Boron-rich layer properties formed by boron spin on dopant diffusion in n-type silicon

Abstract

Starting with N-type base, a p-type emitter is formed using boron spin on dopant (BSoD) which results in formation of boron rich layer (BRL) on top of the emitter and can be used in selective emitter and FF improvements in solar cells. In this work, the morphologies of BRL for varying thicknesses, depending on the diffusion conditions, have been studied to know their impact on emitter formation. The characterizations show that BRL properties are dependent on its thickness and its boron concentration. BRL has amorphous phase with peak boron concentration over 10²¹ atoms/cm³, thickness less than 100 nm, refractive indices of 1.4–1.6 and contact resistance 1.0–6.0 mΩ-cm². The bond properties of the constituent elements of BRL vary depending on the thickness.

Introduction

In recent years, n-type silicon is getting popular in the photovoltaic industry because of its advantages over p-type silicon. It has higher tolerance to metallic impurities (e.g. Fe, Ni, Cu, etc.) and has a higher minority carrier lifetime of holes [1], [2]. Also, light induced degradation (LID) resulting from boron-oxygen complexes under illumination is not seen in case of phosphorus diffused n-type wafers [3]. However, the solar cell industry has not been fully established for n-type solar cells. It is mainly due to the difficulties in emitter formation with p-type dopants, its passivation and contact formation with the existing industrial techniques [4], [5], [6]. Different boron dopant sources [7] with solid (e.g. boron nitride, BN) [8], gaseous (e.g. diborane, B₂H₆) [9], spin on dopants [10] and liquid (e.g. boron tri bromide, BBr₃) [11] forms are used to form the p-type emitter. The most commonly used source is liquid dopant BBr₃ as it results in higher yield and covers most of the silicon industry related to solar cells. The hazardous nature of BBr₃ increases the safety requirements and hence increases the cost of production associated with its diffusion [11]. Other dopant sources, which are not hazardous have also been studied for p-type emitter formation to lower the cost of fabrication of emitter. Boron spin on dopant is one of them and it yields uniform doping due to its diffusion [12]. During the process of boron diffusion, boron-rich layer (BRL) is formed naturally on the emitter surface. It is important to understand the properties of BRL and its impact on the formation of p-type emitters in n-type Si solar cells [13]. There are sources in literature which have shown probable applications of BRL in selective emitter and fill factor improvements [14]. A few reports have discussed the structure of BRL, and its properties for BBr₃ [13], [14], [15] but similar studies for other boron dopants have not been done.

In this work, BRL formed by boron spin on dopant (BSoD) source is studied in detail during the p-type emitter formation in n-type silicon. Different structural behaviours with respect to process step conditions are discussed in terms of its morphology, optical and electrical properties.