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International audience; Energy dispersive—X-ray spectrometry is used to extract a quantitative 3D composition profile of heterostructured nanowires. The analysis of hypermaps recorded along a limited number of projections, with a preliminary calibration of the signal associated with each element, is compared to the intensity profiles calculated for a model structure with successive shells of circular, elliptic or faceted cross-sections. This discrete tomographic technique is applied to II-VI nanowires grown by molecular beam epitaxy, incorporating ZnTe and CdTe and their alloys with Mn and Mg, with typical size down to a few nanometers and Mn or Mg content as low as 10%.

Epitaxially-grown semiconductor quantum dots are promising as single-photon sources due to their compatibility with manufacturing techniques and their ability to be integrated into compact devices. Efficient collection of emitted photons is required for practical use of such nano-emitters. Here, we show the possibility to form photonic wires for efficient extraction of photons by covering nanowire quantum dots with a dielectric coating using atomic layer deposition. We experimentally obtain an Al2O3-based photonic wire with a quantum dot precisely positioned along its axis. Using finite-difference time-domain (FDTD) simulations of ZnO-based photonic structures, we calculate the optimal wire geometry which leads to maximum emitted power along the wire axis. We obtain maximum values of 74% and 52% of the total emitted power for radial and a longitudinal dipoles respectively. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

International audience; Gold-catalyzed ZnTe nanowires were grown at low temperature by molecular beam epitaxy on a ZnTe(111) B buffer layer, under different II/VI flux ratios, including with CdTe insertions. High-resolution electron microscopy and energy-dispersive X-ray spectroscopy (EDX) gave information about the crystal structure, polarity, and growth mechanisms. We observe, under stoichiometric conditions, the simultaneous presence of zinc-blende and wurtzite nanowires spread homogeneously on the same sample. Wurtzite nanowires are cylinder-shaped with a pyramidal-structured base. Zinc-blende nanowires are cone-shaped with a crater at their base. Both nanowires and substrate show a Te-ended polarity. Te-rich conditions favor zinc-blende nanowires, while Zn-rich suppress nanowire growth. Using a diffusion-driven growth model, we present a criterion for the existence of a crater or a pyramid at the base of the nanowires. The difference in nanowire morphology indicates lateral growth only for zinc-blende nanowires. The role of the direct impinging flux on the nanowire's sidewall is discussed

A ZnSe layer grown on a GaAs substrate by molecular beam epitaxy has been analysed by atom probe tomography. The one-dimensional concentration profile shows separation between Zn and Se and between Ga and As at the interface. A comparison of the concentration profile with different interface models suggests that the formation of a Ga 2 + x Se 3 compound at the ZnSe/GaAs interface with fewer vacancies than Ga 2 Se 3 ( x = 0.7). These results show the ability of atom probe tomography to characterize the interface at the atomic scale.

The growth of gold catalyzed ZnSe nanowires, with CdSe insertions, by molecular beam epitaxy is investigated. In situ reflection high energy electron diffraction and ex-situ transmission electron diffraction reveal that both during, the gold dewetting and the nanowire growth, the gold particles remain always in the solid phase. The nanowire growth proceeds by ledge flow at the gold/nanowire interface as observed ex-situ by the presence of two monolayers high steps at the interface. The nanowire diameters present a high homogeneity corresponding to the low dispersion of the gold droplets. Finally, a rather abrupt interface, of less than 1 nm thick, is observed between the ZnSe barrier and the CdSe quantum dot allowing a high confinement of the excitons. All the above observations are compatible with a Vapor–Solid–Solid growth mode.

