PHOTOVOLTAIC SILICON PRODUCED BY THERMAL PLASMA - INFLUENCE OF ATOMIC-HYDROGEN ON OXYGEN ELIMINATION AND PASSIVATION OF THE CRYSTAL DEFECTS

The photovoltaic properties of the polycrystalline silicon depend on t he crystallinity and the purity of the material. The thermal plasma pr ocess gives us an alternative method of silicon preparation since it i s possible to produce an ultrahigh purity with simultaneously a passiv ation of crystalline defects and active impurities. We demonstrate the efficiency of the plasma purification process and particularly the in fluence of the atomic hydrogen in an argon thermal plasma on the photo voltaic properties of silicon. The results of the diffusion lengths me asured by the photoelectrochemical method show that locally it rises u p to 200 mu m. We correlate these photovoltaic measurements with the p roperties 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 microsco py, inductively coupled plasma (ICP), and neutronic activation analyse s. We show that the increase of the purity explains the high measured diffusion lengths. Nevertheless, the thermal conditions of the crystal lization of the silicon, due to the specificity of the plasma, lead to defects such as dislocations for which density is particularly high ( >10(6) dis/cm(2)). The results show that chemical reactions between th e atomic hydrogen of the plasma and the oxygen of the silicon occur. T hey decrease the oxygen content in silicon from 3 x 10(17) at./cm(3) d own to 2 x 10(16) at./cm(3), while the residual hydrogen in silicon is close to 2 x 10(15) at./cm(3). This passivates the dangling bonds of ultrapure silicon with a high thermal stability up to 1000 K, The obje ctive of this paper is to demonstrate that the hydrogen in the plasma modifies the electronic properties of the material to achieve a very g ood photocurrent even though the dislocation density of the silicon is very high.