Search

International audience; The coherent emission of exciton polaritons was proposed as a means of lowering the lasing threshold because it does not require the dissociation of excitons to obtain an electron-hole plasma, as in a classical semiconductor laser based on population inversion. In this work we propose a method to prove clearly the polaritonic nature of lasing by combining experimental measurements with a model accounting for the permittivity change as a function of the carrier density. To do so we use angle resolved photoluminescence to observe the lasing at cryogenic temperature from a polariton mode in a zinc oxide waveguide structure, and to monitor the continuous shift of the polaritonic dispersion towards a photonic dispersion as the optical intensity of the pump is increased (up to 20 times the one at threshold). This shift is reproduced thanks to a model taking into account the reduction of the oscillator strength and the renormalization of the band gap due to the screening of the electrostatic interaction between electrons and holes. Furthermore, the measurement of the carrier lifetime at optical intensities in the order of those at which the polariton lasing occurs enables us to estimate the carrier density, confirming that it is lower than the corresponding Mott density for zinc oxide reported in the literature.

International audience; Single-crystalline Zn3N2 thin films have been grown on MgO (100) and YSZ (100) substrates by plasma-assisted molecular beam epitaxy. Depending on growth conditions, the film orientation can be tuned from (100) to (111). For each orientation, x-ray diffraction and reflection high-energy electron diffraction are used to determine the epitaxial relationships and to quantify the structural quality. Using high-temperature x-ray diffraction, the Zn3N2 linear thermal expansion coefficient is measured with an average of (1.5 ± 0.1) × 10−5 K−1 in the range of 300–700 K. The Zn3N2 films are found to be systematically n-type and degenerate, with carrier concentrations of 1019–1021 cm−3 and electron mobilities ranging from 4 to 388 cm2 V−1 s−1. Low-temperature Hall effect measurements show that ionized impurity scattering is the main mechanism limiting the mobility. The large carrier densities lead to measured optical bandgaps in the range of 1.05–1.37 eV due to Moss–Burstein band filling, with an extrapolated value of 0.99 eV for actual bandgap energy.

International audience; The epitaxial growth of Mg 3 N 2 thin films by molecular beam epitaxy has been recently achieved. This work presents the structural properties of the films, including grains sizes and lattice rotations, as assessed by x-ray diffraction and transmission electron microscopy. The films' microstructure consists of well-aligned columnar grains 10 nm in diameter that nucleate at the film/substrate interface and that display a significant column twist, in the order of 2.5°. As growth proceeds, tilted and twisted mosaic blocs overgrow these columns, as observed in to many other epitaxial semiconductors. Yet, the rocking curves on symmetric reflections display extremely narrow peaks (∼50 arcseconds), revealing a long-range spatial correlation between structural defects that should not be mistakenly considered as a proof of high crystalline quality.

International audience; Surface diffusion is known to be of prime importance in the growth of semiconductor nanowires. In this work, we used ZnMgO layers as markers to analyze the growth mechanisms and kinetics during the deposition of ZnMgO/ZnO multilayered shells by molecular beam epitaxy on previously grown ZnO nanowire cores (so called core-shell heterostructures). Specifically, the influence of the O2 flow sent into the plasma cell on the adatom surface mobility was investigated. By carefully measuring the growth rate on the lateral facets as well as on the top of the nanowires, it is concluded that the surface diffusion length of adatoms, within the used MBE growth conditions, is very low. Such poor surface mobility explains why so few works can be found related to the spontaneous growth (without catalyst) of ZnO nanowires by MBE, contrary to other deposition techniques.

Single-crystalline $\mathrm{M}{\mathrm{g}}_{3}{\mathrm{N}}_{2}$ thin films are grown on MgO (100) substrates with plasma-assisted molecular beam epitaxy. To prevent the oxidation of the $\mathrm{M}{\mathrm{g}}_{3}{\mathrm{N}}_{2}$ films and allow further physical characterization, a polycrystalline MgO cap is deposited in situ. The growth orientation of the $\mathrm{M}{\mathrm{g}}_{3}{\mathrm{N}}_{2}$ films can be tuned from (100) to (111) by changing the growth conditions, and the associated epitaxial relationships have been determined by means of x-ray diffraction. The lattice constant of $\mathrm{M}{\mathrm{g}}_{3}{\mathrm{N}}_{2}$ films has been monitored as a function of temperature from 300 to 900 K, determining thereby the linear thermal expansion coefficient. Transmission measurements indicate an optical bandgap of crystalline $\mathrm{M}{\mathrm{g}}_{3}{\mathrm{N}}_{2}$ around 2.9 eV at room temperature, consistent with diffuse reflectance measurements on micrometric $\mathrm{M}{\mathrm{g}}_{3}{\mathrm{N}}_{2}$ particles. These results demonstrate the possibility of exploiting $\mathrm{M}{\mathrm{g}}_{3}{\mathrm{N}}_{2}$ as a new crystalline semiconductor in the blue-violet range.

