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Results for “semiconductor models”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Wake potential with exchange-correlation effects in semiconductor quantum plasmas

Using the non-relativistic quantum hydrodynamic model, wake potential has been studied in a magnetized semiconductor quantum plasma in the presence of upper hybrid wave which is excited via externally injected electron beam. The quantum effect contains electron exchange and correlation potential, Fermi degenerate pressure, and Bohm potential. It is found that the contribution of quantum mechanical electron exchange and correlation potential significantly modifies the amplitude and the effective length of the oscillatory wake potential. In the electron-hole plasma systems, electron exchange-correlation effects tend to increase the magnitude of the wake potential and are much effective at the distances of the order of Debye-length. The application of the work in context of the semiconductor plasmas have also been analyzed graphically.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Kinetic model for electric-field induced point defect redistribution near semiconductor surfaces

The spatial distribution of point defects near semiconductor surfaces affects the efficiency of devices. Near-surface band bending generates electric fields that influence the spatial redistribution of charged mobile defects that exchange infrequently with the lattice, as recently demonstrated for pile-up of isotopic oxygen near rutile TiO{sub 2} (110). The present work derives a mathematical model to describe such redistribution and establishes its temporal dependence on defect injection rate and band bending. The model shows that band bending of only a few meV induces significant redistribution, and that the direction of the electric field governs formation of either a valley or a pile-up.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Modeling direct band-to-band tunneling: From bulk to quantum-confined semiconductor devices

A rigorous framework to study direct band-to-band tunneling (BTBT) in homo- and hetero-junction semiconductor nanodevices is introduced. An interaction Hamiltonian coupling conduction and valence bands (CVBs) is derived using a multiband envelope method. A general form of the BTBT probability is then obtained from the linear response to the “CVBs interaction” that drives the system out of equilibrium. Simple expressions in terms of the one-electron spectral function are developed to compute the BTBT current in two- and three-dimensional semiconductor structures. Additionally, a two-band envelope equation based on the Flietner model of imaginary dispersion is proposed for the same purpose. In order to characterize their accuracy and differences, both approaches are compared with full-band, atomistic quantum transport simulations of Ge, InAs, and InAs-Si Esaki diodes. As another numerical application, the BTBT current in InAs-Si nanowire tunnel field-effect transistors is computed. It is found that both approaches agree with high accuracy. The first one is considerably easier to conceive and could be implemented straightforwardly in existing quantum transport tools based on the effective mass approximation to account for BTBT in nanodevices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Features of the electric-field distribution in anisotropic semiconductor wafers in a transverse magnetic field

A macroscopic model of the Hall effects and magnetoresistance in anisotropic semiconductor wafers is developed. The results obtained by solving the electrodynamic boundary problem allow the potential and eddy currents in anisotropic semiconductors to be calculated at different current-contact locations, depending on the parameters of the sample material's anisotropy. The results of this study are of great practical importance for investigating the physical properties of anisotropic semiconductors and simulating the electron-transport phenomena in devices based on anisotropic semiconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Theoretical analysis of conditions for observation of plasma oscillations in semiconductors from pulsed terahertz emission

Oscillations of electron-hole plasma generated by femtosecond optical pulse in freestanding semiconductor are studied using hydrodynamic model and Monte Carlo simulations. The conditions required for the observation of coherent plasma oscillations in THz emission from semiconductor are determined. It is shown that several conditions have to be fulfilled in order to observe coherent plasma oscillations. First, the intensity of the optical pulse must exceed some threshold value. Second, the optical absorption depth must exceed the thickness of the built-in electric field region. Third, the generation of electron-hole pairs with uniform illumination is required, i.e., the laser beam with the flattop intensity profile has to be used. It is found that the duration of the optical pulse does not play a vital role in the development of plasma oscillations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Charging and exciton-mediated decharging of metal nanoparticles in organic semiconductor matrices

Gold nanoparticles (Au-NPs) were deposited on the surface of n- and p-type organic semiconductors to form defined model systems for charge storage based electrically addressable memory elements. We used ultraviolet photoelectron spectroscopy to study the electronic properties and found that the Au-NPs become positively charged because of photoelectron emission, evidenced by spectral shifts to higher binding energy. Upon illumination with light that can be absorbed by the organic semiconductors, dynamic charge neutrality of the Au-NPs could be re-established through electron transfer from excitons. The light-controlled charge state of the Au-NPs could add optical addressability to memory elements.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Light Material Interactions in Energy Conversion (Final Report)

