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[en] Screened-pseudopotential calculations of large ((less-or-similar sign)3000 atoms) surface-passivated Ge quantum dots show that below a critical dot diameter that depends on the passivant, the character of the lowest conduction state changes from an L-derived to an X-derived state. Thus, in this size regime, Ge dots are Si-like. This explains the absence, in a pseudopotential description, of a crossing between the band gaps of Si and Ge dots as a function of size, predicted earlier in single-valley effective-mass calculations. The predicted L→X crossing suggests that small Ge dots will have an X-like, red shift of the band gap with applied pressure, as opposed to an L-like blue shift of large dots. (c) 2000 The American Physical Society
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Physical Review. B, Condensed Matter and Materials Physics; ISSN 1098-0121; ; v. 62(4); p. R2275-R2278
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[en] We report a strongly nonlinear pressure dependence of the band gaps and large downward shifts of the conduction band edges as functions of composition in ZnSxTe 1-x and ZnSeyTe 1-y alloys. The dependencies are explained by an interaction between localized A1 symmetry states of S or Se atoms and the extended states of the ZnTe matrix. These results, combined with previous studies of III-N-V materials define a new, broad class of semiconductor alloys in which the introduction of highly electronegative atoms leads to dramatic modifications of the conduction band structure. The modifications are well described by the recently introduced band anticrossing model. (c)
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[en] A dramatic increase of the conduction band electron mass in a nitrogen-containing III-V alloy is reported. The mass is found to be strongly dependent on the nitrogen content and the electron concentration with a value as large as 0.4m0 in In0.08Ga0.92As0.967N0.033 with 6x1019 cm-3 free electrons. This mass is more than five times larger than the electron effective mass in GaAs and comparable to typical heavy hole masses in III-V compounds. The results provide a critical test and fully confirm the predictions of the recently proposed band anticrossing model of the electronic structure of the III-N-V alloys. (c) 2000 American Institute of Physics
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[en] The maximum free-electron concentration is observed to increase dramatically with the nitrogen content x in heavily Se-doped Ga1-3xIn3xNxAs1-x (0≤x<0.033) films. For example, an electron concentration of 7x1019 cm-3 was observed at x=0.033; a value more than 20 times larger than that observed in GaAs films grown under similar conditions. It is shown that the increase is caused by a combination of two effects: (1) a downward shift of the conduction band and (2) an increase of the electron effective mass caused by flattening of the conduction-band minimum. Both these effects are due to modifications to the conduction-band structure caused by an anticrossing interaction of a localized N state and the conduction band of the III-V host. (c) 2000 The American Physical Society
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Physical Review. B, Condensed Matter and Materials Physics; ISSN 1098-0121; ; v. 61(20); p. R13337-R13340
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[en] Electron-hole exchange interactions can lead to spin-forbidden ''dark'' excitons in direct-gap quantum dots. Here, we explore an alternative mechanism for creating optically forbidden excitons. In a large spherical quantum dot made of a diamond-structure semiconductor, the symmetry of the valence band maximum (VBM) is t2. The symmetry of the conduction band minimum (CBM) in direct-gap material is a1, but for indirect-gap systems the symmetry could be (depending on size) a1, e, or t2. In the latter cases, the resulting manifold of excitonic states contains several symmetries derived from the symmetries of the VBM and CBM (e.g., t2xt2=A1+E+T1+T2 or t2xe=T1+T2). Only the T2 exciton is optically active or ''bright,'' while the others A1, E, and T1 are ''dark.'' The question is which is lower in energy, the dark or bright. Using pseudopotential calculations of the single-particle states of Si quantum dots and a direct evaluation of the screened electron-hole Coulomb interaction, we find that, when the CBM symmetry is t2, the direct electron-hole Coulomb interaction lowers the energy of the dark excitons relative to the bright T2 exciton. Thus, the lowest energy exciton is forbidden, even without an electron-hole exchange interaction. We find that our dark-bright excitonic splitting agrees well with experimental data of Calcott et al., Kovalev et al., and Brongersma et al. Our excitonic transition energies agree well with the recent experiment of Wolkin et al. In addition, and contradicting simplified models, we find that Coulomb correlations are more important for small dots than for intermediate sized ones. We describe the full excitonic spectrum of Si quantum dots by using a many-body expansion that includes both Coulomb and exchange electron hole terms. We present the predicted excitonic spectra. (c) 2000 The American Physical Society
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Physical Review. B, Condensed Matter and Materials Physics; ISSN 1098-0121; ; v. 61(19); p. 13073-13087
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[en] In this article, we use Monte Carlo methods to study the interaction of high power laser pulses with electrons in the conduction band of semiconductors. The laser field is represented by a sinusoidal electric field which tends to cause an oscillatory motion in the electrons. The scattering of electrons from the lattice force the electrons to lose phase coherence with the field. The approach is applied to silicon. We use the approach to examine the carrier energy distribution and material breakdown due to the transfer of energy from the laser to the electrons followed by impact ionization. The impact ionization coefficient, α, and its dependence on the laser frequency and field strength is examined and compared to the values in a dc field. In general, the ac value is smaller than the dc value, but at low frequencies and high field strengths, the ac impact ionization coefficient approaches the dc value at the same rms field value. The importance of collisions in the energy transfer process is elucidated. copyright 1997 American Institute of Physics
