Charge transport in oxygen-doped polysilicon layers on Si

2014 It is shown that layers of polysilicon doped with oxygen (polydox) can be used for the passivation of underlying p-n junctions. The conduction mechanism was derived from measurements in layers directly deposited on to silicon crystals. At room temperature we found PooleFrenkel conduction changing at higher temperatures, presumably, to hopping in localized states. REVUE DE PHYSIQUE APPLIQUÉE TOME 13, DÉCEMBRE 1978, 1. How can polydox passivate p-n junctions ? Semi-insulating polysilicon (SIPOS) has been suggested for the passivation of p-n junctions in silicon. Layers of polycristalline silicon were made semiinsulating by adding oxygen during growth [2]. We studied these layers grown at a reduced pressure. We call them polydox which stands for polysilicon doped with oxygen. Figures 1 ând 2 are used to’ demonstrate how polydox layers can passivate underlying p-n junctions. Figure 1 shows a cross-section of a p-n junction near the Si-Si02 interface. If positive charge is present in the Si02 (indicated by crosses) then the depletion layer of the junction is constricted at the interface. Consequently, the breakdown voltage of the junction is lowered. In figure 2 a slightly conducting layer of polydox is present between t6e charged oxide and the silicon crystal. In this case the charge in the oxide induces negative charge in the polydox layer and no constriction of the depletion layer takes place. So, the breakdown voltage of the underlying junction becomes insensitive to the amount of oxide charge. Then the FIG. 1. Cross-section of a p+-n junction covered by a charged S’02 layer. FIG. 2. Cross-section of a p+-n junction with polydox between oxide and silicon. junction is passivated for oxide charge and also for external fields, which have the same effect as a charge. The influence of extemal fields on the breakdown voltage was studied in gated diodes. The result is shown in figure 3. The polydox layer was grown at a pressure of 0.5 torr from a stream of SiH4 and N20 at a temperature of 650 °C. The layer with a thickness of 0.5 ppm was coated with a 0.9 9M Si02 layer by chemical vapour deposition. Figure 3 shows (curve 1) the breakdown voltage VBv as a function of the voltage Yg applied to an Al electrode (gate) on top of the Si02 layer above the place where the p-n junction mtersects the Si-polydox interface. Curve 2 shows VBv of an identical junction without polydox. In the former case no effect of Yg can be seen, hence it can be concluded that good passivation against external fields is obtained. For the passivation effect it is necessary to have polydox layers that are slightly conducting in order, to induce variable amounts of charge in them. Therefore we studied the conduction in more detail. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:019780013012082100