Influence of emitter properties on contact formation to p + silicon

Abstract 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 POCl 3 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 BBr 3 emitter (I: N surface ∼3E19 cm -3 , R sh =50 Ω/▭), and two alternative emitters without BBr 3 diffusion (IIa: N surface ∼1E20 cm -3 , R sh =55 Ω/▭; IIb: N surface ∼1E19 cm -3 , R sh =330 Ω/▭). The specific contact resistance ec is determined using the Transfer Length Method (TLM). With Ag/Al paste e c=9.95 mΩcm 2 on emitter IIa and e c=37.6 mΩcm 2 on IIb could be determined. For the same emitter type a higher N surface therefore results in a lower e c. This cannot be observed for different types of emitters as e c=4.8 mΩcm 2 was measured on emitter I. For a pure Ag paste the same trend can be observed but with e c being at least a factor of four higher, as expected [3] , [4] , [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.