Auger recombination in strained quantum wells

A nonthreshold mechanism for Auger recombination of nonequilibrium carriers in quantum wells with strained layers is investigated theoretically. It is shown that the dependence of the Auger recombination rate on the magnitude of the strain and the height of the heterobarriers for electrons and holes can be analyzed only by calculating the overlap integrals between initial and final particle states microscopically. In quantum wells with strained layers the presence of strain affects qualitatively and quantitatively the electron-hole overlap integral. The dependence of the Auger recombination rate on the quantum well parameters, the magnitude of the stress, and temperature are analyzed for heterostructures based on InGaAsP/InP and InGaAlAs/InP. Fiz. Tekh. Poluprovodn. 31, 358–364 (March 1997)     

Increased radiative recombination of AlGaN-based deep ultraviolet laser diodes with convex quantum wells

An AlGaN-based deep ultraviolet laser diode with convex quantum wells structure is proposed. The advantage of using a convex quantum wells structure is that the radiation recombination is significantly improved. The improvement is attributed to the increase of the effective barrier height for electrons and the reduction of the effective barrier height for holes, which results in an increased hole injection efficiency and a decreased electron leakage into the p-type region. Particularly, comparisons with the convex quantum barriers structure and the reference structure show that the convex quantum wells structure has the best performance in all respects.     

Enhanced radiative recombination in AlGaN quantum wells grown by molecular-beam epitaxy

Dependences of the intensity of cathodoluminescence in multiple Al_0.55Ga_0.45N/Al_0.45Ga_0.55N quantum wells grown by molecular-beam epitaxy on the growth conditions are studied. An increase (by almost two orders of magnitude) in the intensity of the cathodoluminescence peak with an energy of 4.45 eV is observed as the quantum-well layer grows in the conditions of deep depletion with respect to ammonia. In this case, a tendency towards the mode of three-dimensional growth can be inferred from the pattern of diffraction of high-energy electrons; this effect is interpreted using a model of formation of AlGaN quantum dots.     

A numerical calculation of auger recombination coefficients for InGaAsP/InP quantum well heterostructures

Auger recombination coefficients are calculated numerically for InGaAsP/InP quantum well heterostructures. In narrow quantum wells, the quasi-threshold and thresholdless mechanisms mainly contribute to the Auger recombination coefficient. For the processes involving two electrons and a heavy hole (CHCC) or an electron and two heavy holes with a transition of one of the holes to the spin-orbit split-off band (CHHS), the Auger recombination coefficients depend on temperature only slightly in a wide temperature range. The dependence of the Auger coefficient on the quantum well width is analyzed and found to be nonmonotonic.     

Piezoelectric effect in micromachined [001] quantum wells: Theoretical aspects

In this work, we demonstrate the ability to take adBANtage of the piezoelectric effect in systems primarily grown on [001] GaAs substrates. Such an effect can be achieved by making use of elastic relaxation of micromachined strained quantum well structures, the etching direction being carefully chosen. Shear defbrmations present at the edge of released cantilevers can produce a significant piezoelectric field.     

Effects of auger recombination process on laser dynamics

The effects of the Auger recombination process on solid-state laser dynamics are analyzed theoretically. The Auger effects on fluorescence lifetime, relaxation oscillations, spatial hole burning, and output powers are given by examining rate equations including the Auger parameter.     

Band-to-band auger recombination and carrier-carrier scattering in power rectifiers

It is shown that band-to-band Auger recombination has a profound influence on the carrier-concentration profile and on the forward characteristic of p-s-n rectifiers (and thyristors) at high current densities, particularly when the diffusivity is decreased at high carrier concentrations by carrier-carrier scattering.     

Monte Carlo modeling of femtosecond relaxation processes inAlGaAs/GaAs quantum wells

Results are presented from Monte Carlo simulations of the femtosecond relaxation of photoexcited electrons in AlGaAs/GaAs quantum wells. Two experiments are simulated: in one electrons are initially excited at high energies far from equilibrium, and in other electrons are excited at low energies close to the bottom of the band. The effects of electron-electron, polar optical phonon, and intervalley deformation potential scattering are studied. For comparison, subpicosecond relaxation in bulk GaAs is also discussed     

Quasi-analytical study of the energy levels in double quantum wells

In this work we develop an analytical expression to the eigenvalue equation for the double quantum well (DQW), to describe the energy gap between resonant levels. The calculation of the energy gap does not involve the evaluation of the wavefunctions for the DQW nor the transfer or tunneling integrals in contrast to how it is usually done.     

