Terahertz intersubband absorption and conduction band alignment inn-type Si/SiGe multiple quantum wells

Abstract

Absorption due to conduction intersubband transitions is studied in n-type s-Si/SiGe multiquantum wells ͑MQW͒ of different well widths and barrier composition grown by UHV-chemical vapor deposition ͑CVD͒. The measured intersubband transition energies are compared with the theoretical results of a tight-binding model which provides the electronic band structure of the complete MQW system throughout the whole Brillouin zone. Our findings demonstrate both the high quality of the CVD grown MQWs and the effectiveness of the adopted tight-binding model in describing band profiles and electronic structures of SiGe multilayer systems. In particular we have evaluated the conduction band offsets in the investigated structures.

Electron Dynamics in Silicon–Germanium Terahertz Quantum Fountain Structures

“…Starting from Figure 4a, we see that sample 1955 displays an intense ISB absorption (approximately 40% maximum attenuation) centered at 26 meV, compatible with the calculated value of 23 meV, and full width at half-maximum of 4.4 meV that compares well with that found in previous works. 50,51 The much weaker 1 → 2 transition falls at E 12 = 25 meV, well inside the 0 → 1 transition dip, and it is therefore not observable even at room-T. In Figure 4b, the transmission spectra of sample 1953 are shown.…”

“… We verified that the Δ 2 -valley minima in the barriers are higher in energy than level 2, so their presence can be neglected (see Supporting Infromation). We have varied the heterostructure parameters within the experimentally available ranges determined in our previous works. − In germanium, there exist two main nonpolar optical phonon branches that provide fast, although nonresonant, energy relaxation channels to electrons in the excited states: the zone-center phonon at E ph ,Γ = 37 meV, which features negligible momentum transfer to the electrons that therefore remain in the same valley (intravalley phonon relaxation), and the [111] zone-boundary phonon at E ph ,Λ = 25 meV that connects electronic states in two different valleys among the four equivalent valleys centered at the L -point of the Fermi surface of electron-doped germanium (intervalley phonon relaxation). Without entering all the details of the electron–phonon interaction, − here we just recall that E ph ,Λ and E ph ,Γ set the two main energy thresholds for optical phonon emission by electrons: because of energy conservation, the nonradiative ISB transitions with energy smaller than E ph ,Λ are driven by the much slower emission of acoustic phonons .…”

Experimental Evidence for Magnetic Exchange in Di- and Trinuclear Uranium(IV) Ethynylbenzene Complexes

“…This mixture was warmed to room temperature and stirred for 3 h. The yellow precipitate was collected by filtration, dried in vacuo, and recrystallized from hot toluene to afford a yellow-green crystalline solid (0.362 g, 89% based on p-H 2 DEB). Single crystals suitable for X-ray analysis were grown from a concentrated toluene solution maintained at -34 °C for 8 h. Absorption spectrum (toluene) λ max (ε M ): 312 (20700), 329 (15800), 363 (4200), 503 (77), 529 (116), 587 (47), 614 (30), 621 (31), 629 (28), 650 (24), 658 (25), 687 (150), 691 (135), 719 (34), 803 (13), 828 (16), 880 (15), 924 (15), 961 nm (12 L 3 mol -1 3 cm -1 ). 1 [(NN 0 3 ) 3 U 3 (TEB)] (6).…”

“…In addition to fundamental interest in electronic structure, recent work in f-element magnetochemistry is motivated by the potential for these species to contribute to the development of single-molecule magnets (SMMs). − These monodisperse superparamagnetic particles exhibit a thermal barrier to magnetic spin reorientation, and may eventually find use in data storage, , quantum computing, − or refrigeration applications. , However, their exploitation awaits variants that can display magnetic bistability at more practical temperatures than the ∼4.5 K currently observed . Here, incorporation of paramagnetic lanthanide ions have received attention, since spin−orbit coupling and relativistic effects common to those ions can engender the large single-ion anisotropies necessary for slow magnetization relaxation behavior. − Several complexes have properties consistent with SMMs, such as the observation of frequency-dependent out-of-phase alternating current (AC) susceptibility signals. − A drawback to the approach is that the “buried” 4f orbitals in lanthanides participate only weakly in bonding interactions, leading to marginal exchange coupling with neighboring spin centers; − this ultimately limits the maximum temperature at which the magnetic bistability occurs.…”

“…[12][13][14][15] These monodisperse superparamagnetic particles exhibit a thermal barrier to magnetic spin reorientation, and may eventually find use in data storage, 16,17 quantum computing, [18][19][20][21][22][23] or refrigeration applications. 24,25 However, their exploitation awaits variants that can display magnetic bistability at more practical temperatures than the ∼4.5 K currently observed. 15 Here, incorporation of paramagnetic lanthanide ions have received attention, since spin-orbit coupling and relativistic effects common to those ions can engender the large single-ion anisotropies necessary for slow magnetization relaxation behavior.…”