Derek W. Lawther

Vacancy generation resulting from electrical deactivation of arsenic

Electrical deactivation of arsenic in highly doped silicon has been studied using the positron‐beam technique. Direct experimental evidence linking the formation of arsenic‐vacancy complexes (i.e., Asn‐v) to the deactivation process is reported. The average number of arsenic atoms per complex, n≳2, was determined by comparing the observed complex concentrations with those of the deactivated arsenic inferred from Hall‐effect measurements.

Chemical information in positron annihilation spectra

Positron annihilation spectra of arsenic‐ and gold‐implanted silicon are compared with spectra from bulk samples of arsenic and gold. Spectra with strongly reduced background intensities were recorded using a two detector coincidence system with a variable‐energy positron beam. It is shown that features in the high‐momentum region of the spectra (∼514–520 keV) can be identified with particular elements and that this identification is independent of structure, i.e., whether the element forms the bulk or is an implanted impurity. Proportionality between the intensity of characteristic spectral features and the fraction of annihilating positrons is also demonstrated, using the native oxide on a silicon wafer as a test case.

Structure and stability of rapidly quenched Al86Cr14−x Fe x alloys

The structural and thermal properties of melt spun Al86Cr14−x Fex, alloys have been investigated using X-ray diffraction, transmission electron microscopy, electron diffraction and differential thermal analysis. Electron diffraction patterns indicate that all alloys contain quasicrystalline nodules with icosahedral symmetry in a matrix of f cc aluminium. Microscopically the alloys fall into two categories; those with x ⩽ 6 and those withx > 6. The first class alloys shows large (>1μm) icosahedral crystallites with well defined dendritic crystallite growth and sharp quasicrystalline X-ray diffraction peaks. The latter alloys show quasicrystalline crystallites which are predominantly much smaller as well as considerably broadened X-ray diffraction peaks. Thermal analysis measurements indicate that the activation energy for crystallization in these alloys is about 1.5 eV.

Thermal expansion of Bi2.2Sr1.8CaCu2Ox superconductor single crystals

Using a low-temperature stage X-ray diffractometer, we have investigated the temperature dependence of the lattice parameters of single crystal superconductors Bi2Sr1.8CaCu2Ox. The experimental results show that (i) the lattice constants increase linearly with increasing temperature above about 100 K; (ii) thec-axis lattice parameter shows a kink in the superconducting transition region, while thea, b-axis parameters do not show any anomalous behavior in this region; (iii) botha, b, andc show negative thermal expansion coefficients below 40–50 K, which may be related to the characteristics of the Cu-O bond.

On the electrical deactivation of arsenic in silicon

Previous work on thermally induced arsenic deactivation in highly doped silicon has proven the generation of vacancies and suggests the formation of arsenic-vacancy clusters as the deactivation mechanism. Using positron annihilation spectroscopy in the two-detector coincidence geometry, we are able to show that the thermally generated vacancies are indeed surrounded by arsenic atoms.

Transition metal site distributions in binary aluminum-transition metal quasicrystals: AlV and AlCr

Abstract A comprehensive diffraction and thermal analysis investigation of the icosahedral (i) phase formation in binary AlV and AlCr is presented. The observed behavior is compared with similar studies on AlTM (TM ≡ Mn, Fe, Co) with particular emphasis placed upon the existence (or not) of distinct classes of transition metal sites. Contrary to that which is found for the i phase of pseudobinary AlTM1TM2 alloys, it is concluded that no such class distinction is present in the binary alloys. This apparent discrepancy is justified in terms of an indirect coupling between adjacent transition metal sites of a Mackay icosahedron.

Structural and Mössbauer effect studies of quasicrystalline Al-Pd-Cr-Fe alloys

Structural studies of conventionally solidified Al80-2xPd20CrxFex alloys have shown that the formation of a well ordered icosahedral phase occurs for a composition where the free-electron Fermi sphere is coincident with the first pseudo-Brillouin zone boundary. This condition indicates the importance of the Hume-Rothery stabilization criterion for icosahedral phase formation in this system. Mossbauer effect studies show the similarity of the local structural order in stable Al72Pd20Cr4Fe4 to that found in stable quasicrystalline Al-Cu-Fe alloys and indicate a higher degree of structural order compared with that found in metastable icosahedral alloys prepared by non-equilibrium processing techniques.

Physical properties of amorphous Al-Gd-transition metal alloys

Abstract Amorphous alloys of the composition Al 65 Gd 15 Cu 20 and Al 1 Gd 1 Fe 1 have been prepared by rapid quenching from the melt. The alloys have been studied by X-ray diffraction, thermal analysis and SQUID magnetization methods. Thermal analysis results show crystallization temperatures of 647 and 945 K for AlGdCu and AlGdFe, respectively. For AlGdCu, magnetization measurements show Curie-like behaviour with a localized Gd magnetic moment of 8.0μ B . Magnetic measurements of AlGdFe show conventional ferromagnetic behaviour. The Curie temperature is found to be 275 K and a saturation magnetization of 124 emu/g is measured at 4.2 K in an applied magnetic field of 1 T.

Mossbauer effect studies of local Fe environments in crystalline and quasicrystalline Al-Cu-Fe alloys

Abstract AlCuFe alloys of several different stoichiometries have been prepared using a variety of sample preparation methods and have been studied using 57Fe Mossbauer effect spectroscopy. Well ordered icosahedral, phason disordered icosahedral and rhombohedral crystalline phases were investigated. The resulting spectra indicate that the Fe environment in the various phases studied are virtually identical. However, as the sample stoichiometry is varied, two de-stabilizing mechanisms are observed for the icosahedral phase: one involving the formation of a pronounced correlation between the isomer shift and the quadrupole splitting; and the other associated with a reduction of the variance in the isomer shift distribution.

A positron annihilation study of crystalline, quasicrystalline and amorphous Al-Cu-T (T=Fe,V) alloys

Results of a Doppler broadening positron annihilation study on crystalline, quasicrystalline and amorphous Al-Cu-T (T=FeV) alloys are presented. For the first time it is shown that a quasicrystal (Al65Cu20Fe15) can be prepared with a positron trapping site concentration below the sensitivity of positron annihilation spectroscopy. The results and those of similar studies previously reported for quasicrystalline Al-Mn-Si and Al-Cu-Li are discussed in conjunction with possible stabilisation mechanisms.