With ZnTe as an example, we use two different methods to unravel the characteristics of the growth of nanowires by gold-catalyzed molecular beam epitaxy at low temperature. In the first approach, CdTe insertions have been used as markers, and the nanowires have been characterized by scanning transmission electron microscopy, including geometrical phase analysis, and energy dispersive electron spectrometry; the second approach uses scanning electron microscopy and the statistics of the relationship between the length of the tapered nanowires and their base diameter. Axial and radial growth are quantified using a diffusion-limited model adapted to the growth conditions; analytical expressions describe well the relationship between the NW length and the total molecular flux (taking into account the orientation of the effusion cells), and the catalyst-nanowire contact area. A long incubation time is observed. This analysis allows us to assess the evolution of the diffusion lengths on the substrate and along the nanowire sidewalls, as a function of temperature and deviation from stoichiometric flux., Comment: 13 pages, 7 figures

International audience; Epitaxial semiconductor quantum dots are particularly promising as realistic single-photon sources for their compatibility with manufacturing techniques and possibility to be implemented in compact devices. Here, we demonstrate for the first time single-photon emission up to room temperature from an epitaxial quantum dot inserted in a nanowire, namely a CdSe slice in a ZnSe nanowire. The exciton and biexciton lines can still be resolved at room temperature and the biexciton turns out to be the most appropriate transition for single-photon emission due to a large nonradiative decay of the bright exciton to dark exciton states. With an intrinsically short radiative decay time (approximate to 300 ps) this system is the fastest room temperature single-photon emitter, allowing potentially gigahertz repetition rates.

We report the molecular beam epitaxy (MBE) growth of ZnSe nanowires (NWs) on a ZnSe(100) epilayer assisted by gold catalyst. Gold dewetting assists in the formation of nanotrenches along the [0-1-1] direction in the ZnSe buffer layer. Nucleation of the gold catalyst in the trenches leads to the growth of NWs preferentially in directions orthogonal to the trenches. The wires adopt mostly the wurtzite type structure and grow along the c-axis. CdSe quantum dots were inserted in the ZnSe NWs. The CdSe insertions systematically adopt a cubic zinc-blende arrangement with a [111] growth axis, as confirmed by transmission electron microscopy. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

International audience; We report on our development of fabrication of CdSe QD in ZnSe nanowire. We have been able to obtain high quality structures with very good optical properties. This has allowed us to measure photon emission from single quantum dots and to demonstrate photon antibunching. We show that this new type of II-VI quantum dot is very promising for high temperature operation.

ZnSe nanowire heterostructures were grown by molecular beam epitaxy in the vapour-liquid-solid growth mode assisted by gold catalysts. Size, shape and crystal structure are found to strongly depend on the growth conditions. Both, zinc-blende and wurtzite crystal structures are observed using transmission electron microscopy. At low growth temperature, cone-shaped nano-needles are formed. For higher growth temperature, the nanowires are uniform and have a high aspect ratio with sizes of 1-2 $\mu$m in length and 20-50 nm in width as observed by scanning electron microscopy. Growing a nanowire on top of a nano-needle allows us to obtain very narrow nanorods with a diameter less than 10 nm and a low density of stacking fault defects. These results allow us the insertion of CdSe quantum dots in a ZnSe nanowire. An effcient photon anti-bunching was observed up to 220 K, demonstrating a high-temperature single-photon source.

We present a high-temperature single-photon source based on a quantum dot inside a nanowire. The nanowires were grown by molecular beam epitaxy in the vapor-liquid-solid growth mode. We utilize a two-step process that allows a thin, defect-free ZnSe nanowire to grow on top of a broader, cone-shaped nanowire. Quantum dots are formed by incorporating a narrow zone of CdSe into the nanowire. We observe intense and highly polarized photoluminescence even from a single emitter. Efficient photon antibunching is observed up to 220 K, while conserving a normalized antibunching dip of at most 36%. This is the highest reported temperature for single-photon emission from a nonblinking quantum-dot source and principally allows compact and cheap operation by using Peltier cooling.

International audience; Quantum dots inserted inside semiconductor nanowires are extremely promising candidates as building blocks for solid-state based quantum computation and communication. They provide very high crystalline and optical properties and offer a convenient geometry for electrical contacting. Having a complete determination and full control of their emission properties is one of the key goals of nanoscience researchers. Here we use strain as a tool to create in a single magnetic nanowire quantum dot a light-hole exciton, an optically active quasiparticle formed from a single electron bound to a single light hole. In this frame, we provide a general description of the mixing within the hole quadruplet induced by strain or confinement. A multi-instrumental combination of cathodo-luminescence, polarisation-resolved Fourier imaging and magneto-optical spectroscopy, allow us to fully characterize the hole ground state, including its valence band mixing with heavy hole states.

