Molecular Dynamics Studies of Dislocations in CdTe Crystals from a New Bond Order Potential

Cd1-xZnxTe (CZT) crystals are the leading semiconductors for radiation detection, but their application is limited by the high cost of detector-grade materials. High crystal costs primarily result from property non-uniformity that causes low manufacturing yield. While tremendous efforts have been made in the past to reduce Te inclusions / precipitates in CZT, this has not resulted in an anticipated improvement in material property uniformity. Moreover, it is recognized that in addition to Te particles, dislocation cells can also cause electric field perturbation and the associated property non-uniformity. Further improvement of the material, therefore, requires that dislocations in CZT crystals be understood and controlled. Here we use a recently developed CZT bond order potential to perform representative molecular dynamics simulations to study configurations, energies, and mobilities of 29 different types of possible dislocations in CdTe (i.e., x = 1) crystals. An efficient method to derive activation free energies and activation volumes of thermally activated dislocation motion will be explored. Our focus gives insight into understanding important dislocations in the material, and gives guidance toward experimental efforts for improving dislocation network structures in CZT crystals

Observation et structure des parois d'antiphase périodiques dans certains alliages ordonnés

Electron microscopy images of periodic antiphase boundaries in near-stoichiometric Ag3Mg and Cu3Pd corroborate a recent distinction established between long period ordered structures and periodic antiphase structures. The Ag3Mg M = 5/3 structure has been observed for the first time, in good agreement with previous analysis. In Cu3Pd particular attention is drawn to the fine structure of interfaces due to ordering.Les images en microscopie électronique des parois d'antiphase périodiques observées dans les alliages de composition voisine de Ag3Mg et Cu3Pd confirment les différences récemment introduites entre structures ordonnées à longue période et structures à antiphases périodiques. La structure M = 5/3 de Ag3Mg a été observée pour la première fois, et confirme les analyses précédentes. Dans l'alliage Cu3Pd, on a particulièrement étudié la structure fine des interfaces résultant de la mise en ordre

PROPAGATION OF DISLOCATIONS IN LONG PERIOD ORDERED ALLOYS

A partir de considérations géométriques, on montre qu'il existe dans les alliages ordonnés à longue période, des dislocations partielles limitant des fautes d'empilement sans faute d'ordre. En supposant l'énergie de faute d'empilement suffisamment faible, on examine les conséquences de cette situation sur le mode de déformation plastique et on montre qu'elle devrait se produire soit par maclage, soit par propagation de fautes d'empilement sur plusieurs domaines. Un mécanisme de franchissement des parois de domaines est proposé : un groupe de dislocations partielles doit glisser en même temps sur les couches atomiques successives : un arrangement atomique analogue à la nucléation d'un multipole est nécessaire. Les conséquences de ce modèle sont énoncées.From geometrical considerations it is shown that in long period ordered alloys some partial dislocations exist, leaving behind only a stacking fault without destroying the chemical order. Providing that the stacking fault energy is low enough, theoretical implications of the deformation behaviour are examined. It is proposed that plastic deformation should take place either by twinning or propagation of widely extended stacking faults through several domains. A simple mechanism for crossing over domain boundaries is proposed : partial dislocations have to glide together on adjacent layers, in the sense that some convenient dislocation multipole has to be nucleated at the boundary. Some physical consequences are drawn from that model

ELECTRON MICROSCOPY STUDY OF TRANSITION METALS DISULFIDES WITH PYRITE STRUCTURE

Les composés CuS2 et NiSx (1,91 ⩽ x ⩽ 2,1) ont été examinés par microscopie électronique afin de déterminer si les transitions magnétiques et électriques observées sont reliées à un changement de phase cristallographique. Pour CuS2, la transition magnétique à 160 K est liée à la formation d'une surstructure dont la nature est discutée. Dans les sulfures de nickel, par contre, il n'a pas été possible de mettre en évidence une quelconque influence de la stoechiométrie ou de la température (jusque ~ 130 K) sur la structure de ces composés.The compounds CuS2 and NiSx (1.91 ⩽ x ⩽ 2.1) have been examined by electron microscopy in order to determine the correlation between magnetic/electrical transitions and possible crystallographic transformations. In CuS2 the magnetic transition occuring at 160 K is found to be related to the formation of a crystallographic superstructure, the nature of which is discussed. In nickel disulfides, however, no influence is found of either stoichiometry or temperature (down to ~ 130 K) on the crystallographic structure

