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In this article, we have studied the drain current and transconductance of nMOS fully depleted silicon-on-insulator (FDSOI) transistors operating at low temperature (typically < 20 K) when back gate is forward biased. Humps appear in the current, leading to oscillations of the transconductance with gate voltage, owing to mobility discontinuity due to intersubband scattering, in relation with the 2-D subband structure. The conditions for which these specific features appear in thin-film silicon-on-insulator (SOI) devices have been analyzed, by varying the temperature, drain voltage, silicon channel thickness, and gate length.

International audience; Continuous, piezomodulated, and time-resolved optical spectroscopies have been carried out on several samples basically consisting of a 38-nm-wide ZnTe quantum well incorporating monomolecular CdTe islands grown by molecular-beam epitaxy on nominal (001) ZnTe surfaces. During the growth of some samples, five regularly spaced monomolecular layers (single or half-monolayers) of CdTe were inserted in the wide ZnTe quantum wells. Our results indicate that, under certain conditions of deposition, (i) the fractional layers constitute CdTe islands separated by ZnTe “voids,” with in-plane size and spacing quite larger than the exciton Bohr diameter; and (ii) when several fractional monolayers are grown, these islands tend to stack on top of each other. The effects of this vertical self-ordering disappear when the conditions of deposition are changed. Moreover, it is shown that the CdTe inserts act as giant isoelectronic recombination centers, yielding intense and sharp photoluminescence lines with very short decay times (a few tens of picoseconds) in the range of λ≈517 nm.

At ambient temperature monolayer phases of most alkane chain molecules exhibit phases, characterised by the rotation of the chain around the molecular axis. Consequently, these phases are only weakly ordered rather than crystalline. In order to achieve crystalline ordering, the energy of the system needs to be reduced. New cryo-protective liquid subphases extend the accessible range to much lower temperatures than the freezing threshold of water. Monolayers of common surfactants like fatty acids and phospholipids were prepared under constant volume/constant surface pressure conditions and their phase behaviour was studied upon cooling. First insights into film stability and evolution upon cooling were achieved by Langmuir isotherms and GIXOS, while the in-plane ordering was investigated by GIXD. The molecules were found to adapt crystalline phases by a purely entropy driven self-assembly process, reaching packing densities similar to three dimensional single crystals. The evolution of the structure parameters compare to those of bulk alkanes undergoing the rotator-crystalline phase transition. XPCS was employed to relate the surface dynamics of the system to the appearance of crystalline phases. The phase transition was found to be accompanied by a change of the dynamics from propagating to overdamped capillary waves.; A température ambiante, la plupart des monocouches constituées de molécules formées de chaînes alcanes présentent des phases caractérisées par la rotation des chaînes autour de leur axe moléculaire. Afin d'obtenir un ordre cristallin, il est impératif de réduire l'énergie du système. De nouvelles sous-phases liquides utilisant des agents cryoprotectifs permettent d'accéder à des températures plus faibles que la température de glace de l'eau. Les monocouches de surfactants tels que les acides gras ou les phospholipides ont été préparées à volume et pression surfacique constants. L'étude de la stabilité des films par refroidissement a été effectuée par des isothermes de Langmuir et GIXOS et celle de l'ordre dans le plan par GIXD. Les molécules adoptent des phases cristallines qui sont induites par un processus d'autoassemblage d'origine exclusivement entropique et dont la densité de compactage est similaire à celle d'un monocristal à trois dimensions. La technique d'XPCS a enfin été employée pour relier la dynamique de surface du système étudié à la formation des phases cristallines.

International audience; We present optical nonlinearities due to neutral and charged excitons trapped on monomolecular CdTe islands embedded in wide ZnTe–(Zn, Mg)Te quantum wells. These nonlinearities are demonstrated by low-temperature reflectivity and transmission experiments with additional continuous-wave excitation of the sample by different lines of an Ar2+ laser, at various intensities. Such “pumped” transmission measurements show the bleaching and blue-shift of the ground-state heavy-hole exciton, simultaneous to the appearance and strengthening of a lower energy contribution, assigned to a negative trion (X–). These phenomena are explained in terms of (i) space-charge regions in the heterostructure inducing a built-in electric field and (ii) a photo-induced excess of electrons which alters the probability of exciton creation. The experimental results are in good agreement with a simple modeling of the neutralization, by injected carriers, of the initial Stark shift of electronic levels.

International audience; Standard and piezomodulated optical spectroscopy is performed on ZnTe quantum wells embedding integer and fractional monolayers of CdTe. The samples, grown in a molecular-beam-epitaxy setup on the (001) surface of ZnTe substrates, all basically consist of 120-ML-wide ZnTe/(Zn,Mg)Te quantum wells, and some of them contain five equally spaced full or half-monolayers of CdTe, producing monomolecular islands of CdTe “buried” in the wide host ZnTe well. The latter behave as efficient recombination centers for excitons. In order to change the size and the configuration of the islands, various growth parameters have been changed between the different samples, e.g., the growth process (molecular-beam epitaxy of binaries or ternaries, or atomic-layer epitaxy) or the temperature. From spectroscopic measurements, the influence of these parameters is analyzed in detail, in terms of the size of the islands and of their in-plane spacing, or of the vertical correlation between these islands. The internal strain state of the CdTe insertions and the overall photoluminescence efficiency are also studied versus growth conditions.

International audience; Optical spectroscopy at T=2K reveals systematically a low-energy companion to the features of heavy- and light-hole excitons localized on CdTe 1-ML-high islands, buried in wide ZnTe/(Zn,Mg) Te quantum wells. These transitions are assigned to a negatively charged excitonic complex (trion) from magnetoreflectance and photoluminescence results. A mechanism of formation of the trion is proposed. A clear correlation is found between the exciton energies, which depend on the size and strain of the islands, and the trion binding energy, which varies between 1 and 3 meV.

International audience; Samples consisting of a 36–38-nm-wide ZnTe quantum well with barriers made up of short-period ZnTe-MgTe superlattices were studied by low-temperature optical spectroscopy. The samples were grown by molecular-beam epitaxy on nominal (001) ZnTe surfaces. This structure allows the simultaneous observation of the quantization of the center-of-mass motion of both light-hole and heavy-hole excitons. The light- or heavy-hole characters are assigned by piezomodulated reflectivity experiments. The observed quantized energies are in reasonable agreement with the results of a simple model of an infinite quantum well, including “transition layers” near the well-barrier interfaces. This model cannot, however, account quantitatively for the observation of the fundamental resonances slightly below the transverse-exciton energy of bulk ZnTe. This effect is tentatively attributed to the strongly nonparabolic dispersion, in addition to more complex additional boundary conditions, for exciton polaritons with small wave vectors (approximately equal to twice the wave vector of light in the material).

International audience; Samples based on strained CdTe-(Cd,Zn)Te quantum wells, in which heavy-hole excitons are type I and light-hole excitons are usually type II, are studied. Fractional atomic layers of ZnTe have been inserted in CdTe wells in order to trap light-hole excitons and to confer on them a type-I character. This is checked by low-temperature reflectance and piezoreflectance measurements compared to variational calculations of exciton energies. Moreover, reference samples where integer monolayers of ZnTe have been introduced, are also investigated. Comparisons of direct and modulated spectroscopic data of all samples provide a very strong indication that two fractional ZnTe layers, separated by a few CdTe monolayers, tend to grow in an ordered way: successive ZnTe islands appear to avoid growing on top of each other and to adopt a staggered lineup.