Structure of droplet-epitaxy-grown InAs/GaAs quantum dots

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98673/1/ApplPhysLett_98_243115.pd

Surface X-Ray Diffraction Results on the III–V Droplet Heteroepitaxy Growth Process for Quantum Dots: Recent Understanding and Open Questions

In recent years, epitaxial growth of self-assembled quantum dots has offered a way to incorporate new properties into existing solid state devices. Although the droplet heteroepitaxy method is relatively complex, it is quite relaxed with respect to the material combinations that can be used. This offers great flexibility in the systems that can be achieved. In this paper we review the structure and composition of a number of quantum dot systems grown by the droplet heteroepitaxy method, emphasizing the insights that these experiments provide with respect to the growth process. Detailed structural and composition information has been obtained using surface X-ray diffraction analyzed by the COBRA phase retrieval method. A number of interesting phenomena have been observed: penetration of the dots into the substrate (“nano-drilling”) is often encountered; interdiffusion and intermixing already start when the group III droplets are deposited, and structure and composition may be very different from the one initially intended

Pressure - Induced Changes in the Local Structure of KNbO

. The local structure of the perovskite KNbO 3 at 77 K and 300 K under high pressure, up to 15.8 GPa, has been investigated using the X-ray absorption fine structure (XAFS) technique. We found that local distortions exist throughout the measured range which are peaked off-center at the rhombohedral symmetry sites with a width narrower than the separation between the rhombohedral and orthorhombic sites. On the other hand, diffraction indicates an orthorhombic average structure at 300K, and optical measurements suggest a transition to a cubic phase at high pressures. To explain the difference between pressure - induced changes in KNbO 3 structure as measured by XAFS and optical techniques, a pressure - dependent hopping rate in the framework of the eight - site model is proposed. INTRODUCTION Phase transitions in potassium niobate (KNbO 3 ) have been extensively studied, both experimentally and theoretically, since the discovery of its ferroelectric activity in 1949 [1]. Along with oth..

Oxygen Displacement in Cuprates under Ionic Liquid Field-Effect Gating

We studied structural changes in a 5 unit cell thick La1.96Sr0.04CuO4 film, epitaxially grown on a LaSrAlO4 substrate with a single unit cell buffer layer, when ultra-high electric fields were induced in the film by applying a gate voltage between the film (ground) and an ionic liquid in contact with it. Measuring the diffraction intensity along the substrate-defined Bragg rods and analyzing the results using a phase retrieval method we obtained the three-dimensional electron density in the film, buffer layer, and topmost atomic layers of the substrate under different applied gate voltages. The main structural observations were: (i) there were no structural changes when the voltage was negative, holes were injected into the film making it more metallic and screening the electric field; (ii) when the voltage was positive, the film was depleted of holes becoming more insulating, the electric field extended throughout the film, the partial surface monolayer became disordered, and equatorial oxygen atoms were displaced towards the surface; (iii) the changes in surface disorder and the oxygen displacements were both reversed when a negative voltage was applied; and (iv) the c-axis lattice constant of the film did not change in spite of the displacement of equatorial oxygen atoms

Anomalous Interface and Surface Strontium Segregation in (La[subscript 1-y]Sr[subscript y])[subscript 2]CoO[subscript 4±δ]/La[subscript 1-x]Sr[subscript x]CoO[subscript 3-δ] Heterostructured Thin Films

Heterostructured oxides have shown unusual electrochemical properties including enhanced catalytic activity, ion transport, and stability. In particular, it has been shown recently that the activity of oxygen electrocatalysis on the Ruddlesden–Popper/perovskite (La[subscript 1-y]Sr[subscript y])[subscript 2]CoO[subscript 4±δ]/La[subscript 1–x]Sr[subscript x]CoO[subscript 3−δ] heterostructure is remarkably enhanced relative to the Ruddlesden–Popper and perovskite constituents. Here we report the first atomic-scale structure and composition of (La[subscript 1–y]Sr[subscript y])[subscript 2]CoO[subscript 4±δ]/La[subscript 1–x]Sr[subscript x]CoO[subscript 3−δ] grown on SrTiO[subscript 3]. We observe anomalous strontium segregation from the perovskite to the interface and the Ruddlesden–Popper phase using direct X-ray methods as well as with ab initio calculations. Such Sr segregation occurred during the film growth, and no significant changes were found upon subsequent annealing in O[subscript 2]. Our findings provide insights into the design of highly active catalysts for oxygen electrocatalysis.United States. Dept. of Energy (SISGR DESC0002633)King Abdullah University of Science and TechnologyCenter for Clean Water and Clean Energy at MIT and KFUPMIsrael Science Foundation (Grant 1005/11)United States. Dept. of Energy. National Energy Technology Laboratory. Solid State Energy Conversion Alliance Core Technology Program (Funding Opportunity DEFE0009435