The role of electron-electron interaction in the process of charge-carrier capture in deep quantum wells

The role of electron-electron interaction in the process of electron capture to a deep quantum well is investigated. Using two-level and three-level quantum wells as examples, the basic electron-capture mechanisms, i.e., the interaction with optical phonons and the Coulomb electron-electron interaction, are considered, and the corresponding capture probabilities and electron lifetimes are calculated. The effect of Auger recombination on the charge-carrier distribution in a quantum well is also taken into account. With this taken into consideration, a set of rate equations is solved for a nonsteady-state mode, and the time dependences of the electron concentration at the ground energy level in the quantum well are found. The contributions of each of the recombination processes under consideration are shown.     

Dominance of auger recombination in InGaAsP light emitting diode current-power characteristics

Output power saturation in 1.3-µm InGaAsP light emitting diodes with various active layer thickness has been investigated experimentally in a wide temperature range. Nonradiative recombination current with strong injected carrier density dependence, which is responsible for saturation, was found to be proportional to active layer thickness and almost independent of temperature under constant injected carrier density conditions. External quantum efficiency at a constant injected carrier density was found to be independent of active layer thickness. These results indicate strongly that Auger recombination is the dominant nonradiative process in InGaAsP light sources.     

Trapping and recombination processes via deep level T3 in semi-insulating gallium arsenide

Trapping and recombination of free carriers by deep level T₃ has been studied. Occupancy of the level by electrons and dynamics of its filling and emptying as a function of illumination with monoenergetic photons in 0.69–1.55 eV range has been monitored by the thermally stimulated currents method. We have found that level T₃ behaves more like a recombination center than like an ordinary electron trap. Besides trapping free electrons from conduction band, this trap can also communicate with valence band, trapping holes. The capture cross section for trapping a hole is estimated to be comparable or even larger than the capture cross section for trapping an electron. However, in many experimental conditions free electrons are generated more abundantly than free holes, and free carrier mobility and thermal velocity are both much higher for electrons than for holes. Therefore, electron trapping often prevails, so that this frequently detected defect, has been up to now most often perceived as a deep electron trap.     

Modeling of multiple-quantum-well solar cells including capture, escape, and recombination of photoexcited carriers in quantum wells

A self-consistent numerical Poisson-Schrodinger-drift-diffusion solver is described for simulation of multiple-quantum-well (MQW) Al/sub x/Ga/sub 1-x/As-GaAs solar cells. The rates of escape, capture, and recombination of photoexcited carriers in quantum wells embedded in the intrinsic region of a p-i-n device are self-consistently incorporated in the model. The performance of the device for various quantum-well configurations is investigated and the device characteristics are related to the dynamics of capture, escape, absorption, and recombination of carriers in the quantum wells. Our results show that the incorporation of MQWs in the intrinsic region of a p-i-n solar cell can improve the conversion efficiency of non-optimal devices, if the device is designed based on careful consideration of the behavior of the photoexcited carriers in the quantum wells. Specifically, we found out that an Al/sub 0.1/Ga/sub 0.9/As-GaAs cell with multiple quantum wells of 150 /spl Aring/ is more efficient than an identical single bandgap Al/sub 0.1/Ga/sub 0.9/As cell with no quantum wells, but less efficient than a single bandgap GaAs cell without such quantum wells.     

Picosecond photoluminescence measurements of hot carrier relaxation and auger recombination in GaSb

We have measured hot carrier relaxation and recombination of photoexcited carriers in GaSb using picosecond time-resolved photoluminescence. We find that these processes are greatly modified compared to other III–V materials by the conduction band structure which permits a large electron population at the L-minima. Rapid carrier cooling observed at early times is explained by efficient relaxation of electrons within the L-valleys. For carrier densities 10¹⁹cm⁻³ we find that the dominant recombination process at low temperatures (4K) is radiative, but at hihg temperatures Auger recombination becomes dominant. This is manifest in a dramatic reduction of luminescence intensity, faster recombination and very slow energy relaxation.     