A selection of optical experiments is presented, demonstrating the utility of semiconductors in two novel areas of research: spintronics and quantum information. First we show examples of spin manipulation in semiconductor quantum wells. The light is used to generate a spin polarization and to detect it. Next we discuss application of optical methods in studies of carrier-induced ferromagnetism in quantum wells. Finally, we present examples of single quantum dot spectroscopy related to perspectives of application of quantum dots in quantum information, and, in particular, the use of photon correlation measurements as a tool to

The relaxation of the magnetization after a short pulse of magnetic field in n-doped and p-doped (Cd,Mn)Te quantum wells is determined from the giant Zeeman shift of the photoluminescence and transmission lines. This technique allows us to investigate the magnetization dynamics in zero field. In n-doped samples the magnetization relaxation is found to be faster than 20 ns. In case of p-doped samples two components of relaxation are observed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper we report on the structural and magnetic properties of GeMn layers grown on Ge(001). We show that for the optimized Mn concentration (6%) and for optimized growth temperature (close to 130 °C), GeMn samples exhibit a high Curie temperature (higher than 400 K) and Anomalous Hall Effect up to room temperature. Our GeMn layers grown at low temperature (70 °C to 130 °C) are composed of vertical Mn-rich nano-colums. Samples grown at temperatures higher than 130 °C contain GeMn nano-clusters.

An optical study of magnetization relaxation is presented for p-doped quantum wells with magnetic ions. The magnetic anisotropy of the system is controlled by the heavy-light hole splitting, tuned by uniaxial strain. We show that a suppression of the magnetic anisotropy results in an enhancement of the observed relaxation rate by a factor of at least 20. This is explained by the fact that the decrease of the anisotropy results in the lowering of the energy barrier for the domain magnetization flip process. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In the search for high-TC Si or Ge based ferromagnetic semiconductors, we present here the magnetic and transport properties of Ge1–xMnx nano-pillars. These pillars self-organize during the MBE growth of thin Mn doped (6%) Ge films as a consequence of the in-plane and out-of-plane diffusion of Mn atoms. Their composition is close to Ge2Mn and their average diameter and spacing are 3 nm and 10 nm respectively. Magnetic measurements evidence a ferromagnetic phase up to 400 K giving rise to anomalous Hall effect at room temperature. Moreover we evidence a large positive magnetoresistance (up to 7000% at 30 K and 9 T) probably due to the conductivity mismatch between the Mn-rich nano-pillars and the Mn-poor surrounding Ge matrix. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

It has been established that suitable illumination can influence the carrier density in QWs. In several cases, reported so far, light was used to deplete the QW [1,2]. An enhancement of carierr density was also observed in specially designed structures [3,4]. Both cases can be exploited to broaden the experimental possibilities of studying the carrier induced effects. In this work we observed both decrease and increase of the 2D carrier gas density in a simple (Cd,Mn)Te/(Cd,Mg)Te QW. The two effects were achieved for different photon energies of the illumination. Our samples contained 8nm or 10nm QWs with 2D hole gas supplied by surface states. For the sample with 25 nm cap layer thickness, it was possible to tune the hole gas concentration from almost empty well (hole density below 1×10 10 cm -2 ) to 45×10 10 cm -2 . The illumination with 425 nm wavelength almost doubled the hole gas density from the initial 24×10 10 cm -2 . The depletion mechanism was most effective for illumination with the orange (575nm) light. Figure 1 shows the carrier density variation as a function of the illumination photon energy.

