CdTe Quantum Research Articles

Temperature Dependence of Exciton and Trion States in CdTe Quantum Well at High Magnetic Fields

The problem of multiple-electron complexes is one of the most fundamental problems of physics. One of the examples of such complex is a negatively charged exciton (trion), which consists of three particles: two electrons and a hole. Trion states have been under investigation for more than a decade and are now well-studied objects. Still some problems remain unresolved. There are arguments why are triplet trion states, which are known to be dark, observed. To get a better understanding of this one should consider the formation mechanisms of ∗corresponding author

Oscillator Strengths for Neutral, Charged, and Scattered Excitons in CdTe Quantum Wells Containing a 2D Electron Gas

Absorption spectra have been measured for a 10 nm CdTe(0.3%Mn) quantum well with electron concentration ne variable up to 1.5×10 cm−2. Following recent theory appropriate to low ne, here ≈ 0.1/πaB, the spectra are interpreted in a “strong exciton” model, where the initial oscillator strength of the excitonic resonance (X) is conserved, with screening and phase-space filling effects negligible. As ne increases in zero-field and as the filling factor ν increases in magnetic field, the intensity of X is transferred to: (i) trion processes, namely exciton-one electron scattering and the trion resonance (T ), and (ii) quatron processes, namely exciton-two electron scattering and trion-one electron scattering. In magnetic field, the threeand four-body scattering processes become discrete, combined “exciton and cyclotron” and combined “trion and cyclotron” excitations that take all the intensity of X and T for ν >∼ 1 and ν > 2, respectively.

Correlated dynamics of trions and excitons in modulation‐doped CdTe quantum wells

We report on the nonlinear optical dynamical properties of excitonic complexes in CdTe modulation-doped quantum wells, due to many-body interaction among excitons, trions, and electrons. This study was performed by time and spectrally resolved pump–probe experiments as a function of temperature and densities of excitons, trions and electrons. The results reveal that the nonlinearities induced by trions are different from those induced by excitons and in addition they are mutually correlated. We show that the origin of these subtle differences comes from phase-space filling and short-range fermion exchange. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Negatively charged excitons as an optical probe of spin injection from Cd0.73Mg0.24Mn0.03Te spin aligner into a graded bandgap Cd1−xMgxTe/CdTe quantum well structure

The diffusion of spin-polarized electrons through a graded-bandgap Cd1−xMgxTe layer has been studied. The quaternary compound Cd0.73Mg0.24Mn0.03Te diluted magnetic semiconductor (DMS), with a large energy gap (Eg = ∼2 eV), was used as a spin-polarized source in the presence of an external magnetic field. We exploit the spin-dependent feature of trion formation (X−s), i.e. negatively charged excitons in a spin-singlet ground state, as a sensitive probe of electron spin injection into a CdTe quantum well (QW), separated from the injector by a thick non-magnetic graded gap Cd1−xMgxTe layer (which bends the band edges of the structure similar to electrical biasing). Continuous wave and time-resolved magneto-photoluminescence and reflectivity measurements have been performed. Our results show that at small magnetic fields the ratio of the density of X−s and that of neutral excitons, X0, is large when the light is absorbed only in the QW; it becomes small when the absorption occurs in the non-magnetic graded barrier. It is re-enhanced significantly again when the absorption occurs also in the magnetic layer. The latter finding indicates that electrons do not lose completely their initial spin polarization acquired in the DMS layer (due to the giant Zeeman splitting effect in this layer) when they pass through the non-magnetic region into the QW where they enhance the X−s populations. This re-enhancement occurs in spite of a large energy relaxation (∼300 meV) that the carriers must undergo.

Interplay of spin dynamics of trions and two-dimensional electron gas in an-doped CdTe single quantum well

We used the photoinduced Faraday rotation technique to study the low-temperature spin relaxation of excitons, trions, and two-dimensional electron gas in n-doped single CdTe/CdMgTe quantum wells with different concentrations of electrons. A pulsed (ps) optical excitation tuned to an exciton line allows the simple observation of the exciton spin relaxation (18\char21{}36 ps), whereas an excitation resonant with a trion state demonstrates a biexponential spin relaxation. The shorter characteristic time $(40\ifmmode\pm\else\textpm\fi{}10\mathrm{ps})$ then belongs to the photocreated hole spin relaxation, and the longer one $(180\ifmmode\pm\else\textpm\fi{}20\mathrm{ps})$ is attributed to the spin relaxation of the electron gas, initially polarized through the creation of trions by using circularly polarized light. This interpretation is confirmed by the oscillations shown by the photoinduced Faraday rotation in a transverse magnetic field, which provide the Land\'e factor of the free electron in a CdTe quantum well $(|{g}_{e\ensuremath{\perp}}|=1.342\ifmmode\pm\else\textpm\fi{}0.001).$ Finally, the observed interplay of trions and electron gas polarization is quantitatively explained with a system of four rate equations involving the spin populations of trions and those of the electron gas.

