Precipitation of Copper and Cobalt at Grain Boundaries in Silicon
- PDF / 2,648,716 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 111 Downloads / 244 Views
PRECIPITATION OF COPPER AND COBALT AT GRAIN BOUNDARIES IN SILICON U. JENDRICH, H. J. MOLLER, Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106 ABSTRACT The precipitation of copper and (radioactive) cobalt at low energy grain boundaries in polycrystalline silicon and bicrystals is investigated. The metals are diffused in from a surface source between 800 - 1000 OC and the precipitation after cooling down is studied by TEM (for Cu) and MoBbauer spectroscopy (for Co). The precipitates are metal silicides. For copper it is shown that they appear in form of colonies containing hundreds of precipitates with a particle size between 5 - 60 nm. In the grain boundary they nucleate preferentially at dislocations and steps. The distribution and size of the precipitates depend on the cooling rate after the diffusion. In the vicinity of the grain boundary the volume is depleted from the impurities. INTRODUCTION The atomic structure of a grain boundary and the reconstruction of the covalent bonds has been related to their electrical activity [1]. Recent experimental investigations for instance by combined Auger/EBIC [2] and TEM/DLTS [3] measurements have shown, however, that the electronic properties of grain boundaries can also be changed by the presence of impurities. These results raise the question to what extent the electrical properties in general are determined by impurities. In view of the importance of these lattice defects for the electronic properties of polycrystalline semiconductors it seems necessary to investigate the problem further. The transition metals are of particular interest since they are very mobile even at low temperatures and introduce deep levels themselves. However, there is little information available on their interaction with grain boundaries and the influence on their electrical activity. It is therefore the purpose of this study to investigate the behavior of some of the 3d transition elements quantitatively. Copper, nickel and cobalt are the elements with the highest diffusion coefficients and solubilities in silicon among the 3d transition elements and it has been shown that even after quenching they cannot be kept in solution. Their tendency to accumulate at grain boundaries and other lattice defects is therefore expected to be high. In this study results on cobalt and copper will be given. It is well known that copper in silicon precipitates at dislocations ("decoration") during cooling when the solubility is exceeded. The behavior in dislocation free, single crystalline silicon is more complicated [4-12]. Without favorable nucleation sites it precipitates in planar arrangements mostly on 1110) planes in form of colonies. They extend over 0.5 to 100 gim often exhibiting a dendritic shape. The whole colony is usually surrounded by an extrinsic dislocation loop and consists of ten to hundreds of small precipitates, 5 to 30 nm in size. It has been assumed that the first growing particles punch out dislocation loops during precipitation to relea
Data Loading...
