Precision silicon doping with acceptors by temperature gradient zone melting

We present the results of optimization of the temperature gradient zone melting technique, also known as the thermomigration (ThM) technique, aimed on improvement of the quality of p-layers and formation of p-n junctions with sharper boundaries compared to those obtained by conventional thermal diffusion technique. In addition, ThM allows an expansion of the range of doping of silicon substrates with an acceptor impurity, usually limited by the solidus value. The ternary Al-Ga-Si and Al-Sn-Si melts were used as ligatures. The dependences of the migration rate of the liquid zones on temperature and composition for the Al-Ga and Al-Sn solvent metal are presented. The possibility of changing the acceptor concentration from 2·10¹⁹ cm⁻³ to 6·10¹⁹ cm⁻³ is shown. A threshold temperature of 1400 K was experimentally found for the ThM process with stable migration of triple liquid zones in a crystal. X-ray diffractional rocking curves and projection topography confirmed high structural quality of the Si(Al) p-layer with a thickness of 25 µm, obtained by ThM with stable moving liquid zones. The experimentally measured strain Δd/d=2.3×10⁻⁵ for the p-layer and the silicon substrate was used in the substitution model to estimate the Al concentration of 1×10¹⁹ cm⁻³. X-ray topographic images of the layers did not reveal both growth S-defects and misfit dislocations, confirming the high structural perfection of the layers and phase boundaries.