An effective screened Coulomb interaction in a quasi-one-dimensional system

In this paper, we derive an analytical expression for the screened Coulomb potential between charge carriers in quasi-one-dimensional (Q1D) semiconductor structures. As an application, this potential has been used to investigate the screening effect on the binding energy of a neutral donor (D⁰$D^0$) in quantum wires (QWRs). It is found that the screening effect decreases the neutral donor binding energy, and the screening effects are more obvious in wide QWRs than that in narrow ones. Dependence of screening length on temperature and carrier concentration has also been discussed.     

Ground state energy and optical absorption of excitonic trions in two dimensional semiconductors

The electronic and optical properties of the ground state of the excitonic trions corresponding to an exciton bound to an electron or a hole are studied theoretically for 2D semiconductors in the whole range of the electron-to-hole effective mass ratio. The energies are calculated variationally using a 22-terms Hylleraas-type trial wave function. It is confirmed that, as in the 3D case, the 2D trions are stable for any value of the effective mass ratio. The binding energies amount to about ten times those we obtain in the 3D case using the same wave function. Therefore, the experimental observation of the trions should be very more easy in 2D than in 3D semiconductors. We deduce the shape of the lines arising from transitions between free electrons or holes and 2D excitonic trions states and compare it to the 3D results.     

Stability of Charged Exciton States in Quantum Wires

We show that the order of energies of negative (X⁻) and positive (X⁺) trions in quantum wires is determined by the relative electron and hole lateral confinements. For equal electron and hole confinement, X⁺ has a larger binding energy, but a small imbalance towards a stronger hole localization changes the order of the X⁻ and X⁺ recombination lines in the photoluminescence spectrum.     

Strongly confined quantum wire states in strained T-shaped GaAs/InAlAs structures

The confinement energy of T-shaped quantum wires (QWRs), which were fabricated by the cleaved edge overgrowth technique in a way that the QWRs form at the intersection of In_0.2Al_0.8As stressor layers and the overgrown (1 1 0) GaAs quantum well (QW), is examined using micro-photoluminescence spectroscopy. Photoluminescence (PL) signals from individual QWRs can be spatially resolved, since the strained films are separated by 1 μm wide Al_0.3Ga_0.7As layers. We find that due to the tensile strain being transmitted to the QW, the confinement energy of the QWRs rises systematically up to 40 meV with increasing thickness of the stressor layers. By reducing the excitation power to 0.1 μW the QWR PL emission occurs 48 meV redshifted with respect to the QW. All QWR peaks exhibit smooth lineshapes, indicating the absence of pronounced exciton localization.     

Femtosecond study of the interplay between excitons, trions, and carriers in (Cd,Mn)Te quantum wells

We study the absorption by neutral excitons and positively charged excitons (trions) following a femtosecond, circularly polarized, resonant pump pulse. Three populations are involved: free holes, excitons, and trions, all exhibiting transient spin polarization. In particular, a polarization of the gas of free holes is created by the formation of trions. The evolution of these populations is described, including spin flip and trion formation. We evaluate the contributions of phase space filling and spin-dependent screening. We propose a new explanation of the oscillator strength stealing phenomena observed in doped quantum wells, based on the screening of neutral excitons by charge carriers. We have also found that binding holes into charged excitons excludes them from the interaction with the rest of the system, so that oscillator strength stealing is partially blocked.     

Kinetics of an exciton-trion system in shallow GaAs/AlGaAs quantum wells

The formation of three-particle charged exciton complexes (trions) in shallow GaAs/AlGaAs quantum wells in the temperature range 1.7–15 K has been investigated by luminescence spectroscopy and resonant light scattering. The effect of the photon energy and the intensity of additional above-barrier illumination on the trion formation kinetics has been analyzed. It is established that, upon intrawell excitation, illumination leads to the formation of trions when the light photon energy corresponds to the regions of effective formation of trions in the photoluminescence excitation spectra. It is shown that, with an increase in the illumination level, the trion concentration first increases and then reaches a plateau since the quantum well acquires an electric charge whose field equalizes the electron and hole capture rates.     

