M. MacKenzie

Solid state metathesis routes to transition metal carbides

Flame initiated (800 °C, 10 s) or bulk thermal (2 days, 1000 °C) reactions of mixed powders of transition metal halides and CaC 2 or Al 4 C 3 produce transition metal carbides (TiC, ZrC, HfC, V 8 C 7 , NbC, TaC, Cr 3 C 2 , Mo 2 C and WC) in good yields. The carbides were characterised by X-ray powder diffraction, SEM/EDX, FTIR, microelemental analysis, TEM, electron diffraction and ELNES.

Characterization of mineral surfaces using FIB and TEM: A case study of naturally weathered alkali feldspars

Abstract Using a focused ion beam (FIB) instrument, electron-transparent samples (termed foils) have been cut from the naturally weathered surfaces of perthitic alkali feldspars recovered from soils overlying the Shap granite, northwest England. Characterization of these foils by transmission electron microscopy (TEM) has enabled determination of the crystallinity and chemical composition of near-surface regions of the feldspar and an assessment of the influence of intragranular microtextures on the microtopography of grain surfaces and development of etch pits. Damage accompanying implantation of the 30 kV Ga+ ions used for imaging and deposition of protective platinum prior to ion milling creates amorphous layers beneath outer grain surfaces, but can be overcome by coating grains with >85 nm of gold before FIB work. The sidewalls of the foil and feldspar surrounding original voids are also partially amorphized during later stages of ion milling. No evidence was found for the presence of amorphous or crystalline weathering products or amorphous “leached layers” immediately beneath outer grain surfaces. The absence of a leached layer indicates that chemical weathering of feldspar in the Shap soils is stoichiometric, or if non-stoichiometric, either the layer is too thin to resolve by the TEM techniques used (i.e., ≤~2.5 nm) or an insufficient proportion of ions have been leached from near-surface regions so that feldspar crystallinity is maintained. No evidence was found for any difference in the mechanisms of weathering where a microbial filament rests on the feldspar surface. Sub-micrometer-sized steps on the grain surface have formed where subgrains and exsolution lamellae have influenced the propagation of fractures during physical weathering, whereas finer scale corrugations form due to compositional or strain-related differences in dissolution rates of albite platelets and enclosing tweed orthoclase. With progressive weathering, etch pits that initiated at the grain surface extend into grain interiors as etch tubes by exploiting preexisting networks of nanopores that formed during the igneous history of the grain. The combination of FIB and TEM techniques is an especially powerful way of exploring mechanisms of weathering within the “internal zone” beneath outer grain surfaces, but results must be interpreted with caution owing to the ease with which artifacts can be created by the high-energy ion and electron beams used in the preparation and characterization of the foils

Near-simultaneous dual energy range EELS spectrum imaging.

A system that allows the collection of the low loss spectrum and the core loss spectrum, covering different energy regions, at each pixel in a spectrum image is described. It makes use of a fast electrostatic shutter with control signals provided by the spectrum imaging software and synchronisation provided by the CCD camera controller. The system also allows simultaneous collection of the X-ray spectrum and the signals from the imaging detectors while allowing the use of the existing features of the spectrum imaging software including drift correction and sub-pixel scanning. The system allows acquisition of high-quality spectra from both the core and the low loss regions, allowing full processing of the EELS data. Examples are given to show the benefits, including deconvolution, absolute thickness mapping and determination of numbers of atoms per unit area and per unit volume. Possible further developments are considered.

Surface effects in Mn L3,2 x-ray absorption spectra from (Ga,Mn)As

We have identified a Mn-rich layer on the surface on (Ga,Mn)As thin films which significantly influences soft x-ray absorption measurements. The Mn L3,2 x-ray absorption spectra of the untreated films show a strong multiplet structure, consistent with earlier observations and characteristic of MnO. After removal of the surface layer, the multiplet structure is less pronounced and the spectrum is shifted to ∼0.5 eV lower photon energy. Comparison with calculated spectra imply a localized Mn ground state for the untreated sample and a hybridized ground state after etching. In addition, a large x-ray magnetic circular dichroism is observed at the Mn L3,2 edge in the etched film. These results may explain several peculiarities of previously reported x-ray absorption studies from (Ga,Mn)As.

Origins of ferromagnetism in transition-metal doped Si

We present results of the magnetic, structural, and chemical characterizations of Mn+-implanted Si displaying n-type semiconducting behavior and ferromagnetic ordering with Curie temperature, TC, well above room temperature. The temperature-dependent magnetization measured by superconducting quantum interference device from 5 to 800 K was characterized by three different critical temperatures (TC*∼45 K, TC1∼630–650 K, and TC2∼805–825 K). Their origins were investigated using dynamic secondary ion mass spectroscopy and transmission electron microscopy (TEM) techniques, including electron energy loss spectroscopy, Z-contrast scanning TEM imaging, and electron diffraction. We provided direct evidences of the presence of a small amount of Fe and Cr impurities which were unintentionally doped into the samples together with the Mn+ ions as well as the formation of Mn-rich precipitates embedded in a Mn-poor matrix. The observed TC* is attributed to the Mn4Si7 precipitates identified by electron diffraction. Poss...

