Development of a new crystal growth process using a semiconductor powder and reducing the thermal stresses

Abstract

The electron powder ribbon (EPR) method allows one to achieve crystallization of a silicon powder irradiated by an electron beam, giving multicrystalline plates with a mean crystal size from 5 to 50 mm 2. During the elaboration of the plates the unadapted distribution of the electron beam energy leads to the existence of thermal stresses. We studied the thermal conduction during the solidification process of the plate and the influence of the electron beam characteristics on the thermal profiles by modelling the thermal exchanges in the plates using a three-dimensional network. We obtain a system of equations which are solved by computer calculation using the finite difference method. The numerical simulation allows us to follow the evolution of the temperature profiles and to determine the stress induced during the crystallization process. In order to reduce these stresses, a new computer-aided procedure is carried out to control the electron beam and to regulate the irradiation power. Linear temperature profiles during the crystal growth step can be ensured and the thermal imbalance due to the energy losses near the sample edges is reduced to a minimum. Moreover, the goal of the electron beam monitoring is also to decrease the number of grain boundaries in the plates by increasing the grain size, following the promising work of A. Chibani ( Thèse de Doctorat, INSA de Lyon, 1990) on the oriented crystallization of EPR plates with an initial germ.