H. Mariette

Impact of surfaces on the optical properties of GaAs nanowires

The effect of surfaces on the optical properties of GaAs nanowires is evidenced by comparing nanowires with or without an AlGaAs capping shell as a function of the diameter. We find that the optical properties of unpassivated nanowires are governed by Fermi-level pinning, whereas, the optical properties of passivated nanowires are mainly governed by surface recombinations. Finally, we measure a surface recombination velocity of 3 x 10(3) cm s(-1) one order of magnitude lower than values previously reported for (110) GaAs surfaces. These results will serve as guidance for the application of nanowires in solar cell and light emitting devices.

Dynamically stable gallium surface coverages during plasma-assisted molecular-beam epitaxy of (0001) GaN

The Ga surface coverage during the growth of GaN by plasma-assisted molecular-beam epitaxy (PAMBE) has been systematically studied by reflection high-energy electron diffraction as a function of the Ga flux and the substrate temperature. As a consequence, a diagram is depicted, which describes the Ga surface coverage during PAMBE as function of growth conditions. In particular, we show that a region exists in this diagram, in which the Ga surface coverage is independent of fluctuations in the Ga flux or the substrate temperature and which forms a “growth window” for GaN growth. The influence of the Ga surface coverage on the GaN surface morphology and the growth kinetics is discussed.

II–VI quantum dot formation induced by surface energy change of a strained layer

A method for growing self-assembled II–VI quantum dots (QDs) is demonstrated: A highly strained CdTe layer, grown onto Zn(Mg)Te, is covered with an amorphous Te layer which is then desorbed. This induces QD formation, observed as an abrupt change of both the reflection high-energy electron diffraction pattern and the surface morphology studied by atomic force microscopy in an ultrahigh vacuum. The dots are also characterized after capping by microphotoluminescence. This morphology transition, which occurs after and not during the growth, can be understood in terms of variation of the surface energy in presence of the group-VI element, which compensates for the natural trend toward plastic relaxation in II–VI compounds. This method shows the strong influence of the surface energy (and not just the lattice mismatch) in inducing the formation of coherent islands for mismatched systems having a low dislocation formation energy such as CdTe/ZnTe and CdSe/ZnSe.

Anisotropic morphology of nonpolar a-plane GaN quantum dots and quantum wells

The growth of (11–20) or a-plane quantum dots and quantum wells by plasma-assisted molecular-beam epitaxy has been studied. It is shown that Ga-rich conditions lead to the formation of quantum dots, whereas quantum wells are obtained in N-rich conditions. Combining various experimental techniques, it is furthermore demonstrated that quantum dot nucleation along [1–100] and quantum well morphology in the (1–100) plane are influenced by anisotropic growth of AlN buffer layer. Moreover, it is established that peculiar morphological features of quantum dots and quantum wells, in particular the asymmetric shape of quantum dots, are related to the polar character of the [0001] direction in wurtzite nitride material.

Atomic layer epitaxy of CdTe and MnTe

Atomic deposition techniques are investigated for binary semiconductors of the telluride family, namely CdTe and MnTe. An original method for directly determining the CdTe atomic layer epitaxy (ALE) growth rate—in monolayers/cycle—is proposed, consisting in monitoring the reflection high‐energy electron diffraction (RHEED) sublimation intensity oscillations of an ALE grown CdTe layer deposited on a MgTe buffer layer. The ALE CdTe autoregulated growth rate at 0.5 monolayer/cycle (in the substrate temperature domain between 260 and 290 °C) is accounted for on the basis of an atomic model which relies on the alternating c(2×2) Cd and (2×1) Te surface reconstructions during the ALE cycle. RHEED studies on MnTe atomic deposition, together with x‐ray diffraction and transmission electron microscopy on ALE grown CdTe/MnTe superlattices reveal that all deposited Mn atoms are incorporated so that no autoregulated growth can be achieved. Furthermore, less than one or just one monolayer of Mn must be sent on the sur...

STRAIN MAPPING OF ULTRATHIN EPITAXIAL ZNTE AND MNTE LAYERS EMBEDDED IN CDTE

High‐resolution electron microscopy is used to investigate the morphology of ultrathin pseudomorphic (001) ZnTe and MnTe strained layers grown in CdTe. Local distortions of the crystal lattice are measured directly on high‐resolution images by use of image processing software. In the case of ZnTe/CdTe superlattices, the method yields the location of Zn within each place in the heterostructure and the total amount of Zn per period. For MnTe layers embedded in CdTe, one can deduce the atomic morphology of the interfaces which are shown to present a clear asymmetry.

Quantitative characterization of GaN quantum-dot structures in AlN by high-resolution transmission electron microscopy

GaN/AlN heterostructures grown by molecular beam epitaxy are studied by high-resolution transmission electron microscopy (HRTEM). The two-dimensional/three-dimensional Stranski–Krastanow growth mode transition of GaN allows the formation of GaN quantum-dot structures embedded in AlN. The nature of the wetting layer associated with these dots is determined by quantitative HRTEM analysis, based on comparison between interplanar distortion profiles of experimental and simulated images. This study demonstrates a low intermixing between GaN and AlN materials. Such result is also evidenced for the GaN dots.

Optical spin orientation of a single manganese atom in a semiconductor quantum dot using quasiresonant photoexcitation.

A hight degree of spin polarization is achieved for a Mn atom localized in a semiconductor quantum dot using quasi-resonant optical excitation at zero magnetic field. Optically created spin polarized carriers generate an energy splitting of the Mn spin and enable magnetic moment orientation controlled by the photon helicity and energy. The dynamics and the magnetic field dependence of the optical pumping mechanism shows that the spin lifetime of an isolated Mn atom at zero magnetic field is controlled by a magnetic anisotropy induced by the built-in strain in the quantum dots.

Self-assembled zinc blende GaN quantum dots grown by molecular-beam epitaxy

Zinc blende (ZB) GaN quantum dots have been grown by plasma-assisted molecular-beam epitaxy on AlN buffer layers using 3C-SiC(001) substrates. The two- to three-dimensional growth mode transition is studied by following the evolution of the reflection high-energy electron diffraction pattern. ZB GaN island layers are further examined by atomic force microscopy and transmission electron microscopy, extracting a mean island height of 1.6 nm and a mean diameter of 13 nm at a density of 1.3×1011 cm−2. Embedded ZB GaN quantum dots show strong ultraviolet photoluminescence without any thermal quenching up to room temperature.

CdTe/MgTe heterostructures: Growth by atomic layer epitaxy and determination of MgTe parameters

Atomic layer epitaxy (ALE) is investigated for the binary semiconductor MgTe. Reflection high‐energy electron‐diffraction studies on MgTe atomic deposition, together with x‐ray diffraction, high‐resolution transmission electron microscopy, and photoluminescence experiments on ALE‐grown CdTe/MgTe superlattices are reported. They reveal that an autoregulated growth at 0.7±0.1 MgTe monolayer/ALE cycle can be achieved in a substrate temperature range between 260 and 300°C. New values of the zinc‐blende MgTe lattice parameter, aMgTe=6.420 ±0.005 A, of the ratio of the elastic coefficients 2c12c11 (MgTe)=1.06, and of the 300 K MgTe band gap, EG=3.5 eV, are obtained by correlating x‐ray‐diffraction and optical results.