations

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TL;DR: In this article, a new type of material, called polymorphous silicon (pm-Si:H) is obtained, which exhibits enhanced transport properties as compared to state-of-the-art hydrogenated amorphous silicon, and the electron capture cross section of deep gap states is also expected to be lower by a factor of 3-4 to account for photoconductivity results.

Abstract: When silicon thin films are deposited by plasma enhanced chemical vapor deposition in a plasma regime close to that of the formation of powder, a new type of material, named polymorphous silicon (pm-Si:H) is obtained. pmSi:H exhibits enhanced transport properties as compared to state-of-the-art hydrogenated amorphous silicon (a-Si:H). The study of space-charge-limited current in n(+)-i-n(+) structures along with the use of the modulated photocurrent technique, of the constant photocurrent method and of steady-state photoconductivity and dark conductivity measurements allows us to shed some light on the origin of these improved properties. It is shown that the midgap density of states in the samples studied here is at least ten times lower than in a-Si:H, and the electron capture cross section of deep gap states is also expected to be lower by a factor of 3-4 to account for photoconductivity results. An interesting field of theoretical research is now open in order to link these low densities of states and capture cross sections to the peculiar structure of this new material. (C) 1999 American Institute of Physics. [S0021-8979(99)00914-7].

99 citations

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TL;DR: In this paper, a new a-Si:H-like material has been obtained in a radio frequency-powered plasmaenhanced chemical vapor deposition system (RF-PECVD), which was prepared with dilution of silane into He or H2, under high total pressure (≈132 Pa) and high RF power exhibits enhanced electronic transport properties.

Abstract: A new a-Si:H-like material has been obtained in a radio frequency-powered plasma-enhanced chemical vapor deposition system (RF-PECVD). This material prepared with dilution of silane into He or H2, under high total pressure (≈132 Pa) and high RF power exhibits enhanced electronic transport properties. The room temperature electronic mobility-lifetime product is increased by a factor up to 200 compared to hydrogenated amorphous silicon (a-Si:H) prepared under standard deposition conditions (lower pressure, lower RF power). The density of states measured by modulated photocurrent and the deep defect density measured by the constant photocurrent method are both less than that of standard a-Si:H. These transport properties are linked to the structure of this new material deposited under conditions close to those for powder formation. This structure seems to result in a decrease of the deep defect density and capture cross sections.

69 citations

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Chinese Academy of Sciences

^{1}, Tsinghua University^{2}, Gunma University^{3}, University of Paris^{4}, Chongqing University^{5}, University of Cambridge^{6}TL;DR: A selector with a large drive current density of 34 MA cm−2 and a ~106 high nonlinearity, realized in an environment-friendly and earth-abundant sulfide binary semiconductor, GeS, reveals a Ge pyramid-dominated network and high density of near-valence band trap states in amorphous GeS.

Abstract: Selector devices are indispensable components of large-scale nonvolatile memory and neuromorphic array systems. Besides the conventional silicon transistor, two-terminal ovonic threshold switching device with much higher scalability is currently the most industrially favored selector technology. However, current ovonic threshold switching devices rely heavily on intricate control of material stoichiometry and generally suffer from toxic and complex dopants. Here, we report on a selector with a large drive current density of 34 MA cm−2 and a ~106 high nonlinearity, realized in an environment-friendly and earth-abundant sulfide binary semiconductor, GeS. Both experiments and first-principles calculations reveal Ge pyramid-dominated network and high density of near-valence band trap states in amorphous GeS. The high-drive current capacity is associated with the strong Ge-S covalency and the high nonlinearity could arise from the synergy of the mid-gap traps assisted electronic transition and local Ge-Ge chain growth as well as locally enhanced bond alignment under high electric field. Designing efficient selector devices for large-scale nonvolatile memory and neuromorphic array systems remains a challenge. Here, the authors propose a two-terminal ovonic threshold switching selector device with a large drive current density and a high nonlinearity, capable emulating stochastic integrate-and-fire neuron behavior.

60 citations

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TL;DR: In this article, the density of states at the Fermi level N(EF) has been measured on hydrogenated polymorphous (Si:H) silicon samples using both capacitance measurements on Schottky barriers and space-charge-limited current measurements on n+/i/n+ structures.

Abstract: The density of states at the Fermi level N(EF) has been measured on hydrogenated polymorphous (pm-Si:H) silicon samples using both capacitance measurements on Schottky barriers and space-charge-limited current measurements on n+/i/n+ structures. From both techniques, N(EF) values of 7–8×1014 cm−3 eV−1 have been obtained, which is significantly lower than reported in the literature for hydrogenated amorphous silicon (a-Si:H). Such values demonstrate that pm-Si:H is a very low defect density material which should be able to replace a-Si:H in the field of applications like photovoltaics.

60 citations

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1,191 citations

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TL;DR: In this paper, the authors describe the introduction of hydrogenated microcrystalline silicon (μc-Si:H) as novel absorber material for thin-film silicon solar cells.

425 citations

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TL;DR: In this paper, the authors conduct a systematic analysis of Sb 2 S 3 -based photovoltaic devices, highlighting major advancements and most prominent limitations of this technology and provide a roadmap for further Sb2 S 3 technology development.

325 citations

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IBM

^{1}TL;DR: In this article, the authors survey progress in the PCM field over the past five years, ranging from large-scale PCM demonstrations to materials improvements for high-temperature retention and faster switching.

Abstract: We survey progress in the PCM field over the past five years, ranging from large-scale PCM demonstrations to materials improvements for high–temperature retention and faster switching. Both materials and new cell designs that support lower-power switching are discussed, as well as higher reliability for long cycling endurance. Two paths towards higher density are discussed: through 3D integration by the combination of PCM and 3D-capable access devices, and through multiple bits per cell, by understanding and managing resistance drift caused by structural relaxation of the amorphous phase. We also briefly survey work in the nascent field of brain-inspired neuromorphic systems that use PCM to implement non-Von Neumann computing.

302 citations

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TL;DR: In this paper, an elementary empirical model for the distribution of electronic states of an amorphous semiconductor is presented, and the functional form of the optical absorption spectrum is determined, focusing on the joint density of states function, which dominates the absorption spectrum over the range of photon energies.

Abstract: An elementary empirical model for the distribution of electronic states of an amorphous semiconductor is presented. Using this model, we determine the functional form of the optical absorption spectrum, focusing our analysis on the joint density of states function, which dominates the absorption spectrum over the range of photon energies we consider. Applying our optical absorption results, we then determine how the empirical measures commonly used to characterize the absorption edge of an amorphous semiconductor, such as the Tauc gap and the absorption tail breadth, are related to the parameters that characterize the underlying distribution of electronic states. We, thus, provide the experimentalist with a quantitative means of interpreting the physical significance of their optical absorption data.

300 citations