X-ray diffraction has been widely used to characterize the structural properties (strain and structural quality) of semiconductor heterostructures. This work employs hybrid multiple diffraction to analyzer-oriented Zn1−xMgxO layers grown by molecular beam epitaxy on ZnO substrates. In such a low-symmetry material system, additional features appear in symmetric reflection scans, which are described as arising from hybrid multiple diffraction. First, the Bragg conditions necessary for these high-order processes to occur are introduced and applied to explain all the observed satellite reflections, identify the planes that contribute and computea priorithe angles at which they are observed. Furthermore, thanks to this hybrid multiple-diffraction technique, it is possible to determine the layer lattice parameters (in-plane and out-of-plane) in an easy and accurate way by using one single measurement in standard symmetric conditions. The achieved precision is at least as high as that obtained from the combination of symmetric and asymmetric reciprocal space map measurements.

We present research results on the electronic transport in heterostructures based on p-type nickel oxide (NiO) with the n-type oxide semiconductors zinc oxide (ZnO) and cadmium oxide (CdO). NiO is a desirable candidate for application in (opto-)electronic devices. However, because of its small electron affinity, heterojunctions with most n-type oxide semiconductors exhibit conduction and valence band offsets at the heterointerface in excess of 1 eV. ZnO/NiO junctions exhibit a so called type-II band alignment, making electron-hole recombination the only process by which a current can vertically flow through the structure. These heterojunctions are nevertheless shown to be of practical use in efficient optoelectronic devices, as exemplified here by our UV-converting transparent solar cells. These devices, although exhibiting high conversion efficiencies, suffer from two light-activated recombination channels connected to the type-II interface, one of which we identify and analyse in more detail here. Furthermore, CdO/NiO contacts were studied - a heterostructure with even larger band offsets such that a type-III band alignment is achieved. This situation theoretically enables the development of a 2-dimensional electronic system consisting of topologically protected states. We present experiments demonstrating that the CdO/NiO heterostructure indeed hosts a conductive layer absent in both materials when studied separately.

Voigt points represent propagation directions in anisotropic crystals along which optical modes degenerate, leading to a single circularly polarized eigenmode. They are a particular class of exceptional points. Here, we report the fabrication and characterization of a dielectric, anisotropic optical microcavity based on nonpolar ZnO that implements a non-Hermitian system and mimics the behavior of Voigt points in natural crystals. We prove the exceptional-point nature by monitoring the complex-square-root topology of the mode eigenenergies (real and imaginary parts) around the Voigt points. Polarization state analysis shows that these artificially engineered Voigt points behave as vortex cores for the linear polarization and sustain chiral modes. Our findings apply to any planar microcavity with broken cylindrical symmetry and, thus, pave the way for exploiting exceptional points in widespread optoelectronic devices such as vertical cavity surface emitting lasers and resonant cavity light emitting diodes.

When incorporating anisotropic materials, planar microcavities can give rise to exceptional points, i.e. propagation directions along which two otherwise polarization-split photonic modes degenerate in energy and broadening. They are non-Hermitian degeneracies revealing complex-square-root topology and chiral eigenstates. We show computations and experiments on ZnO-based microcavities.

Zinc Oxide (ZnO) has recently attracted considerable attention because of its potential applications in the blue and the UV spectral range. To now, (Zn,Mg)O/ZnO heterostructures have been grown in a c-orientation. The growth of non-polar layers along the a direction [11 20] has been proposed to avoid any built-in electric elds in the c-direction. A series of A-plane (11 20) ZnO quantum wells (QWs) and multiple QWs embedded in (Zn,Mg)O barriers were grown. After the structural and the optical characterizations of the (Zn,Mg)O bu er layers, we compared the PL emission of a-oriented and c-oriented QWs and MQWs. We demonstrated that the a-oriented quantum wells exhibited con nement, but no indication of the quantum con ned Stark e ect was observed, in contrast with c-oriented heterostructures.

