Toward Exotic Silicon Doping with a Low Thermal Budget and Flexible Profile Control by Liquid-Phase Epitaxy.

We investigate semiconductor p-n junction formation by liquid-phase epitaxy (LPE) using metallic pastes incorporating traditional and nontraditional dopants. The LPE technique enables us to control the shape of doping profiles with a low thermal budget through the choice of solvent, total amount of solvent deposited, and process temperature. We focus here on the Al-B, Zn-P, and Sn-Ga chemistries to dope silicon regions using the chemicophysical properties of a low-eutectic-temperature metallic solvent acting as a matrix for the dissolution of a high concentration of a dopant. Additionally, we developed a capping method enabling doping across a large surface area wafer with a tunable thickness well below 1 μm without film dewetting. In good agreement with thermodynamic simulation of the LPE process, we demonstrate B- and Al-doped regions with a sheet resistance ranging from less than 10 to 300 Ω/sq between 650 and 800 °C, which is significantly lower than the typical temperatures of gas-phase doping processes. Comprehensive electrical simulations suggest that LPE p-n junctions with a low carrier recombination activity can be fabricated via the reduction of surface doping concentration and improved surface recombination velocity. Our investigation of exotic LPE chemistries suggests that emitter saturation currents below 50 fA/cm2 could be achieved at doping concentrations relevant to solar cells.