The emerging field of spintronics would be dramatically boosted if room-temperature ferromagnetism could be added to semiconductor nanostructures that are compatible with silicon technology. Here, we report a high-TC (>400 K) ferromagnetic phase of (Ge,Mn) epitaxial layer. The manganese content is 6%, and careful structural and chemical analyses show that the Mn distribution is strongly inhomogeneous: we observe eutectoid growth of well-defined Mn-rich nanocolumns surrounded by a Mn-poor matrix. The average diameter of these nanocolumns is 3 nm and their spacing is 10nm. Their composition is close to Ge2Mn, which corresponds to an unknown germanium-rich phase, and they have a uniaxially elongated diamond structure. Their Curie temperature is higher than 400 K. Magnetotransport reveals a pronounced anomalous Hall effect up to room temperature. A giant positive magnetoresistance is measured from 7,000% at 30K to 200% at 300K and 9 T, with no evidence of saturation.

We present a selection of optical experiments, providing information on several spin-dependent interactions in H-V1 semiconductor structures. Exciton-exciton and exciton-carrier interactions were studied by time-resolved picosecond pump-probe measurements. Several examples of recent studies involving ion-carrier exchange interaction in quantum wells and layers are discussed, concerning the quest for room temperature ferromagnetic semiconductors, spin temperature of Mn ions in (Cd,Mn)Te quantum wells, and spin relaxation in such wells under pulsed magnetic field. Finally, anisotropic electron-hole exchange in semiconductor quantum dots is discussed in the context of efforts to obtain generation of entangled photon pairs in a biexciton-exciton cascade in a semiconductor quantum dot.

The relaxation of the magnetization change induced by a short pulse of magnetic field in p-doped (Cd,Mn)Te quantum wells is determined from the giant Zeeman shift of the photoluminescence line. The characteristic times change by three orders of magnitude upon application of a static magnetic field.

We present new growth conditions for growing high-quality CdSe/ZnSe quantum dots with photoluminescence emission measurable up to room temperature. The surface morphology is characterized in situ by Reflective High Energy Electron Diffraction (RHEED). The key point is the introduction of a new step in the growth process using amorphous selenium to induce a 2D–3D transition of a CdSe strained layer on ZnSe to form the dots. Optical characterizations by photoluminescence of CdSe/ZnSe quantum dots obtained that way, as well as X-ray diffraction results are also discussed here.

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.

We performed pump-probe time-resolved absorption measurements on a modulation p-doped (Cd,Mn)Te quantum well. The density of the 2D hole gas was controlled, in the 10 cm−2 range, by additional cw illumination. Interactions between photocreated neutral (X) and charged (X) excitons and the 2D hole gas were analyzed. We found that the strongest effect is the influence of the carriers on both X and X optical resonances. Neutral and charged excitons are screened by holes, which results in the decrease in their intensities. We conclude from polarization resolved experiments that this screening is spin-dependent: it is more efficient between holes with opposite spins. Binding holes into the charged excitons reduces the screening of the neutral excitons and leads to an enhancement of the neutral exciton intensity. We also analyzed weaker effects, due to exciton–exciton interaction, at a constant hole density. We found that the reduction of the neutral exciton intensity due to two different mechanisms (phase-space filling and biexciton formation) is almost equal. We observed a spin-dependent blue shift of the neutral exciton line in the presence of a population of neutral excitons created by the pump pulse. Due to the attractive interaction between excitons with opposite spins, the corresponding shift of the neutral exciton is smaller than that observed due to the interaction between excitons with the same spins.

Photoluminescence of p-type modulation doped (Cd,Mn)Te quantum wells is studied with carrier density up to 5 X 10(11) cm(-2) at various spin splittings. This splitting can be made larger than the characteristic energies of the system thanks to the giant Zeeman effect. At small spin splitting and regardless of the carrier density, the photoluminescence exhibits a single line, which corresponds to the charged exciton in the singlet state. Above a certain spin splitting, the charged exciton is destabilized in favor of the exciton at vanishing hole density, and in favor of a double line at higher carrier density. It is found here that the charged exciton destabilization energy hardly depends on the carrier density. The double line is found to be band-to-band like, with the same initial state - where the holes have the same spin orientation - and final states that differ by some excitation of the 2D hole gas. In addition, the spin splitting needed to fully polarize the hole gas is twice smaller than expected from the single particle image and gives a unique insight into many-body effects in the hole gas.