We report on the magnetic properties of single crystalline thin films of Zn1−xCoxO ( x = 0.003 – 0.14 ) grown by plasma-assisted molecular beam epitaxy. In order to understand the role of intermediate charge carriers in the magnetic properties of this material two types of films were fabricated, with and without Ga-codoping. Magnetic measurements were made between 2 and 300 K in fields up to 5 T with a Quantum Design SQUID magnetometer. We found that all the tested films exhibit paramagnetic behavior following the Curie–Weiss law, χ = C / ( T − θ ) , with negative Curie–Weiss temperatures and that this behavior holds even under strong n-doping. We show that the magnetization data, M ( H ) , in function of the Co content provide additional evidence in favor of the antiferromagnetic Co–Co interaction in this material. We also observe that these data exhibit an ‘easy plane’ magnetic anisotropy for all the studied Co concentrations. Finally, we develop a simple cluster model, in order to describe the magnetic properties of ZnCoO, which is found to be in good agreement with our experiments.

Zinc oxide (ZnO) has recently attracted considerable attention because of its unique physical properties and its potential applications in the blue and UV spectral range. Up to now, ZnO-based heterostructures have mostly been grown in a c-orientation. The growth of non-polar layers along the a-direction [1120] was proposed to avoid any built-in electric fields in the c-direction. A series of A-plane (1120) ZnO quantum wells (QWs) and multiple QWs embedded in (Zn,Mg)O barriers were grown on an optimized (Zn,Co)O buffer. After the structural and optical characterization of the buffers and the QWs, we have compared the PL emission of a-oriented and c-oriented QWs. From this comparison, we demonstrate that the QWs exhibit confinement but no indication of quantum-confined Stark effect, contrary to what is observed in c-oriented structures.

High-quality ZnMgO/ZnO quantum wells (QWs) showing no phase separation were grown on Al2O3(0001). Growth temperature was shown to play a major role in the stabilisation in the wurtzite phase of ZnMgO barrier layers as well as in their surface morphology. Such effects were seen in X-ray diffraction, RHEED, AFM and PL studies. Whereas the electronic properties of the narrow QWs are governed by quantum confinement effects with excitonic emission energies above that of ZnO, the electronic properties of wider QWs (>3 nm) are governed by the quantum confined Stark effect and can therefore emit several hundreds of meV below the band gap of ZnO. This is a direct consequence of the very large built-in electric field of about 1 MV /cm developed in ZnMgO/ZnO QWs grown along the c-direction. © 2005 Elsevier Ltd. All rights reserved.

The growth of ZnxMn1–xO thin films by plasma-assisted MBE has been investigated. Mn incorporation results in drastic changes in the surface morphology, the distribution of the atoms at the surface and the epilayer growth rates, which have not been reported so far and have not been observed in any other ZnO-based alloy. These marked changes are readily observable in diluted alloys with Mn content below 1%. It is shown that Mn acts as a catalyst for oxygen incorporation into ZnO. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence (PL) and selective photoluminescence (SPL) experiments have been carried out to analyse the near band edge emission (NBE) spectra of unintentionally doped ZnO epilayers. It is shown that the different NBE transitions involve donor centres only. The lines can be divided into two groups. Those lying on the low energy side consist of recombinations of neutral-donor bound exciton complexes, while those lying just below the A free excitonic emission correspond to donor-to-valence band transitions. One of the donors is identified as indium. The emission energy of the In-donor bound exciton, the spectra of the bound exciton excited states, the In-related D°h transition energy, and the electronic spectrum of the In-donor are established. The binding energies of another three donor centres are evaluated. The occurrence of two of these centres is shown to depend on the stoechiometry, as revealed by annealing experiments in an O 2 atmosphere.

We investigated the structural, electrical, and optical properties of ZnO layers grown by plasma-enhanced molecular beam epitaxy on (0001) sapphire. Samples might be divided into two groups according to their growth mode: two-dimensional (2D) or three dimensional (3D). All the samples exhibited a columnar structure, but the size and the misorientation of the subgrains differed between groups. Subgrains of the 3D layers were larger and less in-plane misoriented than those of the 2D layers. Hence, we found that a trade off between roughness and mosaicity exists. A better internal structural quality was obtained for the 3D layers which exhibited a dislocation density of (3-5) x 10 9 cm -2 , a value similar to that of the best quality GaN layers grown by molecular beam epitaxy also on sapphire substrate. However, optical and electrical properties of ZnO layers were not affected by the growth mode. To conclude, despite rougher surfaces, the 3D layers displayed a lower mosaicity along with a lower dislocation density.

