Single-crystalline Si1−xGex:B films with active doping concentration up to 3.5 × 1021 cm−3

High doping level in Si(Ge) is crucial for reducing contact resistance of transistors, which is key for lowering power consumption in advanced nodes. In this paper, we report exceptionally high active boron (B) doping concentrations exceeding 3.0 × 1021 cm−3 in in situ doped Si1−xGex films grown at 300 °C by molecular beam epitaxy. At a fixed chemical B doping concentration of 3.8 × 1021 cm−3, the active doping concentration in Si1−xGex increases with increasing Ge content, reaching a maximum of 3.56 × 1021 cm−3 at x = 0.5, and then decreases beyond this point. A turning point is observed in the resistivity vs active doping concentration curve. The increase in resistivity is attributed to mobility degradation under ultrahigh doping levels. High-resolution transmission electron microscope characterization confirms that the highly doped SiGe films maintain single-crystalline quality. The successful realization of single-crystalline Si1−xGex films with highly active B concentrations over 3.0 × 1021 cm−3 under low growth temperatures has shown the potential for achieving low contact resistivity and low thermal budget in p-type transistors.