State-of-the-art multiparameter characterization of the chemical and field effect passivation of very high lifetime n-Si with n+ front surface field (FSF)

Abstract

An effective characterization approach for n+ front surface field (FSF) passivation has been developed using a sequence of state-of-the-art “corona charge-Kelvin” electrical methods integrated with lifetime based monitoring using QSS-µPCD with a decay control method. The approach was applied to symmetrical test structures on high lifetime n-Si with n+ FSF typically used in high efficiency IBC cells. Test structures with similar SiNx-based top dielectric included structures with and without n+ FSF. Structures without the FSF enabled complete dielectric and interfacial charge characterization. The Dit spectra revealed very low interface trap density with midgap values about 3e10 q/cm2eV. This Dit implies excellent chemical passivation. In addition, classical field-effect response of the effective carrier lifetime vs. corona charge showed a lifetime minimum near zero charge (corresponding to a maximum surface recombination velocity) and an increase of lifetime in inversion and accumulation for the structure without n+ FSF. For n+ FSF structures lifetimes higher than that for undoped structures were measured. The 12ms maximum effective lifetime measured for the n+ FSF structure with QSS-µPCD indicates maximum effective surface recombination around 0.5 cm/s, while for the structure without n+ FSF the maximum effective lifetime was about 1.7ms. For the n+ FSF structure, the field-effect characteristics of τeff demonstrated very high lifetime for zero charge and for positive corona charge. The overall results indicate that excellent n+ FSF passivation is a consequence of three elements: 1. good chemical passivation 2. good surface field passivation and 3. close to optimal dielectric charge controlled field-effect passivation. The surface saturation current measurements revealed J0 = 5.1fA/cm2 that would correspond to VOC ∼ 751mV, consistent with cell results reported for advanced IBC cells. Whole wafer mapping showed good n+ FSF passivation uniformity with J0 from 5 to 7fA/cm2 in 95% of the wafer. The present metrology approach required important technology refinements that are now being introduced in PV2000A tools developed by Semilab SDI.