Transmission electron microscopy and high resolution x-ray diffraction are used to characterize defects in ZnO layers grown by plasma-assisted molecular-beam epitaxy on (0001) sapphire. Two- and three-dimensional types of growth modes are described and the observed mosaic structure is analyzed in each case. It is found that two-dimensional layers exhibit a roughness as low as 6 nm. Their subdomains have small lateral coherence lengths and a mean in-plane misorientation of ±0.4°, leading to an important dislocation density of 1–4×1010 cm−2. On the contrary, it is demonstrated that, through numerous interactions between dislocations, the three-dimensional growth mode leads to a better structural quality with a larger lateral coherence length and a smaller in-plane mosaic spread of ±0.07°. The total dislocation density is consequently reduced by 1 order of magnitude down to 3–5×109 cm−2 and the radical modification of the structure results in a change of the dislocation distribution. Our results thus demonst...

Group V element exchange during the growth of GaAs/Ga 0.51 In 0.49 P quantum wells (QWs) by chemical beam epitaxy is investigated by photoluminescence (PL) experiments as a function of temperature from 4 to 300 K. In order to study the As–P exchange at the direct interface of the GaAs QW, the GaInP 2 surface is exposed to a cracked AsH 3 flow during various times. The QW free exciton energy is deduced at 4 K by fitting the temperature dependence of the PL energy with the classical expression E ( T )= E 0 −a[1+2/{exp( θ /T)−1}]. The calculation of the E 1 HH 1 energies are carried out by taking into account the following phenomena which affect the QW structure: indium segregation, As–P exchange and As and P residual incorporations. It is found that As–P exchange rate decreases with the increase of the cracked AsH 3 exposure time.

We report on the investigation of In surface segregation in GaInP/GaAs and GaInAs/GaAs heterostructures grown by chemical beam epitaxy (CBE). Owing to the peculiarities of CBE growth, it is shown that In segregation can be quantitatively evaluated in real-time by using reflection high-energy electron diffraction (RHEED). A segregation coefficient and its variation with the growth temperature is extracted from the RHEED data. It is used to determine the In composition profiles at the interfaces of GaInAs/GaAs quantum well (QW) structures as a function of the growth temperature. A good agreement is found between optical transition energies calculated from these profiles and the experimental photoluminescence (PL) energies. PL energy shifts observed in GaInP/GaAs QW's as a function of the growth temperature are qualitatively explained by In segregation.

We report on microscopic photoluminescence and photoluminescence excitation of thin Al0.3Ga0.7As/GaAs quantum wells grown on exactly oriented (001) GaAs substrates. The experiments are done at low temperature by selectively exciting a few mu m(2) of the sample with a low excitation intensity. The photoluminescence spectrum performed under these conditions shows sharp peaks appearing on the low-energy side of the main line of about 0.1-meV width. These features are attributed to localized exciton states at fluctuations of the effective quantum well thickness. On the other hand, scanning tunneling microscopy performed on 2x4 reconstructed GaAs (001) surfaces clearly evidences the presence of one-monolayer-deep elongated ''holes'' regularly oriented along the [1(1) over bar0$] direction with an average width of 6 nm. The same features are observed on 2x4 reconstructed Al0.3Ga0.7As surfaces (on which quantum wells are grown), though the width regularity is less pronounced. This strongly suggests that excitons at low temperature are localized in such boxes. We also discuss the very different results obtained using both techniques (micro-photoluminescence and scanning tunneling microscopy) on quantum wells grown on vicinal (001) substrates.

We show that the insertion of extremely narrow AlAs layers in double-barrier GaAs/AlAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As quantum wells results in a variety of electronic configurations, thus providing a powerful tool for tailoring the electronic transitions in GaAs heterostructures. In particular, the transition from type-I to type-II recombination is shown to occur in correspondence with variations by a single monolayer in the thickness of the AlAs and/or GaAs layers. Drastic changes in the recombination lifetimes are correspondingly observed; at the same time, the photoluminescence efficiency is found to be almost independent of the type-I--type-II character of the transition.

We report experimental results on photoluminescence and excitation of photoluminescence of GaAs/Ga0.7Al0.3As quantum wells (19, 38.5 and 77 A) grown on nominal and vicinal GaAs substrates. In order to study the influence of the interface microscopic structure on the optical properties of the studied quantum wells, we perform luminescence experiments at low temperature by exciting selectively a small area of a few μm2. In this way the presence of isolated islands on the interface is evidenced by a fine structure in the photoluminescence and photoluminescence excitation spectra. The difference in localization length of excitons in samples with steps along the [110] and the [110] directions is emphasized.

