The Experts below are selected from a list of 297 Experts worldwide ranked by ideXlab platform
Alex Zunger - One of the best experts on this subject based on the ideXlab platform.
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quantum size induced electronic transitions in quantum dots indirect band gap gaas
Physical Review B, 2008Co-Authors: Junwei Luo, Alberto Franceschetti, Alex ZungerAbstract:We discuss the physical origin of the previously predicted quantum-size-induced electronic transitions in spherical GaAs quantum dots. By using atomistic pseudopotential calculations for freestanding GaAs dots and for GaAs dots embedded in an AlGaAs matrix, we are able to distinguish two types of direct/indirect transitions: (i) in freestanding GaAs dots, the conduction-band minimum changes from $\ensuremath{\Gamma}$-like to $X$-like as the radius of the dot is reduced below 1.6 nm, leading to a direct/indirect transition in reciprocal space. (ii) In GaAs dots embedded in AlAs, the conduction-band minimum changes from dot localized to barrier localized as the radius of the dot is reduced below 4.2 nm, corresponding to a direct-to-indirect transition in real space.
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the inverse band structure problem of finding an atomic configuration with given electronic properties
Nature, 1999Co-Authors: Alberto Franceschetti, Alex ZungerAbstract:Modern crystal-growth techniques, such as molecular beam epitaxy or metal–organic chemical-vapour deposition, are capable of producing prescribed crystal structures, sometimes even in defiance of equilibrium, bulk thermodynamics. These techniques open up the possibility of exploring different atomic arrangements in search of a configuration that possesses given electronic and optical properties1. Unfortunately, the number of possible combinations is so vast, and the electronic properties are so sensitive to the details of the crystal structure, that simple trial-and-error methods (such as those used in combinatorial synthesis2) are unlikely to be successful. Here we describe a theoretical method that addresses the problem of finding the atomic configuration of a complex, multi-component system having a target electronic-structure property. As an example, we predict that the configuration of an Al0.25Ga0.75As alloy having the largest optical bandgap is a (GaAs)2(AlAs)1(GaAs)4(AlAs)1 superlattice oriented in the [201] direction.
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identity of the conduction band minimum in alas 1 gaas 1 001 superlattices intermixing induced reversal of states
Physical Review B, 1992Co-Authors: D B Laks, Alex ZungerAbstract:First-principles pseudopotential calculations on the (001) (AlAs){sub 1}/(GaAs){sub 1} superlattice (SL) shows that {ital partial} intermixing of the Al and Ga atoms relative to the abrupt case lowers its formation energy, making this SL even stabler at low {ital T} than the fully randomized Al{sub 0.5}Ga{sub 0.5}As alloy. Concomitantly, the conduction-band minimum (CBM) reverts from the GaAs {ital L}-derived state, to the {ital X}{sup {ital x}{ital y}}-derived AlAs state. The previously noted discrepancy between theory (pertinent to abrupt SL's and yielding an {ital L}-derived CBM) and experiment (yielding an {ital X}{sup {ital x}{ital y}}-derived CBM) is therefore attributed to insufficient interfacial abruptness in the samples used to date in experimental studies.
Gye Mo Yang - One of the best experts on this subject based on the ideXlab platform.
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ultralow threshold current vertical cavity surface emitting lasers obtained with selective oxidation
Electronics Letters, 1995Co-Authors: Gye Mo Yang, M H Macdougal, P D DapkusAbstract:The authors report InGaAs single quantum well vertical-cavity surface-emitting lasers with an intracavity p-contact fabricated by selective oxidation of AlAs and distributed Bragg reflectors composed of binary materials (AlAs/GaAs). Record low threshold currents of 8.7 µA in ~3 µm square devices and 140 µA in 10 µm square devices with maximum output powers over 1.2 mW are achieved.
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ultralow threshold current vertical cavity surface emitting lasers with alas oxide gaas distributed bragg reflectors
IEEE Photonics Technology Letters, 1995Co-Authors: M H Macdougal, P D Dapkus, V Pudikov, H Zhao, Gye Mo YangAbstract:An electrically-pumped, vertical-cavity, surface-emitting laser (VCSEL) using an AlAs oxide-GaAs DBR above the AlGaAs-GaAs-InGaAs gain region and a conventional AlAs-GaAs DBR below is described. By selective oxidation, devices with current flow apertures of different areas are fabricated, and in 8-/spl mu/m-square devices, threshold currents as low as 0.22 mA are achieved. Being the first electrically-pumped VCSEL to utilize the oxide-based DBR, it demonstrates that the oxide-based DBR is of sufficient quality to realize submilliampere threshold currents. >
Masaaki Nakayama - One of the best experts on this subject based on the ideXlab platform.
