Quantitative high-resolution electron microscopy of defects and interfaces in silicon-based systems
IV.Physikalisches Institut der Georg-August-Universität Göttingen and Sonderforschungsbereich 602, Bunsenstr.13-15, D-37073 Göttingen, Germany
Extended defects in silicon as well as hetero-interfaces play an important role in modern silicon-based microelectronics and the rapidly growing field of solar cell production from crystalline silicon materials. Modern techniques of transmission electron microscopy allow studying the structure and chemistry of defects and interfaces on a quasi-atomic level and hence providing insight into the underlying physics of their formation and properties.
The first part of this contribution summarizes electron microscopy studies on the formation of metal silicide precipitates in crystalline silicon materials containing extended defects (e.g. dislocations), or subjected to the in-diffusion of a high concentration of phosphorus. For defect-free silicon materials it is well known that nickel precipitates as thin platelets consisting of NiSi[2 ]that are bounded by a dislocation with a Burgers vector b = a/4<111>. Such dislocations serve as a reaction channel which allows fast incorporation of nickel atoms during growth. In dislocated materials, however, polyhedral precipitates are observed which strongly modify the dislocation geometry. This indicates that the pre-existing dislocations are an integral part of precipitate growth rather than merely a nucleation site. In particular, a special NiSi2- related precipitate structure is obtained exclusively at dislocations.
Phosphorus-diffusion gettering (PDG) is widely used in solar cell fabrication for the formation of pn-junctions and the simultaneous cleaning of the wafers by the redistribution of metal impurities into the highly P-doped region. Besides the increased solubility of substitutional metal impurities due to electronic effects and pairing with P atoms, silicide formation may play an important role for the gettering process. For Pt in silicon it will be shown that a substantial part of gettered Pt atoms (measured by e.g. secondary ion mass spectroscopy) are due to PtSi precipitates which form during P diffusion either near SiP precipitates or at the interface between silicon and a phosphorus silica glass.
In the second part, high-resolution electron microscopy is used to determine the structure of interfaces between crystalline and amorphous materials. A recently developed technique is used to extract the two-dimensional density of atoms in amorphous solids near crystalline substrates from focal series either by quantitative image matching or by exit wave function restoration. As an example, amorphous germanium is deposited by room-temperature molecular beam epitaxy on hydrogen passivated unreconstructed (111) silicon surfaces. The resulting interface is characterized by (i) a width of about 1.2 nm, (ii) a bond angle distribution slightly smaller than for bulk amorphous germanium; and (iii) atomic positions compatible with tetragonally distorted germanium on silicon.