Type conversion of polycrystalline silicon heavily doped with both boron and phosphorus due to high current conduction

Abstract

This research investigates a new phenomenon in polycrystalline-silicon film which is heavily doped with both phosphorus and boron. By applying high current pulses, n-type layer which is heavily doped with phosphorus and also contains a large amount of boron atoms is converted into a p-type polycrystalline silicon layer. The filamentation in polycrystalline lateral diode due to high current conduction is demonstrated to be an important mechanism that controls the type conversion process. Computer simulation of the formation of the filament is in good agreement with the experimental results. When we apply a high density current pulse of $10^6 A/cm^2$ to the lateral diode, abrupt transition of the current-voltage characteristics from those of a diode to those of a resistor is observed. We can also observe the threshold voltage shift of polycrystalline-silicon gate MOSFET by applying high density current pulse to the lateral diode which is used as a gate electrode. From these experimental results, it is believed that the p-type filament is formed in the $n^+$ region due to high current conduction. The formation of the p-type filament can be explained when we accept the dopant migration in the molten filament. The structure of heavily doped polycrystalline film is examined and existence of boron clustering is found from the SEM analysis of the polycrystalline- silicon film which is subjected to $n^+$ diffusion followed by $p^+$ diffusion.