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[en] We have studied optical transitions at the Γ and L points of the Brillouin zone of GaNxAs1-x and AlyGa1-yNxAs1-x alloys using photomodulation spectroscopy. For GaNxAs1-x with N contents between 0% and 2%, the N-induced shift of the conduction-band L minima is found to be only a fraction of the conduction-band edge shift at the Γ point. The measurements of AlyGa1-yNxAs1-x further show that there is no correlation between the location of the X conduction-band minima and the observed E+ and E- transitions. The results demonstrate that the N-induced interactions between extended Γ, L, and X conduction-band states do not play a significant role in modification of the conduction-band structure of III-N-V alloys. The N-induced change of the conduction-band structure is predominantly influenced by the anticrossing interaction between the extended states of the Γ conduction band and the localized states of nitrogen. (c) 2000 The American Physical Society
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Physical Review. B, Condensed Matter and Materials Physics; ISSN 1098-0121; ; v. 62(7); p. 4211-4214
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[en] This paper examines the band structure and optical selection rules in superlattices with a sinusoidal potential profile. The analysis is motivated by the recent successful fabrication of high quality ZnSe1-xTex superlattices in which the composition x varies sinusoidally along the growth direction. Although the band alignment in the ZnSe1-xTex sinusoidal superlattices is staggered (type II), they exhibit unexpectedly strong photoluminescence, thus suggesting interesting optical behavior. The band structure of such sinusoidal superlattices is formulated in terms of the nearly-free-electron (NFE) approximation, in which the superlattice potential is treated as a perturbation. The resulting band structure is unique, characterized by a single minigap separating two wide, free-electron-like subbands for both electrons and holes. Interband selection rules are derived for optical transitions involving conduction and valence-band states at the superlattice Brillouin-zone center, and at the zone edge. A number of transitions are predicted due to wave-function mixing of different subband states. It should be noted that the zone-center and zone-edge transitions are especially easy to distinguish in these superlattices because of the large width of the respective subbands. The results of the NFE approximation are shown to hold surprisingly well over a wide range of parameters, particularly when the period of the superlattice is short. (c) 2000 The American Physical Society
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Physical Review. B, Condensed Matter and Materials Physics; ISSN 1098-0121; ; v. 61(16); p. 10978-10984
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BAND THEORY, CONDUCTION BANDS, ELECTRONIC STRUCTURE, II-VI SEMICONDUCTORS, INTERFACE STATES, NEARLY-FREE-ELECTRON APPROXIMATION, OPTICAL PROPERTIES, PHOTOLUMINESCENCE, SEMICONDUCTOR MATERIALS, SEMICONDUCTOR SUPERLATTICES, SUPERLATTICES, THEORETICAL DATA, VALENCE BANDS, ZINC SELENIDES, ZINC TELLURIDES
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[en] We have used photoemission methods to directly measure the valence and conduction band offsets at SrTiO3/Si(001) interfaces, as prepared by molecular-beam epitaxy. Within experimental error, the measured values are the same for growth on n- and p-Si, with the entire band discontinuity occurring at the valence band edge. In addition, band bending is much larger at the p-Si heterojunction than at the n-type heterojunction. Previously published threshold voltage behavior for these interfaces can now be understood in light of the present results. (c) 2000 American Institute of Physics
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CONDUCTION BANDS, ELECTRONIC STRUCTURE, EPITAXIAL LAYERS, EXPERIMENTAL DATA, HETEROJUNCTIONS, INTERFACE STATES, INTERFACES, MOLECULAR BEAM EPITAXIAL GROWTH, MOLECULAR BEAM EPITAXY, PHOTOEMISSION, SEMICONDUCTOR-INSULATOR BOUNDARIES, SILICON, STRONTIUM TITANATES, VALENCE BANDS, X-RAY PHOTOELECTRON SPECTRA
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[en] Time resolved emission polarization spectroscopy has been used to study the electron and hole trapping dynamics in room temperature WS2 nanoclusters. The results indicate that radiative recombination of conduction band electrons and valence band holes results in polarized emission. Hole trapping partially depolarizes the emission, and emission from trapped electrons and holes is unpolarized. The kinetics of electron versus hole trapping can be separated by comparison of the depolarization kinetics in the presence and absence of 2,2'-bipyridine. Bipyridine acts as an acceptor for electrons in the conduction band, but not in traps. The decay of the polarized emission and the rise of the unpolarized emission indicate that electron and hole trapping take place on the 300-500 ps and 30 ps time scales, respectively. Time resolved spectral reconstruction results indicate that hole traps are about 3000 cm-1 deep while electron traps are about 270 cm-1 deep. (c) 2000 American Institute of Physics
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CONDUCTION BANDS, DEEP LEVEL TRANSIENT SPECTROSCOPY, DEEP LEVELS, ELECTRON TRAPS, ELECTRONS, EMISSION SPECTROSCOPY, EXPERIMENTAL DATA, HOLE TRAPS, HOLES, MOLECULAR CLUSTERS, NANOSTRUCTURED MATERIALS, PHOTOLUMINESCENCE, POLARIZATION, TIME RESOLUTION, TIME RESOLVED SPECTRA, TRAPPING, TUNGSTEN SULFIDES, VALENCE BANDS
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