Photon Echo from Localized Excitons in Semiconductor Nanostructures

An overview on photon echo spectroscopy under resonant excitation of the exciton complexes in semiconductor nanostructures is presented. The use of four-wave-mixing technique with the pulsed excitation and heterodyne detection allowed us to measure the coherent response of the system with the picosecond time resolution. It is shown that, for resonant selective pulsed excitation of the localized exciton complexes, the coherent signal is represented by the photon echoes due to the inhomogeneous broadening of the optical transitions. In case of resonant excitation of the trions or donor-bound excitons, the Zeeman splitting of the resident electron ground state levels under the applied transverse magnetic field results in quantum beats of photon echo amplitude at the Larmor precession frequency. Application of magnetic field makes it possible to transfer coherently the optical excitation into the spin ensemble of the resident electrons and to observe a long-lived photon echo signal. The described technique can be used as a high-resolution spectroscopy of the energy splittings in the ground state of the system. Next, we consider the Rabi oscillations and their damping under excitation with intensive optical pulses for the excitons complexes with a different degree of localization. It is shown that damping of the echo signal with increase of the excitation pulse intensity is strongly manifested for excitons, while on trions and donor-bound excitons this effect is substantially weaker.     

Strain and piezoelectric fields in embedded quantum wire arrays

Utilizing a recently developed exact closed-form solution for a single polygonal quantum wire (QWR) inclusion problem, we theoretically investigate the elastic strain and electric fields induced by QWR arrays embedded in GaAs. We consider arrays ranging from 1×1 to 7×7 QWRs embedded in infinite substrates and up to 4×7 QWRs in half space substrates where the wires are near a surface. Our results for the infinite substrate indicate that although the elastic fields within any single QWR are similar to the fields in the other wires with only a weak dependence on the number of QWRs, the electric fields (both inside and outside the QWRs) can be significantly different for different array sizes. Due to the existence of the free surface, the half space solutions show that the elastic and electric fields both inside of and outside of the QWRs depend significantly on the number of QWRs, again with the electric field having the stronger dependence. A detailed analysis of the strain and electric fields for embedded QWR arrays is presented and the results could impact the design of proposed strain-modulated electronic devices.     

On binding energy of trions in bulk materials

We study the negatively $T^{?}$ and positively $T^{+}$ charged trions in bulk materials in the effective mass approximation within the framework of a potential model. The binding energies of trions in various semiconductors are calculated by employing Faddeev equation in configuration space. Results of calculations of the binding energies for $T^{?}$ are consistent with previous computational studies and are in reasonable agreement with experimental measurements, while the $T^{+}$ is unbound for all considered cases. The mechanism of formation of the binding energy of trions is analyzed by comparing contributions of a mass-polarization term related to kinetic energy operators and a term related to the Coulomb repulsion of identical particles.     

Stark effect of negatively and positively charged excitons in semiconductor quantum wells

We study the effect of an electric field applied normal to the layers on the binding energy of charged excitons (or trions) in GaAs quantum wells. We find that, in contrast to the neutral exciton, their binding energy is sharply reduced by modest electric fields. The effect is stronger for the positively charged exciton than the negatively charged one. The ionisation of the excess carrier is explained by the field-induced polarisation of the electron and hole subband wave functions.     

Trions in quantum-well structures with two-dimensional electron gas

A study has been made of bound exciton-electron complexes (trions) and unbound exciton-electron states (combined exciton-cyclotron resonance) from reflectance spectra obtained from modulation-doped CdTe/CdMgTe quantum-well structures. It has been established that the contribution of trions to dielectric permittivity is comparable to that of excitons. An analysis is made of the magnetic-field dependence of the parameters describing the contribution to dielectric permittivity due to exciton-cyclotron-resonance states. Fiz. Tverd. Tela (St. Petersburg) 40, 813–815 (May 1998)     

Strain fields in arbitrary shaped quantum wires

The intent of this work is to develop a more generalized approach towards strain field calculations in embedded quantum wires (QWRs). Higher degree polynomials are used to achieve better discretization of QWR in arbitrary shapes and to avoid some of the singular points in the strain field calculations. Calculations are performed for simpler geometries such as triangular and square shaped QWRs to verify the validity of the approach. The same approach is tested for more complicated shapes such as crescent shaped QWRs with and without lateral quantum wells. The strain field distributions, are observed to be similar to those obtained from the analytical expressions. However, in the case of crescent shaped QWRs, the strain distribution is different in the region above the QWR. The difference is the result of the better discretization and of the removed singular points. The use of higher degree polynomials provides better discretization for shapes of interest.     

