J. W. Tomm

Interdot carrier transfer in asymmetric bilayer InAs∕GaAs quantum dot structures

Transient photoluminescence from a series of asymmetric InAs quantum-dot bilayers with a GaAs barrier layer thickness varying from 30 to 60 monolayers between the quantum-dot planes is investigated. The interdot carrier transfer process is analyzed. In the framework of a three-level system, interdot carrier transfer times between 200 and 2500ps are derived and compared with similar data from the literature. Within the semiclassical Wentzel–Kramers–Brillouin approximation, the observed “transfer time-barrier thickness-relation” supports nonresonant tunneling as the microscopic carrier transfer mechanism.

Carrier transfer in self-assembled coupled InAs/GaAs quantum dots

Photoluminescence (PL) spectra and time-resolved PL data from AlGaAs/GaAs superlattice structures containing thin InAs layers of about 1–3 monolayer grown on semi-insulating (001)-oriented GaAs substrates at lowered temperatures are studied. The size distribution of InAs quantum dots (QDs) among different families (modes) is controlled by variation of growth temperature and/or growth interruption. We demonstrate the stabilization of the PL magnitude caused by strong coupling between different modes and the full width at half maximum of “large size” QD modes within a certain temperature interval (50–150 K) due to feeding of the radiative transitions from nonradiative decay and carrier transfer arising from decaying excitonic states of the small size QD modes. Strong competition between different channels of ground state relaxation leads to an oscillating dependence of the PL transient for the small size QD mode. Efficient inter- and intramode tunneling rules out “bottleneck restrictions” for the PL. The pa...

Staircase-like spectral dependence of ground-state luminescence time constants in high-density InAs/GaAs quantum dots

Time-resolved photoluminescence (PL) from InAs/GaAs quantum dots with a bimodal size distribution is used to investigate the dynamic carrier-transfer processes which couple transfer between similarly sized quantum dots and between quantum dots in different size categories. The relationship between the decay time and the emission energy appears staircaselike and the energetic positions of the steps as well as the shape can be correlated to the shape of the steady-state PL emission through a rate-equation theory. These results show how transient PL can be used to investigate the dynamics of carrier transfer in quantum-dot systems.

Far-infrared reflectivity study of lattice dynamics of narrow-gap HgCdMnTe semiconductors

We report the results of a comprehensive study of lattice dynamics by means of far-infrared phonon spectroscopy in quaternary semimagnetic narrow-gap single crystals. A new version of the isodisplacement model is developed which takes into account the peculiarities of the dielectric function of narrow-gap semiconductors. This new approach is used to describe the phonon spectra transformation with composition for quaternary materials and is found to be in excellent agreement with the experimental data. The evidence for better quality and more stabilized crystalline structure of in comparison with ternary is presented and ascribed to the effect of magnetic doping.

Properties of Hg0.7Cd0.3TeCdTe superlattices with semiconducting wells

We present a study of the electro-optical properties of semiconducting Hg1 − xCdxTeCdTe (0.28 ≤ x ≤ 0.30) superlattice (SL) structures by photo- and magneto-luminescence in the Voigt and Faraday configurations. The motivation for this study was to determine and assess the influence of coupling between SL wells on the electrical and emission properties of these structures. The energy bandgap of the SLs was observed to depend very strongly on barrier width. Both transport and magneto-luminescence measurements confirmed the anisotropy of the electron effective mass and identified an excitonic contribution to the PL emission. Studies of the PL integrated intensity on excitation intensity showed that excitonic corrections were required to adequately fit the luminescence data. Optical gains of 80 cm−1 were obtained for an excitation intensity of 100 kW/cm2 indicating that these SLs have the electro-optical properties required for making efficient mid-infrared detectors and laser diodes.

Electron-optical-phonon interaction in the In0.73Ga0.27As–AlAs intersubband laser

The magnetic-field dependence of the operation of a quantum-cascade intersubband laser (QCL) is used to investigate the energetic relaxation of injected electrons through phonon emission. The QCL emits at 3.8μm and incorporates a strain-compensated active region with a large degree of internal strain. Energies of the relevant electron-phonon scattering responsible for the depopulation of the upper laser level are determined from the analysis of the Landau-level spectra. A comparison of those energies with the Raman spectrum of the active region is used to identify which phonon is primarily involved in the electron-phonon scattering. In spite of the low Ga-content in the (In,Ga)As quantum wells and high Al-content in the AlAs∕(In,Al)As composite barriers, the depopulation of the upper laser level appears to be dominated by the resonant electron-GaAs-like-longitudinal-phonon intersubband scattering. In particular, the contribution due to AlAs-like modes is negligible.

On the nature of the excitonic luminescence in narrow-gap Hg1−xCdxTe (x ≊ 0.3)

Exciton luminescence is studied in a series of annealed narrow-gap Hg1−xCdxTe (0.3 < x < 0.4) crystals grown by the travelling heater method (THM). A number of techniques — photoluminescence (PL), PL excitation, magnetoluminescence — was applied to get insight in the action of many-body effects (band gap renormalization, free excitons and their localization) in this narrow-gap material.

Valence band hybridizing in europium-alloyed lead selenide

The quasibinary compounds (Pb,Sr)Se and (Pb,Eu)Se are compared with each other and with PbSe regarding the shape of the interband optical absorption edge and the photoluminescence spectra. No differences in the edge are detected without magnetic field, if scaling with composition of the gap and of the effective masses is taken into account. Thus admixing of d states from the paramagnetic Eu2+ ions should not take place in the composition up to 10 at.% EuSe. In (Pb,Eu)Se an additional luminescence line was detected, and was attributed to transitions from the conduction band to the Eu2+ 4f states. In magnetic fields up to 7 T this line does not shift, whereas the interband line shifts with B. This shift cannot be explained by the Zeeman and spin splitting of the band states alone, but additional spin-spin interaction between the localized 4f and extended valence band states is proposed.

Broadening mechanisms near the E0 transition in narrow-gap Hg1−xCdxTe (0.2 < x < 0.6)

Abstract The near-band-edge optical and photoelectrical properties of Hg 1− x Cd x Te are investigated by transmittance, photoconductive and luminescence spectroscopy. An empirical description of the absorption coefficient below and above the energy gap E g is given. The physical nature of the transitions below E g (Urbach tail) is being discussed. Contributions due to alloy disorder as well as to shallow levels are distinguished.

Identification of the nature of the optical transitions in Hg0.42Cd0.58Te

Abstract Photoluminescence spectra of Hg 0.42 Cd 0.58 Te were measured by exciting the material with intensities from 10 mW cm −2 to 0.5 MW cm −2 and by using different techniques (conventional cw, pulsed with time-dependent detection, FTPL). For the interpretation, additional transmission and photoconductivity measurements are taken into account. It is concluded that the physical concepts for interpreting the luminescence spectra successfully for wide gap materials also apply to Hg 0.42 Cd 0.58 Te. Band-to-band transitions, donor-acceptor pair transitions and recombination via deep levels were clearly identified, whereas the character of the observed excitionic transitions is not completely understood.