Recombination via transition metals in solar silicon: The significance of hydrogen–metal reactions and lattice sites of metal atoms

J Mullins,J D Murphy,M P Halsall,S Leonard,A G Marinopoulos,I D Hawkins,J Coutinho,P Santos,V P Markevich,A R Peaker

doi:10.1002/pssa.201700304

Abstract

The move toward lower cost sources of solar silicon has intensified the efforts to investigate the possibilities of passivating or reducing the recombination activity caused by deep states associated with transition metals. This is particularly important for the case of the slow diffusing metals early in the periodic sequence which are not removed by conventional gettering. In this paper, we examine reactions between hydrogen and transition metals and discuss the possibility of such reactions during cell processing. We analyse the case of hydrogenation of iron in p‐type Si and show that FeH can form under non‐equilibrium conditions. We consider the electrical activity of the slow diffusing metals Ti, V, and Mo, how this is affected in the presence of hydrogen, and the stability of TM‐H complexes formed. Finally, we discuss recent experiments which indicate that re‐siting of some transition metals from the interstitial to substitutional site is possible in the presence of excess vacancies, leading to a reduction in recombination activity.Periodic ordering of the 3d, 4d, and 5d transition metals with adjacent columns of the periodic table. The metals with hydrogen complexes which have had their electronic properties reported in the literature or in this paper are highlighted in green.

Highlights

In silicon technologies much attention has been devoted to contamination by transition metals (TM), those in the 3d series
Many variants on the theme are in production or in development but the end result is that the chemical purification of the feedstock is simplified, for example upgraded metallurgical silicon (UMG) is used, or the inherent purification of the Czochralski method is eliminated using casting
In p-type material we identify the lack of observations of TM-H complexes as being due to long range Coulombic repulsion between the positively charged hydrogen and the positively charged TM

Summary

Introduction

In silicon technologies much attention has been devoted to contamination by transition metals (TM), those in the 3d series. In the case of device fabrication using electronic grade silicon, the concern is largely associated with metals introduced during device processing, electronic grade silicon in its as-grown state contains negligible concentrations of transition metals This is because it derives from polycrystalline feedstock which has been chemically purified very effectively, for example by the Siemens process, and is converted to single crystal from the melt usually by the Czochralski (Cz) method. Many variants on the theme are in production or in development but the end result is that the chemical purification of the feedstock is simplified, for example upgraded metallurgical silicon (UMG) is used, or the inherent purification of the Czochralski method is eliminated using casting This results in a higher concentration of transition metals in the slices used for solar cell fabrication than in electronic grade Si. The significance of specific TMs in solar cells was studied in the 1970’s in what is known as “The Westinghouse Experiment.”. Our principal tools in these studies have been DLTS and related analytical techniques which we describe elsewhere [7]

Formation and thermal stability of hydrogenTM complexes in silicon

Conclusions