ZnO and Zn1−x Mn x O () films with 2 μm thickness were grown on (0001) sapphire substrate by molecular beam epitaxy. X-ray, electronic and optical studies show that films have a single crystalline columnar structure with unevenly distributed impurities and defects at the interfaces and boundaries of columns. ZnO and Zn1−x Mn x O films shows a high-quality hexagonal crystal structure with ZnO cells rotated by 30° relative to the sapphire substrate. We establish that the lateral coherence length obtained from x-ray analysis of Zn1−x Mn x O films is decreased from 900 nm to 400 nm at Mn variation from x = 0 to 0.07, which corresponds to variation of an average column diameter in these films. We find that in Zn1−x Mn x O films the area sizes of coherent phonon decaying are determined by the coherent areas of concentration homogeneity of Mn distributions which are much smaller then the dimensions of the columns. Modeling of ZnO/Al2O3 interface structure and properties was performed by means of first-principle density functional theory calculations. We employ an approach based on the use of large supercells (up to 460 atoms) which makes the simulation of interfaces with very large lattice mismatch possible. In this case amorphization of crystal structure in the vicinity of the interface is appears as a natural result of calculations leading to reduction of internal strains that that originate from the ZnO/Al2O3 lattice mismatch. In all cases the double ZnO layer next closest to the to interface (as well as the upper layers) maintains a nearly perfect wurtzite crystal structure. Based on calculations we propose a new model of interface microstructure which includes Zn- or O-monolayers located between conventional ZnO and Al2O3 surfaces. Adhesion energies of ZnO films to sapphire substrate were calculated for unrotated as well as for 30° rotated domains in the cases of Zn- and O-faced ZnO surfaces both with and without additional Zn- or O-monolayers. Comparison of these quantities suggests that energy gain at interface formation is somewhat larger for 30°- rotated domains than for unrotated ones.

Photoluminescence, reflectivity, and thermally detected optical-absorption (TDOA) experiments have been performed at liquid-helium temperatures on two strained ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs quantum-well (QW) structures grown by metalorganic molecular-beam epitaxy (MOMBE). The QW thicknesses vary from 3--15 ML and 4--16 ML by two monolayer (ML) steps. It is demonstrated that the MOMBE technique allows the thickness to be controlled with an accuracy of 1 ML. The electron--heavy-hole excitonic transitions (${\mathit{e}}_{1}$${\mathrm{hh}}_{1}$) are detected for all the QW's. TDOA enables the light-hole excitonic transition to be observed for the thinnest well in the second sample. The QW excitonic absorption energies are compared with calculations within the framework of the envelope-function approximation by taking into account the strain effects and the indium segregation phenomenon. An accurate determination of the strain conduction-band offset is derived (${\mathit{Q}}_{\mathit{c}}$=0.64\ifmmode\pm\else\textpm\fi{}0.01) and it is found that the indium segregation is very weak at the QW interfaces compared to structures grown by MBE. The light-hole band configuration is type I for the evaluated alloy composition (x=0.21--0.22). Under low excitation intensity, intermediate photoluminescence emissions appear between the ${\mathit{e}}_{1}$${\mathrm{hh}}_{1}$ lines corresponding to nominal thicknesses; they can be associated with fluctuations of 1 ML thickness, which are reported, to our knowledge, for the first time, in this type of QW system. Efficient thermally activated interwell transfer of excitons is also in evidence from photoluminescence experiments. A simple model is used to account for the transfer from narrower to broader wells when the temperature is increased.

Nonpolar ZnO/ZnMgO-based optical microcavities have been grown on (10-10) m-plane ZnO substrates by plasma-assisted molecular beam epitaxy. Reflectivity measurements indicate an exponential increase of the cavity quality factor with the number of layers in the distributed Bragg reflectors. Most importantly, microreflectivity spectra recorded with a spot size in the order of 2 lm show a negligible photonic disorder (well below 1 meV), leading to local quality factors equivalent to those obtained by macroreflectivity. The anisotropic character of the nonpolar heterostructures manifests itself both in the surface features, elongated parallel to the in-plane c direction, and in the optical spectra, with two cavity modes being observed at different energies for orthogonal polarizations.

