Influence of emitter properties on contact formation to p+ silicon

In this study we investigate the influence of the B surface concentration N-surface on the contact formation of Ag and Ag/Al thick film pastes to p(+)-type Si. From literature it is known, that for contacting POCl3 emitters a high P surface concentration is necessary for a reasonable contact resistance [1,2]. This work aims to examine if the same holds for B emitters. For this purpose, three emitters with different B surface concentrations are realized and contacted with pure Ag and Al containing Ag (Ag/Al) thick film pastes: a BBr3 emitter (I: N-surface similar to 3E19 cm(-3), R-sh=500 Omega/square), and two alternative emitters without BBr3 diffusion (IIa: N-surface similar to 1E20 cm(-3), R-sh=55 Omega/square : N-surface similar to 1E19 cm(-3), R-sh=330 Omega/square). The specific contact resistance rho(c) is determined using the Transfer Length Method (TLM). With Ag/Al paste rho(c)=9.95 m Omega cm(2) on emitter IIa and rho(c)=37.6 m Omega cm(2) on IIb could be determined. For the same emitter type a higher Nsurface therefore results in a lower c. This cannot be observed for different types of emitters as rho(c)=4.8 2 m Omega cm(2) was measured on emitter I. For a pure Ag paste the same trend can be observed but with rho(c) being at least a factor of four higher, as expected [3-5]. Electron microscopy analysis reveals that for pure Ag pastes no direct contacts between Ag crystals on the Si surface and the Ag bulk exist. In contrast, for the Ag/Al pastes a direct connection between the Ag/Al particle in contact with the silicon surface and the Ag finger can be found. We propose that the low contact resistance for Ag/Al pastes may be due to direct contacts of Ag/Al spots to the Ag bulk.