A. Belaidi

Conduction band-edge charge densities in II–VI compound semiconductors

Abstract The conduction band-edge charge densities of six zinc-blende II–VI compound semiconductors (CdTe, CdSe, CdS, ZnTe, ZnSe, and ZnS) are calculated using the empirical pseudopotential method (EPM) at selected Γ 1 c , X 1 c and X 3 c points. These charge density profiles are discussed on the basis of the bonding and antibonding behaviour. We predict the susceptibility of II–VI compound semiconductors to band structure modification by insertion of small atoms at their tetrahedral interstitial sites.

Valence and conduction band-edges for SiC and BN

Abstract The gaussian non-local pseudopotential method is used to compute the electronic charge densities at selected k-points for the valence and conduction band-edges of SiC and BN.

Conduction-band-edge densities in GaxAl1−xSb

Abstract The empirical pseudo potential method, combined with the virtual-crystal approximation, is used to compute electronic charge densities at the Гc and Xc k-oints of the conduction-band edge in the ternary semiconductor alloy system (GaxAl1–xSb). We find that these charge densities are strongly dependent upon the stoichiometric coefficient x, particularly at the Xc points. As a consequence, the value of x could change the way that interstitial impurities modify the electronic band structure of the alloy.

Positron study of defects and diffusion induced grain boundary migration in a Pb-Sn system

Abstract The Pb-Sn system discloses a remarkable effect at the intermediate α → α + β phase boundary. The observation of the sharp transient peak in the S-parameter curve which is analogous to those found in thermal capacity curves is, as far as the authors are aware, unique in such positron measurements. This effect is interpreted in terms of positron trapping in defects associated with the strains accompanying the initial stage of transformation. This short note offers a new approch to the underlying phenomenon published in J. Phys. F. Met. Phys., 12 (1982) 813 by A. Belaidi, H. P. Leighly Jr. and R. N. West.

The correlation between charge densities and interstitial impurities in AlP and AlAs

Abstract The empirical pseudopotential method is used to compute electronic charge densities at Γ and X k-points of the valence and conduction band-edges in AlP and AlAs. The charge densities are used to predict the effect of interstitial impurities in the modification of their band structures.

Correlation between the high pressure properties and charge density in Si and Ge

Abstract The electron charge density along the 〈111〉 direction is calculated for Si ana Ge. The results show that this charge density has different profiles for Si and Ge at different high symmetry points in the Brilloin zone. We can explain why the high pressure properties of Si and Ge differ, using these profiles.

Phase transition effects of the Mg-Cd system on the positron annihilation parameters

Positron annihilation measurements made on Mg-Cd alloys are considered. Some pronounced effects at the order-disorder transition were found when the alloy composition was close to stoichiometry. No apparent effects of the order-disorder transformation were exhibited when the alloy was far from the stoichiometric composition. The measured mono-vacancy energies of the system at the stoichiometric composition showed a pronounced decrease when the alloy passed from an ordered to a disordered state.

The density discontinuity associated with the precipitation of the second phase in Pb + 7.45 at.% Sn alloy

Abstract Associated with the precipitation of an Sn-rich phase from a Pb + 7.45 at,% Sn alloy during cooling at a constant rate from an elevated temperature well above the solvus temperature, there is a measurable and precipitous decline in density which begins at a temperature slightly below the solvus temperature for this alloy. This decline takes place over a limited temperature range and is attributed to the formation of excess vacancies created during the nucleation of the precipitation reaction. A similar formation of excess vacancies has been observed using positron annihilation spectrometry in a similar alloy during cooling through the solvus temperature. During this decline in density, the concentration of these vacancies much exceeds the equilibrium vacancy concentration for this alloy. The excess vacancies are unstable and, with continued cooling, quickly anneal out over a narrow temperature range allowing the versus temperature curve to regain linearity.

Positron trapping in Au

Abstract Trapping of positrons by lattice defects has proved to be a powerful tool in probing both the density and the structure of defects. In this work, we are concerned with a three-state trapping model i.e. free (bulk), monovacancy and divacancy states in the case of Au. An attempt is made to calculate the fractions of positrons annihilating in each state and their dependence on both time and sample temperature. A significant divacancy contribution to the annihilation rate is observed for temperatures near melting.

Positron trapping in some metals

Abstract In this work, a three state trapping model has been applied to several metal elements. A good linear trend was found in both E 1v F vs. Tc, E 1v F vs. Tm and Tc vs. Tm curves. Some indication of positron trapping in divacancies is also reported in some elements.