The scientific vision of the “Light-Material Interactions in Energy Conversion” Energy Frontier Research Center (LMI-EFRC) is embodied in our mission statement: ‘to tailor the morphology, complex dielectric structure, and electronic properties of matter so as to sculpt the flow of sunlight and heat, enabling light conversion to electrical energy with unprecedented efficiency’. Founded in 2009, the LMI-EFRC has become a national resource for fundamental optical principles and phenomena relevant to solar energy conversion, addressing new scientific opportunities that enable efficient utilization of the entire visible and infrared solar resource. The LMI-EFRC is directed by Prof. Ralph Nuzzo and centered at its lead institution, the California Institute of Technology. The Center is an integrated and collaborative partnership of research leaders in photonics and photonic materials, comprising 14 faculty investigators that span six academic departments at five institutions, including Profs. Harry Atwater, Andrei Faraon, Nathan Lewis, and Austin Minnich at Caltech; Profs. Ralph Nuzzo, Paul Braun and John Rogers at the University of Illinois at Urbana-Champaign; Prof. Jennifer Lewis at Harvard University; Profs. Mark Brongersma, Jennifer Dionne, and Shanhui Fan at Stanford University; and Profs. Paul Alivisatos, Eli Yablonovitch, and Xiang Zhang at Lawrence Berkeley National Laboratory. The Center is organized into four research groups (RGs) that address scientific themes related to light-matter interactions, with each team spanning multiple institutional partners and designed to meet our scientific objectives. Notable project outcomes that address the LMI Mission and 2014-2018 goals include: (1) RG1 “New Light Management Mechanisms” demonstrated tunable and active nanostructured patterned surfaces (‘metasurfaces’) for control of light absorption, reflection, and propagation. (2) RG2 “Solar Spectrum Control and Conversion” developed colloidal nanocrystals with designer optical properties for fundamental studies of solar spectrum control and conversion. (3) RG3 “Thermal Photon Harvesting” established photonic design principles to harness and control thermal photons in novel conversion systems. (4) RG4 “Programmable Assembly of Photonic and Electronic Architectures” developed powerful new methods for light- and matter-based additive fabrication of electronic and photonic architectures.

14 SOLAR ENERGY↗
DOE OSTI2016

Experimental and theoretical study of rotationally inelastic diffraction of H{sub 2}(D{sub 2}) from methyl-terminated Si(111)

Fundamental details concerning the interaction between H{sub 2} and CH{sub 3}–Si(111) have been elucidated by the combination of diffractive scattering experiments and electronic structure and scattering calculations. Rotationally inelastic diffraction (RID) of H{sub 2} and D{sub 2} from this model hydrocarbon-decorated semiconductor interface has been confirmed for the first time via both time-of-flight and diffraction measurements, with modest j = 0 → 2 RID intensities for H{sub 2} compared to the strong RID features observed for D{sub 2} over a large range of kinematic scattering conditions along two high-symmetry azimuthal directions. The Debye-Waller model was applied to the thermal attenuation of diffraction peaks, allowing for precise determination of the RID probabilities by accounting for incoherent motion of the CH{sub 3}–Si(111) surface atoms. The probabilities of rotationally inelastic diffraction of H{sub 2} and D{sub 2} have been quantitatively evaluated as a function of beam energy and scattering angle, and have been compared with complementary electronic structure and scattering calculations to provide insight into the interaction potential between H{sub 2} (D{sub 2}) and hence the surface charge density distribution. Specifically, a six-dimensional potential energy surface (PES), describing the electronic structure of the H{sub 2}(D{sub 2})/CH{sub 3}−Si(111) system, has been computed based on interpolation of density functional theory energies. Quantum and classical dynamics simulations have allowed for an assessment of the accuracy of the PES, and subsequently for identification of the features of the PES that serve as classical turning points. A close scrutiny of the PES reveals the highly anisotropic character of the interaction potential at these turning points. This combination of experiment and theory provides new and important details about the interaction of H{sub 2} with a hybrid organic-semiconductor interface, which can be used to further investigate energy flow in technologically relevant systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Heterojunctions of model CdTe/CdSe mixtures

We report on the strain behavior of compound mixtures of model group II-VI semiconductors. We use the Stillinger-Weber Hamiltonian that we recently introduced, specifically developed to model binary mixtures of group II-VI compounds such as CdTe and CdSe. We also employ molecular dynamics simulations to examine the behavior of thin sheets of material, bilayers of CdTe and CdSe. The lattice mismatch between the two compounds leads to a strong bending of the entire sheet, with about a 0.5 to 1° deflection between neighboring planes. To further analyze bilayer bending, we introduce a simple one-dimensional model and use energy minimization to find the angle of deflection. The analysis is equivalent to a least-squares straight line fit. We consider the effects of bilayers which are asymmetric with respect to the thickness of the CdTe and CdSe parts. We thus learn that the bending can be subdivided into four kinds depending on the compressive/tensile nature of each outer plane of the sheet. We use this approach to directly compare our findings with experimental results on the bending of CdTe/CdSe rods. To reduce the effects of the lattice mismatch we explore diffuse interfaces, where we mix (i.e. alloy) Te and Se, and estimate the strain response.