Electron cyclotron resonance plasma-assisted molecular-beam epitaxy has been used to grow hexagonal and cubic GaN crystal layers on Si(100). By a combined application of in-situ reflection high-energy electron diffraction, X-ray photoelectron spectroscopy and cross-sectional transmission electron microscopy, the state of the Si(001) surface, the structure of GaN grown on this surface and the orientation relationship between substrate and layer were determined. Substrate cleaning, surface reconstruction and carbon surface contamination were found to have a strong effect on GaN growth in the early stage of epitaxy (up to 2000 Angstrom). While polycrystalline GaN in its hexagonal phase is obtained on a clean Si(001) surface, cubic GaN grows epitaxially on a Si(001)c(4 x 4) surface covered by small three-dimensional cubic beta-SiC crystallites.

This work presents results of near-band gap magnetooptical studies on Zn${}_{1\ensuremath{-}x}$Mn${}_{x}$O epitaxial layers. We observe excitonic transitions in reflectivity and photoluminescence that shift toward higher energies when the Mn concentration increases and split nonlinearly under the magnetic field. Excitonic shifts are determined by the $s,p\ensuremath{-}d$ exchange coupling to magnetic ions, by the electron-hole $s\ensuremath{-}p$ exchange, and the spin-orbit interactions. A quantitative description of the magnetoreflectivity findings indicates that the free excitons $A$ and $B$ are associated with the ${\ensuremath{\Gamma}}_{7}$ and ${\ensuremath{\Gamma}}_{9}$ valence bands, respectively, the order reversed as compared to wurtzite GaN. Furthermore, our results show that the magnitude of the giant exciton splittings, specific to dilute magnetic semiconductors, is unusual: the magnetoreflectivity data are described by an effective exchange energy ${N}_{0}({\ensuremath{\beta}}^{(\mathrm{app})}\ensuremath{-}{\ensuremath{\alpha}}^{(\mathrm{app})})=+0.2\ifmmode\pm\else\textpm\fi{}0.1$ eV, which points to small and positive ${N}_{0}{\ensuremath{\beta}}^{(\mathrm{app})}$. It is shown that both the increase of the gap with $x$ and the small positive value of the exchange energy ${N}_{0}{\ensuremath{\beta}}^{(\mathrm{app})}$ corroborate recent theory describing the exchange splitting of the valence band in a nonperturbative way, suitable for the case of a strong $p\ensuremath{-}d$ hybridization.

In this paper we study the relation between interface roughness and X-ray intensity diffracted by a short period n 1 (AlAs)- n 2 (GaAs)/GaAs(001) superlattice (SPSL). A theoretical approach shows that introducing the same graded composition profile on both sides of the GaAs well equally reduces the intensity of all high order satellites. However, if the profile is not the same at both interfaces, the intensity variation of the satellites is completely different, depending on their order. We show that the extinction rule of satellites is no longer valid and therefore constitutes a very sensitive criterion of the non-equivalence of the direct and reverse interfaces. As an example, we apply this criterion to refine the structure of 8(AlAs)-20(GaAs) SPSL. The best agreement between observed and calculated SL structure factors is obtained for an asymmetric-shape GaAs well. The interface widths are 6 and 4 monolayers, and the best-fit model gives an R factor of 0.028.

In Al0.3Ga0.7As/GaAs quantum wells bordered by ultrathin (one or two monolayers) AlAs layers, the AlAs layer has been shown to provide a well for electrons at high pressure [Phys. Rev. B 45, 11 846 (1992)]. In this work, the same type of structures but grown on (001) substrates misoriented towards (111)Ga, are studied by photoluminescence as a function of pressure, and a comparison is made with those grown on nominal substrates in order to investigate the influence of surface steps on electronic levels. Particular emphasis is given to indirect type-II transitions. An important blueshift of indirect transitions is observed in the case of two AlAs monolayers grown on a vicinal substrate. The size of this shift may be explained considering diffusion of carriers by the interface steps, or also lateral confinement. Inversely, a redshift is observed in the one AlAs monolayer sample. We suggest that this last point may be due to terrace length fluctuations.

Nonradiative recombination processes in thin GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum-well (QW) and double-barrier quantum-well structures have been investigated by means of continuous-wave and time-resolved photoluminescence (PL) measurements. We find that, due to the temperature dependence of the radiative time constant, the Arrhenius plot of the PL intensity cannot be used for extracting the activation energy of the nonradiative channels. In fact it is the temperature dependence of the PL decay time ${\mathit{T}}_{\mathit{L}}$ which directly gives information on the thermal activation of the loss mechanism. The activation energies obtained from the Arrhenius plot of ${\mathit{T}}_{\mathit{L}}$ demonstrate that, at least in the case of thin wells, the main nonradiative mechanism is the thermal escape of the less-confined species of carriers.

