Photovoltaic silicon produced by thermal plasma: Influence of atomic hydrogen on oxygen elimination and passivation of t
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Photovoltaic silicon produced by thermal plasma: Influence of atomic hydrogen on oxygen elimination and passivation of the crystal defects D. Morvan, I. Cazard-Juvernat, and J. Amouroux Laboratoire des r´eacteurs chimiques en phase plasma, Universit´e P&M Curie, ENSCP, 11 rue P&M Curie, 75005 Paris, France (Received 2 March 1998; accepted 2 March 1998)
The photovoltaic properties of the polycrystalline silicon depend on the crystallinity and the purity of the material. The thermal plasma process gives us an alternative method of silicon preparation since it is possible to produce an ultrahigh purity with simultaneously a passivation of crystalline defects and active impurities. We demonstrate the efficiency of the plasma purification process and particularly the influence of the atomic hydrogen in an argon thermal plasma on the photovoltaic properties of silicon. The results of the diffusion lengths measured by the photoelectrochemical method show that locally it rises up to 200 mm. We correlate these photovoltaic measurements with the properties of the crystal (defects and purity) by means of measurements by Fourier transform infrared spectroscopy (FTIR) at low temperature (6 K), four probes resistivity technique, scanning electronic microscopy, inductively coupled plasma (ICP), and neutronic activation analyses. We show that the increase of the purity explains the high measured diffusion lengths. Nevertheless, the thermal conditions of the crystallization of the silicon, due to the specificity of the plasma, lead to defects such as dislocations for which density is particularly high (.106 disycm2 ). The results show that chemical reactions between the atomic hydrogen of the plasma and the oxygen of the silicon occur. They decrease the oxygen content in silicon from 3 3 1017 at.ycm3 down to 2 3 1016 at.ycm3 , while the residual hydrogen in silicon is close to 2 3 1015 at.ycm3 . This passivates the dangling bonds of ultrapure silicon with a high thermal stability up to 1000 K. The objective of this paper is to demonstrate that the hydrogen in the plasma modifies the electronic properties of the material to achieve a very good photocurrent even though the dislocation density of the silicon is very high. I. INTRODUCTION
II. EXPERIMENTAL SETUP
Previous works have shown the efficiency of the thermal plasma technique for silicon purification,1,2 but the recrystallization conditions of the silicon, and particularly the high thermal gradients in the liquid zone of the treated silicon bar, lead to crystal defects. Furthermore, some impurities such as oxygen lead to chemical defects with damage of the crystal and the electronic properties of the silicon. Since a few years of research (Table I) have been done in this field, we review the manufacture of silicon ingots by means of continuous casting or directional crystallization in order to decrease the crystal defects and to increase the size grains as a columnar structure.3,6 – 8 Simultaneously, it is necessary to obtain the h
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