Segregation of dopants

The conditions for stable growth of a crystal are closely associated with segregation of dopants or impurities. Crystal growth theory includes an expression for the effective segregation coefficient K, for a crystal growing from a medium in which the dopant distribution is characterized by a boundary layer of thickness 5 ... 

Russel Ohl of Bell Laboratories discovered and filed patents in 1941 that were granted in 1946 and 1948 " for the p-n junction photovoltaic effect in silicon. The early devices contained rectifying junctions that were accidentally formed by segregation of dopant impurities during cooling. 

Fig. 8.26 Sketch of the solute drag effect produced by the segregation of dopants to the grain boundaries, a Symmetrical distribution of the dopant in the region of a stationary grain boundary, b For a moving boundary, the dopant distribution becomes asymmetrical if the diffusion coefficient of the dopant atoms across the boundary is different from that of the host atoms. The as5nnmetrical distribution produces a drag on the boundary, c Breakaway of the boundary from the dopant leaving a solute cloud behind. Reproduced with permission from [4]. Cop5urght 2003, CRC Press...

Figure 3.34. Electronic structure and dopant distributions in catalysts revealed by cathodoluminescence in electron microscopy unreacted Sb/Sn oxide catalyst with a band-gap of 1.9 eV is shown by the spectrum B. The reacted catalyst at A shows a band-gap of 2.6 eV, demonstrating a peak shift indicative of surface segregation of Sb. (After Boyes et al 1985.)...

The actual incorporation of dopants in silicon is determined by a factor of segregation Keg. It is given by the concentration of dopants in silicon divided by the ratio of partial pressure of dopant gas (Pgopant) and silicon precursor... 

Furthermore, because of the dopant segregation, the dopant boundary layer affects the solidification temperature (Fl) in front of the growth interface. If the thermal gradient Gt is smaller than the gradient of the solidification temperature at the growth front, constitutional supercooling can occur. " This can be described by ... 

The tetragonal RE, Y, and Sc orthophosphates in particular have been widely used as host media for a variety of solid state chemical, spectroscopic, magnetic resonance, neutron and other studies of rare-earth and actinide impurities. These materials have proved to be ideal hosts for the incorporation of other rare-earth dopants (e.g., Er-doped Lu(P04) for microlaser studies). Doped orthophosphates with desired levels of dopants are desirable for both basic investigations and applications. Unfortunately, there are apparently no available quantitative data on the segregation coefficients for the rare earths in the tetragonal orthophosphates. 

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