36 MATERIALS SCIENCE↗
NASA NTRS1998

Computational Modeling of Vertical-Cavity Surface-Emitting Lasers

Vertical-Cavity Surface-Emitting Lasers (VCSELs) have become increasingly important for many applications, such as optical interconnects and optical data storages. In many applications, critical requirements are very often high beam quality and/or high output powers. To help the design and optimization of VCSELs for such applications, it is essential to have a simulation model that includes the time evolution of the transverse space dependence of the hewn. Such models allow arbitrary transverse profiles of laser intensity or carrier density to develop in time without any a priori assumption about the number and types of transverse modes. In addition, extensive research in the past has shown that many-body effects are important in influencing the optical gain and the refractive index of semiconductor quantum wells. It is thus important to include the key many-body effects in simulation models for semiconductor lasers. A computational model, called the Maxwell-Effective Bloch Equations (MEBE) was developed recently that simulates the above features and effects. This model combines the space-time resolution with microscopic many-body effects in a way that is both accurate enough and computationally manageable. This model has been used to simulate edge-emitting high power lasers recently. A new finite-difference algorithm has been developed for solving these equations and will be shown in the presentation. That algorithm will be used to calculate the detailed time evolution of the transverse mode profiles of VCSELs. The VCSELs are based on InGaAs/GaAs quantum well structures. Effects of pumping current and refractive index profiles will be investigated in detail. The simulation results will be compared with other results the use the rate-equation model.

Goorjian, Peter M.↗
NASA NTRS1998

Computational Nonlinear of Vertical-Cavity Surface-Emitting Lasers

Vertical-Cavity Surface-Emitting Lasers (VCSELs) have become increasingly important for many applications, such as optical interconnects and optical data storages. In many applications, critical requirements are very often high beam quality and/or high output powers. To help the design and optimization of VCSELs for such applications, it is essential to have a simulation model that includes the time evolution of the transverse space dependence of the beam. Such models allow arbitrary transverse profiles of laser intensity or carrier density to develop in time without any a priori assumption about the number and types of transverse modes. In addition, extensive research in the past has shown that many-body effects are important in influencing the optical gain and the refractive index of semiconductor quantum wells. It is thus important to include the key many-body effects in simulation models for semiconductor lasers. A computational model, called the Maxwell-Effective Bloch Equations (MEBE) was developed recently that simulates the above features and effects. This model combines the space-time resolution with microscopic many-body effects in a way that is both accurate enough and computationally manageable. This model has been used to simulate edge-emitting high power lasers recently. A new finite-difference algorithm has been developed for solving these equations and will be shown in the presentation. That algorithm will be used to calculate the detailed time evolution of the transverse mode profiles of VCSELs. The VCSELs are based on InGaAs/GaAs quantum well structures. Effects of pumping current and refractive index profiles will be investigated in detail. The simulation results will be compared with other results the use the rate-equation model.

Goorjian, Peter M.↗
DOE OSTI2013

Features of conduction mechanisms in n-HfNiSn semiconductor heavily doped with a Rh acceptor impurity

The crystal structure and electron-density distribution, as well as the energy, kinetic, and magnetic characteristics of n-HfNiSn intermetallic semiconductor heavily doped with a Rh acceptor impurity in the temperature range T = 80-400 K, in the acceptor-concentration range N{sub A}{sup Rh} Almost-Equal-To 9.5 Multiplication-Sign 10{sup 19}-1.9 Multiplication-Sign 10{sup 21} cm{sup -3} (x = 0.005-0.10), and in magnetic fields H {<=} 10 kG are investigated. It is established that doping is accompanied by a simultaneous decrease in concentration, the elimination of donor-type structural defects (to x Almost-Equal-To 0.02), and an increase in the concentration of acceptor-type structural defects (0 < x {<=} 0.10). The dependence of the degree of semiconductor compensation on temperature is revealed. A model of the spatial arrangement of atoms in HfNi{sub 1-x}Rh{sub x}Sn is proposed, and the results of calculating the electron structure based on this model agree with the results of investigations of the kinetic and magnetic characteristics of the semiconductor. The results are discussed within the context of the Shklovskii-Efros model for a heavily doped and compensated semiconductor.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Acceptor impurity activation in III-nitride light emitting diodes