Excitons and Trions in Bilayer van der Waals Heterostructures

Physics of the Solid State - Excitons and trions in bilayer structures based on transition metal dichalcogenides are theoretically studied. Expressions for the effective potential of interparticle...     

Time-resolved magneto-optical properties of InxGa1−xAs V-shaped single quantum wires

In this work we present a time-resolved magneto-luminescence investigation (up to 8 T) of InGaAs V-shaped quantum wires (QWRs) with different In content, as a function of temperature and the applied magnetic field. The states of the wires were investigated by CW PL and quantitatively compared with the results of a numerical solution of the two-dimensional Schrodinger equation. Time-resolved experiments performed in magnetic field at different temperatures indicate the existence of a competition between the electron confinement occurring in deep QWRs at low temperature, and the magnetic confinement prevailing in shallower QWRs.     

Observation of charged excitons in hole-doped carbon nanotubes using photoluminescence and absorption spectroscopy

We report the first observation of trions (charged excitons), three-particle bound states consisting of one electron and two holes, in hole-doped carbon nanotubes at room temperature. When p-type dopants are added to carbon nanotube solutions, the photoluminescence and absorption peaks of the trions appear far below the E11 bright exciton peak, regardless of the dopant species. The unexpectedly large energy separation between the bright excitons and the trions is attributed to the strong electron-hole exchange interaction in carbon nanotubes.     

Photoluminescence inner core excitation in semiconductor quantum structures

We introduce a photoluminescence inner core excitation (PLICE) for the studies of semiconductor quantum structures. This novel method, in which we use synchrotron radiation as tunable excitation source, is expected to facilitate us to obtain electronic and compositional information about buried quantum structures. Here we report experimental results on quantum dots (QDs) and quantum wires (QWRs), in order to demonstrate potential applicability of the method to the semiconductor nanostructure studies.     

Screening by composite charged particles: the case of quantum well trions

We investigate the screening properties of a two-dimensional gas of charged excitons (trions). In a first approach to this complex problem, we determine the Hartree response of these composite charged particles within a random phase approximation, showing the effect of the trion internal structure. Only in the long wave-length limit, trions behave as point charges with mass equal to the sum of the three particle components. For finite wave-vectors, the trion screening strongly deviate from the point charge behavior and can even vanish completely at a nodal wave-vector, due to a compensation between the contribution of the two electrons and the hole within a trion. Predictions are presented for the screening of a Coulomb potential, the scattering by charged impurities and the properties of trionic plasmons.     

Ab initio study of the Fe intra- and inter-layer magnetic order in Fe/Ir(001) superlattices

We reconsider the semiconductor trions from scratch. We first determine the very many “reasonable” ways to write the trions in first quantization. We then select the forms which are easy to relate to physical pictures. In a second part, we derive the corresponding creation operators in second quantization. We pay particular attention to the expression of the X^- trion in terms of exciton and free-electron, as it is the one adapted to future works on many-body effects with trions. Received 27 May 2002 / Received in final form 18 December 2002 Published online 20 June 2003 RID="a" ID="a"e-mail: combescot@gps.jussieu.fr     

Strong Fermi-edge singularity in ultra-high-quality AlGaAs/GaAs quantum wires

We report the observation of a strong Fermi-edge singularity (FES), with the complete suppression of the band-edge peak, in the photoluminescence spectra of ultra-high-quality modulation-doped AlGaAs/GaAs quantum wires (QWRs). We find that the FES effect is very sensitive to the Fermi energy. The strong FES is observed only in QWRs having a Fermi energy of the order of a few meV, and disappears almost completely when the Fermi energy exceeds 10 meV. These results are expected to spark new research activities, both experimentally and theoretically, on many-body effects in one-dimensional electron gas.     

Effective scatterings between electrons,excitons and trions

The final goal of this paper is to derive the effective scattering ruling the time evolution of two semiconductor trions using the many-body formalism for composite fermions we have just proposed. However, to understand the importance of the particle composite nature, their bosonic/fermionic character and their overall charge, we also report on scatterings between free electrons, excitons and trions. This leads us to identify the form factors associated to direct processes involving excitons and trions. For transitions between ground states, these form factors reduce to zero and one respectively, in the small momentum transfer limit.