Boron-Doped Silicon Surfaces From B$_{\bf 2}$H $_{\bf 6}$ Passivated by ALD Al$_{\bf 2}$O$_{\bf 3}$ for Solar Cells

Abstract

A p + -doping method for silicon solar cells is presented whereby boron atoms from a pure boron (PureB) layer deposited by chemical vapor deposition using B 2 H 6 as precursor were thermally diffused into silicon. The applicability of this doping process for the doped surfaces of silicon solar cells was evaluated in terms of surface morphology after thermal diffusion, the boron dopant profiles, and sheet resistances, as well as the recombination parameter J 0p + , when the doped layers were passivated by Al 2 O 3 films prepared by atomic layer deposition. Adequate surface passivation could be achieved with a surface recombination contribution to J 0p + of <20 fA/cm 2 for most samples. However, when a boron-rich layer (BRL) was present at the Si surface, a much higher recombination current density was observed, proving that a BRL was detrimental to the high-quality surface passivation of boron-diffused surfaces. It was found that sufficient O 2 in the furnace during the thermal diffusion process could eliminate any potential residual BRL, while excessive O 2 concentration results in boron depletion and a higher sheet resistance. Therefore, in addition to optimizing the initial PureB layer thickness and thermal budget to control the dopant profiles, the O 2 concentration during the diffusion must also be well controlled.

Direct Observation of the Impurity Gettering Layers in Polysilicon-Based Passivating Contacts for Silicon Solar Cells

“…Distinction of the BRL from c-Si has been reported in the literature using scanning electron microscopy (SEM), TEM, and EDS. 23,26,39,40 However, in the case of partially amorphous and partially polycrystalline silicon (i.e., the recrystallized poly-Si layer in this study), the amorphous BRL 21 could not be distinguished from the amorphous silicon regions in the poly-Si layer (even from diffraction patterns). Moreover, the different layers are in fact intermixed at the interfaces, making it impossible to separate the individual layers from TEM or EDS-STEM.…”

“…This is consistent with our finding that the BRL, from BBr 3 thermal diffusion on polysilicon, is responsible for the gettering effect in boron diffusion-doped polysilicon contacts. Boron doping via spin-on, printed, or chemical vapor deposited surface dopant sources, followed by a drive-in anneal, has also been reported to produce the BRL in silicon surface regions. ,, These techniques, as well as others that cause a significant boron “pile-up” to create the BRL, could potentially be used for doping the polysilicon contacts in order to accomplish both contact formation and impurity gettering effects.…”

PureB diode fabrication using physical or chemical vapor deposition methods for increased back-end-of-line accessibility

“…A customized batch furnace system was developed with the goal of replacing BBr 3 with B 2 H 6 in B deposition/anneal/oxidation cycles used for solar-cell p + -emitter fabrication [13]. The precursor was 0.2% B 2 H 6 in H 2 gas.…”

On the Many Applications of Nanometer-Thin Pure Boron Layers in IC and Microelectromechanical Systems Technology

“…In c-Si solarcell fabrication, doping from borosilicate-glass layers is still commonly used. In this application, boron-rich layers formed at the Si surface reduce carrier lifetime and are necessarily removed [13]. The name "PureB" diodes was introduced to underline that the attractive characteristics of these diodes are the result of a surface coverage of pure B and not B doping of the bulk Si.…”