We describe the magneto-photoluminescence of (Cd,Mn)Te/(Cd,Zn,Mg)Te quantum wells containing a 2D hole gas . The hole population can be tuned either optically in p-i-p structures, by above barrier illumination, or electrically by applying a bias voltage of a few volts on a p-i-n structure. We identify the photoluminescence at low field as due to the positively charged exciton X + , which is destabilized at a given value of the Zeeman splitting, due to its crossing with a state involving a triplet configuration of the trion hole pair. The X + looses its intensity in favor of a double line which exhibits features associated to band-to-band transitions at k=0 and k=k F (wave vector at Fermi energy).

Properties of carrier-induced ferromagnetism in modulation-doped quantum wells of p-type (Cd,Mn)Te are studied by photoluminescence and reflectivity in magnetic field. Valence-band engineering can strongly modify the magnetic properties of a system of localized spins interacting via a 2D hole gas. This is obtained through a strain control of the light-hole (lh)/ heavy-hole (hh) splitting. The axis of easy magnetization can be turned from the growth direction (with a hh gas) to be in-plane (lh gas). It is also found that the strong alloy fluctuations in (Cd,Mn)Te quantum wells containing about 10% of Mn ions are strong enough to significantly modify the magnetic state of the coupled ion-carrier system. A similar effect is observed at moderate Mn content, by restoring the lh/hh degeneracy.

New structures aiming at controlling ferromagnetic properties of diluted magnetic semiconductors quantum wells (QWs) are presented. The carrier density is monitored by applying voltage in p–i–n diode or adjusting distance between QW and surface.

Using photoluminescence and photoluminescence excitation, weinvestigate the magnetic properties of (Cd,Mn)Te quantum wells inserted in pin diodes. Such structures give us the opportunity to tune the carrier density by applying a bias of the order of 1 V. Results obtained in a small magnetic field show the presence of magnetic domains, which persist as long as the quantum well is populated.

New structures aiming at controlling the ferromagnetic properties of diluted magnetic semiconductor quantum wells are presented. The carrier density is controlled by applying a voltage across a p-i-n diode. A new method, creating a 2D hole gas by adjusting the distance between the quantum well and surface, offers opportunities for a broader range of structures.

We present a study of time-dependent transmission spectra of a modulation-doped Cd 1 - x Mn x Te/Cd 1 - y - z Zn y Mg z Te quantum well with variable hole gas concentration. We study the influence of pump pulses on excitonic absorption in subpicosecond time scale. A spectrally broad probe pulse of duration of 40 femtoseconds was used to record the absorption spectra at controlled delay. Studies of temporal evolution of exciton energies revealed coherence decay of linearly polarized excitons and thermalization of non-equilibrium exciton states. We found that a characteristic timescale for thermalization of non-equilibrium populations of photocreated excitons is between 0.8 and 3.6 ps. The timescale of this process depends on the hole concentration in quantum well: for higher hole concentration the decay is faster. Long-lived photo-induced magnetization accompanied by heating of the magnetic system was also observed.

A review of our recent experiments on the studies of spin-lattice relaxation in semimagnetic semiconductor quantum wells (QWs) is given. Three series of magnetic nanostructures are studied: undoped (Cd,Mn)Te/(Cd,Mg)Te QWs; n-doped (Cd,Mn)Te/(Cd,Mg)Te QWs with two-dimensional electron gas; p-doped (Cd,Mn)Te/(Cd,Mg,Zn)Te QWs with two-dimensional hole gas. In the experiments, we use a technique which combines an injection of nonequilibrium phonons and an optical detection of the induced changes via the exciton photoluminescence. We observe that doping induces a significant acceleration of the spin-lattice relaxation process, which is due to spin-flip transitions between Mn ions and free carriers. We demonstrate experimentally that it is possible to tune the value of spin-lattice relaxation time by changing the intensity of the laser excitation in the doped QWs.