In this work, the role of the acceptor doping and the acceptor activation and its impact on the internal quantum efficiency (IQE) of a Gallium Nitride (GaN) based multi-quantum well light emitting diode is studied by microscopic simulation. Acceptor impurities in GaN are subject to a high activation energy which depends on the presence of proximate dopant atoms and the electric field. A combined model for the dopant ionization and activation barrier reduction has been developed and implemented in a semiconductor carrier transport simulator. By model calculations, we demonstrate the impact of the acceptor activation mechanisms on the decay of the IQE at high current densities, which is known as the efficiency droop. A major contributor to the droop is the electron leakage which is largely affected by the acceptor doping.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

A New Method for the Evaluation of Majority and Minority Carrier Contact Resistivity of Polysilicon on Oxide Contacts

In this contribution we present a novel technique for characterizing the majority and minority carrier contact resistivity in polysilicon on oxide passivating contacts. Traditionally, contact resistivity is measured through the Transfer Length Method (TLM). This method presents difficulties when measuring the minority carrier contact due to the presence of a blocking diode which prevents current flow between the pads. Our novel method is performed by creating a unipolar device which has the same contact on both sides (either symmetric n-type or symmetric p-type) and metalizing the device with a grid on one side, and a full metal contact on the back (the same pattern that will be used in a final cell). We then measure the resistance across the device at increasing illumination. Since the resistivity of the wafer is a function of carrier density, when increasing illumination, the wafer resistivity decreases. We can then fit modeling equations to the measured resistivity and extrapolate the fit to infinite illumination where only the contact resistivity is measured to accurately measure both the minority and majority carrier contact resistivity.

current measurement↗
DOE OSTI2014

Beam excited acoustic instability in semiconductor quantum plasmas

The instability of hole-Acoustic waves due to electron beam in semiconductor quantum plasmas is examined using the quantum hydrodynamic model. The quantum effects are considered including Bohm potential, Fermi degenerate pressure, and exchange potential of the semiconductor quantum plasma species. Our model is applied to nano-sized GaAs semiconductor plasmas. The variation of the growth rate of the unstable mode is obtained over a wide range of system parameters. It is found that the thermal effects of semiconductor species have significance over the hole-Acoustic waves.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1988

Models of second-order effects in metal-oxide-semiconductor field-effect transistors for computer applications

Second-order effects in metal-oxide-semiconductor field-effect transistors (MOSFETs) are important for devices with dimensions of 2 microns or less. The short and narrow channel effects and drain-induced barrier lowering primarily affect threshold voltage, but formulas for drain current must also take these effects into account. In addition, the drain current is sensitive to channel length modulation due to pinch-off or velocity saturation and is diminished by electron mobility degradation due to normal and lateral electric fields in the channel. A model of a MOSFET including these considerations and emphasizing charge conservation is discussed.

Benumof, Reuben↗
DOE OSTI2018

Efficiency increased to 15.2% for ultra-thin Cu(In,Ga)Se 2 solar cells

We improved the efficiency of ultra-thin (0.49-um-thick) Cu(In,Ga)Se 2 solar cells to 15.2% (officially measured). To achieve these results, we modified growth conditions from the 3-stage process but did not add post-deposition treatments or additional material layers. The increase in device efficiency is attributed to a steeper Ga gradient in the CIGS with higher Ga content near the Mo back contact, which can hinder electron-hole recombination at the interface. We discuss device measurements and film characterization for ultra-thin CIGS. Modeling is presented that shows the route to even higher efficiencies for devices with CIGS thicknesses of 0.5 um.

14 SOLAR ENERGY↗
DOE OSTI2015

Solitary pulse-on-demand production by optical injection locking of passively Q-switched InGaN diode laser near lasing threshold

In this letter, we investigate the behavior of a Q-switched InGaN multi-section laser diode (MSLD) under optical injection from a continuous wave external cavity diode laser. We obtain solitary optical pulse generation when the slave MSLD is driven near free running threshold, and the peak output power is significantly enhanced with respect to free running configuration. When the slave laser is driven well above threshold, optical injection reduces the peak power. Using standard semiconductor laser rate equation model, we find that both power enhancement and suppression effects are the result of partial bleaching of the saturable absorber by externally injected photons.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