Enhanced band-gap blueshift due to group V intermixing in InGaAsP multiple quantum well laser structures induced by low temperature grown InP

Photoluminescence and cross-sectional transmission electron microscopy, combined with x-ray compositional analysis, have been used to study quantum well intermixing in an InGaAsP quantum well laser structure. Quantum well intermixing is induced by capping the samples with a layer of InP grown at low temperature (300 °C) and subjecting them to rapid thermal anneal treatments in the temperature range 600–800 °C. The presence of the low temperature InP layer, which contains an abundance of nonequilibrium point defects, significantly enhances the intermixing on annealing, producing a large band-gap blueshift. The microscopy results show good broadening with smeared interfaces, and the compositional analysis suggests this can be attributed to the intermixing of group V atoms.

Quantifying the oxidation of AlN using electron energy loss spectroscopy

The successful exploitation of selective oxidation in III-V semiconducting devices depends on the ability to control the nature and extent of the oxidation process over distances of the order of 10 nm. This paper reports an analytical method capable of measuring the nature and extent of the oxidation process in AlN. Analytical electron microscopy and, in particular the near-edge fine structure present on characteristic edges in electron energy loss spectroscopy, is used to measure the extent of surface oxidation of AlN powder in air at room temperature. The oxidation was found to result in the formation of a surface oxidation layer of amorphous Al2O3 and the retention of some of the N produced, most likely in the form of N2 gas. Application of the technique to nanoscale analysis of semiconductors is considered.

Ferromagnetic coupling field reduction in CoFeB tunnel junctions deposited by ion beam

In this paper, junctions with reduced H/sub f/ coupling were fabricated by ion beam deposition and oxidation, using CoFeB electrodes. The CoFeB layer has a strong (111) texture that can be the origin of lower H/sub f/ and coercivity when compared with CoFe. Junctions processed down to 2/spl times/4 /spl mu/m/sup 2/ with 40-thick CoFeB bottom electrodes have 42% of tunneling magnetoresistance (TMR), (R/spl times/A/spl sim/400 /spl Omega/./spl mu/m/sup 2/), H/sub c/ of /spl sim/10 Oe and H/sub f/ of /spl sim/2 Oe. CoFe-based junctions (R/spl times/A/spl sim/500 /spl Omega/./spl mu/m/sup 2/) have lower TMR (/spl sim/35%) and larger H/sub f/ (/spl sim/5-6 Oe) and H/sub c/ (/spl sim/12-14 Oe). Local chemical composition analysis of the cross section indicated Fe-O segregation with very little Co grown on top of the barrier for CoFe-based junctions and not for CoFeB ones.

Chromium nitrides (CrN, Cr2N) from solid state metathesis reactions: effects of dilution and nitriding reagent

The solid state metathesis (SSM) reaction of CrCl3 with Li3N in a sealed evacuated Pyrex ampoule at 445C produces a propagation wave and enables phase pure Cr2N to be isolated. The SSM reaction of CrCl2 and Mg3N2 at 350-500C with dilutant MgCl2 under similar conditions produces single phase CrN. Reaction of chromium oxide with Li3N at 500C produces CrN after 2h and Cr2N after 60h. This is the first report of a metal nitride being formed in a SSM reaction using an oxide starting material. The SSM nitrides were shown to have superior bulk nitridation levels (combustion analysis) and equivalent microscopic compositions (EELS, XRD) to commercial materials. The phases of chromium nitride produced were correlated with reaction enthalpy, dilution conditions and nitriding agent. The products were analysed by X-ray powder diffraction, FTIR, combustion microelemental analysis, SEM/EDAX, XPS and ELNES.

Double-barrier magnetic tunnel junctions with GeSbTe thermal barriers for improved thermally assisted magnetoresistive random access memory cells

Double-barrier magnetic tunnel junction (MTJ) cells incorporating one thermal barrier (GeSbTe) were fabricated for improved thermally assisted magnetic switching. The MTJ has two Al2O3 barriers with a common weakly pinned structure (storage layer) and two pinned layers (reference). The structural quality of the double junction stack and the roughness at the (buffer/thermal barrier) level were investigated and optimized. To minimize the required heating during writing, the blocking temperature (TB) of the storage layer is reduced to 110 °C by thinning the MnIr layer to 80 A, while a strong exchange coupling and TB∼300°C are obtained at the reference layers with a synthetic antiferromagnetically coupled CoFeB∕Ru∕CoFeB structure pinned to 250-A-thick MnIr. For the write experiments, the current flowing through the MTJ (patterned down to 2μm2) increases the temperature above the storage layer TB, under an external field of +∕−80Oe. Current densities <1mA∕μm2 were enough to write in the MTJs with a thermal bar...