Edith Bourret

Reactions of trialkylgalliums with substituted hydrazines leading to the formation of rings and cages: X-ray structures of (iPr2GaNHNMe2)2 and (MeGaNHNtBu)4 ☆

The reaction of trialkylgalliums with substituted hydrazines forms simple Lewis acid–base adducts at room temperature. Subsequent thermolysis of these adducts leads to step by step alkane loss forming first dimeric rings, then tetrameric cages. Thus the dimeric compounds [Me 2 GaNHNMe 2 ] 2 , [Et 2 GaNHNMe 2 ] 2 , [Me 2 GaNHNPh 2 ] 2 , [Et 2 GaNHNPh 2 ] 2 , [ i Pr 2 GaNHNMe 2 ] 2 , and [Me 2 GaNHNH t Bu] 2 , have been synthesized. In addition, [ i Pr 2 GaNHNMe 2 ] 2 has been characterized crystallographically. Although exclusively in the anti conformation in the solid state, the dimers are fluxional in solution at room temperature. The fluxionality is examined in detail by NMR spectroscopy. The tetrameric [MeGaNHN t Bu] 4 has been isolated and crystallographically characterized. It is shown to have a cage structure consisting of two hexagonal and four pentagonal rings.

Functional electronic inversion layers at ferroelectric domain walls

Ferroelectric domain walls hold great promise as functional two-dimensional materials because of their unusual electronic properties. Particularly intriguing are the so-called charged walls where a polarity mismatch causes local, diverging electrostatic potentials requiring charge compensation and hence a change in the electronic structure. These walls can exhibit significantly enhanced conductivity and serve as a circuit path. The development of all-domain-wall devices, however, also requires walls with controllable output to emulate electronic nano-components such as diodes and transistors. Here we demonstrate electric-field control of the electronic transport at ferroelectric domain walls. We reversibly switch from resistive to conductive behaviour at charged walls in semiconducting ErMnO3. We relate the transition to the formation-and eventual activation-of an inversion layer that acts as the channel for the charge transport. The findings provide new insight into the domain-wall physics in ferroelectrics and foreshadow the possibility to design elementary digital devices for all-domain-wall circuitry.

Nucleation and evolution of misfit dislocations in ZnSe/GaAs (001) heterostructures grown by low‐pressure organometallic vapor phase epitaxy

Transmission electron microscopy and x‐ray diffraction were used to study strain relaxation and the evolution of the dislocation structure in ZnSe epilayers grown by low‐pressure organometallic vapor phase epitaxy on a (001) surface of semi‐insulating GaAs. Before the ZnSe growth, the substrate surface was exposed to a flow of tertiarybutylarsine to promote an As‐terminated surface. This surface treatment results in a low density of stacking faults; 60° misfit dislocations were observed at a layer thickness as low as 0.05 μm. This agrees well with the theoretical critical value for misfit dislocation formation in the ZnSe/GaAs system, but is much lower than experimental critical thicknesses reported earlier. Various mechanisms of misfit dislocation generation were observed at different growth stages. The evolution of the dislocation structure is discussed in relation with the morphology of the ZnSe layers.

Growth and characterization of ZnSe grown by organometallic vapor phase epitaxy using diisopropyl selenide and diethyl zinc

Abstract Diisopropyl selenide (DIPSe) and diethyl zinc have been used successfully to grow ZnSe epitaxial layers on GaAs by organometallic vapor phase epitaxy (OMVPE). The epilayers have been characterized by optical microscopy, Rutherford backscattering, X-ray diffraction and photoluminescence. High quality crystalline material exhibiting good surface morphology was obtained in the temperature range 380–500°C and pressure range 30–400 Torr. At temperatures below 440°C and a total reactor pressure of 300 Torr and above, the growth process is controlled by surface reaction kinetics. As the pressure is decreased, surface reaction processes control the growth process up to 580°C. At 100 and 300 Torr, the activation energy values of the thermally activated growth process are similar to those measured using other alkyl Se sources. The crystalline quality was assessed by RBS as a function of thickness of the films. Photoluminescence spectra are analyzed in detail and the effects of sample thickness, precursor ratios and growth temperature are discussed.

Real-time Crystal Growth Visualization and Quantification by Energy-Resolved Neutron Imaging

Energy-resolved neutron imaging is investigated as a real-time diagnostic tool for visualization and in-situ measurements of “blind” processes. This technique is demonstrated for the Bridgman-type crystal growth enabling remote and direct measurements of growth parameters crucial for process optimization. The location and shape of the interface between liquid and solid phases are monitored in real-time, concurrently with the measurement of elemental distribution within the growth volume and with the identification of structural features with a ~100 μm spatial resolution. Such diagnostics can substantially reduce the development time between exploratory small scale growth of new materials and their subsequent commercial production. This technique is widely applicable and is not limited to crystal growth processes.

