The effect of in situ boron doping on the strain relaxation of Si 0.8Ge 0.2 : B/Si heterostructure grown by molecular beam epitaxy

Abstract

The effect of in situ elemental boron doping (boron concentration N B = 1 × 10 19 to 3 × 10 20 cm −3 on the strain relaxation of a Si 0.8Ge 0.2 : B/Si(001) heterostructure grown at 680°C is investigated. Although boron gives rise to lattice contraction in bulk Si, it does not compensate the lattice mismatch between the Si 0.8Ge 0.2 layer and Si substrate. On the contrary, it stimulates the strain relaxation. The strain relaxation of the Si 1- xGe x : B/Si(001) heterostructure mainly gives way to dislocation half-loops generated not in the Si 0.8Ge 0.2 layer, but in the Si buffer layer just below it. This means that heterointerfacial quality may be one of the major control factors of strain relaxation. The strain relaxation induced by boron doping is observed even at N B ∞ 10 19 cm −3, which is in the doping range typically used in Si 1- xGe x / Si heterojunction bipolar transistors (HBTs). Therefore, as well as Ge mole fraction and growth temperature, N B should also be considered in determining critical thickness of a Si 1- xGe x layer grown on a Si substrate.