Possible evidence for overcoordination at semiconductor grain boundaries
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The structure of a near coincidence Ge tilt grain boundary, containing a step, has been derived from a high resolution electron micrograph. There are two possible interpretations of portions of this interface, one of which is the existence of a sheet of fivefold coordinated atoms between the 2 = 19 and X = 27 coincidence misorientations. This finding may represent the first experimental evidence that overcoordinated atoms are present at semiconductor grain boundaries free of a screw dislocation.
I. INTRODUCTION
The atomic arrangement at semiconductor grain boundaries has been investigated extensively both experimentally and theoretically. In general, rebonding occurs in order to lower the energy by reducing the number of dangling bonds. It has been established that the main consequence of rebonding is the appearance of five-membered and seven-membered rings in addition to the standard six-membered perfect crystal rings.1"3 It is believed that threefold-coordinated atoms (dangling bonds) may exist at grain boundaries, manifesting themselves through an electron-spin-resonance (ESR) signal.4 Such atoms with the same ESR signal have been proposed to be the predominant point defect in amorphous Si.5 Overcoordinated atoms until recently have not been considered as possible. It has been pointed out that there are in principle two primitive intrinsic defects, namely threefold-coordinated atoms (one missing bond) and fivefold coordinated atoms (one extra bond), completely analogous to vacancies (one missing atom) and interstitials (one extra atom) in crystals, where fivefold coordinated atoms, labeled floating bonds, were shown to play a role in amorphous Si bonding.6 More recently, molecular dynamics simulations of amorphous Si provided further support for the existence of fivefold coordinated atoms.7 In addition, a parameter-free calculation of the structure of a X = 5/[001]36.9° twist grain boundary in Ge has led to the conclusion that both threefold and fivefoldcoordinated atoms are present.8 Even at free surfaces, it has been pointed out that fivefold coordinated atoms are present.9 In this paper we report on the analysis of a high resolution transmission-electron-microscope image of a particular tilt grain boundary and provide two possible explanations of the structure observed. First, one can invoke the existence of a dislocation with a screw component; hence modeling of the atomic structure at the grain boundary is possible in terms of fourfold coordinated atoms. Second, if no dislocation is invoked, modJ. Mater. Res., Vol. 5, No. 3, Mar 1990
eling of the grain boundary leads naturally to the presence of a series of fivefold-coordinated atoms. It is not currently possible to establish independently whether a dislocation is present at this grain boundary. We believe, however, that in view of recent work that establishes the reality of overcoordination,6"9 the interpretation of the image in terms of overcoordinated atoms is a viable alternative to be considered. II. MICROSCOPY AND GRAIN BOUNDARY STRUCTURE CONSIDERA
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