Wafer Bonding: Light Emitters


Dislocations in Si

M. Reiche and T. Wilhelm


Dislocations represent line defects in the regular crystal lattice of solids and may be regarded as a kind of a ‘native nanostructure’. According to a simple geometric picture, dislocations in the diamond structure of the Si lattice are connected with dangling (broken) bonds (Fig. 1). There exist also many other defects and lattice irregularities in the dislocation core and at/around the dislocations of either intrinsic origin, i.e. kinks, jogs etc., or of extrinsic origin, i.e. impurity atoms; for more details see e.g. [2]. They all are known to cause states in the forbidden gap of Si between the valence and conduction band.


The dislocation-related states can be occupied with electrons in n-type Si forming a negative line charge. To keep space charge neutrality the negatively charged dislocation line in n-Si is surrounded with a cylinder of ionized (positively charged) donor atoms, the so-called Read cylinder. In the opposite case of p-type Si the dislocations form positively charged lines surrounded with ionized (negatively charged) acceptor atoms. A consequence of the structure formed by a charged dislocation line being surrounded with a Read cylinder is the bending of the conduction and valence band, i.e. the appearance of a barrier in the band scheme which is associated with the occurrence of an electric field. This is schematically demonstrated in Fig. 2. In general, the dislocation properties critically depend on the thermal history of dislocations and on the impurity content in the Si material, since these factors determine the density of intrinsic defects in the dislocation core as well as of impurities decorating the dislocation. ‘Clean’ dislocations exhibit only very small electrical activity. Accommodation of even tiny amounts of metal atoms at the dislocations may have pronounced impact on their electrical activity. Predominantly metal and oxygen atoms at the dislocations have been reported to substantially influence their optical and electrical properties. Hydrogen, however, is mostly found to passivate the impact of foreign atoms.


Fig. 1: 60° dislocation after Hornstra [1], with dangling bonds.


Fig. 2: Band scheme of a charged dislocation line and Read cylinder for n-type Si.



[1] J. Hornstra, J. Phys. Chem. Solids 5, 129 (1958).

[2] W. Schröter and H.Cerva, Solid State Phenomena 85-86, 67 (2002).

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