We report on a new spin resonance with a g-factor close to 1.5 observed in n- and p-doped CdMnTe quantum wells with nonmagnetic barriers. The resonance was detected by Fourier transformation of the time-resolved Kerr and Faraday effects measured with 100 fs pulses, in a pump-probe configuration, as a function of magnetic field, temperature, excitation density, and magnetic field orientation in the case of the p-doped sample. The properties of this spin resonance are discussed and possible interpretations are proposed.

Experimental results obtained on Zn 1− x Mn x Te layers and Cd 1− x Mn x Te quantum wells are surprisingly well understood in the framework of a mean field model of the carrier-induced ferromagnetic transition, provided the real structure of the valence band is properly taken into account. Here we put the emphasis on effects leading to deviations from the standard model: RKKY oscillations in Zn 1− x Mn x Te when the Mn concentration is not high enough with respect to the carrier density, electronic disorder and localization at low carrier density in Zn 1− x Mn x Te epilayers and Cd 1− x Mn x Te quantum wells. Importantly, the modulation of magnetic properties through modulation of the carrier density, either optically or using the electric potential in a pin diode, is demonstrated.

Properties of carrier-induced ferromagnetism in modulation-doped quantum wells of p-type (Cd,Mn)Te are assessed by photoluminescence (PL) and PL excitation spectroscopy. Well-chosen illumination makes it possible to deplete the quantum well and thus to control the strength of the ferromagnetic interactions. Spontaneous local magnetization is proportional to the hole concentration p but the Curie temperature T-C varies little over a wide p range, in agreement with the mean-field model for 2D systems. The spatial extension zeta of ferromagnetic correlation is probed by micro- and time-resolved PL and possible mechanisms controlling zeta below T-C are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.

The current status and prospects of research on ferromagnetism in II-VI diluted magnetic semiconductors and their nanostructures are reviewed. So far, in agreement with theoretical predictions, the ferromagnetic ordering has been observed above 1 K for p-type modulation-doped Cd 1-x Mn x Te/Cd 1-y-z Mg y Zn z Te:N heterostructures as well as for Zn 1-x Mn x Te:N and Be 1-x Mn x Te:N epilayers and Bridgman grown Zn 1-x Mn x Te:P but not for n-type films of Zn 1-x Mn x O:Al. Diverse aspects of theoretical models as well as hopes associated with the materials containing magnetic ions other than Mn and encouraging results for II-VI/III-V systems are described.

Atom probe tomography and photoluminescence spectroscopy have been used to study CdxZn1−xSe quantum dots embedded in a ZnSe layer grown on a (001) GaAs substrate. Atom probe tomography analyses show significant cadmium incorporation in the center of the dots surrounded by poor cadmium region. These measurements illustrate that the maximum cadmium concentration in the quantum dots is significantly higher than the concentration estimated by transmission electron microscopy. The composition and size of quantum dots obtained by atom probe tomography have been used to calculate the transition energies including excitonic and strain effects.

We measure the spin–lattice relaxation time of Mn ions in Cd1−xMnxTe/Cd0.66Mg0.27Zn0.07Te (x=0.0035) semimagnetic quantum wells in the presence of a two-dimensional hole gas (2DHG) with sheet densities up to 3×1010 cm−2. We show that the presence of a 2DHG reduces the spin–lattice relaxation time by more than one order of magnitude. This reduction is much stronger than a similar effect induced by the presence of a two-dimensional electron gas, which is due to the heavier hole mass and stronger p–d interaction for holes with Mn ions. We demonstrate experimentally the possibility to tune the spin–lattice relaxation time by varying the density of the 2DHG through above-barrier photoexcitation.

We have studied the growth conditions to obtain the chemically well ordered L1 0 phase of the FePd alloy deposited on ZnSe and CdZnMgTe II–VI semiconductors. We have observed that the segregation of Se or Te atoms at the surface involved a dramatic effect on the formation of the L1 0 phase. The strong anisotropy exhibited along the growth direction allows to observe, by magnetic force microscopy, the perpendicular magnetic configuration with up and down domains. Comparably, we have grown, on the same materials, Cu/Ni multilayers showing a clear perpendicular magnetic anisotropy. Optical measurements on a semimagnetic heterostructure covered by a ferromagnetic metal layer suggest fluctuations of the Fermi level at the interface between the semiconductor and the metal.

