Effect of electron-electron and electron-hole collisions on intraband population inversion of electrons in stepped quantum wells

The effect of intersubband electron-electron (e-e) and electron-hole (e-h) scattering on intraband population inversion of electrons in a stepped InGaAs/AlGaAs quantum well is investigated. The characteristic times of the most probable e-e and e-h processes, which affect the electron densities on the excited levels, are calculated for the temperature range 80–300 K. Dependences of these times on the electron and hole density on the ground levels are studied. Temperature dependences of the intraband inversion of population for two nonequilibrium densities are calculated by solving a system of rate equations. It is shown that the intersubband e-e and e-h scattering only slightly affects the population inversion for electron densities below 1×10¹² cm^?2.     

Carrier heating in quantum wells under optical and current injection of electron-hole pairs

Carrier heating in GaAs/AlGaAs quantum wells (QWs) under optical interband pumping in the spontaneous-emission mode has been studied. The electron temperature was determined as a function of the pumping intensity. The effect of the electric field on the photoluminescence spectrum was examined. The change in the carrier concentration with the drive current in the spontaneous- and stimulated-emission modes in InGaAsSb/InAlGaAsSb QWs was determined from electroluminescence spectra. The rise in the temperature of hot carriers, which results in the increase in the carrier concentration with the drive current, was roughly estimated.     

Electron-electron scattering in stepped quantum wells

A method for calculating the probability of intersubband electron-electron scattering in quantum wells of complex shape is suggested. Numerical data for stepped InGaAs/AlGaAs quantum wells are obtained. The principal mechanisms of electron-electron scattering that exert the strongest effect on the intersubband inversion of population in laser structures are determined.     

Charge-carrier concentration and temperature in quantum wells of laser heterostructures under spontaneous-and stimulated-emission conditions

The charge-carrier concentration and the temperature of hot electrons and holes in quantum-well laser nanostructures in the regimes of spontaneous and stimulated emission are determined as functions of the current density j, with InGaAs/GaAs structures as an example. Under spontaneous-emission conditions, the carrier concentration in the active region of a laser structure grows as the current increases, while carrier heating is insignificant. The spontaneous-emission spectra calculated taking into account forbidden optical transitions agree well with the experimental ones. Under stimulated-emission conditions, the behavior is quite different. When the pump current density is comparatively low (several times above the threshold), the concentration of injected charge carriers levels off and does not grow as the current increases, while the carrier temperature rises considerably. When the current density exceeds the threshold value by orders of magnitude, stabilization of the charge-carrier concentration does not take place; the carrier concentration exhibits a severalfold increase and the carrier temperature rises to about 450 K at j = 80 kA/cm². The number of the charge carriers escaping from the quantum wells into the barriers, which determines the laser efficiency, also increases under these conditions because of the carrier heating. This undesirable effect can be weakened by increasing the depth of the quantum wells.     

Dynamics of photoluminescence and recombination processes in Sb-containing laser nanostructures

The dynamics of interband photoluminescence has been studied at various temperatures and excitation levels in structures with quantum wells based on InGaAsSb alloys and barriers based on AlGaAsSb and AlInGaAsSb alloys. The lifetimes of optically injected charge carriers in quantum wells at various temperatures and levels of optical excitation have been experimentally determined. An increase in the recombination rate in structures with deeper InGaAsSb/AlGaAsSb quantum wells for electrons is attributed to manifestation of resonant Auger recombination. The Auger recombination brings about heating of electrons and holes in lower subbands of dimensional quantization. The temperature of charge carriers in the course of Auger recombination is estimated using the equation for balance of power with accumulation of nonequilibrium optical phonons taken into account. The studied structures were used to fabricate lasers of two types with lasing wavelength of approximately 3 μm; it is shown that the use of a quinary alloy as the material for the barrier leads to an improvement in the characteristics of the lasers.     

Electron heating by a strong longitudinal electric field in quantum wells

The electron heating by a strong longitudinal electric field and the energy losses due to the scattering of nonequilibrium electrons by polar optical phonons in rectangular GaAs/AlGaAs quantum wells are studied. A simple model is suggested to calculate the rate of energy losses due to the scattering of electrons by nonequilibrium optical phonons. Some of the experimental results on the heating of charge carriers in quantum wells are discussed, and it is shown that taking nonequilibrium optical phonons into account significantly improves the agreement between the theoretical and experimental data.     

Modulation of intersubband absorption in tunnel-coupled quantum wells in electric fields

Modulation of absorption of middle-infrared radiation in double tunneling-coupled quantum wells in longitudinal electric fields is studied. A specific feature of the quantum wells is the small separation in energy between the two lower levels. As a consequence, the levels may exhibit “anticrossing” even in low transverse electric fields. An interpretation of the change in intersubband absorption is suggested. The interpretation is based on the assumption that a transverse electric-field component may appear in the structure. The change in the absorption coefficient is calculated taking into account the redistribution of electrons between size-quantization subbands and the changes in the temperature of electrons in the subbands in the longitudinal electric field, as well as the changes in the optical matrix elements, the energies of transitions, and the concentrations of electrons in the subbands in the transverse electric field. The possibility of using the structure for the efficient modulation of middle-infrared light with the photon energy 136 meV is shown.     

Intersubband absorption of light in selectively doped asymmetric double tunnel-coupled quantum wells

The spectrum of equilibrium intersubband absorption has been studied in selectively doped asymmetrical double tunnel-coupled quantum wells designed for research into the modulation of IR light in a longitudinal electric field. The comparison of calculated and experimental spectra at different temperatures is carried out. In calculations, the influence of the space charge on the energy spectra of electrons and the difference in the electron effective mass in different subbands are taken into account. The data obtained on the intersubband absorption spectra in equilibrium conditions and under electron excitation by high-power picosecond pulses of light in the mid-IR range allow us to refine the energy spectrum of electrons in the actual structure.     

A dual-color injection laser based on intra- and inter-band carriertransitions in semiconductor quantum wells or quantum dots

A new type of semiconductor injection laser capable of simultaneously generating radiation in the mid-infrared (MIR) (λ~10 μm) and near-infrared (NIR) (λ~0.9 μm) spectral regions is proposed. The MIR emission is a result of intersubband (intraband) electron transitions within a three-level conduction band in a quantum well or a quantum dot. The NIR emission, on the other hand, is due to conventional interband recombination of injected electrons and holes into the conduction and valence bands, respectively. The conditions for population inversion in the intersubband emission process are determined by an appropriately engineered energy structure for a three-level system in the conduction band of a quantum well or dot structure: for the quantum-well-based system, the structure has an asymmetric funnel shape to provide long electron-phonon lifetime at the third (top) energy level. Under high carrier injection, NIR interband emission depopulates the conduction ground level of the quantum well, thereby stabilizing the electron concentration at this level-a necessary condition fur the operation of the MIR laser. This paper discusses the calculation of the population inversion conditions, the requisite gain, and threshold current for MIR laser operation. We also present a preliminary design of the laser structure with a composite waveguide that accommodates both mid- and NIR stimulated emission     

Effect of the Nature of Fluid and Structure of β-Diketone on Supercritical Extraction of Metal β-Diketonate Complexes

Extraction of uranium with trifluoromethyl alkyl diketones differing in the length of the alkyl chain was studied. The size of the alkyl group only slightly affects the extraction. Extraction of complexes formed by uranyl nitrate with -diketones in pentafluoroethane (Freon-125) was studied. The complexes are efficiently extracted with both super- and subcritical Freon-125. In contrast to extraction of these complexes in supercritical CO₂, in extraction with Freon-125 the complexes do not decompose.