Suppressed boron diffusion in bulk silicon below strained (100) Si _1-xGe _x surfaces during nitrogen annealing

The silicon surface can be the dominant point defect recombination/ generation site for the silicon bulk in many annealing conditions. The presence of SiGe at the surface has been reported to suppress oxidation enhanced boron diffusion in the underlying bulk silicon. Little, however, has been reported about the quantitative effects of strained SiGe surfaces on dopant diffusion although the strain and composition may perturb the equilibrium point defect concentrations in the surface region. In this work, boron diffusion marker layers were epitaxially grown in silicon by rapid thermal chemical vapor deposition followed by an undoped capping layer of either silicon, 45 nm of Si _0.75Ge _0.25, or 5 nm of Si _0.55Ge _0.45. The boron diffusion in the epitaxially grown silicon was subsequently examined after annealing in nitrogen at temperatures between 750 - 825°C, and the boron diffusivity below the Si _1-xGe _x, surface layers was found to be between 2 - 3 times slower than that in the all-silicon samples. The activation energy for the boron diffusivity in the SiGe capped samples was, furthermore, observed to increase by as much as 0.65 eV in the Si _0.75Ge _0.25 capped case, comparable to increases in activation energies reported for the boron diffusivity within strained Si _1-xGe _x containing similar germanium concentrations and strain. This work demonstrates that a Si _1-xGe _x surface can have a non-local effect on boron diffusion in silicon below it during nitrogen annealing.