Nonconventional GaAs/${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As quantum-well structures, with one or two monolayers of AlAs inserted between the GaAs layers and the ${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As barriers (double-barrier quantum wells), have been investigated to observe the quasi-three-dimensional behavior of excitons in thin quantum wells. These samples provide higher confinement energies than single quantum wells of the same thickness, in such a way that relatively thick wells produce shallow carrier subbands. An increase of the photoluminescence lifetime of excitons has been found when narrowing the well thickness. The analysis of intensity and shape of the photoluminescence spectra made us confident in interpreting this increase as an effect of the exciton delocalization, and a qualitative agreement with a simple model for the exciton radiative lifetime is obtained.

The low-temperature direct and indirect optical transitions in double-barrier Al0.3Ga0.7As/AlAs/GaAs single quantum wells, with AlAs widths of 0, 1, and 2 monolayers, have been studied as a function of pressure. It is shown that intense type-II recombinations occur under pressure in AlAs-bordered wells, due to X electron localization in the wells formed by the ultrathin AlAs layers. A fairly accurate description of the luminescence line energies and relative intensities is obtained over the whole pressure range investigated, with use of the envelope-function formalism.

RHEED intensity has been used as a probe of the surface stoichiometry variation occurring in the molecular beam epitaxial (MBE) growth of GaAs when Ga and As are alternatively supplied to the surface. A comparison of the As surface coverage prior to the growth with the coverage reached at the end of each growth cycle gives evidence that As desorption during the Ga deposition sequence becomes significant above ≈ 550°C. An extended As deposition time (with respect to the time required if no As desorption occurred) is then needed to recover, at the end of each growth cycle, the As surface coverage obtained in standard MBE growth using the same growth conditions (temperature and Ga and As4 flux intensities). However, even if the surface stoichiometry can be effectively recovered, a modification of the interface properties is observed through photoluminescence experiments on GaAs quantum wells (QWs) grown at 600°C by this modified MBE procedure.

We have studied the reflectance and photoluminescence spectra of a series of GaAs/AlAs superlattices grown by molecular beam epitaxy on GaAs (0 0 1) substrates exactly oriented or slightly misoriented. It is shown that the substrate misorientation has spectacular effects on the excitonic transition properties, strongly depending on the misorientation direction. The anisotropic dependencies of the exciton broadening and the emission line Stokes-shift are analyzed on the basis of a qualitative understanding of the interface structures.

International audience; Motivated by recent developments in spintronics, we propose an explanation of the single-ion anisotropy of Mn-doped diluted magnetic semiconductors using as an example high-quality ZnO:Mn thin films for which X-band electron-paramagnetic-resonance studies were performed. We derive an analytic formula for the axial parameter D and we prove its validity by the exact diagonalization method. We demonstrate a quantitative agreement between the experimental data and our model. These results bring insights into a long-standing problem of single-ion anisotropy in magnetic solids.

We report an experimental study of the photoluminescence (PL) properties, after both picosecond and continuous-wave excitation, of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum-well structures. The comparison between the PL decay time and the PL integrated intensity allows us to determine the temperature dependence of both the radiative and the nonradiative time constants. We find that the nonradiative processes play an important role and become dominant for T\ensuremath{\ge}100 K. The radiative time constant turns out to be definitely higher than the measured PL decay time and increases by several orders of magnitude as the temperature is raised from T=4 K to room temperature. The experimental results are finally compared with the theories existing in the literature.

A large number of optical oscillations have been observed in the reflectance spectrum of GaAs thin layers grown by molecular beam epitaxy near the exciton energy. They are due to the interference of the exciton-polariton waves and allow a direct measurement of the polariton dispersion curves. The results are analysed in terms of the exciton-polariton theory taking into account the valence band degeneracy and mixing of excitons at k ≠ 0. Good quantitative agreement on the dispersion law is obtained.

We present CW and time-resolved photoluminescence measurements on GaAs/AlGaAs asymmetric double quantum well heterostructures. Several samples (eight) have been considered in order to study the influence, on the carrier tunneling process, of both barrier thickness and energy separation between the levels in the different wells. A comparison between photoluminescence decay time of excitonic recombinations in the narrow well and wide well, under resonant and non resonant excitation, allows us to determine both the electron and hole tunneling times. Evidence of non resonant and LO phonon-assisted electron tunneling, resonant and near-resonant hole tunneling is found and discussed.

