Optical characterization of pseudomorphic AlGaAs/InGaAs/GaAs heterostructures

Pseudomorphic strained-layer AlxGa₁₋xAs/InyGa₁₋yAs/GaAs heterostructures have been studied by means of photoluminescence and Raman scattering. It is established the correlation between the photoluminescence line shape changes and the Raman spectra modification when the quantum well width is below the critical layer thickness estimated to be of 25 nm for y = 0.1. The photoluminescence feature observed for the InGaAs quantum well width equal to 20 nm as extremely narrow exciton-like peak with the full width at half of maximum equal to 1.5 meV at low temperature (T = 6 K) transforms into broad band of the full width at half of maximum equal to 16 meV when the quantum well width reaches the value about of 12 nm. The photoluminescence line shape broadening is accompanied by the modifications of Raman spectra. A new line arising at the spectral position ν = 160 cm⁻¹ is assigned to impurity-induced longitudinal acoustic mode of InyGa₁₋yAs. The changes observed in optical spectra are related to the generation of defects in the under-critical layer thickness region

Suitable factorization of the total intersubband scattering rates for efficient calculation of the current densities and gain characteristics in quantum cascade lasers

Suitable factorization of the intersubband scattering rates is performed for the temperature dependent electron transport model of mid-infrared quantum cascade lasers (QCL). In this case, the total intersubband scattering rate is presented as a sum of individual processes: longitudinal optical phonon, roughness interface, and acoustic phonon scatterings. The individual scattering rate is reduced to a product of the overlap integral for the squared moduli of the envelope functions and the temperature factor that depends on the transition energy and material. This presentation significantly reduces computational efforts in comparison with the ab initio models of full quantum transport in QCL preserving good agreement between the theory and experiment, as well

Configurational resonances in absorption of metal nanoparticles seeded onto a semiconductor surface

In the present study, the optical absorption of a semiconductor surface covered with metallic spheroidal nanoparticles is considered theoretically. The theoretical approach used for calculation of light absorption in such structures is based on the concept of effective susceptibility and made in the frame of Green’s functions theory. The effective susceptibility is taken in an analytical form. The absorption spectra were calculated for coverages with nanoparticles having different shapes and surface densities at different angles of external light incidence. The obtained results predict strong configurational resonances in the spectral range where the absorption is maximum, i.e. resonant enhancement of absorption by nanoparticles of certain shape and surface density. The developed technique of calculation can be further applied to calculation of various effects caused by the excitation of surface plasmons in metallic nanoparticles under applied external fields yielding a wide range of applications of such structures in many fields, such as biomedicine, solar energy, environment protection, and information storage technology. Keywords: Semiconductor surface, Metal nanoparticles, Configurational resonance, Absorption spectrum, Effective susceptibilit

Many-body effects in photoluminescence of heavily doped AlGaAs/InGaAs /GaAs heterostructures

A photoluminescence (PL) study of pseudomorphic modulation-doped AlxGa₁₋xAs/Iny Ga₁₋yAs/GaAs heterostructures possessing high electron density shows a fundamental change of the PL spectrum under excitation density increase. In its high energy tail the PL peak undergoes principal transformations caused by repelling the Fermi-edge singularity (FES) and excitonic states. The character of repelling depends crucially on the excitation density and temperature. At low temperatures an appearance of the FES feature has been observed for the first time under excitation density elevation. This appearance is accompanied by formation of an abrupt high energy edge and occurs far below by intensity the hybridized n = 2 exciton manifestation. Strong screening of the n = 2 exciton state by photoexcited carriers is observed. The PL behavior under excitation density increase and temperature elevation near the Fermi edge is explained in terms of strong carrier density effect on the FES manifestation and is referred to the two-dimensional (2D) electron gas properties not yet explored theoretically

InAs quantum dots embedded into anti-modulation-doped GaAs superlattice structures

Photoluminescence (PL) spectra of anti-modulation-doped GaAs superlattice structures containing thin InAs films of about 1-2.5 monolayers grown on semi-insulating (001)-oriented GaAs substrates at different temperatures are studied. The size distribution of InAs quantum dots (QD's) is found to be bimodal at the higher substrate growth temperature (TG = 505 °C) and is transformed into multimodal for the decreased growth temperature (TG = 420 °C) and growth interruption applied. For the first time we demonstrate the strong coupling between modes, which stabilizes the PL magnitude and the full width at half maximum of large index QD modes within a certain temperature interval (50-150 K) due to feeding of the radiative transitions from non-radiative decay and carrier transfer arising from decaying excitonic states of the low index QD mode

Pseudomorphic modulation-doped AlGaAs/InGaAs/GaAs heterostructures with strong manifestation of many-body effects

Photoluminescence (PL) study of pseudomorphic heavily modulation-doped AlxGa₁₋xAs/InyGa₁-yAs/GaAs heterostructures shows fundamental changes in the PL spectrum under excitation pumping and/or temperature increase. In most the high and low energy tails of the PL feature undergo the principal transformations. High-energy tail peculiarities are related to the repelling of the Fermi-edge singularity (FES) and the excitonic states. The character of repelling depends crucially on the excitation density and/or temperature. At low temperature the origination of the FES feature has been observed for the first time under increasing the excitation density. The FES appearance is accompanied by the formation of an abrupt high energy edge and occurs far below by intensity the hybridized n = 2 exciton manifestation. Strong screening of the n = 2 exciton state by photoexcited carriers is observed, resulting in the 2D electron gas-heavy hole recombination for the second electron subband. The many-body feature is detected in the magnetoluminescence spectrum. This feature develops in magnetic field B = 7 T at low temperature (T = 4.2 K) and is surely detected up to T ≈ 50 K. The LO-phonon side bands for the parent transitions between the Landau levels (LL's) are revealed in the low-energy tail of the PL spectra in magnetic field. The evolution of these phonon side-band with temperature and excitation density is observed. The enhanced strength of phonon side-bands is attributed to an enhanced Frohlich coupling with account of confined phonon and interface modes