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bayesian spectroscopy of admixed photoluminescence spectra with exciton biexciton and electron hole droplet states in a gaas alas type ii superlattice
Journal of Luminescence, 2018Co-Authors: Kazunori Iwamitsu, Masaaki Nakayama, Yoshiaki Furukawa, Masato Okada, I AkaiAbstract:Abstract We have analyzed a photoluminescence (PL) spectrum in a GaAs/AlAs type-II superlattice and have statistically confirmed stable existence of an electron-hole droplet (EHD) state. Although the PL spectrum consists of spectral components of the EHD, exciton and biexciton states, we have succeeded in decomposing the admixed spectrum into the respective components. In addition, posterior probability distributions of spectral parameters have been obtained in all spectral components from 30,000 times samplings through a replica exchange Monte Carlo method. By using mean values of their posterior probabilities, the PL spectrum can be completely reproduced by the sum of these spectral components. The stability energy of the EHD state has a sharp distribution in its posterior probability and has been confirmed to be enough larger than the ambient thermal energy. This fact is a strong evidence for the stable formation of the EHD state in the GaAs/AlAs type-II superlattice.
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intense terahertz radiation from longitudinal optical phonons in gaas alas multiple quantum wells
Applied Physics Letters, 2005Co-Authors: Kohji Mizoguchi, T Furuichi, Osamu Kojima, Masaaki Nakayama, Shingo Saito, A Syouji, Kiyomi SakaiAbstract:We report the generation of intense terahertz (THz) electromagnetic waves from GaAs∕AlAs multiple quantum wells (MQWs) using ultrashort laser pulses. Pronounced THz waves from the coherent GaAs-like longitudinal optical (LO) phonons with a frequency of 8.8 THz were observed in the time region over 6 ps, which indicates the long dephasing time. The power of the THz waves from the coherent GaAs-like LO phonons in the GaAs∕AlAs MQW was enhanced to be about 100 times larger than that of those from coherent LO phonons in an epitaxial-growth GaAs film. We discuss the origin of the enhancement of the THz wave from coherent GaAs-like LO phonon in the MQW.
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characteristics of coupled mode of excitonic quantum beat and coherent longitudinal optical phonon in gaas alas multiple quantum wells
Journal of Luminescence, 2005Co-Authors: T Furuichi, Kohji Mizoguchi, Osamu Kojima, Kouichi Akahane, Naokatsu Yamamoto, Naoki Ohtani, Masaaki NakayamaAbstract:We have investigated the coupled mode of excitonic quantum beats and coherent GaAs-like longitudinal optical (LO) phonons in GaAs/AlAs multiple quantum wells with the splitting energy of heavy- and light-hole excitons close to the LO phonon energy. The time-domain signals show the coherent oscillation of the coupled mode between the quantum beat and the LO phonon with a fast dephasing time and the long-lived coherent GaAs-like LO phonon oscillation. The dependence of the coupled-mode frequency on splitting energy exhibits an anticrossing behavior. We demonstrate that the amplitude of the long-lived coherent LO phonon is manipulated by controlling the coupled mode with a double-pulse excitation technique.
Humphrey J Maris - One of the best experts on this subject based on the ideXlab platform.
