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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

All-Electric Laser Beam Control Based on a Quantum-Confined Heterostructure with an Integrated Distributed Bragg Grating

The characteristics of semiconductor chip modulators providing the angular deflection of a laser beam due to all-electric modulation of the optical characteristics of a semiconductor heterostructure are theoretically studied. Designs for quantum-confined semiconductor waveguide heterostructures with a distributed Bragg reflector integrated on the heterostructure surface are developed. The design of a waveguide structure including 20 periods of coupled asymmetric quantum wells, which provide an optical confinement factor of the waveguide mode of 20%, and an optimized doping profile allowing a uniform electric-field distribution to be maintained in the quantum-well region in the entire range of operating voltages are proposed. In this waveguide structure, the change in the refractive index due to the quantum-confined Stark effect reaches 0.086 when the control signal changes from 0 to 6 V. It is shown that the spatial (angular) scanning of the laser beam emitted from the surface of a distributed Bragg mirror with a divergence of less than 0.1° in the range of 1°–2° is feasible for the proposed waveguide structure design.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Seebeck Coefficients of Layered BiCuSeO Phases: Analysis of Their Hole-Density Dependence and Quantum Confinement Effect

Hole-doped layered BiCuSeO phases include substitutionally doped Bi 1–x AxCuSeO (A = alkali, alkaline earth) as well as vacancy-doped Bi 1-δ Cu 1-γ SeO and Bi 1-δ CuSeO. To probe how their Seebeck coefficients are related to their hole density p, we calculated the Seebeck coefficient for defect-free BiCuSeO as a function of the hole density, which is generated by lowering the Fermi level from the valence band maximum (VBM). In addition, we calculated the Seebeck coefficient for Bi 1-δ CuSeO (δ = 1/32, 1/16) with a large number of Bi vacancies. The Seebeck coefficients of the hole-doped BiCuSeO phases are governed by the electronic states lying within ~0.5 eV from the VBM. These states are composed of largely Cu 3d xz/yz and Se 4p x/y states and possess the character of a uniform one-dimensional (1D) chain rather than a uniform two-dimensional (2D) lattice expected for a layered phase. The observed S-vs-p relationship for Bi 1–x A x CuSeO (A = alkali, alkaline earth) as well as Bi 1-δ Cu 1-γ SeO (δ = 0, 0.025; γ = 0, 0.025) and Bi 1-δ CuSeO (δ = 0.0, 0.025) is very well reproduced by the calculated relationship for defect-free BiCuSeO within the rigid band approximation. The observed S-vs-p relationship reflects the quantum confinement effect of uniform 1D chains, despite that the hole-doped BiCuSeO phases consist of 2D layers, (Cu 2 Se 2 ) 2– and (Bi 2 O 2 ) 2+ . The drastic decrease in the S values of Bi 1-δ CuSeO with large δ (= 0.05, 0.075, 0.10) arises from the loss of the quantum confinement effect in the (Cu 2 Se 2 ) 2– layers; that is, the uniform 1D chain character is lost because of their geometry distortion induced by a large number of Bi vacancies.

Lee, Changhoon↗
DOE OSTI2024

Intraband Cooling and Auger Recombination in Weakly to Strongly Quantum-Confined CsPbBr 3 Perovskite Nanocrystals

Semiconductor nanocrystals (NCs) with size-tuned energy gaps present unique and desirable properties for optoelectronic applications. Recent synthetic advancements offer routes to spheroidal CsPbBr 3 perovskite NCs in the strong quantum confinement regime with narrow size dispersion. Further, using tunable femtosecond laser pulses, we examine intraband carrier relaxation using transient absorption spectroscopy and show that, across the transition from weak to strong confinement, hot carrier lifetime increases compared to larger bulk-like particles. However, further increases of confinement subsequently lead to a reduction of the hot carrier lifetime and increase of the non-radiative Auger recombination rate. Finally, we show that hot carrier lifetimes increase as a function of excess energy above the band gap less sensitively under high confinement in comparison to the bulk. Understanding such unique trends is important for maximizing hot carrier lifetimes for use in next-generation hot carrier devices as well as evaluating the transition from weak to strong confinement.