The discovery of carrier induced ferromagnetism in p-type doped diluted magnetic semiconductors (DMS) with Curie temperatures actually up to 110 K in Ga 1− x Mn x As, as well as the demonstration of high spin injection efficiency observed from II–VI and III–V DMS [1,2] have recently renewed strongly the interest in DMS. In this paper, we present an overview of the recent experiments we have performed on spin injection light emitting diodes using an n-type doped II–VI DMS layer as spin aligner and on p-type doped II–VI magnetic heterostructures: Cd 1− x Mn x Te quantum wells and Zn 1− x Mn x Te epilayers.

Single and multiple quantum wells (QWs) of CdTe/Cd1−xZnxTe, of different composition (x), grown on a Cd1−xZnxTe (001) substrate are studied by Raman spectroscopy at low temperature, under resonance conditions. Two of the three phonon deformation potentials (PDPs) of the LO phonon of CdTe are calculated using the observed phonon frequency shifts due to a lattice-misfit bisotropic strain.

We report on a study of the spin coherence and on the formation dynamics of charged excitons in a p-doped ${\mathrm{C}\mathrm{d}\mathrm{T}\mathrm{e}/\mathrm{C}\mathrm{d}}_{1\ensuremath{-}x\ensuremath{-}y}{\mathrm{Mg}}_{x}{\mathrm{Zn}}_{y}\mathrm{Te}$ quantum well by time-resolved photoluminescence under strictly resonant excitation of either the neutral or the charged exciton transition. The analysis of the decay of the charged exciton photoluminescence polarization and of the oscillation of this polarization when a transverse magnetic field is applied allows us to conclude that the formation of a charged exciton via an exciton state does not affect either the electron spin orientation or its coherence. The radiative lifetime of the charged exciton is directly measured and is found to be \ensuremath{\sim}60 ps. Its formation time is determined from the experiments via a detailed model and is found to be \ensuremath{\sim}65 ps. We also obtain information on the excitonic spin-flip time (12 ps) and the single-carrier spin-flip times within the exciton (electron, 60 ps and hole, 20 ps).

We present a systematic study of the oscillator strength of the positively charged excitons (X+) in Cd1-xMnxTe quantum wells. CW-absorption and time-resolved photoluminescence measurements were combined as two approaches for the determination of the oscillator strength. By varying (in a small magnetic field) the spin subband hole distribution at constant total concentration, we observe an increase of the oscillator strength in absorption, proportional to the hole concentration in one spin subband (the one which allows the X+ formation). The measurements done for different, total hole concentrations show an important decrease of the X+ oscillator strength per carrier when increasing the total concentration. On the contrary the radiative lifetime, measured in time-resolved PL experiment, is found to be constant over the whole range of hole gas concentrations, and equal to the value which we deduce from transmission in the limit of vanishing hole concentration. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

p-type doping of molecular-beam-epitaxy grown layers of the diluted magnetic semiconductor Zn(1−x)MnxTe is achieved by using an active nitrogen cell. The strong interaction between the localized Mn spins and the holes deeply modifies the transport properties (metal–insulator transition, spin-dependent Hall effect). In spite of the weak localization of the carriers at low temperature, the holes clearly induce a ferromagnetic interaction between the localized spins, which is discussed as a function of Mn content and hole concentration.

We report on a detailed study of the dynamics of charged and neutral excitons in modulation p-doped CdTe/CdMgZnTe quantum wells by degenerated four-wave mixing and time-resolved photoluminescence experiments under resonant excitation. The study of the charged and neutral exciton homogeneous linewidth as a function of the temperature and of the quasiparticle densities shows that the charged excitons are localized in the fluctuations of the electrostatic potential induced by the remote dopants. The time-resolved photoluminescence experiments allow us to extract the formation time of the charged exciton (65 ps) and experiments performed in a transverse magnetic field show that neither the spin orientation nor the spin coherence of electrons is destroyed by the formation process of charged excitons via exciton states.