Many body effects and internal transitions of confined excitons in GaAs and CdTe quantum wells

Many body effects contribute significantly to the energy states of electron–hole pairs confined in quantum wells in the presence of excess electrons. We present results of optically detected resonance spectroscopy of the internal transitions of photo-excited electron–hole pairs in the presence of excess electrons for GaAs QWs and CdTe QWs. Compared to the case of isolated negatively charged excitons, excess electrons produce a large blue shift of the internal transitions in modulation-doped GaAs quantum wells (QWs) for filling factor <2, and similar effects are found in CdTe QWs. For filling factor >2 no internal transitions are observed. These measurements demonstrate the strong effects of electron–electron correlations on the internal transitions of charged excitons in these quasi-2D systems and the importance of magnetic translation invariance. In the presence of excess electrons, the observed internal transitions are those of a magnetoplasmon bound to a mobile valence band hole.

Optical implementation of entangled multi-spin states in a CdTe quantum well

Ultrafast optical pulses and coherent techniques are used to create spin entangled states of non-interacting electrons bound to donors in a CdTe quantum well. Our method, relying on the exchange interaction between optically excited holes and the impurities, can in principle, be applied to entangle a large number of spins. Results are presented for resonant excitation of localized excitons below the gap, and above the gap where the signatures of entanglement are significantly enhanced.

Optical detection of electron paramagnetic resonance in CdMnTe single quantum wells

The electron paramagnetic resonance of Mn2+ ions localized in a CdTe quantum well is measured by an all-optical method. The photoluminescence transitions of a two-dimensional electron gas are used to monitor changes in the Mn spin polarization. The local character of this probe and the detection technique permit us to achieve a very high sensitivity for the number of spins detected (better than 108 spins). Two simultaneous effects are observed at resonance: A blueshift of the whole photoluminescence spectrum, and a dramatic transfer of photoluminescence intensity from neutral to negatively charged excitons.

Optically induced multispin entanglement in a semiconductor quantum well.

According to quantum mechanics, a many-particle system is allowed to exhibit non-local behaviour, in that measurements performed on one of the particles can affect a second one that is far away. These so-called entangled states are crucial for the implementation of most quantum information protocols and, in particular, gates for quantum computation. Here we use ultrafast optical pulses and coherent techniques to create and control spin-entangled states in an ensemble of non-interacting electrons bound to donors (at least three) and at least two Mn2+ ions in a CdTe quantum well. Our method, relying on the exchange interaction between localized excitons and paramagnetic impurities, can in principle be applied to entangle an arbitrarily large number of spins.

Magneto-Optical Study of Diluted Magnetic Semiconductor Nanostructures Prepared by Pulsed Laser Deposition

The CdTe/Cd1−xMn x Te quantum well structures have been grown by pulsed laser deposition. X-ray diffraction analysis demonstrates high structural quality of the deposited layers and nanostructures. In the magnetoabsorption spectra the large Zeeman splitting of exciton peak which is corresponded to the heavy-hole exciton transition in the CdTe quantum well layers is revealed. The Faraday rotation spectra of the superlattices is interpreted in framework of the exciton transition for the Cd1−xMn x Te layers at a higher photon energy. Influence of different factors on behavior of the Zeeman splitting and the Faraday rotation in the studied nanostructures is discussed.

Dynamical equilibrium between excitons and trions in CdTe quantum wells in high magnetic fields

The formation of two-dimensional negatively charged excitons (negative trions) out of excitons and free electrons is found to be determined by a dynamical equilibrium. This dynamical equilibrium consists of a chemical equilibrium, relating the trion, exciton, and electron populations, modified by finite formation and recombination times of (charged) excitons, as is evidenced by a magnetic-field-dependent photoluminescence (PL) and far-infrared study of doped CdTe/CdMgTe quantum wells. The data show that the trion formation is entirely driven by the occupation of the spin-split trion, exciton and electron levels. Incorporation of the proposed trion formation scheme into a rate equation model gives a proper description of the experimental data, leading to values of the formation, recombination, and spin-flip times of trions and excitons that are in good agreement with results of time-resolved experiments in the literature. The model elucidates the effect of the heavy-hole splitting on the polarization degree of the trion PL in a magnetic field and the influence of the dark excitons on the trion formation.

Trions and Excitons in CdTe Quantum Wells: Lifetimes, Coherence, Diffusion and Localization

We present the results of a study of the dynamics of trions and excitons in a CdTe modulation-doped quantum well as a function of temperature in the low-density regime. Using three-pulse four-wave mixing technique, we show that, at low temperature, these quasi-particles are inhomogeneously broadened and localized due to disorder effects. At higher temperatures, trions and excitons are mobile and their homogeneous broadening is largely increased as a result of acoustic phonon scattering. Consistently, time-resolved luminescence measurements also reveal a free-particle character for trions at high temperatures.