Dissociation of 1/2 < 112 > perfect dislocations and nature of slip dislocations in ordered Ni3V

Electron microscopy observations show that in the ordered alloy Ni 3V, perfect 1/ 2[112] dislocations are dissociated into three 1/ 6[112] partial dislocations bounding an intrinsic-extrinsic pure stacking fault pair (i.e. without any wrong first neighbour bonds); from this an evaluation of stacking fault energies can be deduced. Furthermore, such pairs should extend under stress and produce 1/ 6[112] partials as the slip dislocations in this crystal structure.Les observations en microscopie électronique dans l'alliage ordonné Ni 3V montrent que les dislocations parfaites de vecteur de Burgers 1/2[112] sont dissociées en trois partielles 1/ 6[112] limitant une paire de fautes d'empilement intrinsèque-extrinsèque dont on a pu déduire une évaluation des énergies de fautes d'empilement (ces fautes sont celles qui n'introduisent pas de mauvaises liaisons entre premiers voisins). Sous l'effet d'une contrainte, ces paires ont tendance à émettre des partielles 1/ 6[112] qui propagent la déformation dans cette structure

On the influence of indium addition on the mechanical properties of gallium arsenide at room temperature

Indentations have been performed on (001) faces of indium alloyed GaAs crystals in darkness and under infrared illumination. The Vickers hardness was measured and the dislocation microstructure around the indents observed by high voltage transmission electron microscopy. A softening effect of indium is evidenced by comparison with the Vickers hardness obtained in the same conditions on undoped GaAs. No significant influence of indium on the dislocation microstructure resulting from indentation in darkness is noted. Indentation under infrared illumination does not reveal any macroscopic photoplastic effect; however, a modification of dislocation microstructure is observed. Whatever the experimental conditions, dislocations appear to experience strong lattice friction. The softening effect of indium addition and the enhanced dislocation mobilities under infrared illumination are discussed in the framework of dislocation glide governed by the Peierls mechanism

EXCITATION-ENHANCED DISLOCATION MOBILITY IN SEMICONDUCTORS

Quantitative features of the enhancement effect by either laser light illumination or electron irradiation on the dislocation motion in semiconductors are surveyed. The experiments show that moving dislocations experience a lattice friction which is lowered under excitation. Theoretical analysis based on the kink diffusion model for dislocation motion indicates that the observed reduction in activation energy corresponds to the electronic energy level in the band gap associated with the straight dislocation sites

An analysis of elementary dislocations in icosahedral Al-Pd-Mn by Large Angle Convergent-Beam Electron Diffraction (LACBED)

Large Angle Convergent Beam Electron Diffraction (LACBED) experiments on dislocations located in the mirror planes of icosahedral AlPdMn single grains show that – as previously inferred from standard image contrast technique – the direction of the Burgers vector is perpendicular to the mirror plane of movement, thus confirming the fact that climb should be an important process for the plasticity mechanisms in icosahedral quasicrystals

In situ TEM study of dislocation mobility in semiconducting materials

In situ straining experiments have been conducted in Toulouse and Grenoble on elemental semiconductors, III-V and II-VI compounds. This article gives a common interpretation of the results obtained, which allows us to discuss the effect of the strength of covalent bonding on the mobility of dislocations under different conditions.Des expériences de déformation in situ de semiconducteurs élémentaires, et de composés III-V et II-VI, ont été réalisées à Toulouse et Grenoble. Cet article donne une interprétation commune des résultats obtenus, ce qui permet de discuter l'effet de la force des liaisons covalentes sur les mobilités des dislocations dans divers cas de figure