On Coincidence Site Lattice Modelling of Twins in the Sphalerite and Chalcopyrite Structures
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1012-Y09-02
On Coincidence Site Lattice Modelling of Twins in the Sphalerite and Chalcopyrite Structures Ken Durose Dept. of Physics, Durham University, South Rd., Durham, DH1 3LE, United Kingdom ABSTRACT Coincidence site lattice (CSL) modelling has been applied to determine the orientations and atomic structures of first order twin boundaries in the chalcopyrite structure. A rotation of 250∞32í about [110] generated a CSL similar to that for the Σ = 3 sphalerite boundary, but with one in two cation columns being metal-metal anti-sites. The models predict one coherent boundary type i.e. ( 112 )M - ( 112 )T, and four lateral types, ( 110 )M -( 118 )T, ( 1110 )M -( 112 )T, ( 114 )M -( 114 )T, (001)M -( 111 )T. Un-relaxed atomic models for the boundaries have been constructed. The coherent boundary is shown to differ from that in sphalerite by the inclusion of one in two Se columns having alternate atoms with either three Cu and one In nearest neighbours or vice versa. All of the [110] tilt boundaries in chalcopyrite are expected to be electrically active. A tentative list of the boundaries ranked by energy, electrical and chemical activity is given, along with comments on the influence of preferred orientation in polycrystalline films in the context of solar cells. INTRODUCTION Twinning, that is the incidence of highly symmetric grain boundaries, is prevalent in the diamond-like semiconductors most often used for thin film solar cells. They are of special interest, both because they occur frequently, and because there is a defined orientational relationship between the matrix lattice and the twinned grain within it. Twin boundaries therefore have low energy and represent the minimum amount of bonding disruption that can be associated with any grain boundary in a polycrystalline semiconductor ñ all other (random) boundaries are more disrupted and have a stronger effect on chemical diffusion, electrical recombination and the inhibition of transport. However, while the twinning relationship is fixed, the interface plane between twin and matrix is, not and several boundary types are possible. Since the twinning relationship is highly symmetric it lends itself to description as a tilt, rotation, shear or in some cases, a reflection. While these are equivalent, the description of twins as special cases of the family of tilt boundaries is the most illuminating, and is now outlined: Small angle tilts (θ) in crystals may be supported by arrays of dislocations of burgers vectors b with spacing s such that θ = b /s. Hence as the angle increases there is an increased density of dislocations, and the boundary energy increases. This trend continues beyond the point of coalescence of individual dislocations, and a curve of boundary energy vs tilt (E - θ) results. For certain special orientations however, the boundaries have anomalously low energies. These correspond to angles for which the lattices of both grains, were they to be superimposed, would share common points - known as the coincidence site lattice (CSL) [1]. The ratio of
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