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Abstract A type-II InAs/AlAs $$_{0.16}$$ 0.16 Sb $$_{0.84}$$ 0.84 multiple-quantum well sample is investigated for the photoexcited carrier dynamics as a function of excitation photon energy and lattice temperature. Time-resolved measurements are performed using a near-infrared pump pulse, with photon energies near to and above the band gap, probed with a terahertz probe pulse. The transient terahertz absorption is characterized by a multi-rise, multi-decay function that captures long-lived decay times and a metastable state for an excess-photon energy of $$>100$$ > 100 meV. For sufficient excess-photon energy, excitation of the metastable state is followed by a transition to the long-lived states. Excitation dependence of the long-lived states map onto a nearly-direct band gap ( $$E{_g}$$ E g ) density of states with an Urbach tail below $$E{_g}$$ E g . As temperature increases, the long-lived decay times increase $$E{_g}$$ > E g . Additionally, Auger (and/or trap-assisted Auger) scattering above the onset of the plateau may also contribute to longer hot-carrier lifetimes. Meanwhile, the initial decay component shows strong dependence on excitation energy and temperature, reflecting the complicated initial transfer of energy between valence-band and defect states, indicating methods to further prolong hot carriers for technological applications.

Abstract Hot electrons established by the absorption of high-energy photons typically thermalize on a picosecond time scale in a semiconductor, dissipating energy via various phonon-mediated relaxation pathways. Here it is shown that a strong hot carrier distribution can be produced using a type-II quantum well structure. In such systems it is shown that the dominant hot carrier thermalization process is limited by the radiative recombination lifetime of electrons with reduced wavefunction overlap with holes. It is proposed that the subsequent reabsorption of acoustic and optical phonons is facilitated by a mismatch in phonon dispersions at the InAs-AlAsSb interface and serves to further stabilize hot electrons in this system. This lengthens the time scale for thermalization to nanoseconds and results in a hot electron distribution with a temperature of 490 K for a quantum well structure under steady-state illumination at room temperature.

Epitaxial NbO2 (1 1 0) films, 20 nm thick, were grown by pulsed laser deposition on Al2O3 (0 0 0 1) substrates. The Ar/O2 total pressure during growth was varied to demonstrate the gradual transformation between NbO2 and Nb2O5 phases, which was verified using x-ray diffraction, x-ray photoelectron spectroscopy, and optical absorption measurements. Electric resistance threshold switching characteristics were studied in a lateral geometry using interdigitated Pt top electrodes in order to preserve the epitaxial crystalline quality of the films. Volatile and reversible transitions between high and low resistance states were observed in epitaxial NbO2 films, while irreversible transitions were found in the case of Nb2O5 phase. Electric field pulsed current measurements confirmed thermally-induced threshold switching. [ABSTRACT FROM AUTHOR]