Influence of Emitter Properties on Contact Formation to P+ Silicon

In this study we investigate the influence of the B surface concentration Nsurface 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: Nsurface∼3E19 cm-3, Rsh=50 Ω/▭), and two alternative emitters without BBr3 diffusion (IIa: Nsurface∼1E20 cm-3, Rsh=55 Ω/▭; IIb: Nsurface∼1E19 cm-3, Rsh=330 Ω/▭). The specific contact resistance ec is determined using the Transfer Length Method (TLM). With Ag/Al paste ec=9.95 mΩcm2 on emitter IIa and ec=37.6 mΩcm2 on IIb could be determined. For the same emitter type a higher Nsurface therefore results in a lower ec. This cannot be observed for different types of emitters as ec=4.8 mΩcm2 was measured on emitter I. For a pure Ag paste the same trend can be observed but with ec 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.