These results contradict existing opinion that for fabrication of silicon sutable for PV cells pure hydrogen or pure neutral gases or vacuum are strictly required. And that for purification of silicon from boron metallurgical methods are not suitable. Obtained results cannot be explainen in terms of traditional understanding of semiconductor silicon production. Explanation of this phenomena one has to search in specific peculirities of the considered process.

The process can be presented as following (fig. 1 ).

Crystallization process occurs on a liquid — solid border. In this case solidified material properties are determined not only by processes at the front of crystallizftion, but by processes at liquid — gas phase frontier as well.

Energy for the process in form of concentrated sunlight flow is transfered to the surface of the melted zone. Therefore, melt temperature near the front of crystallization will be near the Si — melting temperature (namely 1412 oC).

At the same time the temperature at the liquid-gas phase border will be much higher. Thereby the gradient of temperature between a free surface of melted zone and front of crystallization is formed.

Crystallization process occurs on a liquid — solid border. In this case solidified material properties are determined not only by processes at front of crystallizftion, but by processes at liquid — gas phase frontier as well.

Energy for the process in form of concentrated sunlight flow is transfered to the surface of the melted zone. Therefore, melt temperature near the front of crystallization will be closed to the Si-melting temperature (namely 1412 o C). At the same time the temperature at the liquid-gas phase border will be much higher. Thereby the gradient of temperature between a free surface of melted zone and front of crystallization is formed.

On the other hand, the situation on an interface liquid — gas phase will be defined not only by processes of interaction of the fused silicon and its vapor at near surface areas of a gas phase with components of the air environment (oxygen, nitrogen, a moisture) but also by processes of interaction of a surface of the fused silicon with the concentrated sunlight. Therefore one can assume that gas phase composition in a thin layer adjoining to melted silicon will be depend upon not only the diagrame of condition of a comlex system consisting of oxigen-nitrogen-mousture-carbon dioxide-impurities. Very important role can play mechanism of interraction of concentrated solar radiation with a fused silicon surface itself and adjacent to the surface thin gas phase layer.

In these conditions it is possible to expect existence of components not only in atomic, but also in ionized state, capable to active interaction. In this case the conditions arising on the surface can be compared to conditions on a surface of the fused silicon subjected plasma processing.

Distinctive feature of the silicon obtaining process under influence of the concentrated sunlight is the opportunity for fabrication of high quality ingots directly on open air. It allows to lower expenses for expendable materials and to simplify a design of installation.

It is not enough received experimental data for identification of the processes occures in these conditions, and chemical compounds formed in this case.

Probably the process of impurities removal in the given conditions could be explained within the framework of the mechanisms described in [4].

Purification of crystallized silicon under the considered circumstances occurs in two ways:

1- cleaning due to segregation phenomena on border liquid-solid from impurity with low coefficiet of distribution;

2- cleaning due to removal of impurities from a bath of melt into a gas phase through liquid — gas phase interface as volatile substances.

Removal of impurities on both specified ways may be intensified by active mixing within melted zone.

As a result impurities which are concentrating in liquid phase near the border with solidifized silucon because of segregation will be more actively removed from the front of solidification into a volume of liquid silicon, and impurities which are removing into gas phase as a volatile components come from volume to the border with gas phase with higher velocity.

It is obvious, that realization of active mixing in the liquid drop formed on an end face of a growing crystal is extremely difficult, practically impracticable problem because the volume and the form of a bath melt at an end face of an ingot are rather critical parameters for growing of homogeneous on diameter and electrophysical parameters ingots.

Any influences on volume of the fused silicon drop result or to it spilling, or in change of the form of obtained ingots.

The technological processes realized in laboratory scale (solar heating in the concentrator in diameter of 3 m and plasma heating with application of energy sources up to 100 kw) do not allow to use them for large-scale SOG-Si manufacture.