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phonon group velocity and thermal conduction in superlattices
Physical Review B, 1999Co-Authors: Shin-ichiro Tamura, Yukihiro Tanaka, Humphrey J MarisAbstract:With the use of a face-centered cubic model of lattice dynamics we calculate the group velocity of acoustic phonons in the growth direction of periodic superlattices. Comparing with the case of bulk solids, this component of the phonon group velocity is reduced due to the flattening of the dispersion curves associated with Brillouin-zone folding. The results are used to estimate semiquantitatively the effects on the lattice thermal conductivity in Si/Ge and GaAs/AlAs superlattices. For a Si/Ge superlattice an order of magnitude reduction is predicted in the ratio of superlattice thermal conductivity to phonon relaxation time [consistent with the results of P. Hyldgaard and G. D. Mahan, Phys. Rev. B {bold 56}, 10h754 (1997)]. For a GaAs/AlAs superlattice the corresponding reduction is rather small, i.e., a factor of 2{endash}3. These effects are larger for the superlattices with larger unit period, contrary to the recent measurements of thermal conductivity in superlattices. {copyright} {ital 1999} {ital The American Physical Society}
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thermal conductivity measurements of gaas alas superlattices using a picosecond optical pump and probe technique
Physical Review B, 1999Co-Authors: W S Capinski, M Cardona, K Ploog, Humphrey J Maris, T Ruf, D S KatzerAbstract:We present measurements of the lattice thermal conductivity ${\ensuremath{\kappa}}_{\ensuremath{\perp}}$ normal to the interfaces of $(\mathrm{GaAs}{)}_{n}/(\mathrm{AlAs}{)}_{n}$ superlattices with n between 1 and 40 monolayers. The conductivity was measured by an optical pump-and-probe technique in the temperature range of 100 to 375 K. In the experiment, an Al film is deposited onto a superlattice sample, and the rate at which this film cools by conduction into the superlattice is determined. We find a general decrease in ${\ensuremath{\kappa}}_{\ensuremath{\perp}}$ with a reduction of the superlattice period. At 300 K, ${\ensuremath{\kappa}}_{\ensuremath{\perp}}$ of the $(\mathrm{GaAs}{)}_{40}/(\mathrm{AlAs}{)}_{40}$ superlattice is approximately three times less than $\ensuremath{\kappa}$ of bulk GaAs, and ${\ensuremath{\kappa}}_{\ensuremath{\perp}}$ of the $(\mathrm{GaAs}{)}_{1}/(\mathrm{AlAs}{)}_{1}$ superlattice is an order of magnitude less than $\ensuremath{\kappa}$ of bulk GaAs. We discuss the decrease in ${\ensuremath{\kappa}}_{\ensuremath{\perp}}$ compared to the bulk constituents in terms of extrinsic and intrinsic scattering mechanisms.
Shin-ichiro Tamura - One of the best experts on this subject based on the ideXlab platform.
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phonon group velocity and thermal conduction in superlattices
Physical Review B, 1999Co-Authors: Shin-ichiro Tamura, Yukihiro Tanaka, Humphrey J MarisAbstract:With the use of a face-centered cubic model of lattice dynamics we calculate the group velocity of acoustic phonons in the growth direction of periodic superlattices. Comparing with the case of bulk solids, this component of the phonon group velocity is reduced due to the flattening of the dispersion curves associated with Brillouin-zone folding. The results are used to estimate semiquantitatively the effects on the lattice thermal conductivity in Si/Ge and GaAs/AlAs superlattices. For a Si/Ge superlattice an order of magnitude reduction is predicted in the ratio of superlattice thermal conductivity to phonon relaxation time [consistent with the results of P. Hyldgaard and G. D. Mahan, Phys. Rev. B {bold 56}, 10h754 (1997)]. For a GaAs/AlAs superlattice the corresponding reduction is rather small, i.e., a factor of 2{endash}3. These effects are larger for the superlattices with larger unit period, contrary to the recent measurements of thermal conductivity in superlattices. {copyright} {ital 1999} {ital The American Physical Society}
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Phonon group velocity and thermal conduction in superlattices
Physical Review B, 1999Co-Authors: Shin-ichiro Tamura, Yukihiro Tanaka, H. MarisAbstract:With the use of a face-centered cubic model of lattice dynamics we calculate the group velocity of acoustic phonons in the growth direction of periodic superlattices. Comparing with the case of bulk solids, this component of the phonon group velocity is reduced due to the flattening of the dispersion curves associated with Brillouin-zone folding. The results are used to estimate semiquantitatively the effects on the lattice thermal conductivity in Si/Ge and GaAs/AlAs superlattices. For a Si/Ge superlattice an order of magnitude reduction is predicted in the ratio of superlattice thermal conductivity to phonon relaxation time consistent with the results of P. Hyldgaard and G. D. Mahan, Phys. Rev. B 56, 10 754 ~1997!. For a GaAs/AlAs superlattice the corresponding reduction is rather small, i.e., a factor of 23. These effects are larger for the superlattices with larger unit period, contrary to the recent measurements of thermal conductivity in superlattices.
