Electronic and elastic properties of edge dislocations in Si

Abstract

Ab initio, tight-binding, and classical calculations have been done for (a/2)(110) edge dislocation di-0 poles in Si at separations of 7.5 -22. 9 A in unit cells comprising 32 -288 atoms. These calculations show states associated with the cores relatively deep in the band gap ( -0.2 eV) despite the absence of dangling bonds. The shifts in the electronic states depend significantly on separation d and are correlated with a concentration of strain in the cores as the dislocations become more isolated. The strain energies exhibit a logarithmic dependence on d consistent with linear elasticity for all system sizes.Extended defects like interfaces, grain boundaries, and dislocations are receiving increasing attention, particularly in semiconductors, because of their potential impact on device performance. These defects can degrade device performance by serving as undesirable sinks or sources of carriers. ' In the growth of multicomponent structures, lattice mismatch between components can result in the formation of misfit dislocations at the interfaces. At one time, it was believed that the observed electrical properties might arise from intrinsic changes in local bonding, i.e. , dangling bonds or overcoordination. However, many studies of the atomic structures of dislocations and tilt boundaries have revealed structures which retain tetrahedral coordination, with bond and angle distortions too small to introduce localized states in the gap.There have been a few recent ab initio studies of dislocations and related structures in Si. These include the work of DiVincenzo et al. on the X9 twin boundary, of Bigger et al. on the 90 partial dislocation, and of Arias and Joannopoulos on a (110) screw dislocation. In the first two of these, only shallow gap states are reported, and the third is silent as to electronic structure. We present here results of ab initio, tight-binding, and classical calculations of (a/2)(110) edge dislocation dipoles which show states relatively deep in the gap despite the absence of dangling bonds or overcoordination. Calculations were done for dipole separations of 7.5 -22. 9 A along the direction of the Burgers vector in the relaxed samples. For the smallest (32-atom) system, the presence of the dislocations pushes the top of the valence band upward by a small amount into the gap for bulk Si. As the separation increases, this shift increases and converges to a value of almost 0.2 eV above the bulk band edge, much deeper in the gap than reported in previous calculations for systems containing dislocations. We also find that the calculated strain energies for all separations from the smallest to the largest can be fit to a single logarithmic curve of the form predicted by linear elasticity theory, as was found by Arias and Joannopoulos for screw dislocations. A value for the core energy E, is extracted subject to some ambiguity in determining the core radius r, .In the calculations, unit cells containing a dislocation dipole with a separation of n6 sixfold rings along [110](as viewed in projection...

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