Precipitation of Phosphorus, Arsenic, and Boron in Thin Silicon Foils

Diffusion of high concentrations of P, B, and As in thin foils of silicon of (111), (110), and (100) orientations is shown to result in extensive precipitation. Observation of the precipitates through a transmission electron microscope has indicated that phosphorus diffusion gives rise to precipitate platelets. Boron precipitates were found to be of various shapes—rod, platelet, and three-dimensional. Matrix contrast observation showed that small coherent platelets of P and B are both of the “vacancy” type—this is in accordance with the smaller size of B and P atoms in the silicon matrix. Contrast studies for arsenic precipitates were inconclusive. Helical dislocations oriented along 〈220〉 directions and originating from precipitate regions were observed in the phosphorus-diffused foils; boron precipitates showed no such helices. Boron rods and three-dimensional structures, however, were invariably found to be enveloped by dislocation spirals, with Burgers vector different from the usual a/2 〈110〉 type. These dislocation envelopes are thought to be interfacial dislocations. It is concluded that the precipitates are not identifiable as discrete SiP or Si
x
B
y
phases, since these phases would be expected to cause compression in the matrix because their specific volume is higher than that of silicon.