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

Quantum confinement in self-assembled two-dimensional nanoporous honeycomb networks at close-packed metal surfaces

Quantum confinement of a two-dimensional electron gas by supramolecular nanoporous networks is investigated using the boundary elements method based on Green’s functions for finite geometries and electron plane wave expansion for periodic systems. The “particle in a box” picture was analyzed for cases with selected symmetries that model previously reported architectures constructed from organic and metal-organic scattering centers confining surface state electrons of Ag(111) and Cu(111). First, by analyzing a series of cases with systematically defined parameters (scattering geometry, potentials, and effective broadening), we demonstrate how the scattering processes affect the properties of the confined electrons. For the features of the local density of states reported by scanning tunneling spectroscopy (STS), we disentangle the contributions of lifetime broadening and splitting of quantum well states due to coupling of neighboring quantum dots. For each system, we analyze the local electron density distribution and relate it to the corresponding band structure as calculated within the plane-wave expansion framework. Then, we address two experimental investigations, where in one case only STS data and in the other case mainly angle-resolved photoemission spectroscopy (ARPES) data were reported. In both cases, the experimental findings can be successfully simulated. Furthermore, the missing information can be complemented because our approach allows to correlate the information obtained by STS with that of ARPES. The combined analysis of several observations suggests that the scattering potentials created by the network originate primarily from the adsorbate-induced changes of the local surface dipole barrier.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Simulation of an electro-optic modulator based on the quantum-confined Stark effect in a two-layer InAlGaAs rib waveguide on an InP substrate

An electro-optic Mach – Zehnder modulator based on the quantum-confinement Stark effect in layered multiple-quantum-well InAlGaAs structures on InP substrate is numerically simulated. The novelty of the modulator design is the use of a two-layer rib optical waveguides, which provide technologically efficient (loss of 1.5 dB per element) coupling of the modulator with optical fibre. It is shown that, changing the electrode width and the distance between two waveguides in two Mach – Zehnder interferometer arms, one can match the impedance of the modulator with an external 50-Ω load and the velocities of microwave and optical waves in the modulator. The results of the study can be used in integrated optics, optical communication, and radiophotonics devices. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Impact of field-induced quantum confinement on the onset of tunneling field-effect transistors: Experimental verification

The Tunneling Field-Effect Transistor (TFET) is a promising device for future low-power logic. Its performance is often predicted using semiclassical simulations, but there is usually a large discrepancy with experimental results. An important reason is that Field-Induced Quantum Confinement (FIQC) is neglected. Quantum mechanical simulations show FIQC delays the onset of Band-To-Band Tunneling (BTBT) with hundreds of millivolts in the promising line-TFET configuration. In this letter, we provide experimental verification of this delayed onset. We accomplish this by developing a method where line-TFET are modeled using highly doped MOS capacitors (MOS-CAP). Using capacitance-voltage measurements, we demonstrate AC inversion by BTBT, which was so far unobserved in MOS-CAP. Good agreement is shown between the experimentally obtained BTBT onset and quantum mechanical predictions, proving the need to include FIQC in all TFET simulations. Finally, we show that highly doped MOS-CAP is promising for characterization of traps deep into the conduction band.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Assessment of field-induced quantum confinement in heterogate germanium electron–hole bilayer tunnel field-effect transistor

The analysis of quantum mechanical confinement in recent germanium electron–hole bilayer tunnel field-effect transistors has been shown to substantially affect the band-to-band tunneling (BTBT) mechanism between electron and hole inversion layers that constitutes the operating principle of these devices. The vertical electric field that appears across the intrinsic semiconductor to give rise to the bilayer configuration makes the formerly continuous conduction and valence bands become a discrete set of energy subbands, therefore increasing the effective bandgap close to the gates and reducing the BTBT probabilities. In this letter, we present a simulation approach that shows how the inclusion of quantum confinement and the subsequent modification of the band profile results in the appearance of lateral tunneling to the underlap regions that greatly degrades the subthreshold swing of these devices. To overcome this drawback imposed by confinement, we propose an heterogate configuration that proves to suppress this parasitic tunneling and enhances the device performance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Quantum confinement of zero-dimensional hybrid organic-inorganic polaritons at room temperature

