Pierre-Henri Jouneau

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.

Compared growth mechanisms of Zn-polar ZnO nanowires on O-polar ZnO and on sapphire

Controlling the growth of zinc oxide nanowires is necessary to optimize the performance of nanowire-based devices such as photovoltaic solar cells, nano-generators, or light-emitting diodes. With this in mind, we investigate the nucleation and growth mechanisms of ZnO nanowires grown by metalorganic vapor phase epitaxy either on O-polar ZnO or on sapphire substrates. Whatever the substrate, ZnO nanowires are Zn-polar, as demonstrated by convergent beam electron diffraction. For growth on O-polar ZnO substrate, the nanowires are found to sit on O-polar pyramids. As growth proceeds, the inversion domain boundary moves up in order to remain at the top of the O-polar pyramids. For growth on sapphire substrates, the nanowires may also originate from the sapphire/ZnO interface. The presence of atomic steps and the non-polar character of sapphire could be the cause of the Zn-polar crystal nucleation on sapphire, whereas it is proposed that the segregation of aluminum impurities could account for the nucleation of inverted domains for growth on O-polar ZnO.

Visualization of Tm dopant atoms diffused out of GaN quantum dots

GaN quantum dots doped with Tm atoms and embedded in AlN have been characterized by high-angle annular dark-field imaging using a scanning transmission electron microscope. Direct visualization of individual Tm atoms in AlN layers has been achieved. We have found that besides being present in GaN dots, Tm atoms also tend to segregate at AlN barriers. The Tm distribution is related to the capping mechanism of the dots with AlN. A visibility coefficient based on locally integrated, rather than peak, intensities is introduced to determine quantitatively the number of Tm atoms in a given atomic column. Experimental and simulated images show that this visibility presents a reduced sensitivity to the defocus or to the position of the Tm atom within the thin lamella.

INVESTIGATION OF THE EPITAXIAL GROWTH MECHANISM OF ZNTE ON (001) CDTE

The first stages of the growth of highly strained ZnTe on (001) CdTe are investigated by reflection high energy electron diffraction, HRTEM (high resolution transmission electron microscopy), x‐ray photoelectron spectroscopy, and x‐ray double diffraction. A precise study of the factors influencing the critical thickness is presented, with emphasis on the effect of Zn pre‐exposure of the CdTe surface on the subsequent ZnTe growth. Below the critical thickness small lattice distortions attributed to a nontetragonal elastic distortion are detected. An exposure of the (001)CdTe surface to a Zn flux leads to the desorption of the Cd atoms present on the top of the surface and to the formation of a c(2×2) reconstructed surface with half a monolayer of Zn on the top of the surface. Finally, the morphology of an ultrathin strained ZnTe layer embedded in a (001)CdTe matrix will be discussed using results obtained from analysis of the digitized HRTEM image.

Structural recovery of ion implanted ZnO nanowires

Ion implantation is an interesting method to dope semiconducting materials such as zinc oxide provided that the implantation-induced defects can be subsequently removed. Nitrogen implantation followed by anneals under O2 were carried out on zinc oxide nanowires in the same conditions as in a previous study on bulk ZnO [J. Appl.Phys. 109, 023513 (2011)], allowing a direct comparison of the defect recovery mechanisms. Transmission electron microscopy and cathodoluminescence were carried out to assess the effects of nitrogen implantation and of subsequent anneals on the structural and optical properties of ZnO nanowires. Defect recovery is shown to be more effective in nanowires compared with bulk material due to the proximity of free surfaces. Nevertheless, the optical emission of implanted and annealed nanowires deteriorated compared to as-grown nanowires, as also observed for unimplanted and annealed nanowires. This is tentatively attributed to the dissociation of excitons in the space charge region induced by O2 adsorption on the nanowire surface.