On the radiative recombination and tunneling of charge carriers in SiGe/Si heterostructures with double quantum wells

For SiGe/Si(001) epitaxial structures with two nonequivalent SiGe quantum wells separated by a thin Si barrier, the spectral and time characteristics of interband photoluminescence corresponding to the radiative recombination of excitons in quantum wells are studied. For a series of structures with two SiGe quantum wells different in width, the characteristic time of tunneling of charge carriers (holes) from the narrow quantum well, distinguished by a higher exciton recombination energy, to the wide quantum well is determined as a function of the Si barrier thickness. It is shown that the time of tunneling of holes between the Si_0.85Ge_0.15 layers with thicknesses of 3 and 9 nm steadily decreases from ~500 to <5 ns, as the Si barrier thickness is reduced from 16 to 8 nm. At intermediate Si barrier thicknesses, an increase in the photoluminescence signal from the wide quantum well is observed, with a characteristic time of the same order of magnitude as the luminescence decay time of the narrow quantum well. This supports the observation of the effect of the tunneling of holes from the narrow to the wide quantum well. A strong dependence of the tunneling time of holes on the Ge content in the SiGe layers at the same thickness of the Si barrier between quantum wells is observed, which is attributed to an increase in the effective Si barrier height.     

Theoretical study of differential gain in strained quantum wellstructures

The differential gain expected in a strained quantum well (QW) and in a lattice-matched QW is discussed based on a theoretical treatment of the band structures where the band nonparabolicity is taken into account. The differential gain in strained QWs will be larger by about three to four times relative to lattice-matched QWs, and a maximum differential gain of 4-6×10^-15 cm² will be possible in strained QWs. The anisotropy of the subband nonparabolicity in lattice-matched QWs contributes to the larger difference between the two types of QWs. The calculated band-edge effective masses and the calculated laser properties, are compared to the available measurements, and some comments are given for realizing high-speed strained lasers     

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.     

Measurement of radiative and auger recombination rates in p-type InGaAsP diode lasers

Carrier lifetimes and spontaneous emission rates are reported for InGaAsP diode lasers. For active layers Zn-doped in the range 1?2 × 1018/cm3 the radiative recombination constant B is 0.8 × 10?10 cm3/s and the nonradiative constant C is 0.9 × 10?28 cm6/s for a Cnp2 Auger process. For lightly doped lasers the Auger model alone cannot explain the data.     

Weak localization in p-type quantum wells

A weak-localization theory is derived for quantum heterostructures with strong spin-orbit interaction that predicts anomalous magnetoresistance. This theory treats real quantum wells with a few occupied quantum-well subbands. It is shown that in the presence of intense elastic transitions between these subbands the parameters that define the conductivity in classically weak magnetic fields are averaged efficiently. In the opposite limiting case, all the subbands give independent contributions to the anomalous magnetoresistance. Relevant characteristic magnetic fields are calculated for arbitrary ratios between the times for phase breaking and intersubband transitions. Fiz. Tekh. Poluprovodn. 32, 1219–1228 (October 1998)     

Resonance donor states in quantum wells

The energies and wave functions of the resonance states of shallow donors in quantum wells (QWs) are calculated. The calculations are performed in a model of an isolated impurity center using the example of a GaAs/AlGaAs heterostructure. A formula for the probability of a spontaneous emission of polar optical (LO) phonons is derived. It is shown that, in the vicinity of the resonance-state energies, polar electron-phonon interaction is modified. This modification is produced by a hybridization of confinement subbands. Generally, due to hybridization, an electron interacts with phonons simultaneously in two channels (subbands). The absorption cross section for infrared radiation is calculated, taking into account both homogeneous (in the mid-infrared range) and inhomogeneous broadening (in the far-infrared range). The absorption of radiation whose electric field is normal to the heterointerfaces is related to optical transitions to the states near the resonances. Homogeneous broadening of the absorption lines, as well as the LO-phonon scattering rate, depends on the width of the resonance states (the degree of subband hybridization).