Diffusion, localization, and dephasing of trions and excitons in CdTe quantum wells

Excitons and trions in a CdTe modulation-doped quantum well are investigated by picosecond three-pulse four-wave mixing experiments as a function of temperature in the low-density regime. We show that trions are mobile for high enough temperatures. The diffusivity of trions and excitons increases with temperature but, like their mobilities, is limited by acoustic-phonon scattering. Consistently, studying the decay of the interband coherence reveals an increase of the homogeneous broadening as the temperature is raised. At low temperatures, excitonic complexes are localized by both interface roughness scattering and by the electrostatic potential due to remote ionized donors.

Dynamical Equilibrium between Excitons and Trions in CdTe Quantum Well Structures

The trion formation out of excitons and electrons is studied using cw photoluminescence experiments on a 8 nm CdTe quantum well. Measurements as a function of excitation intensity show that for intense non-resonant excitation free electrons and holes should be included in the trion formation process. Temperature dependent measurements suggest that hot excitons contribute to the trion formation. The data are described successfully using a rate equation model which includes free electrons and holes, and the hot excitons.

Dynamics of Excitons and Trions in CdTe Quantum Wells: Direct Observation of Diffusion and Localization

The dynamics of excitons and trions in CdTe modulation-doped quantum wells are studied as a function of temperature by three-pulse four-wave mixing experiments. It is shown that trions are localized by interface roughness scattering and/or by the remote ionized donor potential at low temperature in the low-density regime. The diffusivity of excitons and trions increases with temperature but their mobilities are limited by acoustical phonon scattering. The coherence study reveals that the system is inhomogeneously broadened at low temperature and becomes nearly homogeneous as the temperature is increased.

Negative trions in CdTe quantum wells in the presence of a magnetic field- a numerical study

We present results of numerical calculations of low-lying energy states of the negatively charged exciton in the rectangular CdTe1−xMgxTe/CdTe/Cd1−xMgxTe quantum well in the presence of a perpendicular magnetic field. Specific results for the trion binding energy in 40 Å, 80 Å and 160 Å wide quantum wells are discussed as a function of the magnetic field strength (up to 40 T). These three widths of the quantum wells correspond to three different relationships between the magnetic length, Bohr radius and quantum well width. The 80 Å quantum well is examined in greater detail. In particular, we show the behaviour of the total energy of various trion states, and the behaviour of the allowed optical transitions with an external magnetic field. We discuss also the role of the Zeeman Hamiltonian on the energy spectra of the trion.

Spin Relaxation of Negatively Charged Excitons in CdTe Quantum Wells

Spin Relaxation of Negatively Charged Excitons in CdTe Quantum Wells

Interaction of an electron gas with photoexcited electron–hole pairs in modulation-doped GaAs and CdTe quantum wells

The nature of the correlated electron gas and its response to photo-injected electron–hole pairs in nominally undoped and modulation-doped multiple quantum-well structures was studied by experiment and theory, revealing a new type of optically active excitation, magnetoplasmons bound to a mobile valence hole. These excitations are blue-shifted from the corresponding transition of the isolated charged magnetoexciton X −. The observed blue-shift of X − is larger than that of two-electron negative donor D −, in agreement with theoretical predictions.

Dependence of the activation energies on the well width in CdTe/ZnTe strained single quantum wells

Photoluminescence (PL) measurements on the CdTe/ZnTe strained single quantum wells grown by using the molecular beam epitaxy technique showed that the sharp excitonic peaks corresponding to the transition from the first electronic subband to the first heavy-hole (E 1–HH 1) were shifted to lower energy with increasing well width. The (E 1–HH 1) interband transitions were calculated by using an envelope function approximation taking into account the strain effects, and the values were in reasonable agreement with those obtained from the (E 1–HH 1) excitonic transitions of the PL spectra. The activation energies of the confined electrons in the CdTe quantum well were obtained from the temperature-dependent PL spectra, and their values increased with increasing CdTe well width due to the quantum confinement effect. The present results can help to improve the understanding of the activation energies dependent on the CdTe well width in CdTe/ZnTe single quantum wells.

Experimental measurement of microwave-induced electron spin-flip time

The electron spin resonance (ESR) is optically detected by monitoring the microwave-induced changes in the circular polarization of the neutral exciton (X) and the negatively charged exciton (X−) emission in CdTe quantum wells with low density of excess electrons. We find that the circular polarization of the X and X− emission is a mapping of the spin polarization of excess electrons. By analyzing the ESR-induced decrease in the circular polarization degree of the X emission, we deduce the microwave-induced electron spin-flip time &amp;gt;0.1 μs, which is much longer than the recombination time of X and X−. This demonstrates that the optically detected ESR in type I quantum wells with low density of excess electrons does not obey the prerequisite for the conventional optically detected magnetic resonance.