Atomic arrangement at ZnTe/CdSe interfaces determined by high resolution scanning transmission electron microscopy and atom probe tomography

High resolution scanning transmission electron microscopy and atom probe tomography experiments reveal the presence of an intermediate layer at the interface between two binary compounds with no common atom, namely, ZnTe and CdSe for samples grown by Molecular Beam Epitaxy under standard conditions. This thin transition layer, of the order of 1 to 3 atomic planes, contains typically one monolayer of ZnSe. Even if it occurs at each interface, the direct interface, i.e., ZnTe on CdSe, is sharper than the reverse one, where the ZnSe layer is likely surrounded by alloyed layers. On the other hand, a CdTe-like interface was never observed. This interface knowledge is crucial to properly design superlattices for optoelectronic applications and to master band-gap engineering.

InGaN nanowires with high InN molar fraction: growth, structural and optical properties.

The structural and optical properties of axial GaN/InGaN/GaN nanowire heterostructures with high InN molar fractions grown by molecular beam epitaxy have been studied at the nanoscale by a combination of electron microscopy, extended x-ray absorption fine structure and nano-cathodoluminescence techniques. InN molar fractions up to 50% have been successfully incorporated without extended defects, as evidence of nanowire potentialities for practical device realisation in such a composition range. Taking advantage of the N-polarity of the self-nucleated GaN NWs grown by molecular beam epitaxy on Si(111), the N-polar InGaN stability temperature diagram has been experimentally determined and found to extend to a higher temperature than its metal-polar counterpart. Furthermore, annealing of GaN-capped InGaN NWs up to 800 °C has been found to result in a 20 times increase of photoluminescence intensity, which is assigned to point defect curing.

Atomic Scale Structural Characterization of Epitaxial (Cd,Cr)Te Magnetic Semiconductor

A detailed knowledge of the atomic structure of magnetic semiconductors is crucial to understanding their electronic and magnetic properties, which could enable spintronic applications. Energy-dispersive X-ray spectrometry (EDX) in the scanning transmission electron microscope and atom probe tomography (APT) experiments reveal the formation of Cr-rich regions in Cd1-x Cr x Te layers grown by molecular beam epitaxy. These Cr-rich regions occur on a length scale of 6-10 nm at a nominal Cr composition of x=0.034 and evolve toward an ellipsoidal shape oriented along directions at a composition of x=0.083. Statistical analysis of the APT reconstructed volume reveals that the Cr aggregation increases with the average Cr composition. The correlation with the magnetic properties of such (Cd,Cr)Te layers is discussed within the framework of strongly inhomogeneous materials. Finally, difficulties in accurately quantifying the Cr distribution in the CdTe matrix on an atomic scale by EDX and APT are discussed.

Dopant radial inhomogeneity in Mg-doped GaN nanowires

Using atom probe tomography, it is demonstrated that Mg doping of GaN nanowires grown by Molecular Beam Epitaxy results in a marked radial inhomogeneity, namely a higher Mg content in the periphery of the nanowires. This spatial inhomogeneity is attributed to a preferential incorporation of Mg through the m-plane sidewalls of nanowires and is related to the formation of a Mg-rich surface which is stabilized by hydrogen. This is further supported by Raman spectroscopy experiments which give evidence of Mg-H complexes in the doped nanowires. A Mg doping mechanism such as this, specific to nanowires, may lead to higher levels of Mg doping than in layers, boosting the potential interest of nanowires for light emitting diode applications.

High precision strain mapping of topological insulator HgTe/CdTe

Topological insulators (TIs) represent a class of matter associated with unique electronic and spin properties held by chiral and massless Dirac fermions. The lattice mismatch between the HgTe layer and the CdTe substrate, which is the reason for the TI properties in HgTe/CdTe, is less than 0.4%. Measuring it necessitates an extremely sensitive technique, while a nanometric spatial resolution is mandatory to characterize the strain gradient across the layers. In this letter, we demonstrate the use of nano-beam precession electron diffraction to map the strain of the whole stack, thus experimentally addressing the strain distribution in such systems. Strain maps with a precision of 0.03% and a spatial resolution of 1.9 nm show good agreement with finite element simulations of the expected strain. Strain values and gradients are within the intervals predicted to yield the TI properties of the material.