Optimization of scintillation performance via a combinatorial multi-element co-doping strategy: Application to NaI:Tl

Historically, the discovery and optimization of doped bulk materials has been predominantly developed through an Edisonian approach. While successful and despite the constant progress in fundamental understanding of detector materials physics, the process has been restricted by its inherent slow pace and low success rate. This poor throughput owes largely to the considerable compositional space that needs to be accounted for to fully comprehend complex material/performance relationship. Here, we present a combinatorial approach where doped bulk scintillator materials can be rapidly optimized for their properties through concurrent extrinsic doping/co-doping strategies. The concept that makes use of Design of Experiment, rapid growth and evaluation techniques, and multivariable regression analysis, has been successfully applied to the engineering of NaI performance, a historical but mediocre performer in scintillation detection. Using this approach, we identified a three-element doping/co-doping strategy that significantly improves the material performance. The composition was uncovered by simultaneously screening for a beneficial co-dopant ion among the alkaline earth metal family and by optimizing its concentration and that of Tl+ and Eu2+ ions. The composition with the best performance was identified as 0.1% mol Tl+, 0.1% mol Eu2+ and 0.2% mol Ca2+. This formulation shows enhancement of energy resolution and light output at 662 keV, from 6.3 to 4.9%, and from 44,000 to 52,000 ph/MeV, respectively. The method, in addition to improving NaI performance, provides a versatile framework for rapidly unveiling complex and concealed correlations between material composition and performance, and should be broadly applicable to optimization of other material properties.A combinatorial approach where doped bulk scintillator materials can be rapidly optimized for their properties through concurrent extrinsic doping/co-doping strategies is presented. The concept that makes use of design of experiment, rapid growth, and evaluation techniques, and multivariable regression analysis, has been successfully applied to the engineering of NaI performance, a historical but mediocre performer in scintillation detection. Using this approach, we identified a three-element doping/co-doping strategy that significantly improves the material performance. The composition was uncovered by simultaneously screening for a beneficial co-dopant ion among the alkaline earth metal family and by optimizing its concentration and that of Tl+ and Eu2+ ions. The composition with the best performance was identified as 0.1% mol Tl+, 0.1% mol Eu2+, and 0.2% mol Ca2+. This formulation shows enhancement of energy resolution and light output at 662 keV, from 6.3 to 4.9%, and from 44 000 to 52 000 ph/MeV, res...

Solid-state reaction in Pd/ZnSe thin film contacts

We report on solid‐state reactions in Pd thin film contacts on ZnSe at temperatures below 500 °C. We found that a solid‐state reaction was initiated at the Pd/ZnSe interface by thermal annealing at 200 °C. A tetragonal ternary phase, Pd5+xZnSe, consisting of highly oriented grains was formed as a result of this reaction. This phase is found to be stable up to an annealing temperature of 400 °C. The crystallography and morphology of this ternary Pd–ZnSe phase was studied by x‐ray diffraction and transmission electron microscopy and has similarities to the analogous ternary Pd–GaAs phase formed in the Pd/GaAs contact structure. The Pd/ZnSe interface is found to be thermally more stable than the corresponding Pd/GaAs and Pd/Si structures. Comparisons are made between the systematics of Pd/semiconductor interfacial phenomena on the three semiconductors.

Consequences of Ca Codoping in YAlO3:Ce Single Crystals

The influence of Ca codoping on the optical absorption, photo-, radio-, and thermo-luminescence properties of YAlO3 :Ce (YAP:Ce) crystals has been studied for four different calcium concentrations ranging from 0 to 500 ppm. Ca codoping results in a partial oxidation of Ce3+ into Ce4+ , The luminescence time response under pulsed X-ray excitation of the Ce3+ /Ce4+ admixure clearly demonstrates the role of hole migration on both the rise time and the generally observed slow components. From an application point of view, Ca codoping significantly improves the timing performances, but the induced presence of Ce4+ ions is also the cause of a reduction in scintillation efficiency.

Antisite defects created in neutron irradiated GaP crystals

This article compares the nature of phosphorus antisite defects in as‐grown and neutron irradiated GaP crystals. Electron spin resonance studies indicate that these defects in both kinds of crystals have identical close neighbors consisting of four phosphorus atoms. Neutron irradiation of GaP introduced an additional defect (called WA1), which is linked to a gallium antisite. Characteristic absorption bands and conductivity of neutron irradiated GaP crystals are discussed as well.

A Ga4N8 cage structure formed by reaction of trimethylgallium with phenylhydrazine

Methane elimination during thermolysis of GaMe3 with PhHNNH2 yields sequentially dimeric [Me2Ga{µ-N(H)N(H)Ph}]2, tetrameric [MeGa{µ-N(H)NPh}]4 and, ultimately, GaN; the X-ray structure of the tetramer shows a novel Ga4N8 cage.