We report on the quantum confinement of zero-dimensional polaritons in perovskite-based microcavity at room temperature. Photoluminescence of discrete polaritonic states is observed for polaritons localized in symmetric sphere-like defects which are spontaneously nucleated on the top dielectric Bragg mirror. The linewidth of these confined states is found much sharper (almost one order of magnitude) than that of photonic modes in the perovskite planar microcavity. Our results show the possibility to study organic-inorganic cavity polaritons in confined microstructure and suggest a fabrication method to realize integrated polaritonic devices operating at room temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Quantum confinement effect in Bi anti-dot thin films with tailored pore wall widths and thicknesses

We investigated quantum confinement effects in Bi anti-dot thin films grown on anodized aluminium oxide templates. The pore wall widths (w{sub Bi}) and thickness (t) of the films were tailored to have values longer or shorter than Fermi wavelength of Bi (λ{sub F} = ∼40 nm). Magnetoresistance measurements revealed a well-defined weak antilocalization effect below 10 K. Coherence lengths (L{sub ϕ}) as functions of temperature were derived from the magnetoresistance vs field curves by assuming the Hikami-Larkin-Nagaoka model. The anti-dot thin film with w{sub Bi} and t smaller than λ{sub F} showed low dimensional electronic behavior at low temperatures where L{sub ϕ}(T) exceed w{sub Bi} or t.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Quantum confinement of a hydrogenic donor in a double quantum well: Through diamagnetic susceptibility

Diamagnetic susceptibility of a randomly distributed donor in a GaAs/Al{sub 0.3}Ga{sub 0.7}As Double Quantum Well has been calculated in its ground state as a function of barrier and well width. It is shown that the modification in the barrier and well dimension significantly influences the dimensional character of the donor through modulating the subband distribution and in turn the localization of the donor. The effect of barrier and well thickness on the interparticle distance has also been observed. Interestingly it opens up the possibility of tuning the susceptibility and monitoring the tunnel coupling among the wells.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Effect of (Al)GaAs/AlGaAs quantum confinement region parameters on the threshold current density of laser diodes

An approach has been proposed for choosing parameters (width and depth) of an (Al)GaAs/AlGaAs quantum confinement region via calculation of the threshold current density of a semiconductor laser. A detailed assessment of its components has made it possible to find criteria for optimising the range of quantum well widths so as to minimise the threshold current for lasers with different heterostructure geometries. The data presented in this paper demonstrate the feasibility of further improving output characteristics of semiconductor lasers by optimising the design and technology of quantum well heterostructures. Owing to this, we have simultaneously reduced the threshold current, carrier escape and internal optical loss, which has allowed us to obtain high (60 % to 70 %) efficiencies of a semiconductor laser operating in the spectral range 800 – 850 nm. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Internal Field Shielding and the Quantum Confined Stark Effect in a Single In{sub x}Ga{sub 1-x}N Quantum Disk

Time-integrated and time-resolved microphotoluminescence studies were carried out on In{sub x}Ga{sub 1-x}N quantum disks embedded in GaN nanocolumns grown by molecular beam epitaxy. Emission at {approx}3.33 eV from confined states was detected and observed to blueshift with excitation power; a result of charge screening and the quantum confined Stark effect. The lifetime of the emission was measured to decrease with increasing excitation power, attributed to reduced band bending and resulting increased overlap of the confined electron and hole wave functions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Features of the electroluminescence spectra of quantum-confined silicon p{sup +}-n heterojunctions in the infrared spectral region

The results of studying the characteristics of optical emission in various regions of quantum-confined silicon p{sup +}-n heterojunctions heavily doped with boron are analyzed. The results obtained allow one to conclude that near-infrared electroluminescence arises near the heterointerface between the nanostructured wide-gap silicon p{sup +}-barrier heavily doped with boron and n-type silicon (100), the formation of which included the active involvement of boron dipole centers.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Enhanced thermoelectric performance via quantum confinement in a metal oxide semiconductor field effect transistor for thermal management