Reflectance spectra are studied for both symmetric and asymmetric double-quantum-well structures separated by intermediate barriers with various thicknesses. It is shown that the interwell couplings versus the tunneling of the carriers across the barrier has a significant influence on the optical transition properties. Quantitative analyses are presented, which help to identify a number of transitions involving excited states, in asymmetric double wells. A theoretical model agrees with experimental results in several aspects: the interband transition energies, the transition probabilities, and the broadening of the reflectance structures. The effects of interfacial roughness and alloy disorder, in both symmetric and asymmetric systems, are carefully analyzed, which provides a new insight into the broadening of the optical transitions, due to interfacial morphology.

The great interest for the dynamic processes in coupled quantum wells, due to possible applications for new quantum electronic devices, prescribes improvements in the knowledge of the electronic structure of symmetrical and asymmetrical double quantum wells. In this communication, reflectance properties of GaAs(GaAl)As double quantum wells are studied for various thicknesses of the intermediate barrier and the red and blue shifts of the excitonic transitions are analysed as a function of the coupling between the wells. The identification of the observed transitions results from the calculation of electron-hole envelope function integrals allowing the prediction of the intensity of the reflectivity structures. It is shown that, in some cases, non-intuitive behaviours can happen for asymmetrical structures and that a one monolayer fluctuation at the interfaces has a crucial effect on confined states spectra. This last result demonstrates that the actual interface morphology, as obtained by the usual growth techniques, has to be taken into account in analysing tunneling properties.

In a rather contradictory situation regarding magnetic data on Co-doped ZnO, we have succeeded in fabricating high-quality single crystalline Zn(1-x)Co(x)O (x=0.003-0.07) thin films. This gives us the possibility, for the first time, to examine the intrinsic magnetic properties of ZnO:Co at a quantitative level and therefore to address several unsolved problems, the major one being the nature of the Co-Co interaction in the ZnO structure.

We present a spectroscopic study of ZnCoO layers grown by molecular beam epitaxy on sapphire substrates. ZnCoO is commonly considered as a promising candidate for being a Diluted Magnetic Semiconductor ferromagnetic at room temperature. We performed magneto-optical spectroscopy in the Faraday configuration, by applying a magnetic field up to 11T, at temperatures down to 1.5K. For very dilute samples (x

We report on the magnetic properties of thoroughly characterized Zn(1-x)Co(x)O epitaxial thin films, with low Co concentration, x = 0.003-0.005. Magnetic and EPR measurements, combined with crystal field theory, reveal that isolated Co2+ ions in ZnO possess a strong single ion anisotropy which leads to an "easy plane" ferromagnetic state when the ferromagnetic Co-Co interaction is considered. We suggest that the peculiarities of the magnetization process of this state can be viewed as a signature of intrinsic ferromagnetism in ZnO:Co materials.

Surface segregation processes during the growth of Ga0.5In0.5P/GaAs heterostructures by chemical beam epitaxy have been investigated in real time using reflection high‐energy electron diffraction (RHEED). It is shown that In segregation occurs at both GaInP on GaAs and GaAs on GaInP interfaces. Resulting composition profiles are deduced from the RHEED data.

The evolution of surface roughness and the subsequent plastic relaxation mechanisms have been studied by transmission electron microscopy (TEM) as a function of the thickness of highly strained In0.30Ga0.70As layers on GaAs(001). The following stages have been observed: formation of coherent islands, coalescence of islands, and nucleation of dislocations at the troughs of the surface ripples. Dislocations are thus systematically generated where the highest stress concentrations are expected, according to recent theoretical predictions. It is the first time such a plastic relaxation mechanism has been observed in highly strained semiconductor heterostructures.

Dimethylethylamine alane (DMEAA) has been used as an Al precursor for the growth of AlGaAs alloys and AlGaAs/GaAs quantum wells by metalorganic molecular beam epitaxy (MOMBE). It is shown that DMEAA is a very convenient source in terms of in situ calibration of growth rates and alloy composition by reflection high‐energy electron diffraction. As indicated by secondary ion mass spectroscopy, low residual O and C impurity content is obtained by using this source. GaAs quantum well structures have been characterized by photoluminescence spectroscopy and compared to equivalent structures grown by standard solid‐source molecular beam epitaxy. They are also compared with the results recently reported in the literature for analogous structures grown by MOMBE using alternate aluminum sources.

High strained InxGa1−xAs/GaAs quantum well (QW) structures have been grown by metalorganic molecular beam epitaxy (MOMBE) with the aim of investigating the capability of this epitaxial growth method in terms of thickness control. Emission lines from QWs with thicknesses differing by 1 monolayer (ML) are well resolved by low temperature photoluminescence. It is thus demonstrated that MOMBE allows a thickness control with 1 ML precision.