Abstract The performance of thermoelectric devices is gauged by the dimensionless figure of merit $${{{{{\boldsymbol{ZT}}}}}}$$ ZT . Improving $${{{{{\boldsymbol{ZT}}}}}}$$ ZT has proven to be a formidable challenge given the interdependence of its constitutive quantities, namely Seebeck coefficient, electrical conductivity, thermal conductivity, and temperature. Here, we use quantum confinement to decouple Seebeck coefficient and electrical conductivity to demonstrate an order of magnitude quantum-based enhancement to the thermoelectric figure of merit $${{{{{\boldsymbol{ZT}}}}}}$$ ZT in complimentary metal-oxide-semiconductor field-effect transistors. While most quantum-based enhancement is done through physical confinement using two-dimensional materials, our approach uses electrical confinement. Because of this, our device is more robust than the two-dimensional materials currently used. We further articulate that improvement by as much as a factor of 50 could be achieved in a practical setting. Our approach further provides a path for monolithic integration of on-chip coolers and energy scavengers with virtually no deviation from the fabrication flow of standard electronics.

Oxandale, Samuel W. (ORCID:0000000207484066)↗
DOE OSTI2018

Spatial and Quantum Confinement of Si Nanoparticles Deposited by Laser Electrodispersion onto Crystalline Si

C–V characteristics and DLTS spectra of heterostructures made up of layers of closely packed amorphous Si nanoparticles deposited by laser electrodispersion onto single-crystal p-Si substrates have been examined. The patterns observed in the behavior of the C–V characteristics and DLTS spectra measured in the dark and under illumination with white light at various bias pulse voltages U{sub b} and filling pulse voltages U{sub f} suggest that the spatially localized amorphous Si nanoparticles have an average size of less than 2 nm, which is comparable with the de Broglie electron wavelength, and are characterized by quantum confinement. The ground and excited states of quantum dots are formed and exhibit the Stark effect and effects of electricdipole and controllable metastable occupancy under illumination.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Recombination in GaAs p–i–n Structures with InGaAs Quantum-Confined Objects: Modeling and Regularities

Photovoltaic structures on the basis of GaAs p–i–n junctions with a different number of In{sub 0.4}Ga{sub 0.6}As layers in the space-charge region forming quantum-confined objects are experimentally and theoretically investigated. For all structures, the dependences of the open-circuit voltage on the solar-irradiation concentration are analyzed. It is shown that the implantation of quantum objects leads to the dominance of recombination in them over recombination in the matrix, which manifests itself in a drop in the open-circuit-voltage. An increase in the number of In{sub 0.4}Ga{sub 0.6}As layers leads to a proportional increase in the recombination rate, which is expressed in a proportional increase in the “saturation” current and corresponds to the model proposed in the study.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Effects of strain and quantum confinement in optically pumped nuclear magnetic resonance in GaAs: Interpretation guided by spin-dependent band structure calculations

A combined experimental-theoretical study of optically pumped NMR (OPNMR) has been performed in a GaAs/Al 0.1 Ga 0.9 As quantum well film with thermally induced biaxial strain. The photon energy dependence of the Ga-71 OPNMR signal was recorded at magnetic fields of 4.9 and 9.4 T at a temperature of 4.8-5.4 K. The data were compared to the nuclear spin polarization calculated from differential absorption to spin-up and spin-down states of the conduction band using a modified Pidgeon Brown model. Reasonable agreement between theory and experiment is obtained, facilitating assignment of features in the OPNMR energy dependence to specific interband transitions. Despite the approximations made in the quantum-mechanical model and the inexact correspondence between the experimental and calculated observables, the results provide insight into how effects of strain and quantum confinement are manifested in OPNMR signals

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Enhancement of thermoelectric performance in InAs nanotubes by tuning quantum confinement effect

By using the nonequilibrium Green's function method, we study the thermoelectric properties of InAs nanotubes. The results show that InAs nanotube with a certain internal diameter has much higher ZT value than nanowire due to the enhancement of quantum confinement effect leading to the increase of the power factor S{sup 2}G. The ZT value of InAs nanotube can reach 1.74, which is about three times greater than that of nanowires. Moreover, it is found that the ZT values of InAs nanotubes decrease rapidly with the increase of internal diameter, which results from the rapid increase of phonons thermal conductance due to the “red shift” of low-frequency optical phonon modes.

77 NANOSCIENCE AND NANOTECHNOLOGY↗